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.