Aggregate mass and carbon stocks in a paddy soil after long-term application of chemical or organic fertilizers

The soil organic carbon stock that accumulates in soil aggregates as a result of the practice of fertilizer application is important for sustainable agriculture and for mitigating the effects of climate change. In this study, we investigated how the practice of fertilizer application affects the mass of soil aggregates and carbon stocks in surface and subsoil layers in rice paddy soil. We determined the mass and carbon concentration in different size classes of aggregates in soil in the top 40-cm layer after the long-term application of different fertilizers. We found that the amount of organic carbon in bulk soil and aggregates decreased with increasing soil depth, except free microaggregates (free-m, 250-53 mu m), which showed the opposite trend. Macroaggregates (>250 mu m) were the main type of soil particles in both surface soil and subsoil. Compared with the unfertilized control or chemical fertilizer treatments, manure or straw combined with chemical fertilizers significantly (p < 0.05) increased the mass and carbon concentration in small macroaggregates (2000-250 mu m) and small macroaggregate fractions in the surface soil (0-20 cm), whilst the opposite effect was detected for large macroaggregates (>2000 mu m) and large macroaggregate fractions. In the top 40-cm soil layer, the carbon content in small macroaggregates and most small macroaggregate fractions increased linearly with increasing carbon input rate, but that in large macroaggregates and most large macroaggregate fractions decreased. Overall, small macroaggregates were the dominant particle size class harbouring carbon stocks, and the subsoil in this paddy soil has a high potential for carbon sequestration.