Effects of alley crop planting on soil and nutrient losses in the citrus orchards of the Three Gorges Region

Soil erosion is recognized as one of the major environmental problems in the Three Gorges Region (TGR). We carried out a six years’ study on the effects of different alley crops on runoff, losses of soil and nutrients, and the position related variation of soil nutrients along slope. Two experimental sites were selected with slope of 15° and 32°, respectively. Four treatments were applied at each site: Control (no alley crop); PN (Paspalum notatum as contour alley crop); HC (Hemerocallis citrina as contour alley crop); AH (Arachis hypogaea as contour alley crop). Results indicated that nutrient loss in runoff in the Control treatment was serious, especially the loss of nitrate (site A: NO3-N, 2.32 t ha−1 year−1, NH4-N, 0.79 t ha−1 year−1; site B: NO3-N, 7.41 t ha−1 year−1, NH4-N, 6.97 t ha−1 year−1) and phosphate (site A: 0.10 t ha−1 year−1; site B: 4.69 t ha−1 year−1). However, the nutrient losses by runoff in the other three treatments with alley crops showed significant reduction, especially the treatment of PN (site A: NO3-N, 0.36 t ha−1 year−1, NH4-N, 0.26 t ha−1 year−1, PO4-P, 0.026 t ha−1 year−1; site B: NO3-N, 1.29 t ha−1 year−1, NH4-N, 0.97 t ha−1 year−1, PO4-P, 0.18 t ha−1 year−1). Results of correlation and regression analysis indicated that there were significant relationships between runoff and soil loss in all treatments, which can be described by power function. Alley crop planting greatly reduced the position related variation of soil nutrient status along the slopes compared with Control treatment, especially the PN treatment.