Groundwater: A pathway for terrestrial C and N losses and indirect greenhouse gas emissions

Estimating losses of dissolved carbon (C) and nitrogen (N) via groundwater in an agricultural system provides insights into reducing uncertainties in the terrestrial C and N balances. In addition, quantification of dissolved nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄) in groundwaters beneath agricultural systems is important for global greenhouse gas (GHG) budgets. Dissolved C (DC: dissolved organic carbon (DOC)+CO₂-C+CH₄-C) and dissolved nitrogen (DN: NO₃ ⁻-N+NH₄ ++NO₂ ⁻-N+N₂O-N+N₂) in groundwater were measured in two low permeability (<0.02md⁻¹) and two high permeability (>0.05md⁻¹) aquifers in Ireland. Groundwater in multilevel piezometers was sampled monthly over two years. Mean groundwater discharge to surface water was higher in 2009 (587–836mm) than in 2010 (326–385mm). Dissolved C and N delivery to surface water via groundwater caused substantial losses of terrestrial C and N. The extent of delivery was site specific and depended on N input, recharge and aquifer permeability. Mean dissolved N losses ranged from 8–12% of N input in low permeability to 27–38% in high permeability aquifers. The dominant fraction of DN was NO₃ ⁻-N (84–90% of DN) in high permeability aquifers and N₂ (46–77% of DN) in low permeability aquifers. Indirect N₂O emissions via groundwater denitrification accounted for 0.03–0.12% of N input, which was equivalent to 3–11% of total N₂O emissions. Dissolved C loss to surface waters via groundwater was not significant compared to total carbon (TC) content of the topsoil (0.06–0.18% of TC). Site characteristics contributed greatly to the distribution of N between NO₃ ⁻-N and dissolved N gases, N₂O and N₂. Indirect GHG emissions from groundwater were an important part of farm nutrient budgets, which clearly has implications for national GHG inventories.