Grantee Research Project Results
Final Report: Effects of N Deposition on Gaseous N Loss from Temperate Forest Ecosystems
EPA Grant Number: R827674Title: Effects of N Deposition on Gaseous N Loss from Temperate Forest Ecosystems
Investigators: Groffman, Peter M. , Verchot, Louis V. , Potter, Christopher , Adams, Mary Beth , Fernandez, Ivan , Rustad, Lindsey
Institution: Cary Institute of Ecosystem Studies , University of Maine , USDA
EPA Project Officer: Packard, Benjamin H
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: Aquatic Ecosystems , Ecological Indicators/Assessment/Restoration
Objective:
The objectives of this research project were to: (1) determine the importance of gaseous loss of nitrogen (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; and (4) develop a new and more mechanistic version of the daily National Aeronautics and Space Administration-Carnegie Ames Stanford Approach (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 United States, and to project changes in N gas fluxes with changing air pollution.
Summary/Accomplishments (Outputs/Outcomes):
Although much effort has gone into determining the fate of atmospheric N in temperate forest ecosystems, many uncertainties remain as to where N is stored and which 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 sampled gas fluxes (nitric oxide [NO], nitrous oxide [N2O]) on a monthly basis over two growing seasons (2000 and 2001) at five sites along an N deposition gradient in the Northeast United States: Fernow Experimental Forest (FEF), WV; Catskills State Forest, NY; Hubbard Brook Experimental Forest, NH; Harvard Forest (HF), MA; and Bear Brook watershed (BBW), ME. Sampling took place in both N fertilized and unfertilized plots at four of the five locations. Monthly sampling was augmented by several additional sampling efforts to characterize short-term responses of fluxes to rainfall, diurnal temperature changes, and fertilization events. We also made 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).
The flux and ancillary controlling factor data were 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 was used to assess trends in N cycling over gradients of N deposition in the Northeast United States, and to project changes in N gas fluxes with changing air pollution. The flux measurements to date have revealed some interesting and novel findings. Perhaps most novel is the observation that NO fluxes are significant in northeastern forests and much higher than N2O fluxes, which is frequently the only gas measured. Data from the HF site, where two temperate forest stands have been fertilized since 1988 with different levels of N to simulate elevated rates of atmospheric N deposition, illustrate these results quite well (see Figure 1, Venterea, et al., 2003a). Plots within a red pine stand, which were treated with either low (50 kg N ha-1 y-1) or high (150 kg N ha-1 y-1) levels of N, displayed elevated NO fluxes (100-200 µg N m-2 h-1), but only the high N treatment plot within a mixed hardwood stand displayed elevated NO fluxes. N2O fluxes in the N-treated plots were generally less than 10 percent of NO fluxes. The estimated total NO emissions were similar in the hardwood and pine high N plots, representing 3.0 percent and 3.7 percent, respectively, of total N inputs to the high N plots during June 2000-November 2001. The integrated NO mass flux from the pine low N addition plot represented the greatest proportion (8.3 %) of total N inputs. Estimated emissions of NO from the hardwood and pine control plots represented 1.3 and 2.3 percent, respectively, of total inputs. Total N2O emissions were less than 5 percent of total NO emissions and less than 0.3 percent of total N inputs in all plots. Soil NO fluxes were of the same order of magnitude as the other major N loss mechanism in these plots (i.e., NO3- leaching below the root zone). Typical summertime NO fluxes (100-200 µg N m-2 h-1) were equivalent to 15-45 percent of annual NO3- leaching losses in the high N plots and greater than 100 percent of NO3- losses in the pine low N plots estimated for 1996 (Magill, et al., 2000).
Figure 1. Total Estimated Emissions (kg N -1 y-1) of (a) NO and (b) N2O During June 2000 Through November 2001 Versus Annual N Deposition Rates in Hardwood and Pine Forest Plots. From Venterea, et al. (2003a).
Ancillary measurements of soil N transformations produced several interesting findings related to the mechanisms of N gas production and N saturation in HF soils (Venterea, et al., in press, 2004a). Gross NO production rates increased in a manner that was consistent with nitrification rate increases and represented 5-16 percent of the nitrification rate in organic soils exhibiting symptoms of N saturation. Laboratory NO consumption rates were equivalent to half lives less than or equal to 20 seconds, and estimates of field subsurface NO production indicated that greater than or equal to 96 percent of the NO produced was consumed before being emitted at the soil surface. Diffusion-reaction model simulations were consistent with laboratory and field results in suggesting that NO production, combined with rapid transformation, may have contributed up to 25 percent of total gross NO3- production. Model simulations also suggested that gaseous NO diffusion will influence the proportion of NO that is converted to NO3- and the transport of NO3- through the soil profile.
Measurements at other sites along our regional N deposition gradient confirm the results observed at HF. In high- and low-elevation plots at the FEF, and in hardwood- and softwood-dominated plots at the BBW, NO emissions in N-amended watersheds were higher than emissions in the respective reference watersheds (see Figure 2). Peak emission rates ranged from approximately 1 µg NO-N m h-1 in the softwood plots at BBW to 100 µg NO-N m-2 h-1 in the high- and low-elevation plots at FEF, compared to less than 0.1-20 µg NO-N m h-1 in the control watersheds. Laboratory measurements indicated that nitrification was the dominant source of NO production in soils from both forests.
Figure 2. NO Fluxes in Control and N-Amended Plots at: (a) FEF and (b) BBW (Mean ± Standard Error, n = 2). Bars with different letter designations are significantly different based on least significant difference comparisons of N-amended versus unamended soils within each site and elevation or species with confidence level indicated as follows: * = P < 0.10; ** = P < 0.05; *** = P < 0.01. May 2001 data from FEF high-elevation plots and August 2000 data from BBW softwood control plots were not obtained. (From Venterea, et al., in press, 2004b.)
A comparison of results from all of the sites shows a strong relationship between total N input from deposition and fertilizer and NO flux (see Figure 3). This relationship is not present when the analysis is restricted to the unamended plots (see Figure 3), likely because of landscape controls on flux at each site (discussed below). Still, our results suggest that elevated soil NO emissions are a characteristic response in forests subjected to persistent N inputs. Landscape controls were addressed with specific research at each site (Venterea, et al., 2003a; Venterea, et al., 2003b) and were incorporated into our new model (described below).
Figures 3. Mean Summertime NO Emission Rates in Sites Along an N Deposition Gradient in the Northeastern United States in 2000 and 2001. Panel (a) includes data from unamended and fertilized plots, and panel (b) includes data only from unamended plots.
Modeling and Extrapolation
Gas fluxes are notoriously variable in time and space. This variability greatly complicates our ability to produce estimates of ecosystem-scale annual flux that can be compared with annual N inputs or hydrologic outputs (Groffman, et al., in press, 2004). Although many studies produce such estimates by extrapolating point measurements to larger areas and time scales, we used mechanistic models to produce more refined extrapolations. These models were developed from the field flux and ancillary gross and net N mineralization and nitrification rates data discussed above.
We used the hole-in-the-pipe (HIP) model as the basis for our mechanistic modeling. The HIP model depicts N gas fluxes as a product of gross N fluxes through the soil microbial community, with variable amounts of different gases being emitted as a function of soil environmental conditions (moisture, pH). We adapted and modified the HIP model using new approaches developed by Venterea, et al. (2000) that more accurately depict the links between nitrification, chemical transformations of nitrite, and NO emission (see Figure 4).
Figure 4. Conceptual Diagram Showing How Atmospheric Deposition Leads to Increased Soil:Atmosphere NO Flux in Two Ways: (1) by Adding N, Which Increases Nitrification Rates and Nitrite Production; and (2) by Increasing Soil Acidity, Which Stimulates Chemical Decomposition of Nitrite to NO
We linked our mechanistic model with the NASA-CASA model, which is an aggregated representation of major ecosystem carbon (C) and N transformations (including gas fluxes) that can be run at regional scales when driven by a set of gridded coverages at 1 km spatial resolution (see Figure 4). The N gas emission components of the NASA-CASA model have been reevaluated in the context of our field measurements, and new algorithms have been produced.
Figure 5. The NASA-CASA Ecosystem C and N Process Model Showing How Landscape and Regional Scale Data Can be Used To Drive N Gas Flux Models
Conclusions:
Main Conclusions and Implications
- N gas fluxes in northeastern temperate forest ecosystems are more important than previously thought, especially NO. Fluxes of NO are significant in northeastern forests and much higher than N2O fluxes, which are frequently the only gas measured. NO flux appears to be a significant component of the N budget for these ecosystems and a significant sink for N deposition (up to 10 kg N ha-1 y-1 and 8 % of inputs).
- Atmospheric deposition leads to increased soil:atmosphere NO flux in two ways: (1) by adding N, which increases nitrification rates and nitrite production; and (2) by increasing soil acidity, which stimulates chemical decomposition of nitrite to NO.
- Our model of NO formation and dynamics in the soil indicates that rapid transformation of NO within the soil matrix mitigates NO emissions to the atmosphere, but also serves as a source of soil NO3-, contributing to N saturation in forest ecosystems. There is a strong need for further research on production, consumption, and emission of NO in these forest ecosystems.
- The impact of elevated summertime NO emissions on troposheric processes and potential effects on proximal vegetation may be significant and are worthy of further study. Soil NO emissions can influence local O3 levels, particularly in rural areas where photochemical reaction rates tend to be NOx limited (Stohl, et al., 1996). Recent investigations also have found that broad-scale spatial patterns of tropospheric O3 pollution and forest ecosystem O3 exposure in the Northeastern United States are correlated with atmospheric N deposition rates (Ollinger, et al., 2002). In this context, the consistent association between N deposition and soil NO emissions that we have observed implies other potentially important consequences of atmospheric deposition. That is, persistent atmospheric deposition that results in elevated NO emissions may, in effect, be promoting increased O3-related phytoxicity. As pointed out by Ollinger, et al. (2002), an important aspect of O3 effects on forest vegetation may be the mitigation of increased productivity and carbon storage that otherwise might result as a response to increased N inputs and/or to increases in atmospheric CO2. Whether or not the summertime NO emission rates measured at FEF and BBW (1-200 µg N m-2 h-1) or at HF (100-200 µg N m-2 h-1) contribute significantly to O3 photochemistry is an issue that deserves consideration.
References:
Magill A, Aber J, Berntson G, McDowell W, Nadelhoffer K, Melillo J, Steudler P. Long-term nitrogen additions and nitrogen saturation in two temperate forests. Ecosystems 2000;3(3):238-253.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 15 publications | 5 publications in selected types | All 4 journal articles |
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Venterea RT, Lovett GM, Groffman PM, Schwarz PA. Landscape patterns of net nitrification in a northern hardwood-conifer forest. Soil Science Society of America Journal 2003;67(2):527-539. |
R827674 (2002) R827674 (Final) |
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Venterea RT, Groffman PM, Verchot LV, Magill AH, Aber JD. Gross nitrogen process rates in temperate forest soils exhibiting symptoms of nitrogen saturation. Forest Ecology and Management 2004;196(1):129-142. |
R827674 (2003) R827674 (Final) |
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Supplemental Keywords:
scaling, regional analysis, landscape analysis, nitrous oxide, N2O, nitric oxide, NO, nitrification, microbial, nitrogen, N, N deposition, acid rain, nitrogen oxides, N cycle, environmental biogeochemistry, ecology, modeling, Northeast, volatile organic compounds, VOCs, climate change, geographic information system, GIS, air pollution, air quality, atmospheric models, atmospheric pollutant loads, biogeochemical, ecological exposure, fate and transport, forest ecosystems, forest inventory and analysis, groundwater, nitrogen compounds, nitrogen deposition, regional scale impacts, scaling methods,, RFA, Scientific Discipline, Air, Toxics, Waste, Ecosystem Protection/Environmental Exposure & Risk, Ecology, Ecosystem/Assessment/Indicators, Ecosystem Protection, Environmental Chemistry, climate change, VOCs, Fate & Transport, Ecological Effects - Environmental Exposure & Risk, Forestry, Regional/Scaling, fate and transport, ecological exposure, nitrogen deposition, N deposition, forest ecosystems, forest inventory and analysis, modeling, biogeochemical, air pollution, regional scale impacts, sulfur compounds, atmospheric pollutant loads, GIS, nitrogen compounds, air quality, atmospheric models, nitrogen, acid rain, scaling methodsRelevant Websites:
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Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.