bluuebunny/crossref_metadata_2025 huggingface dataset:
Aerosol fluxes and particle growth above managed grassland
Authors: Nemitz E. , Dorsey J. R. , Flynn M. J. , Gallagher M. W. , Hensen A. , Erisman J.-W. , Owen S. M. , Dämmgen U. , Sutton M. A.
Published: April 2010

Abstract. Particle deposition velocities (11–3000 nm diameter) measured above grassland by eddy covariance during the EU GRAMINAE experiment in June 2000 averaged 0.24 and 0.03 mm s−1 to long (0.75 m) and short (0.07 m) grass, respectively. After fertilisation with 108 kg N ha−1 as calcium ammonium nitrate, sustained apparent upward fluxes of particles were observed. Analysis of concentrations and fluxes of potential precursor gases, including NH3, HNO3, HCl and selected VOCs, shows that condensation of HNO3 and NH3 on the surface of existing particles is responsible for this effect. A novel approach is developed to derive particle growth rates at the field scale, from a combination of measurements of vertical fluxes and particle size-distributions. For the first 9 days after fertilization, growth rates of 11 nm particles of 7.04 nm hr−1 and 1.68 nm hr−1 were derived for day and night-time conditions, respectively. This implies total NH4NO3 production rates of 1.11 and 0.44 μg m−3 h−1, respectively. The effect translates into a small error in measured ammonia fluxes (0.06% day, 0.56% night) and a large error in NH4+ and NO3− aerosol fluxes of 3.6% and 10%, respectively. By converting rapidly exchanged NH3 and HNO3 into slowly depositing NH4NO3, the reaction modifies the total N budget, though this effect is small (<1% for the 10 days following fertilization), as NH3 emission dominates the net flux. It is estimated that 3.8% of the fertilizer N was volatilised as NH3, of which 0.05% re-condensed to form NH4NO3 particles within the lowest 2 m of the surface layer. This surface induced process would at least scale up to a global NH4NO3 formation of ca. 0.21 kt N yr−1 from NH4NO3 fertilisers and potentially 45 kt N yr−1 from NH3 emissions in general.
Concentrations and fluxes of biogenic volatile organic compounds above a Mediterranean macchia ecosystem in western Italy
Authors: Davison B. , Taipale R. , Langford B. , Misztal P. , Fares S. , Matteucci G. , Loreto F. , Cape J. N. , Rinne J. , Hewitt C. N.
Published: April 2010

Abstract. Emission rates and concentrations of biogenic volatile organic compounds (BVOCs) were measured at a Mediterranean coastal site at Castelporziano, approximately 25 km south-west of Rome, between 7 May and 3 June 2007, as part of the ACCENT-VOCBAS field campaign on biosphere–atmosphere interactions. Concentrations and emission rates were measured using the disjunct eddy covariance (DEC) method utilizing three different proton transfer reaction mass spectrometers (PTR-MS) so allowing a comparison between the instruments. The high resolution data from the PTR-MS instruments considerably enhances the original BEMA measurements of the mid 1990s. Depending on the measurement period, the volume mixing ratios were in the range 1.6–3.5 ppbv for methanol, 0.44–1.3 ppbv for acetaldehyde, 0.96–2.1 ppbv for acetone, 0.10–0.14 ppbv for isoprene, and 0.13–0.30 ppbv for monoterpenes. A diurnal cycle in mixing ratios was apparent with daytime maxima for methanol, acetaldehyde, acetone, and isoprene. The fluxes ranged from 370–440 μg m−2 h−1 for methanol, 180–360 μg m−2 h−1 for acetaldehyde, 180–450 μg m−2 h−1 for acetone, 71–290 μg m−2 h−1 for isoprene, and 240–860 μg m−2 h−1 for monoterpenes. From the measured flux data (7 May–3 June) an average basal emission rate for the Macchia vegetation was calculated of 430 μg m−2 h−1 for isoprene and 1100 μg m−2 h−1 for monoterpenes.
Historical records of coastal eutrophication-induced hypoxia
Authors: Gooday A. J. , Jorissen F. , Levin L. A. , Middelburg J. J. , Naqvi S. W. A. , Rabalais N. N. , Scranton M. , Zhang J.
Published: April 2010

Abstract. Under certain conditions, sediment cores from coastal settings subject to hypoxia can yield records of environmental changes over time scales ranging from decades to millennia, sometimes with a resolution of as little as a few years. A variety of biological and geochemical indicators (proxies) derived from such cores have been used to reconstruct the development of eutrophication and hypoxic conditions over time. Those based on (1) the preserved remains of benthic organisms (mainly foraminiferans and ostracods), (2) sedimentary features (e.g. laminations) and (3) sediment chemistry and mineralogy (e.g. presence of sulphides and redox-sensitive trace elements) reflect conditions at or close to the seafloor. Those based on (4) the preserved remains of planktonic organisms (mainly diatoms and dinoflagellates), (5) pigments and lipid biomarkers derived from prokaryotes and eukaryotes and (6) organic C, N and their stable isotope ratios reflect conditions in the water column. However, the interpretation of these indicators is not straightforward. A central difficulty concerns the fact that hypoxia is strongly correlated with, and often induced by, organic enrichment caused by eutrophication, making it difficult to separate the effects of these phenomena in sediment records. The problem is compounded by the enhanced preservation in anoxic and hypoxic sediments of organic microfossils and biomarkers indicating eutrophication. The use of hypoxia-specific proxies, such as the trace metals molybdenum and rhenium and the bacterial biomarker isorenieratene, together with multi-proxy approaches, may provide a way forward. All proxies of bottom-water hypoxia are basically qualitative; their quantification presents a major challenge to which there is currently no satisfactory solution. Finally, it is important to separate the effects of natural ecosystem variability from anthropogenic effects. Despite these problems, in the absence of historical data for dissolved oxygen concentrations, the analysis of sediment cores can provide plausible reconstructions of the temporal development of human-induced hypoxia, and associated eutrophication, in vulnerable coastal environments.
Effect of CO2-related acidification on aspects of the larval development of the European lobster, Homarus gammarus (L.)
Authors: Arnold K. E. , Findlay H. S. , Spicer J. I. , Daniels C. L. , Boothroyd D.
Published: April 2010

Abstract. Oceanic uptake of anthropogenic CO2 results in a reduction in pH termed "Ocean Acidification" (OA). Comparatively little attention has been given to the effect of OA on the early life history stages of marine animals. Consequently, we investigated the effect of culture in CO2-acidified sea water (approx. 1200 ppm, i.e. average values predicted using IPCC 2007 A1F1 emissions scenarios for year 2100) on early larval stages of an economically important crustacean, the European lobster Homarus gammarus. Culture in CO2-acidified sea water did not significantly affect carapace length of H. gammarus. However, there was a reduction in carapace mass during the final stage of larval development in CO2-acidified sea water. This co-occurred with a reduction in exoskeletal mineral (calcium and magnesium) content of the carapace. As the control and high CO2 treatments were not undersaturated with respect to any of the calcium carbonate polymorphs measured, the physiological alterations we record are most likely the result of acidosis or hypercapnia interfering with normal homeostatic function, and not a direct impact on the carbonate supply-side of calcification per se. Thus despite there being no observed effect on survival, carapace length, or zoeal progression, OA related (indirect) disruption of calcification and carapace mass might still adversely affect the competitive fitness and recruitment success of larval lobsters with serious consequences for population dynamics and marine ecosystem function.
Interactions among vegetation and ozone, water and nitrogen fluxes in a coastal Mediterranean maquis ecosystem
Authors: Gerosa G. , Finco A. , Mereu S. , Marzuoli R. , Ballarin-Denti A.
Published: April 2010

Abstract. Ozone, water and energy fluxes were measured over a Mediterranean maquis ecosystem from 5 May until 31 July 2007 by means of the eddy covariance technique. Additional measurements of NOx fluxes were performed by the aerodynamic gradient technique. Stomatal ozone fluxes were obtained from water fluxes by a Dry Deposition Inferential Method based on a big leaf concept. The maquis ecosystem acted as a net sink for ozone. The different water availability between late spring and summer was the major cause of the changes observed in stomatal fluxes, which decreased, together with evapotranspiration, when the season became drier. NOx concentrations were significantly dependent on the local meteorology. NOx fluxes resulted less intense than the ozone fluxes. However an average upward flux of both NO and NO2 was measured. The non-stomatal pathways of ozone deposition were investigated. A correlation of non-stomatal deposition with air humidity and, in a minor way, with NO2 fluxes was found. Ozone risk assessment was performed by comparing the exposure and the dose metrics: AOT40 (Accumulated dose over a threshold of 40 ppb) and AFst1.6 (Accumulated stomatal flux of ozone over a threshold of 1.6 nmol m−2 s−1). AOT40, both at the measurement height and at canopy height was greater than the Critical Level for the protection of forests and semi-natural vegetation (5000 ppb h) adopted by UN-ECE. Also the AFst1.6 value (12.6 mmol m−2 PLA, Projected Leaf Area) was higher than the provisional critical dose of 4 mmol m−2 PLA for forests. The cumulated dose showed two different growth rates in the spring and in the summer periods, while the exposure showed a more irregular behavior in both periods.
Nitrate retention and removal in Mediterranean streams bordered by contrasting land uses: a 15N tracer study
Authors: von Schiller D. , Martí E. , Riera J. L.
Published: April 2010

Abstract. We used 15N-labelled nitrate (NO3−) additions to investigate pathways of nitrogen (N) cycling at the whole-reach scale in three stream reaches with adjacent forested, urban and agricultural land areas. Our aim was to explore among-stream differences in: (i) the magnitude and relative importance of NO3− retention (i.e. assimilatory uptake) and removal (i.e. denitrification), (ii) the relative contribution of the different primary uptake compartments to NO3− retention, and (iii) the regeneration, transformation and export pathways of the retained N. Streams varied strongly in NO3− concentration, which was highest in the agricultural stream and lowest in the forested stream. The agricultural stream also showed the lowest dissolved oxygen (DO) concentration and discharge. Standing stocks of primary uptake compartments were similar among streams and dominated by detritus compartments (i.e. fine and coarse benthic organic matter). Metabolism was net heterotrophic in all streams, although the degree of heterotrophy was highest in the agricultural stream. The NO3− uptake length was shortest in the agricultural stream, intermediate in the urban stream, and longest in the forested stream. Conversely, the NO3− mass-transfer velocity and the areal NO3− uptake rate were highest in the urban stream. Denitrification was not detectable in the forested stream, but accounted for 9% and 68% of total NO3− uptake in the urban and the agricultural stream, respectively. The relative contribution of detritus compartments to NO3− assimilatory uptake was greatest in the forested and lowest in the agricultural stream. In all streams, the retained N was rapidly regenerated back to the water column. Due to a strong coupling between regeneration and nitrification, most retained N was exported from the experimental reaches in the form of NO3−. This study provides evidence of fast in-stream N cycling, although the relative importance of N retention and removal varied considerably among streams. Results suggest that permanent NO3− removal via denitrification may be enhanced over temporary NO3− retention via assimilatory uptake in heterotrophic human-altered streams characterized by high NO3− and low DO concentrations.
Net Loss of CaCO3 from a subtropical calcifying community due to seawater acidification: mesocosm-scale experimental evidence
Authors: Andersson A. J. , Kuffner I. B. , Mackenzie F. T. , Jokiel P. L. , Rodgers K. S. , Tan A.
Published: April 2010

Abstract. Acidification of seawater owing to oceanic uptake of atmospheric CO2 originating from human activities such as burning of fossil fuels and land-use changes has raised serious concerns regarding its adverse effects on corals and calcifying communities. Here we demonstrate a net loss of calcium carbonate (CaCO3) material as a result of decreased calcification and increased carbonate dissolution from replicated subtropical coral reef communities (n=3) incubated in continuous-flow mesocosms subject to future seawater conditions. The calcifying community was dominated by the coral Montipora capitata. Daily average community calcification or Net Ecosystem Calcification (NEC=CaCO3 production – dissolution) was positive at 3.3 mmol CaCO3 m−2 h−1 under ambient seawater pCO2 conditions as opposed to negative at −0.04 mmol CaCO3 m−2 h−1 under seawater conditions of double the ambient pCO2. These experimental results provide support for the conclusion that some net calcifying communities could become subject to net dissolution in response to anthropogenic ocean acidification within this century. Nevertheless, individual corals remained healthy, actively calcified (albeit slower than at present rates), and deposited significant amounts of CaCO3 under the prevailing experimental seawater conditions of elevated pCO2.
Forest floor carbon exchange of a boreal black spruce forest in eastern North America
Authors: Bergeron O. , Margolis H. A. , Coursolle C.
Published: April 2010

Abstract. This study reports continuous automated measurements of forest floor carbon (C) exchange over feathermoss, lichen, and sphagnum micro-sites in a black spruce forest in eastern North America during snow-free periods over three years. The response of soil respiration (Rs-auto) and forest floor photosynthesis (Pff) to environmental factors was determined. The seasonal contributions of scaled up Rs-auto adjusted for spatial representativeness (Rs-adj) and Pff (Pff-eco) relative to that of total ecosystem respiration (Re) and photosynthesis (Peco), respectively, were also quantified. Shallow (5 cm) soil temperature explained 67–86% of the variation in Rs-auto for all ground cover types, while deeper (50 and 100 cm) soil temperatures were related to Rs-auto only for the feathermoss micro-sites. Base respiration was consistently lower under feathermoss, intermediate under sphagnum, and higher under lichen during all three years. The Rs-adj/Re ratio increased from spring through autumn and ranged from 0.85 to 0.87 annually for the snow-free period. The Rs-adj/Re ratio was negatively correlated with the difference between air and shallow soil temperature and this correlation was more pronounced in autumn than summer and spring. Maximum photosynthetic capacity of the forest floor (Pff-max) saturated at low irradiance levels (~200 μmol m−2 s−1) and decreased with increasing air temperature and vapor pressure deficit for all three ground cover types, suggesting that Pff was more limited by desiccation than by light availability. Pff-max was lowest for sphagnum, intermediate for feathermoss, and highest for lichen for two of the three years. Pff normalized for light peaked at air temperatures of 5–8°C, suggesting that this is the optimal temperature range for Pff. The Pff-eco/Peco ratio varied from 13 to 24% over the snow-free period and reached a minimum in mid-summer when both air temperature and Peco were at their maximum. On an annual basis, Pff-eco accounted for 17–18% of Peco depending on the year and the snow-free season totals of Pff-eco were 23–24% that of Rs-adj.
Influence of landscape heterogeneity on spatial patterns of wood productivity, wood specific density and above ground biomass in Amazonia
Authors: Anderson L. O. , Malhi Y. , Ladle R. J. , Aragão L. E. O. C. , Shimabukuro Y. , Phillips O. L. , Baker T. , Costa A. C. L. , Espejo J. S. , Higuchi N. , Laurance W. F. , López-González G. , Monteagudo A. , Núñez-Vargas P. , Peacock J. , Quesada C. A. , Almeida S.
Published: April 2010

Abstract. Long-term studies using the RAINFOR network of forest plots have generated significant insights into the spatial and temporal dynamics of forest carbon cycling in Amazonia. In this work, we map and explore the landscape context of several major RAINFOR plot clusters using Landsat ETM+ satellite data. In particular, we explore how representative the plots are of their landscape context, and test whether bias in plot location within landscapes may be influencing the regional mean values obtained for important forest biophysical parameters. Specifically, we evaluate whether the regional variations in wood productivity, wood specific density and above ground biomass derived from the RAINFOR network could be driven by systematic and unintentional biases in plot location. Remote sensing data covering 45 field plots were aggregated to generate landscape maps to identify the specific physiognomy of the plots. In the Landsat ETM+ data, it was possible to spectrally differentiate three types of terra firme forest, three types of forests over Paleovarzea geomorphologycal formation, two types of bamboo-dominated forest, palm forest, Heliconia monodominant vegetation, swamp forest, disturbed forests and land use areas. Overall, the plots were generally representative of the forest physiognomies in the landscape in which they are located. Furthermore, the analysis supports the observed regional trends in those important forest parameters. This study demonstrates the utility of landscape scale analysis of forest physiognomies for validating and supporting the finds of plot based studies. Moreover, the more precise geolocation of many key RAINFOR plot clusters achieved during this research provides important contextual information for studies employing the RAINFOR database.
Ammonia sources and sinks in an intensively managed grassland canopy
Authors: David M. , Loubet B. , Cellier P. , Mattsson M. , Schjoerring J. K. , Nemitz E. , Roche R. , Riedo M. , Sutton M. A.
Published: April 2010

Abstract. Grasslands represent canopies with a complex structure where sources and sinks of ammonia (NH3) may coexist at the plant level. Moreover, management practices such as mowing, hay production and grazing may change the composition of the sward and hence the source-sink relationship at the canopy level as well as the interaction with the atmosphere. There is therefore a need to understand the exchange of ammonia between grasslands and the atmosphere better, especially regarding the location and magnitude of sources and sinks. Fluxes of atmospheric NH3 within a grassland canopy were assessed in the field and under controlled conditions using a dynamic chamber technique (cuvette). These cuvette measurements were combined with extraction techniques to estimate the ammonium (NH4+) concentration and the pH of a given part of the plant or soil, leading to an estimated ammonia compensation point (Cp). The combination of the cuvette and the extraction techniques was used to identify the potential sources and sinks of NH3 within the different compartments of the grassland: the soil, the litter or senescent "litter leaves", and the functioning "green leaves". A set of six field experiments and six laboratory experiments were performed in which the different compartments were either added or removed from the cuvettes. The results show that the cuvette measurements agree with the extraction technique in ranking the strength of compartment sources. It suggests that in the studied grassland the green leaves were mostly a sink for NH3 with a compensation point around 0.1–0.4 μg m−3 and an NH3 flux of 6 to 7 ng m−2 s−1. Cutting of the grass did not increase the NH3 fluxes of the green leaves. The litter was found to be the largest source of NH3 in the canopy, with a Cp of up to 1000 μg m−3 NH3 and an NH3 flux up to 90 ng m−2 s−1. The litter was found to be a much smaller NH3 source when dried (Cp=160 μg m−3 and FNH3=35 ng m−2 s−1 NH3). Moreover emissions from the litter were found to vary with the relative humidity of the air. The soil was a strong source of NH3 in the period immediately after cutting (Cp=320 μg m−3 and FNH3=60 ng m−2 s−1), which was nevertheless always smaller than the litter source. The soil NH3 emissions lasted, however, for less than one day, and were not observed with sieved soil. They could not be solely explained by xylem sap flow extruding NH4+. These results indicate that future research on grassland-ammonia relationships should focus on the post-mowing period and the role of litter in interaction with meteorological conditions.
Testing the relationship between the solar radiation dose and surface DMS concentrations using in situ data
Authors: Miles C. J. , Bell T. G. , Lenton T. M.
Published: April 2010

Abstract. The proposed strong positive relationship between dimethylsulphide (DMS) concentration and the solar radiation dose (SRD) received into the surface ocean is tested using data from the Atlantic Meridional Transect (AMT) programme. In situ, daily data sampled concurrently with DMS concentrations is used for the component variables of the SRD (mixed layer depth, MLD, surface insolation, I0, and a light attenuation coefficient, k) to calculate SRDinsitu. This is the first time in situ data for all of the components, including k, has been used to test the SRD-DMS relationship over large spatial scales. We find a significant correlation (ρ=0.55 n=65 p<0.01) but the slope of this relationship (0.006 nM/W m−2) is less than previously found at the global (0.019 nM/W m−2) and regional scales (Blanes Bay, Mediterranean, 0.028 nM/W m−2; Sargasso Sea 0.017 nM/W m−2). The correlation is improved (ρ=0.74 n=65 p<0.01) by replacing the in situ data with an estimated I0 (which assumes a constant 50% removal of the top of atmosphere value; 0.5×TOA), a MLD climatology and a fixed value for k following previous work. Equally strong, but non-linear relationships are also found between DMS and both in situ MLD (ρ=0.61 n=65 p<0.01) and the estimated I0 (ρ=0.73 n=65 p<0.01) alone. Using a satellite-retrieved, cloud-adjusted surface UVA irradiance to calculate a UV radiation dose (UVRD) with a climatological MLD also provides an equivalent correlation (ρ=0.67 n=54 p<0.01) to DMS. With this data, MLD appears the dominant control upon DMS concentrations and remains a useful shorthand to prediction without fully resolving the biological processes involved. However, the implied relationship between the incident solar/ultraviolet radiation (modulated by MLD), and sea surface DMS concentrations, is critical for closing a climate feedback loop.
Variability and recent trends in the African terrestrial carbon balance
Authors: Ciais P. , Piao S.-L. , Cadule P. , Friedlingstein P. , Chédin A.
Published: April 2010

Abstract. We modeled the African terrestrial carbon balance over the past century using a spatially resolved process based vegetation model (ORCHIDEE). The model is forced by changing climate and by human-induced changes in land use. It includes a simple parameterization of natural fires, but the natural vegetation dynamics was ignored. The period analyzed is 1901–2002. Overall, we found that the African net terrestrial carbon balance (Net Biome Productivity, NBP) increased from a net CO2 source to the atmosphere of 0.14 Pg C yr−1 in the 1980s to a net sink of 0.15 Pg C yr−1 in the 1990s. The land use flux alone is estimated to be a source of 0.13 Pg C yr−1 caused by deforestation. This implies that climatic trends (mainly increasing precipitation) and CO2 increase (fertilization effect), are causing a sink of 0.28 Pg C yr−1 which offsets the land-use source. We found that the interannual variability of NBP is large, and mostly driven by photosynthesis variability. Over savannas, photosynthesis changes from one year to the next are strongly correlated with rainfall changes (R2=0.77 in northern Africa, and R2=0.42 in southern African savannas). Over forests, such a control by rainfall is not found. The main spatial pattern of interannual variability in NBP and photosynthesis/ecosystem respiration fluxes is related with ENSO, with dryer conditions prevailing over savannas during El Niño and wetter conditions over forests. Climate induced variations in fire emissions respond to this ENSO forcing, but do not determine strongly the NBP interannual variability. Finally, we model that ecosystem respiration variations (mostly autotrophic respiration) are correlated with those of photosynthesis, on interannual as well as on decadal time scales, but this result is uncertain given the potential for acclimation for autotrophic respiration processes.
Sensible and latent heat flux from radiometric surface temperatures at the regional scale: methodology and evaluation
Authors: Miglietta F. , Gioli B. , Brunet Y. , Hutjes R. W. A. , Matese A. , Sarrat C. , Zaldei A.
Published: April 2010

Abstract. The CarboEurope Regional Experiment Strategy (CERES) was designed to develop and test a range of methodologies to assess regional surface energy and mass exchange of a large study area in the South-Western part of France. This paper describes a methodology to estimate sensible and latent heat fluxes on the basis of net radiation, surface radiometric temperature measurements and information obtained from available products derived from the Meteosat Second Generation (MSG) geostationary meteorological satellite, weather stations and ground-based eddy covariance towers. It is based on a simplified bulk formulation of sensible heat flux that considers the degree of coupling between the vegetation and the atmosphere and estimates latent heat as the residual term of net radiation. Estimates of regional energy fluxes obtained in this way are validated at the regional scale by means of a comparison with direct flux measurements made by airborne eddy-covariance. The results show an overall good matching between airborne fluxes and estimates of sensible and latent heat flux obtained from radiometric surface temperatures that holds for different weather conditions and different land use types. The overall applicability of the proposed methodology to regional studies is discussed.
Towards global empirical upscaling of FLUXNET eddy covariance observations: validation of a model tree ensemble approach using a biosphere model
Authors: Jung M. , Reichstein M. , Bondeau A.
Published: April 2010

Abstract. Global, spatially and temporally explicit estimates of carbon and water fluxes derived from empirical up-scaling eddy covariance measurements would constitute a new and possibly powerful data stream to study the variability of the global terrestrial carbon and water cycle. This paper introduces and validates a machine learning approach dedicated to the upscaling of observations from the current global network of eddy covariance towers (FLUXNET). We present a new model TRee Induction ALgorithm (TRIAL) that performs hierarchical stratification of the data set into units where particular multiple regressions for a target variable hold. We propose an ensemble approach (Evolving tRees with RandOm gRowth, ERROR) where the base learning algorithm is perturbed in order to gain a diverse sequence of different model trees which evolves over time. We evaluate the efficiency of the model tree ensemble (MTE) approach using an artificial data set derived from the Lund-Potsdam-Jena managed Land (LPJmL) biosphere model. We aim at reproducing global monthly gross primary production as simulated by LPJmL from 1998–2005 using only locations and months where high quality FLUXNET data exist for the training of the model trees. The model trees are trained with the LPJmL land cover and meteorological input data, climate data, and the fraction of absorbed photosynthetic active radiation simulated by LPJmL. Given that we know the "true result" in the form of global LPJmL simulations we can effectively study the performance of the MTE upscaling and associated problems of extrapolation capacity. We show that MTE is able to explain 92% of the variability of the global LPJmL GPP simulations. The mean spatial pattern and the seasonal variability of GPP that constitute the largest sources of variance are very well reproduced (96% and 94% of variance explained respectively) while the monthly interannual anomalies which occupy much less variance are less well matched (41% of variance explained). We demonstrate the substantially improved accuracy of MTE over individual model trees in particular for the monthly anomalies and for situations of extrapolation. We estimate that roughly one fifth of the domain is subject to extrapolation while MTE is still able to reproduce 73% of the LPJmL GPP variability here. This paper presents for the first time a benchmark for a global FLUXNET upscaling approach that will be employed in future studies. Although the real world FLUXNET upscaling is more complicated than for a noise free and reduced complexity biosphere model as presented here, our results show that an empirical upscaling from the current FLUXNET network with MTE is feasible and able to extract global patterns of carbon flux variability.
A model of Fe speciation and biogeochemistry at the Tropical Eastern North Atlantic Time-Series Observatory site
Authors: Ye Y. , Völker C. , Wolf-Gladrow D. A.
Published: April 2010

Abstract. A one-dimensional model of Fe speciation and biogeochemistry, coupled with the General Ocean Turbulence Model (GOTM) and a NPZD-type ecosystem model, is applied for the Tropical Eastern North Atlantic Time-Series Observatory (TENATSO) site. Among diverse processes affecting Fe speciation, this study is focusing on investigating the role of dust particles in removing dissolved iron (DFe) by a more complex description of particle aggregation and sinking, and explaining the abundance of organic Fe-binding ligands by modelling their origin and fate. The vertical distribution of different particle classes in the model shows high sensitivity to changing aggregation rates. Using the aggregation rates from the sensitivity study in this work, modelled particle fluxes are close to observations, with dust particles dominating near the surface and aggregates deeper in the water column. POC export at 1000 m is a little higher than regional sediment trap measurements, suggesting further improvement of modelling particle aggregation, sinking or remineralisation. Modelled strong ligands have a high abundance near the surface and decline rapidly below the deep chlorophyll maximum, showing qualitative similarity to observations. Without production of strong ligands, phytoplankton concentration falls to 0 within the first 2 years in the model integration, caused by strong Fe-limitation. A nudging of total weak ligands towards a constant value is required for reproducing the observed nutrient-like profiles, assuming a decay time of 7 years for weak ligands. This indicates that weak ligands have a longer decay time and therefore cannot be modelled adequately in a one-dimensional model. The modelled DFe profile is strongly influenced by particle concentration and vertical distribution, because the most important removal of DFe in deeper waters is colloid formation and aggregation. Redissolution of particulate iron is required to reproduce an observed DFe profile at TENATSO site. Assuming colloidal iron is mainly composed of inorganic colloids, the modelled colloidal to soluble iron ratio is lower that observations, indicating the importance of organic colloids.
Methane dynamics in different boreal lake types
Authors: Juutinen S. , Rantakari M. , Kortelainen P. , Huttunen J. T. , Larmola T. , Alm J. , Silvola J. , Martikainen P. J.
Published: April 2010

Abstract. This study explores the variability in concentrations of dissolved CH4 and annual flux estimates in the pelagic zone in a statistically defined sample of 207 lakes in Finland. The lakes were situated in the boreal zone, in an area where the mean annual air temperature ranges from −2.8 to 5.9°C. We examined how lake CH4 dynamics related to regional lake types assessed according to the EU water framework directive. Ten lake types were defined on the basis of water chemistry, color, and size. Lakes were sampled for dissolved CH4 concentrations four times per year, at four different depths at the deepest point of each lake. We found that CH4 concentrations and fluxes to the atmosphere tended to be high in nutrient rich calcareous lakes, and that the shallow lakes had the greatest surface water concentrations. Methane concentration in the hypolimnion was related to oxygen and nutrient concentrations, and to lake depth or lake area. The surface water CH4 concentration was related to the depth or area of lake. Methane concentration close to the bottom can be viewed as proxy of lake status in terms of frequency of anoxia and nutrient levels. The mean pelagic CH4 release from randomly selected lakes was 49 mmol m−2 a−1. The sum CH4 flux (storage and diffusion) correlated with lake depth, area and nutrient content, and CH4 release was greatest from the shallow nutrient rich and humic lakes. Our results support earlier lake studies regarding the regulating factors and also the magnitude of global emission estimate. These results propose that in boreal region small lakes have higher CH4 fluxes per unit area than larger lakes, and that the small lakes have a disproportionate significance regarding to the CH4 release.
Effects of N and P fertilization on the greenhouse gas exchange in two northern peatlands with contrasting N deposition rates
Authors: Lund M. , Christensen T. R. , Mastepanov M. , Lindroth A. , Ström L.
Published: April 2010

Abstract. Peatlands are important ecosystems in the context of biospheric feedback to climate change, due to the large storage of organic C in peatland soils. Nitrogen deposition and increased nutrient availability in soils following climate warming may cause changes in these ecosystems affecting greenhouse gas exchange. We have conducted an N and P fertilization experiment in two Swedish bogs subjected to high and low background N deposition, and measured the exchange of CO2, CH4 and N2O using the closed chamber technique. During the second year of fertilization, both gross primary production and ecosystem respiration were significantly increased by N addition in the northernmost site where background N deposition is low, while gross primary production was stimulated by P addition in the southern high N deposition site. In addition, a short-term response in respiration was seen following fertilization in both sites, probably associated with rapid growth of nutrient-limited soil microorganisms. No treatment effect was seen on the CH4 exchange, while N2O emission peaks were detected in N fertilized plots indicating the importance of taking N2O into consideration under increased N availability. In a longer term, increased nutrient availability will cause changes in plant composition, which will further act to regulate the peatland greenhouse gas exchange.
Distribution of calcifying and silicifying phytoplankton in relation to environmental and biogeochemical parameters during the late stages of the 2005 North East Atlantic Spring Bloom
Authors: Leblanc K. , Hare C. E. , Feng Y. , Berg G. M. , DiTullio G. R. , Neeley A. , Benner I. , Sprengel C. , Beck A. , Sanudo-Wilhelmy S. A. , Passow U. , Klinck K. , Rowe J. M. , Wilhelm S. W. , Brown C. W. , Hutchins D. A.
Published: April 2010

Abstract. The late stage of the North East Atlantic (NEA) spring bloom was investigated during June 2005 along a transect section from 45 to 66° N between 15 and 20° W in order to characterize the contribution of siliceous and calcareous phytoplankton groups and describe their distribution in relation to environmental factors. We measured several biogeochemical parameters such as nutrients, surface trace metals, algal pigments, biogenic silica (BSi), particulate inorganic carbon (PIC) or calcium carbonate, particulate organic carbon, nitrogen and phosphorus (POC, PON and POP, respectively), as well as transparent exopolymer particles (TEP). Results were compared with other studies undertaken in this area since the JGOFS NABE program. Characteristics of the spring bloom generally agreed well with the accepted scenario for the development of the autotrophic community. The NEA seasonal diatom bloom was in the late stages when we sampled the area and diatoms were constrained to the northern part of our transect, over the Icelandic Basin (IB) and Icelandic Shelf (IS). Coccolithophores dominated the phytoplankton community, with a large distribution over the Rockall-Hatton Plateau (RHP) and IB. The Porcupine Abyssal Plain (PAP) region at the southern end of our transect was the region with the lowest biomass, as demonstrated by very low Chla concentrations and a community dominated by picophytoplankton. Early depletion of dissolved silicic acid (DSi) and increased stratification of the surface layer most likely triggered the end of the diatom bloom, leading to coccolithophore dominance. The chronic Si deficiency observed in the NEA could be linked to moderate Fe limitation, which increases the efficiency of the Si pump. TEP closely mirrored the distribution of both biogenic silica at depth and prymnesiophytes in the surface layer suggesting the sedimentation of the diatom bloom in the form of aggregates, but the relative contribution of diatoms and coccolithophores to carbon export in this area still needs to be resolved.
Calcium carbonate saturation in the surface water of the Arctic Ocean: undersaturation in freshwater influenced shelves
Authors: Chierici M. , Fransson A.
Published: April 2010

Abstract. In the summer of 2005, we sampled surface water and measured pH and total alkalinity (AT) underway aboard IB Oden along the Northwest Passage from Cape Farewell (South Greenland) to the Chukchi Sea. We investigated the variability of carbonate system parameters, focusing particularly on carbonate concentration [CO32−] and calcium carbonate saturation states, as related to freshwater addition, biological processes and physical upwelling. Measurements on AT, pH at 15°C, salinity (S) and sea surface temperature (SST), were used to calculate total dissolved inorganic carbon (CT), [CO32−] and the saturation of aragonite (ΩAr) and calcite (ΩCa) in the surface water. The same parameters were measured in the water column of the Bering Strait. Some surface waters in the Canadian Arctic Archipelago (CAA) and on the Mackenzie shelf (MS) were found to be undersaturated with respect to aragonite (ΩAr<1). In these areas, surface water was low in AT and CT (<1500 μmol kg−1) relative to seawater and showed low [CO32−]. The low saturation states were probably due to the likely the effect of dilution due to freshwater addition by sea ice melt (CAA) and river runoff (MS). High AT and CT and low pH, corresponded with the lowest [CO32−], ΩAr and ΩCa, observed near Cape Bathurst and along the South Chukchi Peninsula. This was linked to the physical upwelling of subsurface water with elevated CO2. The highest surface ΩAr and ΩCa of 3.0 and 4.5, respectively, were found on the Chukchi Sea shelf and in the cold water north of Wrangel Island, which is heavily influenced by high CO2 drawdown and lower CT from intense biological production. In the western Bering Strait, the cold and saline Anadyr Current carries water that is enriched in AT and CT from enhanced organic matter remineralization, resulting in the lowest ΩAr (~1.2) of the area.
Photosynthesis of boreal ground vegetation after a forest clear-cut
Authors: Kulmala L. , Pumpanen J. , Vesala T. , Hari P.
Published: April 2010

Abstract. Heather (Calluna vulgaris), rosebay willowherb (Epilobium angustifolium), wavy hair-grass (Deschampsia flexuosa) and raspberry (Rubus idaeus) are typical species at boreal clear-cut sites. In this study, we measured their photosynthesis separately in the growing season of 2005 using a manual chamber. All measured species showed clear and species-specific seasonal cycles of photosynthetic activity (Pmax). The maxima of C. vulgaris and E. angustifolium occurred around June and July, while that of R. idaeus occurred as late as August. A simple model of photosynthetic activity is presented, addressing the photosynthesis of C. vulgaris was mainly explained by temperature history when the soil moisture is high. The activity of deciduous D. flexuosa also followed the temperature history, unlike the activities of E. angustifolium and R. idaeus. During a short drought, some shoots decreased their Pmax levels but none of the species showed similar reactions between individuals. We also observed that the comparison of the whole-plant Pmax or respiration of different-sized individuals were less scattered than the results based on full-grown leaf mass, implying that species-specific rates of photosynthesis at ground level are rather similar regardless of the plant size. Using species composition and continuous temperature and light measurements, we upscaled the species-specific process rates and integrated fixed and respired CO2 of ground vegetation for the entire 2005 growing season. The photosynthetic production per surface area of soil was 760 g C m−2 y−1 at the fertile site and 300 g C m−2 y−1 at the infertile site. During the snow-free period (18 April–21 November), the above ground parts of measured species released 75 g C m−2 y−1 at the infertile site. At the fertile site, E. angustifolium and R. idaeus respired 22 and 12 g C m−2 y−1, respectively.
Carbon and oxygen isotope analysis of leaf biomass reveals contrasting photosynthetic responses to elevated CO2 near geologic vents in Yellowstone National Park
Authors: Sharma S. , Williams D. G.
Published: April 2010

Abstract. In this study we explore the use of natural CO2 emissions in Yellowstone National Park (YNP) in Wyoming, USA to study responses of natural vegetation to elevated CO2 levels. Radiocarbon (14C) analysis of leaf biomass from a conifer (Pinus contortus; lodgepole pine) and an invasive, non-native herb (Linaria dalmatica; Dalmation toadflax) was used to trace the inputs of vent CO2 and quantify assimilation-weighted CO2 concentrations experienced by individual plants near vents and in comparable locations with no geologic CO2 exposure. The carbon and oxygen isotopic composition and nitrogen percent of leaf biomass from the same plants was used to investigate photosynthetic responses of these plants to naturally elevated atmospheric CO2 concentrations. The coupled shifts in carbon and oxygen isotope values suggest that dalmation toadflax responded to elevated CO2 exposure by increasing stomatal conductance with no change in photosynthetic capacity and lodgepole pine apparently responded by decreasing stomatal conductance and photosynthetic capacity. Lodgepole pine saplings exposed to elevated levels of CO2 likewise had reduced leaf nitrogen concentrations compared to plants with no enhanced CO2 exposure, further suggesting widespread and dominant conifer down-regulated photosynthetic capacity under elevated CO2 levels near geologic vents.
Response of temperate grasslands at different altitudes to simulated summer drought differed but scaled with annual precipitation
Authors: Gilgen A. K. , Buchmann N.
Published: April 2010

Abstract. Water is an important resource for plant life. Since climate scenarios for Switzerland predict an average reduction of 20% in summer precipitation until 2070, understanding ecosystem responses to water shortage, e.g. in terms of plant productivity, is of major concern. Thus, we tested the effects of simulated summer drought on three managed grasslands along an altitudinal gradient in Switzerland from 2005 to 2007, representing typical management intensities at the respective altitude. We assessed the effects of experimental drought on above- and below-ground productivity, stand structure (LAI and vegetation height) and resource use (carbon and water). Responses of community above-ground productivity to reduced precipitation input differed among the three sites but scaled positively with total annual precipitation at the sites (R2=0.85). Annual community above-ground biomass productivity was significantly reduced by summer drought at the alpine site receiving the least amount of annual precipitation, while no significant decrease (rather an increase) was observed at the pre-alpine site receiving highest precipitation amounts in all three years. At the lowland site (intermediate precipitation sums), biomass productivity significantly decreased in response to drought only in the third year, after showing increased abundance of a drought tolerant weed species in the second year. No significant change in below-ground biomass productivity was observed at any of the sites in response to simulated summer drought. However, vegetation carbon isotope ratios increased under drought conditions, indicating an increase in water use efficiency. We conclude that there is no general drought response of Swiss grasslands, but that sites with lower annual precipitation seem to be more vulnerable to summer drought than sites with higher annual precipitation, and thus site-specific adaptation of management strategies will be needed, especially in regions with low annual precipitation.
Inter-comparison of ammonia fluxes obtained using the Relaxed Eddy Accumulation technique
Authors: Hensen A. , Nemitz E. , Flynn M. J. , Blatter A. , Jones S. K. , Sørensen L. L. , Hensen B. , Pryor S. C. , Jensen B. , Otjes R. P. , Cobussen J. , Loubet B. , Erisman J. W. , Gallagher M. W. , Neftel A. , Sutton M. A.
Published: April 2010

Abstract. The exchange of Ammonia (NH3) between grassland and the atmosphere was determined using Relaxed Eddy Accumulation (REA) measurements. The use of REA is of special interest for NH3, since the determination of fluxes at one height permits multiple systems to be deployed to quantify vertical flux divergence (either due to effects of chemical production or advection). During the Braunschweig integrated experiment four different continuous-sampling REA systems were operated during a period of about 10 days and were compared against a reference provided by independent application of the Aerodynamic Gradient Method (AGM). The experiment covered episodes before and after both cutting and fertilizing and provided a wide range of fluxes −60–3600 ng NH3 m−2 s−1 for testing the REA systems. The REA systems showed moderate to good correlation with the AGM estimates, with r2 values for the linear regressions between 0.3 and 0.82. For the period immediately after fertilization, the REA systems showed average fluxes 20% to 70% lower than the reference. At periods with low fluxes REA and AGM can agree within a few %. Overall, the results show that the continuous REA technique can now be used to measure NH3 surface exchange fluxes. While REA requires greater analytical precision in NH3 measurement than the AGM, a key advantage of REA is that reference sampling periods can be introduced to remove bias between sampling inlets. However, while the data here indicate differences consistent with advection effects, significant improvements in sampling precision are essential to allow robust determination of flux divergence in future studies. Wet chemical techniques will be developed further since they use the adsorptive and reactive properties of NH3 that impedes development of cheaper optical systems.