Responses of Vegetation and Ecosystem CO2 Exchange to 9 Years of Nutrient Addition at Mer Bleue Bog

Ecosystems DOI: 10.1007/s10021-010-9361-2 Ó 2010 Springer Science+Business Media, LLC Responses of Vegetation and Ecosystem CO2 Exchange to 9 Years of Nutrient Addition at Mer Bleue Bog Sari Juutinen,1,3* Jill L. Bubier,1 and Tim R. Moore2 1Environmental Studies Program, Mount Holyoke College, 50 College Street, South Hadley, Massachusetts 01075, USA; 2Department of Geography, McGill University, 805 Sherbrooke St. W, Montreal, Quebec H3A 2K6, Canada; 3Department of Forest Sciences, University of Helsinki, P.O. Box 27, 00014 Helsinki, Finland ABSTRACT Anthropogenic nitrogen (N) loading has the poten- mum ecosystem photosynthesis (Pgmax) and net CO2 tial to affect plant community structure and func- exchange (NEEmax) were lowered (-19 and -46%, tion, and the carbon dioxide (CO2) sink of peatlands. respectively) in the highest NPK treatment. In the Our aim is to study how vegetation changes, induced following years, while shrub height increased, the by nutrient input, affect the CO2 exchange of a vascular foliar biomass did not fully compensate for nutrient-limited bog. We conducted 9- and 4-year the loss of moss biomass; yet, by year 8 there were no fertilization experiments at Mer Bleue bog, where significant differences in Pgmax and NEEmax between we applied N addition levels of 1.6, 3.2, and the nutrient and the control treatments. At the same 6.4 g N m-2 a-1, upon a background deposition of time, an increase (24–32%) in ecosystem respiration about 0.8 g N m-2 a-1, with or without phosphorus (ER) became evident. Trends in the N-only experi- and potassium (PK). Only the treatments 3.2 and ment resembled those in the older NPK experiment -2 -1 6.4 g N m a with PK significantly affected CO2 by the fourth year. The increasing ER with increas- fluxes. These treatments shifted the Sphagnum moss ing vascular plant and decreasing Sphagnum moss and dwarf shrub community to taller dwarf shrub biomass across the experimental plots suggest that thickets without moss, and the CO2 responses de- high N deposition may lessen the CO2 sink of a bog. pended on the phase of vegetation transition. Overall, compared to the large observed changes in Key words: atmospheric nitrogen deposition; the vegetation, the changes in CO2 fluxes were peatland; carbon; N; P; K; net ecosystem exchange; small. Following Sphagnum loss after 5 years, maxi- Sphagnum; Polytrichum strictum; shrubs. INTRODUCTION to N release from combustion and agriculture. The excess of reactive N has the potential to enhance Atmospheric nitrogen (N) deposition has been plant productivity and alter vegetation composi- elevated in industrialized and populated areas due tion, because N is often the limiting plant nutrient. Therefore, it has been speculated that N deposition Received 23 December 2009; accepted 23 June 2010 could enhance CO2 uptake from the atmosphere Author contributions: SJ conducted the research and analyzed the into ecosystems (Gruber and Galloway 2008; data. JLB and TRM conceived the study and conducted the research, and all three wrote the paper. LeBauer and Treseder 2008). Peatlands have *Corresponding author; e-mail: [email protected] accumulated vast amounts of carbon (C) (Gorham S. Juutinen and others 1991), yet the annual C balance of a peatland is 2007) showed an initial trend of increasing eco- often a very small difference between plant pro- system CO2 uptake in high N [with phosphorus (P) duction and decomposition (for example Alm and and potassium (K)] treatments, which reversed others 1999; Roulet and others 2007). Ombro- after 5 years when the Sphagnum and sparse dwarf trophic bogs are the most nutrient-limited types of shrub community was replaced by a dense shrub peatlands as they receive inputs only from the community having a lower photosynthetic capac- atmosphere (Rydin and Jeglum 2006). The key is- ity. By continuing the study, we wanted to inves- sues are how elevated atmospheric N deposition tigate (1) whether the lower photosynthetic affects the peatland plant communities and whe- capacity and lower net exchange of CO2 were only ther it will increase or decrease the CO2 sink transitional features, and (2) how the change in the function of peatlands. relative contribution of different plant functional Turunen and others (2004) have suggested that groups impacts ecosystem CO2 exchange. elevated N deposition ranging up to 0.8 g N m-2 In this article, we explore the temporal changes in -1 a could have increased C accumulation during vegetation composition and ecosystem CO2 fluxes the last 50 years in ombrotrophic bogs in eastern in a temperate bog, with plots fertilized for Canada. On the other hand, experimental and 8–9 years with N and NPK, and for 4 years with N gradient studies show that deposition higher than only. We investigated the statistical relationships -2 -1 1gNm a influences vegetation composition between vegetation characteristics and CO2 fluxes. in the most nutrient-poor sites, poor fens and bogs, Our hypotheses, following the previous study of and may reduce C accumulation. Typical responses Bubier and others (2007), were that increases in include a reduction in Sphagnum moss abundance dwarf shrub growth would continue in the and invasion or increased abundance of vascular communities affected by the highest fertilization plant species (Heijmans and others 2001; Vitt and treatments, but that increases in ecosystem photo- others 2003; Tomassen and others 2004; Bragazza synthesis would remain small, because a part of the and others 2004; Bubier and others 2007; Limpens new production would be allocated to woody, non- and others 2008). This change may reduce the C photosynthetic tissue. We also hypothesized an in- sequestration in peatlands as Sphagnum mosses are crease in ecosystem respiration because of increas- substantial peat builders through their slow ing shrub biomass and a change in the quality of decomposition, compared to vascular plant tissues substrates for decomposition, from dominantly (Clymo and Hayward 1982; Aerts and others 1999; Sphagnum moss to more easily decomposable shrub Malmer and others 2003; Turetsky 2003; Moore litter, in these altered communities. and others 2007). The coexistence of mosses and vascular plants in bogs is in balance when the moss surface traps the low atmospheric nutrient supply MATERIALS AND METHODS and conversely, shading and litterfall by vascular Study Site and Experimental Set-Up plants hamper moss growth. The balance can be shifted through a process in which saturation of the This study was conducted at Mer Bleue bog near moss layer with N allows more N uptake by vas- Ottawa, Ontario, Canada (45°40¢N, 75°50¢W) with cular plants (Baxter and others 1992; Heijmans and a cool continental climate and a mean annual others 2001; Limpens and Berendse 2003; Malmer temperature of 6.0°C, and mean annual precipita- and others 2003; Bragazza and others 2004; Bubier tion of 944 mm (Canadian Climate Normals 1971– and others 2007). 2000). The experimental site is located in the The general responses of bog vegetation to in- ombrotrophic part of the peatland, where vegeta- creased N deposition are quite well known on the tion is dominated by dwarf shrub species Cham- basis of experiments and long-term monitoring; but aedaphne calyculata Moench, Ledum groenlandicum few studies have assessed the effects of nutrient Oeder, Vaccinium myrtilloides Michx. and Kalmia addition on ecosystem CO2 exchange in nutrient- angustifolia L., and peat mosses Sphagnum magellan- poor peatlands (Saarnio and others 2003; Bubier icum Brid. and Sphagnum capillifolium (Ehrl.) Hedw. and others 2007; Gerdol and others 2008; Lund and Polytricum strictum Brid. is a common, but less others 2009). Most studies have spanned only a abundant, moss. Average aboveground biomass few years, but longer monitoring can show delayed including living moss is about 590 g m-2 (Bubier effects. Nutrient addition may trigger an ecosystem and others 2006). Onset of photosynthesis typically transition, where vegetation changes may not be- occurs soon after the disappearance of snow at the come evident for several years (Wiedermann and end of March to early April, and net CO2 exchange others 2009). Our earlier study (Bubier and others turns to ecosystem loss by November. The bog has Responses of Bog Vegetation and CO2 Fluxes to Fertilization been a long-term C sink of about 20 g C m-2 a-1, August during the summers of 2001, 2003, 2005, and the average contemporary net ecosystem and 2008. Each treatment plot had a pre-installed -2 -1 exchange of CO2 is about 40 g C m a (uptake), 0.6 m 9 0.6 m aluminum collar for the closed based on eddy covariance (EC) measurements made chamber measurements. A 0.5-m tall chamber was on a tower 150 m from the fertilization site (Lafleur used from 2001 to 2005, but was replaced in 2008 by and others 2003; Moore and others 2006; Roulet a 0.9-m tall chamber to accommodate increased and others 2007). At the EC tower, continuous shrub height. Net ecosystem exchange of CO2 (NEE) measurements of water table depth and thermal was measured using a clear plexan chamber equip- regime have been made since 1998. ped with fans and a cooling unit to ensure proper gas Background inorganic wet N deposition in this mixing and humidity and temperature control. region is about 0.8 g N m-2 a-1 (Turunen and Measurements were conducted only when photo- others 2004) and in this experiment the N fertil- synthetic photon flux density (PPFD) exceeded ization levels equal 5, 10, and 20 times the summer 1000 lmol photons m-2 s-1, the level of light satu- time wet N deposition and treatments are abbrevi- ration for photosynthesis. After each NEE mea- ated accordingly (Table 1). Fertilization experiment surement, the chamber was vented and covered I was started in the year 2000 with four treatments, with an opaque shroud to measure ecosystem res- and two more treatments were added in the year piration (ER).

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