Nitrogen Export by Runoff from Agricultural Plots in Two Basins in China

Runoff and sediment yields from agricultural fields are major sources of nitrogen (N) entering lakes in China. Export of sediment and N can be impacted by soil and cropping management practices, but there is relatively little information on N leaving agricultural fields in lake basins in China. Sediment and surface runoff N from a series of field plots in two experimental lake basins were evaluated in situ under simulated rainfall conditions. Objectives of the study were to evaluate the effects of crop cover, slope, and fertilizer application on N in surface runoff and eroded soils. Sediment yields varied from 4.3 to 299.0 g m -2 , depending on management practice. Mean dissolved nitrogen (DN) and total nitrogen (TN) concentrations are 1.35 and 5.4 mg L -1 , respectively, in Lake Taihu basin, while mean DN and TN concentrations are 2.66 and 4.3 mg L -1 , respectively, in Lake Baiyangdian basin. For all experimental plots in two basins, weighted average concentrations of N for total-N, dissolved N and sediment N are 1.0-5.0 mg L -1 , much higher than 0.2 mg L -1 , indicating a problem in lake eutrophication due to high N concentration from agricultural surface runoff. The estimated mean annual export of total N was 6.0 and 14.7 kg ha -1 yr -1 for Baiyangdian and Taihu lake basins, respectively. The study showed that significantly more N (approximately ranging from 10 to 90 of total N) exported was associated with sediment, constituting a long-term source of potentially bioavailable N in lakes.