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 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.
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