Groundwater Derived Greenhouse Gas Emissions From Contrasting Agricultural Settings.

Estimating losses of dissolved carbon (C) and nitrogen (N) via groundwater in an agricultural system provides insights into the reduction of uncertainties in the terrestrial C and N balances. In addition, quantification of dissolved nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄) in groundwater beneath an agricultural system is of huge importance for global greenhouse gas (GHG) budget estimation. Dissolved C (DC: dissolved organic carbon (DOC)+CO₂+CH₄) and dissolved nitrogen (DN: NO₃^--N+NH₄⁺+NO₂^-+N₂O+N₂) in groundwater were measured in two low permeability (<0.02 m d^-1) and two high permeability (>0.05 m d^-1) aquifers in Ireland. Groundwater in multilevel piezometers was sampled monthly over two years (Feb, 2009 to Jan, 2010). Mean groundwater discharge to surface water was higher in 2009 (587-836 mm) than that in 2010 (326-385 mm). 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. Mean N₂O emissions via groundwater ranged from 201- 276 kg of CO₂ equivalence ha^-1 y^-1. The N₂O emissions 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 was not significant compared to total carbon (TC) content of the topsoil (0.06-0.18% of TC). However, CO₂ emissions was an important source of atmospheric CO₂ (mean 537-1041 kg ha^-1 y^-1). Methane emissions via groundwater were 1 to 30 kg of CO₂ equivalence ha^-1 y^-1. Indirect GHG from groundwater were an important part of farm budgets, which clearly has implications for national inventories.