Objectives China is short in potassium resources but rich in rice straw, which contains sufficient potassium resources for recycling. Here, we assessed the quantity of straw K resources and spatial distribution for keeping the soil K balance in China.

Methods Based on China Rural Statistical Yearbook and literature, we assessed the rice straw resources in different cropping seasons at provincial and regional levels. The straw K resources were calculated using the grain-to-straw ratio method. We assessed K input from rice straw incorporation and the contribution to soil K apparent balance.

Results The total output of straw K resources was K₂O 4.939 million tons in 2019, with 13.1%, 14.6%, and 72.4% recorded from early, late, and medium-late rice, respectively. The order of straw K resources distribution was Middle Yangtze River (33.4%)˃Lower Yangtze River (21.7%)>Southwest China (15.3%)>Northeast China (13.1%)>South China (12.8%)>North China (3.7%). Early and late rice were mainly cultivated in Hunan, Jiangxi, Guangdong, and Guangxi; the proportion of early and late rice straw K resources were 25.2% and 24.7% in Hunan, 23.8% and 24.4% in Jiangxi, 18.6% and 19.5% in Guangdong, and 17.2% and 14.8% in Guangxi. The medium-late rice was mainly cultivated in Jiangsu, Heilongjiang, Hubei, Sichuan, Anhui, and Hunan; the proportion of medium-late rice K resources in the provinces were 14.3%, 12.9%, 12.2%, 10.7%, 10.5%, and 8.8%, respectively. The annual K₂O inputs from early rice straw incorporation in Hunan, Jiangxi, Guangdong, and Guangxi were 148.7, 140.6, 143.9, and 145.0 kg/hm². The corresponding K₂O removed by early rice grains were 25.6, 24.2, 24.8, and 25.0 kg/hm², respectively. The K₂O inputs from late rice straw incorporation were 153.4, 145.3, 146.3, and 131.1 kg/hm²; the K₂O removed by late rice grains were 30.3, 28.7, 28.9, and 25.9 kg/hm² in Hunan, Jiangxi, Guangdong, and Guangxi, respectively. Without K fertilization, the soil K apparent balance under the double rice system had a deficit of 357.9, 338.8, 343.9, and 327.0 kg/hm² in Hunan, Jiangxi, Guangdong, and Guangxi, respectively. Straw incorporation reduced the deficits to 55.9, 52.9, 53.6, and 50.8 kg/hm². The K₂O inputs by medium-late rice straw returning were 125.8, 203.8, 201.1, and 189.0 kg/hm², and the K₂O removed by rice grains were 32.6, 29.4, 29.0 and 27.3 kg/hm² in Northeast China, Lower Yangtze River, Middle Yangtze River, and Southwest China, respectively, indicating K surplus if the rice straw was fully returned to the field.

Conclusions More than 70% of rice straw K resources exist in medium-late rice, and the Middle Yangtze River and Lower Yangtze River regions have the largest straw K resources in China. Early and late rice straw incorporation could reduce the soil K deficits, and the full incorporation of medium-late rice straw leads to a surplus of soil K. Therefore, rice straw incorporation is an important way of keeping soil potassium fertility in China.