Isotope-Based Study on Nitrate Sources in a Karst Wetland Water, Southwest China

Abstract

Huixian karst wetland is the largest karst wetland in the subtropical low-altitude areas in China, which is known as the kidney of Lijiang River. This study was focused on researching the NO3− source and N cycling in Huixian karst wetland using multi-isotope, hydro-chemical data, and stable isotope analysis in R model, including 7 groundwater samples, 6 fish pond water samples, and 17 surface river water samples, respectively. Hydro-chemical data showed that the maximum concentrations of NO3− (49.3 mg·L−1) and NH4+ (0.6 mg·L−1) in the groundwater exceeded the limit of Groundwater Quality Standard Class III Standard by 2.5 and 1.2 times, respectively; the maximum concentration of NH4+ (8.2 mg·L−1) in fish pond water exceeded the limit of the Groundwater Quality Standard Class III Standard by 16.3 times; the maximum concentrations of NO3− (24.5 mg·L−1), NO2− (7.3 mg·L−1), and NH4+ (2.4 mg·L−1) in surface river water were 1.2, 7.3, and 17.3 times higher than the limit of the limit of Groundwater Quality Standard Class III Standard, respectively. All water samples were in an aerobic environment with the range of dissolved oxygen (2.6–14.8 mg·L−1). Therefore, nitrification mainly occurred in the process of nitrate transformation, significant denitrification did not. Multi-isotope (δ15N-NO3, δ18O-NO3, and 13CDIC) and stable isotope analysis in R model revealed that manure sewage, ammonium nitrogen fertilizer, and soil nitrogen were the three dominant NO3− sources, contributing to (39.8 ± 6.4)%, (33.4 ± 5.1)%, and (26.8 ± 3.9)%, respectively.