Deepened snow alters soil microbial nutrient limitations in arctic birch hummock tundra

Abstract

Microbial activity in the long arctic cold season is low but cumulatively important. In particular, the size of the microbial biomass and soil solution nutrient pool at the end of winter may control the quantity of nutrients available to plants in the following spring. Microbial starvation and lysis as a result of increasingly severe soluble carbon (C) shortages over winter has been hypothesized as a potential mechanism for microbial nutrient release at thaw. These C shortages may be exacerbated by the warmer temperatures and increased winter precipitation that are consistently predicted for a large part of the low Arctic. In particular, warmer soil temperatures due to deeper snow may increase wintertime microbial activity and organic matter decomposition over the winter, potentially resulting in enhanced nutrient availability to plants in the following growing season. In this study, we investigated nutrient limitations to soil microbial growth and activity in late winter under ambient and experimentally deepened snow (∼0.3 and 1 m respectively) in birch hummock tundra within the Canadian low Arctic. We hypothesized that the build-up of moderately deeper snow over winter would exacerbate soluble C-limitation to microbial growth and activity and increase soluble N accumulation, and thus stimulate the growth of bacteria relative to fungi. We measured the in situ response of the soil microbial biomass and soil soluble pools in control and snow-fenced plots at the end of winter, and then incubated soils from these plots with added C, nitrogen (N) and phosphorus (P) (at 0–15 °C) to characterize nutrient limitations to microbial growth and activity. In late winter, deepened snow increased the microbial pool of N, yet decreased soil pools of dissolved organic N and C, and decreased bacterial counts. Fungal mass and hyphal lengths did not change, but remained dominant under both ambient and deepened snow. Deepened snow exacerbated the soluble C-limitation to microbial growth and reduced the P-limitation for microbial respiration. Fungal mass and hyphal length responses to nutrient addition were larger than the bacterial mass or abundance responses and fungi from under deepened snow responded more than those from under ambient snow, indicating a different potential structural and physiological response to substrate availability for these two soil microbial communities. Our results indicate that deeper snow may increase microbial nutrient pools and can alter the physiological functioning of the soil microbial community in late winter, suggesting that microbial N release and its availability to plants during spring thaw may be enhanced.