Effects Of Crop Cover On Soil Erosion On Slope Land

Soil erosion is a severe global environmental problem. Slope land is the main original place of erosion. As the main cover on slope lands, agricultural crops play an important role in soil erosion processes of slope land. Therefore, the thorough research about the influences of crop cover on erosion is important to control erosion economically and effectively, as well as to improve soil erosion theory. This study took corn, soybean, millet, and wheat, which are widely planted on Loess Plateau, as main materials, using methods such as indoor and outdoor simulated rainfall, infiltration measurement in field, and chemical analysis, investigated the effects of crops on rainfall partitioning, splash detachment, rainfall infiltration, runoff and sediment yield, and nutrient losses from slope land. The results showed that:(1)The canopy of crops partitioned rainfall as there main parts: throughfall, stemflow, and interception storage. As corn, soybean, and millet grew, the ratios of throughfall to total rainfall declined from about 94 to 36%, 95 to 73%, and 85 to 55%, respectively; for wheat, the ratio had no obvious trend as growing (from returning green stage to after heading), and fluctuated between about 76 to 82%. The spatial distribution of throughfall under corn, soybean, and millet was uneven, throughfall intensities at some points under canopy were 2 or 3 times higher than initial rainfall; the distribution of throughfall under wheat canopy was relatively more even. As corn, soybean, and millet grew, the ratios of stemflow to total rainfall increased from about 5 to 68%, 2 to 22%, and 7 to 32%, respectively. The concept of stemflow was not suitable for wheat. Rain water moved along wheat plants was defined as intro-row water, which composed about 18 to 24% in total rainfall amount. A stemflow model for corn which had fairly good physical basis was built in this study. The interception storage (water depth on ground occupied by crops) on corn, soybean, millet, and wheat canopies changed from about 0.02 to 0.4 mm, 0.05 to 1 mm, 0.05 to 0.35 mm, and 0.5 to 1.5 mm, respectively. Interception storage on crop canopy was not a main part in rainfall partitioning process.(2)As crops grew, the ratios of average splash detachment rates under corn, soybean, millet, and wheat canopies to detachment rates on bare soil changed between about 40 to 80%, 25 to 60%, 45 to 89%, and 3 to 12%, respectively. The average detachment rate under soybean canopy decreased as canopy grew; for other three crops, the average detachment rates under canopy changed irregularly during growing stages. Under canopies of the three gramineous crops, the reduction of detachment rates compared to detachment rate on bare soil were greater under rainfall of 80 mm/h intensity than which under rainfall of 40 mm/h intensity; such phenomenon was not obvious under soybean canopy. Splash detachment often occurred at several points under crop canopy, and appeared extremely high spatial variation.(3)During one growing season, crops didn't affect infiltration property of soil significantly. But, rainfall infiltration on slope increased significantly as crops grew. In the growing period, corn, soybean, millet, and wheat enhanced infiltration amount of rainfall by 15 to 51%, 8 to 45%, 5 to 58%, and 110 to 120% compared to bare slope, respectively. The promotion effects of crops on rainfall infiltration were largely caused by slowing down rainfall and runoff movement on slope and prolonging the staying time of runoff on slope with a certain scale, not caused by improving infiltration ability of soil.(4)As corn, soybean, and millet grew, compared to bare soil, crop covers reduced runoff yield by about 11 to 42%, 7 to 38%, and 2 to 43%, and reduced sediment yield by about 27 to 76%, 21 to 78%, and 24 to 84%, respectively. In spring, when wheat started to grow again, the runoff and sediment yield from wheat plots were about 11% and 1.15% of that from bare plots. After heading, those ratios reduced to 2.36% and 0.25%, respectively. Even after harvest, the runoff and sediment yield from wheat plots were at a low level as that before harvest. Soil loss ratios (ratio of soil loss under crop cover to that from bare soil slope) were calculated taking the cover and management factor C in USLE as basis. Soil loss ratios under corn, soybean, and millet cover conditions were decreased exponentially, linearly, and logarithmically as leaf area indexes of the three crop canopies increased, respectively.(5)In this study, nutrient concentrations and enrichment ratios of nutrients in sediment from corn plots were not significantly different from that from bare plots. Under soybean and millet cover, nutrient concentrations and enrichment ratios of nutrients in sediment were higher than that from bare plots, the differences were significant statistically. Nutrients concentrations in top soil on slope didn't decline obviously during one growing season of crops. The influences of different cover conditions, rainfall intensities, and slope gradients on nutrient loss from slope were not caused by changing nutrients concentrations in sediment, but caused by changing total sediment yield. Nutrient loss and sediment loss had the same trend of changing.