Effects of nitrogen-enriched biochar on subtropical paddy soil organic carbon pool dynamics

Agronomic management practices present an opportunity to improve the sustainability of crop production, including reductions of greenhouse gas emissions through impacts on soil organic carbon (SOC) dynamics. We investigated the impacts of contrasting application rates of nitrogen (N)-enriched biochar (4 and 8 t ha^-1) on the concentrations of total and active SOC, microbial biomass carbon (MBC), soil aggregates, and the carbon (C) pool management index (CPMI) as an indicator of soil quality in tillering and mature subtropical early and late rice in China. Soil salinity and soil bulk density increased, and soil water content generally decreased under the application of N-enriched biochar at 4 t ha^-1. Following the application of the biochar, there were greater soil concentrations of SOC and lower concentrations of dissolved organic-C and active labile organic‑carbon, indicating reduced mineralization and enhanced stocks of stable-C. Biochar application (4 and 8 t ha^-1) led to lower soil Ca-SOC concentrations and greater soil Fe(Al)-SOC concentrations. Concentrations of Fe(Al)-SOC were greater under the application of N-enriched biochar at 4 t ha^-1, indicating the bonding capacity of iron‑aluminum oxide and organic carbon provided by biochar improved levels of SOC fixation. The composition of soil aggregates under each treatment was mainly micro-aggregates (<0.25 mm). The greater soil content of macro-aggregates (>0.25 mm) increased under amendment with 4 t of biochar ha ^-1 and the greater SOC content led to greater soil aggregate stability. Levels of C pool activity, C pool index, and CPMI reduced following application of the biochar, while C pool activity index increased slightly, indicating an increase in soil quality. These results indicate that the application of N-enriched biochar during rice cultivation may lead to reductions in SOC mineralization and C emissions and increases in soil C sink capacity, due to greater SOC pool stability, thus improving the sustainability of paddy rice production.