Objective The composition and availability of soil organic nitrogen (N) affect the level of soil fertility. The study aimed to investigate the changes in soil organic N content during different growth stages of rice under different straw returning amounts, and its impact on rice yield, which is of great significance for sustainable agricultural development.

Method The experiment was based on the long-term located experiments, using Bremner acid hydrolysis method and sampling method to investigate the effects of different straw returning amounts (NPKS1, conventional fertilization + 3750 kg hm⁻² straw; NPKS2, conventional fertilization + 7500 kg hm⁻² straw) on the changes of soil organic N components and crop yield at different growth stages of rice.

Result Compared with conventional fertilization (NPK) treatment, the total N content of soil in straw returning treatment increased by 5.42% to 20.81%, but with the increase of straw returning amount, the total N content of soil showed a trend of first increasing and then decreasing. During the rice growing season, there were significant differences in the changes of organic N components in various forms of soil. The content of soil acidic ammonium N showed a trend of first decreasing and then increasing with the growth period of rice, while acidic amino acid N and acidic amino sugar N showed a decreasing trend. The straw returning treatment increased the proportion of acid hydrolyzable unknown N and acid hydrolyzable ammonium N in soil total nitrogen (TN), with NPKS2 treatment showing the highest increase in the proportion of acid hydrolyzable ammonium N to soil TN compared to NPKS1 treatment, reaching 17.00%. The effect of NPKS2 treatment was more significant than that of NPKS1 treatment. In addition, NPKS2 treatment promoted the conversion of non acid hydrolyzable N to available N in paddy soil, increased soil active organic N pool and soil N mineralization capacity, and had a significantly higher effect than NPKS1 treatment. The treatment of returning straw to the field (NPKS1 and NPKS2) increased rice yields by 603.34 and 1101.67 kg hm⁻², possibly by increasing the content of soil acid hydrolyzed ammonium N and the proportion of acid hydrolyzed ammonium N in total soil acid hydrolyzed N at different growth stages of rice, which is beneficial for improving soil N supply capacity and increasing rice yields by 603.34 and 1101.67 kg hm⁻².

Conclusion Returning straw to the field can significantly increase rice yield by changing the organic N composition of paddy soil, especially the content of soil acid hydrolyzed ammonium N and the proportion of soil acid hydrolyzed total N. By increasing the content of soil acid hydrolyzed ammonium N and the proportion of acid hydrolyzed ammonium N in soil acid hydrolyzed total N, the yield of rice can be significantly improved. Moreover, returning a large amount of straw to the field has a better effect on increasing rice yield