T.C.
SELÇUK ÜNİVERSİTESİ FEN BİLİMLERİ ENSTİTÜSÜ
EFFECTS OF COMPOST AND BIOCHAR APPLICATIONS ON SOIL PROPERTIES OF A SANDY SOIL AND CORN PLANT GROWTH (BİYOKÖMÜR VE KOMPOST UYGULAMALARININ KUMLU BİR
TOPRAĞIN ÖZELLİKLERİ İLE MISIR BİTKİSİNİN GELİŞİMİNE ETKİLERİ)
Noel MANIRAKIZA Master’s Thesis
Soil Science and Plant Nutrition Department
August-2019 KONYA All rights reserved
iv
ÖZET
YÜKSEK LİSANS TEZİ
BİYOKÖMÜR VE KOMPOST UYGULAMALARININ KUMLU BİR TOPRAĞIN ÖZELLİKLERİ İLE MISIR BİTKİSİNİN GELİŞİMİNE
ETKİLERİ Noel MANIRAKIZA
Selçuk Üniversitesi Fen Bilimleri Enstitüsü
Toprak Bilimi ve Bitki Besleme Anabilim DalıDanışman Prof. Dr. Cevdet ŞEKER
2019, 87 Sayfa Jüri
Prof. Dr. Cevdet ŞEKER Prof. Dr. Ayşen AKAY
Dr. Öğr. Üyesi Sevim Seda YAMAÇ
Çalışmada, rüzgâr erozyonundan etkilenmiş olan, Karapınar Erozyonla Mücadele istasyonundan, 0-20 cm derinlikten alınan toprak örneği kullanılmıştır. Bu alandaki toprakların temel özellikleri kum ve kireç içeriklerinin yüksekliği, düşük organik madde içerikleri, zayıf strüktürle gelişim göstermeleri, düşük su depolama kapasiteleri ve bu nedenle kuraklık etkisine açık olmalarıdır. Ayrıca bu bölgenin kurak ve yarı kurak iklimin etkisinde bulunması, toprakların fiziksel özelliklerinin zayıflığının yanında, kimyasal ve biyolojik özelliklerinin de yetersizliği nedeniyle biokütle üretimleri sınırlanmakta ve rüzgâr erozyonuna açık hale gelmektedir. Bu nedenle yapılan çalışmada; söz konusu alanda, yaygın olarak yetişen iğde ağaçlarının, budama artıklarından elde edilen kompost ve biyokömürün farklı dozlarının, inkübasyon ve saksı denemesinde, toprak kalitesi özellikleri ile mısır bitkisinin gelişimine etkileri belirlenmiştir. Bu kapsamda, yapılan inkübasyon denemesinde; ağırlık esasına göre % 0, 1, 2 ve 4 dozlarında kompost ve biyokömür ilave edilen toprak örnekleri, tarla kapasitesi nem içeriğinde (29g 100g-1) iki ay süre ile inkübasyona bırakılmıştır. İnkübasyon sonunda uygulamaların toprağın fiziksel, kimyasal ve biyolojik özelliklerine etkileri belirlenmiştir. Ayrıca inkübasyon sonu alınan toprak örneklerinde mısır bitkisi yetiştirilerek, uygulamaların mısır bitkisinin gelişim özelliklerine etkileri de incelenmiştir. İnkübasyon sonunda, kompost ve biyokömür uygulamaları kontrol ile kıyaslandığında; zerre yoğunluğu (Pk), hacim ağırlığı (Pb) ve hava dolu gözeneklilik (AFP) değerlerini düşürürken, porozite (P), tarla kapasitesi (FC), faydalı su içeriği (PAW), agregat stabilitesi (AS), ağırlıklı ortalama çap (MWD), ekstrakte edilebilir K, yarayışlı Mn, yarayışlı fosfor (AP), organik karbon (OC), toplam azot (TN), NH4-N, NO3-N, C/N oranı ve toprak solunumu (SMR) değerlerinin önemli ölçüde artırmıştır. Uygulamaların diğer özellikler üzerine etkileri değişkenlik göstermiştir. Kontrol ile kıyaslandığında, uygulamaların sera şartlarında yetiştirilen mısır bitkisinin bitki boyu ve taze toprak üstü biokütle verimine etkileri önemli olmuş, kompost uygulamaları mısır bitkisinin SPAD değerini, toplam azot, K, Ca, Mg ve Zn içeriklerini önemli ölçüde artırırken, biyokömür uygulamaları sadece SPAD değeri, Mg ve Fe içeriklerini önemli ölçüde artırmış, Cu içeriğini ise azaltmıştır.
Anahtar Kelimeler: Biyokömür, kompost, mısır bitkisi, kumlu kil tın toprak, toprak kalitesi özelikleri
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ABSTRACT MS THESIS
EFFECTS OF COMPOST AND BIOCHAR APPLICATIONS ON SOIL PROPERTIES OF A SANDY SOIL AND CORN PLANT GROWTH
Noel MANIRAKIZA
THE GRADUATE SCHOOL OF NATURAL AND APPLIED SCIENCE OF SELÇUK UNIVERSITY
THE DEGREE OF MASTER OF SCIENCE IN SOIL SCIENCE AND PLANT NUTRITION
Supervisor: Prof. Dr. Cevdet ŞEKER 2019, 87 Pages
Jury
Prof. Dr. Cevdet ŞEKER Prof. Dr. Ayşen AKAY
Dr. Öğr. Üyesi Sevim Seda YAMAÇ
In this study, soil samples taken at 0-20 cm depth from Karapınar Erosion Prevention Station which was affected by wind erosion were used. The main characteristics of the soils of this area are high sand and lime contents, low organic matter contents, weak structure development, low water storage capacity and thus open to drought effect. In addition, since this region is under the influence of arid and semi-arid climate, the weakness of the physical properties of its soils as well as insufficient chemical and biological properties, biomass production is limited, and it becomes vulnerable to wind erosion. For this reason; the effects of different doses of compost and biochar obtained from pruning residues of Elaeagnus trees, which are widely grown in this area on soil quality characteristics and development of corn plant were determined in incubation and pot experiment. In this context, soil samples were mixed with compost and biochar at 0, 1, 2 and 4% by weight basis throughout incubation trial, and were entitled to be incubated for two months at field capacity moisture content (29g 100g-1). At the end of the incubation, the effects of the applications on the physical, chemical and biological properties of the targeted soil were determined. In addition, the effects of the applications on the growth characteristics of the corn plant were investigated by cultivating the corn plant in the soil samples taken after the incubation. At the end of incubation, compost and biochar applications decreased particle density (Pk), bulk density (Pb) and air-filled porosity (AFP) values compared to control, while porosity (P), field capacity (FC), plant available water content (PAW), aggregate stability (AS), mean weight diameter (MWD), extractable K, available Mn, available phosphorus (AP), organic carbon (OC), total nitrogen (TN), NH4-N, NO3-N, C / N ratio and soil respiration (SMR) values significantly increased. The effects of applications on other soil features varied. When compared with the control, the effects of the applications on the plant height and abovegrown plant biomass yield of the corn plant grown in greenhouse conditions were significant. Compost applications significantly increased the SPAD value of the corn plant, total nitrogen, K, Ca, Mg and Zn contents, while the biochar applications only increased SPAD values, Mg and Fe contents and decreased Cu content.
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ACKNOWLEDGEMENTS
Firstly, I would like to express my heartfelt and thanks to the Almighty God, again I would like to acknowledge my supervisor Prof. Dr. Cevdet ŞEKER for his unwavering assistance and advice to accomplish this research.
I would like again to express my much gratitude to our lecturers namely Dr. Hamza NEGİŞ, Dr. İlknur GÜMÜŞ and Research assistant Vildan ERCİ for their help and daily life advices. I would like again to express my deepest gratitude to Turks Abroad and Related Communities Presidency (YTB) for granting me scholarship, as well as Selçuk University Scientific Research Project (BAP) for funding this research. My special thanks are also expressed to my father and mother namely Nyakamwe THEOGENE and Mbabajende DONATHA, respectively as well as my brothers namely Hakizimana LEOPOLD, Habimana FERDINAND and Gakuru Celestin for their assistance, advice and uncountable love that they gave me. Additionally, I would like to give thanks to the PhD student namely Qutaiba ABDULWAHHAB and Ayşe ÇETİN for their diverse assistance during my academic journey.
Noel MANIRAKIZA KONYA-2019
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TABLE OF CONTENT
ÖZET ... iv
ABSTRACT ... v
TABLE OF CONTENT ... vii
1. INTRODUCTION ... 1
2. RESEARCH RESOURCES ... 3
3. MATERIAL AND METHOD ... 7
3.1. Site Characteristics, Preparation of Used Amendments, Laboratory and Greenhouse Experimental Design ... 7
3.2. Laboratory Analysis and Measurements ... 9
3.2.1. Soil analyses ... 9
3.2.2. Plant measurement and analyses ... 12
3.3. Statistical Data Analysis ... 12
4. RESULTS AND DISCUSSIONS ... 13
4.1. Effect of Compost and Biochar on Soil Quality Properties ... 13
4.1.1. Changes in soil physical properties due to compost and biochar applications ... 13
4.1.1.1. Bulk density ... 13
4.1.1.2. Particle density ... 14
4.1.1.3. Total porosity ... 16
4.1.1.4. Water retention at different suction pressures and PAWC ... 18
4.1.1.5. Air filled porosity ... 22
4.1.1.6. Aggregate stability ... 23
4.1.1.7. Mean weight diameter ... 26
4.1.2. Changes in soil chemical properties due to compost and biochar applications ... 30
4.1.3. Changes in soil biological quality properties due to compost and biochar additions ... 40
4.2. Effect of Compost and Biochar on Nutrients Uptake and Maize Plant Growth .. 49
4.2.1. Effect of amendments on growth of maize (Zea mays L.) plant ... 49
4.2.2. Effect of amendments on nutrient uptake by maize (Zea mays L.) plant ... 51
5. CONCLUSION AND RECOMMENDATION ... 57
REFERENCES ... 61
Appendix ... 74
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ABBREVIATIONS AND SYMBOLS
Abbreviations
SCL: Sandy clay loam Pb: Soil bulk density
Pk: Particle density
FC: Field capacity
PWP: Permanent wilting point PAWC: Plant available water content AFP: Air filled porosity
MWD: Mean weight diameter θv: Volumetric water content
OC: Organic carbon AC: Active carbon
AP: Available phosphorous
TN: Total nitrogen
PMN: Potentially mineralized nitrogen SMR: Soil microbial respiration
APFB: Aboveground plant fresh biomass APDB: Aboveground plant dry biomass RFB: Root fresh biomass
RDM: Root dry matter PH: Plant height SG: Stem girth
1. INTRODUCTION
Due to poor soil productivity of a degraded calcareous and alkaline SCL soil, induced by poor aggregation and aggregate stability, poor organic matter, weak structure, low water and nutrients holding capacity, as well as other correlated poor soil physical, chemical and biological quality properties, rational agricultural practices are required for improving the aforementioned difficulties found in a degraded calcareous and alkaline SCL soil in order to enhance agricultural production. Arid and semiarid areas are endowed with a constrained annual precipitation and soil of these regions are subjected to accumulative evaporation and deep percolation losses due to low water holding capacity, excessive organic matter oxidation, and consequent decrease of water storage capacity and aggregation at large.
Soil properties have a definite role in tightening plant root growth, distribution and crop yields as well as productivity of agricultural land as reviewed by Manirakiza and Şeker (2018). Agricultural system has to be substantially productive and resilient, in order to meet escalating global food demand when there are the challenges of limited resource and climate fluctuations (FAO, 2004a). The infusion of organic matter into the soil has long been found to increase productivity and resilience of agricultural land via improving soil properties culminating in increased agricultural production, which in turn help meet global food demand. Compost and biochar are both utmost soil amendments about their uses not only as organic matter source but also source of plant nutrients.
In order to increase water retention capability and aggregation in a soil with a constrained water holding capacity, specifically coarse textured soil endowed with a rapid infiltration and drainage conditions (Bigelow et al., 2004) and poor aggregation, organic materials as soil amendments are highly required. Compost and biochar are amongst soil amendments derived from organic materials, which increase soil organic matter playing role in improving soil aggregation and water holding capacity via decreasing water losses due to deep percolation and evaporation, and herein lies in the root of improving soil water use efficiency and decreasing irrigation frequency. It has been noted that soil amendments effectively play an important role in increasing water retention, decreasing infiltration rate and evaporation as well as enhancing conservation of water under a sandy soil (Al-Omran et al., 1987).
The constraints to soil quality and crop performance found in calcareous and alkaline soils encouraged us to conduct this study. It is in this regard, Eleagnus tree which is grown in the calcareous and alkaline sandy clay loam soil exposed to wind erosion for preventing wind erosion was thought to be used in order to develop in situ solutions. This tree has a woody structure and cannot be directly adopted as soil amendment. Because of this, it was converted into two forms (compost and biochar), which were used as soil amendments to reverse degraded soil quality and crop performance of a calcareous and alkaline sandy clay loam soil exposed to wind erosion. Therefore, the aims of this study were to quantify the effect of compost and biochar both made from Elaeagnus tree applied alone to calcareous and alkaline sandy clay loam soil exposed to wind erosion on soil quality properties and crop performance of maize plant.
2. RESEARCH RESOURCES
Since the inception of farming, crop remainders and organic soil amendments have been employed for improving the aforementioned poor physical properties of a SCL soil; nevertheless, the agronomical benefits are of fine period due to being exposed to increasingly microbial activities (Schneider et al., 2009). As such, organic amendments are endowed with a tremendous organic carbon; still they are rapidly decomposed by soil microorganism (Palm et al., 2001), as the result of rapid organic matter depletion in the soil culminating in decreased soil fertility. Compost as one of the organic amendments, is used to avail suitable soil physical condition, such as improved aggregation and water retention capacity for plant root development resulting in increased agricultural production. Previous study revealed that the infusion of compost into the soil improved soil water storage via decreasing evaporation (Opara-Nadi and Lal, 1986), as well as deep percolation, especially in the SCL soil. The impact of compost on soil physical properties improvement has been investigated by several researchers, and Eusufzai and Fujii (2012) reported that compost addition has a potent impact on improving quality, hydraulic properties and pore size distribution of the soil. Newly, evidenced studies have reflected that compost addition to soil decreased bulk density and particle density, as well increased soil total porosity, soil water retention capacity and plant available water content (Barus, 2016). In contrast, biochar is also considered soil amendment, which has been proven to be recalcitrant to microbial activity than organic materials (Zimmerman, 2010), as a result of long-term organic matter maintenance in the soil and has a tremendous perks in rejuvenating either degraded or nutrients depleted arable land by increasing the availability of agricultural soils and escalating agricultural production, and thereby decreasing needs of agricultural land scale expansion (Blackwell et al., 2009; Barrow, 2012). Biochar as soil amendment possesses a tremendous perks in enhancing soil physical and chemical properties (Luo et al., 2017), due to being endowed with good physical condition, such as high porosity and surface area (Van Zwieten et al., 2010).
Biochar is a material endowed with a sizeable amount of carbon, which is generated from pyrolysis process at a hefty temperature ranging from 300 to 1000oC under either constrained or free-oxygen environment (Verheijen et al., 2010). Biochar’s
composition and structure are affected by employed organic materials and pyrolysis process (i.e. time and temperature) (Crombie et al., 2013; Ronsse et al., 2013; Ippolito et al., 2015; Subedi et al., 2016), culminating in a pronounced differences in biochar properties (Granatstein et al., 2009; DeLuca et al., 2015). Although scaling up temperature during pyrolysis seems to scale down yields and cation exchange capacity of biochar, carbon, potassium and magnesium content, pH and increase specific surface area (Ippolito et al., 2015). It was reported that produced biochar from wood and plant residues are endowed with low nutrients as compared with the one produced from manure and other animal products (Singh et al., 2010; Alburquerque et al., 2014), yet biochar from woody feedstock are coarser and even more recalcitrant than the one from agronomic remainders (Aslam et al., 2014). Rillig and Thies (2012) found that biochar enhances physicochemical soil properties due to being endowed with a high porosity, surface area and able to adsorb organic matter and plant nutrients. This adsorption effect could elucidate that it has the high capability of being adsorbed by water, and thereby increasing water retention capacity of a soil. The potentiality of biochar for improving soil physical properties increasingly raised from being identified as a material endowed with a high porosity (Liang et al., 2006; Hina et al., 2010) and an extensive inward surface area (Kishimoto, 1985; Van Zwieten et al., 2009). Biochar’s porosity increases with pyrolysis’ temperature (Schimmelpfennig and Glaser, 2012) and also depends on biomass types (Hina et al., 2010). In the same way, biochar produced at a high temperature is endowed with a lower nutrient contents and higher micro-porosity, whereas the one produced at low temperature is endowed with a higher nutrient contents and lower micro-porosity (Lehmann and Joseph, 2009) . Biochar has an agronomical benefit in the context of adding organic matter and plant nutrients to the soil system. Biochar is more recalcitrant than other organic amendments due to its aromatic structure and crystalline graphing sheet (Aslam et al., 2014), and its time of recalcitrance in the soil is 10-1000 times more than organic materials (Atkinson et al., 2010). While increasing organic matter into the soil, biochar help increase soil aggregate stability resulting in improved other soil properties. Many studies touching on how biochar as a soil amendment affect soil physical properties were conducted. Previous results revealed that the infusion of biochar into a soil decreased bulk density (Laird et al., 2010; Brewer et al., 2014) and increased soil water holding capacity (Ouyang et al., 2013), increased total porosity (Omondi et al., 2016) and mean weight diameter (Hseu et al., 2014).
The effects of compost and biochar in improving soil properties are well-known and have been even revealed by several researches. Previous research reflected that compost and biochar additions improved soil chemical properties by increasing retention capacity of water and nutrients and biological properties by providing habitat for soil microorganisms resulting in increased microbial activity, and thus enhance soil structural formation and stability (Lorenz et al., 2007; Lal, 2009). Unlike chemical and biological properties betterment, compost and biochar significantly improve soil physical properties (Seker, 2003; Gümüş and Şeker, 2017), also these were reviewed by Manirakiza and Şeker (2018). Compost, cover crop, manure and mulches are the prime organic amendments, which rapidly improve soil fertility by recycling nutrient through microbial activities, and thereby improving nutrient provision to crop (Trujillo, 2002). Application of organic amendments was found to improved soil quality and plant yield under a degraded agricultural land (Dormaar et al., 1988; Sun et al., 1995; Dormaar et al., 1997). Biochar has been reported to improve soil acidity and salinity, as well as mitigating global warming by sequestering carbon in long term (Lehmann et al., 2003; Van Zwieten et al., 2014). In addition, Atkinson et al. (2010) reported that recalcitrance time of biochar is 10-1000 times over that of organic materials in the soil and Paz-Ferreiro et al. (2012) reported that biochar application alter soil biochemical properties. Biochar as a material endowed with a sizeable amount of carbon, is produced by charring biomass at a high temperature in no or limited oxygen environment and has been found to have a positive effect on improving soil biochemical properties due to being endowed with a sizeable amount of plant nutrients and porosity, all of which are responsible for providing habitat to soil microorganism (Lehmann and Joseph, 2009). Several study revealed that biochar addition increased pH, CEC, exchangeable cations (Ca, Mg and K), TN, and AP, decreased soil acidity by lowering acidic cations (i.e. aluminium) (Glaser et al., 2002; Schulz and Glaser, 2012), and thereby indicating betterment in soil biological properties. Other researches also indicated that biochar increased soil OM, pH, AP and decreased leaching of Ca, Mg and N, as well as scaled down aluminium level (Lehmann et al., 2003; Chan et al., 2008b), and consequent improvement in soil microbial activities. Declined concentration of aluminum toxicity due to biochar addition, has been reported in degraded agricultural land (Glaser et al., 2002; Yamato et al., 2006; Van Zwieten et al., 2010).
Biochar is obtained from biomass through pyrogenic process. Biochar is thought as a promising avenue to sequester carbon, decrease emissions of greenhouse gases and improve soil fertility (Lehmann et al., 2006; Vaccari et al., 2011; Liang et al., 2014b) and promote crop growth (Atkinson et al., 2010), and consequent improvement of agricultural production. Previous study revealed that biochar addition increased soil organic carbon (Glaser et al., 2002), soil water retention (Atkinson et al., 2010; Abel et al., 2013), plant nutrient availability and retention (Lehmann et al., 2003; Steiner et al., 2007; Laird, 2008), microbial biomass and functions (Thies and Rillig, 2009), and consequent plant growth and yield increases (Jeffery et al., 2011). It was reported that biochar increased the sequestration of carbon, and consequent decrease of greenhouse gases emissions (Lehmann et al., 2006; Lal, 2011). Enhanced soil chemical, biological and physical quality induced by biochar application, has been reported by (Glaser and Birk, 2012). As could be anticipated, Farrell et al. (2014) reported an inconsistent effect of biochar on acidic and alkaline soil. (Yuan et al., 2011; Glaser and Birk, 2012) found that biochar can act as liming materials once applied to soil, and consequent pH increases. However, decreased or insignificantly affected pH in calcareous soil has been shown by (Van Zwieten et al., 2010; Liu and Zhang, 2012). An upward trend in pH due to biochar has been reported by Chan et al. (2008b), as well as alteration in EC and CEC (DeLuca et al., 2015). P availability in calcareous soil was affected by biochar addition (Farrell et al., 2014). Increased in soil nutrient status (e.g. Ca, Mg and Zn) due to biochar addition, have been revealed by (Major et al., 2010; Gartler et al., 2013). The positive agronomical benefits of organic amendments were also reported by several researchers. Lashari et al. (2013) pointed out that compost and biochar additions significantly improved soil properties, as well as performance and yield of maize crop and the similar trends were corroborated by Lentz et al. (2014). The consistent trends were observed by Nur et al. (2014) where maize growth and yield were increased by virtue of compost and biochar applied in combinations. Agegnehu et al. (2016) reported that compost and biochar improved soil fertility and crop growth, and these findings were in accordance with other researches (Cornelissen et al., 2013; Doan et al., 2015) and also reviewed by Manirakiza and Şeker (2018).
3. MATERIAL AND METHOD
3.1. Site Characteristics, Preparation of Used Amendments, Laboratory and Greenhouse Experimental Design
The two-stage study was carried out on the soil sample taken from the land classified as Xeric Haplogypsid, located at the coordinates of 37.72o N latitude and 33.55o E longitude of the Karapınar Erosion Control Station which was affected by wind erosion (Akça, 2001). A combined sampling was carried out from 0-20 cm depth at 15-20 separate points of the land parcel in question. The air-dried soil sample in the laboratory was passed through a 2 mm and the sieved sample at 4 mm for MWD was used in laboratory analysis and incubation study. Soil sample used in the study has a sandy clay loam textures (60.48% sand, 13.33% silt and 26.19% clay), very high lime content (660 CaCO3 g kg-1), alkali reaction (pH 8.62) and low salt (0.236 mS cm-1) and organic carbon (10.03 g kg-1) content and poor aggregation properties (Table 1 and 2).
In the first stage of the study, 28 pots were employed according to the following experimental design: Two forms of Elaeagnus tree (compost and biochar) x 3 rates of application x 4 replications plus 4 controls in a completely randomized design. The applied rates were: 1, 2 and 4 % (wt/wt) for every applied amendment, which were thoroughly mixed with 3 kg of air-dried soil sieved through 2-mm sieve, subsequently potted in a 5 litre plastic pot, all pots including the control (i.e. 0% application rate) were moistened to exactly FC (0.29 g g-1), then incubated for two months for homogenizing the mixture and weekly water was brought back to FC. Soil samples were collected from every pot for evaluating the responses of soil physical, chemical and biological quality properties to the individually applied two forms of Elaeagnus tree (compost and biochar) at the end of incubation period.
In the second stage of the study, corn plant was grown under greenhouse conditions to determine the effects of post-incubation applications on plant growth. After incubation, 1 kg soil sample base on oven dry weight basis was sampled from every pot, potted in a 1.5 litter pot and then all pots were placed in a greenhouse. Maize (Zea mays L.) plant was chosen as a test crop and four seeds were sown at 2-3 cm in
every pot. Two weeks after germination, seedlings were thinned to 2 plants per pot and maize plant was grown within the period of 77 days, and irrigation was frequently
performed regarding micro pan cab evaporation value. After 77 days of greenhouse experiment, plant growth, leaf chlorophyll content, plant height, stem girth, fresh shoot biomass, dry shoot biomass, fresh root biomass, dry root biomass, the uptake of P, Ca, Mg, K, Fe, Cu and Zn by plant, as well as pH, EC, soil available cations (Ca, Mg, K and Na), extractable micronutrients (Fe, Cu, Mn and Zn) and available phosphorous were determined.
The employed materials at this study were a) compost as soil amendment was produced from pruning residues of acacia tree whereby C/N ratio and moisture were adjusted exactly at 25:1 and 70 % for facilitating microbial activities and detailed composting processes are described by Mücehver et al. (2018) and after composting, obtained compost was passed through 2 mm sieve prior to application; b) Biochar as soil amendments was produced from Elaeagnus tree through slow pyrolysis process (7 o
C min-1 ramp), in which charring of Elaeagnus tree occurred at 450 0C in the muffle furnace as elucidated in detail by Brewer (2012); Mücehver et al. (2018). After pyrolysis process, carbonized product was removed from the muffle furnace and cooled at room temperature, then ground to pass through a 2-mm sieve prior to use. The basic chemical properties of compost are a pH of 8.86, electrical conductivity of 3.60 mS cm-1 and lime content of 58.9 g kg-1 while the one of biochar are pH of 8.95, electrical conductivity of 0.192 mS cm-1 and lime content of 60.2 g kg-1 (Table 2).
Table 1. The physical characteristics of the experimental soil used in the study
Parameters Units Soil sample
Soil texture % of S, C and Si SCL
Bulk density g cm-3 1.19
Particle density g cm-3 2.59
Total porosity cm3 cm-3 0.54 Soil aggregate stability % 24.87
Mean weight diameter mm 0.11
Field capacity g g-1 0.29
Permanent wilting point g g-1 0.12 Plant available water content g g-1 0.17 S: Sand (60.48 %); C: Clay (26.19 %); Si: Silt (13.33 %)
Table 2. The chemical and biological characteristics of soil sample and materials used in the study
Parameters Soil and Materials
Soil Compost Biochar
pH (1:2.5a; 1:10b; 1:20c) 8.68a 8.86b 8.95c EC (mS cm-1) 236a 3593b 192c CaCO3 (g kg-1) 660 58.7 60.2 CEC mmol kg-1 216 nd nd Ca (mg kg-1) 4674 nd nd Mg (mg kg-1) 579 nd nd K (mg kg-1) 688 21940 9452 Na (mg kg-1) 67.08 nd nd Fe (mg kg-1) 4.548 768 234 Mn (mg kg-1) 0.269 175 66.33 Cu (mg kg-1) 0.528 8.33 2.33 Zn (mg kg-1) 0.269 209 138 AP (mg kg-1) 18.38 nd nd Organic carbon (g kg-1) 10.03 236 521
Soil microbial respiration mg CO2 100 g-124 hrs-1) 41.00 nd nd
Active carbon (mg C kg-1) 693 nd nd
Potential mineralizable nitrogen (µg N g -1 week-1) 6.91 nd nd
Total nitrogen (%) 0.103 3.50 2.42
NH4+-N (mg kg-1) 11.20 nd nd
NO3--N (mg kg-1) 12.95 nd nd
C/N ratio 9.78 11.9 36.5
Notes: Not quantified (nd); Extractable cations (Ca, Mg, K and Na); Available micronutrients (Cu, Fe, Zn and Mn); Available phosphorous (AP).
3.2. Laboratory Analysis and Measurements
3.2.1. Soil analyses
3.2.1.1. Soil physical analyses
At the end of incubation period of two months, soil was mixed and subsequently samples were taken from every pot. Prior to analyses of soil physical properties, soil samples were air dried and passed through a 2- mm sieve, except 4-mm sieve for MWD analysis.
Pb was measured through the protocol developed by Jacobs et al. (1964).
cylinder of 100 ml were used. Firstly, dry soil sample was filled into the graduate cylinder of 100 ml up to one-fourth, then simulate natural packing was done by carefully tapping the cylinder five times, and thereafter graduate cylinder was filled with the soil again up to three-fourth and packed similarly. Finally, graduate cylinder was fully filled with the soil and Pb was calculated using the following formula:
Pb=Grams of soil (g) / Volume of soil (cm3).
Particle density was measured through the pycnometer method (Blake and Hartge 1986). Both Pb and Pk were used to calculate total porosity of the soil, and by inference the water holding capacity of the soil (Danielson et al., 1986). The determination of soil aggregate stability was achieved through artificial rainfall simulator device and employed procedures are described in the protocol formulated by Gugino et al. (2009).
MWD is an important indicator of soil particles size distribution, which is also
tied to soil aggregates stability and was determined as described by Elliott (1986). A set of four sieves were employed to get four sizes of aggregates, such as large soil macro-aggregates (2-4mm), macro-macro-aggregates (0.25-2mm) and micro-macro-aggregates (0.053-0.25mm). The separation of the aggregates was achieved through wet-sieve method whereby sieves were shaked electronically up and down movement in nearly 3 cm pure water, 50 times within 10 minutes. The remained aggregates on each sieve were collected, then dried at 1050C for 24 hours and weighted for obtaining the mass of oven dry aggregates (mass of real aggregates + sand) for every sieve. The real mass of aggregates in every aggregates size fraction, such as large soil macro-aggregates (2-4 mm), macro-aggregates (0.25-2mm) and micro-aggregates (0.053-0.25mm) were obtained through subtracting sand content from the measured total aggregates in every aggregates size fraction. The determination of sand content was done through sieving dispersed subsample of aggregates for every sieve with sodium hexametaphosphate by employing 0.053 mm-sieves. The remained sand on the 0.053mm-sieve was oven dried at1050C for 24 hours and weighted, in order to be subtracted from the total weighted aggregates size fraction for every sieve size fraction. The MWD (mm) was calculated through the following formula:
Where n is sieves’ number; xi is the mean diameter of every aggregates size fraction and wi is the proportion of sample’s total weight found in related size fraction.
Water content at different suction pressures was determined using sandbox and pressure plate method (Klute, 1986) and water potential meter device (WP4C) (ASTMD6836-02, 2008). Briefly, specimen per pot was collected in all pots (2 forms of Elaeagnus tree x 3 doses x 4 replicates plus 4 control =28 pots) and compacted (at Pb of 1.4 g cm-3) into a stainless-steel cylinder with diameter and height of 5 and 5 cm respectively. After compaction, specimens were saturated for four days using sandbox and subsequently water retention measurements at saturation (i.e. 0 kPa or Pf=0), FC (33 kPa or Pf=2.5) and PWP (1500 kPa or Pf =4.18) were determined. Prior to applying any pressure to saturated samples, water content at saturation was considered saturated water content. Thereafter, saturated samples were subjected to 33 kPa through pressure plate to get water content at FC. Water content at PWP was calculated from the water retention curve obtained from matric potentials measured by WP4C. Seven samples of 10 g oven dry sample at 1050C for each were respectively moistened to 2, 4, 6, 8,10, 12 and 14 %, then mixed evenly and samples were kept for 24 hrs prior to being red using WP4C and we have tried different amount of soil and we came to conclusion that 10g is fine for WP4C. Thereafter, the red values from the entire samples were used to plot water retention curve and water content at PWP was calculated from the predicted equation from the curve. PAWC was calculated as (FC-PWP). AFP (i.e. sum of drainage and aeration pores) was quantified by subtracting water content at FC from water content at saturation. Water content was gravimetrically measured, and subsequently converted into volumetric water content (θv).
4.2.1.2. Measurements of soil chemical properties
The soil pH was measured through a pH meter on a 1:2.5 soil: water suspension (McLean, 1982). The EC was quantified through a conductivity meter on a 1:2.5 soil: water suspension (Rhoades, 1982). CaCO3 content was determined through Scheibler calcimeter method (Nelson, 1982). CEC was determined through Bower and Wilcox method (Rhoades, 1982). Extractable cations (Ca, Mg, K and Na) were determined through 1 N ammonium acetate extraction method buffered at pH 7 (Thomas, 1982). Available micronutrients (Fe, Cu, Mn and Zn) were determined using DTPA extraction method (Lindsay and Norvell, 1978). AP was determined using sodium bicarbonate method (Nelson and Sommers, 1982). The chemical properties of compost and biochar were determined in the laboratory following the similar method as soil analysis (Table 2).
3.2.1.3. Measurements of soil biological properties
Total organic carbon (OC) was determined through Smith and Weldon wet combustion method (Nelson and Sommers, 1982). TN was determined using dumas dry combustion method (Wright and Bailey, 2001). Concentration of NH4+-N and NO3--N were determined through Kjeldahl method (Keeney and Nelson, 1982). C/N ratio was mathematically calculated from total organic carbon and TN. SMR, AC and PMN were measured through the protocol developed by Gugino et al. (2009).
3.2.2. Plant measurement and analyses
Plant height was measured from the base of shoot to the highest tip of leaf through a ruler and stem girth was measured through a pair of vernier caliper. Leaf chlorophyll content as SPAD value was measured through a portable chlorophyll meter (SPAD-502, Konica-Minolta, Japan). Shoots and roots were separated and rinsed with distilled water and HCl, and then aboveground plant fresh biomass and root fresh biomass were determined using balance. Thereafter, both aboveground plant fresh biomass and fresh root biomass was oven dried at 700C for 48 hrs for determining aboveground plant dry biomass and root dry biomasses by subtracting water content in aboveground plant biomass and root biomass from 100. Shoot P, Ca, Mg, K, Fe, Cu and Zn content were determined after wet digestion with hydrogen peroxide and sulfuric acid through atomic absorption (Watson, 1990). Total nitrogen (TN) was determined using dumas dry combustion method (Wright and Bailey, 2001).
3.3. Statistical Data Analysis
The soil physical, chemical and biological quality properties as well as growth and nutrient uptake by maize (Zea mays L.) plant were statistically subjected to one-way ANOVA analysis using Minitab 16 software and differences between means were considered statistically to be significant at P < 0.05 through Tukey’s test. Data are given as mean value of quadruplicate alongside corresponding standard errors.
4. RESULTS AND DISCUSSIONS
4.1. Effect of Compost and Biochar on Soil Quality Properties
4.1.1. Changes in soil physical properties due to compost and biochar applications
4.1.1.1. Bulk density
The two-month incubation significantly (P<0.001) resulted in a lower Pb in both compost- and biochar-amended SCL soil as presented in Table 3 and Figure 1. As envisioned, applying compost and biochar, substantially decreased Pb and accordingly the decrease in Pb was concomitant of increasing application doses. The values of Pb recorded from compost- amended SCL soil at the rate of 1, 2 and 4% were respectively 0.99, 0.95 and 0.87-fold those of the control, while those recorded from biochar-amended SCL soil at the similar application rates were respectively 0.88, 0.85 and 0.80-fold those of the control (i.e. 1.18 g cm-3). Although both amendments reflected an enormous decrease in Pb, the greatest decrease in Pb was recorded from biochar-amended SCL soil and these results were expected. The application rates of 1, 2 and 4 % for both amendments were statistically fine for improving Pb at small scale within short-term trial (Figure 1). The recorded Pb values for the both amendments varied from 0.95-1.17 g cm-3, which fitted into the range (<1.4 g cm-3) for honing plant growth suggested by USDA-NRCS (2014) in the SCL soil.
All the amendments tested; compost and biochar additions, were able to decrease Pb. The highest downward trend in Pb was found in biochar-amended soil and the lowest in compost-amended soil compared to the control and these results are consistent with (Agegnehu et al., 2015a); the impact might be a result of light material additions (i.e. compost and biochar), which are commonly endowed with a lower bulk density (Ouyang et al., 2013) and increasing of porosity (Barus, 2016) due to highly porous material additions, which might have made the experimental soil become increasingly light and herein lies the root of decreasing Pb. Barnes et al. (2014) found that bulk density of biochar was less than that of experimental soil and also (Liang et al., 2006; Hina et al., 2010) found that biochar is endowed with a high porosity, which might have lightened experimental soil as a results of decreasing Pb in our study and these results were also evidenced by Mukherjee and Lal (2013). In contrast, biochar
amendment stood out from compost in the context of lowering Pb and these findings are in agreement with (Agegnehu et al., 2015a), this may signify that biochar might be substantially lighter than compost. In other words, decrease in Pb was higher by virtue of biochar application in comparison to the one induced by compost application, signifying that bulk density of biochar is lower than that of compost. In contrast to our results, Barus (2016) observed that compost addition stood out from biochar in the context of decreasing Pb; nevertheless, trial was set for 1 month, indicating that biochar addition become more effective in the soil with time prolongation. There was a significant betterment tied to application rates whereby Pb decreased with increasing application rates. In other words, compost application at the rates of 1, 2 and 4 % significantly decreased Pb by 0.01, 0.06 and 0.16 g cm-3, while biochar application at the similar rates decreased Pb by 0.15, 0.18 and 0.23 g cm-3 respectively, and our results are consistent with (Barus, 2016).
Decreases in Pb throughout our study are in accordance with other researches. For instance, Barus (2016) reflected that at 10 t ha-1 application rate, compost and biochar additions lowered Pb by 0.11 and 0.06 g cm-3, respectively. As Pb is one the prominent soil physical properties, Novak et al. (2009) noted that the impact of biochar on the Pb is contingent on biomass type and temperature used during pyrolysis process. The positive perks of compost and biochar additions in the context of improving Pb, have been reported by Agegnehu et al. (2015a). With compost addition, Pb decreased by 0.05 (Agegnehu et al., 2015a) and 0.42 g cm-3 (Emami and Astaraei, 2012) and in the same way, due to biochar addition, Pb decreased by 0.15 (Agegnehu et al., 2015a) and 0.27 g cm-3 (Jien and Wang, 2013). Overall, the application rates of 1, 2 and 4% for either compost or biochar were markedly fine for decreasing Pb and these evidences were reviewed by Aslam et al. (2014).
4.1.1.2. Particle density
Compost and biochar applications were found to be incumbent upon significantly improving Pk at (P<0.001) as illustrated in Table 3 and Figure 1. At the end of experiment, the quantified Pk values from compost-amended SCL soil applied at a rate of 1, 2 and 4% were respectively 1.00, 0.99 and 0.97 times those of the control, while those obtained from biochar-amended SCL soil when applying similar application rates were respectively 0.99, 0.98 and 0.97 times those of the control (i.e. 2.58 g cm-3). As such, the decrease in Pk was concomitant of increasing application rates. Significant
differences were detected in both compost and biochar- amended SCL soil at all applied rates relative to the control, except when applying compost at the rate of 1 %. As expected, reduction in Pk was highest in biochar- amended SCL soil; however, the difference between compost and biochar amendments at 4% application rate was almost insignificant. Overall, when the two amendments were compared at the similar application rates of 1, 2 and 4%, the impact of biochar application outraced that of the compost at large.
Our findings showed that compost and biochar additions seemed able to decrease Pk in the 2-months trial; this might be due to the applied amendments increased organic matter which might have contributed to declined Pk and these results are consistent with (Busscher et al., 2011) who noted that organic matter originated from organic amendments can significantly decrease Pk. Jien and Wang (2013) found that micro-aggregates were flocculated by virtue of biochar addition, and this can be a result of increasing macro-pore at the expenses of micro-pores which also might have been an important reason for decreasing Pk. Another reason might be that particle density of both applied amendments was lower than that of experimental soil, which might have also induced gradual decreases in Pk. Barus (2016) reported that Pk reduction was due to compost addition endowed with low bulk density as compared with experimental soil, and this also might have been a reason of decreasing Pk due to compost addition. These results were expected, since biochar directly affect soil water storage due to its high inner porosity resulting in increased soil pore size distribution and likewise for compost addition. It was found that the application rate of 2 and 4% for compost and 1, 2 and 4% for biochar were noticeably fine for lowering Pk.
Fig 1. Impacts of amendments on bulk density and particle density after incubating*
*: For every amendment, columns with the identical letter are insignificantly different, while those with different letter are significantly different at (P<0.001, one-way ANOVA followed by Tukey’s test).
4.1.1.3. Total porosity
Without any exception, value of soil total porosity was positively impacted by virtue of applied compost and biochar to the SCL soil as presented in Table 3 and Figure 2. Overall, both amendments significantly (P< 0.001) increased soil total porosity and recorded values of soil total porosity in compost-amended SCL soil with the application doses of 1, 2 and 4 % seemed exactly 1.01, 1.03 and 1.09-fold those of the control, while those obtained in biochar-amended SCL soil at the same application doses were respectively 1.10, 1.12 and 1.15-fold those of the control (i.e. 54.17%) and these findings were envisioned. Soil total porosity experienced an upward trend by virtue of applied both amendments and these increments were linear to the application rates. Although compost and biochar additions had a greater values of soil total porosity than control amendment, biochar’s impact on soil total porosity improvement outraced that of the biochar. In other words, the increment of soil total porosity in compost-amended SCL soil ranged from 0.46 to 4. 88 %, while in biochar-compost-amended SCL soil, increment ranged from 5.28 to 7.86 %. It is interesting to mention that biochar addition was more effective than biochar due to recorded value of soil total porosity in
biochar-amended soil at similar application rates of 1, 2 and 4 % were respectively 0.92, 0.93 and 0.95 times those of the compost-amended SCL soil.
Experimental results showed that the applied amendments (i.e. compost and biochar) markedly increased soil total porosity; these increases might be due to the applied amendment increased organic matter (data not given) which might have interacted with mineral fraction as a result of contributing in the betterment of soil aggregation, aggregate stability, Pb and Pk, all of which might have contributed to increased soil total porosity and these reasons are consistent with (Amlinger et al., 2007). Also, increases in soil total porosity might be due to the applied amendments were endowed with a high porosity, for example of biochar as stated by (Liang et al., 2006; Hina et al., 2010) and lower bulk density as stated by (Hati et al., 2007). Additionally, rearranging and forming of macro-and micro-pores due to the applied amendments might be also the utmost other reason for increasing soil total porosity and this reason also is in agreement with (Hseu et al., 2014). Furthermore, the applied amendments might have supported microbial activities by providing microorganisms with habitat and substrate, which in turn might have involved in soil aggregation resulted in forming macro-pores at the expense of micro-pores, and thereby probably being an important reason of increasing soil total porosity over the course of our study, and these results were also evidenced by Barus (2016). Cantón et al. (2009) reported that increasing of soil aggregates stability and aggregates size distribution might be responsible for maintaining genuine size of soil pores and these results corresponded with our results by the fact that the applied amendments increased soil aggregates stability (Figure5) and macro-porosity (data not given), which in turn might have led to increased soil total porosity. Biochar as a material endowed with a hefty micro-pores (Dimoyiannis, 2012; Herath et al., 2013), might have been accountable for increasing soil total porosity. This could also be an indicator of increasing soil water storage capacity and habitat provision for soil micro-organisms resulting in increased soil water holding capacity and soil physical property betterment at large. Higher total porosity was accompanied by increased application rates.
There are other researchers who elucidated the rallies of soil total porosity due to compost and biochar additions. Our findings due to compost addition are in agreement with (Barus, 2016). and that of biochar are also in agreement with (Herath et al., 2013). The positive responses of soil total porosity to compost and biochar additions have been revealed by Barus (2016) and (Burrell et al., 2016), respectively. Soil total
porosity increased by 8.5% due to biochar addition (Omondi et al., 2016), and also both biochar and compost additions increased soil total porosity by 2.97 and 4.64 %, respectively (Barus, 2016). Several studies revealed that biochar application increased soil total porosity (Jien and Wang, 2013; Ouyang et al., 2013; Głąb et al., 2016; Peng et al., 2016; Wang et al., 2017). The two-months experiment showed a sparse increase in soil total porosity due to compost and biochar additions, yet the increases should likely be high in long-terms experiment. Biochar-incubated soil for 5 months and 18 days increased soil total porosity by16-20% as indicated by Hseu et al. (2014) and one with 1 month incubation period increased soil total porosity by 2.97 %; due to biochar being recalcitrant to microbial activities (Hunt et al., 2010), it is clear that its effect in the context of improving soil properties increases with time and also depends on soil types, biomass and used temperature during pyrolysis. As such, it is of great importance to note that 1, 2 and 4 % application rates of either compost or biochar were fine to enhance soil total porosity and these results were reviewed by Aslam et al. (2014).
Fig 2. Impacts of amendments on soil total porosity after incubating
For every amendment, columns with the different letter are significantly different at (P<0.001, one-way ANOVA followed by Tukey’s test.
4.1.1.4. Water retention at different suction pressures and PAWC
As a SCL soil is commonly renowned for low water holding capacity, retained water expressed in θv under the SCL soil at different suction pressures namely 6.3 kPa, 33 kPa and 1500 kPa as impacted by applied amendment is illustrated in Table 3 and Figure 3. Experimental results revealed that retained θv at the suction pressures significantly (P<0.001) increased as compared with the control.
At 6.3 kPa (i.e. at pF=1.8) recorded values of retained θv in compost-amended SCL soil at 1, 2 and 4% application rates were respectively 1.02, 1.01 and 1.00-fold those of the control, while those recorded in biochar-amended SCL soil at similar application rates were respectively 1.01, 1.00 and 1.04-fold those of the control (i.e. 0.436 cm3 cm-3) as presented in figure 3. The responses of water retention capacity to the applied both amendments at low suction pressure of 6.3 kPa (i.e. at pF=1.8) were trivial in the context of increasing water holding capacity. With some exception, significant differences among application rates for every applied form of Eleaegnus tree (i.e. compost and biochar) were observed.
At 33 kPa (i.e. pF=2.5) retained θv in the compost-amended SCL soil at 1, 2 and 4% application rates was respectively 1.07, 1.12 and 1.20 times that of the control (i.e. retained θv increased by 0.02, 0.03 and 0.06 cm3
cm-3 respectively), while that retained in biochar-amended SCL soil was respectively 1.04, 1.08 and 1.20 times that of the control (i.e. retained θv increased by 0.01, 0.02 and 0.06 cm3
cm-3 respectively). All applied amendments were statistically significant (P<0.001) in the magnitude of positively affecting water holding capacity at FC (33 kPa), yet mathematically increments were trivial relative to the control ranging from 0.02 to 0.06 cm3 cm-3 for compost addition and from 0.01 to 0.06 cm3 cm-3 for biochar addition and these increments were proportional to the applied doses (Table3, Figure 3).
At 1500 kPa (i.e. pF=4.18) retained θv in compost-amended SCL soil at 1, 2 and 4% application rates were respectively was 1.158, 1.192 and 1.338 times that of the control (i.e. retained θv increased by 0.018, 0.022 and 0.039 cm3
cm-3 respectively) (Figure 3). Conversely, biochar addition showed a downward trend by decreasing θv by 0.010, 0.009 and 0.009 cm3 cm-3 when the similar rates were applied. Significant differences were observed in all applied amendments (P<0.001). In contrast, compost-amended SCL soil increased water retention capacity, yet biochar-compost-amended one decreased water retention capacity at PWP (1500 kPa).
Since retained θv at FC (33 kPa) was far beyond the one retained at PWP (1500 kPa), it is of great importance to note that PAWC calculated as (retained θv at pF =2.5 – retained θv at pF =4.18) scaled up as reflected in Table3 and Figure 3. Applied both amendments (i.e. compost and biochar) significantly (P< 0.001) increased PAWC, and PAWC was higher in biochar- amended SCL soil than that of compost- amended one. In other words, PAWC in compost-amended SCL soil at 1, 2 and 4% application doses increased respectively by 0.002, 0.013 and 0.018 cm3 cm-3, while the one in
biochar-amended SCL soil at the similar application doses increased by 0.022, 0.033 and 0.065 cm3 cm-3 respectively, in comparison with the control. Significant differences were found in all applied amendments alongside their respective application rates and the betterment of PAWC was proportional to the applied doses, as envisioned. The infusion of either compost or biochar to the experimental soil at the application rates were found to be fine for increasing PAWC ranged from 0.002 to 0.018 cm3 cm-3 for compost addition and 0.022 to 0.065 cm3 cm-3 forbiochar addition in short-term trial at small scale.
Fig 3. Impacts of amendments on volumetric water content at 6.3 kPa, 33 kPa (FC), 1500 kPa (PWP)
and plant available water content (PAWC) after incubating
For every amendment, columns with the identical letter are insignificantly different, while those with different letter are significantly different at (P<0.001, one-way ANOVA followed by Tukey’s test).
At a selected matric potentials, additions of compost and biochar significantly increased θv at 6.3 kPa, 33 kPa (FC) and 1500 kPa (PWP) as well as PAWC as compared with the control (Table 3, Figure 3); this was due to increased organic matter which might have adsorbed a sizeable amount of water and increased specific surface area which also might have increased water storage capacity, decreased Pb and Pk (Figure 1) and also might be due to increasing of pore size distribution (i.e. meso, micro-and macro-pores) as evidenced by Herath et al. (2013) and the aforementioned reasons were also consistent with (Hillel, 1982). This increasing of water retention could be also a prominent indicator of changing soil water retention curve. Our results are in support of other studies, such as Ouyang et al. (2013) who found that water content at FC and PWP increased due to biochar addition in the light of being endowed with a high porosity as stated by (Liang et al., 2006; Hina et al., 2010), and similarly
compost addition increased θv at FC and PWP (Gümüş and Şeker, 2017; Laila, 2011). It is surprisingly to note that there was a decrease in θv at PWP due to biochar addition and retained θv decreased with increasing application; this might be a positive result of increasing PAWC range as presented in Figure 3 at the expense of water content at
PWP. In contrast to our study, Herath et al. (2013) found a controversial effect where
biochar addition increased θv at PWP, still in long-term experiment. The creation of hydrophilic feature for biochar due to the oxidation of carboxylic acid group featured on its external surface might be also another reason for increasing soil water retention capacity in our study as elucidated by Zimmerman (2010). An upward trend in FC due to biochar addition was also reported by Glaser et al. (2002), yet Busscher et al. (2011) reported that applied biochar produced at 7000C in a sandy loam soil did not affect water content at FC, signifying that biochar potential decreases with increasing temperature during pyrolysis process.
The subtraction of water retained at PWP from that retained at FC dubbed ‘PAWC’ was significantly increased due to compost and biochar applications; these increases were attributable to the unparalleled increase in FC and decrease in PWP as elucidated above (Table 3, Figure 3), increase in soil total porosity (Table 3, Figure 2) as far as macro-and micro-porosity are concerned and increase in surface area (data not given), all of which might have been responsible for expanding PAWC range in the SCL soil, and the results are consistent with (Hseu et al., 2014) who said that biochar increased PAWC due to increasing soil micro-pores. For instance, Van Zwieten et al. (2010) stated that biochar is endowed with a sizeable amount of porosity and surface area, which might have been a reason of increasing PAWC and this is presumably for compost addition. The sizeable increases were found in compost-amended SCL soil and the lowest in biochar-amended SCL soil; this might be due to a higher recalcitrance of biochar to microbial activities than compost and herein lies the root of compost being a phenomenon adsorbent than biochar in short-term trial, yet biochar can stand out from compost under long-term trial. For instance, although rate of application differed, biochar addition yielded the high value of PAWC (e.g. PAWC increased by 18 to 89%) compared to our study due to extended incubation period of 168 days (Hseu et al., 2014). This increase in PAWC can be also elucidated in the context of increasing micro-porosity possessing diameter lies between 28.8-0.19 mm at the expenses of macro-porosity having diameter greater than 28.8 mm (Laila, 2011), and macro-macro-porosity as an indicator of AFP measured at 10 kPa suction pressure, when soil water attains its
equilibrium (El-Hady et al., 1990; El- Hady et al., 2006). As expected, Figure 4 shows that AFP decreased with increasing application rates, signifying that micro-porosity increased at the expenses of macro-porosity which might have increased water holding capacity resulted in increased PAWC. This could be an obvious indicator for the proliferation of water storing pores in the SCL soil which is very important for increasing water reservoir in the SCL soil of arid region. The most noticeable quality of compost and biochar additions was that they were fine for enhancing PAWC in a SCL soil as compared with the control in cases of all application rates are concerned. Our results were expected and in accordance with (Laila, 2011). The reported similar findings by (Barus, 2016) indicated that compost and biochar additions increased water retention at FC, PWP and PAWC.
The effectiveness of biochar application in improving soil water holding capacity and PAWC was also validated by (Downie, 2011; Tammeorg et al., 2014). Overall, Increases in PAWC due to compost and biochar addition as presented in Figure 3 could be a solution for drought experienced region by increasing elongation of irrigation frequencies which results in decreased irrigation water need and costs, and these were also stated by El- Hady et al. (2006).
4.1.1.5. Air filled porosity
As AFP in the soil is the prominent indicator for facilitating not only microbial respiration but also crop growth and development culminating in soared both soil profitability and agricultural yields. In this regard, applied compost and biochar as soil amendments significantly (P<0.01for compost and P<0.001for biochar) decreased soil
AFP in the studied SCL soil as presented in Figure 4. Application of compost at 1, 2 and
4 % rates decreased soil AFP by 0.016, 0.029 and 0.065 cm3 cm-3, whereas biochar application at the similar doses scaled down soil AFP by 0.011, 0.016 and 0.059cm3 cm -3
respectively as compared with the control (Table 3, Figure 4). Significant differences were observed in all amendments and soil AFP decreased with increasing application rates, as expected.
Regarding the responses of AFP to the applied amendments (i.e. compost and biochar), AFP experienced a downward trend, which differed between application rates and decreases in AFP were concomitant of increasing application rates in all tested amendments. The AFP results recorded in compost-and biochar-amendment differed, signifying that their impact was not rather the same. Decrease in AFP might be by
virtue of increasing organic matter and micro-porosity at the expenses of macro-porosity, all of which led to expanded range of PAWC (Figure 3). Broadening range of
PAWC as a results increasing micro-porosity at the expenses of macro-porosity was also
reported by Laila ( 2011). Clogging of soil pores due to biochar dust might have been another reason of decreasing macro-pores as a result of decreasing AFP as reviewed by Aslam et al. (2014). As expected, AFP which has to be at least 10% for optimum plant growth and development (Grable and Siemer, 1968) ranged from 12 to 17 % due to compost addition and 13 to 17 % due to biochar addition. With few exception, the recorded AFP values for the both amendments (Figure 4), fitted into the range (> 14%) for honing plant growth as suggested by (Carter, 1988; Drewry, 2006; Mueller et al., 2009) from sandy loam to clay loam soil. Eventually, there is a gap on how compost and biochar additions affect AFP and further research is need for providing enough evidences.
Fig 4. Impacts of amendments on air filled porosity (AFP) after incubating
For every amendment, columns with different letter are significantly different at (P<0.01for compost and P<0.001 for biochar, one-way ANOVA followed by Tukey’s test.
4.1.1.6. Aggregate stability
With some exceptions, compost and biochar additions statistically (P<0.001) showed a significant increase in soil aggregate stability (Table 3, Figure 5). Soil aggregate stability increased linearly with application rates for both amendments and recorded values of soil aggregate stability from compost-amended SCL soil with 1, 2 and 4 % application rates were respectively 1.11, 1.40 and 1.97 times those of the control, while those from biochar-amended SCL soil with similar application doses
were respectively 1.11, 1.74 and 1.97 times those of the control (i.e. 25.25%). Soil aggregate stability was higher in both compost-and biochar amended SCL soil as compared with the control, and both compost-and biochar-amended SCL soil at the same application doses of 1 and 4% indicated almost the same trend. Although the highest values of soil aggregates stability were observed in compost-amended SCL soil, soil aggregate stability in biochar-amended SCL soil was higher than that of compost-amended SCL soil at similar application dose of 2% (Table 3, Figure 5). When the two amendments were compared at the similar application rates of 1, 2 and 4%, compost application increased soil aggregate stability by 1.03, 0. 53 and 1.00 times that of the biochar, respectively. The observation of soil aggregates stability in both compost-and biochar-amended SCL soil at 1 % application rate reflected that the impact was statistically insignificant relative to the control. Statistically, significant differences were noted at 2 and 4% application rates for both compost-and biochar-amended SCL soil as compared to the control. It is interesting to note that 2 and 4% application rates of both compost and biochar statistically were fine for increasing soil aggregates stability as observed in the compost-amended SCL soil, the increase in soil aggregate stability ranged from 2.86 to 24.62 %, while in biochar-amended SCL soil ranged from 2.78 to 24.57 %.
Our findings illustrated a significant increase in soil aggregate stability compared with the control due to compost and biochar infusions; this might be attributable to increasing of organic matter content induced by addition of a high organic matter-rich amendments as also stated by Lehmann and Joseph (2009), adding of nutrients (e.g. Mg2+, K+, Ca2+, Fe2+, Mn+ and others) and improving of microbial pool and activities, all of which might have contributed in soil aggregation and aggregate stability through drawing soil individual particle together into aggregates of all shapes and sizes, and these reasons were under the auspices of Six et al. (2004). This upward trend in soil aggregate stability might be also due to the applied amendments were endowed with higher pore spaces as stated by (Liang et al., 2006; Hina et al., 2010) for supplying a hefty habitat to soil micro-organisms (e.g. play a role of secreting polysaccharide compounds), which might have involved in soil aggregation resulted in the formation and stabilization of aggregates. Our results are in accordance with (Downie et al., 2009) who found that biochar addition increased organic matter which substantially enhanced soil microbial activities, and thereby leading to forming and stabilizing soil aggregates. Furthermore, microorganism bind themselves on the
external surface of the carbon particle, in order to perform their duties, such as decomposing adsorbed pollutants and other organic acid (Rice et al., 1978; De Laat et al., 1985; Kim et al., 1997), which in turn might have contributed in soil aggregation and aggregates stability betterments. Findings of previously conducted study reported that biochar was endowed with a high internal surface area and macro-porosity, which provided biochar ability of increasingly adsorbing soluble cations and providing habitat for the reproduction and growth of soil microorganisms (Pietikäinen et al., 2000), and herein lies the evidences of that biochar might have contributed to soil aggregation and aggregates stability and likewise for the compost addition in our study. Also, interacting of minerals with oxidized carboxylic acid group found at the surface of biochar should be accountable for forming and strengthening bond between soil particles, and thereby enhancing soil aggregates stability (Glaser et al., 2002). Additionally, this could be also an indication for substantially sequestering carbon within the soil which also ties to increased aggregates stability. As expected, aggregates stability increments were higher by virtue of compost application compared to the one obtained with biochar application, reflecting that compost was less recalcitrant to microbial activities than biochar, and this may have led to increased organic matter and other cations that might have made compost addition stand out from biochar addition in the context of increasing soil aggregation and aggregates stability. Herath et al. (2013) stated that soil aggregates formation considered a function of microbial activities and time, still take a long time in case of biochar addition and this might have been a reason why effect of compost measured up that of biochar. The obtained results were envisioned, and also are in accordance with (Annabi et al., 2011; Emami and Astaraei, 2012) who showed that compost addition increased soil aggregate stability and (Herath et al., 2013; Jien and Wang, 2013; Ouyang et al., 2013; Omondi et al., 2016) indicated that biochar application substantially increased soil aggregate stability of the studied soil.
Other researches have reflected that addition of compost increased soil aggregate stability (Albiach et al., 2001; Annabi et al., 2004; Peng et al., 2016; Wang et al., 2017) and similarly, biochar addition increased soil aggregate stability (Burrell et al., 2016; Głąb et al., 2016; Peng et al., 2016; Wang et al., 2017). There was also impact due to applied doses; aggregates stability increments were proportional to the applied rates for both amendments. For example, aggregates stability obtained in either compost- or biochar-incubated SCL soil at 4% application rate was 1.8 times that of obtained at 1% applied rate. The impact of biochar additions on the betterment of soil
