Belowground Impacts of Long-term Warming on Tussock Tundra Soils

Arctic soils are among the largest stores of organic carbon (C). Because arctic warming is predicted to promote decomposition that will stimulate plant growth and changes in community composition, there is great interest in developing mechanistic descriptions of C dynamics as they respond to warming. A warming-driven shift in community structure towards increasing shrub dominance may increase the system's potential for C storage if vegetative C storage increases while microbial decomposition remains constrained by nutrient availability, a limitation that may be exacerbated by increasing plant competition for nutrients. We used a greenhouse experiment at the Toolik Long Term Ecological Research Site to explore the consequences of warming on soil C dynamics. Two decades of growing-season warming increased shrub abundance. While the most overt effects of warming were changing plant species composition and litter inputs, it was the deeper mineral soils that were most perturbed. We also are exploring the impact of long-term warming on early growing season pre-snowmelt microbial community composition in tundra soils. Biogeochemical changes detected in greenhouse mineral soils include: increased soil C:N driven by increase in total C content and increased microbial N coupled with decreased extractable organic nitrogen (N). Because soil temperature effects of greenhouse warming decrease with depth, the driving force in biogeochemical change appears to be the change in plant species composition, perhaps due to increasing plant-derived C inputs at depth. Overall, warming did not cause a net loss of C or N from the system, although over time shrub growth response to warming may be constrained by increasing N-limitation.