Soil carbon cycling in boreal forest wetlands

Methane flux measurements indicate boreal forest wetlands are large contributors to the global CH₄ emission despite the short peak production period, because of the extensive land base covered by wetlands in the north. The Boundary fen, in the boreal forest at the Prince Albert National Park, emitted an average of 0.148 g CH₄ m⁻² d⁻¹ during peak production from the end of June until the end of July, with fluxes reaching 0.531 g CH₄ m⁻² d⁻¹. A fen in the Canwood Forest Reserve showed emissions reaching a high of 0.293 g CH₄ m⁻² d⁻¹ and averaging 0.098 g CH₄ m⁻² d⁻¹ during peak production from the end of June until August 1. These CH₄ fluxes are similar to those reported in freshwater wetlands around the world. Methane fluxes were found to vary in different peat conformations. Areas with floating peat mats and shallow peat under forest had lower CH₄ fluxes relative to open water or deep solid peat locations. Therefore, as a peatland grows with time, the amount of CH₄ flux will change according to the dominant peat conformation type. A small Upland catchment basin in Prince Albert National Park emitted only 0.0087 g CH₄ m⁻² during the summer of 1992 and all of the CH₄ flux occurred during ice thaw from the beginning of May to the end of June. Methane flux may have been inhibited by sulfate-reduction activity in this site as stimulated by high sulfate levels. Wetlands which are only saturated part of the year appear to have low CH₄ fluxes associated with high sulfate-reduction activity. Field and incubation studies examined the effect of environmental factors on CH₄ fluxes. Methane emissions tended to increase as temperature increased, with the greatest increases above 12°C. However, CH₄ production was still evident even at temperatures. as low as 5°C. Methane fluxes were greatest when the water level was at the peat surface or above the peat. Differences in the concentration of NO₃⁻, SO₄²⁻, and H₂PO₄⁻ in interstitial water from the field showed little relationship with measured CH₄ fluxes, but the variations in the amount of these nutrients were small. Incubations of intact peat cores showed CH₄ fluxes to increase greatly with additions of CO₂ substrate, some increase with acetate additions and no significant increase with methanol additions. It appears that the microbial community can utilize CO₂ and acetate to generate more CH₄ and therefore may regularly consume these substrates in the boreal forest wetlands. The greatest increases in CH₄ flux result with temperature increases and substrate additions. ¹³C natural abundance variations were measured in soil and vegetation carbon from three contrasting environments: prairie grassland, parkland forest and boreal forest wetlands. The boreal forest wetlands include the permanently saturated Boundary and Canwood fens and a seasonally saturated Upland catchment basin. The aerobic soils generally become more ¹³C enriched with a higher degree of organic decomposition. ¹³C enrichment was greater in soils under cultivation relative to native soils, and increased with depth possibly because of a greater degree of decomposition under these conditions. In the anaerobic soil, the Upland catchment basin peat deposits had δ¹³CPDB values of organic carbon that were constant with depth, whereas deeper layers of the peat in the Boundary fen had areas of ¹³C enrichment. The ¹³C enrichment may reflect areas of intense CH₄ production in which ¹³C enriched residual substrate is left behind during the production of highly ¹³C depleted CH₄. Carbonates measured in boreal forest wetland soils were dominantly primary carbonates in the aerobic upland soils as indicated by ¹³C values near 0‰ and secondary in the peat (¹³C depleted). Evidence of methane oxidation was shown in the Boundary fen with δ¹³C values as low as -97‰ in carbonate minerals found in floating peat mats. It is postulated that ¹³C depleted CH₄ is oxidized in the mat and reacts with calcium ions to form calcite (identified through x-ray diffraction). Floating peat mats appear to be zones of CH₄ oxidation which lower the CH₄ flux from these areas. Secondary carbonates in the peat of the Upland catchment basin have isotope compositions close to the δ¹³C values of the peat organic carbon (-25‰), indicating their origin is from fermentation and possibly from sulfate-reduction