2139 Special Feature Ecology, 80(7), 1999, pp. 2139���2150 q 1999 by the Ecological Society of America WITHIN-STAND NUTRIENT CYCLING IN ARCTIC AND BOREAL WETLANDS SVEN JONASSON1,3 AND GAIUS R. SHAVER2 1Department of Plant Ecology, University of Copenhagen, ��. Farimagsgade 2D, DK-1353 Copenhagen K, Denmark 2Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts 02543 USA Abstract. The aims of this article are to characterize the nutrient regime in arctic and boreal wetlands and to review published data on various aspects of within-stand nutrient- cycling. Most northern wetlands, such as boreal and arctic ombrogenous bogs and most arctic mesic and wet ecosystem types, are poor in inorganic, plant-available nutrients be- cause mineralization is restricted due to low temperatures and anoxic soils. This is partic- ularly true in the Arctic because nutrient inputs from external sources quite often are small, and nutrient pools in the vegetation also are small. By contrast, the soil and the microbial biomass often contain large stocks of organically fixed nutrients that are largely unavailable to plants. The vegetation is adapted to the low availability of nutrients by the perennial nature of both entire plants and plant tissues, which minimizes the annual nutrient losses and reduces the demand for nutrient uptake to produce new tissues. Furthermore, many of the dominant plant life-forms can use soil organic nitrogen (N), either by direct uptake or through connections with ericoid or ectotrophic mycorrhizae, which reduces competition from other plant groups without the ability to utilize organic N. As soil fertility increases, productivity also increases. However, at least in geogenous (������mineral fed������) Arctic wetlands, vascular-plant turnover time is surprisingly constant across nutrient and moisture gradients. This is mainly because communities with a species stock having high leaf turnover rates generally have slow stem and belowground turnover rates and vice versa. Turnover times of the bulk soil organic matter, by contrast, range from a few decades in the most productive systems to several centuries in the most nutrient-deficient and wettest ecosystem types. This is because decomposition rates of the soils are controlled by varying environmental conditions, and nutrients can be strongly immobilized within the soil���microbial ecosystem compartment if the soil is nutrient poor. The composition of the vegetation regulates the turnover of plants and plant nutrients but plays a minor role for the function of the entire plant���soil system. Hence, characteristics of nutrient cycling at the stand level are relatively unimportant for the function of the overall soil���plant systems, and characteristics of the plants (nutrient resorption, tissue type, longevity, etc.) mirror the functioning of the system rather than play a major role in regulating ecosystem function. However, several uncertainties about the detailed function of the ecosystems are still un- resolved and are highlighted in this compilation. Key words: arctic boreal decomposition leaf longevity microbial nutrient immobilization my- corrhiza nutrient cycling, within-stand and ecosystem nutrient resorption and storage nutrient turn- over soil nutrient characteristics tissue longevity wetlands. INTRODUCTION Geogenous (������mineral-fed������), often-acidic fens and ombrogenous (������rain-fed������) bogs (terminology follows Bridgham et al. 1996) are dominant wetland types in the boreal, subarctic and low-arctic zones, while acidic, wet sedge���moss tundra or circum-neutral tundra mead- ows become increasingly dominant from the low Arctic towards middle Arctic (Frenzel 1983, Bliss and Mat- veyeva 1992). These northern wetlands are dominated by a restricted number of plant functional types (see Manuscript received 28 July 1997 revised 3 May 1998 accepted 26 May 1998 final version received 13 August 1998. For reprints of this Special Feature, see footnote 1, p. 2137. 3 E-mail: svenj@bot.ku.dk Shaver 1995) or growth forms, within which the species are assumed to react similarly to environmental influ- ences and, conversely, also may influence the environ- ment in a similar way (Chapin et al. 1996). Deciduous shrubs or dwarf shrubs and grasses are most common in moderately acidic, eutrophic fens and geogenous tundra wetlands, while evergreen shrubs or dwarf shrubs, sedges, and mosses are the most common plant functional types in acidic wetlands (Malmer et al. 1992, Bridgham et al. 1996). Among the functional types, Sphagnum mosses are mostly restricted in their occurrence to low-pH bogs and fens where they usually are dominant (Verhoeven et al. 1990). As pH and alkalinity increase, they are replaced by so called ������brown������ mosses (Amblystegi-

2140 Ecology, Vol. 80, No. 7 SVEN JONASSON AND GAIUS R. SHAVER Special Feature FIG. 1. Cumulative absorption and throughfall of NO32 and NH41 in Sphagnum capillifolium during an 8-d period of atmospheric deposition to a subarctic ombrogenous mire. Data are from Woodin and Lee (1987), and the figure is mod- ified from Lee and Woodin (1988: Fig. 2). aceae) (Malmer et al. 1992, Bridgham et al. 1996). A succession of sedge species, e.g., of the genuses Eri- ophorum, Carex, and Scirpus, generally dominates dif- ferent sections of the broad interval of pH and soil moisture from bogs to eutrophic fens, and sedges to- gether with grasses are common in tundra wetlands (Bliss and Matveyeva 1992). Evergreen, ericaceous sclerophyllous shrubs and dwarf shrubs occur mostly in low-pH wetlands, and several species, such as Vac- cinium vitis-idaea and Empetrum nigrum also have a distribution along broad soil-moisture gradients from bogs and mesic tundra, to dry forests and arctic heaths (Larsen 1982, Moore 1981). Plant productivity is generally higher in fens than in bogs (Bradbury and Grace 1983) coincident with a gen- eral increase of electrolytes and pH from acidic om- brogenous bogs, through low-pH fens to alkaline, high- pH fens (Sjo ��rs 1950). In tundra, plant productivity de- creases with increasing latitude (Moore 1989, Shaver et al. 1997) and as in fens and bogs (Malmer 1988, Aerts et al. 1992), plant growth usually increases with addition of phosphorus (P) or nitrogen (N) (Shaver and Chapin 1995), suggesting main growth limitation by low availability of these elements. In the following, we first summarize some common and differentiating traits in soil nutrient conditions and nutrient transformations in northern wetlands, with fo- cus on N and P. We then discuss the nutritional char- acteristics of the common plant functional groups, and end with a synthesis of controls of within-stand and ecosystem nutrient cycling. NUTRIENT INPUT AND TRANSFORMATION IN NORTHERN WETLANDS Input and retention of nutrients from external sources Both external and internal sources contribute to the supply of plant nutrients in northern wetlands. Geo- genous wetlands and wet tundra receive external nu- trients from the surrounding watersheds, from ground- water, or by atmospheric deposition of particles or sol- utes in the precipitation, and from atmospheric N2 fix- ation. In contrast, the ombrogenous, precipitation- dominated wetlands receive external nutrients mainly from atmospheric sources. The nutrient input from the atmosphere generally is lower in the arctic and boreal regions than in the temperate region because the air is relatively unpolluted and precipitation is low (Van Cle- ve and Alexander 1981, Chapin 1983). However, some elements can be deposited in large amounts regionally. For instance, sodium (Na) and magnesium (Mg), of oceanic origin, show steep gradients, with high depo- sition near the coast and low deposition in the conti- nental inlands (Malmer 1988, Malmer et al. 1992). Nitrogen and P in precipitation are efficiently trapped by the mosses, particularly by Sphagnum species (Fig. 1), while other elements are less tightly retained (Mal- mer 1992, Lee and Woodin 1988). Atmospheric de- position of N and P, together with N2 fixation, are con- sidered to be the main sources of these elements for mosses in ombrogenous bogs and acidic fens (Malmer 1992). For N in the Arctic, both deposition and mi- crobial N2 fixation generally ranges between 30 and 250 mg��m22��yr21 (Van Cleve and Alexander 1981, Chapin and Bledsoe 1992). These amounts are an order of magnitude lower than in temperate ecosystems (Chapin 1983). Malmer and Nihlga ��rd (1980) estimated the annual input of dry- and wet-deposited N to a sub- arctic mire to be ,100 mg��m22��yr21, with N2 fixation contributing another 180 mg N/m2 annually (Granhall and Selander 1973). Together, this corresponded to 25% of the annual incorporation of N into the plant biomass. Loss of N in outflowing water was insignif- icant, indicating that the mire trapped and immobilized N efficiently in plant biomass, deposited it in litter, and eventually incorporated it into the biologically inert peat in the anaerobic deep peat (catotelm) horizon. Once incorporated in the Sphagnum mosses, the im- mobilization capacity for ions depends on the growth demand of the mosses for the element (Lee and Woodin 1988). This was illustrated by Aerts et al. (1992), who