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 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 (Oₑ and Oₐ 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 predominance 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 (NO₃ ⁻) 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 potentially linked to variation in stream nutrient concentrations and outputs at the watershed scale in these northern hardwood forest ecosystems.