Plant-soil-microbial interactions in a northern hardwood forest

Interactions among plants, soil, hydrology, and microbes regulate nutrient cycling and loss in ecosystems. Variability among these factors is likely t o regulate patterns of productivity and N cycling along topographic gradien ts in forest and other terrestrial ecosystems. Our objectives were to deter mine interrelations among spatial and temporal patterns in microbial biomas s, N transformation rates (net N mineralization, net nitrification, denitri fication potential) and soil, plant, and stream variables along an elevatio nal 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 dif ferences among soil layers in the spatial and temporal variation of these s oil charecteristics and their correlations with one another. Broad patterns in microbial biomass and N transformation rates were correlated in space a nd time and varied with elevation, soil horizon, season, and year. Both mic robial 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 rat es and denitrification enzyme activity were greatest at the highest elevati on site, despite the predominance of beech (Fagus grandifolia) in the canop y at that site, which would be expected to inhibit these activities. Differ ences among years in precipitation may have driven annual variation in N tr ansformation rates, which were correlated with annual variation in litter N content. Elevational patterns in nitrification were broadly correlated wit h elevational patterns in stream nitrate (NO3-) concentration, suggesting a n important link between soil N transformations and nutrients in stream wat er along this elevational gradient. These results indicate that interaction s among plant communities, soil characteristics, and soil microbial communi ties determine spatial and temporal patterns of N transformations, which ar e potentially linked to variation in stream nutrient concentrations and out puts at the watershed scale in these northern hardwood forest ecosystems.