Increased Methane Uptake Under Long-Term Warming in Well-Drained Sub-Arctic Soil A Field Study of Gas Fluxes at Two Icelandic Sites Using Open-top Chambers and Methanotrophy Inhibition

Abstract

Methane (CH4) is the green house gas with the second largest contribution to global warming. Well-drained soils make up 87% of the Arctic and have been observed to be a CH4 sink. There exist several CH4 sources and sinks in the Arctic. One of the CH4 sinks in the Arctic is CH4 oxidation by soil microorganism. Soil CH4 fluxes are depending on two microorganisms; methanotrophs which oxidize CH4 and methanogens which produces CH4. However, there exists knowledge gaps about the size of Arctic CH4 soil uptake, the drivers of the activity level of CH4 consuming and producing microbes along with how the CH4 fluxes will change with global warming. This study measured CH4 fluxes at two sub-Arctic Icelandic sites with well-drained soil under ambient and passively heated conditions. The passive heating was obtained through open-top chambers (OTC’s). CH4 fluxes were measured at one moss-dominated site (Moss site), and one colder, more species rich heath site (Heath site). The results showed a 31,7% increase in CH4 uptake at the Moss site (p<0,05), where the average uptake shifted from 0,790 ± 0,422 in Control plots to 1,04 ± 0,472 mg[CH4]m−2d−1 within the OTC plots. The size of these fluxes is in line with previous studies of CH4 sinks in the Arctic during the growing season. Soil moisture and soil temperature were measured, but neither factor could be determined as a driver behind the increased CH4 uptake. At the Heath site, no change in CH4 uptake was observed when comparing the OTC and Control plots (p=0,84). Further more, the Heath site had a lower CH4 uptake than the Moss site with an average uptake of around 0,037 ± 0,03 mg[CH4]m−2d−1, about a tenth of what previous studies observed in well-drained Arctic soils during the growing season. The small CH4 sink as well as no significant CH4 flux change between the treatments is believed to be due to a late snow melt, leading to higher soil moisture, lower soil temperature and a lower microbial activity. Inhibitions of methanotrophic activity were conducted at the Heath site with acetylene gas to separate the soil’s cooccuring production and consumption of CH4. A slight CH4 production was observed within the soil for some of the inhibitions, indicating that well-drained Arctic soils have active methanogens. However, too few inhibition experiments could be completed for statistical analysis.