Runoff generation and soil detachment in a small sugarcane watershed of Southeast Brazil

Abstract:

Brazil is the world largest sugarcane producer, with a cropped area of approximately 10 million hectares. Soil erosion and its implications are some of the neglected environmental problems in sugarcane fields. In this study a plot of 6 hectares cropped with sugarcane, encompassing a 30 meters of riparian forest bordering a stream was chosen. Two pluviometers were installed in an open area in order to measure the amount and intensity of rain during the rainy season. Surface runoff generation and soil detachment were estimated by plots installed in cultivated and riparian areas. Bathymetry was carried out in the stream channel to estimate the soil mass settled on the stream bed. The sources of organic matter was investigated by carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopic compositions. In the earlier stages of the crop, the rain intensity strongly influenced runoff generation and soil detachment; that were minimized with the plant growth. In riparian areas, runoff and soil detachment occurred only during intense events. The total rainfall was approximately 1300 mm, in one slope of the sugarcane field, approximately 5% of this rainfall drained from the watershed as surface runoff; in the other slope this proportion increased to approximately 11%, and in the riparian forest decreased to less than 2%. The soil detachment was approximately 1,000 g.m^-2 in one sugarcane slope, and increased to approximately 4,800 g m^-2 in the other slope, decreasing in the riparian forest to only 50 g m^-2. The difference in runoff and soil detachment observed between the two sugarcane slopes was due to the sugarcane growth stage and soil properties. Using the bathymetry conducted in the stream channel, we estimated that 3.6 Mg of sediment per hectare were settled on the stream bed during the rainy season (0.6 Mg.ha^-1 per month). The averages δ¹³C and δ¹⁵N of soil from forest areas were -27.6 ± 0.2 ‰ and 4.3 ± 0.4 ‰, respectively. In the sugarcane areas, the averages δ¹³C and the δ¹⁵N increased to -20.4 ± 0.2 ‰ and 6.8 ± 0.1 ‰, respectively. In the stream bed, the average δ¹³C was equal to -23.2 ± 0.2 ‰ and the average δ¹⁵N equal to 5.9 ± 0.1 ‰, indicating that at least 60% of this material was generated in the sugarcane areas.