Objectives Rice production requires large amount of potassium for high yield. For the efficient and sustainable utilization of potassium in red soil paddy field area, we studied the effects of the cooperative returning of rice straw and Chinese milk vetch.

Methods A fertilizer positioning experiment was carried out in Qiyang City, Hunan Province since 2016, under rice and winter Chinese milk vetch (CM) rotation system. The control treatment was no-fertilizer input in rice season and no-cropping after rice harvest during winter season (CK); and the four fertilizer treatments in the rice season included winter fallow (F), returning winter CM merely (CM), returning both rice straw and winter CM (RCM) under normal NPK fertilizer input, and a treatment under 50% reduction of K fertilizer (50%KRCM). At rice harvesting of the third year of the experiment (2019), the yield and plant biomass of rice were weighed, the K content in plants and grains of rice were measured for the calculation of rice potassium uptake, potassium physiological efficiency and potassium apparent balance were calculated. And the rapidly available potassium (AK) and slow-release potassium content (SAK) in topsoil were measured for determine the correlation between rice yield and potassium indicators.

Results Compared with F treatment, the RCM and 50%KRCM treatments increased the annual rice yield by 59.4% and 59.7%; the three CM contained treatments increased the effective panicle number by 53.1%−63.3%. RCM and 50%KRCM treatment were recorded similar rice yield and yield components. Compared with F treatment, RCM and 50%KRCM significantly increased soil AK content by 87.6% and 41.1%, and SAK content by 14.0% and 12.1%, respectively. CM treatment increased AK content, but not increase SAK significantly. 50%KRCM was recorded 24.8% lower AK content but similar SAK content with RCM treatment. Compared with F treatment, all the four fertilization treatments increased the potassium uptake of rice straws and grains, and the potassium surplus (PS). RCM was recorded the highest increment of rice grain K content by 6.7%. The PS was significantly descended in order of RCM, 50%KRCM and CM treatment, and the PS amount of 50%KRCM was 119.7 kg/(hm²·a) less than that of RCM. The PS showed a positive correlation (P<0.05) with the change of soil potassium content, the soil AK would rise 0.25 mg/kg for every 1 kg/hm² of PS increase. Fertilization reduced potassium physiological efficiency, the potassium physiological efficiencies of CM, RCM and F treatments were 10.3%−15.5% lower than that of CK. There was a significant positive correlation between soil AK, plant K uptake, PS and rice yield. Every 1 kg/hm² of K uptake would result 30.5 kg/hm² of rice yield increase, every extra 1 kg/(hm²·a) of PS would result 9.5 kg/hm² of rice yield increase.

Conclusions Cooperative returning of rice straw and Chinese milk vetch is effective to increase rice yield due to more effective panicle number and potassium surplus, and higher soil potassium supply capacity, and the effect has been proved much better than the single returning of Chinese milk vetch. Under the cooperative returning, potassium reduction decreases potassium surplus, that does not show obvious impact on soil potassium supply capacity and rice yield, so is an efficient potassium fertilizer management.