Mapping the redox architecture of the critical zone for quantifying CO2 emissions from groundwater denitrification

While Earth’s critical zone is defined from the canopy to the base of aquifers, the role of groundwater in the greenhouse gas (GHG) budget remains under-represented. Groundwater acts as a massive biogeochemical engine where redox-driven processes play a vital role in nutrient cycles. In agricultural systems where nitrate leaches from the soil layer, the subsurface provides a critical ecosystem service through denitrification. However, this redox-driven process converts nitrate to N₂ by oxidizing reduced materials such as organic carbon and pyrite, thereby producing dissolved inorganic carbon (DIC). Despite its potential to act as an anthropogenic source of CO₂, the climatic implications of groundwater denitrification have not been quantitatively investigated on a large scale.

In this study (recently published in Biogeosciences, 2025), we investigated the DIC increase driven by denitrification across Danish aquifers. Using a national groundwater chemistry dataset and machine learning techniques, we identified eight different redox clusters spanning from oxic to methanogenic conditions. The spatial architecture of these redox clusters was found to be primarily governed by the hydrogeological framework. By combining the clusters with the subsurface structural information, we predicted the predominant denitrification processes at the redox interface, where nitrate is fully reduced to N₂. Our results revealed that about 76% of the area is driven by pyrite oxidation, while the remainder is driven by organic carbon decomposition.

By coupling these findings with a national nitrogen model and the process-specific stoichiometry of N and C, we estimated that groundwater denitrification in Denmark releases approximately 104kt of CO₂ annually. Current IPCC guidelines for GHG accounting cover liming, urea, and other carbon-containing fertilizers as anthropogenic CO₂ sources from agricultural systems. However, our findings indicate that groundwater denitrification generates a CO₂ flux equivalent to nearly half of the emissions from agricultural liming in Denmark (246 kt CO₂ eq. yr^-1), which is currently the predominant source. Crucially, groundwater denitrification exclusively mobilizes “old” geological carbon pools—organic carbon and carbonates—that have been stably stored for millennia. Because these pools are effectively non-renewable on human timescales, this represents a net addition of carbon to the active cycle. We conclude that a quantitative understanding of the coupling of C and N in the deep critical zone must be investigated across diverse global climatic and geological conditions.