Fig. 6 Relationship between (a) δ15N-NO3- and Ln[N-NO3-], and between (b) δ15N-NO3- and δ18O-NO3- , and between (c) DO and pH in Huashan watershed.

3.4 Quantitative Analysis of Nitrate Sources

The SIAR mixing model was applied here to quantify the contribution of AD, NF, MS, and SN to the nitrate content in water (Fig 7a~d). The mean and variance of the nitrogen and oxygen isotopes of nitrate from four potential sources inputted into the isotope mixing model are shown in Table 1. The above qualitative analysis results show that denitrification does not exist in the surface water, and denitrification only existed in groundwater samples which located at the outlet of the watershed, hence, the isotope fractionation is not considered in the SIAR mixing model.
Source identification by SIAR mixing model indicated that the M&S was the main contributor of nitrate to surface water (mean 38.1%), followed by SN (29.6%), NF (20.4%) and AD (11.9%) in November 2021. Nitrate in groundwater mainly comes from NF and SN, followed by M&S, and the contribution of AD is also minimal; the contribution rates were NF (39.8%) > SN (32.5%) > M&S (25.1%) > AD (15.7%). For April 2022, nitrate in surface water mainly comes from NF and SN, followed by M&S, and AD nitrogen is the least; the contributions are NF (38.3%) > SN (29.2%) > M&S (20.8%) > AD (11.7%). The nitrate in groundwater mainly comes from M&S, SN, followed by NF, and the least AD; the contribution rates are M&S (29.4%) > SN (29.8%) > NF (25.1%) > AD (15.7%). According to the Bayesian isotope mixing model, SN contributed the second highest proportion to nitrate in the study area, accounting for an average of 30.4%, with a slightly higher contribution in groundwater than surface water. AD had the smallest contribution rate, averaging 12%, indicating that atmospheric deposition of nitrogen is not the main source of nitrate in the Huashan watershed. In April 2022, there was a significant increase in precipitation in the month before sampling, resulting in a greater contribution of chemical nitrogen fertilizer to nitrate in surface water. In addition, farmers in the Huashan watershed raise poultry, including chickens, ducks, and geese. The excrement from these animals can directly infiltrate into groundwater, leading to a significant impact of MS on groundwater.