Insights into soil and biochar variations and their contribution to soil aggregate status – A meta-analysis

As a dynamic process in the soil, soil aggregation has a direct impact on several vital functions, including microbial activity, greenhouse gas emissions, water storage, and nutrient availability. Biochar as a recalcitrant soil amendment could potentially regulate soil functions, especially aggregation. However, there have been conflicting studies regarding the effectiveness of biochar, its variations, and how it interacts with various soil conditions to affect aggregate status. In this regard a thorough meta-analysis was conducted, considering six categories of significant variables: soil texture, soil organic carbon (SOC), application rate, pyrolysis temperature, feedstock type, and various biochar use methodologies as well as various soil aggregation indices as impacted factors. Based on the results, wood-based biochar showed the highest efficiency above straw and manure-based biochar with a positive effect size of 15.4 % and 17.7 % for mean weight diameter (MWD) and macro-aggregate. The highest geometric mean diameter (GMD) was obtained from biochars pyrolyzed at 550< °C with 19.9 % effect size. Also, low pyrolysis temperature (<450 °C) resulted in the maximum formation of micro-aggregates with a positive effect size of 14.9 %. The moderate application of biochar (10–20 t ha⁻¹) resulted in the lowest micro-aggregates (−8.9 %) and the highest macro-aggregates (24.2 %). The single application of biochar resulted in a positive effect size in the case of macro-aggregate (17.2 %) significantly higher than the combined application of biochar with fertilizer (8.1 %). The highest MWD (12.8 %) and GMD (7.1 %) were obtained from biochar-treated soils with loamy texture. Also, the high availability of SOC (2<%) caused the highest macro-aggregate formation with a positive effect size of 28.2 %. Expanding our knowledge of biochar capability and soil functions could change soil aggregation scenarios, as the variety of biochar pyrolysis processes and its application strategies could directly modify soil's dynamic structure, through inducing functional groups, carbon linkage, and soil particle rearrangement.