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 + N-2) 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. Mean groundwater discharge to surface water was higher in 2009 (587-836 mm) than 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 N-2 (46-77% of ON) 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 N-2. Indirect GHG emissions from groundwater were an important part of farm nutrient budgets, which clearly has implications for national GHG inventories. (C) 2012 Elsevier B.V. All rights reserved.