Fluxes and <sup>13</sup>C isotopic composition of dissolved carbon and pathways of methanogenesis in a fen soil exposed to experimental drought

Abstract

Abstract. The impact of drought and rewetting on carbon cycling in peatland ecosystems is currently debated. We studied the impact of experimental drought and rewetting on intact monoliths from a temperate fen over a period of ~300 days, using a permanently wet treatment and two treatments undergoing drought for 50 days. In one of the mesocosms vegetation had been removed. Net production of CH4 was calculated from mass balances in the peat and emission using static chamber measurements and results compared to 13C isotope budgets of CO2 and CH4 and energy yields of acetoclastic and hydrogenotrophic methanogenesis. Drought retarded methane production after rewetting for days to weeks and promoted methanotrophic activity. Based on isotope and flux budgets, aerobic soil respiration contributed 32–96% in the wet and 86–99% in the other treatments. Drying and rewetting did not shift methanogenic pathways according to δ 13C ratios of CH4 and CO2. Although δ13C ratios indicated a prevalence of hydrogenotrophic methanogenesis, free energies of this process were small and often positive on the horizon scale, suggesting that methane was produced very locally. Fresh plant-derived carbon input apparently supported respiration in the rhizosphere and sustained methanogenesis in the unsaturated zone according to a 13C-CO2 labelling experiment. The study documents that drying and rewetting in a rich fen soil may have little effect on methanogenic pathways but result in rapid shifts between methanogenesis and methanotrophy. Such shifts may be promoted by roots and soil heterogeneity, as hydrogenotrophic methanogenesis occurred locally even when conditions were not conducive for this process in the bulk peat.

Use of wetland buffer areas to reduce nitrogen transport from forested catchments: Retention capacity, emissions of N2O and CH4 and vegetation composition dynamics

“…The reason for the low rate of CH 4 release after rewetting is not fully understood, however, one reason could be the very slow re-establishment of methanogenic bacteria after prolonged aeration (Tuittila et al 2000). The aeration may have restricted methanogenesis to distant anoxic microenvironments, which can result in large spatial heterogeneity in the methanogenic communities and in the CH 4 emissions after rewetting (Knorr et al 2008). However, in restored peatlands used previously for peat extraction, CH 4 production and oxidation potentials have recovered in 4-30 years and even exceeded those of natural sites , Basiliko et al 2007.…”

“…A high peat temperature enhances the emissions (Mikkelä et al 1995, Eriksson et al 2010, and therefore largest emissions usually occur during summer growing season. High productivity of plants and deep rooting plant species support CH 4 production by providing litter, oxygen and root exudates into the peat layer, and also by offering an effective route for the CH 4 transport through the root-shoot pathway (Tuittila et al 2000, Knorr et al 2008, Eriksson et al 2010.…”

Plant-mediated CH<sub>4</sub> transport and contribution of photosynthates to methanogenesis at a boreal mire: a <sup>14</sup>C pulse-labeling study

“…After all, individual compounds of root exudation (acetate), supply of oxygen and presence of alternative electron acceptors may be responsible for methanogenesis (Ström et al 2003;Knorr et al, 2008). Since the former parameters are species-specific, a general relation of CH 4 turnover to vegetation biomass may be a too simplistic approach.…”

“…Almost double amount of emitted 14 C with CO 2 from mesocosms with E. vaginatum as compared to S. palustris hollows might have originated both from respiration and oxidation of CH 4 (Table 1). In addition, oxic conditions and oxygen transport into the rhizosphere could regenerate non-methanogenic electron acceptors such as iron or sulphate oxides (Knorr et al, 2008;Sutton-Grier and Megonigal, 2011), which suppress methanogenesis. This process was reported to be species-specific and did not depend on plant biomass/productivity (Sutton-Grier and Megonigal, 2011).…”

Peatlands and the carbon cycle: from local processes to global implications – a synthesis

“…in the capillary fringe of peat soils. In this zone oxygen transport is impeded, likely leading to spatiotemporally variable mosaic of oxic and anoxic microenvironments with variable respiration rates and pathways (Knorr et al, 2008).…”