Coupled processes involving ammonium inputs, microbial nitrification, and calcite dissolution control riverine...

Rivers in agricultural countries widely suffer from diffuse nitrate (NO₃^-) pollution. Although pollution sources and fates of riverine NO₃^- have been reported worldwide, the driving mechanisms of riverine NO₃^- pollution associated with mineral dissolution in piedmont zones remain unclear. This study combined hydrogeochemical compositions, stable isotopes (δ¹⁸O-NO₃^-, δ¹⁵N-NO₃^-, δ¹⁸O-H₂O, and δ²H-H₂O), and molecular bioinformation to determine the pollution sources, biogeochemical evolution, and natural attenuation of riverine NO₃^- in a typical piedmont zone (Qingshui River). High NO₃^- concentration (37.5 ± 9.44 mg/L) was mainly observed in the agricultural reaches of the river, with ~15.38 % of the samples exceeding the acceptable limit for drinking purpose (44 mg/L as NO₃^-) set by the World Health Organization. Ammonium inputs, microbial nitrification, and HNO₃-induced calcite dissolution were the dominant driving factors that control riverine NO₃^- contamination in the piedmont zone. Approximately 99.4 % of riverine NO₃^- contents were derived from NH₄⁺-containing pollutants, consisted of manure & domestic sewage (74.0 % ± 13.0 %), NH₄⁺-synthetic fertilizer (16.1 % ± 8.99 %), and soil organic nitrogen (9.35 % ± 4.49 %). These NH₄⁺-containing pollutants were converted to HNO₃ (37.2 ± 9.38 mg/L) by nitrifying bacteria, and then the produced HNO₃ preferentially participated in the carbonate (mainly calcite) dissolution, which accounted for 40.0 % ± 12.1 % of the total riverine Ca²⁺ + Mg²⁺, also resulting in the rapid release of NO₃^- into the river water. Thus, microbial nitrification could be a new and non-negligible contributor of riverine NO₃^- pollution, whereas the involvement of HNO₃ in calcite dissolution acted as an accelerator of riverine NO₃^- pollution. However, denitrification had lesser contribution to natural attenuation for high NO₃^- pollution. The obtained results indicated that the mitigation of riverine NO₃^- pollution should focus on the management of ammonium discharges, and the HNO₃-induced carbonate dissolution needs to be considered in comprehensively understanding riverine NO₃^- pollution in piedmont zones.