Effects of Chinese milk vetch incorporation and chemical fertilizer reduction on soil nitrogen supply and rice growth

Objectives Chinese milk vetch (MV) incorporation with reduced chemical fertilizer application is a potential measure for the sustainable production of paddy fields in southern China. This study was designed to investigate how incorporating a proportional amount of Chinese milk vetch to the quantity of chemical fertilizer reduction impacts the aboveground population of rice and soil nitrogen supply. We aim to provide a theoretical basis for achieving green and sustainable rice production.

Methods The experiment commenced in 2018 and had seven treatments, including no fertilizer application (CK), pure chemical fertilizer (F100), and MV incorporation at 22.5 t/hm² together with 100%, 80%, 60%, 40%, and 0% of the pure chemical fertilizer (MVF100, MVF 80, MVF 60, MVF 40, and MV). All the treatments received an equal amount of P fertilizer. Samples (plants and soil) were collected at the peak of tillering, booting, and maturity stages. Data were collected on tiller number, dry matter accumulation and transport, aboveground N accumulation (ANA), N recovery rate (NRE), and soil mineral N content (Nmin). Also, we calculate the apparent N loss in paddy soil.

Results Compared with CK, F100 increased Nmin and ANA and dry matter accumulation by 24.01%–35.35%. F100 increased the total tiller number across the growth stages, mainly in ineffective tillers with reduced panicle rate. Compared with F100, MVF80 (P<0.05) increased Nmin at the booting and maturity stages, promoted dry matter transportation to the panicle, and increased the aboveground dry matter and N accumulation at maturity by 7.14% and 18.74%. It also enhanced the dry matter and N accumulation in panicle by 10.96% and 19.20%, respectively. MVF100 (P<0.05) increased Nmin and ANA at maturity and the aboveground dry matter and N accumulation by 8.52% and 24.54%. Interestingly, the straw dry matter and N accumulation increased by 12.75% and 46.13%, respectively. Compared to F100, MVF60 (P<0.05) increased Nmin at the booting stage and increased the dry matter transfer into panicle after booting. Compared with F100, MVF40 and MV (P<0.05) reduced soil apparent N loss by 27.14% and 63.83%, respectively, while MVF100 increased it by 36.15%.

Conclusions MV incorporation combined with the appropriate amount of chemical fertilizer could improve soil N supply capacity, promote N absorption and accumulation in the key growth period of rice, and increase dry matter accumulation and transport, especially in panicles. The combination of MV and chemical fertilizer did not increase the apparent N loss, which is conducive to attaining green and sustainable rice production. Under MV incorporation, the optimal chemical fertilizer reduction rate is 20%.