Browsing by Author "Groffman, P. M."

Evaluation of N(2)O flux has been one of the most problematic topics in environmental biogeochemistry over the last 10-15 years. Early ideas that we should be able to use the large body of existing research on terrestrial N cycling to predict patterns of N(2)O flux at the ecosystem scale have been hard to prove due to extreme temporal and spatial variability in flux. The vast majority of the N(2)O flux measurement and modeling activity that has taken place has been process lever and field scale, i.e., measurement, analysis and modeling of hourly and daily fluxes with chambers deployed in field plots. It has been very difficult to establish strong predictive relationships between these hourly and daily fluxes and field-scale parameters such as temperature, soil moisture, and soil inorganic N concentrations. In this study, we addressed the question of whether we can increase our predictive understanding of N(2)O fluxes by examining relationships between flux and environmental parameters at larger spatial and temporal scales, i.e., to explore relationships between annual rather than hourly or daily fluxes and ecosystem-scale variables such as plant community and soil type and annual climate rather than field-scale variables such as soil moisture and temperature. We addressed this question by examining existing data on annual fluxes from temperate forest, cropland, and rangeland ecosystems, analyzing both multiyear data sets from individual sites as well as cross-site comparison of single annual flux values from multiple sites. Results suggest that there are indeed coherent patterns in annual N(2)O flux at the ecosystem scale in forest, cropland, and rangeland ecosystems but that these patterns vary by region and only emerge with continuous (at least daily) flux measurements over multiple years. An ecosystem approach to evaluating N(2)O fluxes will be useful for regional and global modeling and for computation of national N(2)O flux inventories for regulatory purposes but only if measurement programs are comprehensive and continuous.

Exotic earthworms from Europe and Asia are invading many northern forests in North America that currently lack native earthworms, providing an opportunity to assess the role of this important group of invertebrates in forest ecosystems. Research on earthworm invasions has focused on changes in soil structure and carbon (C) and nitrogen (N) cycling that occur following invasion. These changes include the mixing of organic and mineral soil horizons, decreases in soil C storage, and equivocal effects on N cycling. Less well studied are changes in the soil foodwebs that accompany earthworm invasion. Soils of north temperate forests harbor a tremendous diversity of microorganisms and invertebrates, whose distribution and abundance can be substantially altered by earthworm invasion. Furthermore, invasive earthworms can affect understory plant communities, raising concerns over the loss of rare native herbs in some areas. The ecological consequences of earthworm invasion are mediated through physical, geochemical, and biological effects. These effects vary with different earthworm species, as well as with the characteristics of the site being invaded. Earthworm invasions may have important interactions with other rapid changes predicted for northern forests in the coming decades, including climate and land-use change, increased nutrient deposition, and other biological invasions.