DIVISION S-3—SOIL BIOLOGY & BIOCHEMISTRY Soil Nitrogen, Microbial Biomass, and Respiration along an Arctic Toposequence

than other regions of the world (Mitchell et al., 1990; Maxwell, 1992). Summer air temperatures in northern To investigate the interactions among mineral N, C availability, Alaska are predicted to increase 3 to 68C over current microbial biomass, and respiration in arctic soils, we sampled soils five times during a growing season from a toposequence on a slope levels, whereas precipitation during the growing season in northern Alaska. The toposequence consisted of six vegetative is predicted to increase 20 to 30% (Maxwell, 1992). The types from the ridge top to the stream bank: lichen heath, dry cassiope, impacts of these potential climate changes on arctic moist carex (Carex spp.), water track, tussock tundra (intertussock), ecosystems are critical for the global C cycle because and riparian. The spatial distribution and temporal variation of soil of the large amount of C stored in these cold regions mineral N, microbial biomass, soil C availability, and C turnover were (Billings et al., 1982; Oechel and Vourlitis, 1994). It is soil type dependent. During the growing season, the concentration generally believed that nutrient availability, especially of soil NH 4 –N decreased in tussock tundra soils but increased in N availability, is the most important factor that will lichen heath soils. Soil C availability at all locations was the highest determine the rates, directions, and magnitudes of C at the beginning of the growing season and declined thereafter. The fluxes and the dynamics of arctic ecosystems under a C availability index (CAI) and the potential C turnover rate increased as soils became wetter. Tussock-forming tundra soil was generally changing climate (e.g., Leadley and Reynolds, 1992; colder than other sites and had high C/N ratios, low amounts of Oechel and Billings, 1992; McKane et al., 1997a). Nutrimineral N, and a low potential C turnover index, and therefore, was ent availability in arctic ecosystems is controlled mainly the least biologically active type. In contrast, water track was the most by decomposition processes, as most of the nutrients biologically active site in the sequence and had the highest C and N are stored in soil organic matter. availability, the highest potential C turnover index, and the highest Many factors influence organic matter decomposimicrobial biomass C and N. The mosaic of diverse plant communities tion. Recent studies on tundra ecosystems suggest that and soil types that comprise arctic landscapes necessitates that accuthe most important factors influencing tundra soil orrate estimates of large-scale C or N budget can only be made by ganic matter decomposition are the quality of the orintegration of all types of plant communities and soils. ganic matter (Nadelhoffer et al., 1991), water conditions, temperature (Oberbauer et al., 1992; Hobbie, 1996), and their interactions (Bridgham et al., 1995). It is well A tundra in northern Alaska forms a rich landknown that microbial biomass is the main biological scape mosaic of diverse soil, topography, lakes, component of most biogeochemical processes in terrescreeks, and vegetation types. The vegetation is repretrial ecosystems (Paul and Voroney, 1980). Microbial sented by an array of different plant growth forms (vasbiomass interacts with ecosystem productivity and nutricular, nonvascular, woody, and herbaceous), which may ent cycling by regulating nutrient availability. Microbial change dramatically across relatively short distances biomass also determines soil C storage and contributes (Bliss, 1981; Chapin and Shaver, 1985). This topographic to atmospheric CO2 via respiration. However, studies variation is strongly related to moisture gradients (e.g., of microbial biomass in arctic ecosystems have been Bliss et al., 1984; Chapin et al., 1988) and the associated scarce (Cheng and Virginia, 1993). chemical, physical, and biological variables, e.g., depth Understanding the potential impacts of climate of permafrost, nutrient availability, soil anoxia, and heat change on arctic ecosystems is critical because large input (Hastings et al., 1989; Shaver and Chapin, 1991; amounts of C are stored below ground in the arctic and Gebauer et al., 1995; Johnson et al., 1996). Thus, beother cold regions of the world. Much current research lowground processes, such as soil respiration, microbial in the Arctic is directed toward developing models for activity, and nutrient mobilization and immobilization, predicting landscape patterns of water discharge, N are key elements that control ecosystem function in availability, vegetation types, C flux, and net primary these ecosystems (Chapin and Shaver, 1989, 1996). productivity (e.g., Reynolds and Leadley, 1992; Leadley Several modeling studies predict that climate change et al., 1996; Shaver, 1996), particularly in relation to in the near future will affect high latitude regions more the potential impacts of climate change (Starfield and Chapin, 1996; McKane et al., 1997a,b). In order for these W. Cheng, Biological Sciences Center, Desert Research Inst., P.O. models to be successful, it is essential that we understand Box 60220, Reno, NV 89506; R.A. Virginia, Environmental Studies the extent of spatial and temporal heterogeneity of soil Program, Dartmouth College, Hanover, NH 03755; S.F. Oberbauer, Dep. of Biological Sciences, Florida Univ., Miami, FL 33199; C.T. processes. At present, such data are lacking. Toward Gillespie, 730 CES/CEVN, 1172 Iceland Ave., Vandenberg Air Force this end, we initiated a study to elucidate patterns of Base, CA 93437-6011; J.F. Reynolds, Dep. of Botany, Duke Univ., soil mineral N, microbial biomass C, soil respiration, Durham, NC 27708; and J.D. Tenhunen, Bayreuther Institut für Ökoand microclimatic conditions in six arctic communities systemforschung, Universität Bayreuth, W-8580 Bayreuth, Germany. Received 4 Oct. 1996. *Corresponding author ([email protected]). Abbreviations: CAI, carbon availability index; LSD, least significant difference; SIR, substrate-induced respiration. Published in Soil Sci. Soc. Am. J. 62:654–662 (1998).