Abstract: 【Objective】The effects of long-term straw combined application of chemical fertilizers on the content of aggregates, distributions of aggregate-associated organic carbon and nitrogen, and soil microbial biomass were studied to reveal the promotion of soil fertility and productivity. 【Method】Soil samples were collected from a 40-year long-term experiment. The research is conducted with 4 treatments：pure chemical fertilizer（NP），straw with chemical fertilizer (S+NP), straw with 1/2 chemical fertilizer (S+1/2NP), and straw with 1/4 chemical fertilizer (S+1/4NP), and the straw is returned with the amount of 3 000 kg·hm^-2, chemical fertilizer NP is pure N 150 kg·hm^-2，P₂O₅ 150 kg·hm^-2 conducted. All soils samples were separated into four aggregate-size classes (>2 mm, 0.25-2 mm, 0.053-0.25 mm and <0.053 mm) by wet sieving. Organic carbon and total nitrogen contents of aggregates and soil microbial biomass content were measured. 【Result】(1) Long-term application of chemical fertilizers with straw reduced soil bulk density while improved the stability of aggregates. Compared with NP, the bulk density of S+NP decreased by 4.7%, while the proportion of aggregates >2 mm in size，the average weight diameter (MWD) and geometric weight diameter (GWD) increased by 254.4%, 76.5% and 91.3%, respectively. (2) In the three chemical fertilizers combined with straw return, the percentage of aggregates >2 mm, MWD and GWD in the S+NP and S+1/2NP were significantly increased by 49.1%-52.4%，19.43%-22.4% 和24.2%-33.3% compared with S+1/4NP, respectively. (3) Compared with NP, S+NP, S+1/2NP and S+1/4NP increased the contribution rate of aggregates >2 mm and >0.25 mm to SOC and total nitrogen, and significantly improved the SOC, SMBC and SMBN content in bulk soil. Among them, the SOC content was highest in S+NP, which was 6.3% and 12.6% higher than that in S+1/2NP and S+1/4NP, respectively. (4) The yield was showed that S+NP>NP>S+1/2NP>S+1/4NP, and S+NP increased wheat yield by 5.83%-83.6% compared with other treatments. (5) Positive correlation was revealed between soil aggregate stability, carbon and nitrogen content and crop yield, and >2 mm aggregate content, MWD and GWD are significantly or extremely significant with the total soil SOC, SMBC content and wheat yield, respectively.【Conclusion】In the dark brown soil area, long-term straw returning and fertilizer application of 150 kg·hm^-2 and P₂O₅ 150 kg·hm^-2 can improve aggregate stability, soil organic carbon content, microbial biomass and yield, and maximize the To achieve the synergistic effect of soil structure improvement, fertility improvement and crop yield increase.

Key words: long-term straw returned, chemical fertilizer , dark brown soil, water-stable aggregates, carbon and nitrogen content, microbial biomass, yield