Long-term application of organic manure and mineral fertilizers on aggregation and aggregate-associated carbon in a sandy loam soil

A long-term field experiment was established in September 1989 to examine the influence of mineral fertilizer and compost on soil fertility in an intensively cultivated fluvo-aquic soil in the North China plain. The study involved seven treatments: compost (CM), half-compost plus half-fertilizer N (HCM), fertilizer NPK (NPK), fertilizer NP (NP), fertilizer NK (NK), fertilizer PK (PK) and control without any fertilizer (CK). The mass and organic C concentration of aggregates (>250 μm macroaggregate, 53–250 μm microaggregate, and <53 μm free silt + clay fraction) and subfractions within aggregates, including intra particulate organic matter (iPOM) and silt + clay subfraction, were measured to evaluate the process of aggregation and organic C accumulation. The long-term application of mineral fertilizers had no obvious effects on the mass proportion of aggregate, and in contrast, compost significantly increased the mass proportion of macroaggregates from 8.8% in CK soil to 17.7–30.8% in compost-added soils. This increase was at the expense of microaggregates and free silt + clay fraction. Soil organic C concentrations were significantly increased in the CM and HCM treatments by 124% and 72%, respectively, but less than 27% increase was observed in mineral fertilizer-added soils over an 18-year period (compared with CK soil). Amendment with mineral fertilizer NPK mainly increased organic C concentrations in macroaggregates and particularly in the free silt + clay fraction. In contrast, compost application mainly accelerated organic C accumulation in macroaggregates by increasing the amount of C in the silt + clay subfraction rather than iPOM, which accounted for 47–58% of the increased organic C in soil. This was because of the mass proportion increase in macroaggregates. The mass proportion of macroaggregates was significantly related with organic C concentration in microaggregate and free silt + clay fractions (P = 0.005). The mass ratio of macroaggregates plus microaggregates to the free silt + clay fraction (P = 0.015) and macroaggregates to microaggregates (P = 0.003) was significantly correlated with organic C concentration in the free silt + clay fraction. These results indicated that the increase of soil organic C in compost-added soil was possibly due first to the enhancement of organic C concentration in the free silt + clay fraction, which in turn promoted the formation of microaggregates and/or macroaggregates. We consider that the increase of organic C concentration in the free silt + clay fraction is likely to play a key role in aggregation and C sequestration.