Effects of Biochar Amendment on Soil Carbon Fractions and Their Decomposition in a North-Subtropical Paddy Field

Corncob-made biochar (C4) was amended to a paddy field which was newly developed from the Yangtze River sediment. After one-year biochar amendment the soil was fractionated to investigate the effects of biochar amendment on soil carbon fractions, as well as the carbon decomposition of such fractions during incubation. The distribution of biochar in soil fractions was quantified based on the different carbon isotope abundances between biochar and native soil organic carbon (nSOC, C3). The results indicate that biochar amendment significantly enhanced the organic matter content of aggregates at all size levels. 76%-90% of biochar existed as free particles in the macroaggregate (>250 μm) and microaggregate (>50-250 μm) fractions; only a minor proportion of biochar showed relatively tight interactions with soil aggregates or < 20 μm mineral particles. These suggest that biochar amendment had not enhanced the formation of soil aggregates. Biochar itself decomposed extremely slowly during incubation of various soil fractions, but its amendment promoted the decomposition of nSOC. To conclude, the application of biochar alone did not significantly promote the soil carbon stabilization of entisols, but instead may accelerate the soil carbon decomposition in the short term.