965 Ecology, 82(4), 2001, pp. 965–978  2001 by the Ecological Society of America PLANT–SOIL–MICROBIAL INTERACTIONS IN A NORTHERN HARDWOOD FOREST PATRICK J. BOHLEN, 1,5 PETER M. GROFFMAN, 1 CHARLES T. DRISCOLL, 2 TIMOTHY J. FAHEY, 3 AND THOMAS G. SICCAMA 4 1 Institute of Ecosystem Studies, Box AB, Millbrook, New York 12545 USA 2 Syracuse University, Department of Civil and Environmental Engineering, Syracuse, New York 13244 USA 3 Cornell University, Department of Natural Resources, Ithaca, New York 14853 USA 4 Yale University, Forestry School, New Haven, Connecticut 06512 USA Abstract. Interactions among plants, soil, hydrology, and microbes regulate nutrient cycling and loss in ecosystems. Variability among these factors is likely to regulate patterns of productivity and N cycling along topographic gradients in forest and other terrestrial ecosystems. Our objectives were to determine interrelations among spatial and temporal patterns in microbial biomass, N transformation rates (net N mineralization, net nitrification, denitrification potential) and soil, plant, and stream variables along an elevational gradient (525–775 m) at the Hubbard Brook Experimental Forest (HBEF), a northern hardwood forest in the White Mountains of New Hampshire, USA. We examined these relationships in the forest floor (O e and O a horizons) and upper mineral soil to assess the contribution of these different layers to overall microbial biomass and N cycling rates and to examine potential differences among soil layers in the spatial and temporal variation of these soil characteristics and their correlations with one another. Broad patterns in microbial biomass and N transformation rates were correlated in space and time and varied with elevation, soil horizon, season, and year. Both microbial biomass and N cycling activities were greater in summer than in fall or spring, although the magnitude of seasonal differences was much greater for the N cycling activities than for microbial biomass. Nitrification rates and denitrification enzyme activity were greatest at the highest elevation site, despite the pre- dominance of beech (Fagus grandifolia) in the canopy at that site, which would be expected to inhibit these activities. Differences among years in precipitation may have driven annual variation in N transformation rates, which were correlated with annual variation in litter N content. Elevational patterns in nitrification were broadly correlated with elevational patterns in stream nitrate (NO 3 - ) concentration, suggesting an important link between soil N transformations and nutrients in stream water along this elevational gradient. These results indicate that interactions among plant communities, soil characteristics, and soil microbial communities determine spatial and temporal patterns of N transformations, which are po- tentially linked to variation in stream nutrient concentrations and outputs at the watershed scale in these northern hardwood forest ecosystems. Key words: elevation gradient; forest soils; hydrology; nitrate; nitrogen transformations; north- ern hardwood forest; nutrient cycling; plant–soil–microbe interactions; soil microbial activity; soil microbial biomass. INTRODUCTION Microorganisms play important roles in regulating ecosystem processes ranging from nutrient cycling, to soil carbon (C) storage, trace gas fluxes, transformation of aqueous solutes, and processing of water pollutants (Mooney et al. 1987, Holden and Firestone 1997, Schlesinger 1997, Groffman and Bohlen 1998). Inter- actions among plants, soil, hydrology, and microor- ganisms regulate nutrient cycling processes in ecosys- tems. These interactions vary in time and space, greatly complicating ecosystem-scale assessments of nutrient Manuscript received 5 April 1999; revised 3 February 2000; accepted 6 March 2000. 5 Present address: MacArthur Agro-Ecology Research Center, 300 Buck Island Ranch Rd., Lake Placid, Florida 33852 USA. E-mail: pbohlen@archbold-station.org loss following disturbance, effects of atmospheric de- position and climate change, and responses to changes in species composition (Ojima et al. 1991, Vitousek 1994). Although ecosystem research has focused on specific nitrogen (N) cycling processes (e.g., N min- eralization, nitrification, stream water nitrate (NO 3 - ) outputs), few studies have evaluated these processes in a broader context of the coupled plant–soil–microbial system. Few studies have addressed the ecosystem- scale controls on microbial biomass and activity, es- pecially in non-agricultural ecosystems. Given the recognized importance of microorganisms in regulating transfers of C, N, and other elements, there is surprisingly little information on variation in microbial biomass and activity in relation to vegetation type, soil factors, climate, and other controllers of eco- system structure and function (Zak et al. 1994). There