Climate warming in the arctic can stimulate soil organic matter decomposition, resulting in more nitrogen available for plants. Deciduous shrubs respond more positively to nitrogen additions than other plants in this region; thus, an increase in temperature could lead to a shift from graminoid to shrub dominance and a shift in the structure and function of the ecosystem. The mechanisms that promote shrub expansion and the consequences of shrub expansion for the biogeochemical cycles of carbon and nitrogen (N) are not well understood. One hypothesis is that snow accumulation by shrubs can indirectly influence N availability by maintaining warmer soil temperatures in winter, allowing soil microbes to stay active longer and supply more N to shrub growth. Alternatively, shrubs may enhance N availability by production of highly decomposable litter, independent of their effects on winter soil temperatures. The objective of our study was twofold: (1) to test whether the amount of snow that accumulates around arctic deciduous shrubs maintains winter soil temperatures high enough to stimulate microbial activity and increase soil N levels and (2) to compare the relative effects of shrubs on N availability via effects on the controls over N mineralization; soil organic matter quality versus the soil microclimate. We directly manipulated snow depth, via snow fences, at three sites that represented a gradient of increasing shrub abundance and height. As an index of N availability, we measured net N mineralization using in situ intact soil cores. To compare the effects of shrubs on controls over N mineralization, we conducted a reciprocal soil core transplant of intact soil cores between the three sites.

Results/Conclusions

Net N mineralization increased with increasing shrub density during both winter (F=17.6463, p<0.0001) and summer (F=10.883, p=0.0003). Snow addition had no effect on net N mineralization in the winter; but surprisingly, winter snow addition had a positive effect on summer mineralization rates (F=5.9021, p=0.0218). Soil organic matter quality had a nine times larger effect on net N mineralization than did soil microclimate in the summer season (F=32.1811, p<0.0001) and only soil organic matter quality had a detectable effect on N mineralization in the winter (F=8.4881, p=0.0008). Our study offers a unique insight into the temporal dynamics of shrub-snow interactions and suggests that summer effects of increased winter snow on N mineralization can be substantial.  Furthermore, shrub abundance effects on the quality of soil organic matter had a much stronger influence than microclimate at our sites.