Effects of Biochar and Maize Roots on CO ₂ Emissions and Microbial Biomass C in Loess Soils

Background Refractory substances such as biochar and labile substances such as dead fine and coarse roots play important roles in soil organic matter dynamics and have key ecological functions. However, interactions between these substances have only rarely been studied. Aim The objectives were to study the effects of biochar and maize roots on CO 2 emissions and microbial biomass C (MBC) in incubation experiments with loess soils from three arable sites. Methods Unaged and H 2 O 2 ‐artificially aged biochars were incubated with maize roots of different sizes and at different rates in field replicates of loess surface soils for approximately 6 months at 15°C and 60% of the water holding capacity. Total (∑CO 2 –C) and maize‐derived cumulative CO 2 –C emissions and C 3 ‐ (autochthonous SOM‐derived) and maize‐derived microbial biomass contents (MBC AS , MBC MD ) were determined, and mixed effects models were calculated. Results ∑CO 2 –C emissions and MBC MD increased significantly with increasing root rates. H 2 O 2 ‐artificially aged biochar either significantly increased ∑CO 2 –C (one site) and decreased MBC AS (two sites) and MBC MD (one site) or had no effect. Different root sizes did not significantly affect the response variables. Unaged biochar significantly increased ∑CO 2 –C emissions and MBC AS relative to control soils and soils with H 2 O 2 ‐artificially aged biochar, probably mainly due to increases in pH and water‐extractable organic C. Conclusions Site variations in biochar effects on biological variables were considerable even for the controlled experiments with a defined sample of loess soils, indicating that a disentanglement of biochar effects according to soil types may not be feasible.