bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of N and P fertilization on the greenhouse gas exchange in two nutrient-poor peatlands
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, probably associated with rapid growth of nutrient-limited soil microorganisms. No treatment effect was seen on the CH4 exchange, while N2O emissions 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 competitive patterns. The related effect on the future net greenhouse gas exchange is likely dependent on the mixture of nutrients being made available and which plant functional types that benefit from it, in combination with other changes related to global warming.
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 approach using an artificial data set derived from the 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 model tree ensemble upscaling and associated problems of extrapolation capacity. We show that the model tree ensemble 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 the model tree ensemble 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 the model tree ensemble 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 a model tree ensemble is feasible and able to extract global patterns of carbon flux variability.
Benthic phosphorus and iron budgets for NW-African slope sediments; biogeochemical processes and the importance of bioturbation
Authors: Küster-Heins K. , de Lange G. J. , Zabel M.
Published: April 2010

Abstract. Despite intensive research on the different domains of the marine phosphorus (P) cycle during the last decades, frequently discussed open questions still exist especially on controlling factors for the benthic behaviour of P and its general distribution in sediment-pore water systems. Steady state or the internal balance of all relevant physical and (bio)geochemical processes are amongst the key issues. In this study we present and discuss an extended data set from surface sediments recovered from three locations on the NW-African continental slope. Pore water data and results from sequential sediment extractions indicate a close relationship between the benthic cycles of P and iron. Accordingly, most of the dissolved phosphate must have been recently released by microbially catalyzed reductive dissolution of iron (oxhydr)oxides. However, rates of release and association of P and iron, respectively, are not directly represented in profiles of element specific sediment compositions. Steady-state based transport-reaction modelling results suggest that particle mixing due to active bioturbation, or rather a physical net downward transport of P associated to iron (oxyhydr)oxides, is an essential process for the balance of the inspected benthic cycles. This study emphasizes the importance of balancing analytical data so as to provide for a more comprehensive interpretation of all relevant processes.
From heterotrophy to autotrophy: a freshwater estuarine ecosystem recovering from hypereutrophication
Authors: Cox T. J. S. , Maris T. , Soetaert K. , Conley D. J. , Van Damme S. , Meire P. , Middelburg J. J. , Vos M. , Struyf E.
Published: April 2010

Abstract. Increased organic matter and nutrient loads have induced major changes in aquatic systems, including hypoxia and algal blooms. In enclosed ecosystems these changes were often not gradual due to non-linear mechanisms. Here we report a 40 year record of eutrophication and hypoxia on an estuarine ecosystem and its recovery from hypereutrophication. We observe a paradoxical increase in chlorophyll a concentrations with decreasing nutrient inputs, and we hypothesise that algal growth was inhibited due to hypereutrophication, either by elevated ammonium concentrations, severe hypoxia or the production of harmful substances in such a reduced environment. We study the dynamics of a simple but realistic mathematical model, incorporating the assumption of algal growth inhibition. It shows an autotrophic equilibrium with low ammonia inputs, and a heterotrophic equilibrium with high ammonia inputs. At intermediate ammonia inputs it displays two alternative stable states. We conclude that the recovery of hypereutrophic riverine/estuarine systems can exhibit threshold-like behaviour.
Synergistic effects of iron and temperature on Antarctic plankton assemblages
Authors: Rose J. M. , Feng Y. , DiTullio G. R. , Dunbar R. B. , Hare C. E. , Lee P. A. , Lohan M. , Long M. , Smith W. O. , Sohst B. , Tozzi S. , Zhang Y. , Hutchins D. A.
Published: April 2010

Abstract. Iron availability and temperature are important limiting factors for the biota in many areas of the world ocean, and both have been predicted to change in future climate scenarios. However, the impacts of combined changes in these two key factors on microbial trophic dynamics and nutrient cycling are unknown. We examined the relative effects of iron addition (+1 nM) and increased temperature (+4°C) on plankton assemblages of the Ross Sea, Antarctica, a region characterized by annual algal blooms and an active microbial community. Increased iron and temperature individually had consistently significant but relatively minor positive effects on total phytoplankton abundance, phytoplankton and microzooplankton community composition, as well as photosynthetic parameters and nutrient drawdown. Unexpectedly, increased iron had a consistently negative impact on microzooplankton abundance, most likely a secondary response to changes in phytoplankton community composition. When iron and temperature were increased in concert, the resulting interactive effects were greatly magnified. This synergy between iron and temperature increases would not have been predictable by examining the effects of each variable individually. Our results suggest the possibility that if iron availability increases under future climate regimes, the impacts of predicted temperature increases on plankton assemblages in polar regions could be significantly enhanced. Such synergistic and antagonistic interactions between individual climate change variables highlight the importance of multivariate studies for marine global change experiments.
Nitrous oxide emissions from riparian forest buffers, warm-season and cool-season grass filters, and crop fields
Authors: Kim D.-G. , Isenhart T. M. , Parkin T. B. , Schultz R. C. , Loynachan T. E. , Raich J. W.
Published: April 2010

Abstract. Denitrification within riparian buffers may trade reduced nonpoint source pollution of surface waters for increased greenhouse gas emissions resulting from denitrification-produced nitrous oxide (N2O). However, little is known about the N2O emission within conservation buffers established for water quality improvement or of the importance of short-term N2O peak emission following rewetting dry soils and thawing frozen soils. Such estimates are important in reducing uncertainties in current Intergovernmental Panel on Climate Change (IPCC) methodologies estimating soil N2O emission which are based on N inputs. This study contrasts N2O emission from riparian buffer systems of three perennial vegetation types and an adjacent crop field, and compares measured N2O emission with estimates based on the IPCC methodology. We measured soil properties, N inputs, weather conditions and N2O fluxes from soils in forested riparian buffers, warm-season and cool-season grass filters, and a crop field located in the Bear Creek watershed in central Iowa, USA. Cumulative N2O emissions from soils in all riparian buffers (5.8 kg N2O-N ha−1 in 2006–2007) were significantly less than those from crop field soils (24.0 kg N2O-N ha−1 in 2006–2007), with no difference among the buffer vegetation types. While N2O peak emissions (up to 70-fold increase) following the rewetting of dry soils and thawing of frozen soils comprised 46–70% of the annual N2O emissions from soils in the crop field, soils in the riparian buffers were less sensitive to such events (3 to 10-fold increase). The ratio of N2O emission to N inputs within riparian buffers (0.02) was smaller than those of crop field (0.07). These results indicate that N2O emission from soils within the riparian buffers established for water quality improvement should not be considered a major source of N2O emission compared to crop field emission. The observed large difference between measured N2O emissions and those estimated using the IPCC's recommended methodology (i.e., 87% underestimation) in the crop field suggests that the IPCC methodology may underestimate N2O emission in the regions where soil rewetting and thawing are common, and that conditions predicted by future climate-change scenarios may increase N2O emissions.
Chemolithoautotrophic production mediating the cycling of the greenhouses gases N2O and CH4 in an upwelling ecosystem
Authors: Farías L. , Fernández C. , Faúndez J. , Cornejo M. , Alcaman M. E.
Published: April 2010

Abstract. Coastal upwelling ecosystems with marked oxyclines (redoxclines) present high availability of electron donors that favour chemoautotrophy, leading in turn to high N2O and CH4 cycling associated with aerobic NH4+ (AAO) and CH4 oxidation (AMO). This is the case of the highly productive coastal upwelling area off Central Chile (36° S), where we evaluated the importance of total chemolithoautotrophic vs. photoautotrophic production, the specific contributions of AAO and AMO to chemosynthesis and their role in gas cycling. Chemoautotrophy (involving bacteria and archaea) was studied at a time-series station during monthly (2002–2009) and seasonal cruises (January 2008, September 2008, January 2009) and was assessed in terms of dark carbon assimilation (CA), N2O and CH4 cycling, and the natural C isotopic ratio of particulate organic carbon (δ13POC). Total Integrated dark CA fluctuated between 19.4 and 2.924 mg C m−2 d−1. It was higher during active upwelling and represented on average 27% of the integrated photoautotrophic production (from 135 to 7.626 mg C m−2d−1). At the oxycline, δ13POC averaged -22.209‰ this was significantly lighter compared to the surface (-19.674‰) and bottom layers (-20.716‰). This pattern, along with low NH4+ content and high accumulations of N2O, NO2- and NO3- within the oxycline indicates that chemolithoautotrophs and specifically AA oxydisers were active. Dark CA was reduced from 27 to 48% after addition of a specific AAO inhibitor (ATU) and from 24 to 76% with GC7, a specific archaea inhibitor, indicating that AAO and maybe AMO microbes (most of them archaea) were performing dark CA through oxidation of NH4+ and CH4. AAO produced N2O at rates from 8.88 to 43 nM d−1 and a fraction of it was effluxed into the atmosphere (up to 42.85 μmol m−2 d−1). AMO on the other hand consumed CH4 at rates between 0.41 and 26.8 nM d−1 therefore preventing its efflux to the atmosphere (up to 18.69 μmol m−2 d−1). These findings show that chemically driven chemoautotrophy (with NH4+ and CH4 acting as electron donors) could be more important than previously thought in upwelling ecosystems and open new questions concerning its future relevance.
Land-use and greenhouse gas balances of peatlands in the Nordic countries – present knowledge and gaps
Authors: Maljanen M. , Sigurdsson B. D. , Guđmundsson J. , Óskarsson H. , Huttunen J. T. , Martikainen P. J.
Published: April 2010

Abstract. This article provides an overview of the effects of land-use on the fluxes of methane (CH4), nitrous oxide (N2O) and carbon dioxide (CO2) of peatlands in the Nordic countries presented in about 100 studies. In addition, the article identifies the gaps in the present knowledge on the greenhouse gas (GHG) balances associated with the land-use of these northern ecosystems. Northern peatlands have accumulated, as peat, a vast amount of carbon from the atmosphere since the last glaciation. However, past land-use and the present climate have evidently changed their GHG balance. The mean annual GHG balances of undisturbed ombrotrophic and minerotrophic peatlands were surprisingly positive (net sources) of 140 and 380 g CO2 eq m−2, respectively, even if the former was a sink of 63 g CO2 eq m−2 when only the CO2 balance was considered. Drainage of such peatlands for agriculture resulted in the most disadvantageous land-use option for the atmosphere, with the net annual GHG balance increasing to 2190, 2280 and 3140 g CO2 eq m−2 for areas drained for grass swards, cereals or those left fallow, respectively. Even after ceasing of the cultivation practices, N2O emissions remained high and together with the other GHGs resulted in net emissions of 1570 and 500 g CO2 eq m−2, in abandoned and afforested peatlands, respectively. Peat extraction sites were also net sources, 730 g CO2 eq m−2. The cultivation of reed canary grass turned the site to net sink of −330 g CO2 eq m−2 but restoration did not (source of 470 g CO2 eq m−2). Data for afforested extraction sites is lacking. Peat soils originally drained for forestry may act as net sinks of 780 g CO2 eq m−2, and when those sites were restored the sink was 190 g CO2 eq m−2. However, more data is needed to confirm this point. Peat soils submerged under water reservoirs had a mean annual emission of 240 g CO2 eq m−2. In general, there is a lack of studies where all three GHGs have been measured at an ecosystem level, especially in the forested peatlands.
Linking the lithogenic, atmospheric, and biogenic cycles of silicate, carbonate, and organic carbon in the ocean
Authors: Smith S. V. , Gattuso J.-P.
Published: April 2010

Abstract. Geochemical theory describes long term cycling of atmospheric CO2 between the atmosphere and rocks at the Earth surface in terms of rock weathering and precipitation of sedimentary minerals. Chemical weathering of silicate rocks takes up atmospheric CO2, releases cations and HCO3− to water, and precipitates SiO2, while CaCO3 precipitation consumes Ca2+ and HCO3− and releases one mole of CO2 to the atmosphere for each mole of CaCO3 precipitated. At steady state, according to this theory, the CO2 uptake and release should equal one another. In contradiction to this theory, carbonate precipitation in the present surface ocean releases only about 0.6 mol of CO2 per mole of carbonate precipitated. This is a result of the buffer effect described by Ψ, the molar ratio of net CO2 gas evasion to net CaCO3 precipitation from seawater in pCO2 equilibrium with the atmosphere. This asymmetry in CO2 flux between weathering and precipitation would quickly exhaust atmospheric CO2, posing a conundrum in the classical weathering and precipitation cycle. While often treated as a constant, Ψ actually varies as a function of salinity, pCO2, and temperature. Introduction of organic C reactions into the weathering-precipitation couplet largely reconciles the relationship. ψ in the North Pacific Ocean central gyre rises from 0.6 to 0.9, as a consequence of organic matter oxidation in the water column. ψ records the combined effect of CaCO3 and organic reactions and storage of dissolved inorganic carbon in the ocean, as well as CO2 gas exchange between the ocean and atmosphere. Further, in the absence of CaCO3 reactions, Ψ would rise to 1.0. Similarly, increasing atmospheric pCO2 over time, which leads to ocean acidification, alters the relationship between organic and inorganic C reactions and carbon storage in the ocean. Thus, the carbon reactions and ψ can cause large variations in oceanic carbon storage with little exchange with the atmosphere.
Seasonal and annual variation of carbon fluxes in a young Siberian larch (Larix sibirica) plantation in Iceland
Authors: Bjarnadottir B. , Sigurdsson B. D. , Lindroth A.
Published: April 2010

Abstract. This study reports 3-year measurements (2004–2006) of net ecosystem exchange (NEE) over a 12–14 year old Siberian larch forest in Iceland established on previously grazed heath land pasture that had been site-prepared prior to planting. The study evaluated interannual and seasonal variation of NEE and its component fluxes, gross primary production (GPP) and ecosystem respiration (Re), with the aim to clarify how climatic factors controlled the site's carbon balance. The young plantation acted as a relatively strong sink for CO2 during all of the three years, with a net sequestration of −375, −566 and −245 g CO2 m−2 for years 2004, 2005 and 2006, respectively. The annual carbon balance was strongly influenced by climatic factors leading to a high inter-annual variability in NEE. This variation was more related to variation in carbon efflux (Re) than carbon uptake (GPP). The abiotic factors that showed the strongest correlation to Re were air temperature during the growing season and soil water potential. The GPP mostly followed the seasonal pattern in irradiance, except in 2005, when the plantation experienced severe spring frost damage that set the GPP back to zero. It was not expected that the rather slow-growing Siberian larch plantation would be such a strong sink for atmospheric CO2 only twelve years after site preparation and afforestation.
Ocean acidification affects iron speciation in seawater
Authors: Breitbarth E. , Bellerby R. J. , Neill C. C. , Ardelan M. V. , Meyerhöfer M. , Zöllner E. , Croot P. L. , Riebesell U.
Published: April 2010

Abstract. Rising atmospheric CO2 is acidifying the surface ocean, a process which is expected to greatly influence the chemistry and biology of the future ocean. Following the development of iron-replete phytoplankton blooms in a coastal mesocosm experiment at 350, 700, and 1050 μatm pCO2, we observed significant increases in dissolved iron concentrations, Fe(II) concentrations, and Fe(II) half-life times during and after the peak of blooms in response to CO2 enrichment, suggesting increased iron bioavailability. If applicable to the open ocean this may provide a negative feedback mechanism to the rising atmospheric CO2 by stimulating marine primary production.
An iron budget during the natural iron fertilisation experiment KEOPS (Kerguelen Islands, Southern Ocean)
Authors: Chever F. , Sarthou G. , Bucciarelli E. , Blain S. , Bowie A. R.
Published: April 2010

Abstract. Total dissolvable iron (TDFe) was measured in the water column above and in the surrounding of the Kerguelen Plateau (Indian sector of the Southern Ocean) during the KErguelen Ocean Plateau compared Study (KEOPS) cruise. TDFe concentrations ranged from 0.90 to 65.6 nmol L−1 above the plateau and from 0.34 to 2.23 nmol L−1 offshore of the plateau. Station C1 located south of the plateau, near Heard Island, exhibited very high values (329–770 nmol L−1). Apparent particulate iron (Feapp), calculated as the difference between the TDFe and the dissolved iron measured on board (DFe) represented 95±5% of the TDFe above the plateau, suggesting that particles and refractory colloids largely dominated the iron pool. This paper presents a budget of DFe and Feapp above the plateau. Lateral advection of water that had been in contact with the continental shelf of Heard Island seems to be the predominant source of Feapp and DFe above the plateau, with a supply of 9.7±2.3×106 and 8.3±6.7×103 mol d−1, respectively. The residence times of 1.7 and 48 days estimated for Feapp and DFe, respectively, indicate a rapid turnover in the surface water. A comparison between Feapp and total particulate iron (TPFe) suggests that the total dissolved fraction is mainly constituted of small refractory colloids. This fraction does not seem to be a potential source of iron to the phytoplankton in our study. Finally, when taking into account the lateral supply of dissolved iron, the seasonal carbon sequestration efficiency was estimated at 154 000 mol C (mol Fe)−1, which is 4-fold lower than the previously estimated value in this area but still 18-fold higher than the one estimated during the other study of a natural iron fertilisation experiment, CROZEX.
Short scale variations in nutrients, ectoenzymatic activities and bottom-up effects on bacterial production and community structure during late summer-autumn transition in the open NW Mediterranean Sea
Authors: Van Wambeke F. , Ghiglione J-F. , Nedoma J. , Mével G. , Raimbault P.
Published: April 2010

Abstract. We examined the vertical and temporal dynamics of nutrients, ectoenzymatic activities under late summer-fall transition period (September–October 2004) in NW Mediterranean Sea in relation to temporal change in factors limiting bacterial production. The depth of the mixed layer (12.8±5.3 m) was extremely stable until the onset of the destratification period after 11 October, creating a zone where diffusion of nutrient from the much deeper phosphacline (69±12 m) and nitracline (50±8 m) was probably strongly limited. However during the second half of the cruise, a shallowing of nutriclines occured, particularly marked for nitracline. Hence, the nitrate to phosphate ratio within the mixed layer, although submitted to a high short term variability, shifted the last week of the cruise from 1.1±1.2 to 4.6±3.8, and nitrate increased by a factor 2 (0.092±0.049 μM). A corresponding switch from more than one limitation (PN) to P-only limitation of bacterial production was observed during the month as detected by enrichment bioassays. Differences in the identity of the limiting nutrient in surface (5 m: N and P at the beginning, strictly P at the end of the study) versus 80 m (labile carbon) influence greatly bacterial community structure shift between these two layers. The two communities (5 and 80 m) reacted rapidly (24 h) to changes in nutrient concentrations by drastic modification of total and active population assemblages resulting in changes in activity. For bacterial production values less than 10 ng C l−1h−1 (associated to deeper layers), aminopeptidase and lipase exhibited higher activity relative to production whereas phosphatase varied in the same proportions than BP on the range of activities tested. Our results illustrate the effect of bottom-up control on bacterial community structure and activities in the epipelagic NW Mediterranean Sea.
Coastal hypoxia responses to remediation
Authors: Kemp W. M. , Testa J. M. , Conley D. J. , Gilbert D. , Hagy J. D.
Published: April 2010

Abstract. The incidence and intensity of hypoxic waters in coastal aquatic ecosystems has been expanding in recent decades coincident with eutrophication of the coastal zone. Because of the negative effects hypoxia has on many organisms, extensive efforts have been made to reduce the size and duration of hypoxia in many coastal waters. Although it has been broadly assumed that reductions in nutrient loading rates would reverse eutrophication and consequently, hypoxia, recent analyses of historical data from European and North American coastal systems suggest little evidence for simple linear response trajectories. We review existing data, analyses, and models that relate variations in the extent and intensity of hypoxia to changes in loading rates for inorganic nutrients and labile organic matter. We also assess existing knowledge of physical and ecological factors regulating oxygen in coastal marine waters and examine a broad range of examples where hypoxia responses to reductions in nutrient (or organic matter) inputs have been documented. Of the 22 systems identified where concurrent time series of loading and O2 were available, half displayed relatively clear and direct recoveries following remediation. We explored in detail 5 well-studied systems that have exhibited complex, non-linear responses to loading, including apparent "regime shifts." A summary of these analyses suggests that O2 conditions improved rapidly and linearly in systems where remediation focused on organic inputs from sewage plants, which were the primary drivers of hypoxia. In larger more open systems where diffuse nutrient loads are more important in fueling O2 depletion and where climatic influences are pronounced, responses to remediation tend to follow non-linear trends that may include hysteresis and time-lags. Improved understanding of hypoxia remediation requires that future studies use comparative approaches and consider multiple regulating factors including: (1) the dominant temporal scales of the hypoxia, (2) the relative contributions of inorganic and organic nutrients, (3) the influence of shifts in climatic and oceanographic processes, and (4) the roles of feedback interactions whereby O2-sensitive biogeochemistry, food-webs, and habitats influence the nutrient and algal dynamics that regulate O2 levels.
Patterns of dissolved organic carbon (DOC) and nitrogen (DON) fluxes in deciduous and coniferous forests under historic high nitrogen deposition
Authors: Sleutel S. , Vandenbruwane J. , De Schrijver A. , Wuyts K. , Moeskops B. , Verheyen K. , De Neve S.
Published: April 2010

Abstract. Numerous recent studies have indicated that dissolved organic carbon (DOC) and nitrogen (DON) play an important role in C and N cycling in natural ecosystems, and have shown that N deposition alters the concentrations and fluxes of dissolved organic substances and may increase leaching losses from forests. Our study was set up to accurately quantify concentrations and flux patterns of DOC, DON and dissolved inorganic nitrogen (DIN) in deciduous and coniferous forest in Flanders under historical high nitrogen deposition. We measured DOC, DON and DIN concentrations at two weekly intervals in a silver birch (SB) stand, a corsican pine (CP) stand and a pine stand with higher N deposition (CPN), and used the SWAP model (calibrated with PEST) for generating accurate water and matter fluxes. The input with precipitation was an important source of DON, but not for DOC. Release of DOC from the forest floor was minimally affected by forest type, but higher N deposition (CPN stand) caused an 82% increase of DOC release from the forest floor. Adsorption to mineral soil material rich in iron and/or aluminum oxyhydroxides was suggested to be the most important process removing DOC from the soil solution, responsible for substantial retention (67–84%) of DOC entering the mineral soil profile with forest floor leachate. Generally, DON was less reactive (i.e. less removal from the soil solution) than DOC, resulting in decreasing DOC/DON ratios with soil depth. We found increased DOC retention in the mineral soil as a result of higher N deposition (84 kg N ha−1 yr−1 additional DOC retention in CPN compared to CP). Overall DON leaching losses were 2.2, 3.3 and 5.0 kg N ha−1 yr−1 for SB, CP and CPN, respectively, contributing between 9–28% to total dissolved N (TDN) leaching. DON loss from SB and CP was not much higher than from unpolluted forests, and its relative contribution to TDN leaching was mainly determined by (large) differences in DIN leaching. The large TDN leaching losses are alarming, especially in the CPN stand that was N saturated.
A model-based assessment of the TrOCA approach for estimating oceanic anthropogenic carbon
Authors: Yool A. , Oschlies A. , Nurser A. J. G.
Published: April 2010

Abstract. The future behaviour of the global ocean as a sink for CO2 is significant for climate change, but it is also important to understand its past by quantifying anthropogenic CO2 (Cant) in the ocean today. Unfortunately, this is complicated by the difficulty of deconvoluting Cant from the natural, unperturbed carbon cycle. Nonetheless, a range of techniques have been devised that perform this separation using the information implicit in other physical, biogeochemical and artificial ocean tracers. One such method is the TrOCA approach, whose parameterisation is derived from relationships between biogeochemical tracers within watermasses defined by age tracers such as CFC-11. TrOCA has a number of methodological advantages, and has been shown to be plausible, relative to other methods, in a number of studies. Here we examine the TrOCA approach by using it to deconvolute the known distribution of Cant from an ocean general circulation model (OGCM) simulation of the industrial period (1864–2004). TrOCA is evaluated at local, regional and global scales, with an emphasis on the wider applicability of the parameterisations derived at these scales. Our work finds that the published TrOCA parameterisation performs poorly when extrapolated beyond its calibration region, either with observational data or (especially) model output. Optimising TrOCA parameters using model output as a synthetic dataset leads to some small improvements, but the resulting TrOCA variants still perform poorly. Furthermore, there are large ranges on the optimised TrOCA parameters suggesting that a "universal" TrOCA parameterisation is not achieveable.
From biota to chemistry and climate: towards a comprehensive description of trace gas exchange between the biosphere and atmosphere
Authors: Arneth A. , Sitch S. , Bondeau A. , Butterbach-Bahl K. , Foster P. , Gedney N. , de Noblet-Ducoudré N. , Prentice I. C. , Sanderson M. , Thonicke K. , Wania R. , Zaehle S.
Published: April 2010

Abstract. Exchange of non-CO2 trace gases between the land surface and the atmosphere plays an important role in atmospheric chemistry and climate. Recent studies have highlighted its importance for interpretation of glacial-interglacial ice-core records, the simulation of the pre-industrial and present atmosphere, and the potential for large climate-chemistry and climate-aerosol feedbacks in the coming century. However, spatial and temporal variations in trace gas emissions and the magnitude of future feedbacks are a major source of uncertainty in atmospheric chemistry, air quality and climate science. To reduce such uncertainties Dynamic Global Vegetation Models (DGVMs) are currently being expanded to mechanistically represent processes relevant to non-CO2 trace gas exchange between land biota and the atmosphere. In this paper we present a review of important non-CO2 trace gas emissions, the state-of-the-art in DGVM modelling of processes regulating these emissions, identify key uncertainties for global scale model applications, and discuss a methodology for model integration and evaluation.
The role of polysaccharides and diatom exudates in the redox cycling of Fe and the photoproduction of hydrogen peroxide in coastal seawaters
Authors: Steigenberger S. , Statham P. J. , Völker C. , Passow U.
Published: April 2010

Abstract. The effect of artificial acidic polysaccharides (PS) and exudates of Phaeodactylum tricornutum on the half-life of Fe(II) in seawater was investigated in laboratory experiments. Strong photochemical hydrogen peroxide (H2O2) production of 5.2 to 10.9 nM (mg C)−1 h−1 was found in the presence of PS and diatom exudates. Furthermore when illuminated with UV light the presence of algal exudates had a net stabilising effect on ferrous iron in seawater (initial value 100 nmol L−1) above that expected from oxidation kinetics. In the dark the PS gum xanthan showed no stabilising effect on Fe(II). The photochemical formation of superoxide (O2−) in presence of diatom exudates and its reducing effect on Fe(III) appears to result in greater than expected concentrations of Fe(II). A model of the photochemical redox cycle of iron incorporating these processes supported the observed data well. Diatom exudates seem to play an important role for the photochemistry of iron in coastal waters.
Modelling the effect of boundary scavenging on Thorium and Protactinium profiles in the ocean
Authors: Roy-Barman M.
Published: April 2010

Abstract. The "boundary scavenging" box model is a cornerstone of our understanding of the particle-reactive radionuclide fluxes between the open ocean and the ocean margins. However, it does not describe the radionuclide profiles in the water column. Here, I present the transport-reaction equations for radionuclides transported vertically by reversible scavenging on settling particles and laterally by horizontal currents between the margin and the open ocean. Analytical solutions of these equations are compared with existing data. In the Pacific Ocean, the model produces "almost" linear 230Th profiles (as observed in the data) despite lateral transport. However, omitting lateral transport biased the 230Th based particle flux estimates by as much as 50%. 231Pa profiles are well reproduced in the whole water column of the Pacific Margin and from the surface down to 3000 m in the Pacific subtropical gyre. Enhanced bottom scavenging or inflow of 231Pa-poor equatorial water may account for the model-data discrepancy below 3000 m. The lithogenic 232Th is modelled using the same transport parameters as 230Th but a different source function. The main source of 232Th scavenged in the open Pacific is advection from the ocean margin, whereas a net flux of 230Th produced in the open Pacific is advected and scavenged at the margin, illustrating boundary exchange. In the Arctic Ocean, the model reproduces 230Th measured profiles that the uni-dimensional scavenging model or the scavenging-ventilation model failed to explain. Moreover, if lateral transport is ignored, the 230Th based particle settling speed may by underestimated by a factor 4 at the Arctic Ocean margin. The very low scavenging rate in the open Arctic Ocean combined with the enhanced scavenging at the margin accounts for the lack of high 231Pa/230Th ratio in arctic sediments.
Environmental controls on carbon fluxes over three grassland ecosystems in China
Authors: Fu Y. , Zheng Z. , Yu G. , Hu Z. , Sun X. , Shi P. , Wang Y. , Zhao X.
Published: April 2010

Abstract. This study compared the CO2 fluxes over three grassland ecosystems in China, including a temperate steppe (TS) in Inner Mongolia, an alpine shrub-meadow (ASM) in Qinghai and an alpine meadow-steppe (AMS) in Tibet. The measurements were made in 2004 and 2005 using the eddy covariance technique. Objectives were to document the different seasonality of net ecosystem exchange of CO2 (NEE) and its components, gross ecosystem photosynthesis (GEP) and ecosystem respiration (Reco), and to examine how environmental factors affect carbon exchange in the three grassland ecosystems. It was warmer in 2005 than in 2004, especially during the growing season (from May to September), across the three sites. The annual precipitation at TS in 2004 (364.4 mm) was close the annual average (350 mm), whereas the precipitation at TS in 2005 (153.3 mm) was significantly below the average. Both GEP and Reco of the temperate steppe in 2005 were significantly reduced by the extreme drought stress, resulting in net carbon release during almost the whole growing season. The magnitude of CO2 fluxes (daily and annual sums) was largest for the alpine shrub-meadow and smallest for the alpine meadow-steppe. The seasonal trends of GEP, Reco and NEE of the alpine shrub-meadow tracked closely with the variation in air temperature, while the seasonality of GEP, Reco and NEE of the temperate steppe and the alpine meadow-steppe was more related to the variation in soil moisture. The alpine shrub-meadow was a local carbon sink over the two years. The temperate steppe and alpine meadow-steppe were acting as net carbon source, with more carbon loss to the atmosphere in warmer and drier year of 2005. Annual precipitation was the primary climate driver for the difference in annual GEP and NEE among the three sites and between the two years. We also found the annual GEP and NEE depended significantly on the growing season length, which was mainly a result of the timing and amount of precipitation for the temperate steppe and the alpine meadow-steppe, but was more linked to the variation in air temperature for the alpine shrub-meadow.
Enhanced biological carbon consumption in a high CO2 ocean: a revised estimate of the atmospheric uptake efficiency
Authors: Matear R. , McNeil B.
Published: April 2010

Abstract. A recent mesocosm study under high CO2 conditions has found phytoplankton carbon consumption is elevated beyond typical Redfield ratios (Riebesell et al., 2007). We investigate the efficacy of this elevated biological carbon consumption to increase global oceanic CO2 uptake from the atmosphere in an ocean general circulation model (OGCM). In the OGCM, elevated biological carbon consumption throughout the ocean increased oceanic CO2 uptake by 46 Pg C during 1800 to 2100 period, which is less than half the value estimated by (Riebesell et al., 2007). Our study's lower ratio of oceanic CO2 uptake from the atmosphere caused by enhanced biological carbon consumption (export production) is due to a more realistic 3-D circulation and the resulting spatial patterns in the re-supply of carbon from the interior ocean to the surface. In our OGCM simulations, despite increased biological carbon export to the ocean interior, some regions like the eastern equatorial Pacific and Southern Ocean actually take up less CO2 from the atmosphere. This is due to the pooling of exported carbon at intermediate depths within these regions (analogous to nutrient trapping) and its subsequent re-supply back to the surface that exceeds the enhanced biological carbon export in the high CO2 world. Thus large-scale increases in biological carbon export can lead to some areas where surface ocean pCO2 increases more rapidly than atmospheric CO2. Furthermore, our results demonstrate that enhancing biological carbon export via other means such as iron fertilization is inefficient in regions like the Southern Ocean because of the rapid vertical re-supply of carbon-rich waters. This vertical resupply of carbon-rich waters in the Southern Ocean dampens the oceanic CO2 uptake efficiency due to enhanced biological carbon consumption to be only 16% and suggests a very low efficacy of biological fertilization in the region.
Temperature response functions introduce high uncertainty in modelled carbon stocks in cold temperature regimes
Authors: Portner H. , Bugmann H. , Wolf A.
Published: April 2010

Abstract. Models of carbon cycling in terrestrial ecosystems contain formulations for the dependence of respiration on temperature, but the sensitivity of predicted carbon pools and fluxes to these formulations and their parameterization is not understood. Thus, we made an uncertainty analysis of soil organic matter decomposition with respect to its temperature dependency using the ecosystem model LPJ-GUESS. We used five temperature response functions (Exponential, Arrhenius, Lloyd-Taylor, Gaussian, Van't Hoff). We determined the parameter uncertainty ranges of the functions by nonlinear regression analysis based on eight experimental datasets from northern hemisphere ecosystems. We sampled over the uncertainty bounds of the parameters and run simulations for each pair of temperature response function and calibration site. The uncertainty in both long-term and short-term soil carbon dynamics was analyzed over an elevation gradient in southern Switzerland. The function of Lloyd-Taylor turned out to be adequate for modelling the temperature dependency of soil organic matter decomposition, whereas the other functions either resulted in poor fits (Exponential, Arrhenius) or were not applicable for all datasets (Gaussian, Van't Hoff). There were two main sources of uncertainty for model simulations: (1) the uncertainty in the parameter estimates of the response functions, which increased with increasing temperature and (2) the uncertainty in the simulated size of carbon pools, which increased with elevation, as slower turn-over times lead to higher carbon stocks and higher associated uncertainties. The higher uncertainty in carbon pools with slow turn-over rates has important implications for the uncertainty in the projection of the change of soil carbon stocks driven by climate change, which turned out to be more uncertain for higher elevations and hence higher latitudes, which are of key importance for the global terrestrial carbon budget.
Ecosystem respiration, vegetation development and soil nitrogen in relation to breeding density of seagulls on a pristine volcanic island, Surtsey, Iceland
Authors: Sigurdsson B. D. , Magnusson B.
Published: April 2010

Abstract. Since its birth in 1963 by volcanic eruption in the North Atlantic Ocean off Iceland, Surtsey has been a unique natural laboratory on how organisms colonize volcanic islands and form ecosystems with contrasting structure and function. In July, 2004, ecosystem respiration rate, soil properties and surface cover of vascular plants were measured on 21 plots distributed among the main plant communities found 40 years after the primary succession started. The plots could be divided into two groups, inside and outside seagull (Larus sp.) colonies found on the island. Vegetation cover of the plots was strongly related to the density of seagull nests within and around them. The occurrence of seagull nests and increased vegetation also coincided with significant increase in ecosystem respiration, soil carbon and nitrogen, and with significantly lower soil pH and soil temperatures. The ecosystem respiration was high inside the gull colonies, similar to the highest fluxes measured in drained wetlands or agricultural fields in Iceland. The most important factor for vegetation succession and ecosystem function on Surtsey seems to be the amount of nitrogen, which was mainly brought in by the seagulls.
A simple ecohydrological model captures essentials of seasonal leaf dynamics in semi-arid tropical grasslands
Authors: Choler P. , Sea W. , Briggs P. , Raupach M. , Leuning R.
Published: April 2010

Abstract. Modelling leaf phenology in water-controlled ecosystems remains a difficult task because of high spatial and temporal variability in the interaction of plant growth and soil moisture. Here, we move beyond widely used linear models to examine the performance of low-dimensional, nonlinear ecohydrological models that couple the dynamics of plant cover and soil moisture. The study area encompasses 400 000 km2 of semi-arid perennial tropical grasslands, dominated by C4 grasses, in the Northern Territory and Queensland (Australia). We prepared 8 yr time series (2001–2008) of climatic variables and estimates of fractional vegetation cover derived from MODIS Normalized Difference Vegetation Index (NDVI) for 400 randomly chosen sites, of which 25% were used for model calibration and 75% for model validation. We found that the mean absolute error of linear and nonlinear models did not markedly differ. However, nonlinear models presented key advantages: (1) they exhibited far less systematic error than their linear counterparts; (2) their error magnitude was consistent throughout a precipitation gradient while the performance of linear models deteriorated at the driest sites, and (3) they better captured the sharp transitions in leaf cover that are observed under high seasonality of precipitation. Our results showed that low-dimensional models including feedbacks between soil water balance and plant growth adequately predict leaf dynamics in semi-arid perennial grasslands. Because these models attempt to capture fundamental ecohydrological processes, they should be the favoured approach for prognostic models of phenology.
Ecosystem carbon exchange of a subtropical evergreen coniferous plantation subjected to seasonal drought, 2003–2007
Authors: Wen X.-F. , Wang H.-M. , Yu G.-R. , Sun X.-M.
Published: April 2010

Abstract. The behavior of whole ecosystems in humid climate zone in response to drought is less well documented. Continuous measurements of carbon dioxide and water vapor exchange using the eddy covariance technique during 2003–2007 were made above a subtropical evergreen coniferous plantation in Southeast China. Seasonal distribution of precipitation and temperature was out of step during the summer, which resulted in droughts frequently. Gross ecosystem photosynthesis (GEP), ecosystem respiration (RE) and net ecosystem production (NEP) showed clear seasonality, with low rates during seasonal drought and in winter. The exponential response of monthly GEP and RE to air temperature provided us an implication that monthly NEP was less sensitive and only increased linearly. However, the function response of monthly NEP and its components (GEP and RE) disappeared under high temperature and low moisture. Monthly and year RE showed a linear and positive dependence on the corresponding GEP, with statistically significant. Annual NEP varied among years from 306 to 430 g C m−2 (mean 395, SD 50). Interannual variability in NEP was controlled primarily by air temperature and the ratio of precipitation and evapotranspiration that affected the RE/GEP ratio, which varied between 0.76 and 0.81 (mean 0.78, SD 0.02).