Influence of hydromorphic soil conditions on greenhouse gas emissions and soil carbon stocks in a Danish temperate forest

Recent research has shown that wet or hydromorphic soils in forests are hotspots for greenhouse gas (GHG) emission of methane (CH4) and nitrous oxide (N2O), and that emission of these gases may offset the mitigation potential from carbon (C) sequestration. However, quantitative evidence at the forest scale is limited. We investigated the role of hydromorphic soils for N2O and CH4 fluxes at the forest district level (Barritskov, 348 ha) by mapping the distribution of upland and hydromorphic soils, measuring the soil carbon and nitrogen stocks and field fluxes of N2O and CH4 for a period of 2 years as well as in laboratory experiments.Field exchange rates of N2O (mean ± standard error of the mean(SE), μg N2O–N m−2 h−1) were similar for hydromorphic (3.8 ± 1.2) and upland soils (3.8 ± 0.4). Although both soil types displayed net CH4 oxidation the average rate (μg CH4–C m−2 h−1) was significantly lower in hydromorphic soils (−5.8 ± 2) compared to the upland soils (−23 ± 1.2). Soil water content (SWC) was, as expected, higher in hydromorphic soils which was consistent with lower uptake of CH4 as well as significantly larger soil carbon stocks in O horizon plus 0–30 cm mineral soil (86 ± 6 versus 66 ± 5 Mg C ha−1 in hydromorphic versus upland). Oxidation rates of CH4 in laboratory incubations at ambient concentration (2 μL L−1) were similar in the two soil types, but the hydromorphic soils oxidised CH4 fastest when incubated at 10,000 μL L−1 CH4: only hydromorphic soils produced CH4. Potential N2O production did not differ between soil types and N2 production was significantly higher in hydromorphic soils, which also had a higher pH > 6.Based on four scenarios, we assessed how reduced ditching might affect the emissions of N2O and CH4 from upland soils. The CH4 sink of the soil decreased in all four reduced ditching scenarios from 1.3 to 7 Mg CO2-equivalent (eqv) y−1. The emissions of N2O and CH4 in the current state and all scenarios comprised only a minute fraction (<1%) of the global warming potential (GWP) of carbon stored in the soil.We conclude that hydromorphic soils are potential hotspots for CH4 production and reduced uptake of atmospheric CH4, but their limited area covered by such soils at Barritskov implies that upland soils are most important in terms of soil C stock and the non-CO2 GHG budget. Ceased drainage activities in upland soils are expected to increase the likelihood of CH4 emissions and reduce soil CH4 uptake.

► Emission of N2O was similar in upland and hydromorphic soils in a Danish forest. ► Methane oxidation in the soil was significantly lower in hydromorphic soils. ► Ceased ditching of soils can lead to decreased net oxidation of methane in the soil. ► Carbon sequestration of forest soils are likely negatively affected by wetter soils.