Legacies: How will greater winter snowfall affect biogeochemical cycling in shrub tundra ecosystems during the summer, and over multiple years?
Arctic and boreal soils contain twice as much carbon as currently found in the atmosphere, making them a great concern regarding global climate change feedbacks. At these high northern latitudes, climate warming and precipitation changes are projected to be largest during autumn and winter where precipitation mainly falls as snow. Snow accumulation effectively insulates soils from extreme cold winter air temperatures, resulting in relatively warm and stable soil temperatures. Deeper snow, as a result of climate change, will therefore lead to warmer soils which may stimulate soil microbial activity, leading to enhanced rates of carbon and nutrient mobilization during the snow-covered cold season. This has led to the hypothesis that greater snow accumulation is a key factor explaining the widespread increases in shrub growth that have recently been observed across the Arctic. However, short-term ecosystem responses to climatic changes are not always sustained over the longer term, making it difficult to predict how deepened snow will impact tundra carbon and nutrient cycling over timescales from decades to centuries. Will deeper snow continue to enhance nutrient mobilization in the long-term, will it lead to greater shrub cover, and how will this affect overall carbon storage above- and belowground?
As expected, winter litter decomposition rates were enhanced under deepened snow, but only in dry and not in wet ecosystems. In contrast with many short-term studies, long-term deepened snow significantly reduced total and soluble mineral soil carbon pools, and although most changes in biogeochemistry were observed in the mineral soil horizon, soluble phosphorous in the organic soil horizon was greatly reduced as well. These reductions may explain the observed decrease in overall growing season ecosystem CO2emissions, since gross ecosystem production and aboveground plant biomass were unaffected.
Our results clearly show that the impact of increased snowfall on biogeochemical cycling will not be confined to the cold season. Surprisingly, the accumulated ‘legacy effect’ of nine years of enhanced winter soil microbial activity seems to have diminished soil carbon storage to such an extent that summer CO2 emissions were consistently reduced. Nitrogen and phosphorous have recently been observed to co-limit birch growth in this tundra ecosystem, and therefore a decline in phosphorous availability may help explain why aboveground plant cover and biomass pools were not stimulated by deeper snow. In conclusion, despite extremely cold temperatures, changes in winter climate are likely to have significant immediate and legacy effects in tundra ecosystems.