Greenhouse gases (CO2, CH4 and N2O) in lowland springs within an agricultural impacted watershed (Po River Plain, northern Italy)

Abstract

In the Po River Plain, nitrogen surplus in permeable soils results in elevated downward nitrogen fluxes, mostly as nitrate. Lowland springs, aligned along interfaces between gravel and sandy soils, recycle part of this nitrogen to the surface and we hypothesised that they may be hot spots of N2O and other greenhouse gases, due to incomplete denitrification in the suboxic environment. In early and late summer 2009, water flow was measured and water samples were collected at the outlet and ∼1 km downstream at 14 springs; physico-chemical parameters [temperature, pH, dissolved inorganic nitrogen (DIN) and dissolved gases (O2, N2O, CH4, CO2)] were analysed. All springs were characterised by elevated nitrate concentrations (154–1411 μ M) and recycled to the surface inorganic nitrogen (∼180 kg N on average). Spring waters were suboxic (40–60% of O2 saturation) and CO2, CH4 and N2O supersaturated (26.6–2399.0, 0.002–1.02 and 0.02–1.02 μ M, respectively). CO2 and N2O underwent a significant degassing process from the supersaturated waters to the atmosphere. Calculated N2O emissions (up to 0.646 g N 2O·m −2·d −1, among the highest reported for aquatic environments) highlight the role of lowland springs as hotspots of N2O. We conclude that lowland springs located in heavily impacted watersheds recycle groundwater nitrate and have an extremely elevated potential as greenhouse gas emitters.