Protection Methods to Reduce Nitrogen and Phosphorus Losses from Sloping Citrus Land in the Three Gorges Area of China

The increased application of chemical fertilizers in citrus orchards and the non-point source nitrogen (N) and phosphorus (P) diffusion threaten the fresh water quality of the Three Gorges Reservoir area of China. A 10-year field trial on sloping citrus lands was conducted to evaluate five protection methods for reducing N and P losses: 1) intercropping with perennial white clover (CW); 2) straw mulching of the soil surface (CS); 3) intercropping with yellow daylily contour hedgerows (CH); 4) an impermeable membrane buried in the soil along the contour lines (CM); and 5) intercropping with a rotation of wheat and peanut (CWP). An area of conventional citrus management was also maintained as the control (CK). The results showed that CM and CH were the most effective methods for reducing surface runoff. The sediment yield were reduced at the highest rate by CW and CH and was also significantly reduced by CS. Reduced runoff volume and sediment yield were the crucial mechanism for the reductions in N and P losses. Compared with the control, CW, CS, CH, and CM reduced annual runoff by 9%, 13%, 25%, and 30%, sediment yield by 77%, 55%, 71%, and 28%, N loss by 10%, 23% 36%, and 37%, and P loss by 39%, 31%, 27%, and 25%, respectively. CW, CS, CH, and CM were effective in reducing N and P losses from the sloping citrus land. However, over the long-term, surface soil nutrient accumulation in CW, CS, and CH diminished the benefit of those methods in reducing N and P losses. In addition, CWP increased soil erosion and nutrient loss, which showed that citrus intercropping with other crops was an unsuitable method for citrus sloping land in the Three Gorges area.