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

Christiansen, Jesper Riis, Gundersen, Per, Frederiksen, Preben, Vesterdal, Lars
Forest ecology and management 2012 v.284 pp. 185-195
carbon, carbon sinks, drainage, forest soils, global warming, greenhouse gas emissions, greenhouse gases, greenhouse soils, highlands, hydromorphic soils, methane, methane production, mineral soils, nitrous oxide, organic horizons, oxidation, risk reduction, soil water content, temperate forests, upland soils
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, 348ha) 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 2years as well as in laboratory experiments. Field exchange rates of N₂O (mean±standard error of the mean(SE), μgN₂O–Nm⁻²h⁻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 (μgCH₄–Cm⁻²h⁻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 0–30cm mineral soil (86±6 versus 66±5MgCha⁻¹ in hydromorphic versus upland). Oxidation rates of CH₄ in laboratory incubations at ambient concentration (2μLL⁻¹) were similar in the two soil types, but the hydromorphic soils oxidised CH₄ fastest when incubated at 10,000μLL⁻¹CH₄: 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 7MgCO₂-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.