bluuebunny/crossref_metadata_2025 huggingface dataset:
Fluxes and 13C isotopic composition of dissolved carbon and pathways of methanogenesis in a fen soil exposed to experimental drought
Authors: Knorr K.-H. , Glaser B. , Blodau C.
Published: April 2010

Abstract. The impact of drought and rewetting on carbon cycling in peatland ecosystems is currently debated. We studied the impact of experimental drought and rewetting on intact monoliths from a temperate fen over a period of ~300 days, using a permanently wet treatment and two treatments undergoing drought for 50 days. In one of the mesocosms vegetation had been removed. Net production of CH4 was calculated from mass balances in the peat and emission using static chamber measurements and results compared to 13C isotope budgets of CO2 and CH4 and energy yields of acetoclastic and hydrogenotrophic methanogenesis. Drought retarded methane production after rewetting for days to weeks and promoted methanotrophic activity. Based on isotope and flux budgets, aerobic soil respiration contributed 32–96% in the wet and 86–99% in the other treatments. Drying and rewetting did not shift methanogenic pathways according to δ 13C ratios of CH4 and CO2. Although δ13C ratios indicated a prevalence of hydrogenotrophic methanogenesis, free energies of this process were small and often positive on the horizon scale, suggesting that methane was produced very locally. Fresh plant-derived carbon input apparently supported respiration in the rhizosphere and sustained methanogenesis in the unsaturated zone according to a 13C-CO2 labelling experiment. The study documents that drying and rewetting in a rich fen soil may have little effect on methanogenic pathways but result in rapid shifts between methanogenesis and methanotrophy. Such shifts may be promoted by roots and soil heterogeneity, as hydrogenotrophic methanogenesis occurred locally even when conditions were not conducive for this process in the bulk peat.
Methylated arsenic and antimony species in suspended matter of the river Ruhr, Germany
Authors: Duester L. , Hartmann L. M. , Hirner A. V.
Published: April 2010

Abstract. The methylated antimony and arsenic species content of sediments derived from a sedimentation bowl of the river Ruhr were monitored over a 12 month period. The most prevalent species detected were monomethylarsenic (MMAs) and monomethylantimony (MMSb). The methylantimony and methylarsenic species concentration was found to be directly correlated to the winter spate. As the biological activity in the water body is generally low at this time of the year, it may be concluded that the concentration maxima in winter originated from the translocation of soil- and sediment particles to the river by heavy rains and the melting of snow. A second maximum in Spring/early Summer was observed for the methylarsenic species, and specifically the dimethylarsenic species (DMAs); this occurred in parallel to the algal bloom. A change in the methylarsenic speciation pattern was observed between April, May and June, with DMAs replacing MMAs as the dominant methylarsenic species. For methylated antimony species no seasonal variation in the species pattern was detected. Taken together these data strongly indicate a higher degree of transformation of arsenic compared to antimony in the Ruhr river system in spring and can be taken as a record for a biogeochemical different behaviour of these two elements which are often treated as equivalent in environmental studies.
Nitrification and inorganic nitrogen distribution in a large perturbed river/estuarine system: the Pearl River Estuary, China
Authors:   Minhan Dai,   Lifang Wang,   Xianghui Guo,   Weidong Zhai,   Qing Li,   Biyan He,   Shuh-Ji Kao
Published: April 2010

Abstract. We investigated the spatial distribution and seasonal variation of dissolved inorganic nitrogen in a large perturbed estuary, the Pearl River Estuary, based on three cruises conducted in winter (January 2005), summer (August 2005) and spring (March 2006). On-site incubation was also carried out for determining ammonium and nitrite oxidation rates (nitrification rates). We observed a year-round pattern of dramatic decrease in NH4+, increase in NO3− but insignificant change in NO2− in the upper estuary at salinity ~0–5. However, species and concentrations of inorganic nitrogen at estuary significantly changed with season. In winter with low runoff the most upper reach of the Pearl River Estuary showed relatively low rates of ammonia oxidation (0–5.4 μmol N L−1 d−1) and nitrite oxidation (0–5.2 μmol N L−1 d−1), accompanied by extremely high concentrations of ammonia (up to >800 μmol L−1) and nitrate (up to >300 μmol L−1). In summer, the upper estuary showed higher nitrification rates (ammonia oxidation rate ~1.5–33.1 μmol N L−1 d−1, nitrite oxidation rate ~0.6–32.0 μmol N L−1 d−1) with lower concentrations of ammonia (<350 μmol L−1) and nitrate (<120 μmol L−1). The Most Probable Number test showed relatively lower nitrifier abundance in summer at most sampling stations, indicating a greater specific nitrification rate per cell in the warm season. Temperatures appeared to control nitrification rates to a large degree in different seasons. In addition to aerobic respiration, nitrification contributed significantly to the consumption of dissolved oxygen (DO) and production of CO2 at the upper estuary. Nitrification-induced DO consumption accounted for approximately up to one third of the total water column community DO consumption in the upper estuary during surveyed periods, boosting environmental stress on this large estuarine ecosystem.
Temporal variability of the anthropogenic CO2 storage in the Irminger Sea
Authors: Pérez F. F. , Vázquez-Rodríguez M. , Louarn E. , Padín X. A. , Mercier H. , Ríos A. F.
Published: April 2010

Abstract. The anthropogenic CO2 (Cant) estimates from cruises spanning more than two decades (1981–2006) in the Irminger Sea area reveal a large variability of the Cant storage rates in the North Atlantic Subpolar Gyre. During the early 1990's, the Cant uptake rates doubled the average rate for 1981–2006, whilst a remarkable drop to almost half that average followed from 1997 onwards. The Cant storage evolution runs parallel to CFC12 inventories and is in good agreement with Cant uptake rates of increase calculated from sea surface pCO2 measurements. The North Atlantic Oscillation shift from a positive to a negative phase in 1996 led to a reduction of the air-sea heat loss in the Labrador Sea. The consequent convection weakening accompanied by an increase in stratification lowered the efficiency of the northern North Atlantic CO2 sink.
Colimitation of decomposition by substrate and decomposers – a comparison of model formulations
Authors: Wutzler T. , Reichstein M.
Published: April 2010

Abstract. Decomposition of soil organic matter (SOM) is limited by both the available substrate and the active decomposer community. The understanding of this colimitation strongly affects the understanding of feedbacks of soil carbon to global warming and its consequences. This study compares different formulations of soil organic matter (SOM) decomposition. We compiled formulations from literature into groups according to the representation of decomposer biomass on the SOM decomposition rate a) non-explicit (substrate only), b) linear, and c) non-linear. By varying the SOM decomposition equation in a basic simplified decomposition model, we analyzed the following questions. Is the priming effect represented? Under which conditions is SOM accumulation limited? And, how does steady state SOM stocks scale with amount of fresh organic matter (FOM) litter inputs? While formulations (a) did not represent the priming effect, with formulations (b) steady state SOM stocks were independent of amount of litter input. Further, with several formulations (c) there was an offset of SOM that was not decomposed when no fresh OM was supplied. The finding that a part of the SOM is not decomposed on exhaust of FOM supply supports the hypothesis of carbon stabilization in deep soil by the absence of energy-rich fresh organic matter. Different representations of colimitation of decomposition by substrate and decomposers in SOM decomposition models resulted in qualitatively different long-term behaviour. A collaborative effort by modellers and experimentalists is required to identify appropriate and inappropriate formulations.
Does ocean acidification induce an upward flux of marine aggregates?
Authors: Mari X.
Published: April 2010

Abstract. The adsorption of anthropogenic atmospheric carbon dioxide (CO2) by the ocean provokes its acidification. This acidification may alter several oceanic processes, including the export of biogenic carbon from the upper layer of the ocean, hence providing a feedback on rising atmospheric carbon concentrations. The effect of seawater acidification on transparent exopolymeric particles (TEP) driven aggregation and sedimentation processes were investigated by studying the interactions between latex beads and TEP precursors collected in the lagoon of New Caledonia. A suspension of TEP and beads was prepared and the formation of mixed aggregates was monitored as a function of pH under increasing turbulence intensities. The pH was controlled by addition of sulfuric acid. Aggregation and sedimentation processes driven by TEP were drastically reduced when the pH of seawater decreases within the expected limits imposed by increased anthropogenic CO2 emissions. In addition to the diminution of TEP sticking properties, the diminution of seawater pH led to a significant increase of the TEP pool, most likely due to swollen structures. A diminution of seawater pH by 0.2 units or more led to a stop or a reversal of the downward flux of particles. If applicable to oceanic conditions, the sedimentation of marine aggregates may slow down or even stop as the pH decreases, and the vertical flux of organic carbon may reverse. This would enhance both rising atmospheric carbon and ocean acidification.
McGill Wetland Model: evaluation of a peatland carbon simulator developed for global assessments
Authors: St-Hilaire F. , Wu J. , Roulet N. T. , Frolking S. , Lafleur P. M. , Humphreys E. R. , Arora V.
Published: April 2010

Abstract. We developed the McGill Wetland Model (MWM) based on the general structure of the Peatland Carbon Simulator (PCARS) and the Canadian Terrestrial Ecosystem Model. Three major changes were made to PCARS: 1. the light use efficiency model of photosynthesis was replaced with a biogeochemical description of photosynthesis; 2. the description of autotrophic respiration was changed to be consistent with the formulation of photosynthesis; and 3. the cohort, multilayer soil respiration model was changed to a simple one box peat decomposition model divided into an oxic and anoxic zones by an effective water table, and a one-year residence time litter pool. MWM was then evaluated by comparing its output to the estimates of net ecosystem production (NEP), gross primary production (GPP) and ecosystem respiration (ER) from 8 years of continuous measurements at the Mer Bleue peatland, a raised ombrotrophic bog located in southern Ontario, Canada (index of agreement [dimensionless]: NEP=0.80, GPP=0.97, ER=0.97; systematic RMSE [g C m−2 d−1]: NEP=0.12, GPP=0.07, ER=0.14; unsystematic RMSE [g C m−2 d−1]: NEP=0.15, GPP=0.27, ER=0.23). Simulated moss NPP approximates what would be expected for a bog peatland, but shrub NPP appears to be underestimated. Sensitivity analysis revealed that the model output did not change greatly due to variations in water table because of offsetting responses in production and respiration, but that even modest temperature increases could lead to converting the bog from a sink to a source of CO2. General weaknesses and further developments of MWM are discussed.
Importance of dissolved organic nitrogen in the North Atlantic Ocean in sustaining primary production: a 3-D modelling approach
Authors: Charria G. , Dadou I. , Llido J. , Drévillon M. , Garçon V.
Published: April 2010

Abstract. An eddy-permitting coupled ecosystem-circulation model including dissolved organic matter is used to estimate the dissolved organic nitrogen (DON) supply sustaining primary production in the subtropical North Atlantic Ocean. After an analysis of the coupled model performances compared to the data, a sensitivity study demonstrates the strong impact of parameter values linked to the hydrolysis of particulate organic nitrogen and remineralisation of dissolved organic nitrogen on surface biogeochemical concentrations. The physical transport of dissolved organic nitrogen contributes to maintain the level of primary production in this subtropical gyre. It is dominated by the meridional component. We estimate a meridional net input of 0.039 molN.m−2.yr−1 over the domain (13°–35° N and 71–40° W) in the subtropical gyre. This supply is driven by the Ekman transport in the southern part and by non-Ekman transport (meridional current components, eddies, meanders and fronts) in the northern part of the subtropical gyre. At 12° N, our estimate (18 kmolN.s-1) confirms the estimation (17.9 kmolN.s-1) made by Roussenov et al. (2006) using a simplified biogeochemical model in a large scale model. This DON meridional input is within the range (from 0.05 up to 0.24 molN.m−2.yr-1) (McGillicuddy and Robinson, 1997; Oschlies, 2002) of all other possible mechanisms (mesoscale activity, nitrogen fixation, atmospheric deposition) fuelling primary production in the subtropical gyre. The present study confirms that the lateral supply of dissolved organic nitrogen might be important in closing the N budget over the North Atlantic Ocean and quantifies the importance of meridional input of dissolved organic nitrogen.
Short-term temporal variations of heterotrophic bacterial abundance and production in the open NW Mediterranean Sea
Authors: Mével G. , Vernet M. , Ghiglione J. F.
Published: April 2010

Abstract. We present the vertical and temporal dynamics of total vs. particle-attached bacterial abundance and activity over a 5 week period under summer to autumn transition in NW Mediterranean Sea. By comparison to previous investigations in the same area but during different seasons, we found that total bacterial biomass and production values were consistent with the hydrological conditions of the summer-fall transition. At a weekly time scale, total bacterial biomass and production in the euphotic layers was significantly correlated with phytoplanktonic biomass. At an hourly time scale, total bacterial biomass responded very rapidly to chlorophyll-a fluctuations, suggesting a tight coupling between phytoplankton and bacteria for resource partitioning during summer-autumn transition. In contrast, no influence of diel changes on bacterial parameters was detected. Episodic events such as coastal water intrusions had a significant positive effect on total bacterial abundance and production, whereas we could not detect any influence of short wind events whatever the magnitude. Finally, we show that particle-attached bacteria can represent a large proportion (until 49%) of the total bacterial activity in the euphotic layer but display rapid and sporadic changes at hourly time scales. This study underlines the value of large datasets covering different temporal scales to clarify the biogeochemical role of bacteria in the cycling of organic matter in open seawater.
The relevance of particulate organic carbon (POC) for carbon composition in the pore water of drained and rewetted fens of the "Donauried" (South-Germany)
Authors: Fiedler S. , Höll B. S. , Freibauer A. , Stahr K. , Drösler M. , Schloter M. , Jungkunst H. F.
Published: April 2010

Abstract. Numerous studies have dealt with carbon (C) concentrations in Histosols, but there are no studies quantifying the relative importance of all individual C components in pore waters. For this study, measurements were made of all the carbon components (i.e., particulate organic carbon, POC; dissolved organic carbon, DOC; dissolved inorganic carbon, DIC; dissolved methane, CH4) in the soil pore water of a calcareous fen under three different water management regimes (re-wetted, deeply and moderately drained). Pore water was collected weekly or biweekly (April 2004 to April 2006) at depths between 10 and 150 cm. The main results obtained were: (1) DIC (94–280 mg C l−1) was the main C-component. (2) POC and DOC concentrations in the pore water (14–125 mg C l−1 vs. 41–95 mg C l−1) were pari passu. (3) Dissolved CH4 was the smallest C component (0.005–0.9 mg C l−1). Interestingly, about 30% of the POM particles were colonized by microbes indicating that they are active in the internal C transfer in the soil profile ("C-Shuttles"). Consequently, it was concluded that POC is at least as important as DOC for internal soil C turnover. There is no reason to assume significant biochemical differences between POC and DOC as they only differ in size. Therefore, both POC and DOC fractions are essential components of C budgets of peatlands. Furthermore dissolved CO2 in all forms of DIC apparently is an important part of peatland C-balances.
Environmental controls of greenhouse gas release in a restoring peat bog in NW Germany
Authors: Glatzel S. , Forbrich I. , Krüger C. , Lemke S. , Gerold G.
Published: April 2010

Abstract. In Central Europe, most bogs have a history of drainage and many of them are currently being restored. Success of restoration as well as greenhouse gas exchange of these bogs is influenced by environmental stress factors as drought and atmospheric nitrogen deposition. We determined the methane and nitrous oxide exchange of sites in the strongly decomposed center and less decomposed edge of the Pietzmoor bog in NW Germany in 2004. Also, we examined the methane and nitrous oxide exchange of mesocosms from the center and edge before, during, and following a drainage experiment as well as carbon dioxide release from disturbed unfertilized and nitrogen fertilized surface peat. In the field, methane fluxes ranged from 0 to 3.8 mg m−2 h−1 and were highest from hollows. Field nitrous oxide fluxes ranged from 0 to 574 μg m−2 h−1 and were elevated at the edge. A large Eriophorum vaginatum tussock showed decreasing nitrous oxide release as the season progressed. Drainage of mesocosms decreased methane release to 0, even during rewetting. There was a tendency for a decrease of nitrous oxide release during drainage and for an increase in nitrous oxide release during rewetting. Nitrogen fertilization did not increase decomposition of surface peat. Our examinations suggest a competition between vascular vegetation and denitrifiers for excess nitrogen. We also provide evidence that the von Post humification index can be used to explain greenhouse gas release from bogs, if the role of vascular vegetation is also considered. An assessment of the greenhouse gas release from nitrogen saturated restoring bogs needs to take into account elevated release from fresh Sphagnum peat as well as from sedges growing on decomposed peat. Given the high atmospheric nitrogen deposition, restoration will not be able to achieve an oligotrophic ecosystem in the short term.
Short term changes in zooplankton community during the summer-autumn transition in the open NW Mediterranean Sea: species composition, abundance and diversity
Authors: Raybaud V. , Nival P. , Mousseau L. , Gubanova A. , Altukhov D. , Khvorov S. , Ibañez F. , Andersen V.
Published: April 2010

Abstract. Short term changes in zooplankton community were investigated at a fixed station in offshore waters of the Ligurian Sea (Dynaproc 2 cruise, September–October 2004). Mesozooplankton was sampled with vertical WP2 hauls (200 µm mesh-size) and large mesozooplankton, macrozooplankton and micronekton with a BIONESS multinet sampler (500 µm mesh-size). Temporal variations of total biomass, species composition and abundance of major taxa were studied. Intrusions of low salinity water masses were observed two times during the cruise. The first one, which was the most important, was associated with changes in zooplankton community composition. Among copepods, the abundance of Calocalanus, Euchaeta, Heterorhabdus, Mesocalanus, Nannocalanus, Neocalanus, Pleuromamma and also calanoid copepodites increased markedly. Among non-copepod taxa, only small ostracods abundance increased. After this low salinity event, abundance of all taxa nearly returned to their initial values. The influence of salinity on each zooplankton taxon was confirmed by a statistical analysis (Perry's method). Shannon diversity index, Pielou evenness and species richness were used to describe temporal variations of large copepod (>500 µm) diversity. Shannon index and Pielou evenness decreased at the beginning of the low salinity water intrusions, but not species richness. We suggest that low salinity water masses contained its own zooplankton community and passed through the sampling area, thus causing the replacement of zooplankton population.
Importance of long-term monitoring for detecting environmental change: lessons from a lowland river in south east England
Authors: Burt T. P. , Howden N. J. K. , Worrall F. , Whelan M. J.
Published: April 2010

Abstract. Rising nitrate concentrations in rivers and groundwater in regions with intensive agriculture have been of concern for several decades. Long records of nitrate concentration are rare; in the UK, few pre-date 1974. Records from the River Stour from 1937 are analysed using "moving-windows", to explore the effects of window width on trend detection and process inference. Nitrate concentrations rose sharply from the late 1950s in response to widespread ploughing of grassland, but have fallen since the early 1980s, suggesting that widespread nitrogen control measures, which were introduced after the start of the downward trend, may not be the principal driver for recent change. Short windows (5 years) are too noisy; longer windows (10, 15 and 30 years) identify signals (i.e. trends) more reliably. Since system response times can be of the order of 20–30 years, management decisions made on shorter data sets (even up to 15 years) could be misleading.
Chemical sensing of plant stress at the ecosystem scale
Authors: Karl T. , Guenther A. , Turnipseed A. , Patton E. G. , Jardine K.
Published: April 2010

Abstract. Significant ecosystem-scale emissions of methylsalicylate (MeSA), a semivolatile plant hormone thought to act as the mobile signal for systemic acquired resistance (SAR) (Park et al., 2006), were observed in an agroforest. Our measurements show that plant internal defence mechanisms can be activated in response to temperature stress and are modulated by water availability on large scales. Highest MeSA fluxes (up to 0.25 mg/m2/h) were observed after plants experienced ambient night-time temperatures of ~7.5°C followed by a large daytime temperature increase (e.g. up to 22°C). Under these conditions estimated night-time leaf temperatures were as low as ~4.6°C, likely inducing a response to prevent chilling injury (Ding et al., 2002). Our observations imply that plant hormones can be a significant component of ecosystem scale volatile organic compound (VOC) fluxes (e.g. as high as the total monoterpene (MT) flux) and therefore contribute to the missing VOC budget (de Carlo et al., 2004; Goldstein and Galbally, 2007). If generalized to other ecosystems and different types of stresses these findings suggest that semivolatile plant hormones have been overlooked by investigations of the impact of biogenic VOCs on aerosol formation events in forested regions (Kulmala et al., 2001; Boy et al., 2000). Our observations show that the presence of MeSA in canopy air serves as an early chemical warning signal indicating ecosystem-scale stresses before visible damage becomes apparent. As a chemical metric, ecosystem emission measurements of MeSA in ambient air could therefore support field studies investigating factors that adversely affect plant growth.
Effect of UV radiation and temperature on the emission of methane from plant biomass and structural components
Authors: Vigano I. , van Weelden H. , Holzinger R. , Keppler F. , Röckmann T.
Published: April 2010

Abstract. The recently reported finding that plant matter and living plants produce significant amounts of the important greenhouse gas methane under aerobic conditions has led to an intense scientific and public controversy. Whereas some studies question the up-scaling method that was used to estimate the global source strength, others have suggested that experimental artifacts could have caused the reported signals, and in fact one study has recently reported the absence of CH4 emissions from plants based on an isotope labeling experiment. Here we show – using several independent experimental analysis techniques – that dry and detached fresh plant matter, as well as several structural plant components, emit significant amounts of methane upon irradiation with UV light and/or heating. Emissions from UV irradiation are almost instantaneous, indicating a direct photochemical process. Long-time irradiation experiments demonstrate that the size of the CH4 producing reservoir is large, exceeding potential interferences from degassing or desorption processes by several orders of magnitude. A dry leaf of a pure 13C plant produces 13CH4 at a similar rate as dry leaves of non-labeled plants produce non-labeled methane.
The response of the terrestrial biosphere to urbanization: land cover conversion, climate, and urban pollution
Authors: Trusilova K. , Churkina G.
Published: April 2010

Abstract. Although urban areas occupy a relatively small fraction of land, they produce major disturbances of the carbon cycle through land use change, climate modification, and atmospheric pollution. In this study we quantify effects of urban areas on the carbon cycle in Europe. Among urbanization-driven environmental changes, which influence carbon sequestration in the terrestrial biosphere, we account for: 1) proportion of land covered by impervious materials, 2) local urban meteorological conditions, 3) urban CO2-dome, and 4) elevated atmospheric nitrogen deposition. We use the terrestrial ecosystem model BIOME-BGC to estimate fluxes of carbon exchange between the biosphere and the atmosphere in response to these urban factors. We analysed these four urbanization-driven changes individually, setting up our model in such a way that only one of the four was active at a time. From these model simulations we found that fertilization effects from the CO2-dome and the atmospheric nitrogen deposition made the strongest positive contributions to the carbon uptake (0.023 Pg/year and 0.039 Pg/year, respectively), whereas, the impervious urban land and local urban meteorological conditions resulted in a reduction of carbon uptake (–0.006 Pg/year and –0.007 Pg/year, respectively). The synergetic effect of the four urbanization-induced changes was an increase of the carbon sequestration in Europe of 0.056 Pg/year.
Sub meso scale phytoplankton distribution in the north east Atlantic surface waters determined with an automated flow cytometer
Authors: Thyssen M. , Garcia N. , Denis M.
Published: April 2010

Abstract. Phytoplankton cells in the size range ~1–50 μm were analysed in surface waters using an automated flow cytometer, the Cytosub (http://www.cytobuoy.com), from the Azores to the French Brittany during spring 2007. The Cytosub records the pulse shape of the optical signals generated by phytoplankton cells when intercepted by the laser beam. A total of 6 distinct optical groups were resolved during the whole transect, and the high frequency sampling (15 min) provided evidence for the cellular cycle (based on cyclic changes in cell size and fluorescence) and distribution changes linked to the different water characteristics crossed in the north east Atlantic provinces. Nutrient concentrations and mixed layer depth varied from west to east, with a decrease in the mixed layer depth and high nutrient concentrations in the middle of the transect as well as near the French coast. Data provided a link between the sub meso scale processes and phytoplankton patchiness, some abundance variations due to the cellular cycle can be pointed out. The high frequency spatial sampling encompasses temporal variations of the phytoplankton abundance, offering a better insight into phytoplankton distribution.
Sinking rates of particles in biogenic silica- and carbonate-dominated production systems of the Atlantic Ocean: implications for the organic carbon fluxes to the deep ocean
Authors: Fischer G. , Karakas G.
Published: April 2010

Abstract. The flux of materials to the deep sea is dominated by larger, organic-rich particles with sinking rates varying between a few meters and several hundred meters per day. Mineral ballast may regulate the transfer of organic matter and other components by determining the sinking rates, e.g. via particle density. We calculated particle sinking rates from flux patterns and alkenone measurements applying sediment trap experiments from the Atlantic Ocean. We obtained higher particle sinking rates in carbonate-dominated production systems, both regionally and seasonally. During a summer coccolithophorid bloom in the Cape Blanc coastal upwelling off Mauritania, sinking rates reached almost 570 m per day, most probably due the fast sedimentation via zooplankton fecal pellets. During the recurring winter-spring blooms off NW Africa and in opal-rich production systems of the Southern Ocean, sinking rates of larger particles, most probably diatom aggregates, appeared to be lower. Although a tight and overall coupling between dust supply and particle sinking rates was not observed, it remains noticeable that the highest sinking rates occurred mostly in the dust-rich ocean regions off NW Africa. We obtained increasing sinking rates with depth. By using a seven-compartment biogeochemical model, it was shown that deep ocean organic carbon fluxes at a mesotrophic sediment trap site off Cape Blanc can be captured fairly well using seasonal variable particle sinking rates. Our model provides a total organic carbon flux of 0.29 Tg per year down to 3000 m off the NW African upwelling region between 5 and 35° N.
Contribution of different grass species to plant-atmosphere ammonia exchange in intensively managed grassland
Authors: Mattsson M. , Herrmann B. , Jones S. , Neftel A. , Sutton M. A. , Schjoerring J. K.
Published: April 2010

Abstract. Species diversity in grasslands usually declines with increasing input of nitrogen from fertilizers or atmospheric nitrogen deposition. Conversely, species diversity may also impact the build-up of soil nitrogen pools. Limited information is available on how plant-atmosphere ammonia exchange is related to species diversity in grasslands. We have here investigated grass species abundance and different foliar nitrogen pools in 4-year-old intensively managed grassland. Apoplastic pH and NH4+ concentrations of the 8 most abundant species were used to calculate stomatal NH3 compensation points. Apoplastic NH4+ concentrations differed considerably among the species, ranging from 13 to 117 μM, with highest values in Festuca pratensis. Also apoplastic pH values varied, from pH 6.0 in Phleum pratense to 6.9 in Dactylis glomerata. The observed differences in apoplastic NH4+ and pH resulted in a large span of predicted values for the stomatal NH3 compensation point which ranged from 0.20 to 6.57 nmol mol−1. Three species (Lolium perenne, Festuca pratensis and Dactylis glomerata) had sufficiently high NH3 compensation points and abundance to contribute to the NH3 emission of the whole field. At the same time, other grass species such as Phleum pratense and Lolium multiflorum had NH3 compensation points below the atmospheric NH3 concentration and could thus contribute to NH3 uptake from the atmosphere. Evaluated across species, leaf bulk-tissue NH4+ concentrations correlated well (r2=0.902) with stomatal NH3 compensation points calculated on the basis of the apoplastic bioassay. This suggests that leaf tissue NH4+ concentrations combined with data for the frequency distribution of the corresponding species can be used for predicting the NH3 exchange potential of a mixed grass sward.
Methanotrophy potential versus methane supply by pore water diffusion in peatlands
Authors: Hornibrook E. R. C. , Bowes H. L. , Culbert A. , Gallego-Sala A. V.
Published: April 2010

Abstract. Low affinity methanotrophic bacteria consume a significant quantity of methane in wetland soils in the vicinity of plant roots and at the oxic-anoxic interface. Estimates of the efficiency of methanotrophy in peat soils vary widely in part because of differences in approaches employed to quantify methane cycling. High resolution profiles of dissolved methane abundance measured during the summer of 2003 were used to quantify rates of upward methane flux in four peatlands situated in Wales, UK. Aerobic incubations of peat from a minerotrophic and an ombrogenous mire were used to determine depth distributions of kinetic parameters associated with methane oxidation. The capacity for methanotrophy in a 3 cm thick zone immediately beneath the depth of nil methane abundance in pore water was significantly greater than the rate of upward diffusion of methane in all four peatlands. Rates of methane diffusion in pore water at the minerotrophic peatlands were small (<10%) compared to surface emissions during June to August. The proportions were notably greater in the ombrogenous bogs because of their typically low methane emission rates. Methanotrophy appears to consume entirely methane transported by pore water diffusion in the four peatlands with the exception of 4 of the 33 gas profiles sampled. Flux rates to the atmosphere regardless are high because of gas transport through vascular flora, in particular, at the minerotrophic sites. Cumulative rainfall amount 3-days prior to sampling correlated well with the distance between the water table level and the depth of 0 μmol l−1 methane, indicating that precipitation events can impact methane distributions in pore water. Further work is needed to characterise the kinetics of methane oxidation spatially and temporally in different wetland types in order to determine generalized relationships for methanotrophy in peatlands that can be incorporated into process-based models of methane cycling in peat soils.
Plant physiological and environmental controls over the exchange of acetaldehyde between forest canopies and the atmosphere
Authors: Jardine K. , Harley P. , Karl T. , Guenther A. , Lerdau M. , Mak J. E.
Published: April 2010

Abstract. We quantified fine scale sources and sinks of gas phase acetaldehyde in two forested ecosystems in the US. During the daytime, the upper canopy behaved as a net source while at lower heights, reduced emission rates or net uptake were observed. At night, uptake generally predominated thoughout the canopies. Net ecosystem emission rates were inversely related to foliar density which influenced the extinction of light and the acetaldehyde compensation point in the canopy. This is supported by branch level studies revealing much higher compensation points in the light than in the dark for poplar (Populus deltoides) and holly oak (Quercus ilex) implying a higher light/temperature sensitivity for acetaldehyde production relative to consumption. The view of stomata as the major pathway for acetaldehyde exchange is supported by strong linear correlations between branch transpiration rates and acetaldehyde exchange velocities for both species. In addition, natural abundance carbon isotope analysis of gas-phase acetaldehyde during poplar branch fumigation experiments revealed a significant kinetic isotope effect of 5.1±0.3‰, associated with the uptake of acetaldehyde. Similar experiments with dry dead poplar leaves showed no fractionation or uptake of acetaldehyde, confirming that this is only a property of living leaves. We suggest that acetaldehyde belongs to a potentially large list of plant metabolites where stomatal conductance can exert long term control over both emission and uptake rates due to the presence of both source(s) and sink(s) within the leaf which strongly buffer large changes in concentrations in the substomatal airspace due to changes in stomatal conductance. We conclude that the exchange of acetaldehyde between plant canopies and the atmosphere is fundamentally controlled by ambient acetaldehyde concentrations, stomatal conductance, and the acetaldehyde compensation point.
CO2 enrichment increases nutrient leaching from model forest ecosystems in subtropical China
Authors: Liu J. X. , Zhang D. Q. , Zhou G. Y. , Faivre-Vuillin B. , Deng Q. , Wang C. L.
Published: April 2010

Abstract. The effect of high atmospheric CO2 concentrations on the dynamics of mineral nutrient is not well documented, especially for subtropical China. We used model forest ecosystems in open-top chambers to study the effects of CO2 enrichment alone and together with N addition on the dynamics of soil cations and anions. Two years of exposure to a 700 ppm CO2 atmospheric concentration resulted in increased annual nutrient losses by leaching below 70 cm soil profile. Compared to the control, net Mg2+ losses increased by 385%, K+ by 223%, Ca2+ by 167% and N-NO3- by 108%, respectively. Increased losses following exposure to elevated CO2 were related to both faster soil weathering/organic matter decomposition and greater amounts of water leaching during high rainfall as a consequence of higher soil moisture. Net annual nutrient losses in the high CO2 concentration chambers reached 22.2 kg ha−1 year−1 for K+, 171.3 kg ha−1 year−1 for Ca2+, 8.2 kg ha−1 year−1 for Mg2+ and about 2 kg ha−1 year−1 for N-NO3-. The N addition alone had no significant effect on the mineral nutrient leaching losses. However, addition of N together with the high CO2 treatment significantly reduced mineral nutrient losses. We hypothesize that forests in subtropical China might suffer nutrient limitation and reduction in plant biomass under elevated CO2 concentration due to mineral leaching losses in the future.
Assessing seasonality of boreal coniferous forest CO2 exchange by estimating biochemical model parameters from micrometeorological flux observations
Authors: Thum T. , Aalto T. , Laurila T. , Aurela M. , Lindroth A. , Vesala T.
Published: April 2010

Abstract. The biochemical seasonality of the northern boreal coniferous forests was investigated by means of inversion modelling using eddy covariance data. Eddy covariance data was used to optimize the biochemical model parameters. Our study sites consisted of three Scots pine (l. Pinus sylvestris) forests and one Norway spruce (l. Picea abies) forest that were located in Finland and Sweden. We obtained temperature and seasonal dependence for the biochemical model parameters: the maximum rate of carboxylation (Vc(max) and the maximum rate of electron transport (Jmax). Both of the parameters were optimized without assumptions about their mutual magnitude. The values obtained for the biochemical model parameters were similar at all the sites during summer time. To describe seasonality, different temperature fits were made for the spring, summer and autumn periods. During summer, average Jmax across the sites was 54.0 μmol m−2 s−1 (variance 31.2 μmol m−2 s-1) and Vc(max) was 12.0 μmol m−2 s−1 (variance 6.6 μmol m−2 s-1) at 17°C. The sensitivity of the model to LAI was also studied. Simulation runs were done to study the effect of the seasonality implemented in the model using different temperature fits. The impact of seasonality on annual GPP was 15%, which corresponded to an increase of 2°C in air temperature.
Temporal variability in bioassays of ammonia exchange potential in relation to plant and soil nitrogen parameters in intensively managed grassland
Authors: Mattsson M. , Herrmann B. , David M. , Loubet B. , Riedo M. , Theobald M. R. , Sutton M. A. , Bruhn D. , Neftel A. , Schjoerring J. K.
Published: April 2010

Abstract. The exchange of ammonia between crop canopies and the atmosphere depends on a range of plant parameters and climatic conditions but little is known about effects of management factors. We have here investigated the ammonia exchange potential of a grass sward dominated by Lolium perenne in response to cutting and fertilization. Tall grass showed a low potential for NH3 emission before cutting. During re-growth after cutting, leaf tissue concentrations of NO3−, NH4+, soluble N and total N increased along with apoplastic NH4+ concentrations. In contrast, apoplastic pH decreased resulting in largely unaltered NH3 emission potential. A high potential for NH3 emission was shown by the plant litter. Fertilization with 100 kg N ha−1 one week after cutting caused the apoplastic NH4+ concentration of the newly emerging leaves to increase dramatically. The apoplastic NH4+ concentration peaked the day after the fertiliser was applied and thereafter decreased over the following 10 days until reaching the same level as before fertilisation. A positive correlation was found between NH4+ concentrations in leaf apoplast, bulk tissue and litter throughout the experimental period. Leaf soluble N was negatively correlated with apoplastic NH4+ concentration whereas total N was weakly correlated with NH4+ concentrations in leaf tissue and soil.
Soil biogenic emissions of nitric oxide from a semi-arid savanna in South Africa
Authors: Feig G. T. , Mamtimin B. , Meixner F. X.
Published: April 2010

Abstract. Soils of arid and semi-arid ecosystems are important biogenic sources of atmospheric nitric oxide (NO), however, there is still a shortage of measurements from these systems. Here we present the results of a laboratory study of the biogenic emission of NO from four different landscape positions of the Kruger National Park (KNP), a large conservation area in a semi-arid region of South Africa. Results show that the highest net potential NO fluxes come from the low lying (footslope) landscape regions, which have the largest nitrogen stocks and highest rates of nitrogen input into the soil. Net potential NO fluxes from midslope and crest regions were considerably lower. The maximum release of NO occurred at fairly low soil moisture contents of 10%–20% water filled pore space. Using soil moisture and temperature data obtained in situ at the Kruger National Park flux tower site, net potential NO fluxes obtained in the laboratory were converted to field fluxes for each of the four landscape positions for the period 2003 to 2005. The highest field NO flux is from footslope positions, during each of these years and emissions ranged from 1.5–8.5 kg ha−1 yr−1 (in terms of mass of nitrogen). Remote sensing and Geographic Information Systems techniques were used to up-scale field NO fluxes on a regional basis indicating that the highest emissions occurred from the midslope positions, due to their large geographical extent in the considered research area. Emissions for the KNP Skukuza land type (56 000 ha) ranged from 20×103 kg in 2004 to 34×103 kg in 2003. The importance of landscape characteristics in the determination of regional biogenic NO soil emissions is emphasized.
Anthropogenic and biophysical contributions to increasing atmospheric CO2 growth rate and airborne fraction
Authors: Raupach M. R. , Canadell J. G. , Le Quéré C.
Published: April 2010

Abstract. We quantify the relative roles of natural and anthropogenic influences on the growth rate of atmospheric CO2 and the CO2 airborne fraction, considering both interdecadal trends and interannual variability. A combined ENSO-Volcanic Index (EVI) relates most (~75%) of the interannual variability in CO2 growth rate to the El-Niño-Southern-Oscillation (ENSO) climate mode and volcanic activity. Analysis of several CO2 data sets with removal of the EVI-correlated component confirms a previous finding of a detectable increasing trend in CO2 airborne fraction (defined using total anthropogenic emissions including fossil fuels and land use change) over the period 1959–2006, at a proportional growth rate 0.24% y−1 with probability ~0.9 of a positive trend. This implies that the atmospheric CO2 growth rate increased slightly faster than total anthropogenic CO2 emissions. An extended form of the Kaya identity relates the increase in the CO2 growth rate (1.9% y−1 over 1959–2006) to the growth rates of four global driving factors: population (contributing +1.7% y−1); per capita income (+1.8% y−1); the total carbon intensity of the global economy (−1.7% y−1); and airborne fraction (averaging +0.2% y−1 with strong interannual variability). Together, the recent (post-2000) increase in growth of per capita income and decline in the negative growth (improvement) in the carbon intensity of the economy will drive a significant acceleration in the CO2 growth rate over coming decades, unless these recent trends reverse. To achieve an annual reduction rate in total emissions of −2% y−1 (which would halve emissions in 35 years) in the presence of a per-capita income growth rate of 2% y−1 and a population growth rate of 1% y−1, it is necessary to achieve a decline in total carbon intensity of the economy at a rate of around −5% y−1, three times the 1959–2006 average.
CO2 budgeting at the regional scale using a Lagrangian experimental strategy and meso-scale modeling
Authors: Sarrat C. , Noilhan J. , Lacarrère P. , Masson V. , Ceschia E. , Ciais P. , Dolman A. , Elbers J. , Gerbig C. , Jarosz N.
Published: April 2010

Abstract. An atmospheric Lagrangian experiment for regional CO2 budgeting with aircraft measurements took place during the CarboEurope Regional Experiment campaign (CERES) in south-western France, in June, 2005. The atmospheric CO2 aircraft measurements taken upstream and downstream of an active and homogeneous pine forest revealed a CO2 depletion in the same air mass. This field experiment is analyzed with a meteorological meso-scale model interactively coupled with a surface scheme, allowing plant assimilation, ecosystem respiration, CO2 anthropogenic emissions and sea fluxes. First, the model is carefully validated against observations close to the surface and in the atmospheric boundary layer. Then, the carbon budget is evaluated using the numerous CERES observations and as well as the modeling results in order to estimate the relative contribution of each physical process. A good agreement is shown in terms of the estimation of the regional CO2 surface flux by the Eulerian meso-scale model budget and by the observations at the surface flux sites. A Lagrangian estimation of regional CO2 surface flux from aircraft observations is more difficult due to several sources of uncertainty. In our case, probable errors are due to the determination of CO2 vertical profile measurements and owing to the difficulties in monitoring the meteorological condition evolution during several hours.
On the trophic regimes of the Mediterranean Sea: a satellite analysis
Authors: D'Ortenzio F. , Ribera d'Alcalà M.
Published: April 2010

Abstract. The ten years of the SeaWiFS satellite surface chlorophyll concentration observations, presently available, were used to characterize the biogeography of the Mediterranean Sea and the seasonal cycle of the surface biomass in different areas of the basin. The K-means cluster analysis was applied on the satellite time-series of chlorophyll concentration. The resulting coherent patterns were then explained on the basis of the present knowledge of the basin functioning. Winter biomass enhancements were shown to occur in most of the basin and last for 2–3 months depending on the region. Classical spring bloom regimes were also observed, regularly in the North Western Mediterranean, and intermittently in four others specific areas. The analysis confirmed that the Mediterranean Sea is an ideal area to evaluate the impacts of the external physical forcing on the marine ecosystem functioning.