Abstract: 【Objective】The results of carbon sequestration studies on combining straw returning with nitrogen fertilizer are controversial. Aimed at such problem, our experiment was carried out to reveal the effect of combining straw returning with nitrogen fertilizer on Carbon sequestration capacity and mechanism of farmland, and provide a reference for the future research.【Method】Based on 11 years of long-term positioning experiments, this paper adopts split-zone design, the main treatment included straw returning to soil and removal straw from field, and the subplots included three N application rate which are no nitrogen (N0), 168 kg·hm^-2 (N168, nitrogen), 336 kg·hm^-2 (N336, excessive nitrogen application).【Result】Compared with wheat without nitrogen fertilizer, wheat yield increased by 14.4%-19.5% with nitrogen fertilizer. The effect of straw returning to the field on yield was not significant. Straw returning significantly increased the cumulative input of soil carbon by 70.8% (P<0.05), but had no significant effect on soil organic carbon storage. Compared with no nitrogen application, nitrogen application significantly increased soil carbon accumulation input and soil organic carbon storage by 7.7%-8.5% (P<0.05) and 4.73%-8.11% (P<0.05), respectively. The application of nitrogen fertilizer significantly increased the carbon fixation rate by 32.7%-56.1% (P<0.05), and the excessive application of nitrogen (N336) significantly increased the soil carbon fixation efficiency by 51.8% (P<0.05); Straw returning to the field did not significantly improve the soil carbon fixation rate, but significantly reduced the carbon fixation efficiency by 30.9% (P<0.05). Both nitrogen application and straw returning can improve soil carbon pool capacity, and N0 and N168 have reached carbon saturation. The content of soluble organic carbon (DOC), microbial biomass carbon (MBC) and easily oxidized organic carbon (EO) in the soil increased by 4.61%, 11.2% and 4.51% respectively after returning straw to the field. Compared with no nitrogen application (N0), DOC of nitrogen application (N168) and excessive nitrogen application (N336) increased by 14.12% and 29.54% respectively; MBC decreased by 14.02% and 28% respectively on average; EO increased by 8.19% and 11.53% respectively. Straw returning to the field is beneficial to the improvement of soil DOC/SOC and microbial entropy. Applying nitrogen fertilizer is beneficial to the increase of DOC/SOC, but reduces the microbial entropy. Both straw returning and nitrogen fertilizer application had no effect on soil EO/SOC. Both straw returning and nitrogen application were beneficial to the improvement of macroaggregates (>0.25 mm), and straw returning significantly increased the organic carbon content of macroaggregates by 5.2%. The average weight diameter (MWD) and geometric average diameter (GMD) of aggregates under non-return showed a trend of first increasing and then decreasing with the increase of nitrogen level, while under return, it showed an increase with the increase of nitrogen level. Straw return increased the MWD and GMD of aggregates by 8.8% and 7.5% respectively, and the application of nitrogen fertilizer increased the MWD and GMD by 14.1%-22.7% and 16.8%-23.4% respectively compared with no nitrogen application. Both straw returning and nitrogen application can improve the distribution of organic carbon in large aggregates.【Conclusion】Straw return with nitrogen fertilizer can increase carbon input, increase activated organic carbon content, reduce microbial activity, and improve the protection of organic carbon by aggregates.

Key words: straw return, nitrogen fertilizer, tillage layer, carbon storage, activated carbon components, aggregates