Effect of combined application of Chinese milk vetch and nitrogen fertilizer on nitrogen uptake, utilization and dry matter accumulation in early rice

This study evaluated the effect of combined application of Chinese milk vetch (Astragalus sinicus L.) and different levels of nitrogen (N) fertilizer on dry matter accumulation and N use efficiency of early rice in order to determinate the suitable N level under application of Chinese milk vetch. Field treatments were set with four N application levels (90 kg·hm^-2, 120 kg·hm^-2, 150 kg·hm^-2 and 180 kg·hm^-2) under 22 500 kg·hm^-2 Chinese milk vetch using the winter fallow field with general N fertilizer level (150 kg·hm^-2) as the control. In the experiment, Chinese milk vetch was planted in winter and incorporated into soil in spring before early rice transplant, while N fertilizer was applied as base fertilizer, tillering fertilizer and heading fertilizer with 5:3:2 rate. The rice plants were sampled at tillering, booting, heading, filling and maturity stages of the early rice. Dry matter weight and N content were assayed, and the relative indexes were calculated. The results showed that all N application treatments had higher dry matter accumulation than the control. Specifically, the treatments with Chinese milk vetch combined with 90 kg (N)·hm^-2 and 120 kg (N)·hm^-2 increased dry matter (9.65 t·hm^-2 and 9.97 t·hm^-2) respectively by 11.18% and 14.86% compared to the control. Dry matter accumulation increased with duration of growth and peaked twice, one at sowing-tillering stage (19.26%-24.77%) and the other at heading-filling stage (45.23%-52.75%). The combined application of Chinese milk vetch and N fertilizer supported N integration that was beneficial to rice growth. The amounts of N accumulation of early rice treated with Chinese milk vetch and N fertilizer were higher than the control, with increases of 6.95%-18.68%. Among the treatments, that of Chinese milk vetch combined with 90 kg (N)·hm^-2 had the highest production efficiency of dry matter and N harvest index, with respective increases of 3.94%-14.08% and 6.65%-14.90% over other treatments. The results indicated that combined application of Chinese milk vetch with 90 kg (N)·hm^-2 or 120 kg (N)·hm^-2 was satisfactory because they reduced N fertilizer and increased N use efficiency at the same time, which was the ideal fertilization model for the study area.