Ecosystem processes are controlled by a variety of abiotic and biotic factors. For example, decomposition and nitrogen cycling rates are controlled by both climatic gradients and plant functional traits. The objective of this study was to determine the relative importance of abiotic and biotic factors that have been proposed to influence nitrification rates. I measured nitrification potential using the soil-slurry method, soil temperature, soil carbon:nitrogen (C:N) mass ratio, total soil N, litter mass, and soil texture on 82 plots in ponderosa pine forests near Flagstaff, Arizona. In addition to these abiotic factors, I was interested in determining the relative importance of four potentially important biotic factors: ponderosa pine density, understory litter quantity (i.e., above- and below-ground herbaceous standing crop), functional diversity (i.e., a multivariate index that accounts for both the composition of the community and the functional traits expressed in the community), and litter quality. Litter quality was quantified as a community-aggregated trait, which reflects the location of the community along the global ‘leaf economics spectrum’. I used structural equation modeling (SEM) to evaluate the importance of these factors in the context of a multivariate model.
Results/Conclusions
Model results suggest that abiotic factors, such as soil temperature, soil C:N mass ratio, and total soil N are the dominant factors controlling nitrification potential in these forest soils. However, after statistically controlling for these abiotic factors, community-level litter quality was an important predictor of nitrification potential. Despite a significantly positive bivariate relationship between functional diversity and nitrification potential, functional diversity was not an important predictor of nitrification potential in the context of the model. This study shows that the leaf economics spectrum is the primary axis of specialization within the flora and that this spectrum has consequences for ecosystem processes. This model supports Grime’s ‘mass ratio’ theory, which proposes that plant species controls on ecosystem processes are in proportion to their relative input to primary production and that functional diversity is less important for this critical link in the internal cycling of nitrogen.
