Research paperInventory of apparent nitrogen and phosphorus balance and risk of potential pollution in typical sloping cropland of purple soil in China-A case study in the Three Gorges Reservoir region

- Phosphorus balance is put in previous model and used to calculate nutrients balance.
- Recycling nitrogen and phosphorus is concerned as the source of nutrients balance.
- Input of nitrogen and phosphorus by parent-rock weathering are taken into account.

Agricultural non-point source pollution of nitrogen (N) and phosphorus (P) exerts a key influence on aquatic environment safety. Purple soil and sloping cropland make up the highest proportion of all soil types and land-use pattern in the up and middle reaches of Yangtze River. Therefore, a study on nutrients balance for risk assessment in sloping cropland of purple soil will contribute to the understanding of the causes of eutrophication in aquatic environment in China. The Organization for Economic Cooperation and Development (OECD) have developed the soil surface nitrogen balance indicator to estimate potential N pollution through measuring the difference between agricultural inputs and outputs. Based on this indicator, we established a framework, a methodology, and a database for calculating the N and P balance for typical sloping cropland of purple soil in TGR region, and evaluated the impact of nutrients balance on agricultural production and environment. For the basic watershed of Xinzheng unit, which mainly consists of sloping cropland of purple soil, N and P inputs were 755 and 107 kg ha−1 a−1, with 68.4% of N and 56.1% of P inputs originating from chemical fertilizer. The N and P outputs were 482 and 60 kg ha−1 a−1; the N in crops and P in straw accounted for 23.5% and 45.6% of total outputs, respectively. The N and P surplus intensities were 273 and 47 kg ha−1 a−1, much higher than the respective risk thresholds of 180 kg ha−1 a−1 and 35 kg ha−1 a−1, indicating a risk of N and P potential pollution in aquatic environments via cropland discharge. It is essential to utilize proper fertilization methods and improve fertilizer use efficiency in sloping cropland of purple soil in China.