B23A-0391
Pre-Anthropocene nitrogen cycle: balanced mass and isotope fluxes
One of the features of the present-day global nitrogen cycle is the return to the atmosphere (taken as δ15N = 0 ‰) and surface waters of isotopically lighter N (δ15N < 0) primarily from denitrification on land and in the ocean. With the N residence time in the atmosphere of about 18 million years, the N mass and isotope fluxes were likely to be balanced at geologically longer time scales if the isotopic composition of the atmosphere did not vary. For Pre-Anthropocene time, we construct a balanced N cycle model of mass and isotopic fluxes, based on 14 nitrogen reservoirs in the domains of the atmosphere, land, and ocean. Atmosphere, the largest N reservoir, supplies N to land and ocean domains mainly by nitrogen fixation, deposition, and dissolution, and these fluxes are balanced by denitrification and volatilization back to the atmosphere. The land and ocean biotic reservoirs (N-fixing plants, non-N-fixing plant, and marine biota) interact with atmospheric N2 and dissolved inorganic nitrogen (DIN, consisting of N2, NO3-, and NH4+) in soil and ocean waters through N fixation and DIN uptake. Remineralization of dissolved organic N and particulate organic N in water and sediments produces nitrate and ammonia of DIN. Land and ocean domains are linked by river transport that carries both dissolved and particulate nitrogen to the oceanic coastal zone. As the reported δ15N values of reservoirs and fractionation factors (ε = δ15Nproduct - δ15Nsource) in natural systems vary greatly, the isotope-mass balances and fractionation factors in the cycle were obtained by numerical iteration of the flux equations. Within the mass-balanced N cycle model, the isotope fractionation factors of the interreservoir fluxes are sensitive to the δ15N reservoir values. The resultant fractionation factors (ε) are mostly within the reported ranges. In general, culture studies of N isotope fractionation suggest that the magnitude of fractionation is smaller (less negative) under N-limited growth conditions where biota utilizes the heavier nitrogen, making the isotopic fractionation smaller. Our balanced, integrated N-cycle model shows faster turnover times and greater isotopic fractionation in biologically mediated processes, consistent with the data on individual reservoirs and processes.
B23A-0392
Modeling the effects of N deposition, precipitation variability, and soil texture on winter annual production and fire risk in Southern California deserts
Fire risk in deserts is increased by high production of annual grasses and forbs that create a continuous fine fuel bed in the interspaces between shrubs. Interspace production is influenced by many factors including water and N availability and soil texture, and so the DayCent model was used to investigate how production of herbaceous annuals change along gradients of these production-forcing factors. DayCent was calibrated on the interspace vegetation from a creosote bush scrub site in Joshua Tree National Park and validated on a second creosote site within the Park with different soils and climate. The DayCent model was well calibrated on the first site, but validation on the second site showed that the model is sensitive to soil clay content such that soils with low clay contents lose soil C and N during model equilibration. Despite discrepancies between modeled and observed soil C and N pools, relative response of production to N fertilization was well represented by the model. Thus, DayCent can be used to determine conservative estimates of fire risk in the desert under increased precipitation and N deposition. Fire risk simulations indicate that interspace vegetation is strongly limited by water availability when mean annual precipitation is less than 21 cm yr-1. Under simulated N deposition of 8 kg-N ha-1, approximating the most polluted regions in these deserts, fire risk increases to 62-76% under a range of soil textures indicating that under increased N deposition fire risk is high in years of average to above-average rainfall.
B23A-0393
Coupled isotopic and simulation modeling of gaseous nitrogen losses from tropical rainforests
Gaseous nitrogen (N) losses remove fixed N from the biosphere and play an important role in regulating Earth's climate system. Current techniques for directly measuring gaseous N fluxes are still limited, however, and many uncertainties remain. We combined natural isotopic and simulation modeling (DAYCENT; daily version of CENTURY) to examine the extent to which N isotopes offer meaningful constraint to estimates of large-scale gaseous N emissions from terrestrial ecosystems. The isotope model considers two scenarios: in the first, soil δ15N is a linear function of fraction of gaseous N losses; in the second, underexpression of the isotope effect of denitrification is considered and soil 15N/14N is determined by both the fraction of gaseous losses and the proportion of nitrate consumed locally by denitrification. We examined the coupled simulation and isotope-based model along two Hawaiian rainforest gradients which span a range of tropical rainfall climates, soil biogeochemical ages and ecosystem 15N/14N. Under most conditions (MAP < 4050 mm and age > 2100 yr), modeled soil 15N/14N ratios agreed reasonably well with measurements (r2 = 0.53), consistent with full expression of a field-calibrated isotope effect (scenario 1). In very wet sites (MAP > 4050 mm), locally complete consumption of nitrate appears to lower the effective isotope effect of denitrification at ecosystem levels, resulting in soil 15N/14N ratios that approach those of the N inputs (i.e., scenario 2). Replacing DAYCENT simulation results with field-based measures of N gas fluxes (NOx + N2O) yielded consistently lower estimates of soil 15N/14N ratios across the forests, pointing to a missing gas N loss term (i.e., N2), inadequate coverage of spatial and temporal heterogeneity by empirical measures or both. These results demonstrate the potential for soil N isotopes to constrain N gas fluxes at large geographic scales, implying a quantitative tracer for gaseous N losses from terrestrial ecosystems.
B23A-0394
Trade-offs between nitrous oxide emission and C-sequestration in the soil: the role of earthworms
The rapidly rising concentrations of the greenhouse gas carbon dioxide (CO2) in the atmosphere has spurred the interest in soils as a potential carbon (C) sink. However, there are many reports indicating that C- sequestration is often negated by elevated emissions of the potent greenhouse gas nitrous oxide (N2O). It is not yet clear what the driving factors behind this trade-off are, nor how it can be avoided. We suggest that earthworm activity may be partly responsible for the trade-off. Earthworm activity is increasingly recognized as being beneficial to C-sequestration through stabilization of SOM. We report experimental results suggesting that they can also lead to strongly elevated N2O-emissions. In a first experiment, dried grass residue (Lolium perenne) was applied at the top of a loamy soil or mixed through the soil, and N2O-emission was followed for three months. Treatments included presence of the epigeic earthworm Lumbricus rubellus and the anecic earthworm Aporrectodea longa. Cumulative N2O-emissions increased significantly for both species. The strongest effect was measured for L. rubellus, where N2O-emissions significantly increased from 55.7 to 789.1 micro g N2O-N kg- 1 soil. This effect was only observed when residue was applied on top of the soil. In a second experiment we determined the effect of epigeic (L. rubellus) and endogeic (Aporrectodea caliginosa) earthworms on N2O-emissions for two different soil types (loam and sand) in the presence of 15N-labeled radish residue (Raphanus sativus subsp. oleiferus). Both species showed significant increases in N2O-emissions, which differed with residue application method and soil type. N2O- emissions were generally larger in loamy soils and the strongest effect was measured for A. caliginosa when residue was mixed into the soil, increasing emissions from 1350.1 to 2223.2 micro g N2O-N kg- 1 soil. L. rubellus only resulted in elevated N2O-emissions when residue was applied on top. These studies make it clear that elevated N2O-emissions due to earthworm activity is a widespread phenomenon, and that the nature of earthworm-induced effect is largely controlled by its feeding habit and interactions with other species. Our results contribute to understanding the important but intricate relations between (functional) biodiversity and the soil greenhouse gas balance.
B23A-0395
Aridland Emissions of Reactive Nitrogen Gases: The Importance of Abiotic Processes
Gaseous reactive nitrogen compounds play an important role in the atmospheric chemistry defining climate and air quality. Our current understanding of the production of trace N gases at the soil surface is largely based on the microbial processing of soil N. However, measurements in the Mojave Desert show substantial soil emissions of nitrogen oxides and ammonia under conditions that are unlikely to support biological activity: extremely high temperature and low moisture conditions. Summertime patterns of trace N gases efflux show strong diurnal patterns, on average midday maximum fluxes were 75 times higher than nighttime lows. Diurnal flux patterns appear to most closely match daily changes in the intensity of solar radiation and not soil temperature, suggesting a direct role of solar radiation in driving abiotic formation of reactive nitrogen gases. Experimental blocking of incoming radiation resulted in rapid and dramatic reductions in fluxes of both nitrogen oxides (NO + NOy) and ammonia. On average, midday fluxes dropped by 70 and 35% for nitrogen oxides and ammonia, respectively. The chronology of this response closely matched changes in surface soil temperature (1cm) indicating that extreme surface soil temperatures (65-80° C) caused by incoming solar radiation may contribute to reactive N gas loss from these soils. Following a simulated 20 mm precipitation event, fluxes increased 40-fold, yielding total reactive N gas losses of 175 ± 23 ng N m- 2 s-1 composed of an equal mixture of nitrogen oxides and ammonia. Responses to blocking incoming radiation were even stronger in irrigated soils with 65-70% reductions in emissions for all gas species. Results suggest that under high light and temperature conditions a series of abiotic processes yield considerable reactive N gas production, especially following pulse rain events. Such abiotic processes likely dominate reactive N gas loss from the Mojave Desert and these non-biological sources release approximately 0.5 kg N ha-1 of reactive N into the atmosphere during summer months. Given the relevance and sensitivity of trace N gases to global climate change, characterization of abiotic mechanisms driving reactive N gas loss in arid environments will be essential to understanding and predicting current and future biosphere-atmosphere exchange of reactive nitrogen gas species.
B23A-0396
Nitrous Oxide From Fertilized Cropland in the New York State as Affected by Different Practices
The detrimental loss of N from N fertilizers is a serious environmental problem of modern agriculture. Amongst the threats that elevated N use poses to the environment, the increase in net N2O production is of special concern, due to its persistent atmospheric effect as a greenhouse gas and as a contributor to ozone destruction. Long-term monitoring of N2O flux was conducted on a mid-size dairy farm in central New York to determine N2O emission levels from agricultural practices that are common for the region. The technique used was Tunable Diode Laser Absorption Spectrometer (TDLAS) coupled with 3D sonic anemometer; continuous eddy flux observations were conducted throughout three years on an alfalfa field in 2006, corn field in 2007, and on a split alfalfa/corn site in 2008 (ongoing). All fields were fertilized after/before growing seasons with dairy manure only. The results showed highly variable, instantaneous nature of N2O flux, with peak values reached after strong and sharp increases in both soil moisture and temperature. Manure spreading affected N2O fluxes only when factors of temperature and moisture were not limiting: at temperatures below 5°C, manure spreading had no pronounced effect on the N2O release. Average flux values were higher for corn (0.059μg m-2 s-1) then for alfalfa (0.045μg m-2 s-1), which was possibly related to manure application during spring thaw and its further incorporation with the plowing on the corn field. The greatest flux value for 2006-2007 of 0.76μg m-2 s-1 was also observed for the corn field during a spring thawing event.
B23A-0397
A Coupled Land Surface-Subsurface Biogeochemical Model for Aqueous and Gaseous Nitrogen Losses
In recent years concern has grown over the contribution of nitrogen (N) fertilizers to nitrate (NOB3PB-P) water pollution and atmospheric pollution of nitrous oxide (NB2BO), nitric oxide (NO), and ammonia (NHB3B). Characterizing the amount and species of N losses is therefore essential in developing a strategy to estimate and mitigate N leaching and emission to the atmosphere. Indeed, transformations of nitrogen depend strongly on water content, soil temperature, and nitrogen concentration. Land surface processes therefore have to be taken into account to properly characterize N biogeochemical cycling. However, most current nitrogen biogeochemical models take the land surface as the upper boundary by lumping the complex processes above the surface as known boundary conditions. In this study, an extant subsurface mechanistic N cycle model (TOUGHREACT-N) was coupled with the community land model (CLM). The resulting coupled model extends the modeling capability of TOUGHREACT-N to include the important energy, momentum, and moisture dynamics provided by CLM. The coupled model showed a significant impact of land-surface diurnal forcing on soil temperature and moisture and on nitrogen fluxes. We also discuss field applications of the model and discuss how temporal dynamics of nitrogen fluxes are affected by land surface processes.
B23A-0398
Relationship between N2O Fluxes from an Almond Soil and Denitrifying Bacterial Populations Estimated by Quantitative PCR
Cultivated soils emit substantial quantities of nitrous oxide (N2O), a greenhouse gas with almost 300 times the radiative forcing potential of CO2. Agriculture-related activities generate from 6 to 35 Tg N2O-N per year, or about 60 to 70% of global production. The microbial processes of nitrification, denitrification and nitrifier denitrification are major biogenic sources of N2O to the atmosphere from soils. Denitrification is considered the major source of N2O especially when soils are wet. The microbial N transformations that produce N2O depend primarily on nitrogen (N) fertilizer, with water content, available carbon and soil temperature being secondary controllers. Despite the fact that microbial processes are responsible for N2O emissions, very little is known about the numbers or types of populations involved. The objective of this study was to relate changes in denitrifying population densities, using quantitative PCR (qPCR) of functional genes, to N2O emissions in a fertilized almond orchard. Quantitative PCR targeted three specific genes involved in denitrification: nirS, nirK and nosZ. Copy numbers of the genes were related back to population densities and the portion of organisms likely to produce nitrous oxide. The study site, a 21.7 acre almond orchard fitted with micro-sprinklers, was fertigated (irrigated and fertilized simultaneously) with 50 lbs/acre sodium nitrate in late March 2008, then irrigated weekly. Immediately after the initial fertigation, fluxes of N2O and CO2, moisture content, inorganic N and denitrification gene copy numbers were measured 6 times over 24 days. Despite the fact that N2O emissions increased following fertigation, there was no consistent increase in any of the targeted genes. The genes nirK and nirS ranged from 0.4-1.4 × 107 and 0.4-1.4 × 108, whereas nosZ ranged from 2-8 × 106 copy numbers per g soil, respectively. Considerable variation, compounded by the small sample sizes used for DNA analysis, made it difficult to discern trends over time. High spatial variability was also observed with one of the field replicates have a substantially higher flux of N2O. This replicate also had the highest water filled pore space (WFPS) and water content, factors that likely favored denitrification. Water saturation of soil air space, optimal at >60% for denitrification to occur, was relatively low in the other field replicates. Thus, the low N2O flux measurements and gene copy numbers agreed in supporting the hypothesis denitrification was relatively low under the environmental conditions of these particular almond soils.
B23A-0399
Tree-Ring Nitrogen Isotopes As Environmental Monitoring Tools – Inferring Air Quality Changes And Climate Effects
Anthropogenic emissions of atmospheric nitrogen greatly increased over the last 150 years, however the monitoring of nitrous oxide concentration in North America started only recently, generally during the last 30 years. Could the geochemical characteristics of tree rings be used to infer past changes in nitrogen cycles of temperate regions? To address this question we use long-term series (125 years) of nitrogen stable isotopes (δ15N) obtained from rings of pine (Pinus strobus) and beech (Fagus grandifolia) trees in the Montreal region (western Quebec), and of beech specimens in the Georgian Bay Islands National Park (central Ontario). Reliability tests of N concentrations in wood treated for removal of soluble materials reveal that the reproducibility from tree to tree is poor, and that the concentrations in both Pine and Beech trees change in the heartwood-sapwood transition zones. We therefore reject N concentration as environmental indicator. Alternatively, the N stable isotopes pass all reliability tests. In Montreal, short-term δ15N fluctuations correlate directly with precipitation and inversely with temperature. A long-term decreasing isotope trend suggests progressive changes in soil chemistry after 1951. A pedochemical change is also inferred for the Georgian Bay site on the basis of a positive δ15N trend initiated after 1971. At both sites, the long-term δ15N series correlate with a proxy for NOx emissions, and the δ13C values of the same ring series suggest that all studied trees have been stressed by phytotoxic pollutants. We propose that the contrasted long-term δ15N changes of Montreal and Georgian Bay reflect deposition of NOx emissions from cars and coal-power plants, with higher proportions from coal burning in Georgian Bay. This interpretation is conceivable because recent monitoring indicates that coal-power plant NOx emissions play an important role in the annual N budget in Ontario, but they seem negligible on the Quebec side. This research suggests that tree-ring nitrogen isotopes may record regional climatic conditions and anthropogenic perturbations of the N cycle.
B23A-0400
Foliar Uptake of Atmospheric Reactive Nitrogen Pollution Along an Urban-Rural Gradient in New York State
Vegetation is an important sink for atmospheric reactive nitrogen (N) pollution in terrestrial ecosystems, and when soil N is limiting, foliar N uptake can be a source of plant-available N. A proxy for pollution derived N, and in particular foliar assimilated N, would be useful to quantify the impact of the foliar uptake pathway on plant metabolism. Nitrogen stable isotope ratios (15N/14N) are practical for this purpose because forms of plant-available N often have varying isotopic compositions. However, the mechanisms driving differences in foliar N isotopic composition (δ15N) are still unresolved. Current understanding of foliar δ 15N suggests these values primarily represent the integration of the soil water solution δ15N, direct foliar uptake of atmospheric reactive N, within-plant fractionations, and fractionation due to the fungus to root transfer in mycorrhizae. In this study, we investigated the influence of direct foliar uptake, soil solution δ 15N, and mycorrhizae on foliar δ15N in seedlings of two dominant Northeastern tree species, red maple (Acer rubrum) and red oak (Quercus rubra), along an N deposition gradient in New York State. Using a potted plant mesocosm system, we compared foliar δ15N values directly to soil solution δ15N values while controlling for mycorrhizal associations. Both species showed higher foliar δ15N when exposed to fractionation by mycorrhizal associations. Overall, A. rubrum showed higher foliar δ15N than Q. rubra across all sites. In both species, patterns of foliar δ15N values were coupled with soil solution δ15N values across the N deposition gradient. Additionally, increasing atmospheric N deposition was correlated with higher foliar δ15N values in Q. rubra, but not in A. rubrum. Using a mixing model, we estimated that Q. rubra seedlings incorporated up to 7% of their assimilated N via direct foliar uptake of atmospheric N pollution. However, foliar uptake was not detectable in A. rubrum seedlings. Results suggest that the use of foliar δ15N values may be an effective tool to estimate the magnitude of foliar uptake of pollutant N compounds under some circumstances.
B23A-0401
δ15Nitrogen in Tree Rings of Temperate Forest Trees: Indicators of Past Changes in Soil Nitrogen Cycling
Forest disturbances can result in a temporary increase in soil nitrification and nitrate (N03-) losses from forest soils to nearby streams. Nitrate leaching can alter soil nutrient balances, cause soil acidification and contribute to declines in biodiversity of nearby aquatic ecosystems. Previous studies have shown that clear-cutting induced nitrification, followed by NO3- losses from soils, can lead to elevated δ15N values of remaining nitrogen in soils and foliage. As trees reforest an area following a clear- cut, nitrification rates tend to decline, bringing soil and foliar δ15N values to background levels. In this study, we sought to determine whether δ15N values of bole wood of trees could be used as an indicator of past pulses of elevated nitrification followed by NO3- leaching. Since trees obtain nitrogen from the soil, we expected that immediately following a clear-cut, δ15N in tree rings would spike and then return to pre-disturbance levels. We sampled bole wood from individuals of American Beech (Fagus grandifolia) and sugar maple (Acer saccharum), two dominant tree species of northeastern hardwood forests. Wood was collected from mature trees growing in forest sites without recent disturbance and from trees growing in a previously clear-cut forest at the Hubbard Brook Experimental Forest in New Hampshire. Our results show that for both species, δ15N values were greater in wood from the previously cut watershed compared to the undisturbed sites for the first three years following the forest cut. After three years, there was no significant difference between the disturbed and undisturbed forested watersheds. These results suggest that δ15N of bole wood from sugar maple and American beech trees can be used as indicators of past pulses of nitrification followed by NO3- losses, illustrating that δ15N within tree rings can be used as an indicator of past disturbances and changes in soil nitrogen cycling in temperate forest ecosystems.
B23A-0402
Concentrations and Fluxes of Water-Soluble Reactive Nitrogen Gases and Aerosol Compounds Above a Forest Canopy
In summer 2007 we measured concentration gradients of NH3, HNO3, HONO and related aerosol species NH4+ and NO3- as well as SO2, and aerosol SO42- above a spruce canopy in south-east Germany (50° 09"N, 11° 52"E, 775m asl). Measurements were performed as part of an intensive observation period within the framework of the EGER (ExchanGE processes in mountainous Regions) project. NH3, HNO3, HONO, SO2, aerosol NH4+, aerosol NO3-, and aerosol SO42- were measured using the Gradient Analyzer for Aerosols and Gases, mounted on a tower. Water-soluble gases and aerosol species were collected simultaneously at two different heights by two rotating wet-annular denuders and two Steam-Jet Aerosol Collectors, respectively. Samples were analysed on-line via ion chromatography and flow injection analysis. To our knowledge this was the first time that these gas and aerosol species were measured simultaneously and with high time resolution (30 min) above a forest canopy. Data accuracy and precision is provided by a rigorous data screening, including the use of an internal standard, careful error estimation and repeated in- field blanks. Gradient precision of the measurements are derived from extended periods of side-by-side sampling of the sample boxes (n = 257). NH3 mixing ratios reached their maximum in the late afternoon with 2 to 3 ppb and their minimum in morning hours with 0.25 ppb, whereas aerosol NH4+ mirrored this behaviour with maximum values late night and early morning with 4 up to 8 ppb and minimum values in the afternoon, around 0.5 ppb and less. HNO3 and aerosol NO3- diel cycles also mirrored each other, HNO3 maxima during late afternoon ( above 1 ppb) and minimum during night and early morning with less than 0.2 ppb and aerosol NO3- maxima during night ( around 2 ppb, up to 6 ppb) and minima during afternoons with 0.5 ppb. Patterns of aerosol NH4+ and aerosol NO3- in the time series are apparently closely related. NH3 gradients indicate bidirectional fluxes, whereas HNO3 gradients are indicating net deposition. These gradients may be biased by micrometeorology and chemistry. For example, gradients in NH3 and HNO3 may be product of a phase change in the thermodynamic equilibrium between NH3, HNO3 and particulate NH4NO3, induced by a temperature and/or humidity gradient above the forest canopy. The equilibrium will be investigated for the pure NH3- HNO3- NH4NO3 triad as well as for more complex inorganic aerosol mixtures and chemical timescales will be compared to turbulent timescales, to estimate the potential of chemical interferences affecting the gradient. If compounds react sufficiently slow and may therefore be treated as passive tracers, prerequisites for the application of micrometeorological methods to derive fluxes from gradients will be investigated.
B23A-0403
Use of Laboratory and Remote Sensing Techniques to Estimate Vegetation Patch Scale Emissions of Nitric Oxide From an Arid Kalahari Savanna
The biogenic emission of nitric oxide (NO) from the soil has an important impact on a number of environmental issues, such as the production of tropospheric ozone, the cycle of the hydroxyl radical (OH) and the production of NO. Arid regions cover a significant proportion of the earth's surface; however there have been relatively few studies on the biogenic emissions of NO from these ecosystems. In this study we collected soils from four differing vegetation patch types (Pan, Annual Grassland, Perennial Grassland and Bush Encroached) in an arid savanna ecosystem in the Kalahari (Botswana). A laboratory incubation technique was used to determine the net potential NO flux from the soils as a function of the soil moisture and the soil temperature. The net potential NO emissions were then up-scaled for the year 2006 and a region (185km x 185km) of the southern Kalahari. For that we used (a) the net potential NO emissions measured in the laboratory, (b) the vegetation patch distribution obtained from Landsat NDVI measurements, (c) estimated soil moisture contents obtained from ENVISAT ASAR measurements, and (d) the soil surface temperature estimated using MODIS MOD11A2 8 day land surface temperature measurements. There are differences in the net potential NO fluxes between differing vegetation patches which range from 0.27 ng m-2 s-1 in the Pan patches to 2.95 ng m-2 s-1 in the Perennial Grassland patches. Up- scaling results in NO fluxes of up to 323 g ha-1 month-1, where the highest up-scaled NO fluxes occurred in the Perennial Grassland patches, and the lowest in the Pan patches. A marked seasonal pattern was observed where the highest fluxes occurred in the austral summer months (January and February 2006) while the minimum fluxes occurred in the austral winter months (June and July 2006) where the up-scaled NO fluxes were less than 1.8 g ha-1 month-1. Over the course of the year the mean NO emission for the up-scaled region was 0.54 kg ha-1 yr-1, which accounts for a loss of up to 7.4% of the nitrogen input to the region through atmospheric deposition and biological nitrogen fixation. The biogenic emission of NO from the soil is therefore an important mechanism of N loss from this arid savanna ecosystem and has the potential to play an important role in the production of tropospheric ozone and the OH cycle.
B23A-0404
A mechanistic, globally-applicable model of plant nitrogen uptake, retranslocation and fixation
Nitrogen is one of the nutrients that can most limit plant growth, and nitrogen availability may be a controlling factor on biosphere responses to climate change. We developed a plant nitrogen assimilation model based on a) advective transport through the transpiration stream, b) retranslocation whereby carbon is expended to resorb nitrogen from leaves, c) active uptake whereby carbon is expended to acquire soil nitrogen, and d) biological nitrogen fixation whereby carbon is expended for symbiotic nitrogen fixers. The model relies on 9 inputs: 1) net primary productivity (NPP), 2) plant C:N ratio, 3) available soil nitrogen, 4) root biomass, 5) transpiration rate, 6) saturated soil depth,7) leaf nitrogen before senescence, 8) soil temperature, and 9) ability to fix nitrogen. A carbon cost of retranslocation is estimated based on leaf nitrogen and compared to an active uptake carbon cost based on root biomass and available soil nitrogen; for nitrogen fixers both costs are compared to a carbon cost of fixation dependent on soil temperature. The NPP is then allocated to optimize growth while maintaining the C:N ratio. The model outputs are total plant nitrogen uptake, remaining NPP available for growth, carbon respired to the soil and updated available soil nitrogen content. We test and validate the model (called FUN: Fixation and Uptake of Nitrogen) against data from the UK, Germany and Peru, and run the model under simplified scenarios of primary succession and climate change. FUN is suitable for incorporation into a land surface scheme of a General Circulation Model and will be coupled with a soil model and dynamic global vegetation model as part of a land surface model (JULES).
B23A-0405
Patterns in Nitrogen Cycling Across Diverse California Soils Subjected to Nitrogen Inputs
California receives the highest rates of anthropogenic nitrogen (N) deposition in the western United States. Changes in N cycling with N deposition are likely to be strongly influenced by native soil C and N content. We performed a ten-month long laboratory incubation using soils from 33 California ecosystems, including deserts, wetlands, grasslands, shrublands and forests, which ranged widely in soil C (0.07-5.40%) and N (0.009-0.35%) content. Plant-free soils were maintained at field capacity and four treatment levels of N (0, 5, 20, 80 kg-N/ha/y equivalent) were added as ammonium nitrate solution weekly. We measured gross N cycling rates, dissimilatory NO3 reduction to NH4 (DNRA), and soil C and N content initially and after ten months. Soil N cycling rates did not respond to the N addition treatments but differed by ecosystem both initially and after incubation. Initially, wetlands and forests had the highest gross mineralization rates, and shrublands had the highest gross nitrification rates. After ten months, desert soils exhibited lower gross nitrification rates and wetland soils exhibited higher DNRA rates compared to all other ecosystems (p< 0.001). Inorganic N pools and soil C content were strongly predictive of N cycling rates though the controls on N cycling rates generally differed before and after incubation. Across biomes, initial gross mineralization rates were best predicted by soil NH4 and NO3 concentrations whereas final rates were best predicted by final C content (R2= 0.39 and 0.43, respectively). Initial gross nitrification rates were best predicted by the combination of soil NH4 concentrations, total soil N content, and DNRA rates (R2= 0.49). However, final gross nitrification rates were best predicted by initial C content and final NO3 concentrations (R2= 0.73). In contrast, both initial and final DNRA rates were best predicted by final soil NH4 and NO3 concentrations (R2= 0.83 and 0.73, respectively). Neither final soil NH4 and NO3 concentrations nor final soil C content were correlated with N addition. These results suggest that microbial processes were relatively insensitive to N inputs in the long-term laboratory incubation.
B23A-0406
Topographic Controls of CH4 and N2O Fluxes from Deciduous Forest Soils, southern Quebec
Forest landscapes are not homogenous, but consist of a mosaic of well and poorly drained soils determined by topography. Variation in topography and drainage classes influences biogeochemical controllers of CH4 and N2O fluxes from soils such as moisture, N mineralization and production and decomposition of organic carbon. We measured N2O and CH4 fluxes from two deciduous forests near Montreal, along transects running from high-elevation, well-drained to low-elevation, poorly-drained soils. One site is an old-growth and the other, a mature semi-managed forest dominated by American beech and sugar maple trees. In-situ gas fluxes were measured bi-weekly using static chambers from May 2006 to May 2008. Well-drained soils consume 13 ± 1 while poorly-drained soils emit 92 ± 22 g CH4 m-2 y-1. CH4 consumption in well-drained soils in the old-growth forest site was more than twice as large than in the semi-managed forest due mainly to higher soil porosities in the old-growth forest supporting faster diffusion of CH4 into consumption sites. Soil moisture, temperature and CO2 emission rates were the main controllers of hourly CH4 fluxes at plot scale both in well and poorly-drained soils. Well-drained soils emitted 37 ± 8 and poorly-drained soils emitted 115 ± 35 mg N2O-N m-2 y-1. On certain sampling dates, atmospheric N2O was consumed ranging from -237 to -9 mg N2O-N m-2 y-1. N2O consumption was mainly driven by higher soil organic C substrate, low soil NO3 contents under anoxic conditions. Background, seasonal and even-based N2O fluxes were apparent from these sites. Hourly N2O flux rates at plot scale were highly variable and did not correlated with any environmental variable; however, averaged annual emission rates were markedly controlled by soil C:N ratios, N mineralization rates and soil NO3 contents. Our data shows that seasonal CH4 fluxes at watershed scale can be estimated using soil moisture, temperature and drainage class, while annual N2O fluxes can be estimated using soil C:N ratios. Topographic position need to be accounted when modeling CH4 and N2O fluxes from forested landscapes in Canada.
B23A-0407
Using The Ecosys Mathematical Model to Simulate Topographic Effects on Spatial Variability of Nitrous Oxide Emissions from an Agricultural Soil
Calculation of emission factors (EFs) for nitrous oxide (N2O) is complicated by their large spatial variability. The objective of this study was to test the hypotheses that spatial variation in N2O emissions can be explained by (1) spatial and temporal variation in soil water-filled pore space (WFPS) among topographic positions that shed or collect water according to topographically-driven water movement, and (2) spatial variation in soil properties which may themselves be caused by topographically driven water movement. These hypotheses have been incorporated into a detailed processed-based, three-dimensional mathematical model of terrestrial ecosystems, ecosys. We simulated emissions using ecosys at different spatial scales – meter, fetch and field, using a 20 x 20 matrix of 36m x 36m grid cells from a digital elevation model (DEM) to represent topography of a fertilized agricultural field in Ottawa, Canada. Modeled results were compared to fluxes measured with chambers placed at different topographic positions to measure spatial variability of N2O emissions at the meter scale, and with stationary and mobile flux towers with tunable diode lasers (TDL) and a flux-gradient technique to assess spatial N2O variability at the fetch scale. Most modeled and measured emissions occurred during a 10-day interval during late spring/early summer, due to a combination of fertilizer N application, rainfall and rising soil temperatures. Coefficients of spatial variation (CSVs) amongst 4 chamber replicates (2 x 3 m grid) during emission events were 28 to 195%, indicating that spatial variation of N2O occurs at a very small spatial scale. Modeled annual CSVs at the field scale rose from 25% (uniform soil) to 101% when soil properties in the model were allowed to vary according to results from a field soil survey. The modeled EF (uniform soil properties) assumed for 112 kg N ha-1 was larger in an area of the field with lower topography (0.3%) compared to one with higher (0.1%). EFs were comparatively low because nitrification of fertilizer N occurred in slightly cooler soil temperatures compared to long-term normals for this site. These results show the importance of the use of 3-dimensional models such as ecosys at an hourly time-step with input from DEMs, to fully capture the large spatial and temporal variability of N2O at different spatial scales even in seemingly flat (0.2% slope) landscapes.
B23A-0408
Climate warming and N deposition in a temperate ecosystem: does winter warming increase N losses?
Changes in soil freezing dynamics caused by climate warming may reduce the retention of atmospheric N deposition over winter in temperate ecosystems as a result of either increased rates of N mineralization at a time when plants roots are largely inactive or as a consequence of increases in the intensity and frequency of soil freeze-thaw cycles. We examined the recoveries of trace amounts of 15N-labeled nitrate and ammonium added to the soil surface at spring thaw in the context of a factorial warming and N addition experiment designed to simulate future climate and N deposition rates in a temperate old field. We sampled plant shoots, roots and bulk soil the fall after the addition of the label and again the next spring to examine both the fate of N added at spring melt and the retention and redistribution of this N over winter, respectively. In the fall, recovery of 15N from the added ammonium was approximately double that of the added nitrate in both roots and soil but there were no significant effects of either the warming or nitrogen addition treatments on recovery of the added 15N. Over winter, there was a net loss of the label from soil in the N addition plots but a net increase of the label in soil from control plots that received no added nitrogen, whereas losses of the label from roots and shoots were equal for both the treatment and control. In spring, both roots and shoots contained approximately one quarter more of the label in warmed plots than in ambient temperature plots, whereas both plant shoots and soil contained approximately one quarter less label in nitrogen addition plots than in control plots. Overall, these results reveal that contrary to our hypothesis that warming would enhance ecosystem losses of nitrogen by disrupting soil freezing dynamics, increased N retention in warmed soil instead offset N losses in nitrogen enriched soils after one year.
B23A-0409
Source Apportionment of Sulfur and Nitrogen Species at Rocky Mountain National Park using Modeled Conservative Tracer Releases and Tracers of Opportunity
Visibility degradation and changes in ecosystem function in Rocky Mountain National Park (RMNP) are occurring because of emissions of nitrogen and sulfate species along the Front Range of the Colorado (CO) Rocky Mountains, as well as sources farther east and west. The nitrogen compounds include oxidized, reduced, inorganic, and organic nitrogen. The Rocky Mountain Atmospheric Nitrogen and Sulfur study (RoMANS) was initiated to better understand the origins of sulfur and nitrogen species as well as the complex chemistry occurring during transport from source to receptor. This included a source apportionment assessment to identify the relative contributions to atmospheric sulfur and nitrogen species in RMNP from within and outside of the state of CO; from emission sources along the Colorado Front Range; as well as the relative contributions from mobile sources, agricultural activities, and large and small point sources within CO. As part of the study, a monitoring program was conducted for two 5-week periods, one during the spring, the other during late summer. Monitoring data of ammonium/ammonia, nitrogen oxide/nitrates, and sulfur dioxide/sulfates were combined with tracers of opportunity and modeled releases of conservative tracers from source regions around the United States to apportion these species to their respective sources, using a variety of receptor modeling tools. The preliminary results show that during the spring a larger fraction of nitrogen and sulfur species came from sources within CO than during the summer. Specifically, during the spring, sources within CO contributed more than 80% of the ambient NH3, NH4+, HNO3, and particulate NO3-; 50% of SO2 and 30% of particulate SO4= came from sources within CO. During the summer period CO sources contributed to about 75% of the ambient NH3 and 55–60% of the NH4+ and HNO3; 30% of SO2, and 20% of particulate SO4=.
B23A-0410
The Nitrogen Budget for a Mediterranean Chamise (Adenostoma fasciculatum L.) Dominated Watershed, Sierra Nevada, CA
Chaparral ecosystems in Southern California receive among the highest rates of atmospheric N deposition in the country. Elevated N additions are known to induce deleterious effects to both terrestrial and aquatic ecosystems such as direct effects on plant physiology, soil acidification, and eutrophication. We studied N dynamics in a 4.3-hectare Chamise dominated watershed located in the foothills of the western Sierra Nevada to document the response of seasonally dry Mediterranean catchments to elevated rates of N deposition. During our study, we monitored stream water DIN, DON, DOC, base cations, and chloride during the 2003 and 2004 water years. Preliminary results suggest that despite elevated N inputs of approximately 10 kg N ha1 yr-1 based on Fenn collectors, only about 0.02 kg N ha-1 yr-1 were exported in streamwater. DIN export patterns were typical of Mediterranean ecosystems exhibiting a strong nitrate flush during storms at the onset of the wet season, but with DIN concentrations remaining elevated regardless of decreases in stream discharge. Further research will provide a better understanding of N dynamics as we integrate measurements of DON, DOC, base cations, and chloride.
B23A-0411
Riparian invasive alters stream nitrogen dynamics.
Invasive species may be most likely to have strong effects on the ecosystem they invade when they contribute a new function such as nitrogen (N) fixation. Russian olive (Eleagnus angustifolia) is a non-native invasive tree which is rapidly spreading along riparian corridors in the American West. Russian olive is a nitrogen fixing plant due to a symbiotic relationship with Actinomycetes and is invading systems that frequently lack a strong native N fixer. The contribution of reactive N by these invasive riparian plants to soils may also be altering N cycling and processing in the adjacent streams. We measured nutrient limitation via periphyton growth on nutrient diffusing substrates and nitrate uptake using short term nitrate additions in Deep Creek, ID. Measurements were made in three reaches along a Russian olive invasion gradient, with an upstream reference reach that has no Russian olive and two downstream invaded reaches, one with moderate density and one with high density. Periphyton growth in Deep Creek was significantly N limited in the reference reach while the moderately invaded reach showed no significant limitation and the highly invaded reach was significantly P limited. The nitrate uptake velocity (Vf) for both of the invaded reaches was an order of magnitude less than the reference reach, implying that biological demand for nitrate is significantly less in the invaded reaches than the reference. Considering the current extent of Russian olive invasion and its continued rapid spread, possible alteration of N cycling in waterways may have important implications for the management of both this invasive species and management of nutrient pollution in waters of the western U.S.