Topographic Patterns of Above- and Belowground Production and Nitrogen Cycling in Alpine Tundra

Topography controls snowpack accumulation and hence growing-season length, soil water availability, and the distribution of plant communities in the Colorado Front Range alpine. Nutrient cycles in such an environment are likely to be regulated by interactions between topographically determined climate and plant species composition. We investigated variation in plant and soil components of internal N cycling across topographic gradients of dry, moist, and wet alpine tundra meadows at Niwot Ridge, Colorado. We expected that plant production and N cycling would increase from dry to wet alpine tundra meadows, but we hypothesized that variation in N turnover would span a proportionately greater range than productivity, because of feedbacks between plants and soil microbial processes that determine N availability. Plant production of foliage and roots increased over topographic sequences from 280 g·m22·yr21 in dry meadows to 600 g·m22·yr21 in wet meadows and was significantly correlated to soil moisture. Contrary to our expectation, plant N uptake for production increased to a lesser degree, from 3.9 g N·m22·yr21 in dry meadows to 6.8 g N·m22·yr21 in wet meadows. In all communities, the belowground component accounted for the majority of biomass, production, and N use for production. Allocation belowground also differed among communities, accounting for 70% of total production and 80% of N use for production in dry meadows compared to 55% of production and 65% of N use for production in moist meadows. Variation in microbial processes was highly related to soil moisture, and we found very consistent relationships among microbial respiration, gross N mineralization, and N immobilization among communities. These results indicate that the topographic soil moisture gradient is in fundamental control of the patterns of N turnover among communities and that differences in plant species do not appear to be as important.