Soil erosion processes and nutrient loss. II. The effect of surface contact cover and erosion processes on enrichment ratio and nitrogen loss in eroded sediment

Thirty-five erosion experiments, involving four levels of surface contact cover by corn stalks and corn leaves (the latter represented by flat metal sheets) on three slopes, were carried out under simulated rainfall to investigate the effect of fractional surface contact cover and type on the loss and enrichment ratio (ER) of nitrogen in eroded sediment. All experiments were in a tilting flume of the simulated rainfall facility with a sandy clay loam soil. Experiments with rainfall detachment as the only erosion process were conducted on a low slope of 0.1%, to prevent entrainment occurring. The simulated rainfall rate was 100 mm h-1, and sediment samples were collected at the flume exit for up to 40 min. In experiments with entrainment as the only erosion process, clear water was applied as runon at the top of the flume. A stream power of 0 33 W m-2 was used and maintained with entrainment alone and in experiments with rainfall and runon combined for both 3 and 6% slopes. Sediment samples were fractionated through a series of sieves and total nitrogen was analysed for each size range to give the enrichment ratio (ER). The aggregate size or settling velocity characteristics, enrichment ratio (ER), and total nitrogen loss of the eroded sediment varied considerably with slope and cover types for the different erosion experiments. As cover by corn stalks increased, the settling velocity characteristics of eroded sediment became finer; the degree of this fineness was greater than when simulated leaves provided the same cover. For the rainfall detachment alone experiments, values of ER were greater than unity for both cover types and slopes, and greater than values for all other experiments. For the combined rainfall and runon experiments, ER was higher for corn stalks than simulated leaves. For experiments with entrainment alone, values of ER were close to unity for both cover types and slope, even by the early sampling time of 0.6 min. It may be concluded that the effectiveness of cover in reducing nutrient loss lies in reducing sediment loss, not in reducing ER. When rainfall detachment and entrainment were applied together, sediment concentration and total nitrogen loss were substantially increased over the sum of the contribution of rainfall detachment and entrainment acting alone. This finding indicates synergism in nutrient loss between these two erosion processes.