Objectives During the long-term cultivation of rice, a large amount of chemical fertilizer application led to the deterioration of soil structure. We studied the effects of different amounts of rice straw biochar and chemical fertilizer on paddy soil structure and carbon and nitrogen distribution, and explored the optimal fertilization management method in paddy field.

Methods A positional rice field experiment was conducted for four consecutive years in Quzhou of Zhejiang province for four years. The treatments included no fertilizer control (CK), conventional chemical fertilizer treatment (NPK), and combined application of rice straw biochar 22.5 t·hm⁻² (NPK+1%B), 45 t·hm⁻² (NPK+2%B) and 90 t·hm⁻² (NPK+4%B) with the conventional NPK applications, respectively. At rice harvest, 0-20 cm top soil samples were collected for the analysis of soil aggregate stability, organic carbon and total nitrogen contents, and the contents of aggregates in sizes of >2 mm, 0.25～2 mm, 0.053～0.25 mm, and <0.053 mm, respectively.

Results Compared with NPK treatment, straw treatments increased the > 0.25 mm aggregate contents by 2.43%-7.99%, decreased <0.25 mm aggregates by 2.93%-9.63%, increased soil organic carbon by 16.4%～45.16%, and total nitrogen by 14.67%～36.69% (P<0.05), respectively; biochar treatments also enhanced the contents of organic carbon and total nitrogen in >2 mm and 0.25～2 mm aggregate size fractions by 23.96%～70.87%, 19.44%～47.56%, 21.87%～49.27% and 17.18%～37.47% (P<0.05); However, biochar had no effect on the contents of organic carbon and total nitrogen in the 0.053～0.25 mm and <0.053 mm aggregate size fractions, indicated that the increased organic carbon and nitrogen were mainly accumulated in >0.25 mm aggregates. NPK+4%B treatment was recorded the best effects on all the above indices. Compared with NPK treatment, straw biochar application amount did not change soil C/N ratios significantly, but increased the C/N ratios of all aggregate size fractions (P<0.05), and NPK+4%B treatment resulted the higher C/N ratios in all aggregate size fractions. In addition, biochar treatments increased the organic carbon and nitrogen contribution rates of the >2 mm aggregates by 6.54%～17.43% and 3.67%～7.28%, and those of the 0.25～2 mm aggregates by 5.16%～11.55% and 0.57%～1.83%, decreased those of the >2 mm aggregates by 7.25%～29.27% and 15.93%～33.68%, and those of 0.25～2 mm aggregates by 10.26%～26.80% and 12.61%～30.78%.

Conclusions The application of high amount of rice straw biochar could significantly increased the soil total organic carbon and nitrogen, increased the stability of >0.25 mm aggregates, enhance the the contribution rates of organic carbon and total nitrogen in large aggregates. The increased magnitudes were positively correlated to the application amounts of rice straw biochar, thus increase the sequestration potential of organic carbon and total nitrogen in paddy soils, which is of great significance in ensuring the structure stability of paddy soils and maintaining the high yields of rice.