Influence of grass soil cover on water runoff and soil detachment under rainfall simulation in a sub-humid South African degraded rangeland

摘要

In most regions of the world overgrazing plays a major role in land degradation and thus creates a major threat to natural ecosystems. Several feedbacks exist between overgrazing, vegetation, soil infiltration by water and soil erosion that need to be better understood. In this study of a sub-humid overgrazed rangeland in South Africa, the main objective was to evaluate the impact of grass cover on soil infiltration by water and soil detachment. Artificial rains of 30 and 60mmh-1 were applied for 30min on 1m2 micro-plots showing similar sandy-loam Acrisols with different proportions of soil surface coverage by grass (Class A: 75-100%; B: 75-50%; C: 50-25%; D: 25-5%; E: 5-0% with an outcropping A horizon; F: 0% with an outcropping B horizon) to evaluate pre-runoff rainfall (Pr), steady state water infiltration (I), sediment concentration (SC) and soil losses (SL). Whatever the class of vegetal cover and the rainfall intensity, with the exception of two plots probably affected by biological activity, I decreased regularly to a steady rate <2mmh-1 after 15min rain. There was no significant correlation between I and Pr with vegetal cover. The average SC computed from the two rains increased from 0·16gL-1 (class A) to 48·5gL-1 (class F) while SL was varied between 4gm-2h-1 for A and 1883gm-2h-1 for F. SL increased significantly with decreasing vegetal cover with an exponential increase while the removal of the A horizon increased SC and SL by a factor of 4. The results support the belief that soil vegetation cover and overgrazing plays a major role in soil infiltration by water but also suggest that the interrill erosion process is self-increasing. Abandoned cultivated lands and animal preferred pathways are more vulnerable to erosive processes than simply overgrazed rangelands.