Insight into the impact of biogeochemical reactions of groundwater nitrogen on chemical weathering and carbon cycling

Groundwater nitrate pollution is a global environmental issue impacted by complex biogeochemical processes. The biogeochemical behavior of nitrogen in groundwater can significantly influence the hydrogeochemical processes and the carbon cycle. This study, taking the Jinghuiqu Irrigation District in China as an example, analyzed the biogeochemical processes of nitrate in groundwater, and discussed their effects on groundwater chemical weathering and the carbon cycle by using groundwater chemistry, multiple isotopes (δ¹⁵N-NO₃⁻, δ¹⁸O-NO₃⁻, and δ¹⁸O-H₂O), and microbial techniques. Results indicated that nitrification predominantly drives the biogeochemical processes of nitrogen in groundwater in this area. Anthropogenic nitrogen inputs enhanced the geochemical weathering of sediments in shallow groundwater systems through nitrification. As nitrification increased nitrate concentrations, the net CO₂ sink gradually shifted to a net CO₂ source. Under the influence of nitrification, the CO₂ consumption decreases, leading to a reduction in the carbon sink. The average CO₂ consumption rates of carbonate weathering and silicate weathering were 1.10 × 10⁵ mol/km²/yr and 0.60 × 10⁵ mol/km²/yr, respectively. Additionally, energy released during nitrification may promote microbial metabolic processes related to the carbon cycle. Correlation analysis of the nitrogen cycle and carbon cycle pathways revealed a significant association (p ≤ 0.05) between nitrification and both the reductive tricarboxylic acid cycle and the Calvin-Benson-Bassham cycle. Therefore, nitrification significantly influences the nitrogen cycle and may indirectly affect the carbon cycle. This research enhances our understanding of how the biogeochemical processes of groundwater nitrogen impact hydrochemistry and the carbon cycle, providing scientific insights for addressing climate change and ecosystem management.