Showing papers in "Biogeochemistry in 2011"

Accelerated microbial organic matter mineralization following salt-water intrusion into tidal freshwater marsh soils

Abstract: The impact of salt-water intrusion on microbial organic carbon (C) mineralization in tidal freshwater marsh (TFM) soils was investigated in a year-long laboratory experiment in which intact soils were exposed to a simulated tidal cycle of freshwater or dilute salt-water. Gas fluxes [carbon dioxide (CO2) and methane (CH4)], rates of microbial processes (sulfate reduction and methanogenesis), and porewater and solid phase biogeochemistry were measured throughout the experiment. Flux rates of CO2 and, surprisingly, CH4 increased significantly following salt-water intrusion, and remained elevated relative to freshwater cores for 6 and 5 months, respectively. Following salt-water intrusion, rates of sulfate reduction increased significantly and remained higher than rates in the freshwater controls throughout the experiment. Rates of acetoclastic methanogenesis were higher than rates of hydrogenotrophic methanogenesis, but the rates did not differ by salinity treatment. Soil organic C content decreased significantly in soils experiencing salt-water intrusion. Estimates of total organic C mineralized in freshwater and salt-water amended soils over the 1-year experiment using gas flux measurements (18.2 and 24.9 mol C m−2, respectively) were similar to estimates obtained from microbial rates (37.8 and 56.2 mol C m−2, respectively), and to losses in soil organic C content (0 and 44.1 mol C m−2, respectively). These findings indicate that salt-water intrusion stimulates microbial decomposition, accelerates the loss of organic C from TFM soils, and may put TFMs at risk of permanent inundation.

Soil organic phosphorus in lowland tropical rain forests

Abstract: Phosphorus is widely considered to constrain primary productivity in tropical rain forests, yet the chemistry of soil organic phosphorus in such ecosystems remains poorly understood. We assessed the composition of soil organic phosphorus in 19 contrasting soils under lowland tropical forest in the Republic of Panama using NaOH–EDTA extraction and solution 31P nuclear magnetic resonance spectroscopy. The soils spanned a strong rainfall gradient (1730–3404 mm y−1) and contained a wide range of chemical properties (pH 3.3–7.0; total carbon 2.8–10.4%; total phosphorus 74–1650 mg P kg−1). Soil organic phosphorus concentrations ranged between 22 and 494 mg P kg−1 and were correlated positively with total soil phosphorus, pH, and total carbon, but not with annual rainfall. Organic phosphorus constituted 26 ± 1% (mean ± STD error, n = 19) of the total phosphorus, suggesting that this represents a broad emergent property of tropical forest soils. Organic phosphorus occurred mainly as phosphate monoesters (68–96% of total organic phosphorus) with smaller concentrations of phosphate diesters in the form of DNA (4–32% of total organic phosphorus). Phosphonates, which contain a direct carbon–phosphorus bond, were detected in only two soils (3% of the organic phosphorus), while pyrophosphate, an inorganic polyphosphate with a chain length of two, was detected in all soils at concentrations up to 13 mg P kg−1 (3–13% of extracted inorganic phosphorus). Phosphate monoesters were a greater proportion of the total organic phosphorus in neutral soils with high concentrations of phosphorus and organic matter, whereas the proportion of phosphate diesters was greater in very acidic soils low in phosphorus and organic matter. Most soils did not contain detectable concentrations of either myo- or scyllo-inositol hexakisphosphate, which is in marked contrast to many temperate mineral soils that contain abundant inositol phosphates. We conclude that soil properties exert a strong control on the amounts and forms of soil organic phosphorus in tropical rain forests, but that the proportion of the total phosphorus in organic forms is relatively insensitive to variation in climate and soil properties. Further work is now required to assess the contribution of soil organic phosphorus to the nutrition and diversity of plants in these species-rich ecosystems.

Ecoenzymatic stoichiometry of recalcitrant organic matter decomposition: the growth rate hypothesis in reverse

Abstract: The flow of carbon and nutrients from plant production into detrital food webs is mediated by microbial enzymes released into the environment (ecoenzymes). Ecoenzymatic activities are linked to both microbial metabolism and environmental resource availability. In this paper, we extend the theoretical and empirical framework for ecoenzymatic stoichiometry from nutrient availability to carbon composition by relating ratios of β-1,4-glucosidase (BG), acid (alkaline) phosphatase (AP), β-N-acetylglucosaminidase (NAG), leucine aminopeptidase (LAP) and phenol oxidase (POX) activities in soils to measures of organic matter recalcitrance, using data from 28 ecosystems. BG and POX activities are uncorrelated even though both are required for lignocellulose degradation. However, the ratio of BG:POX activity is negatively correlated with the relative abundance of recalcitrant carbon. Unlike BG, POX activity is positively correlated with (NAG + LAP) and AP activities. We propose that the effect of organic matter recalcitrance on microbial C:N and C:P threshold element ratios (TER) can be represented by normalizing BG, AP and (NAG + LAP) activities to POX activity. The scaling relationships among these ratios indicate that the increasing recalcitrance of decomposing organic matter effectively reverses the growth rate hypothesis of stoichiometric theory by decreasing carbon and nutrient availability and slowing growth, which increases TERN:P. This effect is consistent with the narrow difference between the mean elemental C:N ratios of soil organic matter and microbial biomass and with the inhibitory effect of N enrichment on rates of decomposition and microbial metabolism for recalcitrant organic matter. From these findings, we propose a conceptual framework for bottom-up decomposition models that integrate the stoichiometry of ecoenzymatic activities into general theories of ecology.

Biological and photochemical transformations of amino acids and lignin phenols in riverine dissolved organic matter

Abstract: The photo- and bio-degradation of dissolved organic matter (DOM) in water from the Broad River were investigated in laboratory experiments using a solar simulator to control the intensity and exposure of samples to irradiation. The water samples included a natural assemblage of microorganisms, and the daily exposure of samples to irradiation was varied to distinguish the relative contributions of photochemical and biological degradation. Concentrations of dissolved organic carbon (DOC) and specific components of DOM, including chromophoric DOM (CDOM), lignin phenols and amino acids, were monitored to investigate the reactivity and predominant pathway of degradation of these DOM components. Biodegradation was primarily responsible for the overall remineralization of DOC and losses of the amino acid component of DOM, whereas photodegradation was primarily responsible for losses of the chromophoric and lignin phenol components of DOM. The rates of photodegradation of lignin phenols were strongly influenced by the presence of methoxy groups on the aryl ring. Syringyl (S) phenols have two methoxy substitutions, vanillyl (V) phenols have one methoxy substitution, and p-hydroxy (P) phenols are not substituted with methoxy groups. Photochemical decay constants were highest for S phenols, lowest for P phenols and followed a consistent pattern (S > V > P) in the experiments. The carbon-normalized yields of amino acids and lignin phenols were found to be useful molecular indicators of the highly reactive (i.e. labile) components of biodegradable and photodegradable DOM, respectively.

An Absorbing Markov Chain approach to understanding the microbial role in soil carbon stabilization

Abstract: The number of studies focused on the transformation and sequestration of soil organic carbon (C) has dramatically increased in recent years due to growing interest in understanding the global C cycle. While it is readily accepted that terrestrial C dynamics are heavily influenced by the catabolic and anabolic activities of microorganisms, the incorporation of microbial biomass components into stable soil C pools (via microbial living cells and necromass) has received less attention. Nevertheless, microbial-derived C inputs to soils are now increasingly recognized as playing a far greater role in stabilization of soil organic matter than previously believed. Our understanding, however, is limited by the difficulties associated with studying microbial turnover in soils. Here, we describe the use of an Absorbing Markov Chain (AMC) to model the dynamics of soil C transformations among three microbial states: living microbial biomass, microbial necromass, and C removed from living and dead microbial sources. We find that AMC provides a powerful quantitative approach that allows prediction of how C will be distributed among these three states, and how long it will take for the entire amount of initial C to pass through the biomass and necromass pools and be moved into atmosphere. Further, assuming constant C inputs to the model, we can predict how C is eventually distributed, along with how much C sequestrated in soil is microbial-derived. Our work represents a first step in attempting to quantify the flow of C through microbial pathways, and has the potential to increase our understanding of the microbial role in soil C dynamics.

The effects of deforestation and climate variability on the streamflow of the Araguaia River, Brazil

Abstract: Deforestation changes the hydrological, geomorphological, and biochemical states of streams by decreasing evapotranspiration on the land surface and increasing runoff, river discharge, erosion and sediment fluxes from the land surface. Deforestation has removed about 55% of the native vegetation and significantly altered the hydrological and morphological characteristics of an 82,632 km2 watershed of the Araguaia River in east-central Brazil. Observed discharge increased by 25% from the 1970s to the 1990s and computer simulations suggest that about 2/3 of the increase is from deforestation, the remaining 1/3 from climate variability. Changes of this scale are likely occurring throughout the 2,000,000 km2 savannah region of central Brazil.

Effects of nitrogen additions on above- and belowground carbon dynamics in two tropical forests

Abstract: Anthropogenic nitrogen (N) deposition is increasing rapidly in tropical regions, adding N to ecosystems that often have high background N availability. Tropical forests play an important role in the global carbon (C) cycle, yet the effects of N deposition on C cycling in these ecosystems are poorly understood. We used a field N-fertilization experiment in lower and upper elevation tropical rain forests in Puerto Rico to explore the responses of above- and belowground C pools to N addition. As expected, tree stem growth and litterfall productivity did not respond to N fertilization in either of these N-rich forests, indicating a lack of N limitation to net primary productivity (NPP). In contrast, soil C concentrations increased significantly with N fertilization in both forests, leading to larger C stocks in fertilized plots. However, different soil C pools responded to N fertilization differently. Labile (low density) soil C fractions and live fine roots declined with fertilization, while mineral-associated soil C increased in both forests. Decreased soil CO2 fluxes in fertilized plots were correlated with smaller labile soil C pools in the lower elevation forest (R2 = 0.65, p < 0.05), and with lower live fine root biomass in the upper elevation forest (R2 = 0.90, p < 0.05). Our results indicate that soil C storage is sensitive to N deposition in tropical forests, even where plant productivity is not N-limited. The mineral-associated soil C pool has the potential to respond relatively quickly to N additions, and can drive increases in bulk soil C stocks in tropical forests.

Release of CO2 and CH4 from lakes and drainage ditches in temperate wetlands

Abstract: Shallow fresh water bodies in peat areas are important contributors to greenhouse gas fluxes to the atmosphere. In this study we determined the magnitude of CH4 and CO2 fluxes from 12 water bodies in Dutch wetlands during the summer season and studied the factors that might regulate emissions of CH4 and CO2 from these lakes and ditches. The lakes and ditches acted as CO2 and CH4 sources of emissions to the atmosphere; the fluxes from the ditches were significantly larger than the fluxes from the lakes. The mean greenhouse gas flux from ditches and lakes amounted to 129.1 ± 8.2 (mean ± SE) and 61.5 ± 7.1 mg m−2 h−1 for CO2 and 33.7 ± 9.3 and 3.9 ± 1.6 mg m−2 h−1 for CH4, respectively. In most water bodies CH4 was the dominant greenhouse gas in terms of warming potential. Trophic status of the water and the sediment was an important factor regulating emissions. By using multiple linear regression 87% of the variation in CH4 could be explained by PO4 3− concentration in the sediment and Fe2+ concentration in the water, and 89% of the CO2 flux could be explained by depth, EC and pH of the water. Decreasing the nutrient loads and input of organic substrates to ditches and lakes by for example reducing application of fertilizers and manure within the catchments and decreasing upward seepage of nutrient rich water from the surrounding area will likely reduce summer emissions of CO2 and CH4 from these water bodies.

Frontiers and challenges in soil respiration research: from measurements to model-data integration

Abstract: Soil respiration, the flux of CO2 from the soil to the atmosphere represents a major flux in the global carbon cycle. Our ability to predict this flux remains limited because of multiple controlling mechanisms that interact over different temporal and spatial scales. However, new advances in measurement and analyses present an opportunity for the scientific community to improve the understanding of the mechanisms that regulate soil respiration. In this paper, we address several recent advancements in soil respiration research from experimental measurements and data analysis to new considerations for model-data integration. We focus on the links between the soil–plant-atmosphere continuum at short (i.e., diel) and medium (i.e., seasonal-years) temporal scales. First, we bring attention to the importance of identifying sources of soil CO2 production and highlight the application of automated soil respiration measurements and isotope approaches. Second, we discuss the need of quality assurance and quality control for applications in time series analysis. Third, we review perspectives about emergent ideas for modeling development and model-data integration for soil respiration research. Finally, we call for stronger interactions between modelers and experimentalists as a way to improve our understanding of soil respiration and overall terrestrial carbon cycling.

Assessment of the eutrophication status of the Great Barrier Reef lagoon (Australia)

Abstract: Current scientific consensus is that inshore regions of the central and southern Great Barrier Reef, Australia, are at risk of impacts from increased nutrient (as well as sediment and pesticide) loads delivered to Reef waters. Increases in the discharge of water quality contaminants to the Reef are largely a consequence of the expansion of agricultural practices in northern Queensland catchments following European settlement in the 1850s. In particular, the presence of elevated chlorophyll a and nutrient concentrations in many parts of the inshore Great Barrier Reef together with intense and extensive phytoplankton blooms following the discharge of nutrient-rich river flood waters suggest that the central and southern inshore area of the Great Barrier Reef is likely to be significantly impacted by elevated nutrient loads. The biological consequences of this are not fully quantified, but are likely to include changes in reef condition including hard and soft coral biodiversity, macroalgal abundance, hard coral cover and coral recruitment, as well as change in seagrass distribution and tissue nutrient status. Contemporary government policy is centered around promotion and funding of better catchment management practices to minimize the loss of catchment nutrients (both applied and natural) and the maintenance of a Reef wide water quality and ecosystem monitoring program. The monitoring program is designed to assess trends in uptake of management practice improvements and their associated impacts on water quality and ecosystem status over the next 10 years. A draft set of quantitative criteria to assess the eutrophication status of Great Barrier Reef waters is outlined for further discussion and refinement.

Cell-wall polysaccharides play an important role in decay resistance of Sphagnum and actively depressed decomposition in vitro

Abstract: Sphagnum-dominated peatlands head the list of ecosystems with the largest known reservoirs of organic carbon (C). The bulk of this C is stored in decomposition-resistant litter of one bryophyte genus: Sphagnum. Understanding how Sphagnum litter chemistry controls C mineralization is essential for understanding potential interactions between environmental changes and C mineralization in peatlands. We aimed to separate the effects of phenolics from structural polysaccharides on decay of Sphagnum. We measured aerobic microbial respiration of different moss litter types in a lab. We used chemical treatments to step-wise remove the chemical compounds thought to be important in decay-resistance in three taxonomically distant moss genera. We also focused on the effect of Sphagnum-specific cell-wall pectin-like polysaccharides (sphagnan) on C and N mineralization. Removing polymeric lignin-like phenolics had only negligible effects on C mineralization of Sphagnum litter, but increased mineralization of two other bryophyte genera, suggesting a minor role of these phenolics in decay resistance of Sphagnum but a major role of cell-wall polysaccharides. Carboxyl groups of pectin-like polysaccharides represented a C-source in non-Sphagnum litters but resisted decay in Sphagnum. Finally, isolated sphagnan did not serve as C-source but inhibited C and N mineralization instead, reminiscent of the effects reported for phenolics in other ecosystems. Our results emphasize the role of polysaccharides in resistance to, and active inhibition of, microbial mineralization in Sphagnum-dominated litter. As the polysaccharides displayed decay-inhibiting properties hitherto associated with phenolics (lignin, polyphenols), it raises the question if polysaccharide-dominated litter also shares similar environmental controls on decomposition, such as temperature or nutrient and water availability.

Snow depth, soil freezing and nitrogen cycling in a northern hardwood forest landscape

Abstract: Increases in soil freezing associated with decreases in snow cover have been identified as a significant disturbance to nitrogen (N) cycling in northern hardwood forests. We created a range of soil freezing intensity through snow manipulation experiments along an elevation gradient at the Hubbard Brook Experimental Forest (HBEF) in the White Mountains, NH USA in order to improve understanding of the factors regulating freeze effects on nitrate (NO3 −) leaching, nitrous oxide (N2O) flux, potential and in situ net N mineralization and nitrification, microbial biomass carbon (C) and N content and respiration, and denitrification. While the snow manipulation treatment produced deep and persistent soil freezing at all sites, effects on hydrologic and gaseous losses of N were less than expected and less than values observed in previous studies at the HBEF. There was no relationship between frost depth, frost heaving and NO3 − leaching, and a weak relationship between frost depth and winter N2O flux. There was a significant positive relationship between dissolved organic carbon (DOC) and NO3 − concentrations in treatment plots but not in reference plots, suggesting that the snow manipulation treatment mobilized available C, which may have stimulated retention of N and prevented treatment effects on N losses. While the results support the hypothesis that climate change resulting in less snow and more soil freezing will increase N losses from northern hardwood forests, they also suggest that ecosystem response to soil freezing disturbance is affected by multiple factors that must be reconciled in future research.

An open system framework for integrating critical zone structure and function

Abstract: The ''critical zone'' includes the coupled earth surface systems of vegetation, regolith and groundwater that are essential to sustaining life on the planet. The function of this zone is the result of complex interactions among physical, chemical and biological processes and understanding these interac- tions remains a major challenge to earth system sciences. Here we develop an integrated framework based on thermodynamic theory to characterize the critical zone as a system open to energy and mass fluxes that are forced by radiant, geochemical, and elevational gradients. We derive a statement that demonstrates the relative importance of solar radia- tion, water, carbon, and physical/chemical denudation mass fluxes to the critical zone energy balance. Within this framework we use rates of effective energy and mass transfer (EEMT; W m -2 ) to quantify the relevant flux-gradient relations. Synthesis of existing data demonstrates that variation in energetics associ- ated with primary production and effective precipita- tion explains substantial variance in critical zone structure and function. Furthermore, we observe threshold behavior in systems that transition to primary production predominance of the energy flux term. The proposed framework provides a first order approximation of non-linearity in critical zone pro- cesses that may be coupled with physical and numerical models to constrain landscape evolution.

The temperature responses of soil respiration in deserts: a seven desert synthesis

Abstract: The temperature response of soil respiration in deserts is not well quantified. We evaluated the response of respiration to temperatures spanning 67°C from seven deserts across North America and Greenland. Deserts have similar respiration rates in dry soil at 20°C, and as expected, respiration rates are greater under wet conditions, rivaling rates observed for more mesic systems. However, deserts differ in their respiration rates under wet soil at 20°C and in the strength of the effect of current and antecedent soil moisture on the sensitivity and magnitude of respiration. Respiration increases with temperature below 30°C but declines for temperatures exceeding 35°C. Hot deserts have lower temperature sensitivity than cold deserts, and insensitive or negative temperature sensitivities were predicted under certain moisture conditions that differed among deserts. These results have implications for large-scale modeling efforts because we highlight the unique behavior of desert soil respiration relative to other systems. These behaviors include variable temperature responses and the importance of antecedent moisture conditions for soil respiration.

Denitrification, N-fixation and nitrogen and phosphorus fluxes in different benthic habitats and their contribution to the nitrogen and phosphorus budgets of a shallow oligotrophic sub-tropical coastal system (southern Moreton Bay, Australia)

Abstract: Denitrification, N-fixation, and dissolved inorganic and organic fluxes of nitrogen (N) and phosphorus (P) were measured in each of the major benthic habitat types of a shallow oligotrophic sub-tropical coastal system, and N and P budgets were constructed to quantify the importance of each habitat to N and P cycling in the whole ecosystem. The productivity/respiration (p/r) ratio (trophic status) of the habitats was an important control on the rates, direction (uptake, efflux) and composition (dissolved inorganic N (DIN), dissolved organic N (DON), N2) of N fluxes across the sediment–water interface, with an efflux below p/r = 1.5 and an uptake above p/r = 1.5. The Zostera Seagrass Community was the most important habitat for N loss via net N2 effluxes (denitrification; 48%). Denitrification rates in seagrass were higher than those previously measured in temperate regions, most likely due to greater availability of NH4 + for coupled nitrification–denitrification. Yabby Shoals (sub-tidal shoals inhabited by burrowing shrimp, Trypaea australiensis) accounted for the second largest loss of N via denitrification, the largest recycling of DIN and dissolved inorganic P (DIP; statistically significant only during the dark in summer) across the sediment–water interface and the second largest uptake of DON (statistically significant only in summer). This study highlighted that shallow subtropical coastal systems have a complex mosaic of benthic habitats and that some less ‘iconic’ habitats (i.e. non-seagrass) also make an important functional contribution that controls the flow of N and P through the whole ecosystem.

Influence of hydrological connectivity of riverine wetlands on nitrogen removal via denitrification

Abstract: Wetland ecosystems in agricultural areas often become progressively more isolated from main water bodies. Stagnation favors the accumulation of organic matter as the supply of electron acceptors with water renewal is limited. In this context it is expected that nitrogen recycling prevails over nitrogen dissipation. To test this hypothesis, denitrification rates, fluxes of dissolved oxygen (SOD), inorganic carbon (DIC) and nitrogen and sediment features were measured in winter and summer 2007 on 22 shallow riverine wetlands in the Po River Plain (Northern Italy). Fluxes were determined from incubations of intact cores by measurement of concentration changes or isotope pairing in the case of denitrification. Sampled sites were eutrophic to hypertrophic; 10 were connected and 12 were isolated from the adjacent rivers, resulting in large differences in nitrate concentrations in the water column (from <5 to 1,133 μM). Benthic metabolism and denitrification rates were investigated by two overarching factors: season and hydrological connectivity. SOD and DIC fluxes resulted in respiratory quotients greater than one at most sampling sites. Sediment respiration was coupled to both ammonium efflux, which increased from winter to summer, and nitrate consumption, with higher rates in river-connected wetlands. Denitrification rates measured in river-connected wetlands (35–1,888 μmol N m−2 h−1) were up to two orders of magnitude higher than rates measured in isolated wetlands (2–231 μmol N m−2 h−1), suggesting a strong regulation of the process by nitrate availability. These rates were also significantly higher in summer (9–1,888 μmol N m−2 h−1) than in winter (2–365 μmol N m−2 h−1). Denitrification supported by water column nitrate (DW) accounted for 60–100% of total denitrification (Dtot); denitrification coupled to nitrification (DN) was probably controlled by limited oxygen availability within sediments. Denitrification efficiency, calculated as the ratio between N removal via denitrification and N regeneration, and the relative role of denitrification for organic matter oxidation, were high in connected wetlands but not in isolated sites. This study confirms the importance of restoring hydraulic connectivity of riverine wetlands for the maintenance of important biogeochemical functions such as nitrogen removal via denitrification.

Dissolved organic matter composition in a fragmented Mediterranean fluvial system under severe drought conditions

Abstract: In Mediterranean regions, drought is one of the main factors shaping fluvial ecosystems. Droughts cause a shift from lotic to lentic conditions, triggering a gradual fragmentation of the longitudinal hydrological continuum, and a severe alteration of water chemical properties. However, within a biogeochemical perspective, little is known about how and to which extend droughts modify the chemical properties of dissolved organic matter (DOM). In this study, the variability of DOM properties along a fragmented fluvial system is explored, during a summer severe drought, by means of (a) the ratio between dissolved organic carbon and nitrogen concentrations (DOC:DON); (b) DOC bioavailability (BDOC) and (c) DOM optical properties (SUVA index, fluorescence index, and excitation–emission fluorescence matrices). DOM and water measurements were collected from isolated water parcels that became disconnected from the fluvial continuum at different times, and were compared with data obtained in the following autumn, when the fluvial continuum was re-established. Analysis of DOM chemical properties evidenced that these properties during drought clearly differed from those observed in autumn, but changes did not follow an arbitrary pattern. Thus, the sampling sites with lotic water bodies showed DOM properties similar to those observed in autumn reflecting the dominance of terrestrial inputs. But, once hydrological fragmentation occurred, there was a gradual increase in the contribution of autochthonous DOM as the time elapsed since the pools were established, and the geochemical conditions shifted from oxidized to reduced conditions. In consequence, the fragmentation of fluvial continuum generates a set of distinct biochemical hot spots (i.e., each water parcel), revealing that extreme drought greatly amplifies the qualitative heterogeneity of organic matter in a fluvial system.

Getting the measure of eutrophication in the Baltic Sea: towards improved assessment principles and methods

Abstract: The eutrophication status of the entire Baltic Sea is classified using a multi-metric indicator- based assessment tool. A total of 189 areas are assessed using indicators where information on reference conditions (RefCon), and acceptable devi- ation (AcDev) from reference condition could be combined with national monitoring data from the period 2001-2006. Most areas (176) are classified as 'affected by eutrophication' and only two open water areas and 11 coastal areas are classified as 'unaf- fected by eutrophication'. The classification is made by application of the recently developed HELCOM Eutrophication Assessment Tool (HEAT), which is described in this paper. The use of harmonized

Summer nitrate uptake and denitrification in an upper Mississippi River backwater lake: the role of rooted aquatic vegetation

Abstract: In-stream nitrogen processing in the Mississippi River has been suggested as one mechanism to reduce coastal eutrophication in the Gulf of Mexico. Aquatic macrophytes in river channels and flood plain lakes have the potential to temporarily remove large quantities of nitrogen through assimilation both by themselves and by the attached epiphyton. In addition, rooted macrophytes act as oxygen pumps, creating aerobic microsites around their roots where coupled nitrification–denitrification can occur. We used in situ 15N–NO3 − tracer mesocosm experiments to measure nitrate assimilation rates for macrophytes, epiphyton, and microbial fauna in the sediment in Third Lake, a backwater lake of the upper Mississippi River during June and July 2005. We measured assimilation over a range of nitrate concentrations and estimated a nitrate mass balance for Third Lake. Macrophytes assimilated the most nitrate (29.5 mg N m−2 d−1) followed by sediment microbes (14.4 mg N m−2 d−1) and epiphytes (5.7 mg N m−2 d−1). Assimilation accounted for 6.8% in June and 18.6% in July of total nitrate loss in the control chambers. However, denitrification (292.4 mg N m−2 d−1) is estimated to account for the majority (82%) of the nitrate loss. Assimilation and denitrification rates generally increased with increasing nitrate concentration but denitrification rates plateaued at about 5 mg N L−1. This suggests that backwaters have the potential to remove a relatively high amount of nitrate but will likely become saturated if the load becomes too large.

Effects of aquatic vegetation type on denitrification

Abstract: In a microcosm 15N enrichment experiment we tested the effect of floating vegetation (Lemna sp.) and submerged vegetation (Elodea nuttallii) on denitrification rates, and compared it to systems without macrophytes. Oxygen concentration, and thus photosynthesis, plays an important role in regulating denitrification rates and therefore the experiments were performed under dark as well as under light conditions. Denitrification rates differed widely between treatments, ranging from 2.8 to 20.9 μmol N m−2 h−1, and were strongly affected by the type of macrophytes present. These differences may be explained by the effects of macrophytes on oxygen conditions. Highest denitrification rates were observed under a closed mat of floating macrophytes where oxygen concentrations were low. In the light, denitrification was inhibited by oxygen from photosynthesis by submerged macrophytes, and by benthic algae in the systems without macrophytes. However, in microcosms with floating vegetation there was no effect of light, as the closed mat of floating plants caused permanently dark conditions in the water column. Nitrate removal was dominated by plant uptake rather than denitrification, and did not differ between systems with submerged or floating plants.

Variations in microbial isotopic fractionation during soil organic matter decomposition

Abstract: The soil microbial biomass (SMB) is known to participate in key soil processes such as the decomposition of soil organic matter (SOM) However, its contribution to the isotopic composition of the SOM is not clear yet Shifts in the 13C and 15N natural abundances of the SMB and SOM fractions (mineralised, water soluble and non-extractable) were investigated by incubating an unamended arable soil for 6 months Microbial communities were also studied using Fatty Acid Methyl Ester specific isotope analysis The SMB was significantly 13C and 15N-enriched relative to other fractions throughout the incubation However, significant isotopic variations with time were also observed due to the rapid consumption of relatively 13C-enriched water soluble compounds The increase in the difference in SMB and water soluble 15N compositions as the water soluble C/N ratio decreased, indicated a shift from N assimilation to N dissimilation during the incubation These changes also induced modifications of the microbial community structure Once the system reached a steady-state (after 1 month), the isotopic trends appeared to corroborate those obtained in long term experiments in the field in that there was a constant microbial isotopic fractionation leading to a 13C and 15N enrichment of the SOM over the long-term This work also suggests that caution must be exercised when interpreting short term incubation studies since perturbations associated with experimental set-up can have an important effect on C and N dynamics, microbial fractionation of 13C and 15N and microbial community structure

Spatial and temporal variation in phosphorus budgets for 24 watersheds in the Lake Erie and Lake Michigan basins

Abstract: We estimated net anthropogenic phosphorus inputs (NAPI) to 18 Lake Michigan (LM) and 6 Lake Erie (LE) watersheds for 1974, 1978, 1982, 1987, and 1992. NAPI quantifies all anthropogenic inputs of P (fertilizer use, atmospheric deposition, and detergents) as well as trade of P in food and feed, which can be a net input or output. Fertilizer was the dominant input overall, varying by three orders of magnitude among the 24 watersheds, but detergent was the largest input in the most urbanized watershed. NAPI increased in relation to area of disturbed land (R2 = 0.90) and decreased with forested and wetland area (R2 = 0.90). Export of P by rivers varied with NAPI, especially for the 18 watersheds of LM (R2 = 0.93), whereas the relationship was more variable among the six LE watersheds (R2 = 0.59). On average, rivers of the LE watersheds exported about 10% of NAPI, whereas LM watersheds exported 5% of estimated NAPI. A comparison of our results with others as well as nitrogen (N) budgets suggests that fractional export of P may vary regionally, as has been reported for N, and the proportion of P inputs exported by rivers appears lower than comparable findings with N.

Comparisons of watershed sulfur budgets in southeast Canada and northeast US: new approaches and implications

Abstract: Most of eastern North America receives elevated levels of atmospheric deposition of sulfur (S) that result from anthropogenic SO2 emissions from fossil fuel combustion. Atmospheric S deposi- tion has acidified sensitive terrestrial and aquatic ecosystems in this region; however, deposition has been declining since the 1970s, resulting in some recovery in previously acidified aquatic ecosystems. Accurate watershed S mass balances help to evaluate the extent to which atmospheric S deposition is retained within ecosystems, and whether internal cycling sources and biogeochemical processes may be affecting the rate of recovery from decreasing S atmospheric loads. This study evaluated S mass balances for 15 sites with watersheds in southeastern Canada and northeastern US for the period 1985 to 2002. These 15 sites included nine in Canada (Turkey

Biogeochemical processes along a nutrient gradient in a tropical ombrotrophic peatland

Abstract: Biogeochemical properties, including nutrient concentrations, carbon gas fluxes, microbial biomass, and hydrolytic enzyme activities, were determined along a strong nutrient gradient in an ombrotrophic peatland in the Republic of Panama. Total phosphorus in surface peat decreased markedly along a 2.7 km transect from the marginal Raphia taedigera swamp to the interior sawgrass swamp, with similar trends in total nitrogen and potassium. There were parallel changes in the forest structure: basal area decreased dramatically from the margins to the interior, while tree diversity was greatest at sites with extremely low concentrations of readily-exchangeable phosphate. Soil microbial biomass concentrations declined in parallel with nutrient concentrations, although microbes consistently contained a large proportion (up to 47%) of the total soil phosphorus. Microbial C:P and N:P ratios and hydrolytic enzyme activities, including those involved in the cycles of carbon, nitrogen, and phosphorus, increased towards the nutrient-poor wetland interior, indicating strong below-ground nutrient limitation. Soil CO2 fluxes and CH4 fluxes did not vary systematically along the nutrient gradient, although potential soil respiration determined on drained soils was lower from nutrient-poor sites. Soil respiration responded strongly to drainage and increased temperature. Taken together, our results demonstrate that nutrient status exerts a strong control on above and below-ground processes in tropical peatlands with implications for carbon dynamics and hence long term development of such ecosystems.

Microbial carbon oxidation rates and pathways in sediments of two Tanzanian mangrove forests

Abstract: Temporal and spatial variations in benthic metabolism and anaerobic carbon oxidation pathways were assessed in an anthropogenically impacted (Mtoni) and a pristine (Ras Dege) mangrove forest in Tanzania. The objectives were (1) to evaluate how benthic metabolism is affected by organic carbon availability; (2) to determine the validity of diffusive release of CO2 as a measure benthic carbon oxidation; and (3) to assess the partitioning of anaerobic carbon pathways and factors controlling the availability of electron acceptors (e.g. oxidized iron). Microbial carbon oxidation measured as diffusive exchange of O2 and CO2 (32–67 and 28–115 mmol m−2 day−1, respectively) showed no specific temporal patterns. Low intertidal sediments at Mtoni fed by labile algal carbon of anthropogenic origin had higher diffusive CO2 release than high intertidal sediments that primarily received less reactive mangrove detritus. Diffusive release of CO2 apparently underestimated total sediment carbon oxidation due to CO2 loss from deep sediments via emission through biogenic structures (i.e. crab burrows and pneumatophores) and porewater seepage into creeks. We propose that diffusive fluxes in the present mangrove sediments are roughly equivalent to depth-integrated reactions occurring in the upper 12 cm. Anaerobic carbon oxidation was dominated by FeR irrespective of anthropogenic influence in sediments where the oxidizing effects of biogenic structures increased the Fe(III) level. More than 80% of the anaerobic carbon oxidation in Mtoni and Ras Dege sediments was due to FeR when reactive Fe(III) exceeded 30 μmol cm−3. The anthropogenic influence at Mtoni was primarily noted as up to one order of magnitude higher denitrification than at Ras Dege, but this process always accounted for less than 1% of total carbon oxidation. It is noteworthy that organic and nutrient enrichment of anthropogenic origin in Mtoni has no measurable effect on microbial processes, other than carbon oxidation in the low intertidal area and denitrification throughout the forest, and indicates a strong resilience of mangrove environments towards disturbances.

Comparison of five methods for assessing impacts of nutrient enrichment using estuarine case studies

Abstract: There are several approaches for assessing nutrient enrichment impacts in marine waters including the OSPAR* Comprehensive Procedure, the TRIX* ranking process, the WFD, ASSETS* and EPA NCA*. They differ in definitions and application, but all use key indicators for evaluating eutrophication status. Data from two estuaries in the United Kingdom (UK) were used to test the hypothesis that these five methods would result in the same outcome. The intent is to provide managers with information for selecting an appropriate method, insight about how each method works, how results from different methods compare and a basis for improvement of methods. Results from all approaches indicate that both estuaries required management intervention, but detailed results differed. Methods using more than one biological indicator show that secondary biological impacts were minimal, with the exception of moderate macroalgal problems in the Medway. Comparison of final results was difficult due to differences in: timeframes of data analysis (seasonal versus annual), characteristics included in indicator metrics (concentration, spatial coverage, frequency of occurrence), and methods for combining indicators to determine eutrophication status. This comparison suggests that use of annual data, inclusion frequency of occurrence, spatial coverage and, secondary biological indicators in the index, and a multi-category rating scale results in a more representative assessment.

Thinning affects Pinus sylvestris needle decomposition rates and chemistry differently depending on site conditions

Abstract: Changes in mass and chemical composition of Pinus sylvestris senescent needles were studied over a 5 year period in Mediterranean (MF) and Continental forests (CF) in the Pyrenees under varying levels of thinning (P0: reference, no thinning; P20: removal of 20% basal area, P30: removal of 30% basal area). Decomposition rates were higher in MF (k = 0.423 year−1) than in CF (k = 0.245 year−1). However, the maximum decomposition limit was higher in MF (87.9%) compared to CF (78.1%). The relative importance and timing of rainfall, and cellulose and lignin abundance on the decomposition process was similar among both sites. However, air temperature and degree-days only affected CF (the colder site) during the initial stages of decomposition, while litter moisture was significant only in MF (the drier site) in the latter stages of decomposition. Nutrient and carbon dynamics showed temporal patterns similar to those reported in higher latitudes (except for Ca), however, indicators of N mineralization such as C/N and lignin/N at the study sites were lower than values reported in the literature. Decreases in decomposition rates after thinning were higher in MF than in CF, indicating that this ecosystem could, in the short term, be more sensitive to human intervention. Thinning had similar temporary qualitative effects at both sites, slowing decomposition, increasing N and P immobilization and decreasing Ca immobilization. However, quantitative effects of thinning were site dependent in that the magnitude of nutrient immobilization was higher in CF. A conceptual model is presented to explain effects of thinning on litter N dynamics. These temporary changes are not trivial as nutrient immobilization and accumulated organic matter losses over a thinning cycle may affect tree growth particularly during short rotations and intensive fast-growing plantations. Under similar nutrient availability conditions, sites where nutrient release occurs faster may show higher post-thinning tree growth rates.

Carbonate recrystallization in root-free soil and rhizosphere of Triticum aestivum and Lolium perenne estimated by 14C labeling

Abstract: Under arid and semiarid conditions, pedogenic (secondary) carbonates are formed in soil by precipitation of Ca2+ from soil parent material with dissolved CO2 originating from root and rhizomicrobial respiration. δ13C values of secondary CaCO3 record the photosynthetic pathway of former vegetation and is therefore used as a tool for paleoenvironmental studies. The time scale of pedogenic carbonate formation as well as the influence of several environmental factors are crucial, yet poorly known. We estimated the recrystallization rate of pedogenic carbonate by the 14C isotopic exchange method. 14CO2 was assimilated by plants, respired into the rhizosphere and subsequently incorporated into secondary carbonate by recrystallization of primary loess carbonate. With ascending number of 14CO2 pulses, the amount of rhizosphere 14C recovered in loess CaCO3 increased linearly, leading to recrystallization rates of 3.2 × 10−5 and 2.8 × 10−5 day−1 for wheat and ryegrass, respectively. In loess close to roots, recrystallization rates more than twice as high were obtained. Extrapolating these rates we showed that several hundred years are necessary for complete recrystallization of primary loess CaCO3 in root-free substrate, assuming that both primary and secondary carbonate is recrystallized several times. In contrast, the process probably takes only decades in rhizosphere loess if carbonaceous encrustations form around the root, impeding repeated recrystallization. This indicates the importance of rhizosphere processes (e.g. respiration of roots and microorganisms, exudation) for secondary carbonate formation.

Cross-stream comparison of substrate-specific denitrification potential

Abstract: Headwater streams have a demonstrated ability to denitrify a portion of their nitrate (NO3−) load but there has not been an extensive consideration of where in a stream this process is occurring and how various habitats contribute to total denitrification capability. As part of the Lotic Intersite Nitrogen Experiment II (LINX II) we measured denitrification potential in 65 streams spanning eight regions of the US and draining three land-use types. In each stream, potential denitrification rates were measured in common substrate types found across many streams as well as locations unique to particular streams. Overall, habitats from streams draining urban and agricultural land-uses showed higher potential rates of denitrification than reference streams draining native vegetation. This difference among streams was probably driven by higher ambient nitrate concentrations found in urban or agricultural streams. Within streams, sandy habitats and accumulations of fine benthic organic matter contributed more than half of the total denitrification capacity (mg N removed m−2 h−1). A particular rate of potential denitrification per unit area could be achieved either by high activity per unit organic matter or lower activities associated with larger standing stocks of organic matter. We found that both small patches with high rates (hot spots) or more widespread but less active areas (cool matrix) contributed significantly to whole stream denitrification capacity. Denitrification estimated from scaled-up denitrification enzyme assay (DEA) potentials were not always dramatically higher than in situ rates of denitrification measured as 15N gas generation following 24-h 15N–NO3 tracer additions. In general, headwater streams draining varying land-use types have significant potential to remove nitrate via denitrification and some appear to be functioning near their maximal capacity.

Metabolism of different benthic habitats and their contribution to the carbon budget of a shallow oligotrophic sub-tropical coastal system (southern Moreton Bay, Australia)

Abstract: The major benthic habitats in a shallow oligotrophic sub-tropical coastal system were mapped, benthic productivity and respiration were measured seasonally (summer, winter) in each open water habitat, and an annual carbon budget was constructed using measured, modelled and literature fluxes to estimate the functional importance of each major benthic habitat to the whole ecosystem. Stable Zostera Seagrass Communities covered 16% of the open water system but made little contribution to whole system metabolism. In contrast, ephemeral Halophila Seagrass Communities covered only 8% of the open water system but contributed 46% of the net productivity (p). The less ‘iconic’ Inter- and Sub-tidal Pimpama Shoals also only had a small areal extent (10%) but accounted for 50% of the net benthic production. Similarly, Yabby Shoals only covered 27% of the open water system but accounted for 89% of the net respiration (r). Budget estimates suggest that lateral import of organic matter, most likely tidally transported phytoplankton trapped in seagrass beds, across the Broadwater boundaries was required to balance the carbon budget if any reasonable estimate of burial was invoked. However, budget errors make it difficult to distinguish this import from zero. This study demonstrated that shallow subtropical coastal systems have a complex mosaic of benthic habitats, and that some of the less ‘iconic’ habitats (i.e. non-seagrass, non-mangrove) also make an important functional contribution that controls the flow of energy and nutrients through the whole ecosystem and determines the net ecosystem metabolism and possible exchanges with adjacent systems.