Distribution of rice straw potassium resources across China and the importance of rice straw incorporation to soil potassium balance

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.