Combined returning of milk vetch and rice straw improves fertilizer nitrogen recovery and fertility of yellow-mud paddy soil

Objectives  Rice straw and milk vetch (Astragalus sinicus L.) plants are important organic nutrient resources in southern China. Thus, the effects of their combined incorporation to field on rice yield, soil fertility, nutrient uptake, and their optimum replacement proportion for chemical fertilizer were studied.

Methods  A field experiment was conducted in a yellow-mud paddy in Fujian for four consecutive years. The experiment involved six different treatments with uniform total N input (135 kg/hm²) but varying replacement percentage of N from rice straw-milk vetch plants (RM), i.e. 0% (RM0, CK), 20% (RM20), 40% (RM40), 60% (RM60), 80% (RM80) and 100% (RM100). The rice yield, soil fertility, nutrient uptake, and utilization efficiencies were analyzed.

Results  All treatments with rice straw-milk vetch plants (RM) produced higher rice yield to varying degrees than CK, but the highest increment in the grain (15.4%) and straw (23.6%) yields of rice were found under RM20 treatment, and the yield increments due to RM incorporation declined with replacement percentage higher than 20%. The highest number of effective panicles was also found under RM20 treatment. Compared with CK, all RM treatments increased the uptake of N, P and K by 2.4%–15.4%, 2.6%–17.4%, and 2.0%–22.3%, respectively. Similarly, all RM treatments, except RM100, increased the N recovery rates by 2.0–13.5 percent points. The highest increments in the N, P and K, and N recovery rate were found under the RM20 treatment. The RM replacements increased the physiochemical (pH, organic matter, total N, available N, available P, readily available K) and biochemical (microbial biomass carbon, urease and acid phosphatase activity) properties of soil to different extents, but decreased soil bulk density.

Conclusions  The continuous and combined incorporation of rice straw and milk vetch to the field improves soil fertility, nutrient uptake and productivity in yellow-mud paddy soil. More importantly, replacing 20%–40% of chemical fertilizer with combined rice straw and milk vetch plants is recommended for imcreasing rice yield and soil fertility, and reduction in chemical fertilizer application rates.