Effects of N Deposition on Gaseous N Loss from Temperate Forest EcosystemsEPA Grant Number: R827674
Title: Effects of N Deposition on Gaseous N Loss from Temperate Forest Ecosystems
Investigators: Groffman, Peter M. , Adams, Mary Beth , Fernandez, Ivan , Potter, Christopher , Rustad, Lindsey , Verchot, Louis V.
Institution: Cary Institute of Ecosystem Studies , USDA Forest Service , University of Maine
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1999 through September 30, 2002
Project Amount: $894,361
RFA: Regional Scale Analysis and Assessment (1999) RFA Text | Recipients Lists
Research Category: Ecosystems , Ecological Indicators/Assessment/Restoration
Description:Objectives: We are seeking funds to increase our understanding the effects of N deposition on gaseous N loss from temperate forest ecosystems. Our specific objectives are to:
- Determine the importance of gaseous loss of N from temperate forest ecosystems;
- Determine the impacts of N deposition on gaseous loss of N from these ecosystems;
- Test a mechanistic model that relates N gas emissions to N availability and soil moisture content;
- Develop a new and more mechanistic version of the daily NASA-CASA ecosystem model for N gas emissions that can be applied at the regional level using satellite remote sensing and other spatial data sets in a geographic information system (GIS) format. This new simulation model will be used to assess trends in N cycling over gradients of N deposition in the northeast U.S. and to project changes in N gas fluxes with changing air pollution.
Project Summary: While much effort has gone into determining the fate of atmospheric N in temperate forest ecosystems, many uncertainties remain as to just where N is stored and what specific biogeochemical processes and pathways influence N retention and/or loss. The effects of chronic, low-level N inputs probably produce subtle changes in N cycling, so detection of the effect of increased inputs is difficult. The most common approach to examining the effects of chronic N deposition is to determine whether N is "leaking" to streams and groundwater. However, this approach is useful only for identifying severely impacted areas and does not help identify susceptible areas before major changes occur.
One of the largest areas of uncertainty in our understanding of the fate of atmospheric N in temperate forest ecosystems is gaseous loss. Several studies have examined N2O and/or NO loss in temperate forest soils and the effects of N deposition on emissions, but there has been no systematic effort to quantify the total gaseous N loss from these ecosystems. This flux may be large and may be very sensitive to N deposition. If so, gaseous N loss may be a better indicator of the impacts of N deposition on the N cycle of an ecosystem than is NO3- leaching.
Approach:We will sample gas fluxes on a monthly basis at five sites along an N deposition gradient in the northeast U.S: Fernow Experimental Forest, WV; Catskills State Forest, NY; Hubbard Brook Experimental Forest, NH; Harvard Forest, MA and Bear Brook Watershed, ME. We will make several additional measurements of factors known to control flux rates (e.g., N pool sizes and turnover rates, denitrification rates, soil temperature, soil pH, and soil moisture). At each of these sites, watersheds or plots are currently being artificially enriched or will be enriched during the course of this study with additional simulated N deposition in the form of fertilizer. Measurements of gas fluxes and controlling factors in these N enriched plots will extend the power to test our hypotheses and model the mechanisms controlling the fluxes of these gases. These data will then be used to develop a new and more mechanistic version of the daily NASA-CASA ecosystem model for N gas emissions that can be applied at the regional level using satellite remote sensing and other spatial data sets in a geographic information system (GIS) format. The model currently predicts N oxide flux from soil in a submodel based on the hole-in-the-pipe (HIP) model. In the proposed research, we will further adapt the NASA-CASA model to the northeast US by refining the hole-in-the-pipe formulation and applying it across a 10 state region (ME, NH, VT, MA, RI, CT, NY, NJ, PA, WV. Possible refinements for the NASA-CASA model include separating the N flow pipes for nitrification and denitrification and improving the mechanisms of the allocation scheme for partitioning available mineral N among plant, microbial and soil organic matter sinks. This new simulation model will be used to assess trends in N cycling over gradients of N deposition in the northeast U.S. and to project changes in N gas fluxes with changing air pollution.
The proposed studies are important for several reasons. One of the least well understood facets of the complex N cycle is loss of N in gaseous forms from the soil. Current thinking holds that temperate forests are not important sources of N oxides to the atmosphere. However, the soil N2O source from these ecosystems is still uncertain (estimates range from 0.1 to 2.0 Tg yr-1). The importance of the soil source of NO is less well understood, but we know that soil emissions of NO in the US account for only about 10% of total sources, but summer soil emissions in rural areas can be important enough to induce O3 formation where the atmospheric concentration of volatile organic compounds is high (i.e. above forests). Whether or not temperate forest soils are important sources of nitrogen oxides to the atmosphere on a global scale, quantifying this flux is important from the perspective of understanding the fate of N deposited on these ecosystems from the atmosphere.
Expected Results:The consequences of N deposition on N oxide production in soils have not yet been addressed by the scientific community. The research that we propose here will take advantage of existing N enrichment studies to provide this systematic and regional evaluation, not only of the flux of N oxides from the soil to the atmosphere, but also on the factors controlling the spatial variation in these fluxes. The modeling effort will permit us to place constraints on the regional contribution of the soil source of N gases and make projections of the impact of increasing N saturation on the regional atmospheric budget of these gases.
On the practical side, regional quantification of gaseous N loss and improved understanding of the spatial variability of this process will be important for development of critical load tolerances for N deposition and for evaluating the importance of gaseous loss to atmospheric loading of trace gases. The concept of critical loads has been developed as a means to establish limits for pollution inputs to ecosystems from a purely ecological perspective, which is then used by policy makers to establish target loads. Mass balance models have been successfully used in the establishment of critical loads for sulfur deposition, but strong and variable biotic retention of N within forest ecosystems has complicated their application to N loading. The mass balance approach generally assumes that gaseous N loss is negligible, which we feel is erroneous. The research that we propose here will provide fundamental information required for the refinement of the mass balance approach and quantification of this often ignored and probably important flux.