Vegetation Leachate During Arctic Thaw Enhances So

Authors: Kate M Buckeridge Sean M Schaeffer Joshua P Schimel

Publish Date: 2015/12/29

Volume: 19, Issue: 3, Pages: 477-489

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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 ecosystemsKMB conceived of and designed the study performed research analyzed the data and wrote the paper SMS conceived of and designed the study performed research analyzed the data and wrote the paper and JPS conceived of and designed the study and wrote the paperDuring spring thaw in the Arctic litter and vegetation is leached by the melting snowpack Wickland and others 2012 It is usually assumed that because soils are still frozen that most of this leachate is flushed laterally into streams and lakes where the leachate provides an important nutrient subsidy Michaelson and others 1998 Amon and others 2012 However although 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 ecosystemsThere are two alternate hypotheses for how leachate may affect microbial processes in the surface soil at thaw The first is that the dissolved organic C in leachate provides a pulse of highquality resources that fuels activity of resourcelimited microbes as it does in incubations of boreal vegetation leachate without soil Wickland and others 2007 when leachate is added to summer alpine soils Cleveland and others 2004 or during the spring freshet in Arctic aquatic ecosystems Crump and others 2003 Judd and others 2006 Balcarczyk and others 2009 where terrestrial plant C dominates snowmelt inputs Amon and others 2012 This hypothesis is consistent with tundra ecosystems being strongly nutrientlimited Shaver and Chapin 1980 Sistla and others 2012 additions of C N and P alter aboveground community composition Hobbie and others 2005 belowground community composition Clemmensen and others 2006 Deslippe and others 2011 and ecosystem C N and P storage Jonasson and others 1996 Mack and others 2004 Alternatively leachate may represent a small enough proportion of the total amount of labile material mobilized at soil thaw that it may be effectively irrelevant as a fuel for soil microbes enduring the environmental stress of thaw Northern vegetation leachate DOC appears to range in biodegradability with deciduous shrub leaves and mosses having a greater proportion of readily mineralizable DOC than the more common evergreen leaves and stems and deciduous shrub stems Wickland and others 2007 In addition thaw leachate additions occur when soils are frozen albeit at temperatures that allow reduced microbial activity −2 to 0°C McMahon and others 2009 and already replete with autochthonous soil solution thaw nutrients Judd and Kling 2002 Buckeridge and Grogan 2010 These alternate hypotheses represent of course end points of a spectrum of effects rather than being discrete alternatives Distinguishing which pattern holds true or rather where along the spectrum of possibilities is important to understand the factors that drive the abrupt pulse of microbial activity that occurs at thawThe quality of leachates as microbial substrates is controlled by the chemical composition of the vegetation Cleveland and others 2004 Wickland and others 2007 Arctic landscapes contain a juxtaposition of different ecosystem types each with its own patterns of vegetation productivity nutrient content Giblin and others 1991 Shaver and Chapin 1991 and DOC quality and concentration Armstrong and others 2012 As the spring thaw is a critical time for nutrient release in arctic ecosystems the interaction of fresh plant leachates with the extant soil microbial community provides a possible driver that could either stimulate mineralization of remaining native organic matter priming or could serve as a substrate for growth and so immobilize nutrients that might otherwise be lostArctic ecosystems also vary in their disturbance history A common disturbance in the Arctic results from permafrost thaw on slopes melting at the top of the permafrost layer can allow the thawed soil to detach and slump downslope exposing fresh material which can then continue to thaw—these thermoerosional features are known as retrogressive thaw slumps or more commonly thermokarsts—and are common in northern Alaska Bowden and others 2008 Gooseff and others 2009 and western Canada Lantz and Kokelj 2008 Landscape succession after thermokarst is characterized by rapid initial ~50 years plant growth on the denuded mineral soil which only accumulates a new organic horizon slowly Pizano and others 2014 The rapid regrowth early in recovery requires a large supply of nutrients Arctic ecosystems are strongly dependent on the recycling of nutrients from organic matter Chapin and Bloom 1976 so it is unclear where those nutrients come from in this bare soil and what processes regulate nutrient mineralization through the recovery of these thermokarst systems Vitousek and Reiners 1975 Rastetter and others 2013 Spring thaw vegetation leachates are a potential source of nutrients and this allochthonous upslope source may be particularly important in early low organic matter stages of ecosystem recoveryWe evaluated the effects of fresh plant leachates on soil microbial processes in Alaskan tundra systems that were undisturbed or in various stages of the thawslump/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 We collected soils from a variety of arctic ecosystem types and in one from several stages of thermokarst disturbance/recovery we incubated them through thaw with and without added vegetation leachate We measured respiration through the incubation and soil and microbial nutrient pools and gross N mineralization at the end of the thaw incubation

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