气候变化领域集成服务门户

Arctic tundra ecosystems are warming disproportionately in the winter, including a delayed autumn soil freeze-up. Because microbial processes are extremely sensitive to change in temperature below freezing, overwinter warming strongly stimulates decomposition and nutrient mineralization and ultimately promotes the conversion of sedge-dominated tussock tundra into shrub tundra. We characterized the responses of decomposition in shrub tundra and tussock tundra soils to changes in the rate and extent of freezing using laboratory simulations to study a range of active layer freeze-up scenarios that included variation in time held above 0 degrees C (an acclimation period) and final temperature (-2 degrees C to -20 degrees C). We hypothesized that shrub soil decomposers would be more sensitive to the rate of the freezing process than tussock soil decomposers, but less sensitive to the extent of freezing. Although freezing strongly influenced microbial processes, the effects of freezing tended to not significantly vary across the range freezing regimes tested. Unexpectedly, the tussock-derived soil decomposer community was more sensitive than shrub-derived soil to the freezing treatments. Freezing conditions stimulated tussock soil to release more water-extractable organic carbon (WEOC) that was composed of a greater proportion of microbially derived materials than shrub soil under the same conditions. Freezing-driven changes in the tussock WEOC pool coincided with reduced microbial decomposer biomass, while the shrub decomposer biomass was relatively insensitive to the freezing treatments. These findings suggest that the microbial community of shrub-dominated soils is more resistant to the soil freezing process than the tussock-dominated decomposer community and increases available soil N while reducing labile C release as the soils freeze. Because autumn nutrient dynamics set the stage for overwinter tundra biogeochemical conditions, increasing shrub dominance in tussock tundra may therefore promote both plant N availability and decomposer C limitation during thaw.