The influence of spring temperatures and snow depth on arctic tundra plant growth and soil nutrient dynamics

Background/Question/Methods

Higher spring temperatures and earlier snowmelt have the potential to alter the carbon (C) balance of terrestrial ecosystems because they can significantly influence both plant growth and decomposition during the growing season. This is of particular concern in arctic tundra soils, which contain large stores of C and may act as a significant CO₂ source with warming. To determine how changes in the timing of snowmelt and higher spring temperatures affect plant growth and soil microbial activity and nutrient dynamics, we are conducting a factorial accelerated snowmelt × warming experiment in a moist acidic tundra community in the Alaskan arctic. We are following plant phenology, root growth and rhizodeposition, soil nitrogen (N) availability, microbial biomass and enzyme activity, and ecosystem respiration to develop an enhanced mechanistic understanding of the implications for a lengthening of the arctic growing season. We hypothesized that accelerated snowmelt would allow plants to start growing earlier, resulting in earlier root growth and plant N uptake from these nutrient poor soils, potentially exacerbating N limitation to decomposer microorganisms and reducing their activities.

Results/Conclusions

Contrary to our hypothesis, accelerated snowmelt resulted in delayed root growth in relation to the date of snowmelt, and reduced growing season root productivity. Our data suggest that low air temperatures following snowmelt may have inhibited plant growth, as plants were unprotected from swings in air temperature without the insulating snowpack. The fact that warming in combination with accelerated snowmelt alleviated the inhibition of root growth supports this conclusion. Also in contrast to our predictions, we found that greater root growth was associated with elevated, not reduced, soil N availability. Our data suggest that C-rich root exudates stimulate microbial N acquisition and can actually increase N availability even as roots and microbes are taking up more N. These results suggest that as long as there is potentially available N in organic matter, energy limitation (in the form of labile C) to decomposer microorganisms will be one of the fundamental controls on how arctic tundra soils will respond to climate change. Because root exudates are one of soil microbes’ primary labile C sources in this soil, any climate changes that impact root growth are likely to alter nutrient availability and microbial activity. The broader implication of our results is that increased spring climate variability is likely to have significant effects on decomposition rates and nutrient availability, mediated by—and with feedbacks to—plant growth and phenology.