Effects of rainfall and runoff on the groundwater quality in farmland and poplar forestland in the area of Hung-tse Lake

The loss of nitrogen and phosphorus in agriculture has become an important cause of groundwater pollution. To explore and compare the losses of nitrogen and phosphorus from runoff in wheat fields, rice fields, and poplar forestland, the surface nutrient losses and groundwater quality were monitored in the area of Hung-tse Lake for one year, by setting up monitoring plots and monitoring wells on farmland and poplar forestland. The main results were as follows:1) the surface soil water content before and after precipitation in the forestland was lower than that in the wheat field. The average soil water content in wheat fields was 8.95% higher than that before rain, whereas it was 4.05% higher than that before rain in forestland. 2) The total loss of nitrate nitrogen, ammonium nitrogen, and available phosphorus in the surface soil of the wheat field were 63.53 mg·kg^-1, 5.61 mg·kg^-1, and 57.43 mg·kg^-1; while those in the poplar forestland were 16.78 mg·kg^-1, 2.45 mg·kg^-1, and 0.73 mg·kg^-1, respectively. The total loss of nitrate nitrogen, ammonium nitrogen, soluble phosphorus, and particulate phosphorus in the surface water of the rice field was 8.32 mg·L^-1, 27.44 mg·L^-1, 2.39 mg·L^-1, and 2.99 mg·L^-1, and the total loss of nitrogen and phosphorus in the poplar forestland was significantly lower than that in the farmland during the monitoring period. 3) There was a close relationship between the soil nutrient loss and rainfall in farmland. The soil nutrient loss in farmland increased logarithmically with an increase in rainfall. However, the loss of surface nutrients in the poplar forestland was almost unaffected by rainfall. 4) The theoretical minimum rainfall generating runoff in farmlands (wheat field:3.3 mm; rice field:4.2 mm) was much lower than that of the poplar forestland (22.8 mm). The concentration of ammonium nitrogen, orthophosphate, total nitrogen, nitrate nitrogen, total phosphorus, and soluble phosphorus were significantly correlated with rainfall in wheat field. 5) There was a significant correlation between the nutrient concentrations in farmland runoff and nitrogen and phosphorus contents in groundwater (P < 0.05). The nitrogen and phosphorus contents in the groundwater of poplar forestland, which had no significant correlations with the nutrient concentrations in the runoff, remained at a relatively stable level. Compared with farmlands, forestland can better control nutrient losses caused by runoff, alleviate groundwater pollution, and contribute to the control of agricultural non-point source pollution.