Grantee Research Project Results
1998 Progress Report: The Effects of Anthropogenic Nitrogen Deposition on the Functioning of Alpine and Subalpine Ecosystems Nitrogen Cycling and Trace Gas Fluxes
EPA Grant Number: R823442Title: The Effects of Anthropogenic Nitrogen Deposition on the Functioning of Alpine and Subalpine Ecosystems Nitrogen Cycling and Trace Gas Fluxes
Investigators: Schmidt, Steven K. , Williams, Mark , Bowman, William
Institution: University of Colorado at Boulder
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1995 through September 1, 1998
Project Period Covered by this Report: October 1, 1997 through September 1, 1998
Project Amount: $329,280
RFA: Exploratory Research - Environmental Biology (1995) RFA Text | Recipients Lists
Research Category: Biology/Life Sciences , Human Health , Aquatic Ecosystems
Objective:
Our major objective is to gain a mechanistic understanding of how nitrogen fertilization affects the functioning of high elevation ecosystems.Progress Summary:
Controls on methane fluxes in the alpine.The purpose of Ann West's dissertation research has been to identify the effects nitrogen deposition and other variables on soil CH4 fluxes in alpine tundra. She has focused on CH4 consumption by the drier plant communities. Our understanding of the effects of nitrogen on soil CH4 consumption has been limited because of lack of basic information about the methanotrophic bacteria responsible for the soil CH4 sink. Therefore, her approach has been to investigate the effect of various soil manipulations on soil CH4 oxidation rates. Careful analysis of the effects of these manipulations offer insight into the ecology of soil methanotrophs.
Analysis of the field CH4 fluxes indicated that CH4 consumption in drier alpine meadows is stimulated by precipitation (West et al., 1998). This has been demonstrated only in a few other ecosystems. Previously, such stimulation by moisture additions has been attributed to relief of osmotic stress on soil microbes. However, a delay of several days before stimulation by wetting demonstrated that relief of water stress could not be the complete explanation in alpine tundra systems (West and Schmidt, 1998).
In this system alpine tundra system, acetate and formate stimulate soil CH4 oxidation. Therefore, stimulation of soil CH4 oxidation by moisture additions may depend on the release of carbon substrates. These carbon substrates may support transitory anaerobic conditions in which CH4 production occurs. This CH4 production in turn stimulates CH4 -consuming microorganisms. Supporting this idea, anaerobic pre-incubations stimulate soil CH4 oxidation. Furthermore, the methanogenic inhibitor lumazine prevented stimulation of soil CH4 oxidation by acetate additions or by anaerobic pre-incubations. Although the consumption of atmospheric CH4 by soil microorganisms is itself an aerobic process, these studies indicate that soil CH4 oxidation depends partially on the anaerobic mineralization of carbon substrates to CH4.
The relationship between soil CH4 consumption and soil carbon metabolism means that the inhibitory effects of nitrogen deposition on soil CH4 consumption may be caused in part by changes in soils carbon availability. Nitrogen fertilization alters plant carbon allocation and C:N ratios of plant biomass. In turn, these changes alter the quantity and quality of carbon substrates available to the soil microbial community, ultimately altering the potential soil CH4 production and consumption. Our data indicate that in fertilized plots, not only are soil CH4 oxidation rates suppressed, but the response of soil CH4 oxidation to acetate additions are also weakened. This corresponds with a drop in the population of soil microorganisms which can utilize acetate for growth.
Fate and effects of the early season N-pulse.
A considerable portion of our recent efforts focused on quantifying the effects of inorganic and organic N compounds released during and after snowmelt (Brooks et al. 1997, 1998, Lipson et al. 1998). Much of the N deposition to our system ends up in the winter snowpack and is released as a pulse of N during snowmelt. We have quantified trace gas fluxes prior to and during snowmelt (Brooks et al. 1997). Fluxes were significant when soil temperatures under the snow ranged from -5!C to 0!C, but here was no significant relationship between flux of either CO2 or N2O within this temperature range. The highest CO2 fluxes were observed at sites which received a hard freeze before a consistent snopwpack developed. The seasonal flux of CO2 was related to both the severity of the freeze and the duration of snow cover (Brooks et al. 1997). In contrast to the pattern observed with CO2 loss, N2O losses were related to the length of time soils were covered by a consistent snowpack. Over-winter N2O losses ranged from 0.2 to 17 mg N m-2 with the highest fluxes being observed at sites with the most snow cover. These N2O fluxes ranged from half to ten times higher than fluxes observed during the summer (Brooks et al. 1997).
We have also been trying to quantify the microbial community shifts that coincide with the flux of N from the alpine snowpack. Using the chloroform-fumigation technique, Brooks et al. (1998) showed that there is a decrease in microbial biomass N during snowmelt which coincided with an increase in inorganic N levels in K. myosuroides-dominated meadows. In more recent work, Lipson et al. (1998) demonstrated that the pulse of inorganic N was preceded by peaks in soluble protein and amino acid levels. Using different methods from those of Brooks et al. (1998), Lipson et al. (1998) also showed that there was a decline in total microbial biomass during snowmelt in three consecutive years and that rates of amino acid production were highest just after snowmelt. We concluded from the above studies that the turnover of the large wintertime microbial community releases N early in the growing season.
Our preliminary studies indicate that the decline in microbial biomass during the snowmelt season is followed by the development of a summer adapted microbial community and that the summer and winter communities are dominated by different organisms. Measurements of heterotrophic microbial biomass using the substrate-induced growth-response method (SIGR) (Colores et al. 1996) in conjunction with combinations of fungal and bacterial inhibitors showed that the responsive biomass in the winter was dominated by fungi, while bacteria and fungi were of almost equal importance in the summer. In addition to this evidence, the most probable numbers (MPN) of organisms capable of growth on media with phenolic compounds (vanillic acid or salicylic acid) as sole C-sources were disproportionately higher in colder periods compared to other carbon sources, indicating a seasonal change in substrate preferences. Finally, MPN estimates of microbial population size agreed well with SIGR, substrate-induced respiration (SIR), and the chloroform fumigation-extraction method during the warm part of the year, but MPNs greatly underestimated the population size during colder periods. This indicates that the winter community is harder to extract and culture. We have tentatively concluded that the summer community is dominated by more readily-culturable bacteria, a great number of which are Gram-negative rhizosphere dwellers that grow rapidly on simple substrates, and that the winter community is dominated by fungal species that rely on complex C-sources such as cellulose and tannins.
Journal Articles on this Report : 12 Displayed | Download in RIS Format
Other project views: | All 12 publications | 12 publications in selected types | All 12 journal articles |
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Bowman WD, Schardt JC, Schmidt SK. Symbiotic N2-fixation in alpine tundra: ecosystem input and variation in fixation rates among communities. Oecologia 1996;108(2):345-350. |
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Brooks PD, Williams MW, Schmidt SK. Microbial activity under alpine snowpacks, Niwot Ridge, Colorado. Biogeochemistry 1996;32(2):93-113. |
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Brooks PD, Schmidt SK, Williams MW. Winter production of CO2 and N2O from alpine tundra: environmental controls and relationship to inter-system C and N fluxes. Oecologia 1997;110(3):403-413. |
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Brooks PD, Williams MW, Schmidt SK. Inorganic nitrogen and microbial biomass dynamics before and during spring snowmelt. Biogeochemistry 1998;43(1):1-15. |
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Colores GM, Schmidt SK, Fisk MC. Estimating the biomass of microbial functional groups using rates of growth-related soil respiration. Soil Biology and Biochemistry 1996;28(12):1569-1577. |
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Fisk MC, Schmidt SK. Microbial responses to nitrogen additions in alpine tundra soils. Soil Biology and Biochemistry 1996;28(6):751-755. |
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Fisk MC, Schmidt SK, Seastedt TR. Topographic patterns of above- and belowground production and nitrogen cycling in Alpine tundra. Ecology 1998;79(7):2253-2266. |
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Jaeger III CH, Monson RK, Fisk MC, Schmidt SK. Seasonal partitioning of nitrogen by plants and soil microorganisms in an alpine ecosystem. Ecology 1999;80(6):1883-1891. |
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Lipson DA, Schmidt SK, Monson RK. Links between microbial population dynamics and nitrogen availability in an alpine ecosystem. Ecology 1999;80(5):1623-1631. |
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Mullen RB, Schmidt SK, Jaeger III CH. Nitrogen uptake during snowmelt by the snow buttercup, Ranunculus adoneus. Arctic, Antarctic, and Alpine Research 1998;30(2):121-125. |
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West AE, Schmidt SK. Wetting stimulates atmospheric CH4 oxidation by alpine soil. FEMS Microbiology Ecology 1998;25(4):349-353. |
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West AE, Brooks PD, Fisk MC, Smith LK, Holland EA, Jaeger III CH, Babcock S, Lai RS, Schmidt SK. Landscape patterns of CH4 fluxes in an alpine tundra ecosystem. Biogeochemistry 1999;45(3):243-264. |
R823442 (1998) |
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Supplemental Keywords:
RFA, Scientific Discipline, Waste, Geographic Area, Ecosystem Protection/Environmental Exposure & Risk, Bioavailability, Ecology, Ecosystem/Assessment/Indicators, Ecosystem Protection, exploratory research environmental biology, Chemical Mixtures - Environmental Exposure & Risk, Environmental Chemistry, State, Chemistry, Ecological Effects - Environmental Exposure & Risk, Ecological Effects - Human Health, Biology, Ecological Indicators, fate and transport, ecological exposure, fate, urban air toxics, urbanization, nitrogen deposition, urban ecosystems, plants, Alpine Lakes, atmospheric contaminants, Colorado (CO)Relevant Websites:
http://spot.Colorado.EDU/~schmidts/research.html
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.