Excess nitrate (NO3
–) loading in terrestrial
and aquatic ecosystems can result in critical environmental and health
issues. NO3
–-rich groundwater has been
recorded in the Guanzhong Plain in the Yellow River Basin of China
for over 1000 years. To assess the sources and fate of NO3
– in the vadose zone and groundwater, numerous
samples were collected via borehole drilling and field surveys, followed
by analysis and stable NO3
– isotope quantification.
The results demonstrated that the NO3
– concentration in 38% of the groundwater samples exceeded the limit
set by the World Health Organization. The total NO3
– stock in the 0–10 m soil profile of the orchards
was 3.7 times higher than that of the croplands, suggesting that the
cropland-to-orchard transition aggravated NO3
– accumulation in the deep vadose zone. Based on a Bayesian mixing
model applied to stable NO3
– isotopes
(δ15N and δ18O), NO3
– accumulation in the vadose zone was predominantly
from manure and sewage N (MN, 27–54%), soil N (SN, 0–64%),
and chemical N fertilizer (FN, 4–46%). MN was, by far, the
greatest contributor to groundwater NO3
– (58–82%). The results also indicated that groundwater NO3
– was mainly associated with the soil and
hydrogeochemical characteristics, whereas no relationship with modern
agricultural activities was observed, likely due to the time delay
in the thick vadose zone. The estimated residence time of NO3
– in the vadose zone varied from decades to centuries;
however, NO3
– might reach the aquifer
in the near future in areas with recent FN loading, especially those
under cropland-to-orchard transition or where the vadose zone is relatively
thin. This study suggests that future agricultural land-use transitions
from croplands to orchards should be promoted with caution in areas
with shallow vadose zones and coarse soil texture.
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