Arctic soils are among the largest stores of organic carbon (C). Because rapid Arctic warming is predicted to promote decomposition that will stimulate plant growth and community composition shifts as well as decomposition, there is great interest in developing mechanistic descriptions of the system’s C dynamics as it responds 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 the 1989 Toolik Long Term Ecological Research Site  moist acidic tundra greenhouse experiment to explore the  consequences of growing season warming on soil across horizons.

Results/Conclusions

Two decades of growing season warming is demonstrated to drive increased shrub abundance. Our study suggests that while the most overt effects of warming are plant species composition shifts and change in litter inputs to the upper soil horizons, it is the deeper mineral soils that are the most perturbed by the warming driver. Biogeochemical changes detected in greenhouse mineral soils include: increased soil C:N driven by increase in C content 4.7 to 5.3 % and increased microbial N (300%of  control) coupled with  decreased extractable organic nitrogen (N) (65% control) and increased C mineralization rates  (150% of  control).  Because soil temperature effects of greenhouse warming decrease with depth, the driving force in biogeochemical change appears to be driven by change in plant species composition, perhaps due to increasing plant-derived C inputs at depth.  Overall, warming does not result in 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.