The emissions of nitrous oxide and methane from natural soil temperature gradients in a volcanic area in southwest Iceland

Abstract

Nitrous oxide (N2O) and methane (CH4) emissions were measured along three natural geothermal soil temperature (Ts) gradients in freely drained upland soils in a volcanic area in Iceland. Two of the Ts gradients (underneath a grassland (GN) and a forest site (FN), respectively) were recently formed (in May 2008) and thus subjected to relatively short-term warming. The third Ts gradient, underneath another grassland site (GO), had been subjected to long-term soil warming (over at least 45 years). The N2O and CH4 emissions were measured using the static chamber method. In addition, subsurface soil gas concentrations (5–20 cm) were studied. N2O emissions from GN were slightly higher than those from GO in the temperature elevation range up to +5 °C, while CH4 uptake rates were similar. Under moderate soil warming (<+5 °C) there were no significant increases in gas flux rates within any of the sites, but when soil warming exceeded +20 °C, both N2O and CH4 emissions increased significantly at all sites. While net uptake of CH4 (up to −0.15 mg CH4 m−2 h−1) and occasional N2O uptake (up to −12 μg N2O m−2 h−1) were measured in the unwarmed plots at all sites, net emissions were only measured from the warmest plots (up to 2600 μg N2O m−2 h−1 and up to 1.3 mg CH4 m−2 h−1). The subsurface soil N2O concentrations increased with soil warming, indicating enhanced N-turnover. Subsurface soil CH4 concentrations initially decreased under moderate soil warming (up to +5 °C), but above that threshold they also increased significantly. A portion of the N2O and CH4 emitted from the warmest plots may, however, be geothermally derived, this should be further confirmed with isotope studies. In conclusion, our research suggests that moderate increases in soil temperature (up to +5 °C) may not significantly increase N2O and CH4 emissions at these upland soils, both in the short and longer term. However, warming trends exceeding +5 °C as predicted for 2100 in pessimistic scenarios may cause increased trace gas emissions and thus significant positive feedbacks to climate change.