Long-Term Straw Returning Strategies Alter Soil Organic Carbon Sequestration During the Incorporation of Fresh Straw

Continuous straw returning is one of the important measures for soil organic carbon (SOC) sequestration in China, where most straw is returned to fields directly or used as animal feed before being returned to the field as livestock manure. However, the impacts of different long-term straw returning strategies on net soil carbon change during fresh straw incorporation are not fully understood. Here, we compared the soil chemical properties and microbial community after nine years of straw returning using different strategies including mineral fertilisers (NPK), mineral fertilisers plus manure (NPKM), and mineral fertilisers plus straw (NPKS). Then, the response of the SOC balance to the incorporation of fresh 13 C-labelled straw was determined during a 45-day incubation period. After nine years of straw returning, the soil subjected to NPKS showed significantly higher SOC contents (9.57 g kg -1 ) than other treatments. A broad spectrum of substrate utilisation by soil microbes was detected in soils amended with straw or manure relative to NPK, and a noticeable isolated position on the microbial utilisation of C sources was found between NPKM and NPKS. Fresh straw incorporation led to the highest rate of SOC mineralisation and the highest rate of straw decomposition in NPKS and NPKM soils, respectively. The amounts of straw-derived SOC (2.41-2.59 g C/kg soil) overcompensated for the native SOC losses (0.91-1.37 g C/kg soil) by mineralisation, resulting in net C sequestration in all treatments, and the lowest net SOC change was detected in NPKS soils (1.21 g C/kg soil). Overall, the SOC content showed a significant ( p <0.01 ) correlation with native soil C mineralisation, while the soil microbial index played a greater role in influencing fresh straw sequestration ( p <0.01 ). Our study emphasises that the stimulated native SOC mineralisation may contribute to the saturation limits of SOC pools in soils with high SOC contents.