Effect of forest fire on the fluxes of CO2, CH4 and N2O in boreal forest soils, interior Alaska

[1] Flux measurements at sites of mixed hardwood and black spruce stands from an area (C4) of the Caribou-Poker Creek Research Watershed (CPCRW), interior Alaska, in the summer seasons of 1998, 1999, and 2000 are used to estimate the fluxes of CO2, CH4, and N2O before and after forest fires. The FROSTFIRE burning experiment was executed in typical boreal forest from 8 to 15 July 1999. The forest fire significantly decreased soil CO2 and N2O emissions, by at most 50%. On the contrary, CH4 flux from the soil increased from 7 to 142%, suggesting that the forest fire plays a role in accelerative thawing of the frozen soil, and subsequently the release of CH4 from permafrost. Most of the CH4 was oxidized in the soil after the fire; however, some was released from the soil when the permafrost maximally thawed at the end of August 1999 and September 2000. Relationships between the fluxes of trace gases and soil temperature before and after the fire showed good exponential correlations, indicating that soil temperature was one of the factors determining the fluxes of trace gases on boreal forest soils. Also, the higher soil temperature after the fire may be led to the enhanced diffusion of CO2, CH4 and N2O by microbial activity between the atmosphere and the forest soils, and to the increased fluxes of trace gases in burned black spruce stand soils. In order to understand the roles of moss and lichen mats on the black spruce stands, the net respiration by mosses and lichens was estimated with light and dark chamber measurements. Net respiration corresponds to 42 to 58% of the total soil respiration before fire. Therefore, the net respiration by moss and lichen layers was responsible for one-half of total soil CO2 emissions. The maximum regional net respiration rate by moss and lichen mats on black spruce forest floors of central Alaska was 0.018 ± 0.009 GtC/yr, an important source of atmospheric CO2 in boreal forests. After the prescribed burn, soil respiration was attributable only to respiration by roots and microbes. The microbial respiration estimated after the fire is almost three times as high as that the calculated respiration before the fire. This finding indicates the post-fire condition may stimulate microbial respiration because of higher nutrients and substrates in remnant soils and enhanced soil temperature. The microbial respiration can be estimated 14.7 tC/ha in burned black spruce stands over a decade after the fire, suggesting burned black spruce forests in central Alaska are a crucial source of atmospheric CO2.