Showing papers in "Biogeochemistry in 2003"

Groundwater and pore water inputs to the coastal zone

Abstract: Both terrestrial and marine forces drive underground fluid flows in the coastal zone. Hydraulic gradients on land result in groundwater seepage near shore and may contribute to flows further out on the shelf from confined aquifers. Marine processes such as tidal pumping and current-induced pressure gradients may induce interfacial fluid flow anywhere on the shelf where permeable sediments are present. The terrestrial and oceanic forces overlap spatially so measured fluid advection through coastal sediments may be a result of composite forcing. We thus define “submarine groundwater discharge” (SGD) as any and all flow of water on continental margins from the seabed to the coastal ocean, regardless of fluid composition or driving force. SGD is typically characterized by low specific flow rates that make detection and quantification difficult. However, because such flows occur over very large areas, the total flux is significant. Discharging fluids, whether derived from land or composed of re-circulated seawater, will react with sediment components. These reactions may increase substantially the concentrations of nutrients, carbon, and metals in the fluids. These fluids are thus a source of biogeochemically important constituents to the coastal ocean. Terrestrially-derived fluids represent a pathway for new material fluxes to the coastal zone. This may result in diffuse pollution in areas where contaminated groundwaters occur. This paper presents an historical context of SGD studies, defines the process in a form that is consistent with our current understanding of the driving forces as well as our assessment techniques, and reviews the estimated global fluxes and biogeochemical implications. We conclude that to fully characterize marine geochemical budgets, one must give due consideration to SGD. New methodologies, technologies, and modeling approaches are required to discriminate among the various forces that drive SGD and to evaluate these fluxes more precisely.

Nitrogen vs. phosphorus limitation across an ecotonal gradient in a mangrove forest

Abstract: Mangrove forests are characterized by distinctive tree-height gradientsthat reflect complex spatial, within-stand differences in environmentalfactors,including nutrient dynamics, salinity, and tidal inundation, across narrowgradients. To determine patterns of nutrient limitation and the effects ofnutrient availability on plant growth and within-stand nutrient dynamics, weused a factorial experiment with three nutrient treatment levels (control, N,P)and three zones along a tree-height gradient (fringe, transition, dwarf) onoffshore islands in Belize. Transects were laid out perpendicular to theshoreline across a mangrove forest from a fringe stand along the seaward edge,through a stand of intermediate height, into a dwarf stand in the interior ofthe island. At three sites, three trees were fertilized per zone for 2yr. Although there was spatial variability in response, growth byR. mangle was generally nitrogen (N) -limited in thefringe zone;phosphorus (P) -limited in the dwarf zone; and, N- and/or P-limited in thetransition zone. Phosphorus-resorption efficiency decreased in all three zones,and N-resorption efficiency increased in the dwarf zone in response to Penrichment. The addition of N had no effect on either P or N resorptionefficiencies. Belowground decomposition was increased by P enrichment in allzones, whereas N enrichment had no effect. This study demonstrated thatessential nutrients are not uniformly distributed within mangrove ecosystems;that soil fertility can switch from conditions of N to P limitation acrossnarrow ecotonal gradients; and, that not all ecological processes respondsimilarly to, or are limited by, the same nutrient.

Global patterns of dissolved N, P and Si in large rivers

Abstract: The concentration of dissolved inorganic nitrogen (DIN), dissolved nitrate-N, Total-N (TN), dissolved inorganic phosphate (DIP), total phosphorus (TP), dissolved silicate-Si (DSi) and their ratios in the world's largest rivers are examined using a global data base that includes 37% of the earth's watershed area and half its population. These data were compared to water quality in 42 subbasins of the relatively well-monitored Mississippi River basin (MRB) and of 82 small watersheds of the United States. The average total nitrogen concentration varies over three orders of magnitude among both world river watersheds and the MRB, and is primarily dependent on variations in dissolved nitrate concentration, rather than particulate or dissolved organic matter or ammonium. There is also a direct relationship between the DIN:DIP ratio and nitrate concentration. When nitrate-N exceeds 100 μg-at l−1, the DIN:DIP ratio is generally above the Redfield ratio (16:1), which implies phosphorus limitation of phytoplankton growth. Compared to nitrate, the among river variation in the DSi concentration is relatively small so that the DSi loading (mass/area/time) is largely controlled by runoff volume. The well-documented influence of human activities on dissolved inorganic nitrogen loading thus exceeds the influences arising from the great variability in soil types, climate and geography among these watersheds. The DSi:nitrate-N ratio is controlled primarily by nitrogen loading and is shown to be inversely correlated with an index of landscape development – the “City Lights” nighttime imagery. Increased nitrogen loading is thus driving the world's largest rivers towards a higher DIN:DIP ratio and a lower DSi:DIN ratio. About 7.3 and 21 % of the world's population lives in watersheds with a DSi:nitrate-N ratio near a 1:1 and 2:1 ratio, respectively. The empirical evidence is that this percentage will increase with further economic development. When the DSi:nitrate-N atomic ratio is near 1:1, aquatic food webs leading from diatoms (which require silicate) to fish may be compromised and the frequency or size of harmful or noxious algal blooms may increase. Used together, the DSi:nitrate-N ratio and nitrate-N concentration are useful and robust comparative indicators of eutrophication in large rivers. Finally, we estimate the riverine loading to the ocean for nitrate-N, TN, DIP, TP and DSi to be 16.2, 21, 2.6, 3.7 to 5.6, and 194 Tg yr−1, respectively.

Nutrients in the Changjiang and its tributaries

Abstract: Dissolved and particulate, organic and inorganic N, P and Si weremeasured in the main stream and 15 major tributaries of the Changjiang (YangtzeRiver) in April–May 1997. The nutrient concentrations are related towaterdischarge, suspended particulate matter, anthropogenic activities etc. Thenutrient levels were quite low in the upper reaches, and significantlyincreasedin the main stream in a region of 2000–3000 km inland from the rivermouth. The northern tributaries contribute more nutrients to the Changjiangthanthe southern tributaries. Based on atomic ratios of N, P and Si, the limitingnutrient in the Changjiang drainage basin was P. The nutrient yields in theChangjiang and its major tributaries indicated high rates of transport ofnutrients within the watersheds. Concentrations of nitrate in the Changjianghave increased, but there have been no systematic trends for phosphate andsilicic acid since 1980. The DIN/P ratios and DIN/Si ratios increased. TheDIN/Pand DIN/Si ratios may be expected to continue to increase after construction ofthe “Three Gorges Dam”, which will exercise a great deal ofinfluence on theecological environment of the Changjiang estuary and its adjacent sea.

Controls of streamwater dissolved inorganic carbon dynamics in a forested watershed

Abstract: Iinvestigated controls of stream dissolved inorganic carbon (DIC) sources andcycling along a stream size and productivity gradient in a temperate forestedwatershed in northern California. Dissolved CO2 (CO2(aq))dynamics in heavily shaded streams contrasted strongly with those of larger,open canopied sites. In streams with canopy cover > 97%, CO2 (aq)was highest during baseflow periods (up to 540 μM) and wasnegatively related to discharge. Effects of algal photosynthesis on CO2(aq) were minimal and stream CO2 (aq) was primarily controlledby groundwater CO2 (aq) inputs and degassing losses to theatmosphere. In contrast to the small streams, CO2 (aq) in larger,open-canopied streams was often below atmospheric levels at midday duringbaseflow and was positively related to discharge. Here, stream CO2(aq) was strongly influenced by the balance between autotrophic andheterotrophic processes. Dynamics of HCO3 − werelesscomplex. HCO3 − and Ca2+ were positivelycorrelated, negatively related to discharge, and showed no pattern with streamsize. Stable carbon isotope ratios of DIC (i.e. δ13C DIC)increased with stream size and discharge, indicating contrasting sources of DICto streams and rivers. During summer baseflows, δ13C DIC were13C-depleted in the smallest streams (minimum of−17.7‰) due to the influence of CO2 (aq) derived frommicrobialrespiration and HCO3 − derived from carbonateweathering. δ13C DIC were higher (up to −6.6‰)inthe larger streams and rivers due to invasion of atmospheric CO2enhanced by algal CO2 (aq) uptake. While small streams wereinfluenced by groundwater inputs, patterns in CO2 (aq) and evidencefrom stable isotopes demonstrate the strong influence of stream metabolism andCO2 exchange with the atmosphere on stream and river carbon cycles.

Distribution of mercury, methyl mercury and organic sulphur species in soil, soil solution and stream of a boreal forest catchment

Abstract: Concentrations of methyl mercury, CH3Hg (II), total mercury, Hgtot = CH3Hg (II) + Hg (II), and organic sulphur species were determined in soils, soil solutions and streams of a small (50 ha) boreal forest catchment in northern Sweden. The CH3Hg (II)/Hgtot ratio decreased from 1.2–17.2% in the peaty stream bank soils to 0.4–0.8% in mineral and peat soils 20 m away from the streams, indicating that conditions for net methylation of Hg (II) are most favourable in the riparian zone close to streams. Concentrations of CH3Hg (II) bound in soil and in soil solution were significantly, positively correlated to the concentration of Hgtot in soil solution. This, and the fact that the CH3Hg (II)/Hgtot ratio was higher in soil solution than in soil may indicate that Hg (II) in soil solution is more available for methylation processes than soil bound Hg (II). Reduced organic S functional groups (Org-SRED) in soil, soil extract and in samples of organic substances from streams were quantified using S K-edge X-ray absorption near-edge structure (XANES) spectroscopy. Org-SRED, likely representing RSH, RSSH, RSR and RSSR functionalities, made up 50 to 78% of total S in all samples examined. Inorganic sulphide [e.g. FeS2 (s)] was only detected in one soil sample out of 10, and in none of the stream samples. Model calculations showed that under oxic conditions nearly 100% of Hg (II) and CH3Hg (II) were complexed by thiol groups (RSH) in the soil, soil solution and in the stream water. Concentrations of free CH3Hg+ and Hg2+ ions in soil solution and stream were on the order of 10−18 and 10−32M, respectively, at pH 5. For CH3Hg (II), inorganic bi-sulphide complexes may contribute to an overall solubility at concentrations of inorganic sulphides higher than 10−9M, whereas considerably higher concentrations of inorganic sulphides (lower redox-potential) are required to increase the solubility of Hg (II).

Isotopic composition of nitrate-nitrogen as a marker of riparian and benthic denitrification at the scale of the whole Seine River system

Abstract: Nitrogen budgets established for large river systems reveal that up to 60% of the nitrate exported from agricultural soils is eliminated, either when crossing riparian wetlands areas before even reaching surface waters, or within the rivers themselves through benthic denitrification. The study of nitrogen isotope ratios of riverine nitrates could offer an elegant means to assess the extent of denitrification and thus confirm these budgets, as it is known that denitrification results in a natural 15N enrichment of residual nitrates. The results reported here, for the Seine river system (France), demonstrate the feasibility of this isotopic approach at the scale of large watersheds. On the basis of in situ observations carried out in a large storage reservoir in the upstream Seine catchment (Der Lake), where intensive benthic denitrification occurs, as well as on the basis of laboratory experiments of denitrification under controlled conditions, it is shown that the isotopic discrimination associated with benthic denitrification is minimal (e of NO3-N ranging from −1.5 to −3.6‰), probably because the rate-limiting step of the process consists of nitrate diffusion through the water-sediment interface. Riparian denitrification on the contrary, when it implies nitrate reduction during convective transfer through reducing environements, causes a much more significant isotopic enrichment of 15N of residual nitrate (e about −18‰). The authors report measurements of nitrogen isotopic composition of nitrate from rivers of various stream orders in the Seine river system under summer conditions. Anomalies in the data with respect to the values expected from the mixture of the various sources of nitrate are here attributed to riparian denitrification. However, the authors show that because of the patchy distribution of actively denitrifying riparian zones within the drainage network, the isotopic signature conferred to residual nitrate in river water intrinsically provides only a minimum estimate of the extent of denitrification.

Long term records of riverine dissolved organic matter

Abstract: This presents the longest, consistent records of dissolved organic carbon in rivers ever published. This study presents long-term records of organic matter as indicated by water colour that were constructed for three catchments in Northern England for as far back as 1962. Observations show that there have been large increases in DOC concentrations over the period of study with in one case a doubling of the concentration over a period of 29 years. However, in one of the catchments no significant change was observed over a 31-year period. All catchments show common inter-annual control on carbon release in response to droughts, but no step increases in DOC concentrations were observed in response to such perturbations with pre-drought levels being restored within a period 3–4 years. Observed increasing trends do not correlate with changes in river discharge, pH, alkalinity or rainfall, but do coincide with increasing average summer temperatures in the region. The times series of DOC concentration over the period of the record appears stationary, but the distribution of daily values suggests a change in sources of colour over the increasing trend. The evidence supports a view that increases in carbon release are in equilibrium with temperature increases accentuated by land-use factors.

Modelling the production and transport of dissolved organic carbon in forest soils

Abstract: DyDOC describes soil carbon dynamics, with a focus on dissolved organic carbon (DOC). The model treats the soil as a three-horizon profile, and simulates metabolic carbon transformations, sorption reactions and water transport. Humic substances are partitioned into three fractions, one of which is immobile, while the other two (hydrophilic and hydrophobic) can pass into solution as DOC. DyDOC requires site-specific soil characteristics, and is driven by inputs of litter and water, and air and soil temperatures. The model operates on hourly and daily time steps, and can simulate carbon cycling over both long (hundreds-to-thousands of years) and short (daily) time scales. An important feature of DyDOC is the tracking of 14C, from its entry in litter to its loss as DO14C in drainage water, enabling information about C dynamics to be obtained from both long-term radioactive decay, and the characteristic 14C pulse caused by thermonuclear weapon testing during the 1960s ("bomb carbon"). Parameterisation is performed by assuming a current steady state. Values of a range of variables, including C pools, annual DOC fluxes, and 14C signals, are combined into objective functions for least-squares minimisation. DyDOC has been applied successfully to spruce forest sites at Birkenes (Norway) and Waldstein (Germany), and most of the parameters have similar values at the two sites. The results indicate that the supply of DOC from the surface soil horizon to percolating water depends upon the continual metabolic production of easily leached humic material. In contrast, concentrations and fluxes of DOC in the deeper soil horizons are controlled by sorption processes, involving comparatively large pools of leachable organic matter. Times to reach steady state are calculated to be several hundred years in the organic layer, and hundreds-to-thousands of years in the deeper mineral layers. It is estimated that DOC supplies 89% of the mineral soil carbon at Birkenes, and 73% at Waldstein. The model, parameterised with "steady state" data, simulates short-term variations in DOC concentrations and fluxes, and in DO14C, which are in approximate agreement with observations.

Sources and fluxes of submarine groundwater discharge delineated by radium isotopes

Abstract: This paper reports the results derived fromradium isotopes of a submarine groundwaterdischarge (SGD) intercomparison in thenortheast Gulf of Mexico. Radium isotopesamples were collected from seepage meters,piezometers, and surface and deep ocean waters.Samples collected within the near-shore SGDexperimental area were highly enriched in allfour radium isotopes; offshore samples wereselectively enriched. Samples collected fromseepage meters were about a factor of 2–3higher in radium activity compared to theoverlying waters. Samples from piezometers,which sampled 1–4 meters below the sea bed were1–2 orders of magnitude higher in radiumisotopes than surface waters. The twolong-lived Ra isotopes, 228Ra and226Ra, provide convincing evidence thatthere are two sources of SGD to the study area:shallow seepage from the surficial aquifer andinput from a deeper aquifer. A three end-membermixing model can describe the Ra distributionin these samples. The short-lived radium isotopes, 223Ra and224Ra, were used to establish mixing ratesfor the near-shore study area. Mixing wasretarded within 3 km of shore due to a strongsalinity gradient. The product of the mixingrate and the offshore 226Ra gradientestablished the 226Ra flux. This flux mustbe balanced by Ra input from SGD. The flux ofSGD within 200 m of shore based on the226Ra budget was 1.5 m3 min−1.This flux agreed well with other estimatesbased on seepage meters and 222Rn.

Enumeration of Fe(II)-oxidizing and Fe(III)-reducing bacteria in the root zone of wetland plants: Implications for a rhizosphere iron cycle

Abstract: Iron plaque occurs on the roots of most wetland and submersed aquatic plant species and is a large pool of oxidized Fe(III) in some environments. Because plaque formation in wetlands with circumneutral pH has been largely assumed to be an abiotic process, no systematic effort has been made to describe plaque-associated microbial communities or their role in plaque deposition. We hypothesized that Fe(II)-oxidizing bacteria (FeOB) and Fe(III)-reducing bacteria (FeRB) are abundant in the rhizosphere of wetland plants across a wide range of biogeochemical environments. In a survey of 13 wetland and aquatic habitats in the Mid-Atlantic region, FeOB were present in the rhizosphere of 92% of the plant specimens collected (n = 37), representing 25 plant species. In a subsequent study at six of these sites, bacterial abundances were determined in the rhizosphere and bulk soil using the most probable number technique. The soil had significantly more total bacteria than the roots on a dry mass basis (1.4 × 109 cells/g soil vs. 8.6 × 107 cells/g root; p < 0.05). The absolute abundance of aerobic, lithotrophic FeOB was higher in the soil than in the rhizosphere (3.7 × 106/g soil vs. 5.9 × 105/g root; p < 0.05), but there was no statistical difference between these habitats in terms of relative abundance (∼1% of the total cell number). In the rhizosphere, FeRB accounted for an average of 12% of all bacterial cells while in the soil they accounted for < 1% of the total bacteria. We concluded that FeOB are ubiquitous and abundant in wetland ecosystems, and that FeRB are dominant members of the rhizosphere microbial community. These observations provide a strong rationale for quantifying the contribution of FeOB to rhizosphere Fe(II) oxidation rates, and investigating the combined role of FeOB and FeRB in a rhizosphere iron cycle.

Patterns and mechanisms of soil acidification in the conversion of grasslands to forests

Abstract: Grassland to forest conversions currently affect some of the world's most productive regions and have the potential to modify many soil properties. We used afforestation of native temperate humid grassland in the Pampas with eucalypts as an experimental system to 1) isolate forest and grassland imprints on soil acidity and base cation cycling and 2) evaluate the mechanisms of soil acidification. We characterized soil changes with afforestation using ten paired stands of native grasslands and Eucalyptus plantations (10–100 years of age). Compared to grasslands, afforested stands had lower soil pH (4.6 vs.5.6, p < 0.0001) and ∼40% lower exchangeable Ca (p < 0.001) in the top 20 cm of the soil. At three afforested stands where we further characterized soil changes to one meter depth, soil became increasingly acidic from 5 to 35 cm depth but more alkaline below ∼60 cm compared to adjacent grasslands, with few differences observed between 35 and 60 cm. These changes corresponded with gains of exchangeable acidity and Na in intermediate and deeper soil layers. Inferred ecosystem cation balances (biomass + forest floor + first meter of mineral soil) revealed substantial vertical redistributions of Ca and Mn and a tripling of Na pools within the mineral soil after afforestation. Soil exchangeable acidity increased 0.5–1.2 kmolc.Ha−1.yr−1 across afforested stands, although no aboveground acidic inputs were detected in wet + dry deposition, throughfall and forest floor leachates. Our results suggest that cation cycling and redistribution by trees, rather than cation leaching by organic acids or enhanced carbonic acid production in the soil, is the dominant mechanism of acidification in this system. The magnitude of soil changes that we observed within half a century of tree establishment in the Pampas emphasizes the rapid influence of vegetation on soil formation and suggests that massive afforestation of grasslands for carbon sequestration could have important consequences for soil fertility and base cation cycles.

Factors controlling concentration, export, and decomposition of dissolved organic nutrients in the Everglades of Florida

Abstract: Water draining from the Everglades marshes of southern Florida containshigh concentrations of dissolved organic C (DOC), N (DON), and in somelocations, P (DOP). These dissolved organic nutrients carry over 90% of the Nand organic C, and about 25% of the P exported downstream in the Everglades.Ourobjectives were to describe the most important aspects of the origin and fateofdissolved organic matter (DOM) in the Everglades, and to describe the processescontrolling its concentration and export. Concentrations of dissolved organicnutrients are influenced by local plant production, decomposition, and sorptionequilibrium with peat. The drained peat soils of the Everglades AgriculturalArea and the more productive marshes of the northern Everglades produce some ofthe highest concentrations of DOC and DON in the Everglades watershed. Inportions of the marshes of the northern Everglades, P enrichment was correlatedwith higher local DOC and DON concentrations and greater production of solubleplant matter. Microbial degradation of Everglades DOM was very slow; less than10% of the DOC was lost after 6 months of incubation in the laboratory andsupplements of inorganic nutrients failed to speed the decomposition. Exposureto solar radiation increased the subsequent decay rate of the remaining DOC(25%in 6 mo.). Solar radiation alone mineralized 20.5% of the DOC, 7%of the DON, and degraded about 50% of the humic substances over 21 days insterile porewater samples and thus degraded DOM faster than microbialdegradation. The humic substances appeared to inhibit biodegradation of theother fractions of the DOC since hydrophilic organic acids decomposed fasterwhen isolated from the humic substances.The fate of DOC and DON is closely linked as indicated by a generally narrowrange of C/N ratios. In contrast, high concentrations of DOP were associatedwith P enrichment (at least in pore water). The DOC was composed of about 50%humic substances, 33% hydrophilic acids, and 15% hydrophilic neutralsubstances,typical of DOC from other environments, despite the fact that it originatesfroma neutral to slightly alkaline peatland. Despite high exports of DON (3.9g m−2 y−1 from one area), themarshes of the northern Everglades are a sink for DON on a landscape scale. Theagricultural fields of the Everglades Agricultural Area, however, exported netquantities of DON. High concentrations of DOC desorbed from the agriculturalsoils when water with no DOC was added. Sorption experiments indicated thathighconcentrations of dissolved organic matter flowing into the marshes from theEverglades Agricultural Area could suppress the further desorption ofadditionalsoluble organic matter through physicochemical mechanisms. While biologicalfactors, plant production and microbial decomposition are important inproducingpotentially soluble organic nutrients, physicochemical sorption equilibria,hydrology, and degradation by solar radiation are also likely to control theexport of this material on the landscape scale.

Spatial and temporal variation in soil CO2 efflux in an old-growth neotropical rain forest, La Selva, Costa Rica

Abstract: Our objectives were to quantify and compare soil CO2 efflux of two dominant soil types in an old-growth neotropical rain forest in the Atlantic zone of Costa Rica, and to evaluate the control of environmental factors on CO2 release We measured soil CO2 efflux from eight permanent soil chambers on six Oxisol sites Three sites were developed on old river terraces (‘old alluvium’) and the other three were developed on old lava flows (‘residual’) At the same time we measured soil CO2 concentrations, soil water content and soil temperature at various depths in 6 soil shafts (3 m deep) Between ‘old alluvium’ sites, the two-year average CO2 flux rates ranged from 1173 to 1289 mg C m−2 h−1 Significantly higher soil CO2 flux occurred on the ‘residual’ sites (1411 to 1842 mg C m−2 h−1) Spatial differences in CO2 efflux were related to fine root biomass, soil carbon and phosphorus concentration but also to soil water content Spatial variability in CO2 storage was high and the amount of CO2 stored in the upper and lower soil profile was different between ‘old alluvial’ and ‘residual’ sites The major factor identified for explaining temporal variations in soil CO2 efflux was soil water content During periods of high soil water content CO2 emission decreased, probably due to lower diffusion and CO2 production rates During the 2-year study period inter-annual variation in soil CO2 efflux was not detected

Biotic controls on CO2 and CH4 exchange in wetlands – a closed environment study

Abstract: Wetlands are significant sources of the important greenhouse gas CH4. Here we explore the use of an experimental system developed for the determination of continuous fluxes of CO2 and CH4 in closed ecosystem monoliths including the capture of (CO2)-C-14 and (CH4)-C-14 following pulse labelling with (CO2)-C-14. We show that, in the ecosystem studied, ebullition (bubble emission) may account for 18 to 50% of the total CH4 emission, representing fluxes that have been difficult to estimate accurately in the past. Furthermore, using plant removal and C-14 labelling techniques, we use the system to detail the direct influence of vascular plants on CH4 emission. This influence is observed to be dependent on the amount of vascular plants present. The results that may be produced using the presented experimental set-up have implications for an improved understanding of wetland ecosystem/atmosphere interactions, including possible feedback effects on climate change. In recent years much attention has been devoted to ascertaining and subsequently using the relationship between net ecosystem productivity and CH4 emission as a basis for extrapolation of fluxes across large areas. The experimental system presented may be used to study the complex relationship between vascular plants and CH4 emission and here we show examples of how this may vary considerably in nature between and even within ecosystems.

Isotope variability of particulate organic matter at the PN section in the East China Sea

Abstract: The source of particulate organic matter at the PN section in the East China Sea has been evaluated using stable carbon and nitrogen isotopes. The results showed that the δ13C and δ15N compositions varied from −19 to −31‰ and 0.7–9.5‰ respectively, and the isotope compositions were statistically distinct, enabling, by use of a simple components mixing equations, assessment of the ability of each tracer to estimate the terrestrial, Kuroshio Water, marine and remineralized sources' contributions. The dominance of terrestrial inputs of the Changjiang could be observed 250 km far from the river mouth in the East China Sea. In the shelf water column, the remineralization of biogenic organic matter becomes an important source except for the terrigenous and marine sources. The estimation of sources recorded by δ13C data was partly confirmed by equivalent δ15N and C/N compositions that reflected greater control by organic matter diagenesis and biological processing. However, the lighter contribution of δ13C data of the Kuroshio samples also indicates the alteration of the isotope values by microbial or other processes. The net export flux of POC in the PN section is estimated to be 4.1 kmol C/s and the annual export is 129 Gmol C/yr, which is account for 20% of the East China Sea.

Sulfur cycling and seagrass (Posidonia oceanica) status in carbonate sediments

Abstract: Sulfur cycling was investigated in carbonate-rich and iron-poor sediments vegetated with Posidonia oceanica in oligotrophic Mediterranean around Mallorca Island, Spain, to quantify sulfate reduction and pools of sulfide in seagrass sediments. The oxygen penetration depth was low (< 4.5 mm) and sulfate reduction rates were relatively high (0.7–12 mmol m−2d−1). The total pools of reduced sulfides were remarkably low (< 5 mol S m−2) indicating a fast turnover of reduced sulfides in these iron-poor sediments. The sulfate reduction rates were generally higher in vegetated compared to bare sediments possible due to enhanced sedimentation of sestonic material inside the seagrass meadows. The sulfate reduction rates were positively correlated with the seasonal variation in water temperature and negatively correlated with the shoot density indicating that the microbial activity was controlled by temperature and release of oxygen from the roots. The pools of reduced sulfides were low in these iron-poor sediments leading to high oxygen consumption for reoxidation. The sediments were highly anoxic as shown by relatively low oxygen penetration depths (< 4.5 mm) in these low organic sediments. The net shoot recruitment rate was negative in sediments enriched with organic matter, suggesting that organic matter enrichment may be an important factor for seagrass status in these iron-depleted carbonate sediments.

Spatial and temporal distributions of submarine groundwater discharge rates obtained from various types of seepage meters at a site in the Northeastern Gulf of Mexico

Abstract: Direct measurements of submarine groundwaterdischarge (SGD) were taken by three different(continuous heat, heat pulse, and ultrasonic)types of automated seepage meters as well asstandard Lee-type manually operated meters. SGD flux comparisons and the spatial andtemporal variations in groundwater flow wereanalyzed. Seepage rates measured by thedifferent meters agree relatively well witheach other (more than 80% agreement). Comparisons of flux rates as a function ofdistance offshore using exponentialapproximations show that more than fivemeasurement locations (200 m offshore) areneeded for a precise integrated estimation ofSGD offshore within an accuracy of ±10%. Thedominant period of seepage variations isestimated to be about 12 hours, which closelymatches the semidiurnal tides in this area. Our analysis also shows that short durationmeasurement periods may cause significantunderestimates or overestimates of the dailyaveraged groundwater flow rates (±25%–±60% difference when the measurement durationis less than 12 hours). Thus, continuousmeasurements of SGD using automated seepagemeters with high time resolution should enableus to evaluate temporal and spatial variationsof dissolved material transports viagroundwater pathways. Such inputs may affectbiogeochemical phenomena in the coastal zone.

Seasonal oscillation of microbial iron and sulfate reduction in saltmarsh sediments (Sapelo Island, GA, USA)

Abstract: Seasonal variations in anaerobic respiration pathways were investigated at three saltmarsh sites using chemical data, sulfate reduction rate measurements, enumerations of culturable populations of anaerobic iron-reducing bacteria (FeRB), and quantification of in situ 16S rRNA hybridization signals targeted for sulfate-reducing bacteria (SRB). Bacterial sulfate reduction in the sediments followed seasonal changes in temperature and primary production of the saltmarsh, with activity levels lowest in winter and highest in summer. In contrast, a dramatic decrease in the FeRB population size was observed during summer at all sites. The collapse of FeRB populations during summer was ascribed to high rates of sulfide production by SRB, resulting in abiotic reduction of bioavailable Fe(III) (hydr)oxides. To test this hypothesis, sediment slurry incubations at 10, 20 and 30 °C were carried out. Increases in temperature and labile organic carbon availability (acetate or lactate additions) increased rates of sulfate reduction while decreasing the abundance of culturable anaerobic FeRB. These trends were not reversed by the addition of amorphous Fe(III) (hydr)oxides to the slurries. However, when sulfate reduction was inhibited by molybdate, no decline in FeRB growth was observed with increasing temperature. Addition of dissolved sulfide adversely impacted propagation of FeRB whether molybdate was added or not. Both field and laboratory data therefore support a sulfide-mediated limitation of microbial iron respiration by SRB. When total sediment respiration rates reach their highest levels during summer, SRB force a decline in the FeRB populations. As sulfate reduction activity slows down after the summer, the FeRB are able to recover.

Controls on the release of dissolved organic carbon and nitrogen from a deciduous forest floor investigated by manipulations of aboveground litter inputs and water flux

Abstract: Although dissolved organic matter (DOM) released from the forest floor plays a crucial role in transporting carbon and major nutrients through the soil profile, its formation and responses to changing litter inputs are only partially understood. To gain insights into the controlling mechanisms of DOM release from the forest floor, we investigated responses of the concentrations and fluxes of dissolved organic carbon (DOC) and nitrogen (DON) in forest floor leachates to manipulations of throughfall (TF) flow and aboveground litter inputs (litter removal, litter addition, and glucose addition) at a hardwood stand in Bavaria, Germany. Over the two-year study period, litter manipulations resulted in significant changes in C and N stocks of the uppermost organic horizon (Oi). DOC and DON losses via forest floor leaching represented 8 and 11% of annual litterfall C and N inputs at the control, respectively. The exclusion of aboveground litter inputs caused a slight decrease in DOC release from the Oi horizon but no change in the overall leaching losses of DOC and DON in forest floor leachates. In contrast, the addition of litter or glucose increased the release of DOC and DON either from the Oi or from the lower horizons (Oe + Oa). Net releases of DOC from the Oe + Oa horizons over the entire manipulation period were not related to changes in microbial activity (measured as rates of basal and substrate-induced respiration) but to the original forest floor depths prior to manipulation, pointing to the flux control by the size of source pools rather than a straightforward relationship between microbial activity and DOM production. In response to doubled TF fluxes, net increases in DOM fluxes occurred in the lower forest floor, indicating the presence of substantial pools of potentially soluble organic matter in the Oe + Oa horizons. In contrast to the general assumption of DOM as a leaching product from recent litter, our results suggest that DOM in forest floor leachates is derived from both newly added litter and older organic horizons through complex interactions between microbial production and consumption and hydrologic transport.

Biogeochemistry of unpolluted forested watersheds in the Oregon Cascades: temporal patterns of precipitation and stream nitrogen fluxes

Abstract: We analyzed long-term organic and inorganic nitrogen inputs and outputs in precipitation and streamwater in six watersheds at the H.J. Andrews Experimental Forest in the central Cascade Mountains of Oregon. Total bulk N deposition, averaging 1.6 to 2.0 kg N ha−1 yr−1, is low compared to other sites in the United States and little influenced by anthropogenic N sources. Streamwater N export is also low, averaging <1 kg ha−1 yr−1. DON is the predominant form of N exported from all watersheds, followed by PON, NH4-N, and NO3-N. Total annual stream discharge was a positive predictor of annual DON output in all six watersheds, suggesting that DON export is related to regional precipitation. In contrast, annual discharge was a positive predictor of annual NO3-N output in one watershed, annual NH4-N output in three watersheds, and annual PON output in three watersheds. Of the four forms of N, only DON had consistent seasonal concentration patterns in all watersheds. Peak streamwater DON concentrations occurred in November-December after the onset of fall rains but before the peak in the hydrograph, probably due to flushing of products of decomposition that had built up during the dry summer. Multiple biotic controls on the more labile nitrate and ammonium concentrations in streams may obscure temporal DIN flux patterns from the terrestrial environment. Results from this study underscore the value of using several watersheds from a single climatic zone to make inferences about controls on stream N chemistry; analysis of a single watershed may preclude identification of geographically extensive mechanisms controlling N dynamics.

Emission of gaseous nitrogen oxides from an extensively managed grassland in NE Bavaria, Germany - II. Stable isotope natural abundance of N2O

Abstract: In a one-year field study (June 1998 to April 1999), we quantified N2O and NOx emissions from an extensively managed grassland in NE Bavaria (Germany) in unfertilized controls and after application of slurry or mineral N (calcium ammonium nitrate), respectively Emissions were measured every 2–4 weeks, with additional daily measurements for 10 days after each fertilizer application The closed chamber method was used for N2O and the open chamber method for NOx measurements Fertilizer applications resulted in significantly increased N2O emission rates in comparison to the low annual mean of the control plots (14 μmol m−2 h−1) Episodical emission peaks during the summer were attributed to high microbial activity after rainfall Mineral N fertilization resulted in the highest emission rates Cumulative annual N2O emissions were 112 kg N ha−1 a−1 for the mineral N, 88 kg N ha−1 a−1 for the slurry and 34 kg N ha−1 a−1 for the control plots This represents 105% (mineral N) and 72% (slurry) of the applied nitrogen The fertilizer-induced N2O emission factors on this extensively managed grassland are high in comparison to emission factors on intensively managed grassland and substantially higher than the 15% estimate used by the global emission inventory NOx emissions increased after the first fertilizer application in summer, but not after the two following fertilizations in fall and spring Differences between treatments were not significant Annual NOx emissions were 19 kg N ha−1 a−1 for both, mineral N and slurry plots and 15 kg N ha−1 a−1 for the controls, representing 05% of the N applied with each fertilizer The ratio of emitted NOx to N2O was 1:47 for both fertilized treatments (based on N-atoms)

Rates and regulation of microbial iron reduction in sediments of the Baltic-North Sea transition

Abstract: The rates and pathways of anaerobic carbon mineralization processes were investigated at seven stations, ranging from 10 to 56 m water depth, in the Kattegat and Belt Sea, Denmark Organic carbon mineralization coupled to microbial Mn and Fe reduction was quantified using anaerobic sediment incubation at two stations that were widely separated geographically within the study area Fe reduction accounted for 75% of the anaerobic carbon oxidation at the station in the northern Kattegat, which is the highest percentage so far reported from subtidal marine sediment By contrast, sulfate reduction was the dominant anaerobic respiration pathway (95%) at the station in the Great Belt Dominance of Fe reduction was related to a relatively high sediment Fe content in combination with active reworking of the sediment by infauna The relative contribution of Fe reduction to anaerobic carbon oxidation at both stations correlated with the concentration of poorly crystalline Fe(III), confirming that the concentration of poorly crystalline Fe(III) exerts a strong control on rates of Fe reduction in marine sediments The dependence of microbial Fe reduction on concentrations of poorly crystalline Fe(III) was used to quantify the importance of Fe reduction at sites where anaerobic incubations were not applied This study showed that Fe reduction is an important process in anaerobic carbon oxidation in a wider area of the seafloor in the northern and eastern Kattegat (contribution 60 – 75%) By contrast, Fe reduction is of little significance (6 – 25%) in the more coarse-grained sediments of the shallower western and southern Kattegat, where a low Fe content was an important limiting factor, and in fine-grained sediments of the Belt Sea (4 – 28%), where seasonal oxygen depletion limits the intensity of bioturbation and thereby the availability of Fe(III) A large fraction of the total deposition of organic matter in the Kattegat and Belt Sea occurs in the northern Kattegat, and we estimate 33% of benthic carbon oxidation in the whole area is conveyed by Fe reduction

Mechanisms underlying export of N from high-elevation catchments during seasonal transitions

Abstract: Mechanisms underlying catchment export of nitrogen (N) during seasonal transitions (ie, winter to spring and summer to autumn) were investigated in high-elevation catchments of the Sierra Nevada using stable isotopes of nitrate and water, intensive monitoring of stream chemistry and detailed catchment N-budgets We had four objectives: (1) determine the relative contribution of snowpack and soil nitrate to the spring nitrate pulse, (2) look for evidence of biotic control of N losses at the catchment scale, (3) examine dissolved organic nitrogen ( DON) export patterns to gain a better understanding of the biological and hydrological controls on DON loss, and (4) examine the relationship between soil physico-chemical conditions and N export At the Emerald Lake watershed, nitrogen budgets and isotopic analyses of the spring nitrate pulse indicate that 50 to 70% of the total nitrate exported during snowmelt (ca April to July) is derived from catchment soils and talus; the remainder is snowpack nitrate The spring nitrate pulse occurred several weeks after the start of snowmelt and was different from export patterns of less biologically labile compounds such as silica and DON suggesting that: (1) nitrate is produced and released from soils only after intense flushing has occurred and (2) a microbial N-sink is operating in catchment soils during the early stages of snowmelt DON concentrations varied less than 20–30% during snowmelt, indicating that soil processes tightly controlled DON losses

Elevated CO2 and water depth regulation of methane emissions: Comparison of woody and non-woody wetland plant species

Abstract: Elevated CO2 has been shown to increase methane emissions in herbaceous wetlands, but it is not clear that this will occur in wetlands dominated by woody plants or in wetlands that are not inundated. We determined the effects of elevated CO2 and water table position on methane emission and oxidation rates from plant-soil microcosms planted with a woody tree, Taxodium distichum, or an emergent aquatic macrophyte, Orontium aquaticum. Experiments were conducted in replicate glasshouses (n = 2) at CO2 concentrations of either 350 or 700 ppmv. Plants were grown from seed and subjected to two water level depths, flooded (+5 cm above the soil surface) and non-flooded (−10 cm for T. distichum and −6 cm for O. aquaticum). Elevated CO2 increased whole-plant photosynthetic rates in both water table treatments. Methane emission rates increased by 62 to 69% in the T. distichum treatment and 27 to 29% in the O. aquaticum treatment. Whole-plant photosynthesis and biomass were strongly correlated with methane emissions (r2≥ 0.75, P ≤ 0.01). This relationship provides evidence of a tight coupling between plant and microbial activity and suggests that similar relationships from other wetland studies measured at ambient CO2 can be extrapolated into the future. In the O. aquaticum, non-flooded treatment, methanotrophy consumed 14 and 22% (replicate glasshouses) of the methane produced in the ambient treatment compared to 29 and 36% in the elevated CO2 treatment. However, there was no significant methane oxidation detected in the flooded treatment. We concluded that woody and non-woody wetland ecosystems growing in a future CO2-enriched atmosphere will emit more methane regardless of water table position, but the degree of stimulation will be sensitive to changes in water table position, particularly in forested wetlands.

Submarine groundwater discharge estimates at a Florida coastal site based on continuous radon measurements

Abstract: The direct discharge of groundwater into thecoastal zone has received increased attentionin the last few years as it is now recognizedthat this process represents an importantpathway for material transport. Assessingthese material fluxes is difficult, as there isno simple means to gauge the water flux. Weestimated the changing flux of groundwaterdischarge into a coastal area in the northeastGulf of Mexico (Florida) based on continuousmeasurements of radon concentrations over aseveral day period. Changing radon inventorieswere converted to fluxes after accounting forlosses due to atmospheric evasion and mixing. Radon fluxes are then converted to groundwaterinflow rates by estimating the radonconcentration of the fluids discharging intothe study domain. Groundwater flow was also assessed via seepagemeters, radium isotopes, and modeling duringthis period as part of an ``intercomparison''study. The radon results suggest that the flowis: (1) highly variable with flows ranging from∼5 to 50 cm/day; and (2) strongly influenced bythe tides, with spikes in the flow every 12hours. The discharge estimates and pattern offlow derived from the radon model matches theautomated seepage meter records very closely.

Relationships between DOC bioavailability and nitrate removal in an upland stream: An experimental approach

Abstract: The Catskill Mountains of southeastern New York State have among the highest rates of at- mospheric nitrogen deposition in the United States Some streams draining Catskill catchments have shown dramatic increases in nitrate concentrations while others have maintained low nitrate concentra- tions Streams in which exchange occurs between surface and subsurface (ie hyporheic) waters are thought to be conducive to nitrate removal via microbial assimilation and/or denitrification Hyporheic exchange was documented in the Neversink River in the southern Catskill Mountains, but dissolved organic carbon (DOC) and nitrate (NO3 ) losses along hyporheic flowpaths were negligible In this study, Neversink River water was amended with natural, bioavailable dissolved organic carbon (BDOC) (leaf leachate) in a series of experimental mesocosms that simulated hyporheic flowpaths DOC and N dy- namics were examined before and throughout a three week BDOC amendment In addition, bacterial production, dissolved oxygen demand, denitrification, and six extracellular enzyme activities were mea- sured to arrive at a mechanistic understanding of potential DOC and NO3 removal along hyporheic flowpaths There were marked declines in DOC and complete removal of nitrate in the BDOC amended mesocosms Independent approaches were used to partition NO3 loss into two fractions: denitrification and assimilation Microbial assimilation appears to be the predominant process explaining N loss These results suggest that variability in BDOC may contribute to temporal differences in NO3 export from streams in the Catskill Mountains

Net carbon flux from agriculture: Carbon emissions, carbon sequestration, crop yield, and land-use change

Abstract: There is a potential to sequester carbon in soil by changing agricultural management practices. These changes in agricultural management can also result in changes in fossil-fuel use, agricultural inputs, and the carbon emissions associated with fossil fuels and other inputs. Management practices that alter crop yields and land productivity can affect the amount of land used for crop production with further significant implications for both emissions and sequestration potential. Data from a 20-year agricultural experiment were used to analyze carbon sequestration, carbon emissions, crop yield, and land-use change and to estimate the impact that carbon sequestration strategies might have on the net flux of carbon to the atmosphere. Results indicate that if changes in management result in decreased crop yields, the net carbon flux can be greater under the new system, assuming that crop demand remains the same and additional lands are brought into production. Conversely, if increasing crop yields lead to land abandonment, the overall carbon savings from changes in management will be greater than when soil carbon sequestration alone is considered.

Acidic deposition, nutrient leaching and forest growth

Abstract: Studies in Germany and confirmed in North America established that the forest decline that developed in the late 1970's and 80's resulted from a deficiency in one or more of the nutrient cations: Ca2+, Mg2+, and K+. These nutrients are essential to the structure of the foliage, to photosynthesis and to the growth of the trees. The reactions and mechanisms involved in the entry of nutrients to the soil, their storage, and rate of transfer to the soil solution, and through it, to the fine roots and to the leaves at the top of the tree are reviewed. The continuing material balance studies carried out on a watershed at the Hubbard Brook Experimental Forest in New Hampshire allow a unique analysis of the changes caused in these nutrient transfers by acid rain. The nutrient cations are stored in the soil by adsorption on negatively charged clay, and the presence of an acid is required for their release to the soil solution. In pre-industrial times this acid was H2CO3, which was subsequently displaced from the soil solution by H2SO4 and HNO3, as a result of acid deposition. The effect of the increased concentration of the negatively charged SO4 2− and NO3 − anions seeping through the soil, compared with that of the HCO3 − that had been previously present, resulted in a substantially increased rate of transfer of an equivalent of Ca2+ and other positively charged nutrient cations from the soil to the soil solution. The increased concentration of Ca2+ in the soil solution resulted in both an initial increase in the rate of biomass growth and in a simultaneous increase in the rate of Ca2+ loss in the effluent soil solution from the watershed. It was found that this increased rate of removal of Ca2+ from the watershed soil had become greater than its rate of input to the soil from weathering and from dust and rain. As a result, the large Ca2+ inventory that had built up in the soil as a result of the reduced leaching in the years prior to the entry of acid rain, that started in about the1880's, was eventually depleted in the hardwood forest at Hubbard Brook in the 1980's, about 100 years later. With insufficient Ca2+ available for its continuing transfer, net biomass growth on the watershed stopped. This resulted from the rate of tree mortality becoming equal to that of the small incremental growth of a few trees on the watershed. The future growth of forests is at risk from the long-term effects of acid deposition. The fundamental nature of the reactions involved indicates that similar growth anomalies are occurring in other forests impacted by acid rain. These changes from normal biomass growth can affect the amount of CO2 stored in the biomass, of importance to our understanding of Global Warming.

Effect of land use change on soil carbon in Hawaii

Abstract: Organic carbon storage and turnover were altered in soils formed from volcanic ash (Andisols) as a result of conversion of tropical forest to pasture and sugarcane cropland. Changes in soil carbon storage after approximately a century of each land use were estimated using stable carbon isotope values and carbon contents. Total organic carbon stored in soils varied as a result of management, with pasture soils showing net carbon gain and sugarcane soils showing net carbon loss. In pasture soils, increases in carbon at depth (40 to 80 cm) are below the rooting zone of the introduced (C4) vegetation, and have stable carbon isotopic values indicative of forest (C3) plants. Within the pasture rooting zone (0–40 cm) the isotopic data reveals that additions of pasture (C4) organic matter have been offset by losses of C3 carbon. The concentration of Fe/Al oxides (soil minerals that bind with organic matter to form oxide-humus complexes) appear to control the quantity of carbon stored in soils, as well as the difference in the depth and magnitude of carbon storage changes that occur with each type of land use change. Sugarcane land use appears to induce dissociation of Fe/Al oxide-humus complexes and loss of oxide-associated organic matter from the profile. In pastures, Fe/Al oxide-humus complexes are translocated to deeper horizons in the soils, resulting in greater profile carbon storage and longer apparent turnover time of carbon stored below 50 cm depth. In this high precipitation region, carbon losses from the soil appear to occur via downward transport, either as colloids or in solution, in addition to the generally assumed pathway of flux to the atmosphere as CO2.