1. Determine the importance of gaseous loss of N from temperate forest ecosystems;
2. Determine the impacts of N deposition on gaseous loss of N from these ecosystems;
3. Test a mechanistic model that relates N gas emissions to N availability and soil moisture content;
4. 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.

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 processes and pathways influence N retention and/or loss. One of the largest areas of uncertainty is gaseous loss. This flux may be large and may be very sensitive to N deposition. To accomplish our objectives, we will sample gas fluxes (NO, N₂O, N₂) on a monthly basis at five sites along an N deposition gradient in the northeast U.S: Fernow Experimental Forest (FN), WV; Catskills State Forest (CS), NY; Hubbard Brook Experimental Forest (HB), NH; Harvard Forest (HF), MA and Bear Brook Watershed (BB), ME. We are also making 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). We are sampling in both N fertilized and unfertilized plots at each location. 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 across a 10 state region (ME, NH, VT, MA, RI, CT, NY, NJ, PA, WV.) using satellite remote sensing and other spatial data sets in a 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.

One of our first tasks was to establish experimental designs to capture the main factors influencing N gas fluxes at our different sites, e.g., topographic position, N availability, species composition. Gas flux measurements began in summer 2000. Regional data sets from modeling and scaling are being assembled.

Because sampling began in summer 2000, only preliminary results are available. However, several encouraging trends emerge from the research:

N gas fluxes may be more important in northeastern temperate forests than previously thought. Data from the Harvard Forest site (Figure 1) shows that NO fluxes are significant relative to N inputs to the site and are much larger than N2O fluxes previously measured at this site. An NO flux rate of 10 ng N cm-2 h-1, extrapolated over 270 days, equals 6.5 kg N ha-1 y-1, which is equivalent to 13% of the annual fertilizer input to the low N plots and nearly 5% of annual fertilizer input to the "high N" plots. See Figure 1.	Figure 1. (Click for larger image.)
N gas fluxes appear to be sensitive to atmospheric deposition. In addition to responding to fertilizer inputs (Figure 1), in the first month when we had data from all five sites along our N deposition gradient (August 2000), fluxes were highest at the high N deposition sites. See Figure 2.	Figure 2. (Click for larger image.)
There appear to be coherent ecosystem controllers of N gas fluxes. For example, plots dominated by different vegetation in the Catskills, NY consistently have distinct patterns of NO flux (Figure 2). These patterns are consistent with other N cycling data collected in these and other studies, i.e. sugar maple has more active cycling than oak or beech. These results suggest that our ecosystem modeling and regional scaling approaches will work. See Figure 3.	Figure 3. (Click for larger image.)