Naturally rainfall-induced changes in runoff-associated nitrogen and phosphorus losses in purple soil area: roles of land disturbance and plot length

Severe soil erosion occurs in southwestern China owing to the large expanses of human disturbance and sloping land. This field monitoring study was conducted during the rainy season to record the rainfall events, runoff, sediment yield, nitrogen, and phosphorous loss in 20-, 40-, and 60-m plots under conditions of artificial disturbance or natural restoration on a 15° slope in the purple soil area of southwestern China. The concentrations and loss amounts of total nitrogen (TN), total dissolved nitrogen (TDN), ammonium-nitrogen (NH4-N) and nitrate-nitrogen (NO₃-N), total phosphorus (TP), total dissolved phosphorus (TDP) and orthophosphate (PO₄-P) were comparatively determined. The highest N concentration was observed in long duration and soft rainfall events across all plots. The highest P concentration in artificial disturbed plots was found in long duration and intensive rainfall events while it was recordeds for measured variables were dominantly recorded under the long duration and lowest soft rainfall events in naturally restored plots intensity., while The the highest loss amounts for N and P in different forms for these variablesalmostmostly appeared under high rainfall intensity. Land disturbances differed orthophosphate PO₄-P concentration in 20--m plot and and loss amounts of of measured variables N and P with different forms across in all plots. Plot lengths differed total dissolved phosphorus TDP concentration in natural restored plot and loss amounts of total dissolved nitrogenTDN and orthophosphate PO₄-P in artificially disturbed plots. Naturally restoration reduced loss amounts of total nitrogen and total phosphorus by 69.4%62.14-79.05% and 79.28-83.43% TN, 68.8% TDN, 71.2% NH₄-N, 74.3% NO₃-N, 81.5% TP, 71.9% TDP and 70.0% PO₄-P loss amounts comparedrelative to artificial disturbance, respectively. There were significant interrelationships among N and P concentrations in different forms in two land disturbance plots, while nitrate-NO₃-nitrogenN concentration hadwas significantly negatively negative correlatedion with rainfall intensity and runoff rate in artificialally disturbanceed plots. Rainfall intensity was logarithmically correlated with TN, NO₃-N concentrations in artificially disturbed plots and with NO₃-N concentration in naturally restored plots. Runoff rate was logarithmically correlated with TN, TDN and NO₃-N concentrations in artificially disturbed plots. Our results highlight the effects of land disturbance and plot length on nutrient losses in sloping land.