Green manuring combined with rice straw return enhances secondary and micronutrients in rice grains in Southern China

Green manure (GM)-rice rotation is an effective practice for yield-increasing and soil-fertilizing in southern China, while the effects of GM, and the co-utilization with rice straw return on secondary and micronutrients in rice grain remains unclear. This study systematically investigated the regulatory effects of the co-utilization of GM and rice straw combined with reduced nitrogen (N) fertilizer application on grain secondary and micronutrients based on a field experiment in the southern Jiangsu rice-growing area. Results showed that the five-year average rice yield under GM increased by 5.65% relative to that under winter fallow (WF). The co-utilization of GM and rice straw with 40% reduction of N fertilizer (GMS_N60) was identified as the optimal mode. It achieved a rice yield increased by 16.87% compared with the WF with straw return and conventional N fertilization (WFS_N100), and the accumulation of N, phosphorus (P), and potassium (K) improved simultaneously. Green manuring promoted the uptake of secondary and micronutrients in rice grain and silicon (Si) in straw. The average contents of calcium (Ca), iron (Fe), copper (Cu), molybdenum (Mo), and selenium (Se) in rice grain increased by 8.08%, 17.75%, 3.99%, 13.56%, and 20.25%, respectively, and the rice straw Si content increased by 5.24% under GM relative to that under WF. The contents of zinc (Zn) and Mo under GMS_N60 increased by 8.38% and 19.12%, respectively, relative to those under WFS_N100. Correlation analysis indicated that GM facilitated the synergistic absorption of magnesium (Mg), manganese (Mn), and Zn. This study elucidates the roles of GM in stabilizing and increasing rice yields and enhancing the nutritional quality of rice, providing important support for high-yield and high-quality rice production in southern China.