Biochar increases soil microbial biomass but has variable effects on microbial diversity: A meta-analysis

Biochar has been extensively studied as a soil amendment for carbon sequestration and for improving soil quality; however, a systematic understanding of the responses of soil microbial biomass and diversity to biochar addition is lacking. Here, a meta-analysis of 999 paired data points from 194 studies shows that biochar increases microbial biomass but has variable effects on microbial diversity. Generally, the effects of biochar on microbial biomass are dependent on biochar properties, while that on microbial diversity is dependent on soil properties. The application of biochar, particularly that produced under low temperature and from nutrient-rich feedstocks, could better increase soil microbial biomass (based on phospholipid fatty acid analysis (MBCPLFA)) and diversity. The increases of total microbial biomass with biochar addition are greater in the field than in laboratory studies, in sandy than in clay soils, and when measured by fumigation-extraction (MBCFE) than by MBCPLFA. The bacterial biomass only significantly increases in laboratory studies and fungal biomass only in soils with pH ≤ 7.5 and soil organic carbon ≤30 g kg−1. The increases in total microbial diversity with biochar addition were greater in acidic and sandy soils with low soil organic carbon content and in laboratory incubation studies. In addition, long-term and low-rate addition of biochar always increases microbial diversity. To better guide the use of biochar as a soil amendment, we suggest that establishing long-term and field studies, using a standard method for measuring microbial communities, on different soil types should be our emphasis in future research.