bluuebunny/crossref_metadata_2025 huggingface dataset:
Atmospheric drivers of storage water use in Scots pine
Authors: Verbeeck H. , Steppe K. , Nadezhdina N. , Op De Beeck M. , Deckmyn G. , Meiresonne L. , Lemeur R. , Čermák J. , Ceulemans R. , Janssens I. A.
Published: April 2010

Abstract. In this study we determined the microclimatic drivers of storage water use in Scots pine (Pinus sylvestris L.) growing in a temperate climate. The storage water use was modeled using the ANAFORE model, integrating a dynamic water flow and – storage model with a process-based transpiration model. The model was calibrated and validated with sap flow measurements for the growing season of 2000 (26 May–18 October). Because there was no severe soil drought during the study period, we were able to study atmospheric effects. Incoming radiation was the main driver of storage water use. The general trends of sap flow and storage water use are similar, and follow more or less the pattern of incoming radiation. Nevertheless, considerable differences in the day-to-day pattern of sap flow and storage water use were observed, mainly driven by vapour pressure deficit (VPD). During dry atmospheric conditions (high VPD) storage water use was reduced. This reduction was disproportionally higher than the reduction in measured sap flow. Our results suggest that the trees did not rely more on storage water during periods of atmospheric drought, without severe soil drought. A third important factor was the tree water deficit. When storage compartments were depleted beyond a threshold, storage water use was limited due to the low water potential in the storage compartments. The maximum relative contribution of storage water to daily transpiration was also constrained by an increasing tree water deficit.
Dissolution of coccolithophorid calcite by microzooplankton and copepod grazing
Authors: Antia A. N. , Suffrian K. , Holste L. , Müller M. N. , Nejstgaard J. C. , Simonelli P. , Carotenuto Y. , Putzeys S.
Published: April 2010

Abstract. Independent of the ongoing acidification of surface seawater, the majority of the calcium carbonate produced in the pelagial is dissolved by natural processes above the lysocline. We investigate to what extent grazing and passage of coccolithophorids through the guts of copepods and the food vacuoles of microzooplankton contribute to calcite dissolution. In laboratory experiments where the coccolithophorid Emiliania huxleyi was fed to the rotifer Brachionus plicatilis, the heterotrophic flagellate Oxyrrhis marina and the copepod Acartia tonsa, calcite dissolution rates of 45–55%, 37–53% and 5–22% of ingested calcite were found. We ascribe higher loss rates in microzooplankton food vacuoles as compared to copepod guts to the strongly acidic digestion and the individual packaging of algal cells. In further experiments, specific rates of calcification and calcite dissolution were also measured in natural populations during the PeECE III mesocosm study under differing ambient pCO2 concentrations. Microzooplankton grazing accounted for between 27 and 70% of the dynamic calcite stock being lost per day, with no measurable effect of CO2 treatment. These measured calcite dissolution rates indicate that dissolution of calcite in the guts of microzooplankton and copepods can account for the calcite losses calculated for the global ocean using budget and model estimates.
North Pacific-wide spreading of isotopically heavy nitrogen from intensified denitrification during the Bølling/Allerød and post-younger dryas periods: evidence from the Western Pacific
Authors: Kao S. J. , Liu K. K. , Hsu S. C. , Chang Y. P. , Dai M. H.
Published: April 2010

Abstract. Sedimentary δ15N record for the past 30 ka buried in the Okinawa Trough in the western North Pacific mimicking the pattern from the Eastern Tropical North Pacific (ETNP), but the values (4.4–5.8‰) and the amplitude of the variation were much smaller than those (9–17‰) of the previous site. All but three values in the record were lower than the mean δ15N (5.6‰) of nitrate in the upper 800 m of the Kuroshio water suggesting additional inputs of isotopically light nitrogen from N2-fixation. The peak values of δ15N occurred during the Bølling/Allerød period and the warming period right after the Younger Dryas, synchronous to those found in the Eastern North Pacific. It is highly probable the high δ15N values are originated from the influence of the intensified denitrification in the ETNP during the warming periods. These new data represent the sedimentary record most distant from the intensive denitrifying zone in the ETNP and may serve as critical constraints to better quantify the nitrogen budget in the last climate cycle.
Microbiology and atmospheric processes: biological, physical and chemical characterization of aerosol particles
Authors: Georgakopoulos D. G. , Després V. , Fröhlich-Nowoisky J. , Psenner R. , Ariya P. A. , Pósfai M. , Ahern H. E. , Moffett B. F. , Hill T. C. J.
Published: April 2010

Abstract. The interest in bioaerosols has traditionally been linked to health hazards for humans, animals and plants. However, several components of bioaerosols exhibit physical properties of great significance for cloud processes, such as ice nucleation and cloud condensation. To gain a better understanding of their influence on climate, it is therefore important to determine the composition, concentration, seasonal fluctuation, regional diversity and evolution of bioaerosols. In this paper, we will review briefly the existing techniques for detection, quantification, physical and chemical analysis of biological particles, attempting to bridge physical, chemical and biological methods for analysis of biological particles and integrate them with aerosol sampling techniques. We will also explore some emerging spectroscopy techniques for bulk and single-particle analysis that have potential for in-situ physical and chemical analysis. Lastly, we will outline open questions and further desired capabilities (e.g., in-situ, sensitive, both broad and selective, on-line, time-resolved, rapid, versatile, cost-effective techniques) required prior to comprehensive understanding of chemical and physical characterization of bioaerosols.
Branch xylem density variations across Amazonia
Authors: Patiño S. , Lloyd J. , Paiva R. , Quesada C. A. , Baker T. R. , Santos A. J. B. , Mercado L. M. , Malhi Y. , Phillips O. L. , Aguilar A. , Alvarez E. , Arroyo L. , Bonal D. , Costa A. C. L. , Czimczik C. I. , Gallo J. , Herrera R. , Higuchi N. , Horna V. , Hoyos E. J. , Jimenez E. M. , Killeen T. , Leal E. , Luizão F. , Meir P. , Monteagudo A. , Neill D. , Núñez-Vargas P. , Palomino W. , Peacock J. , Peña-Cruz A. , Peñuela M. C. , Pitman N. , Priante Filho N. , Prieto A. , Panfil S. N. , Rudas A. , Salomão R. , Silva N. , Silveira M. , Soares de Almeida S. , Torres-Lezama A. , Turriago J. D. , Vásquez-Martínez R. , Schwarz M. , Sota A. , Schmerler J. , Vieira I. , Villanueva B. , Vitzthum P.
Published: April 2010

Abstract. Measurements of branch xylem density, Dx, were made for 1466 trees representing 503 species, sampled from 80 sites across the Amazon basin. Measured values ranged from 240 kg m−3 for a Brosimum parinarioides from Tapajos in West Pará, Brazil to 1130 kg m−3 for an Aiouea sp. from Caxiuana, Central Pará, Brazil. Analysis of variance showed significant differences in average Dx across the sample plots as well as significant differences between families, genera and species. A partitioning of the total variance in the dataset showed that geographic location and plot accounted for 33% of the variation with species identity accounting for an additional 27%; the remaining "residual" 40% of the variance accounted for by tree to tree (within species) variation. Variations in plot means, were, however, hardly accountable at all by differences in species composition. Rather, it would seem that variations of xylem density at plot level must be explained by the effects of soils and/or climate. This conclusion is supported by the observation that the xylem density of the more widely distributed species varied systematically from plot to plot. Thus, as well as having a genetic component branch xylem density is a plastic trait that, for any given species, varies according to where the tree is growing and in a predictable manner. Exceptions to this general rule may be some pioneers belonging to Pourouma and Miconia and some species within the genera Brosimum, Rinorea and Trichillia which seem to be more constrained in terms of this plasticity than most species sampled as part of this study.
Carbon dynamics in aboveground coarse wood biomass of wetland forests in the northern Pantanal, Brazil
Authors: Schöngart J. , Arieira J. , Felfili Fortes C. , Cezarine de Arruda E. , Nunes da Cunha C.
Published: April 2010

Abstract. This is the first estimation on carbon dynamics in the aboveground coarse wood biomass (AGWB) of wetland forests in the Pantanal, located in Central Southern America. In four 1-ha plots in stands characterized by the pioneer species Vochysia divergens Pohl (Vochysiaceae) forest inventories (trees ≥10 cm diameter at breast height, DBH) have been performed and converted to predictions of AGWB by five different allometric models using two or three predicting parameters (DBH, tree height, wood density). Best prediction has been achieved using allometric equations with three independent variables. Carbon stocks (50% of AGWB) vary from 7.4 to 100.9 Mg C ha−1 between the four stands. Carbon sequestration differs 0.50–4.24 Mg C ha−1 yr−1 estimated by two growth models derived from tree-ring analysis describing the relationships between age and DBH for V. divergens and other tree species. We find a close correlation between estimated tree age and C-stock, C-sequestration and C-turnover (mean residence of C in AGWB).
Response of carbon fluxes to water relations in a savanna ecosystem in South Africa
Authors: Kutsch W. L. , Hanan N. , Scholes R. J. , McHugh I. , Kubheka W. , Eckhardt H. , Williams C.
Published: April 2010

Abstract. The principal mechanisms that connect carbon fluxes with water relations in savanna ecosystems were studied by using eddy covariance in a savanna ecosystem at Kruger National Park, South Africa. Since the annual drought and rewetting cycle is a major factor influencing the function of savanna ecosystems, this work focused on the close inter-connection between water relations and carbon fluxes. Data from a nine-month measuring campaign lasting from the early wet season to the late dry season were used. Total ecosystem respiration showed highest values at the onset of the growing season, a slightly lower plateau during the main part of the growing season and a continuous decrease during the transition towards the dry season. The regulation of canopy conductance was changed in two ways: changes due to phenology during the course of the growing season and short-term acclimation to soil water conditions. The most constant parameter was water use efficiency that was influenced by VPD during the day but the VPD response curve of water usage did change only slightly during the course of the growing season and decreased by about 30% during the transition from wet to dry season. The regulation of canopy conductance and photosynthetic capacity were closely related. This observation meets recent leaf-level findings that stomatal closure triggers down-regulation of Rubisco during drought. Our results may show the effects of these processes on the ecosystem scale.
Mass extinctions past and present: a unifying hypothesis
Authors: Wooldridge S. A.
Published: April 2010

Abstract. Enzymes are often referred to as the "agents of life" – a very apt term, since essentially all life processes are controlled by them. Typically, these enzymes only function across a narrow band of environmental conditions, particularly temperature and pH. Ambient conditions that challenge these operating conspecifics trigger enzyme dysfunction. Here, it is proposed that the pH-dependent inactivation of a single enzyme, urease, provides a unifying kill-mechanism for at least four of the "big five" mass extinctions of the past 560 million years. The triggering of this kill-mechanism is suggested to be sensitive to both gradualistic and catastrophic environmental disturbances that cause the operating pH of urease-dependent organisms to cross enzymatic "dead zones", one of which is suggested to exist at ~pH 7.9. For a wide range of oceanic and terrestrial ecosystems, this pH threshold coincides with an atmospheric CO2 partial pressure (pCO2) of ~560 ppmv – a level that at current CO2 emission trajectories may be exceeded as early as 2050. The urease hypothesis thus predicts an impending Anthropocene extinction event of equivalence to the "big five" unless future atmospheric pCO2 levels can be stabilised well below 560 ppmv. Immediate scientific discussion and testing is required to confirm the validity of the urease hypothesis.
Iodine speciation and cycling in limnic systems: observations from a humic rich headwater lake (Mummelsee)
Authors: Gilfedder B. S. , Petri M. , Biester H.
Published: April 2010

Abstract. Iodine undergoes several redox changes in the natural environment, existing as iodate, iodide, and covalently bound to organic matter. While considerable attention has been given to iodine speciation and cycling in the marine environment, very little is known about iodine cycling and speciation in terrestrial fresh water systems. Here we show iodine speciation (measured by IC-ICP-MS) data from one year of monthly sampling of a small humic rich lake in the Black Forest (Mummelsee) under varying redox conditions. The aim was to elucidate the seasonal cycles of iodine species in the lake water column and to quantify both inorganic and organic iodine species. A sediment core was also collected for iodine analysis. Total iodine levels in the Mummelsee averaged 1.93±0.3 μg l−1. Organo-I was the dominant species in the lake, making up on average 85±7% of the total iodine. No strong seasonal variation in organo-I was observed, with only small variations occurring in the epi- and hypolimnion. Iodide was scavenged from the epilimnion during the summer and autumn, which could be related to (micro)biological uptake and co-precipitation. This was also suggested by the high iodine levels in the sediment core (av. 11.8±1.7 mg kg −1). In the hypolimnion, a strong flux of iodide was observed from the sediments into the water column during anoxic and hypoxic conditions, observed during the summer, autumn and, in the bottom 2 m, the winter. This iodide flux and is thought to occur during decomposition of biological material. Iodate levels in the epilimnion increased consistently over the year, whereas it was reduced below detection limits in the hypolimnion during low oxygen conditions. The winter partial turnover lead to reintroduction of oxygen into the hypolimnion and the formation of iodate and organo-I, as well as removal of iodide. In conclusions, iodine cycling in the Mummelsee was controlled by organo-I, although redox conditions and perhaps biological activity were also important, particularly in the hypolimnion during stratification.
Carbonate precipitation by the thermophilic archaeon Archaeoglobus fulgidus: a model of carbon flow for an ancient microorganism
Authors: Robbins L. L. , Van Cleave K. A. , Ostrom P.
Published: April 2010

Abstract. Microbial carbonate precipitation experiments were conducted using the archaeon bacteria Archaeoglobus fulgidus to determine chemical and isotopic fractionation of organic and inorganic carbon into mineral phases. Carbonate precipitation was induced in two different experiments using A. fulgidus to determine the relative abundance of organically derived carbon incorporated into carbonate minerals as well as to define any distinct phases or patterns that could be attributed to the precipitation process. One experiment used a medium containing 13C-depleted organic carbon and 13C-enriched inorganic carbon, and the other used a 14C-labeled organic carbon source. Results indicated that 0.9–24.8% organic carbon was incorporated into carbonates precipitated by A. fulgidus and that this process was mediated primarily by pH and CO2 emission from cells. Data showed that the carbon in the CO2 produced from this microorganism is incorporated into carbonates and that the rate at which precipitation occurs and the dynamics of the carbonate precipitation process are strongly mediated by the specific steps involved in the biochemical process for lactate oxidation by A. fulgidus.
Phytoplankton blooms induced/sustained by cyclonic eddies during the Indian Ocean Dipole event of 1997 along the southern coasts of Java and Sumatra
Authors: Rahul Chand Reddy P. , Salvekar P. S.
Published: April 2010

Abstract. The Indonesian archipelago is the gateway in the tropics connecting two oceans (Pacific and the Indian Ocean) and two continents (Asia and Australia). During the Indian Ocean Dipole 1997, record anomalous and unanticipated upwelling had occurred along the southern coasts of Java and Sumatra causing massive phytoplankton blooms. But the method/mode/process for such anomalous upwelling was not known. Using monthly SeaWifs chlorophyll-a anomalies, TOPEX Sea Surface Height (SSH) anomalies, Sea Surface Temperatures (SST) and currents from a state-of-the-art OGCM, we report the presence of a series of cyclonic eddies along southern coasts of Sumatra and Java during November, December 1997 and January 1998. Upwelling caused by these cyclonic eddies, as also supported by the SSH and SST anomalies, has been responsible for the phytoplankton blooms to persist and dissipate during the 3 months (November, December 1997 and January 1998).
Soil carbon and plant diversity distribution at the farm level in the savannah region of Northern Togo (West Africa)
Authors: Sebastià M.-T. , Marks E. , Poch R. M.
Published: April 2010

Abstract. In western Africa, soil organic matter is a source of fertility for food provision and a tool for climate mitigation. In the Savannah region, strong soil degradation linked to an increase in population threatens organic matter conservation and agricultural yield. Soil degradation is also expected to impact biodiversity and, with it, increase the vulnerability of ecosystem goods and services, including the storage of soil organic carbon. Studies of land use, plant species composition and soil fertility were conducted for a conservation project at a demonstration farm in Northern Togo (West Africa), host to various management regimes. Results showed a low organic matter content of the surface soil horizons, often around 0.5%. The highest values were found in a sacred forest within the farm (2.2%). Among crops, rice had the highest soil organic matter, around 1%. In a survey of grasslands, pastures showed the highest organic matter content, with vegetation composition differing from grazed fallows and abandoned grasslands. Plant species richness showed a positive relationship with soil organic matter (R2adj=41.2%), but only by the end of the wet season, when species richness was also highest. Sampling date had a strong effect on vegetation composition. Results showed a strong influence of human activity on soil formation and distribution, and also on plant diversity. The soil characteristics found under the permanent forest suggest a high potential of the soils of the region for improvement of both agricultural yields and as a potential carbon sink relevant to global change policies.
Diel variability of heterotrophic bacterial production and UV doses in the South East Pacific
Authors: Van Wambeke F. , Tedetti M. , Duhamel S. , Sempéré R.
Published: April 2010

Abstract. Diel variability of heterotrophic bacterial production (BP) was investigated in the South East Pacific from October to December 2004 during the BIOSOPE cruise. Three sites differing by their trophic status were studied: Marquesas Islands (MAR; 08° S, 141° W), the centre of the South Pacific Gyre (SPG) (GYR; 26° S, 114° W) and the eastern part of the SPG (EGY; 32° S, 91° W). At the three sites, diel variability of BP ranged from 17 to 40% and from 13 to 22% for volumetric surface (5 m) and integrated (to Ze and Zm) data, respectively. The main feature we observed was at 5 m, an abrupt increase (×2 to ×4) in leucine activity during the afternoon-sunset period (12:00–18:00 at the site MAR and 15:00–21:00 at the site GYR) and lowest activities recorded between 10:00 and 14:00. To assess the potential influence of solar ultraviolet radiation (UVR: 280–400 nm) on this BP diel variability, we determined, from in situ optical measurements, the mean tri-hourly ultraviolet B (UVB, 305 nm) and ultraviolet A (UVA, 380 nm) doses (irradiances integrated over time) within the mixed layer (Hm(UVB) and Hm(UVA), respectively). The wavelengths 305 nm and 380 nm were used as biologically effective wavelengths for the induction of DNA damages (cyclobutane pyrimidine dimers: CPDs) and photoenzymatic repairs (PERs), respectively. In the SPG, daily Hm(UVB) and Hm(UVA) were 0.6 and 14 kJ m−2 nm−1, respectively. The latter were probably the highest daily doses ever measured in the marine environment. The Hm(UVB)/Hm(UVA) ratio (Q) increased by 58, 117 and 46% from 06:00–09:00 to 12:00–15:00, and decreased by 36, 26 and 16% from 12:00–15:00 to 15:00–18:00 at the sites MAR, GYR and EGY, respectively. The relationship between Q and BP suggested a significant influence of UVR on the diel variability of BP (BP decreased when Q increased) at the site GYR from the surface waters to Zm, likely in relation with its hyper-oligotrophic status. Therefore, possible alternance of CPD and PER periods attributed to Q ratio, as well as a strong lags between process of autotrophic production with their associated dissolved organic carbon (DOC) release and heterotrophic utilization of organic matter could explain such diel variations.
Use of laboratory and remote sensing techniques to estimate vegetation patch scale emissions of nitric oxide from an arid Kalahari savanna
Authors: Feig G. T. , Mamtimin B. , Meixner F. X.
Published: April 2010

Abstract. The biogenic emission of nitric oxide (NO) from the soil has an important impact on a number of environmental issues, such as the production of tropospheric ozone, the cycle of the hydroxyl radical (OH) and the production of NO. In this study we collected soils from four differing vegetation patch types (Pan, Annual Grassland, Perennial Grassland and Bush Encroached) in an arid savanna ecosystem in the Kalahari (Botswana). A laboratory incubation technique was used to determine the net potential NO flux from the soils as a function of the soil moisture and the soil temperature. The net potential NO emissions were up-scaled for the year 2006 and a region (185 km×185 km) of the southern Kalahari. For that we used (a) the net potential NO emissions measured in the laboratory, (b) the vegetation patch distribution obtained from Landsat NDVI measurements, (c) estimated soil moisture contents obtained from ENVISAT ASAR measurements and (d) the soil surface temperature estimated using MODIS MOD11A2 8 day land surface temperature measurements. Differences in the net potential NO fluxes between vegetation patches occur and range from 0.27 ng m−2 s−1 in the Pan patches to 2.95 ng m−2 s−1 in the Perennial Grassland patches. Up-scaling the net potential NO fluxes with the satellite derived soil moisture and temperature data gave NO fluxes of up to 323 g ha−1 month−1, where the highest up-scaled NO fluxes occurred in the Perennial Grassland patches, and the lowest in the Pan patches. A marked seasonal pattern was observed where the highest fluxes occurred in the austral summer months (January and February) while the minimum fluxes occurred in the austral winter months (June and July), and were less than 1.8 g ha−1 month−1. Over the course of the year the mean NO emission for the up-scaled region was 0.54 kg ha−1 yr−1, which accounts for a loss of up to 7.4% of the nitrogen (N) input to the region through atmospheric deposition and biological N fixation. The biogenic emission of NO from the soil is therefore an important mechanism of N loss from this arid savanna ecosystem and has the potential to play an important role in the production of tropospheric ozone and the OH cycle.
Leaf isoprene emission in a subarctic wetland sedge community
Authors: Ekberg A. , Arneth A. , Hakola H. , Hayward S. , Holst T.
Published: April 2010

Abstract. High latitude wetlands play an important role for the surface-atmosphere exchange of carbon dioxide (CO2) and methane (CH4), but fluxes of biogenic volatile organic compounds (BVOC) in these ecosystems have to date not been extensively studied. This is despite BVOC representing a measurable proportion of the total gaseous C fluxes at northern locations and in the face of the high temperature sensitivity of these systems that requires a much improved process understanding to interpret and project possible changes in response to climate warming. We measured emission of isoprene and photosynthetic gas exchange over two growing seasons (2005–2006) in a subarctic wetland in northern Sweden with the objective to identify the physiological and environmental controls of these fluxes on the leaf scale. The sedge species Eriophorum angustifolium and Carex rostrata were both emitters of isoprene, and springtime emissions were first detected after an accumulated diurnal mean temperature above 0°C of about 100 degree days. Maximum measured growing season standardized (basal) emission rates (20°C, 1000 μmol m−2 s−1) were 1075 (2005) and 1118 (2006) μg C m−2 (leaf area) h−1 in E. angustifolium, and 489 (2005) and 396 (2006) μg C m−2 h−1 in C. rostrata. Over the growing season, basal isoprene emission varied in response to the temperature history of the last 48 h. Seasonal basal isoprene emission rates decreased also with leaf nitrogen (N), which may be explained by the typical growth and resource allocation pattern of clonal sedges as the leaves age. The observations were used to model emissions over the growing season, accounting for effects of temperature history, links to leaf assimilation rate and the light and temperature dependencies of the cold-adapted sedges.
Phosphorus burial in the ocean over glacial-interglacial time scales
Authors: Tamburini F. , Föllmi K. B.
Published: April 2010

Abstract. The role of nutrients and their impact on primary productivity and the fluctuations in atmospheric CO2 over glacial-interglacial periods are intensely debated. One of the cornerstones is the role of phosphorus (P; in the form of phosphate). Suggestions as to the importance of P evolved from an earlier proposal that P actively participated in changing productivity rates and therefore climate change, to the most recent one that changes in the glacial ocean inventory of phosphorus were small and not influential if compared to other macronutrients, such as nitrogen. Using new data coming from a selection of ODP sites, we illustrate oceanic P sedimentary phases distribution and reactive P burial fluxes, and we show how P burial fluxes changed over the last glacial-interglacial period. Concentrations of reactive P are generally lower during glacial times, while mass accumulation rates (MAR) show higher variability. On a global scale, glacial burial fluxes of reactive P are lower than those during interglacial periods by 7–10%, because lack of burial of reactive P on the glacial reduced shelf was apparently not compensated by burial in other regions of the ocean. Using the calculated changes in P burial, we try to infer their possible impact on the phosphate inventory in the world oceans. Using a simple mathematical approach, we find that these changes alone could have increased the phosphate inventory of glacial ocean waters by 20–40% compared to interglacial stages. Variations in the C/P reactive ratios and in the P sedimentary phases distribution at the investigated sites seem to indicate that at the onset of interglacial stages, shallower sites experienced an increase in reactive P concentrations. This seems to point to P-richer waters at glacial terminations, supporting the shelf-nutrient hypothesis and giving phosphorus a role as a potential player in climate change.
A mathematical modelling of bloom of the coccolithophore Emiliania huxleyi in a mesocosm experiment
Authors: Joassin P. , Delille B. , Soetaert K. , Borges A. V. , Chou L. , Engel A. , Gattuso J.-P. , Harlay J. , Riebesell U. , Suykens K. , Gregoire M.
Published: April 2010

Abstract. A dynamic model has been developed to represent biogeochemical variables and processes observed during a bloom of Emiliania huxleyi coccolithophore. This bloom was induced in a mesocosm experiment during which the ecosystem development was followed over a period of 23-days through changes in various biogeochemical parameters such as inorganic nutrients (nitrate, ammonium and phosphate), total alkalinity (TA), dissolved inorganic carbon (DIC), partial pressure of CO2 (pCO2), dissolved oxygen (O2), photosynthetic pigments, particulate organic carbon (POC), dissolved organic carbon (DOC), Transparent Exopolymer Particles (TEP), primary production, and calcification. This dynamic model is based on unbalanced algal growth and balanced bacterial growth. In order to adequately reproduce the observations, the model includes an explicit description of phosphorus cycling, calcification, TEP production and an enhanced mortality due to viral lysis. The model represented carbon, nitrogen and phosphorus fluxes observed in the mesocosms. Modelled profiles of algal biomass and final concentrations of DIC and nutrients are in agreement with the experimental observations.
Seasonal and interannual study of volatile reduced sulfur compounds (VRSC) in coastal environment: the Bay of Quiberon (Brittany, France)
Authors: Cozic-Houly A. , Viollier E. , Sarazin G. , Knoery J.
Published: April 2010

Abstract. Seasonal and annual variability of hydrogen sulfide (H2S), carbonyl sulfide (OCS), methane thiol (MeSH), dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) concentrations and supporting parameters (e.g., phytoplanktonic cells abundance) were investigated in a coastal marine environment, the Bay of Quiberon (Brittany, France) from July 2004 to August 2006. The sampling was conducted in the water column, within two meters of the sediment water interface (SWI). Minimum and maximum values were <0.1–1.6 nmol L−1 for H2S, <0.1–4.2 nmol L−1 for OCS, <0.1–7.8 nmol L−1 for MeSH, <0.1–17.5 nmol L−1 for DMS and <0.1–1.7 nmol L−1 for DMDS. Vertical carbonyl sulfide distribution showed seasonal variations with lower concentration near the SWI in winter and bottom enrichments near sediments in summer. Vertical sulfide distribution not seems to be influenced by the shallow sediments. The likely influence of Dinophyceae abundance on the MeSH, DMS and DMDS concentrations was evident for the 3-summer monitored period.
Local ecosystem feedbacks and critical transitions in the climate
Authors: Rietkerk M. , Brovkin V. , van Bodegom P. M. , Claussen M. , Dekker S. C. , Dijkstra H. A. , Goryachkin S. V. , Kabat P. , van Nes E. H. , Neutel A.-M. , Nicholson S. E. , Nobre C. , Petoukhov V. , Provenzale A. , Scheffer M. , Seneviratne S. I.
Published: April 2010

Abstract. Global and regional climate models, such as those used in IPCC assessments, are the best tools available for climate predictions. Such models typically account for large-scale land-atmosphere feedbacks. However, these models omit local vegetation-environment feedbacks that are crucial for critical transitions in ecosystems. Here, we reveal the hypothesis that, if the balance of feedbacks is positive at all scales, local vegetation-environment feedbacks may trigger a cascade of amplifying effects, propagating from local to large scale, possibly leading to critical transitions in the large-scale climate. We call for linking local ecosystem feedbacks with large-scale land-atmosphere feedbacks in global and regional climate models in order to yield climate predictions that we are more confident about.
Ventilation of subterranean CO2 and Eddy covariance incongruities over carbonaceous ecosystems
Authors: Were A. , Serrano-Ortiz P. , Moreno de Jong C. , Villagarcía L. , Domingo F. , Kowalski A. S.
Published: April 2010

Abstract. Measurements of CO2 fluxes with Eddy Covariance (EC) systems are ongoing over different ecosystems around the world, through different measuring networks, in order to assess the carbon balance of these ecosystems. In carbonaceous ecosystems, characterized by the presence of subterranean pores and cavities, ventilation of the CO2 accumulated in these cavities and pores can act as an extra source of CO2 exchange between the ecosystem and the atmosphere. In this work we analyse the effect of the subterranean heterogeneity of a carbonaceous ecosystem on measurements of CO2 fluxes by comparing measurements from two EC systems with distinct footprints. Results showed that both EC systems agreed for measurements of evapotranspiration and of CO2 in periods when respiratory and photosynthetic processes were dominant (biological periods), with a regression slope of 0.99 and 0.97, respectively. However, in periods when the main source of CO2 comes from the ventilation of subterranean pores and caves (abiotic periods) agreement is not good, with a regression slope of 0.6. Ground-penetrating radar measurements of the sub-surface confirmed the existence of high sub-surface heterogeneity that, combined with different footprints, lead to differences in the measurements of the two EC systems. These results show that measurements of CO2 fluxes with Eddy covariance systems over carbonaceous ecosystems must be taken carefully, as they may not be representative of the ecosystem under consideration.
Significant non-linearity in nitrous oxide chamber data and its effect on calculated annual emissions
Authors: Stolk P. C. , Jacobs C. M. J. , Moors E. J. , Hensen A. , Velthof G. L. , Kabat P.
Published: April 2010

Abstract. Chambers are widely used to measure surface fluxes of nitrous oxide (N2O). Usually linear regression is used to calculate the fluxes from the chamber data. Non-linearity in the chamber data can result in an underestimation of the flux. Non-linear regression models are available for these data, but are not commonly used. In this study we compared the fit of linear and non-linear regression models to determine significant non-linearity in the chamber data. We assessed the influence of this significant non-linearity on the annual fluxes. For a two year dataset from an automatic chamber we calculated the fluxes with linear and non-linear regression methods. Based on the fit of the methods 32% of the data was defined significant non-linear. Significant non-linearity was not recognized by the goodness of fit of the linear regression alone. Using non-linear regression for these data and linear regression for the rest, increases the annual flux with 21% to 53% compared to the flux determined from linear regression alone. We suggest that differences this large are due to leakage through the soil. Macropores or a coarse textured soil can add to fast leakage from the chamber. Yet, also for chambers without leakage non-linearity in the chamber data is unavoidable, due to feedback from the increasing concentration in the chamber. To prevent a possibly small, but systematic underestimation of the flux, we recommend comparing the fit of a linear regression model with a non-linear regression model. The non-linear regression model should be used if the fit is significantly better. Open questions are how macropores affect chamber measurements and how optimization of chamber design can prevent this.
Soil respiration at mean annual temperature predicts annual total across vegetation types and biomes
Authors: Bahn M. , Reichstein M. , Davidson E. A. , Grünzweig J. , Jung M. , Carbone M. S. , Epron D. , Misson L. , Nouvellon Y. , Roupsard O. , Savage K. , Trumbore S. E. , Gimeno C. , Curiel Yuste J. , Tang J. , Vargas R. , Janssens I. A.
Published: April 2010

Abstract. Soil respiration (SR) constitutes the largest flux of CO2 from terrestrial ecosystems to the atmosphere. There still exist considerable uncertainties as to its actual magnitude, as well as its spatial and interannual variability. Based on a reanalysis and synthesis of 72 site-years for 58 forests, plantations, savannas, shrublands and grasslands from boreal to tropical climates we present evidence that total annual SR is closely related to SR at mean annual soil temperature (SR MAT), irrespective of the type of ecosystem and biome. This convergence is to be theoretically expected for non water-limited ecosystems within most of the globally occurring range of annual temperature variability and sensitivity (Q10). We further show that for seasonally dry sites where annual precipitation (P) is lower than potential evapotranspiration (PET), annual SR can be predicted from wet season SR MAT corrected for a factor related to P/PET. Our finding indicates that it is sufficient to measure SR MAT for obtaining a highly constrained estimate of its annual total. This should substantially increase our capacity for assessing the spatial distribution and interannual variation of soil CO2 emissions across ecosystems, landscapes and regions, and thereby contribute to improving the spatio-temporal resolution of a major component of the global carbon cycle.
The effects of management on ammonia fluxes over a cut grassland as measured by use of dynamic chambers
Authors: David M. , Roche R. , Mattsson M. , Sutton M. A. , Dämmgen U. , Schjoerring J. K. , Cellier P.
Published: April 2010

Abstract. Grassland management may lead to strong modification of the canopy structure and hence fluxes of carbon and nitrogen in the soil-plant-atmosphere system. Mowing or grazing removes green leaves, which are often a sink for ammonia. Consequently, the ratio between actively growing leaves and senescing/dead parts of the plants is strongly changed in favour of the latter, which may constitute a large source of ammonia. Moreover, fertilisers are a known source of ammonia through direct volatilisation. The effects of grassland management, e.g. growing, cutting and fertilisation, on ammonia emission were investigated using a dynamic chamber. This technique made it possible to monitor ammonia emissions in the field at the plant level. With ammonia-free air at the inlet, the ammonia emissions from mature sward did not exceed 4 ng NH3 m−2 s−1. They were approximately 20 times larger above a sward re-growing after cutting and 200 times larger after fertilisation, where 0.5–1.0% of the applied inorganic nitrogen fertiliser was lost by volatilisation. Cutting implied three main changes in ammonia sources and sinks within the canopy: (i) physiological changes with nitrogen remobilisation to the growing leaves and increase in senescence, (ii) changes in compartment proportions with only 5% of green leaves remaining after cutting as opposed to equal proportions of dead leaves as green leaves before cutting, (iii) microclimate changes within the canopy especially for litter with higher turbulence, temperature, and alternation of dry (day) and wet (night) conditions after cutting. These changes promoted ammonia volatilisation from the litter, which could account for the increased ammonia loss following cutting. Another potential source was the wounded surfaces of the stubble which may have emitted ammonia during bleeding and evaporation of sap containing significant levels of ammonium. These results showed that the contribution of litter and drying cut sward on the ammonia balance of grassland is very significant, as well as their interaction with microclimatic conditions. This could apply to most natural and managed ecosystems and could be especially significant in the former. Consequently, further studies on ammonia fluxes should have a 0focus on this part of the canopy.
Mercury concentrations and pools in four Sierra Nevada forest sites, and relationships to organic carbon and nitrogen
Authors: Obrist D. , Johnson D. W. , Lindberg S. E.
Published: April 2010

Abstract. This study presents data on Hg concentrations, stochiometric relations to carbon (C) and nitrogen (N), and Hg pool sizes in four Sierra Nevada forest sites of similar exposure and precipitation regimes, and hence similar atmospheric deposition, to evaluate how ecosystem parameters control Hg retention in ecosystems. In all four sites, the largest amounts of Hg reside in soils which account for 94–98% of ecosystem pools. Hg concentrations and Hg/C ratios increase in the following order: Green Needles/Leaves < Dry Needles/Leaves < Oi litter < Oe litter < Oa litter. Stochiometric relations show negative correlations between Hg and C (r2=0.58) and N and C (r2=0.64) in decomposing litter, but a positive correlation between litter Hg and N (r2=0.70). These inverse relations may reflect preferential retention of N and Hg over C during decomposition, or may be due to older age of decomposed litter layers which are exposed to longer-term atmospheric Hg deposition in the field. The results indicate that litter Hg levels depend on decomposition stage and may not follow generally observed positive relationships between Hg and organic C. Mineral soil layers show strong positive correlations of Hg to C across all sites and soil horizons (r2=0.83), but Hg concentrations are even more closely related to N with a similar slope to that observed in litter (r2=92%). Soil N levels alone explain over 90% of Hg pool sizes across the four Sierra Nevada forest sites. This suggest that soil organic N and C groups provide sorption sites for Hg to retain atmospheric deposition. However, the patterns could be due indirect relationships where high soil N and C levels reflect high ecosystem productivity which leads to corresponding high atmospheric Hg deposition inputs via leaf litterfall and plant senescence. Our results also show that two of the sites previously affected by prescribed burning and wildfires show significant depletion of above-ground Hg pools but that belowground Hg pools remain unaffected. We conclude that sequestration of Hg in remote Sierra Nevada forest sites is strongly co-determined by ecosystem parameters with C and N pools being excellent determinants for the pool sizes of Hg.
Calcification, a physiological process to be considered in the context of the whole organism
Authors: Findlay H. S. , Wood H. L. , Kendall M. A. , Spicer J. I. , Twitchett R. J. , Widdicombe S.
Published: April 2010

Abstract. Marine organisms that produce calcium carbonate structures are predicted to be most vulnerable to a decline in oceanic pH (ocean acidification) based on the understanding that calcification rates will decrease as a result of changes in the seawater carbonate chemistry thereby reducing carbonate ion concentration (and associated saturation states). Coastal seas are critical components of the global carbon cycle yet little research has been conducted on acidification impacts on coastal benthic organisms. Here, a critical appraisal of calcification in six benthic species showed, contrary to popular predictions, calcification can increase, and not decrease, in acidified seawater. Measuring the changes in calcium in isolated calcium carbonate structure as well as structures from live animals exposed to acidified seawater allowed a comparison between a species' ability to calcify and the dissolution affects across decreasing levels of pH. Calcium carbonate production is dependant on the ability to increase calcification thus counteracting an increase in dissolution. Comparison with paleoecological studies of past high carbon dioxide (CO2) events presents a similar picture. This conclusion implies that calcification may not be the critical process impacted by ocean acidification; particularly as all species investigated displayed physiological trade offs including reduced metabolism, health, and behavioural responses, in association with this calcification upregulation, which possess as great a threat to survival as an inability to calcify.
Improved understanding of drought controls on seasonal variation in Mediterranean forest canopy CO2 and water fluxes through combined in situ measurements and ecosystem modelling
Authors: Keenan T. , García R. , Friend A. D. , Zaehle S. , Gracia C. , Sabate S.
Published: April 2010

Abstract. Water stress is a defining characteristic of Mediterranean ecosystems, and is likely to become more severe in the coming decades. However, our current understanding of how soil moisture controls ecosystem functioning is not sufficient to adequately constrain model parameterisations. Canopy-scale flux data from four forest ecosystems with Mediterranean-type climates were analysed in order to determine the physiological controls on carbon and water flues through the year. Stomatal and non-stomatal limitations on photosynthesis were separated, and new parameterisations were derived and implemented in two independent modelling approaches. The effectiveness of the two approaches to ecosystem process-based modelling, one a dynamic global vegetation model (ORCHIDEE), and the other a forest growth model (GOTILWA+), was assessed and modelled canopy responses to seasonal changes in soil moisture were analysed with respect to in situ flux measurements. In contrast to commonly held assumptions, we find that stomatal control does not dominate photosynthesis under natural seasonally-developing soil moisture stress. However, applying a soil moisture dependency to photosynthetic capacity results in accurate prediction of both carbon and water fluxes under all soil moisture levels encountered in the field. The new parameterisation has important consequences for simulated responses of carbon and water fluxes to seasonal soil moisture stress, and will greatly improve our ability to anticipate future impacts of climate changes on the functioning of Mediterranean ecosystems.
Seasonal variation in ecosystem parameters derived from FLUXNET data
Authors: Groenendijk M. , van der Molen M. K. , Dolman A. J.
Published: April 2010

Abstract. The carbon dioxide sink is in a complex way related to weather and climate. In order to better understand the relationship and feedbacks, we present a methodology to simulate observed carbon dioxide flux data with a simple vegetation model (5PM) with weekly varying model parameters. The model parameters explain the interaction between vegetation and seasonal climate more general than the flux data. Two parameters (Rref and E0) are related to ecosystem respiration and three parameters (Jm, α and λ) to photosynthesis and transpiration. We quantified the weekly variability of ecosystem parameters as a function of vegetation type and climate region. After statistical quality checks 121 FLUXNET sites were available for analysis of the weekly varying model parameters. The simulations of these sites have high correlation coefficients (r2=0.6 to 0.8) between the observed and simulated carbon and water fluxes. With weekly parameters we determined average seasonal cycles for the different combinations of vegetation type and climate regions (PFTs). The variation between PFTs is large, which provides an excellent dataset to study the differences in ecosystem characteristics. In general we observed that needleleaf forests and grasslands in warmer climates have relatively constant parameter values during the year. Broadleaf forests in all climate regions have large seasonal variation for each of the five parameters. In boreal regions parameter values are always lower than in temperate regions. A large seasonality of the model parameters indicates a strong relation between vegetation and climate. This suggests that climate change will have the largest impact on the terrestrial carbon fluxes in boreal regions and for deciduous forests, and less for grasslands and evergreen forests.