Effects of chemical fertilizer reduction on rice yield and soil fertility in yellow-mud paddy field under the continuous return of milk vetch

Objectives We studied the effects of different chemical fertilizer reduction rates on rice yield and paddy field fertility under the long-term return of milk vetch (Astragalus sinicus) for efficient and sustainable rice production.

Methods The field experiment was carried out in Fujian Province from 2009 to 2019, with cropping system of single-rice cultivation followed by milk vetch planting. There were seven fertilization treatments: no fertilizer application (CK), conventional chemical fertilization rate (100%F), applying 100%, 80%, 60% and 40% of the conventional chemical fertilization rate under the return of milk vetch (M+100%F, M+80%F, M+60%F, M+40%F), and milk vetch (M) only. Rice grain and straw samples were collected at the rice harvest stage to detect the N, P, and K content, rice grain yield and amino acid content from 2017 to 2019. Soil samples at 0−20 cm depth were collected in 2019 at tillering and maturity stage for the determination of soil bulk density, pH, organic carbon, microbial biomass nitrogen (MBN) and soluble nitrogen.

Results Compared with CK, the rice grain and straw yields in all the fertilizer treatments (P<0.05) increased by 11.4%−21.3% and 17.1%−39.0%, respectively. Compared with 100%F, M+100%F and M+80%F increased rice yield by 3.7% and 3.0%, straw yield by 5.1% and 5.6%, while the other treatments produced similar grain and straw yields. M+80%F increased N uptake by 75.4% and 5.7% at peak tillering and maturity stage, increased the total and essential amino acid contents in grains by 5.2% and 6.7%, while the M+less than 60%F decreased the total and essential amino acid contents. Compared with the 100%F treatment, treatments including M increased soil nutrient contents by 7.3%−4.3%, soil MBN by −1.0%−23.5%, total N by 9.4%−14.1%, and organic matter by 7.9%−12.0%.

Conclusions On the basis of annual returning of 18000−22500 kg/hm² milk vetch to the soil, applying 60% of the conventional rate of chemical fertilizer in the middle and low yield paddy field could maintain rice grain and straw yield. Applying 80% of the conventional chemical fertilizer rate could improve soil properties, stimulate rice N uptake, yield and nutritional quality, indicating it the optimum application rate.