Appropriate fertilization increases carbon and nitrogen sequestration and economic benefit for straw-incorporated upland farming

Long-term fertilization management is not only an important source of nutrients to ensure food productivity, but also significantly affects soil organic carbon (SOC) and total nitrogen (TN) sequestration in agroecosystems, as well as strongly affects soil aggregate structure and microbiological characteristics. Straw incorporation is expected to promote SOC and TN. However, how long-term fertilization managements change soil aggregate distribution, SOC and TN components, and microbiological characteristics in straw-returned dryland fields are unknown, and how the quantitative analysis of the relationship between SOC and TN and biological factors, as well as their relative importance at an aggregate scale, are equally unknown. In addition, whether fertilization management can achieve higher crop productivity and economic efficiency while achieving higher soil carbon and nitrogen sequestration is a topic of greater concern to farmers. A 10-year in situ field fertilization experiment with five N application rates (0, 75, 150, 225, and 300 kgNha(-1), denoted as N0, N75, N150, 225, and N300, respectively) was conducted on straw-returned dryland. The wet sieving method was used to investigate the soil aggregates distribution in aggregate fractions (>5, 5-2, 2-1, 1-0.5, 0.5-0.25, and < 0.25 mm) at topsoil (0-20 cm) at different N application rates. The results showed that fertilization significantly increased the proportion of macro-aggregates (>0.25 mm) and mean weight diameter. Then, K2CrO7-H2SO4 oxidation and Kjeldahl digestion methods were used to determine the SOC and TN contents and calculate the contributing rate of aggregate size fractions in SOC and TN. It was found that most SOC and TN are distributed in macro-aggregate (especially 2-1 mm). N225 significantly increased the SOC and TN in bulk soil and aggregate scale, especially the macro-aggregate contribution rate to SOC and TN. At the same time, chloroform fumigation extraction and the 3, 5-dinitrosalicylic acid method were adopted for the determination of soil microbial biomass and enzyme activity. It was found that microbial activity characteristics were consistent with SOC and TN dynamics at bulk soil and aggregate scales. Through further variation partitioning analysis demonstrated microbial biomass and enzyme activities accounting for 26 % and 34 % of the variation in SOC and TN. Finally, by calculating economic benefits, N225 maximizes crop yield and economic efficiency. Overall, reduced N application rate (N225) improves the soil's bioactive functioning, promotes carbon and nitrogen sequestration by promoting aggregation and improving the microbial biomass and enzyme activities in macro-aggregates, as well as enhancing crop yields and maximizing economic benefits in straw-returned dryland fields. These results would provide new insights into fertilization management strategies for improving soil conditions in terms of optimizing the interaction of soil aggregate structure-microbial activity characteristics-SOC and TN sequestration, as well as achieving sustainable agricultural development in straw-returned dryland fields.