Short-Term Irrigation Level Effects on Residual Nitrate in Soil Profile and N Balance from Long-Term Manure and Fertilizer Applications in the Arid Areas of Northwest China

A field experiment was conducted to determine the effects of long-term applications of fertilizers and manure (1982 to 2003) and short-term irrigation level (2002 and 2003) on accumulation of nitrate nitrogen (NO3-N) in soil at Zhangye Oasis, China. The treatments included manure (M) and no manure (M0) as main plots; check (Ck), nitrogen (N), nitrogen + phosphorus (NP), and nitrogen + phosphorus + potassium (NPK) as subplots; and two amounts of irrigation (I1 and I2) as subsubplots. The application of N alone resulted in large NO3-N accumulation in soil, accounting for 6% of the applied N, and the lowest crop N recovery. Application of manure resulted in increased NO3-N in the soil profile compared to treatment with no manure, and the MN treatment resulted in the greatest amount of NO3-N in soil. Nitrogen applied with P and/or K reduced the amount of NO3-N in soil in both manure and no-manure treatments compared with N only. The unaccounted N was greatest (60%) in the N-alone treatment and lowest (30%) in the NPK treatment. When manure plus fertilizer were applied together, the unaccounted N ranged from 35%-42%. Based on results from only 2 years, greater amounts of irrigation (I2) caused greater leaching of NO3-N in the soil profile compared with I1, especially in treatments receiving manure. The implications of these findings are that these high amounts of accumulated NO3-N in surface and subsoil layers can be a potential threat to surface water, underground water, and air quality in the long run. This accumulated N in the soil profile can be used as a source of available N for future crops and should be recycled by using proper crop, soil, fertilizer, and water-management strategies/practices. The findings also suggest the need for further research to make an effective and efficient use of this accumulated NO3-N in the soil profile in order to save cost of N fertilizer application to future crops.