Nitrate reduction in groundwater as an overlooked source of agricultural CO 2 emissions

Abstract. Nitrate pollution from agriculture poses a global environmental and public health threat. Nitrate levels in water can be reduced through denitrification, which increases dissolved inorganic carbon (DIC) via organic carbon mineralization and/or carbonate dissolution. This DIC potentially acts as a net anthropogenic source to atmospheric CO2; however, its overall impact remains unclear. This study quantified CO2 production from denitrification in Denmark, utilizing extensive observational datasets and national-scale modelling tools. We identified dominant denitrification processes in groundwater and predicted a national process map. Our results indicate that hydrogeology plays a central role in determining the dominant processes. CO2 production from denitrification in groundwater varied spatially, depending on nitrogen leaching and the denitrification processes. We estimated that denitrification in groundwater produces about 204 kt of CO2-eq. yr-1 as DIC, and ~50 % would be emitted to atmosphere. The Intergovernmental Panel on Climate Change (IPCC) guidelines account CO2 emissions related to agriculture from liming, urea, and other C-containing fertilizers, and these were 250, 1 and 4 kt of CO2-eq. yr-1, respectively, for Denmark in 2020. Although CO2 is a minor agricultural GHG emission (2 % of the total), our findings suggest that the agricultural GHG inventories should include denitrification-related CO2 emissions.