Effects of Farming Practices on Soil Nitrogen and Phosphorus Concentrations and Its Loss in the Drawdown Area of the Tributary Embayment of the Three Gorges Reservoir

As the Three Gorges Reservoir (TGR) periodically operates at low water levels, its drawdown area has been utilized for cultivation by local farmers due to the overlap of the non-inundated period and the crop-growth period. However, traditional agricultural planting may affect the aquatic environment of the TGR area. To explain the effects of agricultural farming and abandoned farming on the water environment, a study was conducted in the drawdown area in an embayment of the Pengxi River (a tributary of the TGR). Corn, potato, and peanut fields were investigated for nitrogen and phosphorus content in surface soil, during the farming period (March to September 2018) and the conversion period (March to September 2019). Nitrogen and phosphorus balance models were constructed for farmland and abandoned farmland, to compare and analyze the budgets and loss risk of nitrogen and phosphorus from soil in the drawdown area. The results showed that the ammonia nitrogen (NH4+-N), total phosphorus (TP), and inorganic phosphorus (IP) content of soil in the corn field varied significantly across different planting periods. The concentrations of ammonium nitrogen and nitrate nitrogen (NO3--N) were significantly higher in farmland soil than in abandoned farmland soil, and the concentrations of total phosphorus (TP), inorganic phosphorus (IP), and calcium-bound phosphorus (Ca-P) were significantly lower in farmland soil than in abandoned farmland soil. The different soils were ranked according to the intensity of nitrogen and phosphorus surplus as follows:corn field>potato field>peanut field. The apparent surplus values in the different farmland soils were 76.89 kg ·hm-2(corn field), 51.92 kg ·hm-2(potato field), and 43.74 kg ·hm-2(peanut field) for nitrogen, and 79.69 kg ·hm-2(corn field), 75.76 kg ·hm-2(potato field), and 17.78 kg ·hm-2(peanut field) for phosphorous. Overall, the surplus intensities of nitrogen and phosphorus in all three croplands were higher than the respective risk thresholds, indicating potential nitrogen and phosphorus pollution in the three farmland types. Agricultural farming in the drawdown area may therefore increase the risk of nitrogen and phosphorus loss and is not conducive to the protection of the aquatic environment.