Soil erosion and losses of nitrogen and phosphorus in the Loess Plateau, Wangjiagou, Lishi, People's Republic of China

This study investigates the linkages between the processes of sediment transfer and the movement of nitrogen and phosphorus in the hilly loess region of China's Loess Piateau. Field work was carried out in a representative first-order drainage basin, the Yangdaogou, during 1987 and 1988. Soil loss in the Yandaogou equalled 203.4 Mg ha-1 in 1987 and 176.9 Mg ha-1 in 1988. The mean sediment concentration of the runoff was 505 kg m-3 and concentrations were observed to reach 1000 kg m-3. The principal sediment transfer process is hyperconcentrated overland flow that was observed to carry very large amounts of suspended sediment throughout the storm hydrograph. The majority of plotted discharge-concentration relationships showed counter-clockwise hysteresis where concentrations in the recession limb exceed the rising limb. Reductions in flow velocity resulted in only limited sediment deposition and virtually all entrained sediment is exported from the basin. Calculated Reynold's Numbers indicate that the hyperconcentrated flow is laminar. Sheet and rill erosion appears to be the most important sediment source followed by channel erosion, tunnel erosion, and the remobilization of sediments displaced by mass movement.;Total phosphorus losses were 73.7 kg ha-1 in 1988 with 99% of the load in particulate form. This rate is among the highest reported and is a function of the high rate of soil loss rather than of high P concentrations in the eroded sediment. Losses and concentrations of dissolved orthophosphate are comparable to those found elsewhere. However the available P content of the loess soils is low and any losses of soluble P should be considered serious.;In general, despite a rather significant influx of sediment from other processes, mass movement, channel erosion and tunnel erosion, it appears that the contribution of surface erosion to the sediment yield is large and the associated loss of nutrients is serious. Hyperconcentrated flow maintains eroded organic matter and sediment associated nutrients in suspension and the material is readily exported from basins. Conservation strategies that inhibit the initiation of hyperconcentrated flow on the upper slopes are thus critical for conserving both soil and nutrients.;Nitrogen losses from the Yangdaogou in runoff were 76.4 kg ha-1 in 1987 and 44.1 kg ha-1 in 1988. These data are among the highest reported in the literature. Each year more than 98% of the total load was in the sediment-associated form. The presence of hyperconcentrated flow is significant as virtually all suspended solids are efficiently exported from the basin. Within-basin transfers of N appear to be slight. Nitrogen balance information suggests that the biological fixation of atmospheric N by the shrub Caragana korshinskii is a major N source to the non-cultivated part of the basin.