The influence of grazing and cultivation on runoff, soil erosion, and soil nutrient export in the central highlands of Ethiopia

Background: Grazing by livestock and cultivation have been considered as two important causes of soil erosion and nutrient export. However, there has been limited evidence that grazing or cultivation matters to soil erosion and nutrient export in Ethiopia. Hence, this study was conducted in the Galesa watershed in Ethiopia to examine the effects of grazing and cultivation on runoff, soil loss, and nutrient export. Daily values of runoff, soil erosion, and nutrient outflow were measured for three consecutive years following standard procedures. Independent t test was performed to check if the means of runoff, soil loss, and nutrient loss from grazing and cultivated lands were significantly different. Moreover, repeated analysis of variance (ANOVA) was used to test if mean values of runoff, soil loss, and nutrient export varied significantly over the study years. Results: Although the average annual runoff depth was 7.8% higher in grazing land (GL), soil erosion was significantly lower (39%) in GL as compared to cultivated land (CL). Similarly, sediment and runoff-associated annual losses of total nitrogen (N), available phosphorus (P), exchangeable potassium (K), and organic carbon (OC) were low in the GL treatments. Lowest losses of total N (9.30 kg ha-1 year-1 ), available P (0.83 kg ha-1 year-1 ), and exchangeable K (1.84 kg ha-1 year-1 ) were recorded in GL treatment. Likewise, lowest losses of sediment-associated total N (32.8 kg ha-1 year-1 ), available P (0.39 kg ha-1 year-1 ), exchangeable K (0.23 kg ha-1 year-1 ), and soil organic carbon (630 kg ha-1 year-1 ) were recorded from GL over the 3 years of experimentation. Conclusion: Our results indicate that cultivation increased soil erosion as compared to grazing. Although there were significant reductions in soil erosion and nutrient export from grazing lands compared with cultivated lands, the absolute losses were still high. This implies the need for grazing land management using appropriate physical and biological erosion control measures to increase productivity and reduce soil erosion as well as nutrient export.