Identifying diffused nitrate sources in a stream in an agricultural field using a dual isotopic approach

Nitrate (NO3−) pollution is a severe problem in aquatic systems in Taihu Lake Basin in China. A dual isotope approach (δ15NNO3− and δ18ONO3−) was applied to identify diffused NO3− inputs in a stream in an agricultural field at the basin in 2013. The site-specific isotopic characteristics of five NO3− sources (atmospheric deposition, AD; NO3− derived from soil organic matter nitrification, NS; NO3− derived from chemical fertilizer nitrification, NF; groundwater, GW; and manure and sewage, M&S) were identified. NO3− concentrations in the stream during the rainy season [mean ± standard deviation (SD) = 2.5 ± 0.4 mg/L] were lower than those during the dry season (mean ± SD = 4.0 ± 0.5 mg/L), whereas the δ18ONO3− values during the rainy season (mean ± SD = + 12.3 ± 3.6‰) were higher than those during the dry season (mean ± SD = + 0.9 ± 1.9‰). Both chemical and isotopic characteristics indicated that mixing with atmospheric NO3− resulted in the high δ18O values during the rainy season, whereas NS and M&S were the dominant NO3− sources during the dry season. A Bayesian model was used to determine the contribution of each NO3− source to total stream NO3−. Results showed that reduced N nitrification in soil zones (including soil organic matter and fertilizer) was the main NO3− source throughout the year. M&S contributed more NO3− during the dry season (22.4%) than during the rainy season (17.8%). AD generated substantial amounts of NO3− in May (18.4%), June (29.8%), and July (24.5%). With the assessment of temporal variation of diffused NO3− sources in agricultural field, improved agricultural management practices can be implemented to protect the water resource and avoid further water quality deterioration in Taihu Lake Basin.