B31A-0053
In situ isotope signals of nitrate show how the gross nitrification was high in the surface soils in a humid temperate forest in Japan
In order to elucidate the intensity and mechanism of the impact of atmospherically derived nitrate on the soil nitrogen dynamics, dual isotope measurements on nitrate (δ15N and δ18O) in soil waters were conducted in a forested watershed in central Japan. Soil waters have been sampled from various horizons of the soil profiles to elucidate the vertical distributions of nitrogen transformation processes. In general, the δ18O of nitrate in rainwater is remarkably higher than that produced by nitrifying bacteria in soils. Accordingly, the δ18O can often be used as an index of the impact of the atmospherically derived nitrate. While soil waters in <20cm depth had a strong signal of the atmospheric nitrate, the δ18O-nitrate in soil water decreased in the deeper soil horizons, indicating that the dominant source of nitrate in this soil profile was nitrification. The net nitrate production of this soil profile was about 18 kg-N ha-1year-1, and deposited nitrate was about 6 kg-N ha-1year-1. Assuming that the annual mean δ18O of deposited nitrate was 60 ‰, and the mean value of bacterially produced nitrate in soil was about 0 ‰, the average value for soil nitrate pool could be ~15 ‰. However, the observed δ18O of the soil and groundwater was 0 to 6 ‰ and remarkably smaller than the above estimation based on annual mass balance. This suggests that the gross nitrification was sufficiently higher than net nitrification rate, and the major portion of nitrate produced in soil was reused by microbes. Previous studies in temperate forests have shown from laboratory experiments using isotope dilution method that gross nitrification in soils was several times greater than net nitrification. We could verify this phenomenon from our δ18O-nitrate measurements of the field samples.
B31A-0054
Effect of a nitrogen-carbon interaction on terrestrial carbon fluxes estimated by biosphere model
It is important for the global warming to accurately understand the terrestrial carbon fluxes at global scale. Estimating spatial and temporal patterns in the carbon fluxes, recently, many global biosphere models were proposed and developed. However, since the model analyses have always some uncertainties. One of the major uncertainties is an effect of nitrogen cycle on the carbon cycle, as nitrogen largely controls carbon dynamics as plant and soil microbe nutrients. A goal of this study is to investigate the effect of terrestrial carbon-nitrogen interaction on NPP using new biosphere model. Firstly, a new nitrogen cycle model was constructed including twelve main nitrogen flows (nitrogen fixation, deposition, nitrifications, volatilization, nitrate leaching, plant uptake, allocation, translocation, retranslocation, soil organic and inorganic nitrogen dynamics), and fourteen pools (three biomass, four litter fall, five soil organic, and two inorganic). Secondly, the nitrogen model was integrated to the existing biosphere model, BEAMS (Biosphere model integrating Eco-physiological And Mechanistic approaches using Satellite data) [Sasai et al., 2005, 2007]. The new biosphere model was run for 20 years (1982-2001) at a global scale. The inputs datasets used were NCEP/NCAR re-analysis and fPAR/LAI based on NOAA/AVHRR produced by Boston University. The two-dimensional distributions of monthly GPP and NPP were calculated. And, the GPP estimates by the original and new BEAMS were compared with ground measurements at flux-tower sites. We compared seasonal changes in GPP between the new model and eddy covariance measurements at flux sites. As a result, the GPP estimates had good agreement with the GPP measurements (r2 = 0.91). In view of a comparison in GPP between the measurements and the original BEAMS (r2 = 0.84), the new model is better than the original BEAMS. Especially, we could observe an indisputable improvement of the new model on a seasonal change in the growing and falling seasons of forest, so that the original BEAMS tends to exaggerate GPP in these stages. In spatial variation in GPP and NPP, a comparison in GPP estimates between the two models was shown that global spatial patterns are roughly much the same, but annual NPP trends are different. Especially, in case of southern Africa and South America, NPP by the original BEAMS showed decreasing trend (-1.8gC/m2/yr), whereas NPP by the new model were turned around (+3.6). We would confirm that nitrogen cycle is largely affected to carbon cycle, and biosphere model need to be developed for gradually increasing the affinity of nitrogen cycle.
B31A-0055
Carbon-Nitrogen Cycle Model of Terrestrial Ecosystem for Plot Scale: Application to Cool Temperate Deciduous Broad-leaved Forest and Cool Temperate Evergreen Coniferous Forest in Central Japan
By developing a biogeochemical model that fully includes carbon cycle and nitrogen cycle in a single framework, we can consider nitrogen nutrition as a limiting elements in the model and evaluate atmosphere-ecosystem exchange of trace greenhouse gases in addition to carbon dioxide (i.e. methane and nitrous oxide). In this study, a process-based model simulating carbon-nitrogen cycle was developed and applied to a cool-temperate deciduous broad-leaved forest in Takayama (36N, 137E, 1420m ASL) and a cool-temperate evergreen coniferous forest in Fujiyoshida (35N, 138E, 1030m ASL), central Japan. Models of nitrogen cycle were introduced into a revised terrestrial ecosystem model Sim-CYCLE, which is a simple box-type carbon cycle model simulating net ecosystem CO2 exchange on the basis of ecophysiological relationships. Using the model, photosynthetic and respiratory CO2 fluxes were simulated during the periods from 1948 to 2004 at daily step using time-series climate data. On average during the last 10 years, the model estimated that the temperate forest absorbed net CO2 and CH4 at rates of 804.53 g CO2 m-2 yr-1 and 0.34 g CH4 m-2 yr-1, and net released N2O at a rate of 0.02 g N2O m-2 yr-1, respectively. Based on the 100-year GWP of greenhouse gases in IPCC (2001), the forest was estimated to have a negative (i.e. ameliorating) effect of GWP by 807.74 g CO2 (equivalent) m-2 yr-1. Because the results seem reasonable, we are working on scaling-up of the model to regional scale, in our forthcoming studies.
B31A-0056
Water Soluble Organic Nitrogen in atmospheric aerosol samples from urban, sub-urban and pristine areas of Venezuela
Concentrations of water soluble organic nitrogen (WSON) were determined in atmospheric total suspended particles (TSP) collected between September of 2005 and May of 2006, in an urban continental (Caracas, 10° 29' 09'' N, 66° 53' 48'' W), an urban coastal (Catia la mar, 10° 35' 47'' N, 67° 01' 45'' W), a sub-urban coastal (Osma, 10° 32' N, 67° 28' W), a suburban continental (Altos de Pipe, 10° 23' 41'' N, 63° 59' 10'' W), a pristine coastal (Isla de Aves, 15° 40' N, 63° 36' W) and a pristine continental (La Gran Sabana National Park, 5° 41' 30'' N, 61° 34' 20'' W) areas of Venezuela. TSP samples were collected using a Hi-Vol airborne particle sampler. TSP were impacted on a fiberglass filter pretreated under 400° C for 4 hours to minimize organic nitrogen contamination. Ultra sound water extractions of the sample filters were performed and their NH4+, NO2- and NO3- concentrations were determined by ion exchange liquid chromatography. The water extracts were UV digested and the nitrogen inorganic ions were analyzed after the UV exposure. WSON concentrations were calculated by the difference between the inorganic nitrogen concentrations before and after UV digestion. Ninety five percent of the aerosol samples collected in the suburban and pristine areas showed a WSON concentration range from 0.03 to 0.6 μg/m3 whereas in urban areas the range was 0.21 to 1.09 μg/m3. These concentration values are on the same order of magnitude than the previously found in other tropical and subtropical areas. The contribution of aerosol WSON to the total soluble nitrogen in the coastal urban, sub-urban and pristine areas ranged from 23 to 67%, while in Caracas was smaller (38±8%, n=5). Therefore, aerosol WSON provides an important source of nitrogen to these pristine and suburban ecosystems, which could potentially have implications on the nutrient cycling. There was a statistically significant linear correlation between the aerosol WSON and the water soluble inorganic nitrogen (WSIN) for the urban coastal, sub-urban and pristine zones (R2= 0.81, n=22). This correlation could be explained by a possible source of secondary water soluble organic aerosols derived by the reaction between biogenic volatile organic compounds (VOCs), such as isoprene, and nitrogen oxides (NOx) present in the atmosphere of these regions. Such correlation was not found in Caracas, possibly due to the fact that in this city the major source of VOCs is fossil fuel combustion which produces mostly non soluble aliphatic VOCs. These compounds could most likely produce low water soluble secondary organic nitrogen aerosols.
B31A-0057
Nitrate Deficits by Denitrification and Partial Nitrification Processes in the Pacific Ocean
The model to separate the nitrate deficits produced by the denitrification (dN") and by partial nitrification processes (dN; Li et al, 2006) was applied to the Pacific WOCE data from the cruises P19 and P21E, which traversed the oxygen minimum zones north and south of the eastern equatorial Pacific Ocean. Results are consistent with those obtained from the Arabian Sea. Two dN" maxima coincide with two nitrite maxima and two O2 minima in the north-south cross sections of those parameters. Two dN maxima coincide with nitrate or phosphate maxima, and lie within the lower oxycline below the oxygen minimum zones. The dN production within the upper oxycline is appreciable, but not as prominent as in the lower oxycline. The dN" and dN maxima are in direct contact with the continental shelf and slope sediments, indicating some nitrate deficit inputs from sediments. The nitrate deficit in the bottom water of the Bering Sea is confirmed to be sediment inputs.
B31A-0058
Nitrogen Cycle Modeling: a Mechanistic Estimate of N-losses From Agricultural Fields Over the Seasonal Time Period
The biogeochemical cycle of nitrogen and production of NO, N2O, and CO2 gas and NO2- and NO3- ions in nutrient-enriched agricultural fields is mediated by soil microbial activity, the hydrological cycle, plant dynamics, and climatic forcing. Understanding how NO, N2O, CO2 gases and NO2- and NO3- ions are released from agricultural fields to the environment is a key factor in controlling the green-house effect and water contamination, and assumes ever greater importance in view of the foreseen increase in biofuel, food, and fiber production. To address these issues we have developed a mechanistic model (TOUGHREACT-N) for various nitrification and denitrification pathways, multiple microbial biomass dynamics, heat and water flows, and various chemical reactions at local and kinetic equilibrium. The soil column is represented in a 1D framework, with hydraulic properties described by a water tension-saturation model. Biotic and abiotic reactions are assumed to follow Michaelis-Menten kinetics, while a consortium of several micro-organismal strains is assumed to follow multiple Monod growth kinetics accounting for electron donor, electron acceptor, and inhibitor concentrations. Water flow is modeled with the Darcy-Richards equation, while nutrient transport is modeled by Fickian advective and diffusive processes in both gaseous and liquid phases. Heat flow is modeled with the Fourier equation. Plant dynamics is taken into account by coupling TOUGHREACT-N with CERES to determine water and nutrient uptake, and soil carbon accumulation. TOUGHREACT-N was calibrated against field measurements to assess pathways of N losses following fertilization. A good agreement between field observations and model predictions was found. We identified two dominant time scales in the system response that depended on plants dynamics. Before plants have substantial impact on soil nutrients and moisture content, N losses are characterized by rapid increases as a function of water application rate and fertilizer amount and application depth. Under reference fertilization and irrigation practices, approximately 1.64% and 1.61% of the total applied N is lost as N-NO(g) and N-N2O(g), respectively, while losses of N-N2(g), N-NO2-, and N-NO3- where several orders of magnitude smaller. When plants grow, pulses in N losses became smoother due to nutrient and water uptake. Contrarily to predictions of non- mechanistic, coarse-scale models (e.g., CASA, CENTURY) N losses are predominantly non-linearly increasing with fertilizer and water application amount, and with fertilizer application depth, thus invoking a revision of long- term estimates of nitrogen and carbon balances at global scales
B31A-0059
Why do Different Anthropogenic Sources of Atmospheric Nitrate have Distinctive Isotopic Signatures?
Do different sources of atmospheric nitrate (power plants, vehicles, agricultural emissions) have distinctive isotopic signatures? To answer this question, we conducted a national survey of nitrate isotopes in wet deposition samples collected throughout the USA from 156 NADP sites. Archived samples from the year 2000 were pooled into bimonthly composites and analyzed for d15N and d18O, with a subset analyzed for D17O. In this presentation, we present our current thoughts about why the different sources have distinctive isotopic signatures, focusing mainly on oxygen isotopes of nitrate. Our original conceptual model was based largely on (1) Heaton's 1990 paper that showed that NOx emissions from power plants in South Africa had d15N values significantly higher than exhaust from vehicles, and (2) anecdotal data from several studies showing higher d15N and/or d18O values of nitrate in precipitation downwind of areas dominated by power plants. Our model proposed that atmospheric nitrate derived from near- surface sources (e.g., vehicle and biogenic emissions) would obtain d18O and D17O signatures predominantly from near-surface O2 produced by photosynthesis (averaging +23 permil), whereas NOx exiting power plant stacks would circulate higher into the atmosphere and obtain nitrate d18O and D17O signatures predominantly from tropospheric O3 (+95 and +35 permil, respectively). We speculated that the source discrimination seen in NOx emissions would be maintained in the atmospheric nitrate ultimately derived from it, despite potential isotopic fractionation during conversion to nitrate. Furthermore, because of the likelihood of temporal and spatial variation in the compositions of NOx exiting individual combustion engines due to changes in temperature, fuel types, and other operating conditions, we suspected that source signatures would best be determined after the NOx had been converted to nitrate. Hence, we proposed to determine source signatures primarily from statistical analysis of a large set of wet deposition samples, instead of using emission samples. The most striking aspect of our data are the distinct but different spatial patterns shown by the d15N, d18O, and D17O values. We observe strong correlations between d15N and power plant NOx, suggesting this source is well mixed relative to vehicle emissions, which appear to be largely deposited near roadways (Elliott et al., in press). Several recent papers have developed models to explain seasonal nitrate isotopic variations in wet and dry deposition in localized areas. In general, these models explain the variations in terms of seasonality in oxidative reactions in the atmosphere, not source signatures. We will evaluate these models and our original conceptual model with our various large data sets, and present the current state of our understanding of the isotopic signatures of different sources and how they are established during combustion, transport, mixing, and atmospheric reactions during the conversion of NOx emissions to nitrate in deposition.
B31A-0060
Climatic Controls on Leaf Nitrogen Content and Implications for Biochemical Modeling.
Leaf nitrogen (N) content, expressed as percent total nitrogen per unit of leaf dry mass, is a widely used parameter in biochemical modeling, due mainly to its role as a potentially limiting factor for photosynthesis. The amount of nitrogen, however, does not occur in a fixed amount in every leaf, but rather varies continuously with the leaf life cycle, in constant response to soil-root-stem-leaf-climate interactions and demand for growth. Moreover, while broad data on leaf N has become available it is normally measured under ambient conditions with consequent difficulty for distinguishing between genetic and time specific environmental effects. In the present work we: 1) Investigate the theoretical variation of leaf mass, specific heat capacity and leaf thickness of full sun-expanded leaves as a regulatory mechanism to ensure thermal survival along with long-term climatic radiation/temperature gradient; and discuss nitrogen and carbon controls on leaf thickness. 2) Based on possible states of partition between nitrogenous and non-nitrogenous components of a leaf we further derive probability density functions (PDFs) of nitrogen and carbon content and assess the effect of water and nutrient uptake on the PDFs. 3) Translate the results to spatially explicit representation over the conterminous USA at 1 km spatial resolution by providing maximum potential values of leaf N of fully expanded leaf optimally suited for long term climatic averages values and soils conditions. Implications for potential presence of inherently slow/fast growing species are discussed along with suitability of results for use by biochemical models.
B31A-0061
Non-Linear Nitrogen Cycling and Ecosystem Calcium Depletion Along a Temperate Forest Soil Nitrogen Gradient
Understanding how N availability influences base cation stores is critical for assessing long-term ecosystem sustainability. Indices of nitrogen (N) availability and the distribution of nutrients in plant biomass, soil, and soil water were examined across ten Douglas-fir (Pseudotsuga menziesii) stands spanning a three-fold soil N gradient (0-10 cm: 0.21 – 0.69% N, 0-100 cm: 9.2 – 28.8 Mg N ha-1) in the Oregon Coast Range. This gradient is largely the consequence of historical inputs from N2-fixing red alder stands that can add 100-200 kg N ha-1 yr-1 to the ecosystem for decades. Annual net N mineralization and litterfall N return displayed non-linear relationships with soil N, increasing initially, and then decreasing as N-richness increased. In contrast, nitrate leaching from deep soils increased linearly across the soil N gradient and ranged from 0.074 to 30 kg N ha-1 yr-1. Soil exchangeable Ca, Mg, and K pools to 1 m depth were negatively related to nitrate losses across sites. Ca was the only base cation exhibiting concentration decreases in both plant and soil pools across the soil N gradient, and a greater proportion of total available ecosystem Ca was sequestered in aboveground plant biomass at high N, low Ca sites. Our work supports a hierarchical model of coupled N-Ca cycles across gradients of soil N enrichment, with microbial production of mobile nitrate anions leading to depletion of readily available Ca at the ecosystem scale, and plant sequestration promoting Ca conservation as Ca supply diminishes. The preferential storage of Ca in aboveground biomass at high N and low Ca sites, while critical for sustaining plant productivity, may also predispose forests to Ca depletion in areas managed for intensive biomass removal. Long-term N enrichment of temperate forest soils appears capable of sustaining an open N cycle and key symptoms of N-saturation for multiple decades after the cessation of elevated N inputs.
B31A-0062
Compound-Specific Stable Isotopes of Organic Compounds From Lake Sediments Track Recent Environmental Changes in an Alpine Ecosystem, Rocky Mountain National Park, Colorado (United States of America)
Sediments from high altitude lakes in the North American Cordillera reveal rapid changes in both the composition of diatom communities and bulk organic δ15N over the past c. 60 years. In this study, compound- specific nitrogen, carbon, and hydrogen isotope records from Sky Pond, an alpine lake in Rocky Mountain National Park (Colorado, United States of America), were used to identify the factors contributing to ecological change. Our results from the nitrogen isotopic compositions of purified algal chlorins indicate that the magnitude of isotopic change is larger than implied from bulk organic δ15N, and support a substantial shift in nitrogen (N)-cycling in the region. Temporal changes in the growth characteristics of lichen surrounding Sky Pond, as well as a -60‰ excursion in δD values of algal-derived palmitic acid, are coincident with changes in N-cycling, indicating alterations in catchment hydrology and nutrient delivery. The confluence of these trends is attributed to an increase in anthropogenic N deposition caused by both expansion anthropogenic influences and temporal changes in regional hydrology associated with snow, glacier, and permafrost melt.
B31A-0063
Enriched groundwater seeps in Vermont's forest ecosystems: sources or sinks for nitrate?
It is well known that wetter areas in the landscape can be hotspots for denitrification but it is not well appreciated that enriched seeps could also be hotspots for nitrification. Such seeps are high in calcium and pH relative to surrounding soils, reflecting less weathered bedrock or till. The various N fluxes may differ markedly between poorly drained soils in or near the seep and well drained soils away from the seep. We are investigating this in a study in two watersheds in Vermont comparing in-seep soils with nearby well drained soils. The sites are the two enriched groundwater seep areas at Brush Brook ‘G' watershed and Sleepers River Research Watershed in Vermont. The soils at both site are sandy loams and vegetation comprises of Northern hardwoods mainly sugar maple. Gross N transformation rates of the soils were measured using the 15N isotopic pool dilution method. Rates were variable but there were many instances of gross nitrification rates being higher in nearly-saturated seep soils compared with drier upland soils. Denitrification rates were calculated by measuring the change in the 15N of both N20 and N2 during the gross nitrification incubation. Both nitrification and denitrification were occurring in the same cores. Rates varied seasonally and in response to soil moisture. Thus the seeps may alternatively be sources or sinks for nitrate.
B31A-0064
Lime and Soil Moisture Effects on Nitrogen gas Loss Following Fertilizer Application
The loss of nitrogen from fertilizer application through ammonia volatilization and nitrous oxide emissions are of major environmental concern. Liming has been regarded as a mitigation option for lowering soil nitrogen gas emissions following the addition of fertilizers. A mechanistic nitrogen-cycle model (TOUGHREACT-N) has been developed to simulate the interaction of water saturation variation with biogeochemical processes, and the balance between liming and soil buffering capacity. The model was tested with data from a laboratory soil incubation following the addition of synthetic urine (500 kg N ha-1). Simulation results agreed well with measured N2O emissions and soil inorganic-N concentrations. The study indicated that liming significantly increase NH3 volatilization, while the reduction in cumulative N2O emissions depended strongly on water regime. The cumulative N2O emissions under relatively dry conditions were reduced by up to 243% with liming. However, the cumulative N2O and N2 emissions were predicted to increase by up to 346% following liming because the resulting NO3--N pools (from enhanced nitrification) were susceptible to enhanced N2O and N2 losses during subsequent water application. Consequently, short-term (i.e., days ¡§C weeks) gains made in reducing soil N2O emissions by liming can be offset, and potentially reversed, by emissions later in the growing season. We describe an approach using the modeling framework to optimize N gas reductions using liming under various edaphic, crop type, fertilizer and irrigation application rates, and climate conditions.
B31A-0065
Understanding nitrogen removal processes within river networks over annual time scales: implications of saturation.
River networks are an important control of nutrient exports between terrestrial and coastal systems. We explored how this control is simultaneously influenced by runoff variability, elevated N inputs, and aquatic process saturation using a daily time step river network N removal model. The model assumes a saturating denitrification function derived from recent denitrification measurements in headwater stream channels experiencing a range of loading rates. We assumed the denitrification function could be applied over the entire range of stream sizes within a full river network-drainage basin context, and over time assuming rates vary only with temperature. The model was applied to a single suburbanizing watershed (Ipswich River, MA, 400km2 watershed area) using specified runoff conditions and at the global scale using runoff from the Land Model (LM) of the GFDL AM2/LM2 supplemented by a new river discharge and mass-transport model. We used a frequency analysis to characterize the distribution of river network N removal over annual time scales as a function of flow conditions. Percent of inputs removed can be significant under low flow conditions (>70%) but is constrained over annual time scales because most nonpoint N inputs enter the network when hydraulic conditions limit removal capacity. The effects of N saturation within the river system are most evident at moderate flows below mean annual discharge where the relative influences of biological activity and hydraulic factors on removal are balanced. Both increasing runoff conditions and increasing N inputs shift N removal to larger downstream systems. Greater climate variability and N inputs to river systems would lead to disproportionate changes in N exports to the coastal zone.
B31A-0066
The effect of soil freezing on nitrogen dynamics: A snow manipulation study in a forested watershed in the snow belt of western New York, USA
Despite decades of research on the effects of chronically high nitrogen (N) deposition on various ecosystems, there is a knowledge gap in our understanding how atmospheric N is processed in soil, especially during winter. Understanding soil processes such as nitrification is important since nitrate is the most mobile form of N, and production of it in the soil during winter can result in substantial N leaching to surface waters. In this study, we manipulated the snowpack depth to test the effects of soil freezing on in situ rates of nitrification and N mineralization with reference and snow manipulation treatment plots at Point Peter Brook Watershed (PPBW), located in the snow belt of western, NY. One pair of treatment and reference plots was located in the riparian zone of PPBW while another pair was located on the hillslope. The snow manipulation treatment, which simulated a smaller snowpack, as is likely to occur due to the increased importance of thaws, sleet, and rain-on- snow events, induced soil freezing that lasted over the entire winter in both treatment plots. The treatment plots at both landscape positions had significantly greater N mineralization than reference plots. The effect of soil freezing on nitrification was less clear with the riparian treatment plot having greater nitrification rates than reference plot while the hillslope treatment plot had smaller nitrification rates than the reference plot. The discrepancy is likely attributed to soil moisture conditions with hillslope soils being dryer and having a lower heat capacity. As elevated N deposition continues in complex environments and climate change alters such environments, especially during winter, the effect of soil freezing should be considered when evaluating differences in N dynamics in temperate ecosystems.
B31A-0067
Mechanistic Representation of the N Isotope Composition of Pools and Fluxes in a Coupled Soil and Plant System: Model Development, Testing, and Application
The composition and location of 15N atoms on N2O molecules has been used to characterize soil biological N cycling, N2O surface emissions, and N2O atmospheric transport and fate. However, the complexity and interdependency of soil microbial processes (e.g., nitrification, denitrification, mineralization, decomposition), physical processes (e.g., soil moisture and temperature dynamics, aqueous and gaseous advection and diffusion), and abiotic chemical processes (e.g., dissolution and precipitation, pH buffering) makes interpretation of the isotopic composition of measured N2O pools and surface fluxes very difficult. We report here on the development of a mechanistic model (TOUGHREACT-N) that attempts to account for these interactions and aids in the interpretation of 15N soil and plant measurements. The model accounts for the various N exchanges in nitrification, dentrification, decomposition, and mineralization; dynamics of several microbial populations; soil moisture and heat dynamics; multiple chemical reactions in solution and with the mineral phase; and exchanges with the overlying atmosphere. Interactions of soil N and water with plants are currently being implemented. We briefly describe the successful testing of TOUGHREACT-N in agricultural fields under a number of fertilizer and irrigation treatments. The model is then used to interpret the 15N composition of N pools and surface fluxes in published experiments. Our results indicate that the previously unaccounted transport and microbial processes substantially increase the uncertainty of source partitioning (e.g., between nitrification and denitrification). We also demonstrate several ways in which the model can be used to design sampling protocols to improve measurement interpretation.
B31A-0068
Diurnal, Seasonal and Inter-annual Variations of N2O Fluxes from Perennial Vineyard Soils in California, USA.
The USA emits 1562 million metric tons of carbon equivalents a year, whereby this value is projected to rise by an estimated 14 % in 2012. California is the 12th major global emitter of greenhouse gases, emitting approximately 500 million metric tons of carbon equivalents a year. 84 % of greenhouse gas emissions are from CO2, 7 % and 6 % from N2O and CH4 respectively and approximately 8 % of these emissions are derived from agricultural activities. The concentration of nitrous oxide (N2O) within the atmosphere has been increasing at a rate of approximately 0.27 % per year and has mainly been attributed to agricultural practices such as land-use changes, biomass burning, nitrogen fertilization, livestock and manure management. Agriculture related activities generate from 6 to 35 Tg N2O-N per year, or about 60 to 70 % of global production. The primary biogenic sources of N2O are from terrestrial soils, which are thought to be a major source of N2O to the atmosphere and mainly involve the microbial nitrogen transformations brought about by nitrification and denitrification. The aim of this study was to quantify the seasonal and inter-annual variability of N2O emissions and nitrogen cycling from a conventionally tilled wine grape vineyard in Napa, California during a two year closed static chamber study and to also investigate the diurnal N2O flux pattern and effects of fertilization management practices on emissions within a table grape vineyard in Delano, California. Preliminary data shows that the annual N2O fluxes were influenced by soil properties, management practices and weather such as precipitation events where increases in N2O emissions were observed after irrigation or fertilization practices and immediately following rainfall. Vineyard floor and vine management will be discussed in terms of the significance management practices have upon the release of N2O emissions from vineyard soils where the high water and nitrogen fertilizer usage within these perennial cropping systems may be significant sources of N2O production. The data obtained from this study will be utilized to contribute to and quantify regional greenhouse gas budgets which may not have been included in previous budgets in relation to policy implications and potential mitigation strategies on a regional and global scale.
B31A-0069
Modeling Evaluation of Retention and Release of Atmospheric Deposition N in Mixed Coniferous Forests, San Bernardino National Forest, CA
Atmospheric N deposition usually benefits N limited forests; however, sustained high deposition may cause N saturation in ecosystems. This study presents a modeling analysis on soil-plant ecosystem retention of N deposition and its releases using DAYCENT at two mixed coniferous forest sites, Camp Paivika (CP) and Barton Flats (BF). These sites represent high and low levels of N depositions (currently about 70 and 10 kgN ha-1 yr-1) in the San Bernardino Mountains, California. In addition to differences in N deposition CP has annual precipitation of 105.7 cm while BF average 48.7 cm. Using available information, DAYCENT was calibrated to simulate biomass and SOM accumulation reasonably well, but underestimated litter C likely due to lack of ozone injury induced litter-fall in the model. The model also competently predicted biomass C/N ratios. Model showed that the ecosystem retained 80-90% of deposition N with SOM the largest retention pool (40-50%) at both sites. At the high-N and wet CP, the litter stored higher percentage (20-25%)of deposition N than plants (10-15%), while in reverse at the low-N and dry BF site. This difference could be related to plant N uptake per biomass production (1.8 gN per kgC at CP and 1.2 gN per kgC at BF) and immobilization/mineralization ratios (0.83 at CP and 0.74 at BF) due to the contrasted litter/SOM accumulation and soil wetness. It showed that about 40% of added N was captured by plants for about 10 years since N deposition, and up to 50% was incorporated into litter in about 20 years. While it took 40~50 years for SOM pool to immobilize 40% of total N deposited. Meanwhile soil inorganic N was gradually accumulating up to 9 mg kg-1 at CP while only 1.5 mg kg-1 at BF. The model demonstrated that a large amount of N was released to the atmosphere (5-15 kgN ha-1 at CP and <1-4 kgN ha-1 at BF), because of seasonal inconsistency of N addition and plant production, and high soil saturation during winter period. However, N losses into aquatic systems occurred in the late winters of the wet years, and were therefore highly variable from negligible to 5-12 kgN ha-1 when high precipitation occurred at CP, but only 0.1-0.3 kgN ha-1 at BF. This modeling analysis demonstrated that forest atmospheric deposition induced N saturation and possible environmental impacts are a comprehensive expression of fast plant physiological responses with lagged soil biogeochemical processes. These responses occur in a context of environmental driving forces, specifically the precipitation regime in this study.
B31A-0070
Linking N Cycling to Microbial Function Within Soil Microenvironments in Cover Crop Systems
Cover crops have emerged as a crop management strategy to achieve agricultural sustainability and maintain environmental quality. Thus, fundamental knowledge of microbial-mediated C and N cycling is vital to understanding soil organic matter (SOM) dynamics in cover cropped agroecosystems. We investigated the effects of short-term cover crop-C input on N processing by microbial communities within SOM microenvironments and in bulk soil, across a gradient of organic to conventional crop management. We hypothesized that cover crop C and N inputs promote soil aggregation, which increases the abundance of ammonia oxidizing bacteria (AOB) and stimulates greater microbial cycling of N within soil microenvironments, thereby leading to potential increases in N stabilization coupled with decreases in N loss. Our hypothesis was tested on the long-term organic, low-input, and conventional maize-tomato rotations at the Center for Integrated Farming Systems experiment (Davis, CA). We collected soil samples (0-15cm) across the cover crop and subsequent maize growing seasons and then isolated three SOM fractions soil: coarse particulate organic matter (cPOM; >250um), microaggregates (53-250um), and silt-and-clay (<53um). Total C and N were measured on both bulk soil and SOM fractions. Real-time polymerase chain reaction (PCR) using primers for the functional genes, amoA and nosZ, were employed to quantify AOB and denitrifier population sizes, respectively. We also measured gross ammonification and nitrification rates in short-term 15N-incubations of the bulk soil to link cover crop induced N cycling to N-transforming bacteria. Total soil C and N concentrations and soil aggregation were higher in the organic than conventional and low-input systems. The amoA and no Z copy numbers g-1 dry soil were highest in the microaggregate fraction and similar between the cPOM and silt-and-clay fractions, among all cropping treatments. Abundances of AOB and denitrifiers were lower in bulk soil from the conventional and low- input than organic system. Our study indicates that long-term, annual cover crop inputs to the organic system lead to greater aggregation and development of microaggregate structures. Consequently, the abundance of nitrifiers and denitrifiers as well as the rates of ammonification and nitrification are augmented in the organic system compared to the conventional, which does not receive a cover crop, and the low-input system, which receives cover crops only in alternate years. These results shed light on the specific mechanisms governing short-term N stabilization versus losses under long-term crop management.
B31A-0071
Species Effects on Stand-Level Nutrient Economy of a Costa Rican Rain Forest
In tropical ecosystems, successional forests are rapidly replacing old growth forests as the dominant forest type. This shift in successional status combined with projected changes in climate could result in a significant change in the species composition of tropical forests. How changes in species composition could affect stand-level nutrient economy is not well understood. Using species-specific leaf litter nutrient and productivity data combined with randomly generated dominance scenarios, we investigated species effects on leaf litter nutrient inputs. We conducted this research in a 1-ha secondary forest stand (30-yr in 2003) in northeastern Costa Rica. We measured senesced leaf N and P contents of the nine dominant canopy tree species within the study plot and scaled the results to the stand level using % basal area (BA) as a proxy for relative litter contribution (Sum[total leaf litterfall x % BAsp x nutrient concentrationsp]). We created different dominance scenarios using Monte Carlo generated BA distributions of the nine species. We then selected all scenarios in which one of the nine species accounted for greater than 30% of the BA. This allowed us to create communities with each of the nine species as dominant while varying the composition of the remaining tree community. Both N and P leaf litter inputs differed significantly when the dominant species changed from the current forest community. The change in N inputs was relatively small in relation to the potential change in leaf litter P inputs. P inputs decreased by 23% when Vochysia ferruginea, a shade-intolerant late pioneer species, was dominant. When Casearia arborea, a shade-tolerant species, was the dominant species there was 6% increase in leaf litter P inputs. Our results demonstrate that changes in leaf litter N and P cycling will likely occur as land use and climate change alter forest community composition.
B31A-0072
Bacterial community structure and nitrogen transformation in hyporheic zones of arid-land streams
Hyporheic zones of desert streams can be areas of high biological activity and consequent nutrient transformation, particularly where land use change increases nutrient concentrations in a stream. Does hyporheic bacterial community composition vary, and does this biotic heterogeneity covary with water and nutrient supply? Bromide (Br-) and 15N-NO3- was injected for 24 hr in six streams (three "natural" reference streams, three streams in agricultural/urbanized catchments) in New Mexico and Arizona, USA. Four transects of 3 to 4 wells were placed along a longitudinal gradient within the study reach, and from these hyporheic water and gas samples were collected during and after each experiment. Gas samples were analyzed for O2, 15N2O, and 15N2. Hyporheic water samples were analyzed for major cations and anions, DOC, 15NO3- and 15NH4+. Bacterial diversity of hyporheic water was assessed using Denaturing Gradient Gel Electrophoresis (DGGE). There was high spatial and temporal variability in hyporheic bacterial community structure, connection with surface water and nutrient concentrations both within and among streams. For example, mean subsurface DGGE band richness per stream ranged from 9 to 21, and surface water comprised between 0 to 100 percent of hyporheic water in each well. There were strong differences in bacterial richness between streams (ANOVA, p < 0.001); however, this variability appeared related to salinity rather than differences in land use or nutrient concentration. 15NH4+ levels were higher in modified stream than reference stream subsurface waters, suggesting dissimilatory nitrate reduction to ammonium (DNRA) may be an important process in these hyporheic sediments. Our results to date suggest that though hyporheic microbial community structure is highly heterogeneous, this biological variability may be due to different factors than variability in stream nitrogen cycling function. Further work will identify dominant sequences within these bacterial communities and investigate within-stream heterogeneity.
B31A-0073
Nitrite Production from Ammonium in Riparian Wetland Soils and Determination of Rates of Consumption of Nitrite and Ammonium in-situ
Nitrite accumulation in soil has been observed under a variety of conditions. The accumulation of NO2- in soils is possible during nitrification or denitrification depending on the soil conditions. This study presents a series of experiments performed at different scales that demonstrate accumulation and production of nitrite under iron reducing conditions, in absence of an initial nitrate pool. These experiments were performed either directly in a riparian wetland in New Jersey, or in laboratory experiments with soils from the same location. The rates of utilization of nitrite and ammonium in situ determined by performing pushpull experiments were of the same magnitude, supporting the observation of a steady state nitrite pool under such anaerobic conditions. An isotope experiment performed with 15NH4 conclusively showed ammonium to be the source of nitrite under the experimental conditions and existence of a pathway linking ammonium and dinitrogen gas under anaerobic conditions.