Returning rice straw to the field increased the yield and nutrient accumulation of green manure crops in a typical paddy field in South China

  Objectives  Returning leguminous green manure (GM) to the field promotes high and stable crop yield and sustainable agricultural development. Here, we studied the suitable patterns of returning rice straw to the field combined with chemical N fertilization for high growth and nutrient accumulation in GM planted during the fallow season in winter.

  Methods  Pot experiments with the same treatments were conducted on salinized paddy soil in Guangxi, purple soil in Sichuan, and yellow-brown soil in Anhui. The three levels of rice straw turnover were 0, 3000, and 6000 kg/hm² (S0, S1 and S2), and the N fertilization rates were 0 and 45 kg/hm² (N0 and N45), giving a total of 6 treatments. At the full flowering stage of GM, the fresh biomass was harvested and weighed; the plant and soil samples were collected to determine plant nutrient accumulation and soil physical and chemical properties.

  Results  Compared with no straw and no fertilization control, returning straw and N application increased the GM fresh yield by 63.0%–66.0%, 35.2%–53.8% and 103.6%–117.1%, N accumulation by 19.1%–41.5%, 43.7%–67.2% and 65.1%–70.5%, P accumulation by 12.1%–68.9%, 31.4%–57.3% and 37.9%–45.3%, K accumulation by 22.0%–58.7%, 30.7%–35.0% and 89.2%–102.9% in salinized paddy soil, purple soil, and yellow-brown soil, respectively. In salinized paddy soil and purple soil, returning rice straw (S1 and S2) increased the GM fresh yield and the accumulation of N, P, K, but the combined application of N fertilization did not improve them. In yellow brown soil, the total return of rice straw reduced the yield and nutrient accumulation of GM, and the return of rice straw combined with N fertilizer significantly increased the yield and nutrient accumulation of GM. In salinized paddy soil, RS1N45 and RS2N45 (P < 0.05) increased soil available P content. In yellow-brown soil, a higher straw rate resulted in higher soil pH, available K and P . In contrast, moderate straw rate (S1) resulted in the highest soil total N. According to the partial least squares path model analysis, returning straw and soil type had (P < 0.05) significant positive effects on the GM fresh yield and NPK accumulation. Conversely, N application only had a (P < 0.05) positive effect on GM fresh yield. Returning straw showed higher effects on GM fresh yield in salinized paddy and purple soil, while N application showed higher effects in yellow-brown soil.l least squares path model analysis, returning straw and soil type had (P < 0.05) significant positive effects on the GM fresh yield and NPK accumulation. Conversely, N application only had a (P < 0.05) positive effect on GM fresh yield. Returning straw showed higher effects on GM fresh yield in salinized paddy and purple soil, while N application showed higher effects in yellow-brown soil.

  Conclusions  Returning straw and N application could increase the fresh yield and N, P, K accumulation in green manures and soil inorganic N content. In salinized paddy and purple soil, returning straw to the field could increase the yield and nutrient accumulation in leguminous green manure and maintain available soil nutrients without N application. In yellow-brown soil, returning the whole rice straw to the field should be combined with N application for sustainable yield and nutrient accumulation in green manure. This would also alleviate soil inorganic N and available P and K decrease.