Groundwater Contamination with NO 3-N in a Wheat-Corn Cropping System in the North China Plain

Abstract

The North China Plain, where summer corn ( Zea mays L.) and winter wheat ( Triticum aestivum L.) are the major crops grown, is a major agricultural area in China. Permeable soils make the region susceptible to groundwater pollution by NO 3-N, which is applied to fields in large amounts of more than 400 kg NO 3-N ha −1 as fertilizer. A field experiment was established in 2002 to examine the relationship among N fertilization rate, soil NO 3-N, and NO 3-N groundwater contamination. Two adjacent fields were fertilized with local farmers' N fertilization rate (LN) and double the normal application rate (HN), respectively, and managed under otherwise identical conditions. The fields were under a traditional summer corn/winter wheat rotation. Over a 22-month period, we monitored NO 3-N concentrations in both bulk soil and soil pore water in 20{40 cm increments up to 180 cm depth. We also monitored NO 3-N concentrations in groundwater and the depth of the groundwater table. No significant differences in soil NO 3-N were observed between the LN and HN treatment. We identified NO 3-N plumes moving downward through the soil profile. The HN treatment resulted in significantly higher groundwater NO 3-N, relative to the LN treatment, with groundwater NO 3-N consistently exceeding the maximum safe level of 10 mg L −1, but groundwater NO 3-N above the maximum safe level was also observed in the LN treatment after heavy rain. Heavy rain in June, July, and August 2003 caused increased NO 3-N leaching through the soil and elevated NO 3-N concentrations in the groundwater. Concurrent rise of the groundwater table into NO 3-N- rich soil layers also contributed to the increased NO 3-N concentrations in the groundwater. Our results indicate that under conditions of average rainfall, soil NO 3-N was accumulated in the soil profile. The subsequent significantly higher-than-average rainfalls continuously flushed the soil NO 3-N into deeper layers and raised the groundwater table, which caused continuous groundwater contamination with NO 3-N. The results suggest that under common farming practices in the North China Plain, groundwater contamination with NO 3-N was likely, especially during heavy rainfalls, and the degree of groundwater contamination appeared to be proportional to the N application rates. Decreasing fertilization rates, splitting fertilizer inputs, and optimizing irrigation scheduling had potential to reduce groundwater NO 3-N contamination.