Objectives  Nitrogen is an important nutrient element limiting soil productivity. We investigated the relationship between the dynamics of soil N in reddish paddy fields, and N supply capacity and intensity under long-term application of green manure and straw, to optimize the utilization of these organic materials for sustainable rice production.

  Methods  The paddy soils developed from the Quaternary red clay, were collected from the long-term field experiment located in the Science and Technology Park of Jiangxi Agricultural University, initiated in 1981. The field experiment had four treatments: no fertilizer (CK); single application of chemical fertilizer (F); returning milk vetch to field as basal fertilizer for early rice + chemical fertilizer (MF); and returning milk vetch and straw together to late rice field + chemical fertilizer (MSF). After harvesting of late rice in 2019, the soil organic N components, total N, mineralizable N, microbial biomass N, and release characteristics of available N in 0–20 cm soil layer were analyzed.

  Results  Fertilization significantly increased the content of various forms of soil N. Under the same N, P₂O₅, and K₂O nutrient input, compared with F treatment, MF and MSF treatments increased soil total N by 15.03% and 24.35%, mineralized N by 35.73% and 58.02%, and microbial biomass N by 21.73% and 36.73%, respectively (P < 0.05). The soil supply intensity of available N was in the order of MSF>MF>F>CK, and the supply capacity of MSF and MF treatment was 19.01% and 25.22% higher than that of F treatment. Of the soil organic N components, the total hydrolysable N under MF and MSF treatments was markedly higher than that of CK and F treatments; the hydrolysable ammonia N, hydrolysable amino acid N, and unknown hydrolysable N contents under MSF were 36.02%, 33.52%, and 26.58% higher than those of F treatment, however, there was no significant difference in soil non-hydrolysable N among treatments. The stepwise linear regression and path analysis showed that soil microbial biomass N and hydrolysable ammoniacal N were the main sources of soil mineralizable N, and soil microbial biomass N was the main direct contributor of available N.

  Conclusions  Under the same nutrient input, long-term replacement of chemical fertilizer input with green manure and straw effectively increased the microbial biomass N and hydrolysable ammonia N in the red paddy soil, this strategy also promoted the mineralization of soil organic nitrogen, and increased the content of available N thereby, improving the N supply capacity and intensity of paddy field. The effect of returning green manure and straw together is better than that of returning green manure alone.