Soil carbon cycling in boreal forest wetlands

Abstract

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. l3C 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 l3C enriched with a higher degree of organic decomposition. l3C 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.