Soil organic carbon fractions and its association with water-stable aggregates under different fertilization management practices

Agricultural soil organic carbon (SOC) plays a critical role in soil ecosystems, and SOC sequestration is a key ecosystem service. This study investigated the SOC fractions, including mineral-incorporated organic carbon (MSC), intra-particulate organic carbon (iPOC) and microbial biomass carbon (MBC), in bulk soil and water-stable aggregates, as influenced by soil fertilization managements, using three treatments: no fertilizer (CK), chemical fertilizer (FR) and organic manure (OM). Fertilizer application significantly increased SOC content and its fractions (MSC, iPOC and MBC) compared to CK, and the effect of organic manure amendment had more impact on the SOC accumulation than chemical fertilizer. Macroaggregate (>250 mu m)-associated SOC, MSC and iPOC (including fine and coarse iPOC) increased by 22%-283%, 18%-84% and 45%-232%, respectively, in FR and OM treatments compared with CK. In addition, the iPOC:MSC ratio values (%) within all size aggregates were all increased in the FR (about 84%-183%) and OM (about 103%-230%) treatments compared to CK (about 65%-146%). Meanwhile, MBC:SOC ratio values were the highest (about 2.16%-3.07%) in small macroaggregates (250-2000 mu m), but decreased sharply (about 2%-2.35%) in large macroaggregates (>2000 mu m) in different treatments. Our findings suggested that macroaggregates were the main sites of carbon sequestration and that carbon became more unstable with the increase of SOC in aggregates. Furthermore, the microbial biomass showed obvious differentiation characteristics in macroaggregates (>250 mu m). Specifically, small macroaggregates (250-2000 mu m) contained more microbial biomass, and large macroaggregates (>2000 mu m) were inclined to inhibit microbial activity.