Runoff and soil and nutrient losses from gravel mulching: A field experiment with natural rainfall on the Loess Plateau of China

Gravel mulch has been used effectively to conserve agricultural water for three centuries in the semiarid Loess Plateau of northwestern China. In this study, gravel mulch field experiments on the slopes of the plateau were carried out to evaluate the effects of different particle sizes of gravel mulch and different slope gradients on surface runoff, soil erosion, and nutrient losses under natural rainfall throughout 2017 and 2018. Experiment plots (3 m × 5 m) on slopes of 5°, 10°, 15°, and 30° were implemented; in each plot, three gravel mulch treatments were investigated: fine gravel (2 to 5 mm in size, FG), coarse gravel (40 to 60 mm, CG), and the control that lacked any mulching (CK). We found that gravel mulching on soil significantly reduced both the amounts of runoff and soil loss under natural rainfall conditions in 2017 and 2018, for which the soil loss-suppression effect was most pronounced under FG mulching. Applying FG also significantly increased the rainfall threshold to generate runoff, and caused the fewest runoff events during the two years of monitoring. Coarse gravel mulching caused a strong reduction in soil loss on the relatively flat slopes (i.e., 5° and 10°) whereas its effect was weak on relatively steep slopes (i.e., 15° and 30°). The respective effects of FG and CG mulching on the losses of various nutrients from soil also differed markedly. The gravel mulching significantly increased the amounts of total nitrogen (TN), potassium (K), and nitrate-nitrogen (NO3-N) in the runoff but greatly decreased those of iron (Fe), copper (Cu), and zinc (Zn). Furthermore, the amount of nutrients lost in runoff increased with a steeper slope, with notably greater variability of nutrient elements in the runoff on the steepest slopes. Accordingly, FG mulching is recommended for use in arid and semiarid areas from the perspective of water, soil, and nutrient conservation.