http://www.biogeosciences-discuss.net/ Biogeosciences Discussions Latest Articlesenhttp://www.biogeosciences-discuss.net/9/5793/2012/<b>Stratigraphy and paleoenvironments of the early to middle Holocene Chipalamawamba Beds (Malawi Basin, Africa)</b><br /><br />Biogeosciences Discussions, 9, 5793-5822, 2012<br /><br />Author(s): B. Van Bocxlaer, W. Salenbien, N. Praet, and J. Verniers<br /><br />We describe the Chipalamawamba Beds, early to middle Holocene deposits at the southern margin of long-lived Lake Malawi. The beds are exposed because of downcutting of the upper Shire River. The Chipalamawamba sediments are medium to coarse, yellow to brown sands deposited in lenses varying in horizontal extent from a few meters to several hundreds of meters. Four units are recognized; the first three mainly contain lacustrine sediments deposited during lake high-stands about 10.6–9.7 cal ka BP (Unit 1), 7.6–6.5 cal ka BP (Unit 2) and 5.9–5.3 cal ka BP. Sediments of Unit 4 top units 1 to 3, are coarser and display regular foresets and oblique-bedding, suggesting deposition in riverine environments after installation of the Shire River (~5.0 ka BP). Freshwater mollusk assemblages and bioturbations regularly occur in the lacustrine sediments, but are largely absent from Unit 4. Diverse and often contradicting hypotheses on the lake levels of Lake Malawi have been proposed for the early and middle Holocene. The Chipalamawamba Beds allow straight-forward recognition of water levels and provide strong evidence for oscillating lake levels during this period, rather than continuous high or low levels. Sedimentation rates have been high and individual shell beds have typically been deposited during a few decades. Because the Chipalamawamba Beds contain a sequence of mollusk assemblages with intervals between subsequent shell beds ranging from a century to a few millennia, they enable paleontological analysis of the fauna with unusually high temporal resolution. That some mollusk lineages inhabiting Lake Malawi are in the early stages of diversification and radiation increases the paleobiological relevance of these beds.Mon, 21 May 2012 00:00:00 +0200http://www.biogeosciences-discuss.net/9/5755/2012/<b>Production, partitioning and stoichiometry of organic matter under variable nutrient supply during mesocosm experiments in the tropical Pacific and Atlantic Ocean</b><br /><br />Biogeosciences Discussions, 9, 5755-5791, 2012<br /><br />Author(s): J. M. S. Franz, H. Hauss, U. Sommer, T. Dittmar, and U. Riebesell<br /><br />Oxygen-deficient waters in the ocean, generally referred to as oxygen minimum zones (OMZ), are expected to expand as a consequence of global climate change. Poor oxygenation is promoting microbial loss of inorganic nitrogen (N) and increasing release of sediment-bound phosphate (P) into the water column. These intermediate water masses, nutrient-loaded but with an N deficit relative to the canonical N:P Redfield ratio of 16:1, are transported via coastal upwelling into the euphotic zone. To test the impact of nutrient supply and nutrient stoichiometry on production, partitioning and elemental composition of phytoplankton-derived dissolved (DOC, DON, DOP) and particulate (POC, PON, POP) organic matter, three nutrient enrichment experiments were conducted with natural phytoplankton communities in shipboard mesocosms, during research cruises in the tropical waters of the South East Pacific and the North East Atlantic. Maximum accumulation of POC and PON was observed under high N supply conditions, indicating that primary production was controlled by N availability. The stoichiometry of photoautotrophic biomass was unaffected by nutrient N:P supply during exponential growth under nutrient saturation, while it was highly variable under conditions of nutrient limitation and closely correlated to the N:P supply ratio, although PON:POP of accumulated phytoplankton generally exceeded the supply ratio. Phytoplankton N:P composition was constrained by a general lower limit of 5:1. Channelling of assimilated P into DOP appears to be the mechanism responsible for the consistent offset of cellular stoichiometry relative to inorganic nutrient supply and nutrient drawdown, as DOP build-up was observed to intensify under decreasing N:P supply. Low nutrient N:P conditions in coastal upwelling areas overlying O₂-deficient waters seem to represent a net source for DOP, which may stimulate growth of diazotrophic phytoplankton. These results demonstrate that microalgal nutrient assimilation and partitioning of phytoplankton-derived organic matter between the particulate and the dissolved phase are controlled by the N:P ratio of upwelled nutrients, implying substantial consequences for nutrient cycling and organic matter pools in the course of decreasing nutrient N:P stoichiometry.Tue, 15 May 2012 00:00:00 +0200http://www.biogeosciences-discuss.net/9/5719/2012/<b>Spatial patterns of some trace elements in four Swedish stream networks</b><br /><br />Biogeosciences Discussions, 9, 5719-5753, 2012<br /><br />Author(s): J. Temnerud, A. Düker, S. Karlsson, B. Allard, K. Bishop, J. Fölster, and S. Köhler<br /><br />Four river basins in Southern Sweden, catchment size 0.3 to maximum 127 km² (median 1.9), were sampled in October 2007. The 243 samples were analysed for 26 trace elements (Ag, As, Au, Ba, Be, Bi, Cd, Co, Cr, Cu, Ga, Ge, In, La, Li, Mo, Ni, Pb, Sb, Se, Sn, Tl, Ti, U, V and Zn) to identify spatial patterns within drainage networks. The sampling design made it possible to compare the difference between 40 stream reaches, 53 stream junctions with 107 tributaries vs. downstream reaches and 36 lakes with 77 inlets vs. outlets comparisons. The largest concentration differences (at reaches, junctions and lakes) were observed for lakes, with outlets usually having lower concentration compared to the inlets for As, Ba, Be, Bi, Cd, Co, Cr, Ga, Ge, Ni, Pb, Sn, Ti, Tl, U, V and Zn. Significantly lower concentrations were observed for Co and Cd when comparing headwaters with downstream sites in each catchment. We found no evidence for elevated concentration of heavy metals Cd, Cu, Zn or Pb in the two acidified areas of South-west Sweden. Common factor analysis revealed that As, Bi, Cr, Ga, Ge, Tl, V co-vary positively with Al, Fe and total organic carbon (TOC) and negatively with La, Li and pH. A better understanding of the quantitative removal of organic carbon and iron will aid in understanding metal fluxes from landscapes rich in organic matter and iron.Tue, 15 May 2012 00:00:00 +0200http://www.biogeosciences-discuss.net/9/5695/2012/<b>Alaskan soil carbon stocks: spatial variability and dependence on environmental factors</b><br /><br />Biogeosciences Discussions, 9, 5695-5718, 2012<br /><br />Author(s): U. Mishra and W. J. Riley<br /><br />The direction and magnitude of soil organic carbon (SOC) changes in response to climate change depend on the spatial and vertical distributions of SOC. We estimated spatially-resolved SOC stocks from surface to C horizon, distinguishing active-layer and permafrost-layer stocks, based on geospatial analysis of 472 soil profiles and spatially referenced environmental variables for Alaska. Total Alaska state-wide SOC stock was estimated to be 77 Pg, with 61% in the active-layer, 27% in permafrost, and 12% in non-permafrost soils. Prediction accuracy was highest for the active-layer as demonstrated by highest ratio of performance to deviation (1.5). Large spatial variability was predicted, with whole-profile, active-layer, and permafrost-layer stocks ranging from 1–296 kg C m⁻², 2–166 kg m⁻², and 0–232 kg m⁻², respectively. Temperature and soil wetness were found to be primary controllers of whole-profile, active-layer, and permafrost-layer SOC stocks. Secondary controllers, in order of importance, were: land cover type, topographic attributes, and bedrock geology. The observed importance of soil wetness rather than precipitation on SOC stocks implies that the poor representation of high-latitude soil wetness in Earth System Models may lead to large uncertainty in predicted SOC stocks under future climate change scenarios. Under strict caveats described in the text and assuming temperature changes from the A1B Intergovernmental Panel on Climate Change emissions scenario, our geospatial model indicates that the equilibrium average 2100 Alaska active-layer depth could deepen by 11 cm, resulting in a thawing of 13 Pg C currently in permafrost. The equilibrium SOC loss associated with this warming would be highest under continuous permafrost (31%), followed by discontinuous (28%), isolated (24.3%), and sporadic (23.6%) permafrost areas. Our high resolution mapping of soil carbon stock reveals the potential vulnerability of high-latitude soil carbon and can be used as a basis for future studies of anthropogenic and climatic perturbations.Mon, 14 May 2012 00:00:00 +0200http://www.biogeosciences-discuss.net/9/5659/2012/<b>Nitrate leaching from short-hydroperiod floodplain soils</b><br /><br />Biogeosciences Discussions, 9, 5659-5694, 2012<br /><br />Author(s): B. Huber, J. Luster, S. M. Bernasconi, J. Shrestha, and E. Graf Pannatier<br /><br />Numerous studies have shown the importance of riparian zones to reduce nitrate (NO₃^&ndash;) contamination coming from adjacent agricultural land. Much less is known about nitrogen (N) transformations and nitrate fluxes in riparian soils with short hydroperiods (1–3 days of inundation) and there is no study that could show whether these soils are a N sink or source. <br></br> Within a restored section of the Thur River in NE Switzerland, we measured nitrate concentrations in soil solutions as an indicator of the net nitrate production. Samples were collected along a quasi-successional gradient from frequently inundated gravel bars to an alluvial forest, at three different depths (10, 50 and 100 cm) over a one-year period. Along this gradient we quantified N input (atmospheric deposition and sedimentation) and N output (leaching) to create a nitrogen balance and assess the risk of nitrate leaching from the unsaturated soil to the groundwater. <br></br> Overall, the main factor explaining the differences in nitrate concentrations was the variability in soil texture and volumetric water content (VWC) at field capacity (FC). In subsoils with high VWC at FC and VWC near 100 % FC, high nitrate concentrations were observed, often exceeding the Swiss and EU groundwater quality criterions of 400 and 800 μmol l⁻¹, respectively. High sedimentation rates of river-derived nitrogen led to apparent N retention up to 200 kg N ha⁻¹ yr⁻¹ in the frequently inundated zones. By contrast, in the mature alluvial forest, nitrate leaching exceeded total N input most of the time. As a result of the large soil N pools, high amounts of nitrate were produced by nitrification and up to 94 kg N-NO₃^&ndash; ha⁻¹ yr⁻¹ were leached into the groundwater. Thus, during flooding when water fluxes are high, nitrate from soils can contribute up to 11 % to the total nitrate load in groundwater.Mon, 14 May 2012 00:00:00 +0200http://www.biogeosciences-discuss.net/9/5625/2012/<b>Simulating the effects of phosphorus limitation in the Mississippi and Atchafalaya River plumes</b><br /><br />Biogeosciences Discussions, 9, 5625-5657, 2012<br /><br />Author(s): A. Laurent, K. Fennel, J. Hu, and R. Hetland<br /><br />The continental shelf of the northern Gulf of Mexico receives high dissolved inorganic nitrogen and phosphorus loads from the Mississippi and Atchafalaya rivers. The nutrient load results in high primary production in the river plumes and contributes to the development of hypoxia on the Texas-Louisiana shelf in summer. While phytoplankton growth is considered to be typically nitrogen-limited, phosphorus limitation has been observed in this region during periods of peak river discharge in spring and early summer. Here we investigate the presence, spatio-temporal distribution and implications of phosphorus limitation in the plume region using a circulation model of the northern Gulf of Mexico coupled to a multi-nutrient ecosystem model. Results from a 7 yr simulation (2001–2007) compare well with available observations and suggest that phosphorus limitation develops every year between the Mississippi and Atchafalaya deltas. Model simulations show that phosphorus limitation results in a delay and westward shift of a fraction of river-stimulated primary production. The consequence is a reduced flux of particulate organic matter to the sediment near the Mississippi delta, but enhanced fluxes westward in the Atchafalaya and far-field regions. Two discharge scenarios with altered river phosphate concentrations (&pm;50 %) reveal a significant variation (&pm;40 % in July) in the spatial extent of phosphorus limitation with changes in phosphate load.Mon, 14 May 2012 00:00:00 +0200http://www.biogeosciences-discuss.net/9/5579/2012/<b>Modelling contrasting responses of wetland productivity to changes in water table depth</b><br /><br />Biogeosciences Discussions, 9, 5579-5623, 2012<br /><br />Author(s): R. F. Grant, A. R. Desai, and B. N. Sulman<br /><br />Responses of wetland productivity to changes in water table depth (WTD) are controlled by complex interactions among several soil and plant processes, and hence are site-specific rather than general in nature. Hydrological controls on wetland productivity were studied by representing these interactions in connected hummock and hollow sites in the ecosystem model <i>ecosys</i>, and by testing CO₂ and energy fluxes from the model with those measured by eddy covariance (EC) during years with contrasting WTD in a shrub fen at Lost Creek, WI. Modelled interactions among coupled processes for O₂ transfer, O₂ uptake, C oxidation, N mineralization, N uptake and C fixation by diverse microbial, root, mycorrhizal and shoot populations enabled the model to simulate complex responses of CO₂ exchange to changes in WTD that depended on the WTD at which change was occurring. At the site scale, greater WTD caused the model to simulate greater CO₂ influxes and effluxes over hummocks vs. hollows, as has been found at field sites. At the landscape scale, greater WTD caused the model to simulate greater diurnal CO₂ influxes and effluxes under cooler weather when water tables were shallow, but also smaller diurnal CO₂ influxes and effluxes under warmer weather when water tables were deeper, as was also apparent in the EC flux measurements. At an annual time scale, these diurnal responses to WTD in the model caused lower net primary productivity (NPP) and heterotrophic respiration (<i>R</i>_h), but higher net ecosystem productivity (NEP = NPP &ndash; <i>R</i>_h), to be simulated in a cooler year with a shallower water table than in a warmer year with a deeper one. This difference in NEP was consistent with those estimated from gap-filled EC fluxes in years with different water tables at Lost Creek and at similar boreal fens elsewhere. In sensitivity test of the model, annual NEP declined with increasing WTD in a year with a shallow water table, but rose in a year with a deeper one. The model thus provided an integrated set of hypotheses for explaining site-specific and sometimes contrasting responses of wetland productivity to changes in WTD as found in different field experiments.Mon, 14 May 2012 00:00:00 +0200http://www.biogeosciences-discuss.net/9/5551/2012/<b>Separating agricultural and non-agricultural fire seasonality at regional scales</b><br /><br />Biogeosciences Discussions, 9, 5551-5577, 2012<br /><br />Author(s): B. I. Magi, S. Rabin, E. Shevliakova, and S. Pacala<br /><br />The timing and length of burning seasons in different parts of the world depend on climate, land cover characteristics, and human activities. In this study, global fire data from satellite-based instruments are used in conjunction with global gridded distributions of agricultural land cover types (defined as the sum of cropland and pasture area) to separate the seasonality of agricultural burning practices from that of non-agricultural fire. The results presented in this study show that agricultural and non-agricultural land experience broadly different fire seasonality patterns that are not always linked to climate conditions. We highlight these differences on a regional basis, examining variations in both agricultural land cover and associated cultural practices to help explain our results. While we discuss two land cover categories, the methods can be generalized to derive seasonality for any number of land uses or cover types. This will be useful as global fire models evolve to be fully interactive with land use and land cover change in the next generation of Earth system models.Fri, 11 May 2012 00:00:00 +0200http://www.biogeosciences-discuss.net/9/5509/2012/<b>Controlling factors of the OMZ in the Arabian Sea</b><br /><br />Biogeosciences Discussions, 9, 5509-5550, 2012<br /><br />Author(s): L. Resplandy, M. Lévy, L. Bopp, V. Echevin, S. Pous, V. V. S. S. Sarma, and D. Kumar<br /><br />In-situ observations indicate that the Arabian Sea oxygen minimum zone (OMZ) is only weakly influenced by the strong seasonal cycle of ocean dynamic and biogeochemistry forced by the asian monsoon system and it is spatially decorrelated from the coastal upwelling systems where the biological production is the strongest. In this study we examine the factors controlling the seasonality and the spatial distribution of the OMZ in the Arabian Sea using a coupled bio-physical model. We find that the oxygen concentration in the OMZ displays a seasonal cycle with an amplitude of 5–15 % of the annual mean oxygen concentration. The OMZ is ventilated by lateral ventilation along the western boundary current and in the coastal undercurrent along India during the summer monsoon and by coastal downwelling and negative Ekman pumping during the fall intermonsoon and winter monsoon. This ventilation is counterbalanced by strong coastal upwelling and positive Ekman pumping of low oxygen waters at the base of the OMZ during the spring intermonsoon. Although the factors controlling the OMZ seasonality are associated with the men circulation, we find that mesoscale dynamics modulates them by limiting the vertical ventilation during winter and enhancing it through lateral advection during the rest of the year. Processes explaining the establishment and spatial distribution of the OMZ were quantified using a perturbation experiment initialised with no OMZ. As expected, the oxygen depletion is triggered by strong biological activity in central Arabian Sea during winter and in western and eastern boundary coastal upwelling systems during summer. We find that the 3-D ocean dynamic largely controls the spatial distribution of the OMZ. The eastward shift ensues from the northward lateral transport of ventilated waters along the western and eastern coasts and the advection offshore of low oxygen waters formed in the upwelling system.Thu, 10 May 2012 00:00:00 +0200http://www.biogeosciences-discuss.net/9/5471/2012/<b>High-resolution interpolar difference of atmospheric methane around the Last Glacial Maximum</b><br /><br />Biogeosciences Discussions, 9, 5471-5508, 2012<br /><br />Author(s): M. Baumgartner, A. Schilt, O. Eicher, J. Schmitt, J. Schwander, R. Spahni, H. Fischer, and T. F. Stocker<br /><br />Reconstructions of past atmospheric methane concentrations are available from ice cores from both, Greenland and Antarctica. The difference observed between the two polar methane concentration levels is a valuable additional parameter which allows to constrain the geographical location of the responsible methane sources. Here we present new high-resolution methane records from the North Greenland Ice Core Project (NGRIP) and the European Project for Ice Coring in Antarctica (EPICA) Dronning Maud Land (EDML) ice cores covering Termination 1, the Last Glacial Maximum, and parts of the last glacial back to 32 000 years before present. Due to the high-resolution records the synchronisation between the ice cores from NGRIP and EDML is considerably improved and the interpolar concentration difference of methane is determined with unprecedented precision and temporal resolution. Relative to the mean methane concentration, we find a rather stable positive interpolar difference throughout the record with its minimum value of 3.7 &plusmn; 0.7 % between 21 900–21 200 years before present, which is higher than previously estimated in this interval close to the Last Glacial Maximum. This implies that Northern Hemisphere boreal wetland sources were never completely shut off during the peak glacial. Starting at 21 000 years before present, i.e. severval millenia prior to the transition into the Holocene, the relative interpolar difference becomes even more positive and stays at a fairly stable level of 6.5 &plusmn; 0.8 % during Termination 1. We hypothesise that the anti-correlation observed in the monsoon records from the Northern and Southern Hemispheres induces a methane source redistribution within lower latitudes, which could explain parts of the variations in the interpolar difference.Wed, 09 May 2012 00:00:00 +0200http://www.biogeosciences-discuss.net/9/5445/2012/<b>The role of alkalinity generation in controlling the fluxes of CO₂ during exposure and inundation on tidal flats</b><br /><br />Biogeosciences Discussions, 9, 5445-5469, 2012<br /><br />Author(s): P. A. Faber, A. J. Kessler, J. K. Bull, I. D. McKelvie, F. J. R. Meysman, and P. L. M. Cook<br /><br />Dissolved inorganic carbon, CO₂ and alkalinity fluxes from intertidal sediments were investigated during periods of exposure and inundation, using laboratory core incubations, field data and reactive transport model simulations. In the incubations and field data, it was found that during periods of alkalinity production, the flux of dissolved inorganic carbon (DIC) out of the sediment was significantly greater during inundation periods. This alkalinity production was attributed to the accumulation of reduced sulfur species within the sediment. This finding was supported by computational simulations which indicated that large amounts of sulfate reduction and reduced solute burial (FeS) induce an alkalinity flux from the sediment during high tide conditions. As the fate of sulfide is controlled by Fe, the sensitivity of alkalinity flux to Fe concentrations was investigated, and it was found that amount of reactive Fe in the sediment was a major driver of net alkalinity production. The finding the CO₂ fluxes can be significantly lower than total metabolism during exposure has implications for how total metabolism is quantified on tidal flats.Wed, 09 May 2012 00:00:00 +0200http://www.biogeosciences-discuss.net/9/5415/2012/<b>Partitioning of soil water among canopy trees during a soil desiccation period in a temperate mixed forest</b><br /><br />Biogeosciences Discussions, 9, 5415-5443, 2012<br /><br />Author(s): M. Meißner, M. Köhler, L. Schwendenmann, and D. Hölscher<br /><br />Complementary resource use is considered an important mechanism in the study of biodiversity effects. Here we explore how species identity, species mixture and tree size influence the vertical partitioning of soil water among canopy trees during a soil desiccation period. In the Hainich forest, Germany, the species <i>Fagus sylvatica, Tilia sp.</i> and <i>Fraxinus excelsior</i> were studied in single- and three-species mixed clusters, each consisting of three co-dominant trees situated within a larger mixed forest stand. Vertical soil water uptake depth was assessed by analyzing the hydrogen stable isotope composition (deuterium, &delta; D) of water from depth intervals throughout the soil profile and in tree xylem water. For single species clusters, a mixing model suggested that <i>Fagus</i> distinctively drew water from soil depths of 0.3–0.5 m, <i>Tilia</i> from 0.3–0.5 m and 0.5–0.7 m and <i>Fraxinus</i> mainly used water from 0.5–0.7 m. In mixed clusters, the uptake patterns of <i>Fagus</i> and <i>Tilia</i> were similar to those of the single-species clusters (mainly uptake form 0.3–0.5 m), but <i>Fraxinus</i> showed a different uptake pattern. <i>Fraxinus</i> in mixture had a somewhat homogenously distributed uptake over the soil depths 0.2–0.7 m. For single species clusters, there was no correlation between main soil water uptake depth and tree diameter, irrespective of variations in tree size. In contrast, for mixed clusters there was a significant decrease in the main uptake depth with increasing tree size (<i>P</i><0.001, <i>R</i>²_adj = 0.73), irrespective of species mix. In consequence, soil water partitioning was strongest where species were mixed and tree size varied. We further analyzed whether single and mixed-species clusters differed in the level of water uptake, e.g. due to complementarity, but our soil water budgeting did not indicate any such differences. A possible explanation might be that the volume of water used is predominantly governed by properties at the stand level, such as aerodynamic roughness, than by processes acting at the meter scale between neighbouring trees. With respect to application, we assume that the upcoming close-to-nature forestry approach for the area, which fosters mixed stands of heterogonous diameters, may result in enhanced complementarity in soil water uptake among canopy trees.Wed, 09 May 2012 00:00:00 +0200http://www.biogeosciences-discuss.net/9/5373/2012/<b>Imbalanced nutrients as triggers for black shale formation in a shallow shelf setting during the OAE 2 (Wunstorf, Germany)</b><br /><br />Biogeosciences Discussions, 9, 5373-5413, 2012<br /><br />Author(s): M. Blumenberg and F. Wiese<br /><br />During the oceanic anoxic event 2 (OAE 2) in the mid-Cretaceous widespread black shale (BS) formation occurred, reflecting perturbations in major biogeochemical cycles. Here we present geochemical and biomarker data of the OAE 2 from a shelf setting situated at about 100 to 150 water depth (Wunstorf, Germany). Our data support that processes inducing BS deposition were related to orbital cyclicity in Wunstorf and that they were not restricted to the time of the OAE 2 carbon isotope excursion. Correlations of total organic carbon (TOC) and &delta;¹⁵N and high relative abundances of functionalized hopanoids (incl. 2-methylated structures) suggest that BS were formed during times of imbalanced nutrients with high phosphorus inputs and increased (cyano)bacterial nitrogen fixation. Periods of BS formation were also characterized by enhanced growth of dinoflagellates and bacteriovorous ciliates, the latter supporting the presence of a stratified water body. The lack of biomarkers specific for green-sulfur bacteria excludes photic zone euxinia during OAE 2 in Wunstorf. Conflicting maturities and biomarker distributions in kerogen and extractable organic matter and, interestingly, a negative correlation of the diagenetically resistant 2-methyl hopane hydrocarbons with TOC indicate a complex depositional setting at Wunstorf. In Wunstorf this might have been induced by high continental run-off during BS formation and the accompanying mobilisation of refractory OM from the shelfs and near shore areas.Wed, 09 May 2012 00:00:00 +0200http://www.biogeosciences-discuss.net/9/5347/2012/<b>Impact of mire reclamation on export potential and characteristics of dissolved carbons in the Sanjiang Plain, Northeast China</b><br /><br />Biogeosciences Discussions, 9, 5347-5371, 2012<br /><br />Author(s): Y. D. Guo, C. C. Song, Y. Z. Lu, Y. Y. Song, and Z. M. Wan<br /><br />As an important dissolved organic carbon (DOC) reservoir, the mires in the Sanjiang Plain, Northeast China, have been suffering from large scale of reclamation, and thus elevated loss and degradation since the 1960s. This study compares the export dynamics of the dissolved carbons, as well as the chemical characteristics of DOC, in the natural mire, degraded mire and drainage ditches during the growing seasons from 2008 to 2010 with the aim to clarify the final effects of the longterm reclamation on the export dynamics of the dissolved carbons. Results show that the average concentrations of total dissolved carbon (TC) and DOC are much higher in natural mires than that in degraded mire and drainage ditches. The DOC concentration for natural mires, about 35.53 ± 5.15 mg l⁻¹ on average, is nearly 2.39 times of that in degraded mire (14.84 ± 4.21 mg l⁻¹) and 2.77 times of the average value in ditches (12.84 ± 4.49 mg l⁻¹). Similarly, the hydrophobic fraction and SUVA₂₅₄ of DOC also represent lower values in the degraded mire and ditches, which suggests that mire reclamation has resulted not only in the reduced DOC concentrations but also in the reduced chemical stability. Whereas the inorganic dissolved carbons (DIC) exhibits obvious increased trends in drainage ditches in comparison to natural mires. Analyses of exitation-emission fluorescence spectra reveal that the reclamation has greatly altered the DOC composition with more biological organic substances exporting from the Sanjiang Plain. The presence of protein- and tryptophan-like substances in the ditches indicates there has been extensive agricultural pollution in the surface waters. Changes in the hydrological regime of the mire landscapes by sustained agriculture activities are deemed the prodominant reason, and the trends in the export dynamics of dissolved carbons will keep on if mire reclamation continues in the future.Mon, 07 May 2012 00:00:00 +0200http://www.biogeosciences-discuss.net/9/5321/2012/<b>Regional analysis of groundwater nitrate concentrations and trends in Denmark in regard to agricultural influence</b><br /><br />Biogeosciences Discussions, 9, 5321-5346, 2012<br /><br />Author(s): B. Hansen, T. Dalgaard, L. Thorling, B. Sørensen, and M. Erlandsen<br /><br />The act of balancing between an intensive agriculture with a high potential for nitrate pollution and a~drinking water supply almost entirely based on groundwater is a challenge faced by Denmark and similar regions around the globe. Since the 1980s, regulations implemented by Danish farmers have succeeded in optimizing the N (nitrogen) management at farm level. As a result, the upward agricultural N surplus trend has been reversed, and the N surplus has reduced by 30–55 % from 1980 to 2007 depending on region. The reduction in the N surplus served to reduce the losses of N from agriculture, with documented positive effects on nature and the environment in Denmark. In groundwater, the upward trend in nitrate concentration was reversed around 1980, and a larger number of downward nitrate trends were seen in the youngest groundwater compared with the oldest groundwater. However, on average, approximately 48 % of the oxic monitored groundwater has nitrate concentrations above the groundwater and drinking water standards of 50 mg l^&minus;1. Furthermore, trend analyses show that 33 % of all the monitored groundwater has upward nitrate trends, while only 18 % of the youngest groundwater has upward nitrate trends according to data sampled from 1988–2009. A regional analysis shows a correlation between a high level of N surplus in agriculture, high concentrations of nitrate in groundwater and the largest number of downward nitrate trends in groundwater in the livestock-dense northern and western parts of Denmark compared with the south-eastern regions with lower livestock densities. These results indicate that the livestock farms dominating in northern and western parts of Denmark have achieved the largest reductions in N surpluses. Groundwater recharge age determinations allow comparison of long-term changes in N surplus in agriculture with changes in oxic groundwater quality. The presented data analysis is based on groundwater recharged from 1952–2003, but sampled from 1988–2009. Repetition of the nitrate trend analyses at five-year intervals using dating of the groundwater recharged in the coming years and a longer time series of the nitrate analyses can reveal the evolution in nitrate leaching from Danish agriculture during the past 10 yr. Similar analyses can be carried out to compare with other regions internationally.Fri, 04 May 2012 00:00:00 +0200http://www.biogeosciences-discuss.net/9/5287/2012/<b>Partitioning of soil CO₂ efflux in un-manipulated and experimentally flooded plots of a temperate fen</b><br /><br />Biogeosciences Discussions, 9, 5287-5319, 2012<br /><br />Author(s): S. Wunderlich and W. Borken<br /><br />Peatlands store large amounts of organic carbon, but the carbon stock is sensitive to changes in precipitation or water table manipulations. Restoration of drained peatlands by drain blocking and flooding is a common measure to conserve and augment the carbon stock of peatland soils. Here, we report to what extent flooding affected the contribution of heterotrophic and rhizosphere respiration to soil CO₂ efflux in a grass-dominated mountain fen, Germany. Soil CO₂ efflux was measured in three un-manipulated control plots and three flooded plots in two consecutive years. Flooding was achieved by permanent irrigation during the growing seasons. Radiocarbon signatures of CO₂ from different sources including soil CO₂ efflux, incubated peat cores and live grass roots were repeatedly analyzed for partitioning of soil CO₂ efflux. Additionally, heterotrophic respiration and its radiocarbon signature were determined by eliminating rhizosphere respiration in trenched subplots (only control). In the control plots, rhizosphere respiration determined by ¹⁴C signatures contributed between 47 and 61% during the growing season, but was small (4%) immediately before budding. Trenching revealed a smaller rhizosphere contribution of 33% (2009) and 22% (2010) during growing seasons. <br><br> Flooding reduced annual soil CO₂ efflux of the fen by 42% in 2009 and by 30% in 2010. The reduction was smaller in 2010 mainly through naturally elevated water level in the control plots. A 1-week interruption of irrigation caused a strong short-lived increase in soil CO₂ efflux, demonstrating the sensitivity of the fen to water table drawdown near the peat surface. The reduction in soil CO₂ efflux in the flooded plots diminished the relative proportion of rhizosphere respiration from 56 to 46%, suggesting that rhizosphere respiration was slightly more sensitive to flooding than heterotrophic respiration. We conclude that the moderate decrease in rhizosphere respiration following flooding arises from a gradual change in vegetation in this fen ecosystem.Wed, 02 May 2012 00:00:00 +0200http://www.biogeosciences-discuss.net/9/5249/2012/<b>Parameter-induced uncertainty quantification of soil N₂O, NO and CO₂ emission from Höglwald spruce forest (Germany) using the LandscapeDNDC model</b><br /><br />Biogeosciences Discussions, 9, 5249-5286, 2012<br /><br />Author(s): K.-H. Rahn, C. Werner, R. Kiese, E. Haas, and K. Butterbach-Bahl<br /><br />Assessing the uncertainties of simulation results of ecological models is becoming of increasing importance, specifically if these models are used to estimate greenhouse gas emissions at site to regional/national levels. Four general sources of uncertainty effect the outcome of process-based models: (i) uncertainty of information used to initialise and drive the model, (ii) uncertainty of model parameters describing specific ecosystem processes, (iii) uncertainty of the model structure and (iv) accurateness of measurements (e.g. soil-atmosphere greenhouse gas exchange) which are used for model testing and development. <br><br> The aim of our study was to assess the simulation uncertainty of the process-based biogeochemical model LandscapeDNDC. For this we set up a Bayesian framework using a Markov Chain Monte Carlo (MCMC) method, to estimate the joint model parameter distribution. Data for model testing, parameter estimation and uncertainty assessment were taken from observations of soil fluxes of nitrous oxide (N₂O), nitric oxide (NO), and carbon dioxide (CO₂) as observed over a 10 yr period at the spruce site of the Höglwald Forest, Germany. By running four independent Markov Chains in parallel with identical properties (except for the parameter start values), an objective criteria for chain convergence developed by Gelman et al. (2003) could be used. <br><br> Our approach showed that by means of the joined parameter distribution, we were able not only to limit the parameter space and specify the probability of parameter values, but also to assess the complex dependencies among model parameters used for simulating soil C and N trace gas emissions. This helped to improve the understanding of the behaviour of the complex LandscapeDNDC model while simulating soil C and N turnover processes and associated C and N soil-atmosphere exchange. <br><br> In a final step the parameter distribution of the most sensitive parameters determining soil-atmosphere C and N exchange were used to obtain the parameter-induced uncertainty of simulated N₂O, NO and CO₂ emissions. These were compared to observational data of the calibration set (6 yr) and an independent validation set of 4 yr. <br><br> The comparison showed that most of the annual observed trace gas emissions were in the range of simulated values and were predicted with a high certainty (Residual mean squared error (RMSE) NO: 2.5 to 21.3 g N ha^&minus;1 d^&minus;1, N₂O: 0.2 to 21.4 g N ha^&minus;1 d^&minus;1, CO₂: 5.8 to 12.6 kg C ha^&minus;1 d^&minus;1). However, LandscapeDNDC simulations were sometimes limited to accurately predict observed seasonal variations in fluxes.Fri, 27 Apr 2012 00:00:00 +0200http://www.biogeosciences-discuss.net/9/5205/2012/<b>Remote sensing the dynamics of suspended particles in the Mackenzie River plume (Canadian Arctic Ocean)</b><br /><br />Biogeosciences Discussions, 9, 5205-5248, 2012<br /><br />Author(s): D. Doxaran, J. Ehn, S. Bélanger, A. Matsuoka, S. Hooker, and M. Babin<br /><br />Climate change significantly impacts Arctic shelf regions in terms of air temperature, ultraviolet radiation, melting of sea ice, precipitation, thawing of permafrost and coastal erosion. A direct consequence is an increase in Arctic river discharge with an expectation of increased delivery of organic carbon sequestered in high-latitute soils since the last glacial maximum. Monitoring the fluxes and fate of this terrigenous organic carbon is problematic in such sparsely populated regions unless remote sensing techniques can be developed to an operational stage. <br><br> The main objective of this study is to develop an ocean colour algorithm to operationally monitor dynamics of suspended particulate matter (SPM) on the Mackenzie River continental shelf (Canadian Arctic Ocean) using satellite imagery. The water optical properties are documented across the study area and related to concentrations of SPM and particulate organic carbon (POC). Robust SPM and POC:SPM proxies are identified, such as the light backscattering and attenuation coefficients, and relationships are established between these optical and biogeochemical parameters. Following a semi-analytical approach, a regional SPM quantification relationship is obtained for the inversion of the water reflectance signal into SPM concentration. This relationship is validated based on independent field optical measurements. It is successfully applied to a selection of MODIS satellite data which allow estimating fluxes at the river mouth and monitoring the extension and dynamics of the Mackenzie River surface plume in 2009, 2010 and 2011. Good agreement is obtained with field observations representative of the whole water column in the river delta zone within which terrigenous SPM is mainly constrained (out of short periods of maximum river outflow). Most of the seaward export of SPM is observed to occur within the west side of the river mouth. <br><br> Future work require the validation of the developed SPM regional algorithm based on match-ups with field measurements, then the routine application to ocean colour satellite data in order to better estimate the fluxes and fate of SPM and POC delivered by the Mackenzie River to the Arctic Ocean.Fri, 27 Apr 2012 00:00:00 +0200http://www.biogeosciences-discuss.net/9/5177/2012/<b>Molecular analysis of the microbial community structures in water-flooding petroleum reservoirs with different temperatures</b><br /><br />Biogeosciences Discussions, 9, 5177-5203, 2012<br /><br />Author(s): L.-Y. Wang, R.-Y. Duan, J.-F. Liu, S.-Z. Yang, J.-D. Gu, and B.-Z. Mu<br /><br />Temperature is one of the most important environmental factors regulating the activity and determining the composition of the microbial community. Analysis of microbial communities from six water-flooding petroleum reservoirs at temperatures from 20 to 63 &deg;C by 16S rRNA gene clone libraries indicates the presence of physiologically diverse and temperature-dependent microorganisms in these subterrestrial ecosystems. In high-temperature petroleum reservoirs, most of the archaeal sequences belong to the thermophilic archaea including the genera <i>Thermococcus</i>, <i>Methanothermobacter</i> and <i>Thermoplasmatales</i>, most of the bacterial sequences belong to the phyla <i>Firmicutes</i>, <i>Thermotogae</i> and <i>Thermodesulfobacteria</i>; in low-temperature petroleum reservoirs, most of the archaeal sequences are affiliated with the genera <i>Methanobacterium</i>, <i>Methanoculleus</i> and <i>Methanocalculus</i>, most of the bacterial sequences to the phyla <i>Proteobacteria</i>, <i>Bacteroidetes</i> and <i>Actinobacteria</i>. Canonical correspondence analysis (CCA) revealed that temperature, mineralization, ionic type as well as volatile fatty acids showed correlation with the microbial community structures. These organisms may be adapted to the environmental conditions of these petroleum reservoirs over geologic time by metabolizing buried organic matter from the original deep subsurface environment and became the common inhabitants in subsurface environments.Fri, 27 Apr 2012 00:00:00 +0200http://www.biogeosciences-discuss.net/9/5153/2012/<b>Water supply patterns in two agricultural areas of Central Germany under climate change conditions</b><br /><br />Biogeosciences Discussions, 9, 5153-5176, 2012<br /><br />Author(s): M. H. Tölle, C. Moseley, O. Panferov, G. Busch, and A. Knohl<br /><br />Increasing emissions of greenhouse gases and increasing prices for fossil fuels have highlighted the demand for CO₂ "neutral" renewable energy sources, e.g. short rotation forestry systems used for bioenergy. These systems might be vulnerable to changes in temperature, precipitation and occurrence of extreme weather events. To estimate success or failure of such short rotation coppices in a certain area we need regional climate projections and risk assessment. Changes of water supply patterns in two agriculturally extensively used regions in Central Germany (around Göttingen and Großfahner) with different climate conditions but both in the temperate climate zone are explored. The study is carried out under present conditions as well as under projected climate change conditions (1971–2100) using A1B and B1 climate scenarios downscaled for Europe. Analysis of precipitation bias shows regional differences: a strong bias in Göttingen area and a weaker bias in the Großfahner area. A bias correction approach, Quantile mapping, is applied to the ensemble results for both areas for winter and summer seasons. By using quantile regression on the seasonal Standardized Precipitation Indices (SPIs) as indicator for water supply conditions we found that precipitation is expected to increase in winter in all quantiles of the distribution for Göttingen area during the 21th century. Heavy precipitation is also expected to increase for Großfahner area suggesting a trend to wetter extremes in winter for the future. This winter precipitation increase could trigger runoff and soil erosion risk enhancing the severity of floods. Increasing winter availability of water could enhance local water supply in spring. For both areas no significant change in summer was found over the whole time period. Although the climate change signal of the SPI indicate mild dryer conditions in summer at the end of the 21st century which may trigger water shortage and summer drying associated with above–average temperatures in the future. Even though both study areas are close together Großfahner area was found to be the least affected one by changes indicating that small spatial scale differences matter. These developments were found in all examined simulation runs. This study highlighted the regional differences in the vulnerability to water surplus or deficit risks in a temperate system which emphasizes the need in impact studies to focus on proper consideration of local and regional environmental conditions as well as adaption and mitigation of management for agriculture.Fri, 27 Apr 2012 00:00:00 +0200