18-year grass hedge effect on soil water loss and soil productivity on sloping cropland

Grass hedges planted along the contour of slope cropland can greatly reduce water and soil loss, but the effect of long-term grass hedge cultivation on soil productivity is unclear. The objective of this study was to evaluate the temporal and spatial characteristics of soil thickness, soil fertility and grain yield on the slope croplands with long-term grass hedge cultivation. Four grass hedges, vetiver (Vetiveria ziaaniodes), false indigo (Amsopha fruticose), alfalfa (Medicago sativa) and Chinese alpine rush (Eulaliopisi binata), were grown for 18 years on slope croplands in the purple-soil area of Sichuan province, China. Maize was planted in the alley between the grass hedges. The results showed that the plots with grass hedges decreased runoff by 50.1%-60.7% and soil loss by 82%-91.6% compared with the control plots without grass hedges. The vetiver hedge was the most effective in reducing soil loss among the four grass hedges. The soil thickness increased 7-31 cm in the upslope position above the grass hedges and decreased 7.5-34.5 cm in the downslope position below the grass hedges compared to those at the beginning of the experiment 18 years ago. A series of less slope gradient terraces were gradually built after grass hedge planted. The gradient decrease rate was higher in the first 9 years, and the rate became lower in the next 9 years after grass hedge planted. The soil on and above the grass hedges had higher organic matter content (SOM) than the soil below the grass hedges. The maize yield had a positive correlation with SOM and soil thickness. The maize yields in the grass hedge treatments were 0%-11.3% less than that in the control. Water, nutrient and sunlight competition was the main impact factor negatively affecting the maize yield, and grass hedges retained soil thickness benefits for increasing maize yields. In addition, the grass hedges occupied space otherwise used for crop cultivation, so the plot maize yields in the grass hedge treatments were 12.7%-22.8% less than that in the control. To ensure crop production and environmental quality, it was recommended that grass hedges should be applied on slope cropland with serious soil erosion. In addition, grass species with higher soil and water conservation efficiency, smaller canopy, lower demand for water and nutrients should be selected, such as vetiver grass hedge.