Nutrient absorption and transport characteristics of rice under different nitrogen reduction ratios in a Chinese milk vetch and rice rotation system in southern Henan Province, China

Objectives Chinese milk vetch (CMV) and rice rotation is a common cropping system in south Henan rice production area. We studied the nutrient absorption and transport characteristics of rice, and the soil nutrient supply capability under the condition of incorporating all CMV and reducing different proportion of nitrogen fertilizer input, to provide a theoretical basis for N fertilizer management in the rotation system.

Methods The long-term CMV and rice rotation experiment was established in Xinyang, Henan Province in 2008. Six treatments were included - no fertilization (CK), conventional fertilization without burying CMV (N100), and burying fresh CMV before rice transplanting, and conventional N rate of 100%, 80%, 60%, and 40% (GN100, GN80, GN60 and GN40). The rice yield, biomass, N, phosphorus (P), and potassium (K) nutrient absorption and basic soil nutrients at different stages were measured.

Results There was no significant difference in rice yield among N100, GN40, GN60, GN80, and GN100 treatments, while GN40 and GN60 treatments increased agronomic efficiency of N fertilizer by 213.41% and 113.70% compared with N100; GN40, GN60, and GN80 treatments increased N partial productivity by 162.17%, 75.69%, and 34.39%, respectively. Compared with N100 treatment, GN80 and GN100 treatments straw biomass increased by 41.06% and 17.22% at heading stage and by 28.22% and 20.92% at maturity stage, respectively. GN60, GN80, and GN100 treatments increased N absorption in grain by 31.34%, 31.79%, and 20.13%; GN60 and GN80 treatments increased P absorption in grain by 14.23% and 14.61%, respectively; GN80 treatment increased K absorption in grain by 10.64% at the maturity of rice. Compared with N100 treatment, GN60 treatment increased N transport capacity by 12.54 kg/hm²; GN40, GN60, and GN80 increased P transport capacity by 11.32 kg/hm², 23.02 kg/hm² and 18.14 kg/hm²; P transport rate by 35.92, 41.22, and 39.91 percentage points; P contribution rate to grain was increased by 39.36, 74.60 and 57.93 percentage points, respectively. Compared with N100 treatment, GN40, GN60, GN80, and GN100 treatments met the sufficient supply of soil mineral nitrogen during rice growth. GN60 and GN80 treatments significantly reduced the content of soil available P at tillering stage, jointing stage and heading stage; GN40, GN60, GN80, and GN100 treatments significantly decreased the content of soil available K at tillering stage, booting stage and maturity stage, respectively. The random forest analysis showed that soil available P at each growth stage and N, P, and K absorption at rice tillering stage had a great contribution to rice yield. Linear regression analysis revealed that soil nutrient at tillering stage and booting stage of rice and aboveground N, P, and K absorption at each growth stage of rice were significantly and positively correlated with rice yield.

Conclusions Under the CMV and rice rotation system in southern Henan Province, reducing 40% of conventional chemical N input improves the soil N and carbon pool storage, stimulates the absorption and transport to grain of NPK, keeps rice yield stable, and improves the agronomic efficiency and partial factor productivity of N fertilizer.