Soil Carbon and Nitrogen Dynamics in Two Agricultural Soils Amended with Manure‐Derived Biochar

Biochar has been promoted as a means to sequester C and improve soil quality. Biochar produced from agricultural waste streams and recycled as a soil amendment also provides a strategy for improved nutrient management in agricultural systems. The effects of biochar amendment on soil C and N cycling remain poorly constrained. This study aimed to examine the effects of biochar on soil C and N storage, N mineralization, and soil physiochemical properties. Soils were collected from a field experiment in which biochar derived from poultry manure was applied for 2 yr in two croplands differing in soil texture (sandy and silt‐loam). Samples from biochar‐amended and control soils were physically fractionated to separate water‐stable soil aggregates and analyzed for C and N. Biochar amendments increased total soil C by 16 (sandy soil) and 30% (silt‐loam soil). These increases were observed in aggregate size classes associated with short‐term C and N storage in silt‐loam soils and intermediate‐term C and N storage in sandy soils. Net N mineralization rates observed in a short‐term incubation were small or negative (1.79 and −24.7 μg N g −1 soil for sandy and silt‐loam soils, respectively), indicating little or no new N mineralization from biochar over short timescales. Biochar amendment had a positive impact on cation exchange capacity at both sites, increasing it by 7 and 11% in the silt‐loam soil and sandy soil, respectively. These results suggest that biochar amendments to cropping systems can improve the capacity of soil to retain nutrients and store C and N. Core Ideas Generating biochar from manure is a strategy for sustainable nutrient management. Manure‐derived biochar was applied in two agricultural soil contexts. Biochar increased soil C and N, with more pronounced effects with higher clay content. Biochar improved soil physiochemical properties, including cation exchange capacity. Biochar provided a source of plant available N, with lower nitrification rates in sandy soils.