Tracking the fate of Arctic carbon: Will Arctic shrub expansion result in a loss or gain of soil carbon?

Background/Question/Methods

Northern circumpolar soils cover 16% of the total land surface area yet account for nearly 50% of the estimated global organic carbon (C) pool. Unprecedented rates of warming may convert the Arctic from a net sink to net source of atmospheric C as soil organic matter (SOM) decomposes more rapidly. Additionally, the abundance of woody shrubs is projected to increase as the climate warms, potentially increasing the total amount of labile C added to the system. Our ability to predict the response of arctic C cycling is limited by significant uncertainties in our understanding of processes that may counteract or enhance SOM loss and how this may vary with vegetation shifts. Our research objective is to improve our mechanistic understanding of the effects of labile C inputs into Arctic tundra soils. We added isotopically enriched ¹³C-glucose to soils dominated by two dominant arctic vegetation: Betula nana, a woody dwarf birch species, and Eriophorum vaginatum, a ubiquitous tussock-forming sedge. We hypothesized labile C additions would stimulate loss of native SOM from soils under Betula nana vegetation more than Eirophorum vaginatum. We measured ¹³CO₂ efflux within the first two weeks and one month following C additions. We will measure ¹³C-glucose incorporation to microbial biomass using chloroform direct fumigation and will trace the remainder of our label into soil pore space, heavy, and light soil fractions using GC-IRMS and solid-state ¹³C-NMR following SPT density flotation.

Results/Conclusions

Preliminary results indicate greater respiration and ¹³C-glucose utilization by E. vaginatum with no short-term native SOM loss by either vegetation type. Overall, only ~10% of ¹³C-glucose was measured in CO₂ efflux, and no priming was observed beneath either vegetation type. Our ¹³CO₂ efflux results suggest new inputs of labile carbon will not result in an immediate destabilization of native soil organic matter. Our future plans are to track the remaining C into soil pools and microbial biomass.