1 Introduction
The Yangtze River basin, comprises approximately one-fifth of China’s land area, and is the most important base of grain production. It is distributed across subtropical-monsoon climate regions with four seasons, including a hot rainy season. Annual precipitation ranges from 400 mm to 2590 mm, with half occurring during summer as high intensity storms that result in drying and wetting cycles. The annual temperature ranges from -4 °C to 20 °C, and the highest and the lowest temperatures occur in June to August and December to the following February, respectively. This climate provides favorable conditions for paddy-upland crop rotation systems. Paddy-upland rotation refers to planting rice and upland crops in the same field. Upland crops, including rape, wheat, peanuts, cotton, and potatoes, are planted in the dry season from October to the next April, while rice is planted in April to October. In the Yangtze River basin, rice-rape and rice-wheat are the most vital cropping systems, accounting for 51.3% of the total rice yield (Zhang et al., 2013) and more than 90% of the total rapeseed production (Ren et al., 2013).
The soils of the Yangtze River basin are characterized by low organic matter and acidic pH, with moderate levels of phosphorus and potential deficiency of available potassium (K). In fact, three-fourths of paddy soils in China are deficient in K (Rengel & Damon, 2008; Römheld & Kirkby, 2010). The main reasons for K deficiency are biomass removal from the soil in the form of grain, straw, or hay (Smil, 1999) and unbalanced K fertilizer application (Cong et al., 2016). Leaching and runoff of K also contribute to decreased soil K (Rengel & Damon, 2008). Under highly intensive cropping systems and drying and wetting cycles, soil K is transformed between four forms, namely solution K, exchangeable K, nonexchangeable K, and structural K (Sparks, 1987). Soil K has changed greatly over the past 30 years due to changes in planting conditions. The Chinese national soil testing and fertilizer recommendation program, started in 2005, has been widely used in practice, and the recommended fertilizer rates have improved soil nutrient contents. Additionally, K cycles in plant-soil systems have changed because of environmental impacts, including balanced fertilization, recycling of straw, increased of atmospheric deposition, and decreased of soil K fixation. This paper summarizes changes in soil K and environmental impacts in the Yangtze River basin in China over the past 30 years, aiming at providing information for optimal K management in agricultural systems.