Response of microbial communities to biochar-amended soils:a critical review

Application of biochar to soils changes soil physicochemical properties and stimulates the activities of soil microorganisms that influence soil quality and plant performance.Studying the response of soil microbial communities to biochar amendments is important for better understanding interactions of biochar with soil,as well as plants.However,the effect of biochar on soil microorganisms has received less attention than its influences on soil physicochemical properties.In this review,the following key questions are discussed:(i)how does biochar affect soil microbial activities,in particular soil carbon(C)mineralization,nutrient cycling,and enzyme activities?(ii)how do microorganisms respond to biochar amendment in contaminated soils?and(iii)what is the role of biochar as a growth promoter for soil microorganisms?Many studies have demonstrated that biochar-soil application enhances the soil microbial biomass with substantial changes in microbial community composition.Biochar amendment changes microbial habitats,directly or indirectly affects microbial metabolic activities,and modifies the soil microbial community in terms of their diversity and abundance.However,chemical properties of biochar,(especially pH and nutrient content),and physical properties such as pore size,pore volume,and specific surface area play significant roles in determining the efficacy of biochar on microbial performance as biochar provides suitable habitats for microorgan-isms.The mode of action of biochar leading to stimulation of microbial activities is complex and is influenced by the nature of biochar as well as soil conditions.

Effects of biochar on soil water retention curves of compacted clay during wetting and drying

Compacted clay possesses a low water permeability and has been widely used in geo-environmental facilities such as landfill cover systems.Recent studies revealed the potential applications of compacted biochar-amended clay(BAC)as an alternative landfill cover soil.However,the effects of biochar on the soil water retention curves(SWRCs)of clay at low suction are not known.This study can help fill this knowledge gap by measuring the wetting and drying SWRCs of clay and BAC(20%biochar).Soils were compacted in instrumented soil columns and subjected to a wetting and drying cycle,and soil water content and suction were measured along with the soil depth.BAC was saturated faster when compared with clay.Upon drying,the soil water content of clay at 65 mm depth dropped to almost zero,while about 5%of water was retained in BAC.It showed that biochar increased the soil water retention capacity of compacted clay upon drying.SWRCs showed that biochar-clay composite possesses a higher soil water retention capacity at a low soil suction range(<1000 kPa)compared with clay alone.It revealed the benefits of using compacted biochar-clay composite as a hydraulic barrier to minimize desiccation-induced cracks and potentially promote its serviceability,especially in arid and semi-arid regions.The composite is also a carbon sink material that can reduce landfill gas emissions and pollutant leaching.