A preliminary determination of the plot-scale fertilization tolerance of an Entisol in southwest China

Purple soil of China is suitable for planting a variety of food and cash crops, owing to its rich nutrients and high natural fertility. In purple soil areas, farmers often apply excessive fertilizers to fields to maintain yields in continuous cropping systems. However, excessive fertilizer application can result in high nutrient concentrations in soils and nutrient pollution in the water environment. The fertilization tolerance (FT) of purple soil should be determined by considering critical values to both maximize crop production and minimize environmental risk of nitrogen (N) and phosphorus (P) loss. In this study, farmland fields with 18% slope plots planted with the typical crop of corn (Zea mays L.) were used to investigate the effects of varied fertilizer application levels on crop yields and the main soil fertility indexes, as well as on N and P loss from purple soil. The main results showed that corn yield increased with fertilization level when the fertilization amount ranged from 375 to 1,500 kg ha^–1 and declined with overfertilization (1,875 kg ha^–1). The grain yield, which increased with fertilization levels in 2011, was not significantly higher than in 2012 (p > 0.05), while the 1,000 grain weight in 2011 was significantly higher than in 2012 (p < 0.05). The actual amounts of soil ammonifying bacteria, nitrifying bacteria, denitrifying bacteria, and organic P bacteria during the various growth periods were significantly greater at the fertilization amounts of 1,125 kg ha^–1 and 1,500 kg ha^–1 than in other treatments both in 2011 and 2012 (p < 0.05). Compared to the fertilization amount of 750 kg ha^–1, urease and alkaline phosphatase activities with the fertilization amounts of 1,125 kg ha^–1 and 1,500 kg ha^–1 significantly increased in both 2011 and 2012 (p < 0.05), while significant reductions in both years were observed at the fertilization amounts of 1,875 kg ha^–1 and 375 kg ha^–1. Total N and total phosphorous concentrations in runoff water rose with an increase in fertilization rate in both 2011 and 2012. The effects of three types of fertilization on crop yield, soil fertility, and N and P loss were standardized to values ranging from 0 to 1, with an integrated index of these effects then evaluated to determine the FT, which is here defined as the amount of fertilizer applied to a soil that maintains not only high soil productivity but also high soil fertility and water environmental security. In this study, the range of FT was 750 to 1,120 kg ha^–1 (organic compound fertilizer of potassium nitrate [KNO₃]) at plot scale. Therefore, the FT for purple soil ensures not only high output but also that the quality of the regional soil and water environment reaches certain quality requirements.