No abstract

A comparative (15)N-tracer study of nitrogen dynamics in headwater streams from biomes throughout North America demonstrates that streams exert control over nutrient exports to rivers, lakes, and estuaries. The most rapid uptake and transformation of inorganic nitrogen occurred in the smallest streams. Ammonium entering these streams was removed from the water within a few tens to hundreds of meters. Nitrate was also removed from stream water but traveled a distance 5 to 10 times as long, on average, as ammonium. Despite low ammonium concentration in stream water, nitrification rates were high, indicating that small streams are potentially important sources of atmospheric nitrous oxide. During seasons of high biological activity, the reaches of headwater streams typically export downstream less than half of the input of dissolved inorganic nitrogen from their watersheds.

Nitrous oxide (N 2 O) is a potent greenhouse gas that contributes to climate change and stratospheric ozone destruction. Anthropogenic nitrogen (N) loading to river networks is a potentially important source of N 2 O via microbial denitrification that converts N to N 2 O and dinitrogen (N 2 ). The fraction of denitrified N that escapes as N 2 O rather than N 2 (i.e., the N 2 O yield) is an important determinant of how much N 2 O is produced by river networks, but little is known about the N 2 O yield in flowing waters. Here, we present the results of whole-stream 15 N-tracer additions conducted in 72 headwater streams draining multiple land-use types across the United States. We found that stream denitrification produces N 2 O at rates that increase with stream water nitrate (NO 3 − ) concentrations, but that <1% of denitrified N is converted to N 2 O. Unlike some previous studies, we found no relationship between the N 2 O yield and stream water NO 3 − . We suggest that increased stream NO 3 − loading stimulates denitrification and concomitant N 2 O production, but does not increase the N 2 O yield. In our study, most streams were sources of N 2 O to the atmosphere and the highest emission rates were observed in streams draining urban basins. Using a global river network model, we estimate that microbial N transformations (e.g., denitrification and nitrification) convert at least 0.68 Tg·y −1 of anthropogenic N inputs to N 2 O in river networks, equivalent to 10% of the global anthropogenic N 2 O emission rate. This estimate of stream and river N 2 O emissions is three times greater than estimated by the Intergovernmental Panel on Climate Change.H umans have more than doubled the availability of fixed nitrogen (N) in the biosphere, particularly through the production of N fertilizers and the cultivation of N-fixing crops (1). Increasing N availability is producing unintended environmental consequences including enhanced emissions of nitrous oxide (N 2 O), a potent greenhouse gas (2) and an important cause of stratospheric ozone destruction (3). The Intergovernmental Panel on Climate Change (IPCC) estimates that the microbial conversion of agriculturally derived N to N 2 O in soils and aquatic ecosystems is the largest source of anthropogenic N 2 O to the atmosphere (2). The production of N 2 O in agricultural soils has been the focus of intense investigation (i.e., >1,000 published studies) and is a relatively well constrained component of the N 2 O budget (4). However, emissions of anthropogenic N 2 O from streams, rivers, and estuaries have received much less attention and remain a major source of uncertainty in the global anthropogenic N 2 O budget.Microbial denitrification is a large source of N 2 O emissions in terrestrial and aquatic ecosystems. Most microbial denitrification is a form of anaerobic respiration in which nitrate (NO 3 − , the dominant form of inorganic N) is converted to dinitrogen (N 2 ) and N 2 O gases (5). The proportion of denitrified NO 3 − that is converted to N 2 O rather than N 2 (h...

Wetlands of the Amazon River basin are globally significant sources of atmospheric methane. Satellite remote sensing (passive and active microwave) of the temporally varying extent of inundation and vegetation was combined with field measurements to calculate regional rates of methane emission for Amazonian wetlands. Monthly inundation areas for the fringing floodplains of the mainstem Solimõ es/Amazon River were derived from analysis of the 37 GHz polarization difference observed by the Scanning Multichannel Microwave Radiometer from 1979 to 1987. L-band synthetic aperture radar data (Japanese Earth Resources Satellite-1) were used to determine inundation and wetland vegetation for the Amazon basin (o500 m elevation) at high (May-June 1996) and low water (October 1995). An extensive set of measurements of methane emission is available from the literature for the fringing floodplains of the central Amazon, segregated into open water, flooded forest and floating macrophyte habitats. Uncertainties in the regional emission rates were determined by Monte Carlo error analyses that combined error estimates for the measurements of emission and for calculations of inundation and habitat areas. The mainstem Solimõ es/Amazon floodplain (54-701W) emitted methane at a mean annual rate of 1.3 Tg C yr À1 , with a standard deviation (SD) of the mean of 0.3 Tg C yr À1 ; 67% of this range in uncertainty is owed to the range in rates of methane emission and 33% is owed to uncertainty in the areal estimates of inundation and vegetative cover. Methane emission from a 1.77 million square kilometers area in the central basin had a mean of 6.8 Tg C yr À1 with a SD of 1.3 Tg C yr À1 . If extrapolated to the whole basin below the 500 m contour, approximately 22 Tg C yr À1 is emitted; this mean flux has a greenhouse warming potential of about 0.5 Pg C as CO 2 . Improvement of these regional estimates will require many more field measurements of methane emission, further examination of remotely sensed data for types of wetlands not represented in the central basin, and process-based models of methane production and emission.

We measured denitrification and total nitrate uptake rates in a small stream (East Fork of Walker Branch in eastern Tennessee) using a new field 15 N tracer addition and modeling approach that quantifies these rates for entire stream reaches. The field experiment consisted of an 8-h addition of 99 atom% K 15 NO 3 and a conservative solute tracer. Two 15 N tracer addition experiments were performed on consecutive days, the first under ambient NO concentra- (Ͼ99%) and comprised about 16% (Ϯ10%) of total NO uptake rate under ambient NO concentrations and about1% (Ϯ1%) of total NO 3 uptake rate with NO addition. Denitrification rate expressed on a mass flux basis was AcknowledgmentsWe thank Jeff Houser, Ramie Wilkerson, and Erica Lewis for their help in the field and laboratory; Suzanne Thomas for analysis of 15 N samples at the Marine Biological Lab; and Melody Bernot and Jennifer Tank for kindly providing the gas sampling vials. We also appreciate advice on sampling and sample analysis from Jennifer Tank and Melody Bernot. We thank David Harris, Stable Isotope Laboratory, University of California, Davis, for performing most of the 15 N analysis. We benefited greatly from discussions with Jim McClelland on transformations of the isotope data and with Wil Wollheim on model development. We also thank two anonymous reviewers for their constructive comments on an earlier version of the manuscript.

Over 13 million ha of former cropland are enrolled in the US Conservation Reserve Program (CRP), providing well-recognized biodiversity, water quality, and carbon (C) sequestration benefits that could be lost on conversion back to agricultural production. Here we provide measurements of the greenhouse gas consequences of converting CRP land to continuous corn, corn–soybean, or perennial grass for biofuel production. No-till soybeans preceded the annual crops and created an initial carbon debt of 10.6 Mg CO 2 equivalents (CO 2 e)·ha −1 that included agronomic inputs, changes in C stocks, altered N 2 O and CH 4 fluxes, and foregone C sequestration less a fossil fuel offset credit. Total debt, which includes future debt created by additional changes in soil C stocks and the loss of substantial future soil C sequestration, can be constrained to 68 Mg CO 2 e·ha −1 if subsequent crops are under permanent no-till management. If tilled, however, total debt triples to 222 Mg CO 2 e·ha −1 on account of further soil C loss. Projected C debt repayment periods under no-till management range from 29 to 40 y for corn–soybean and continuous corn, respectively. Under conventional tillage repayment periods are three times longer, from 89 to 123 y, respectively. Alternatively, the direct use of existing CRP grasslands for cellulosic feedstock production would avoid C debt entirely and provide modest climate change mitigation immediately. Incentives for permanent no till and especially permission to harvest CRP biomass for cellulosic biofuel would help to blunt the climate impact of future CRP conversion.

This study examined a community of stream bivalves (unionids and fingernail clams) in a second-order woodland stream in southern Michigan using both the natural abundance of 15 N and a 6-week whole-stream 15 N enrichment experiment, as part of the Lotic Intersite Nitrogen eXperiment (LINX). Objectives included addressing what made up the diet of these bivalves and whether suspended algae consumed by bivalves were derived from pelagic phytoplankton imported from an upstream lake or attached algae sloughed from instream surfaces. Within the examination of bivalve diets, we considered whether suspension-and/or deposit-feeding modes were employed and whether bivalves selectively assimilated the algal and microbial portions of bulk material they ingested. All 12 unionid species reached a level of 15 N enrichment greater than the bulk suspended organic matter. Sphaerium striatinum (Sphaeriidae) were enriched to levels greater than all presumed food sources. Suspended algae were derived both from sloughed epilithon and pelagic phytoplankton originating from lentic waters upstream. A mixing model suggested that unionids were consuming 80% deposited and 20% suspended material. Alternatively, these bivalves were preferentially assimilating the highly enriched living component of suspended and/or benthic organic matter rather than assimilating the bulk material. These results advance our understanding of freshwater bivalve-feeding ecology, which is necessary if conservation efforts of these increasingly threatened organisms are to succeed.

Abstract. This study examines dissolved 02, CO2 and CH4 in waters of the Pantanal, a vast savanna floodplain in Brazil. Measurementsare presented for 540 samples from throughout the region, ranging from areas of sheet flooding to sluggish marsh streams to the major rivers of the region. Dissolved 02 is often strongly depleted, particularly in waters filled with emergent vascular plants, which are the most extensive aquatic environment of the region. Median Or concentrations were 35 PM for vegetated waters, 116 PM for the Paraguay River, 95 PM for tributary rivers, and 165 ,uM for open lakes (atmospheric equilibrium, 230-290 PM). Airwater diffusive fluxes were calculated from dissolved gas concentrations for representative vegetated floodplain waters, based on data collected over the course of an annual cycle. These fluxes reveal about twice as much CO2 evasion as can be accounted for by invasion of 02 (overall means in nmol cm-= s-': 02 0.18, CO2 0.34, and CH4 0.017). Methanogenesis is estimated to account for ca. 20% of the total heterotrophic metabolism in the water column and sediments, with the remainder likely due mostly to aerobic respiration. Anaerobic respiration is limited by the low concentrations of alternate electron acceptors. We hypothesize that 02 transported through the stems of emergent plants is consumed in aerobic respiration by plant tissues or microorganisms, producing CO2 that preferentially dissolves into the water, and thus explaining most of the excess CO2 evasion. This hypothesis is supported by measurements of gases in submersed stems of emergent plants.