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 (N ₂O), carbon dioxide (CO ₂) and methane (CH ₄) in groundwaters beneath agricultural systems is important for global greenhouse gas (GHG) budgets. Dissolved C (DC: dissolved organic carbon (DOC)+CO ₂-C+CH ₄-C) and dissolved nitrogen (DN: NO ₃ ^--N+NH ₄ ⁺+NO ₂ ^--N+N ₂O-N+N ₂) 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 NO ₃ ^--N (84-90% of DN) in high permeability aquifers and N ₂ (46-77% of DN) in low permeability aquifers. Indirect N ₂O emissions via groundwater denitrification accounted for 0.03-0.12% of N input, which was equivalent to 3-11% of total N ₂O 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 NO ₃ ^--N and dissolved N gases, N ₂O and N ₂. Indirect GHG emissions from groundwater were an important part of farm nutrient budgets, which clearly has implications for national GHG inventories.