Soil aggregation and soil aggregate stability regulate organic carbon and nitrogen storage in a red soil of southern China.

Soil aggregation plays a critical role in the maintenance of soil structure, as well as in its productivity. Fertilization influences soil aggregation, especially by regulating soil organic carbon (SOC) and total nitrogen (TN) contents in aggregate fractions. The present study evaluated the influence of three contrasting fertilizer regimes (unfertilized control -CK-, mineral fertilization -NPK- and manure combined with NPK -NPKM) on soil aggregate stability, aggregate-associated organic carbon and total nitrogen sequestration and mineralization of SOC. Soil samples from (20 cm) depth were collected from a long-term fertilization experiment and analysed for size distribution ranging (>250 μm, 250-53 μm and <53 μm sizes), SOC and TN contents, as well as for mineralization of bulk and aggregate associated-SOC. Both NPK and NPKM fertilizations significantly enhanced SOC and TN contents in bulk soil and its constituent aggregates of >250 μm, 250-53 μm and <53 μm sizes, as compared to CK. Long-term NPK and NPKM increased SOC and TN stock in bulk soil by 45 and 98%, and by 70 and 144%, respectively, as compared to CK. Similarly, higher values of SOC and TN stock in all aggregate fractions was observed with the application of NPKM. Application of NPK and NPKM for 26 years significantly increased aggregate stability, which was positively correlated with total SOC contents in terms of mean weight diameter (MWD) (Adj. R2 = 0.689, p < 0.03) and geometric mean diameter (GMD) (Adj. R2 = 0.471, p < 0.24). Moreover, higher scores regarding cumulative mineralization for bulk soil and aggregate associated OC were observed with the application of NPK and NPKM. Irrespective of treatments, higher cumulative C-mineralization was observed for macro-aggregates (>250 μm size) followed by 250-53 μm and <53 μm size aggregates. Interestingly, a highly positive correlation was observed between aggregate stability and the cumulative amount of mineralization for bulk soil and aggregate fractions, with R2 ranging from 0.84 to 0.99. This study evidenced that long-term fertilization of NPK and NPKM can improve soil aggregation, stability and associated OC and TN stock in aggregates, as well as aggregate-associated OC mineralization, which was further governed by aggregate size.