Biochar Effects On Soil Physical Properties,Greenhouse Gases Emission,and Extractable Heavy Metals:A Meta-Analysis

The challenge of increasing food production for the rapidly increasing population has led to practices that contribute to the degradation of soil and increase in the atmospheric concentration of greenhouse gasses.Land degradation manifested as heavy metal contamination of agricultural and urban soils,loss of organic matter leading to poor soil structure,compaction,and loss of soil fertility all contribute to low agricultural productivity.Biochar use as a soil amendment has received increasing attention for its effects on carbon sequestration and greenhouse gas emission mitigation as well as on the improvement of soil fertility.It has also been proposed as a means of remediating heavy metal contaminated soils and improvement of microbial biomass and community structure.However,there had not yet been a general assessment of biochar effects on soil physical properties to allow strong recommendations for biochar's practical use for soil quality improvement for global application.There has also been a paucity of studies that quantify biochar effects on greenhouse gas mitigation for a wide range of soil experimental and biochar property conditions to allow for robust conclusions on biochar's practical use in global agriculture.The objectives of this study were,therefore(i)to quantitatively examine the effect size of biochar amendment on soil greenhouse gas emission,heavy metals and selected physical properties and(ii)to identify the main factors that influence the response of soil physical properties,greenhouse gas emission,and heavy metals to biochar amendment.To achieve our objectives,we employed meta-analysis which is a critical analytical methodology that compares results across studies to reveal common response trends.The study was divided into four parts.For the first study,we performed a meta-analysis of data available in literature published by October 2015 and quantified biochar effects on selected soil physical properties.The literature data covered a range of feedstocks,pyrolysis temperature,soil and experimental conditions.Results showed that biochar amendment significantly improved all the soil physical properties tested.On average,soil bulk density was significantly reduced by 7.6%whereas soil porosity significantly increased by 8.4%,aggregate stability by 8.2%,available water holding capacity(AWC)by 15.1%and saturated hydraulic conductivity by 25.2%.Furthermore,the changes in soil bulk density were negatively correlated to porosity and AWC.Also,these effects were greater in coarse-textured soils than in fine-textured soils.While the size of biochar effect on soil physical properties varied with the amount of biochar added,changes in bulk density only correlated with application rates of crop residue and wood biochar.Overall,biochar amendment could likely improve soil hydrological properties though the extent varying with biochar and soil conditions.Use of biochar thus could offer a viable option to improve moisture storage and water use efficiency in soils poor in organic carbon in arid/semiarid zones.In the second study,the meta-analysis covered data available in literature published by 31st October 2016 on greenhouse gas emission.The literature data covered the biochar,soil and experimental characteristics as described earlier but also had information on flooded and non-flooded soil conditions for the case of methane emission.Results showed that biochar amendment significantly reduced methane and nitrous oxide emissions but increased carbon dioxide emission.On average,methane emission was significantly reduced by 11%,nitrous oxide emission reduced by 28%whereas the carbon dioxide emission considerably increased by 18%.Furthermore,the changes in greenhouse gas emission were greater in high pyrolysis temperature biochars than low pyrolysis temperature biochars.Also,the influence of biochar was profound under waterlogged conditions that are more prone to methanogenesis and enhanced emission.Biochar is thus a promising amendment for reduction of greenhouse gasses especially methane in rice producing areas that contribute to large amounts of methane to the atmosphere.Biochar effects on greenhouse gases were shown to be greater in lab incubations although field experiments were relatively fewer.For the third study,the meta-analysis covered data available in literature published by 31st October 2016 and quantified biochar effects on heavy metal reduction.Results showed that biochar amendment significantly reduced extractable heavy metals.On average,extractable heavy metals were significantly reduced by 33%.The greatest reduction among the heavy metals was shown in cadmium and lead by 35%and 41%respectively.Furthermore,the changes in heavy metal reduction were greater in high pyrolysis temperature biochar than in low-temperature ones.Biochar effects were shown to be greater in lab incubations although field experiments were relatively fewer.The fourth part comprised a comparison of extents of effects based on two major purposes namely soil hydrological properties and environmental remediation including greenhouse gas emission and reduction of extractable heavy metal.The comparison showed greater biochar effects on environmental remediation than on soil hydrologic properties improvement.The study provides evidence of biochar potential in enhancing soil hydrological properties,reduction of greenhouse gas emission,and heavy metal remediation.The study showed that the biochar related factors that influence the responses soil hydrological properties,greenhouse gas emission,and heavy metal include the feedstock source,pyrolysis temperature,and application rate.The greenhouse gas emission mitigation changes were however shown to be initially high but reduced over time.The mechanism for the reduced greenhouse mitigation ability dynamics with time remains unclear.There is,therefore,an urgent need for long-term experiments to determine the biochar aging effects on greenhouse gas emission.