Consequences of the ‘greening’ of the high Arctic: A tradeoff between plant cover and microbial activity?

Background/Question/Methods

While climate warming in the high Arctic is expected to increase shrub plant cover and stimulate decomposition of soil organic matter, it remains unclear how these variables will interact to influence ecosystem carbon and nutrient cycling. The uncertain response of microbial community composition to warmer temperatures and more plant cover further confounds this relationship. How will greater plant growth affect belowground carbon: Will plant inputs stimulate microbial activity and cause soil respiration to outpace storage, or will more plant shading lower soil temperature and curtail the predicted increase in microbial activity? To evaluate microbial responses to greater plant cover in the high Arctic, we characterized soil biogeochemical properties and bacterial community composition along a vegetation cover gradient in northwest Greenland. Specifically, we evaluated pH, temperature, moisture, bulk density, extractable nutrient pools, water extractable organic carbon and nitrogen, and total microbial biomass. Vegetation cover was classified using the Normalized Difference Vegetation Index (NDVI), and vegetation cover classes were used as a proxy for warming.

Results/Conclusions

We found that with greater plant cover, soil moisture increases and soil temperature decreases significantly. Extractable nutrients (NO₃^-, NH₄⁺, PO₄^3-) and extractable organic C and N generally increase with vegetation cover. Despite increases in available carbon and nutrients, microbial biomass carbon in both horizons ultimately decreased with plant cover. Moreover, the relative proportion of microbial biomass carbon to water extractable organic carbon decreases with vegetation cover, indicating that decomposers in more vegetated soils cannot readily use available carbon. Our data suggests that despite an increase in available substrate in high vegetation cover soils, the insulating properties of vegetation (i.e. high moisture and low temperatures) ultimately limit decomposer activity. We hypothesize that as plant cover in the high Arctic increases with climate warming, decomposer activity – initially heightened by higher temperatures – will ultimately be curtailed by the insulating properties of vegetation. This hindered nutrient mineralization may reduce availability to plants, resulting in a decline in plant cover until soils warm and dry to reach conditions more optimal for microbial processing. Soils may thus oscillate and maintain a moderate vegetation cover equilibrium.