Ecosystems Center
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The Ecosystems Center carries out research in ecosystems that range from the Arctic to the Antarctic, from Brazil to Martha’s Vineyard. In the Alaskan Arctic, scientists study the effect of warmer temperatures on tundra, stream and lake ecosystems. On the Arctic rivers of Eurasia, they measure how freshwater discharge is changing as the climate warms. On the western Antarctic peninsula, research focuses on the responses of the marine coastal ecosystem to rapid climate warming. In the western Amazon in Brazil, researchers assess how much the clearing of tropical forests will change the amount of greenhouse gas released into the atmosphere, while on the island of Martha’s Vineyard, scientists used controlled burns to restore coastal ecosystems. In central Massachusetts and in Abisko, Sweden, soil warming experiments are conducted to assess the forest’s response to climate warming. In northeastern Massachusetts, scientists study how changes in rural land use and urban development affect the flow of nutrients and organic matter into New England estuaries. In Boston Harbor, they measure the transfer of nitrogen from sediments to the water column as the harbor recovers from decades of sewage addition.
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BookLimnology of tundra ponds, Barrow, Alaska(Dowden, Hutchinson & Ross, 1980) Hobbie, John E.This book is a report of investigations of several small ponds on the arctic tundra near Barrow, Alaska. The main study, which ran from 1971 through 1973, was funded from three sources: The National Science Foundation, the State of Alaska through the University of Alaska, and individual companies and members of the petroleum industry. The NSF funding was under the joint sponsorship of the U.S. Arctic Research Program (Division of Polar Programs) and the U.S. International Biological Program (Ecosystem Analysis Program). The U.S. Tundra Biome Program was under the overall direction of Jerry Brown of the U .S. Army Cold Regions Research and Engineering Laboratory and consisted of aquatic and terrestrial sections.
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ArticleInformation technology and libraries(IAMSLIC, 2000) Melillo, Jerry M.
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ArticleFamily- and genus-level 16S rRNA-targeted oligonucleotide probes for ecological studies of methanotrophic bacteria(American Society for Microbiology, 2001-10) Gulledge, Jay ; Ahmad, Azeem ; Steudler, Paul A. ; Pomerantz, William J. ; Cavanaugh, Colleen M.Methanotrophic bacteria play a major role in the global carbon cycle, degrade xenobiotic pollutants, and have the potential for a variety of biotechnological applications. To facilitate ecological studies of these important organisms, we developed a suite of oligonucleotide probes for quantitative analysis of methanotroph-specific 16S rRNA from environmental samples. Two probes target methanotrophs in the family Methylocystaceae (type II methanotrophs) as a group. No oligonucleotide signatures that distinguish between the two genera in this family, Methylocystis and Methylosinus, were identified. Two other probes target, as a single group, a majority of the known methanotrophs belonging to the family Methylococcaceae (type I/X methanotrophs). The remaining probes target members of individual genera of the Methylococcaceae, including Methylobacter, Methylomonas, Methylomicrobium, Methylococcus, and Methylocaldum. One of the family-level probes also covers all methanotrophic endosymbionts of marine mollusks for which 16S rRNA sequences have been published. The two known species of the newly described genus Methylosarcina gen. nov. are covered by a probe that otherwise targets only members of the closely related genus Methylomicrobium. None of the probes covers strains of the newly proposed genera Methylocella and "Methylothermus," which are polyphyletic with respect to the recognized methanotrophic families. Empirically determined midpoint dissociation temperatures were 49 to 57°C for all probes. In dot blot screening against RNA from positive- and negative-control strains, the probes were specific to their intended targets. The broad coverage and high degree of specificity of this new suite of probes will provide more detailed, quantitative information about the community structure of methanotrophs in environmental samples than was previously available.
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ArticleBacterioplankton community shifts in an Arctic lake correlate with seasonal changes in organic matter source(American Society for Microbiology, 2003-04) Crump, Byron C. ; Kling, George W. ; Bahr, Michele ; Hobbie, John E.Seasonal shifts in bacterioplankton community composition in Toolik Lake, a tundra lake on the North Slope of Alaska, were related to shifts in the source (terrestrial versus phytoplankton) and lability of dissolved organic matter (DOM). A shift in community composition, measured by denaturing gradient gel electrophoresis (DGGE) of 16S rRNA genes, occurred at 4°C in near-surface waters beneath seasonal ice and snow cover in spring. This shift was associated with an annual peak in bacterial productivity ([14C]leucine incorporation) driven by the large influx of labile terrestrial DOM associated with snow meltwater. A second shift occurred after the flux of terrestrial DOM had ended in early summer as ice left the lake and as the phytoplankton community developed. Bacterioplankton communities were composed of persistent populations present throughout the year and transient populations that appeared and disappeared. Most of the transient populations could be divided into those that were advected into the lake with terrestrial DOM in spring and those that grew up from low concentrations during the development of the phytoplankton community in early summer. Sequencing of DNA in DGGE bands demonstrated that most bands represented single ribotypes and that matching bands from different samples represented identical ribotypes. Bacteria were identified as members of globally distributed freshwater phylogenetic clusters within the {alpha}- and ß-Proteobacteria, the Cytophaga-Flavobacteria-Bacteroides group, and the Actinobacteria.
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ArticleRapid screening for freshwater bacterial groups by using reverse line blot hybridization(American Society for Microbiology, 2003-10) Zwart, Gabriel ; van Hannen, Erik J. ; Kamst-van Agterveld, Miranda P. ; Van der Gucht, Katleen ; Lindstrom, Eva S. ; Van Wichelen, Jeroen ; Lauridsen, Torben ; Crump, Byron C. ; Han, Suk-Kyun ; Declerck, StevenThe identification of phylogenetic clusters of bacteria that are common in freshwater has provided a basis for probe design to target important freshwater groups. We present a set of 16S ribosomal RNA gene-based oligonucleotide probes specific for 15 of these freshwater clusters. The probes were applied in reverse line blot hybridization, a simple method that enables the rapid screening of PCR products from many samples against an array of probes. The optimized assay was made stringent to discriminate at approximately the single-mismatch level. This made 10 of the probes highly specific, with at least two mismatches to the closest noncluster member in the global database. Screening of PCR products from bacterioplankton of 81 diverse lakes from Belgium, The Netherlands, Denmark, Sweden, and Norway showed that the respective probes were reactive against 5 to 100% of the lake samples. Positive reactivity of six highly specific probes showed that bacteria from actinobacterial clusters ACK-M1 and Sta2-30 and from verrucomicrobial cluster CLO-14 occurred in at least 90% of the investigated lakes. Furthermore, bacteria from alpha-proteobacterial cluster LD12 (closely related to the marine SAR11 cluster), beta-proteobacterial cluster LD28 and cyanobacterial cluster Synechococcus 6b occurred in more than 70% of the lakes. Reverse line blot hybridization is a new tool in microbial ecology that will facilitate research on distribution and habitat specificity of target species at relatively low costs.
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PreprintEffects of ozone on net primary production and carbon sequestration in the conterminous United States using a biogeochemistry model( 2003-11-25) Felzer, Benjamin S. ; Kicklighter, David W. ; Melillo, Jerry M. ; Wang, C. ; Zhuang, Qianlai ; Prinn, Ronald G.The effects of air pollution on vegetation may provide an important control on the carbon cycle that has not yet been widely considered. Prolonged exposure to high levels of ozone, in particular, has been observed to inhibit photosynthesis by direct cellular damage within the leaves and through possible changes in stomatal conductance. We have incorporated empirical equations derived for trees (hardwoods and pines) and crops into the Terrestrial Ecosystem Model to explore the effects of ozone on net primary production and carbon sequestration across the conterminous United States. Our results show a 2.6 – 6.8% mean reduction for the U.S. in annual Net Primary Production (NPP) in response to modeled historical ozone levels during the late 1980s-early 1990s. The largest decreases (over 13% in some locations) occur in the Midwest agricultural lands, during the mid-summer when ozone levels are highest. Carbon sequestration since the 1950s has been reduced by 18 - 38 Tg C yr-1 with the presence of ozone. Thus the effects of ozone on NPP and carbon sequestration should be factored into future calculations of the US carbon budget.
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ArticleMicrobial biogeography along an estuarine salinity gradient : combined influences of bacterial growth and residence time(American Society for Microbiology, 2004-03) Crump, Byron C. ; Hopkinson, Charles S. ; Sogin, Mitchell L. ; Hobbie, John E.Shifts in bacterioplankton community composition along the salinity gradient of the Parker River estuary and Plum Island Sound, in northeastern Massachusetts, were related to residence time and bacterial community doubling time in spring, summer, and fall seasons. Bacterial community composition was characterized with denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S ribosomal DNA. Average community doubling time was calculated from bacterial production ([14C]leucine incorporation) and bacterial abundance (direct counts). Freshwater and marine populations advected into the estuary represented a large fraction of the bacterioplankton community in all seasons. However, a unique estuarine community formed at intermediate salinities in summer and fall, when average doubling time was much shorter than water residence time, but not in spring, when doubling time was similar to residence time. Sequencing of DNA in DGGE bands demonstrated that most bands represented single phylotypes and that matching bands from different samples represented identical phylotypes. Most river and coastal ocean bacterioplankton were members of common freshwater and marine phylogenetic clusters within the phyla Proteobacteria, Bacteroidetes, and Actinobacteria. Estuarine bacterioplankton also belonged to these phyla but were related to clones and isolates from several different environments, including marine water columns, freshwater sediments, and soil.
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PreprintTerrestrial C sequestration at elevated CO2 and temperature : the role of dissolved organic N loss( 2004-06-07) Rastetter, Edward B. ; Perakis, Steven S. ; Shaver, Gaius R. ; Agren, Goran I.We used a simple model of carbon–nitrogen (C–N) interactions in terrestrial ecosystems to examine the responses to elevated CO2 and to elevated CO2 plus warming in ecosystems that had the same total nitrogen loss but that differed in the ratio of dissolved organic nitrogen (DON) to dissolved inorganic nitrogen (DIN) loss. We postulate that DIN losses can be curtailed by higher N demand in response to elevated CO2, but that DON losses cannot. We also examined simulations in which DON losses were held constant, were proportional to the amount of soil organic matter, were proportional to the soil C:N ratio, or were proportional to the rate of decomposition. We found that the mode of N loss made little difference to the short-term (<60 years) rate of carbon sequestration by the ecosystem, but high DON losses resulted in much lower carbon sequestration in the long term than did low DON losses. In the short term, C sequestration was fueled by an internal redistribution of N from soils to vegetation and by increases in the C:N ratio of soils and vegetation. This sequestration was about three times larger with elevated CO2 and warming than with elevated CO2 alone. After year 60, C sequestration was fueled by a net accumulation of N in the ecosystem, and the rate of sequestration was about the same with elevated CO2 and warming as with elevated CO2 alone. With high DON losses, the ecosystem either sequestered C slowly after year 60 (when DON losses were constant or proportional to soil organic matter) or lost C (when DON losses were proportional to the soil C:N ratio or to decomposition). We conclude that changes in long-term C sequestration depend not only on the magnitude of N losses, but also on the form of those losses.
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ArticleIncreasing river discharge in the Eurasian Arctic : consideration of dams, permafrost thaw, and fires as potential agents of change(American Geophysical Union, 2004-09-17) McClelland, James W. ; Holmes, Robert M. ; Peterson, Bruce J. ; Stieglitz, MarcDischarge from Eurasian rivers to the Arctic Ocean has increased significantly in recent decades, but the reason for this trend remains unclear. Increased net atmospheric moisture transport from lower to higher latitudes in a warming climate has been identified as one potential mechanism. However, uncertainty associated with estimates of precipitation in the Arctic makes it difficult to confirm whether or not this mechanism is responsible for the change in discharge. Three alternative mechanisms are dam construction and operation, permafrost thaw, and increasing forest fires. Here we evaluate the potential influence of these three mechanisms on changes in discharge from the six largest Eurasian Arctic rivers (Yenisey, Ob', Lena, Kolyma, Pechora, and Severnaya Dvina) between 1936 and 1999. Comprehensive discharge records made it possible to evaluate the influence of dams directly. Data on permafrost thaw and fires in the watersheds of the Eurasian Arctic rivers are more limited. We therefore use a combination of data and modeling scenarios to explore the potential of these two mechanisms as drivers of increasing discharge. Dams have dramatically altered the seasonality of discharge but are not responsible for increases in annual values. Both thawing of permafrost and increased fires may have contributed to changes in discharge, but neither can be considered a major driver. Cumulative thaw depths required to produce the observed increases in discharge are unreasonable: Even if all of the water from thawing permafrost were converted to discharge, a minimum of 4 m thawed evenly across the combined permafrost area of the six major Eurasian Arctic watersheds would have been required. Similarly, sensitivity analysis shows that the increases in fires that would have been necessary to drive the changes in discharge are unrealistic. Of the potential drivers considered here, increasing northward transport of moisture as a result of global warming remains the most viable explanation for the observed increases in Eurasian Arctic river discharge.
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PreprintTight coupling between leaf area index and foliage N content in arctic plant communities( 2004-09-17) van Wijk, Mark T. ; Williams, Mathew ; Shaver, Gaius R.The large spatial heterogeneity of arctic landscapes complicates efforts to quantify key processes of these ecosystems, for example productivity, at the landscape level. Robust relationships that help to simplify and explain observed patterns, are thus powerful tools for understanding and predicting vegetation distribution and dynamics. Here we present the same linear relationship between leaf area index and total foliar nitrogen, the two factors determining the photosynthetic capacity of vegetation, across a wide range of tundra vegetation types in both Northern-Sweden and Alaska between leaf area indices of 0 and 1 m2 m-2, which is essentially the entire range of leaf area index values for the Arctic as a whole. Surprisingly, this simple relationship arises as an emergent property at the plant community level, whereas at the species level a large variability in leaf traits exists. As the relationship between LAI and foliar N exists among such varied ecosystems, the arctic environment must impose tight constraints on vegetation canopy development. This relationship simplifies the quantification of vegetation productivity of arctic vegetation types as the two most important drivers of productivity can now be estimated reliably from remotely sensed NDVI images.
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PreprintFuture effects of ozone on carbon sequestration and climate change policy using a global biogeochemical model( 2004-10-29) Felzer, Benjamin S. ; Reilly, John M. ; Melillo, Jerry M. ; Kicklighter, David W. ; Sarofim, Marcus C. ; Wang, C. ; Prinn, Ronald G. ; Zhuang, QianlaiExposure of plants to ozone inhibits photosynthesis and therefore reduces vegetation production and carbon sequestration. The reduced carbon storage would then require further reductions in fossil fuel emissions to meet a given CO2 concentration target, thereby increasing the cost of meeting the target. Simulations with the Terrestrial Ecosystem Model (TEM) for the historical period (1860-1995) show the largest damages occur in the Southeast and Midwestern regions of the United States, eastern Europe, and eastern China. The largest reductions in carbon storage for the period 1950-1995, 41%, occur in eastern Europe. Scenarios for the 21st century developed with the MIT Integrated Global Systems Model (IGSM) lead to even greater negative effects on carbon storage in the future. In some regions, current land carbon sinks become carbon sources, and this change leads to carbon sequestration decreases of up to 0.4 Pg C yr-1 due to damage in some regional ozone hot spots. With a climate policy, failing to consider the effects of ozone damage on carbon sequestration would raise the global costs over the next century of stabilizing atmospheric concentrations of CO2 equivalents at 550 ppm by 6 to 21%. Because stabilization at 550 ppm will reduce emission of other gases that cause ozone, these additional benefits are estimated to be between 5 and 25% of the cost of the climate policy. Tropospheric ozone effects on terrestrial ecosystems thus produce a surprisingly large feedback in estimating climate policy costs that, heretofore, has not been included in cost estimates.
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ArticleN budgets and aquatic uptake in the Ipswich River basin, northeastern Massachusetts(American Geophysical Union, 2004-11-05) Williams, Michael ; Hopkinson, Charles S. ; Rastetter, Edward B. ; Vallino, Joseph J.We calculated N budgets and conducted nutrient uptake experiments to evaluate the fate of N in the aquatic environment of the Ipswich River basin, northeastern Massachusetts. A mass balance indicates that the basin retains about 50% of gross N inputs, mostly in terrestrial components of the landscape, and the loss and retention of total nitrogen (TN) in the aquatic environment was about 9% of stream loading. Uptake lengths of PO4 and NH4 were measurable in headwater streams, but NO3 uptake was below detection (minimum detection limit = 0.05 μM). Retention or loss of NO3 was observed in a main stem reach bordered by wetland habitat. Nitrate removal in urban headwater tributaries was because of water withdrawals and denitrification during hypoxic events and in ponded wetlands with long water residence times. A mass balance using an entire river network indicates that basin-wide losses due to aquatic denitrification are considerably lower than estimates from several recent studies and range from 4 to 16% of TDN in stream loading. Withdrawals for domestic use restrict the runoff of headwater catchments from reaching the main stem during low base flow periods, thereby contributing to the spatial and temporal regulation of N export from headwater tributaries.
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ArticleLong-term survival of adult Arctic grayling (Thymallus arcticus) in the Kuparuk River, Alaska(National Research Council Canada, 2004-12-23) Buzby, Karen M. ; Deegan, Linda A.In many long-lived species such as Arctic grayling (Thymallus arcticus), population growth rate is most sensitive to changes in adult survival probabilities. Understanding the factors that regulate adult survival in this species should provide insight into the population dynamics of this and other long-lived Arctic species. Using the program MARK, we analyzed 17 years of mark–recapture data to estimate survival rates for Arctic grayling in the Kuparuk River, Alaska, from 1985 to 2000. Mean annual survival rates overall ranged from 0.39 to 1.0 and averaged 0.71 ± 0.05 for resident and 0.75 ± 0.05 for nonresident fish. Spending the summer in the more productive fertilized zone of the experimental reach had no influence on survival despite higher productivity on all trophic levels and consistently higher growth rates in Arctic grayling. None of the environmental (stream temperature, discharge, winter severity, and incidence of drought) or population parameters (growth, condition factor, and mean fish size) that we examined explained significant amounts of variance in survival rates. The lack of responsiveness of survival to annual environmental conditions was unexpected and suggests that multiyear factors or life history tactics that maintain survival at the expense of growth and fecundity likely determine survival.
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PreprintA stable isotope simulator that can be coupled to existing mass balance models( 2005-01-07) Rastetter, Edward B. ; Kwiatkowski, Bonnie L. ; McKane, Robert B.To facilitate the simulation of isotope dynamics in ecosystems, we developed software to model changes in the isotopic signatures of the stocks of an element using the output from any parent model that specifies the stocks and flux rates of that element based on a mass balance approach. The software alleviates the need to recode the parent model to incorporate isotopes. This parent model can be a simple mass balance spreadsheet of the system. The isotopic simulations use a linear, donor-controlled approximation of the fluxes in the parent model, which are updated for each time step. These approximations are based on the output of the parent model, so no modifications to the parent model are required. However, all fluxes provided to the simulator must be gross fluxes, and the user must provide the initial isotopic signature for all stocks, the fractionation associated with each flux, and the isotopic signature of any flux originating from outside the system. We illustrate the use of the simulator with two examples. The first is based on a model of the carbon and nitrogen mass balance in an eight-species food web. We examine the consequences of using the steady-state assumption implicit in multi-source mixing models often used to map food webs based on 13C and 15N. We also use the simulator to analyze a pulse chase 15N-labeling experiment based on a spreadsheet model of the nitrogen cycle at the Harvard Forest Long Term Ecological Research site. We examine the constraints on net vs. gross N mineralization that are necessary to match the observed changes in the isotopic signatures of the forest N stocks.
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PreprintSpecies compositional differences on different-aged glacial landscapes drive contrasting responses of tundra to nutrient addition( 2005-01-17) Hobbie, Sarah E. ; Gough, Laura ; Shaver, Gaius R.In the northern foothills of the Brooks Range, Alaska, moist non-acidic tundra dominates more recently deglaciated upland landscapes, whereas moist acidic tundra dominates older upland landscapes. In previous studies, experimental fertilization of moist acidic tussock tundra greatly increased the abundance and productivity of the deciduous dwarf shrub Betula nana. However, this species is largely absent from moist non-acidic tundra. These two common upland tundra community types exhibited markedly different responses to fertilization with nitrogen and phosphorus. In moist acidic tundra, cover of deciduous shrubs (primarily B. nana) increased after only 2 years, and by 4 years vascular biomass and above-ground net primary productivity (ANPP) had increased significantly, almost entirely because of Betula. In moist non-acidic tundra, both biomass and ANPP were again significantly greater, but no single species dominated the response to fertilization. Instead, the effect was due to a combination of several small, sometimes statistically non-significant responses by forbs, graminoids and prostrate deciduous shrubs. The different growth form and species' responses suggest that fertilization will cause carbon cycling through plant biomass to diverge in these two tundra ecosystems. Already, production of new stems by apical growth has increased relative to leaf production in acidic tundra, whereas the opposite has occurred in non-acidic tundra. Secondary stem growth has also increased as a component of primary production in acidic tundra, but is unchanged in non-acidic tundra. Thus, fertilization will probably increase carbon sequestration in woody biomass of B. nana in acidic tundra, while increasing carbon turnover (but not storage) of non-woody species in non-acidic tundra. These results indicate that nutrient enrichment can have very different consequences for plant communities that occur on different geological substrates, because of differences in composition, even though they share the same regional species pool. Although the specific edaphic factors that maintain compositional differences in this case are unknown, variation in soil pH and related variability in soil nutrient availability may well play a role.
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ArticleChina's changing landscape during the 1990s : large-scale land transformations estimated with satellite data(American Geophysical Union, 2005-01-27) Liu, Jiyuan ; Tian, Hanqin ; Liu, Mingliang ; Zhuang, Dafang ; Melillo, Jerry M. ; Zhang, ZengxiangLand-cover changes in China are being powered by demand for food for its growing population and by the nation's transition from a largely rural society to one in which more than half of its people are expected to live in cities within two decades. Here we use an analysis of remotely sensed data gathered between 1990 and 2000, to map the magnitude and pattern of changes such as the conversion of grasslands and forests to croplands and the loss of croplands to urban expansion. With high-resolution (30 m) imagery from Landsat TM for the entire country, we show that between 1990 and 2000 the cropland area increased by 2.99 million hectares and urban areas increased by 0.82 million hectares. In northern China, large areas of woodlands, grasslands and wetlands were converted to croplands, while in southern China large areas of croplands were converted to urban areas. The land-cover products presented here give the Chinese government and international community, for the first time, an unambiguous understanding of the degree to which the nation's landscape is being altered. Documentation of these changes in a reliable and spatially explicit way forms the foundation for management of China's environment over the coming decades.
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ArticleAggregate measures of ecosystem services : can we take the pulse of nature?(Ecological Society of America, 2005-02) Meyerson, Laura A. ; Baron, Jill ; Melillo, Jerry M. ; Naiman, Robert J. ; O'Malley, Robin I. ; Orians, Gordon ; Palmer, Margaret A. ; Pfaff, Alexander S. P. ; Running, Steven W. ; Sala, Osvaldo E.National scale aggregate indicators of ecosystem services are useful for stimulating and supporting a broad public discussion about trends in the provision of these services. There are important considerations involved in producing an aggregate indicator, including whether the scientific and technological capacity exists, how to address varying perceptions of the societal importance of different services, and how to communicate information about these services to both decision makers and the general public. Although the challenges are formidable, they are not insurmountable. Quantification of ecosystem services and dissemination of information to decision makers and the public is critical for the responsible and sustainable management of natural resources.
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ArticleControls on nitrogen cycling in terrestrial ecosystems : a synthetic analysis of literature data(Ecological Society of America, 2005-05) Booth, Mary S. ; Stark, John M. ; Rastetter, Edward B.Isotope pool dilution studies are increasingly reported in the soils and ecology literature as a means of measuring gross rates of nitrogen (N) mineralization, nitrification, and inorganic N assimilation in soils. We assembled data on soil characteristics and gross rates from 100 studies conducted in forest, shrubland, grassland, and agricultural systems to answer the following questions: What factors appear to be the major drivers for production and consumption of inorganic N as measured by isotope dilution studies? Do rates or the relationships between drivers and rates differ among ecosystem types? Across a wide range of ecosystems, gross N mineralization is positively correlated with microbial biomass and soil C and N concentrations, while soil C:N ratio exerts a negative effect on N mineralization only after adjusting for differences in soil C. Nitrification is a log-linear function of N mineralization, increasing rapidly at low mineralization rates but changing only slightly at high mineralization rates. In contrast, NH4+ assimilation by soil microbes increases nearly linearly over the full range of mineralization rates. As a result, nitrification is proportionately more important as a fate for NH4+ at low mineralization rates than at high mineralization rates. Gross nitrification rates show no relationship to soil pH, with some of the fastest nitrification rates occurring below pH 5 in soils with high N mineralization rates. Differences in soil organic matter (SOM) composition and concentration among ecosystem types influence the production and fate of mineralized N. Soil organic matter from grasslands appears to be inherently more productive of ammonium than SOM from wooded sites, and SOM from deciduous forests is more so than SOM in coniferous forests, but differences appear to result primarily from differing C:N ratios of organic matter. Because of the central importance of SOM characteristics and concentrations in regulating rates, soil organic matter depletion in agricultural systems appears to be an important determinant of gross process rates and the proportion of NH4+ that is nitrified. Addition of 15N appears to stimulate NH4+ consumption more than NO3− consumption processes; however, the magnitude of the stimulation may provide useful information regarding the factors limiting microbial N transformations.
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PreprintFate of nitrogen in riparian forest soils and trees : an N-15 tracer study simulating salmon decay( 2005-05-23) Drake, Deanne C. ; Naiman, Robert J. ; Bechtold, J. ScottWe introduced a 15N-NH4+ tracer to the riparian forest of a salmon-bearing stream (Kennedy Creek, Washington, USA) to quantify the cycling and fate of a late-season pulse of salmon-N, and, ultimately, mechanisms regulating potential links between salmon abundance and tree growth. The 15N tracer simulated deposition of 7.25 kg of salmon (fresh) to 4, 50-m2 plots. We added NH4+ (the initial product of salmon carcass decay) and other important nutrients provided by carcasses (P, S, K, Mg, Ca) to soils in late October 2003, coincident with local salmon spawning. We followed the 15N tracer through soil and tree pools for one year. Biological uptake of the 15N tracer occurred quickly; 64% of the 15N tracer was bound in soil microbiota within 14 days, and roots of the dominant riparian tree, western redcedar (Thuja plicata), began to take up 15N tracer within 7 days. Root uptake continued through the winter. The 15N tracer content of soil organic matter reached a maximum of ~52%, 5 weeks after the application, and a relative equilibrium of ~40% within 5 months. Six months after the addition, in spring 2004, at least 37% of the 15N tracer was found in tree tissues: ~23% in foliage, ~11% in roots and ~3% in stems. Within the stems, xylem and phloem sap contained ~96% of the tracer N, and ~4% was in structural xylem N. After one year, at least 28% of the 15N tracer was still found in trees, and loss from the plots was only ~20%. The large portion of tracer N taken up in the fall and reallocated to leaves and stems the following spring provides mechanistic evidence for a 1-year lagged tree-growth response to salmon nutrients. Salmon nutrients have been deposited in the Kennedy Creek system each fall for centuries, but the system shows no evidence of nutrient saturation. Rates of N uptake and retention are a function of site history, disturbance, and may also be the result of a legacy effect, in which annual salmon nutrient addition may lead to increased efficiency of nutrient uptake and use.
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PreprintMethods for measuring denitrification : diverse approaches to a difficult problem( 2005-07-15) Groffman, Peter M. ; Altabet, Mark A. ; Bohlke, John K. ; Butterbach-Bahl, Klaus ; David, Mark B. ; Firestone, Mary K. ; Giblin, Anne E. ; Kana, Todd M. ; Nielsen, Lars Peter ; Voytek, Mary A.Denitrification, the reduction of the nitrogen (N) oxides, nitrate (NO3-) and nitrite (NO2-), to the gases nitric oxide (NO), nitrous oxide (N2O) and dinitrogen (N2), is important to primary production, water quality and the chemistry and physics of the atmosphere at ecosystem, landscape, regional and global scales. Unfortunately, this process is very difficult to measure, and existing methods are problematic for different reasons in different places at different times. In this paper, we review the major approaches that have been taken to measure denitrification in terrestrial and aquatic environments and discuss the strengths, weaknesses and future prospects for the different methods. Methodological approaches covered include; 1) acetylene-based methods, 2) 15N tracers, 3) direct N2 quantification, 4) N2/Ar ratio quantification, 5) mass balance approaches, 6) stoichiometric approaches, 7) methods based on stable isotopes, 8) in situ gradients with atmospheric environmental tracers and 9) molecular approaches. Our review makes it clear that the prospects for improved quantification of denitrification vary greatly in different environments and at different scales. While current methodology allows for the production of accurate estimates of denitrification at scales relevant to water and air quality and ecosystem fertility questions in some systems (e.g., aquatic sediments, well defined aquifers), methodology for other systems, especially upland terrestrial areas, still needs development. Comparison of mass balance and stoichiometric approaches that constrain estimates of denitrification at large scales with point measurements (made using multiple methods), in multiple systems, is likely to propel more improvement in denitrification methods over the next few years.