Abstract
Leachate from litter and vegetation penetrates permafrost surface soils during thaw before being exported to aquatic systems. We know this leachate is critical to ecosystem function downstream and hypothesized that thaw leachate inputs would also drive terrestrial microbial activity and nutrient uptake. However, we recognized two potential endpoint scenarios: vegetation leachate is an important source of C for microbes in thawing soil; or vegetation leachate is irrelevant next to the large background C, N, and P pools in thaw soil solution. We assessed these potential outcomes by making vegetation leachate from frozen vegetation and litter in four Arctic ecosystems that have a variety of litter quality and soil C, N, and P contents; one of these ecosystems included a disturbance recovery chronosequence that allowed us to test our second hypothesis that thaw leachate response would be enhanced in disturbed ecosystems. We added water or vegetation leachate to intact, frozen, winter soil cores and incubated the cores through thaw. We measured soil respiration throughout, and soil solution and microbial biomass C, N, and P pools and gross N mineralization immediately after a thaw incubation (−10 to 2°C) lasting 6 days. Vegetation leachate varied strongly by ecosystem in C, N, and P quantity and stoichiometry. Regardless, all vegetated ecosystems responded to leachate additions at thaw with an increase in the microbial biomass phosphate flush and an increase in soil solution carbon and nitrogen, implying a selective microbial uptake of phosphate from plant and litter leachate at thaw. This response to leachate additions was absent in recently disturbed, exposed mineral soil but otherwise did not differ between disturbed and undisturbed ecosystems. The selective uptake of P by microbes implies either thaw microbial P limitation or thaw microbial P uptake opportunism, and that spring thaw is an important time for P retention in several Arctic ecosystems.
Highlights
During spring thaw in the Arctic, litter and vegetation is leached by the melting snowpack (Wickland and others 2012)
We evaluated the effects of fresh plant leachates on soil microbial processes in Alaskan tundra systems that were undisturbed or in various stages of the thaw-slump/recovery cycle during the critical spring thaw period; we tested the following hypotheses: (1) leachates are important stimulants to soil microbial activity and nutrient immobilization at thaw; and (2) the leachate response is enhanced in disturbed ecosystems and diminishes as ecosystems recover and organic matter accumulates
Because of the differences in vegetation chemistry, the leachates, which were added at a constant volume to reflect what would happen in situ, varied in their relative contribution to labile soil C, N, and P stocks (Table 3)
Summary
During spring thaw in the Arctic, litter and vegetation is leached by the melting snowpack (Wickland and others 2012). Mineral soil remains frozen during this early spring snowmelt, the soil surface organic layer thaws enough to allow the leachate to percolate (Hinzman and others 1991); the nutrients may be taken up and metabolized by microorganisms in this zone (Qualls and others 2002; Cleveland and others 2004) These leachate inputs occur at a time associated with wide swings in microbial biomass and activity—such as a burst of soil organic matter mineralization, a crash in microbial biomass, and accelerated community turnover (Schimel and Clein 1996; Edwards and others 2006; Buckeridge and others 2013). It is unclear how important the flush of plant and litter leachates are to fueling microbial processes during the rapid and intense spring thaw in Arctic ecosystems
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