Dynamics of Soil Organic Carbon Fractions and Aggregates in Vegetable Cropping Systems

Fertilisers significantly affect crop production and crop biomass inputs to soil organic carbon (SOC). However, the long-term effects of fertilisers on C associated with aggregates are not yet fully understood. Based on soil aggregate and SOC fractionation analysis, this study investigated the long-term effects of organic manure and inorganic fertilisers on the accumulation and change in SOC and its fractions, including the C concentrations of free light fraction, intra-aggregate particulate organic matter (POM) and intra-aggregate mineral-associated organic matter (MOM). Long-term manure applications improved SOC and increased the concentrations of some C fractions. Manure also accelerated the decomposition of coarse POM (cPOM) into fine POM (fPOM) and facilitated the transformation of fPOM encrustation into intra-microaggregate POM within macroaggregates. However, the application of inorganic fertilisers was detrimental to the formation of fPOM and to the subsequent encrustation of fPOM with clay particles, thus inhibiting the formation of stable microaggregates within macroaggregates. No significant differences were observed among the inorganic fertiliser treatments in terms of C concentrations of MOM, intra-microaggregate MOM within macroaggregate (imMMOM) and intra-microaggregate MOM (imMOM). However, the long-term application of manure resulted in large increases in C concentrations of MOM (36.35%), imMMOM (456.31%) and imMOM (19.33%) compared with control treatment.