Biochar addition rate influences soil microbial abundance and activity in temperate soils

Summary Biochar ( BC ) amendment to soils is a proposed strategy to improve soil fertility and mitigate climate change. However, before this can become a recommended management practice, a better understanding of the impacts of BC on the soil biota is needed. We determined the effect of addition rates (0, 1, 5, 10 and 20% by mass) of a fast‐pyrolysis wood‐derived BC on the extraction efficiency ( EE ), abundance and temporal dynamics of phospholipid fatty acids ( PLFAs , microbial community biomarkers) in four temperate soils during a 1‐year incubation. Additionally, the effects on microbial mineralization/incorporation of BC‐C were determined by measuring CO 2 efflux and the BC contribution to CO 2 and PLFA‐C using the natural 13 C abundance difference between BC and soils. Biochar addition proportionally increased microbial abundance in all soils and altered the community composition, particularly at the greatest addition rate, towards a more gram‐negative bacteria‐dominated (relative to fungi and gram‐positive) community. Though chemically recalcitrant, the BC served as a substrate for microbial activity, more so at large addition rates and in soil with little organic matter. Microbial utilization of BC‐C for growth could only partially explain the observed increase in microbial biomass, suggesting that other, potentially abiotic, mechanisms were involved. The strong decrease in PLFA EE (−77%) in all soils with biochar addition emphasizes the need to measure and correct for EE when using PLFA biomarkers to estimate soil microbial responses to BC additions. Overall, our study provides support for BC use as a soil amendment that potentially stimulates microbial activity and growth.