Effects of shallow groundwater table fluctuations on nitrogen in the groundwater and soil profile in the nearshore vegetable fields of Erhai Lake, southwest China

Abstract

Erhai Lake is the second-largest freshwater lake on the Yunnan–Guizhou Plateau in the southwest China. Water quality protection for the lake is necessary because it is a potable water supply for more than one million people. However, nitrogen exchanges between the soil profile (SP) and shallow groundwater (SW) accelerate nitrogen loss from the soil into groundwater in the nearshore vegetable fields of Erhai Lake due to excessive nitrogen accumulation in the SP and shallow and variable groundwater tables. More nitrogen enrichment in the groundwater increases the nitrogen input to lake water from groundwater migration and threatens the water safety of the water in Erhai Lake. However, there is a lack of information on the effect of fluctuations in the SW table on nitrogen concentrations in the SW and SP in this region. Field monitoring of plots at different elevations was conducted to monitor the SW table and nitrogen concentrations in the SW and SP from June 2015 to May 2016. With increasing SW depth, TN and NO3−–N concentrations in the SW decreased exponentially, and the NH4+–N concentration increased. The NO3−–N, DTN, and NH4+–N concentrations in the SP were positively linearly correlated with SW depth. With an increase in the NO3−–N, DTN, and NH4+–N concentrations in the soil of the groundwater level fluctuation zone, NO3−–N, TN, and NH4+–N concentrations in the SW showed exponential increases. Multiple factors, including nitrogen application rates, properties of the SP, rainfall, and topography, affected the SW table and the nitrogen distribution in the SP. Then, TN, NO3−–N, and NH4+–N concentrations in the SW presented temporal-spatial variations via nitrogen exchange between the SP and SW. The SW table fluctuations significantly affected the nitrogen concentrations in the SW and SP. These results provide a scientific basis for regulating seasonal SW tables and preventing nitrogen loss from the SP.