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.