Limited variation in proportional contributions of auto- and heterotrophic soil respiration, despite large differences in vegetation structure and function in the Low Arctic

Arctic ecosystems contain vast stores of soil carbon (C), yet our understanding of the factors controlling CO₂ efflux from tundra soils remains poor. Partitioning soil respiration (R_S) into heterotrophic (R_H) and autotrophic (R_A) sources can help elucidate the relative contributions from microbial breakdown of soil organic matter (SOM) and root and rhizospheric activities—two processes that can have contrasting effects on long-term soil C stocks. Using two techniques, we quantified the magnitudes, relative proportions and environmental drivers of R_H and R_A in four common arctic vegetation types in West Greenland. We employed a trenching method in large patches of Betula nana, Salix glauca, mixed-shrub (equal mix of Betula and Salix) and graminoids dominated by Poa spp. At a nearby location, we introduced ¹³CO₂ to Betula- and graminoid-dominated plots. The difference in the autotrophic proportion (R_A/R_S) between methods was minimal, providing confidence that our more extensive trenching approach provided accurate estimates of R_A and R_H. Despite contrasting microclimate conditions, large differences in vegetation structure and wide variation in R_S, there were minimal differences in mean R_A/R_S (0.40–0.48 across all vegetation types). Our results suggest that R_A/R_S may be more conservative than previously thought for low-productivity ecosystems. We suggest that partitioning R_S into R_A and R_H may be a useful tool to identify ecosystems that have fallen out of equilibrium and may be poised to either gain or lose soil C.