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Landcover changes have altered the natural carbon cycle; however, most landcover studies focus on either forest conversion to agriculture or urban, rarely both. We present differences in dissolved organic carbon (DOC) concentrations and dissolved organic matter (DOM) molecular composition within Upper Mississippi River Basin low order streams and rivers draining one of three dominant landcovers (forest, agriculture, and urban). Streams draining forest and urban landcovers have greater DOC concentrations, likely driven by differences in carbon sourcing, microbial processing, and soil disturbance. Using Fourier transform-ion cyclotron resonance mass spectrometry, 24% of assigned molecular formulae are common across all landcovers. Relative abundances of N-,S- heteroatomic formulae (CHON, CHOS, CHONS) are higher for agricultural and urban streams, with agricultural stream DOM having more N-containing formulae compared to urban stream DOM, which has more S-containing formulae. Higher N-,S- heteroatomic formulae abundance, along with enrichment in aliphatic, N-aliphatic, and highly unsaturated and phenolic (low O/C) compound categories within agricultural and urban stream DOM are likely to result from increased anthropogenic inputs, autochthonous production, and microbial processing associated with agricultural and urban impacts. Reduced N-,S- heteroatomic formulae abundances in forested stream DOM, along with enrichments in condensed aromatics, polyphenolics, and highly unsaturated phenolic (high O/C) compound categories, likely reflect greater contributions from surrounding organic-rich forest soil and vegetation. Overall, landcover change from forested to agriculture lowers DOC concentrations and changes from forested to agriculture or urban increases autochthonous, and presumably more biolabile, DOM contributions with ramifications for stream biogeochemical cycling. [ABSTRACT FROM AUTHOR]

The spatial distribution of soil phosphorus (P) is important to both biogeochemical processes and the management of agricultural landscapes, where it is critical for both crop production and conservation planning. Recent advances in the availability of large environmental datasets together with big data analytical tools like machine learning have created opportunities for evaluating and predicting spatial patterns in complex environmental variables like soil P. Here, we apply a random forest machine learning model to publicly available soil P datasets together with nearly 300 geospatial attributes summarizing aspects of soil type, land cover, land use, topography, nutrient inputs, and climate to predict total soil P at a 100 m grid scale for the Upper Mississippi River Basin (UMRB), USA. The UMRB is one of the most intensively farmed regions in the world and is characterized by widespread water quality degradation arising from P-associated eutrophication. Although potentially complex interacting drivers determine total soil P, the predictive accuracy of our random forest model was relatively high (R2 = 0.58 and RMSE = 129.3 for an independent validation dataset). At the regional scale represented by our model, the variables with the greatest comparative importance for predicting soil P included a combination of soil sample depth, land use/land cover, underlying soil physical and geochemical properties, landscape features (such as slope, elevation and proximity to the stream network), nutrient inputs, and climate-related factors. An important product of this research is a fine-scale (100 m) raster data layer of predicted total soil P values for the UMRB for public use. This dataset can be used to improve conservation planning and modeling efforts to improve water quality in the region. [ABSTRACT FROM AUTHOR]

The evolution of biogeochemistry, retraces the important historical steps in part, covered by Gorham (Biogeochemistry 13:199–239, 1991) in the 18–19th centuries—with new emergent linkages and trends in 20–21st centuries. In the post-phlogiston period, key synthetic connections are made between weathering, atmospheric chemistry, carbon cycling, and climate change. Early work in the 19th century, focused on weathering and the importance of organisms in the exchange of carbon dioxide between the rocks and the atmosphere, provided foundations for new analytical approaches. The role microbes in connecting abiotic and biotic processes begins to emerge, based largely on the existing knowledge of stoichiometry in agricultural soils and plants. This in part, leads to the founding of ecology and its linkages with evolution and biogeography. Verandsky boldly emerges in the 20th century, with his concepts of a biosphere and a noosphere, as concerns begin to arise about human impacts on nature. The development of organic geochemistry as a discipline, allowed for new roots to develop in the evolution of biogeochemistry through linkages between short and long-term carbon cycles. In the 20th century, a new interesting stoichiometry emerges in biogeochemistry—as related to the Green Revolution, human population growth, and eutrophication problems. The advent of long-term and large-scale experiments help to constrain the complexity of non-linearity and regional differences in fluxes and rates in biogeochemical work. A new age begins in the 21st century whereby molecular approaches (e.g. omics) combined with large-scale satellite, monitoring, survey, observatory approaches are combined in the development of Earth System models. These new connections with ecological/evolutionary genetics are one of the more dramatic and important aspects of biogeochemistry in modern times. [ABSTRACT FROM AUTHOR]

Dissolved organic matter (DOM) is recognized for its importance in freshwater ecosystems, but historical reliance on DOM quantity rather than indicators of DOM composition has led to an incomplete understanding of DOM and an underestimation of its role and importance in biogeochemical processes. A single sample of DOM can be composed of tens of thousands of distinct molecules. Each of these unique DOM molecules has their own chemical properties and reactivity or role in the environment. Human activities can modify DOM composition and recent research has uncovered distinct DOM pools laced with human markers and footprints. Here we review how land use change, climate change, nutrient pollution, browning, wildfires, and dams can change DOM composition which in turn will affect internal processing of freshwater DOM. We then describe how human-modified DOM can affect biogeochemical processes. Drought, wildfires, cultivated land use, eutrophication, climate change driven permafrost thaw, and other human stressors can shift the composition of DOM in freshwater ecosystems increasing the relative contribution of microbial-like and aliphatic components. In contrast, increases in precipitation may shift DOM towards more relatively humic-rich, allochthonous forms of DOM. These shifts in DOM pools will likely have highly contrasting effects on carbon outgassing and burial, nutrient cycles, ecosystem metabolism, metal toxicity, and the treatments needed to produce clean drinking water. A deeper understanding of the links between the chemical properties of DOM and biogeochemical dynamics can help to address important future environmental issues, such as the transfer of organic contaminants through food webs, alterations to nitrogen cycling, impacts on drinking water quality, and biogeochemical effects of global climate change. [ABSTRACT FROM AUTHOR]

Lawns are a common ecosystem type in human-dominated landscapes which can have negative impacts on water quality due to fertilizer applications, but also host a range of ecosystem services. While many studies have addressed water and nitrogen (N) dynamics in lawns, few have considered how topography interacts with human behaviors to control these dynamics. Our overarching objective was to determine if mesoscale topography (hillslopes within lawns) interacts with human behavior (fertilizer use) influencing patterns of N mobilization and removal in lawns. To that end, we measured several hydrobiogeochemical characteristics associated with N dynamics along topographic gradients in fertilized and unfertilized residential and institutional lawns. We found topographic gradients affect the hydrobiogeochemistry of lawns, with significant effects of landscape position (top versus toe slope versus bottomland swales), but with direction and strength of the effect often varying among different lawn types (exurban versus suburban front yards versus suburban backyards versus institutional). Fertilizer application did not affect the hydrobiogeochemical properties of lawns. Rather, results from this study suggest lawns in suburban front yards were at greatest risk of N mobilization due to a complex suite of characteristics including proximity to impervious surfaces, swales with low saturated infiltration rates, and potential vulnerability to N deposition from vehicles. This study highlights the need to consider landscape controls of water and N fluxes and how they interact with human behaviors to better understand how these landscapes function. These results contribute to the emerging understanding of the structure, function and environmental impacts of lawns. [ABSTRACT FROM AUTHOR]

Non-point source pollution of phosphorus (P) is a primary cause of eutrophication of aquatic ecosystems, and poses a persistent management challenge due to the dynamic and poorly understood processes controlling the transport and transformation of P at the watershed scale. We examined phosphorus inputs, retention, and riverine losses in 62 diverse watersheds that included a wide range of land cover and use (minimally disturbed to human dominated) and human P inputs in Minnesota, USA. Fertilizer inputs from row crop cultivation were the dominant source of P to agricultural watersheds. A large majority of P inputs was retained in watershed soils or removed in agricultural products. However, fertilizer input was the most important factor associated with average annual river transport of total, dissolved, and particulate phosphorus (PP). Mean annual runoff increased total and dissolved P yields and decreased P retention. Dissolved P made up a significant portion of annual river loads at sites with high rates of P inputs and river TP export, with the ratio of dissolved to PP export increasing with crop cover and fertilizer inputs. PP export rose with larger extents of eroding bluffs near channels. Bluffs constitute only a small proportion of watershed area, but make a disproportionately high contribution to sediment loads due to their close proximity to river channels that are increasingly affected by human and climate-driven changes to river hydrology. Together, our results suggest that rising discharge and flow variability due to climate change and agricultural intensification coupled with high rates of P inputs will maintain elevated fluvial P export into the future. Without reductions in P inputs and reductions in both soluble P losses and stream bank erosion, reversal of water quality degradation will be difficult to achieve. [ABSTRACT FROM AUTHOR]

Soluble nutrient flux is increasingly implicated in the degradation of receiving water quality. With high-frequency sensors, river discharge along with nitrate and orthophosphate concentrations were collected over 2 years. We examined how storms (16 events) affected the dissolved nutrient flux in two watersheds with contrasting landcover—urban and agricultural. The adjacent watersheds, both < 7 km2, had very similar soil, slope, relief, and physiography. Wastewater is conveyed outside of the watersheds minimizing this nutrient source. Cumulative nitrate and orthophosphate fluxes in the agricultural watershed were substantially higher than the urban watershed. In both watersheds, a disproportionate amount of discharge, higher orthophosphate concentrations and flux occurred during the highest flow events—approximately 50–70% of flux occurring in the highest 5% of daily discharge. The nitrate flux was dampened compared to discharge during storm events. Baseflow accounted for 66% of the nitrate flux and had higher or comparable concentrations than storm events. The pattern of flux and concentration within storm events also differed for the two solutes. Nitrate exhibited positive hysteresis (higher concentrations on the rising limb of the hydrograph) and magnification of flux during the first flush of storm runoff. In contrast, orthophosphate concentrations were lower on the rising limb and flux during the first flush of storm runoff was dampened. In addition to targeting source reductions, orthophosphate flux may require runoff reducing strategies (i.e., enhanced infiltration) from large runoff events. First flush stormwater management practices combined with enhancement/protection of groundwater nitrate sinks (e.g., riparian wetlands) may help reduce nitrate fluxes. [ABSTRACT FROM AUTHOR]

Watershed urbanization increases the concentrations of major ions in downstream freshwater ecosystems. Non-point source ions from human activities and the chemical weathering of infrastructure are efficiently transported by stormwater runoff through subsurface pipes directly into streams. While the increase in mean streamwater solute concentrations in urban watersheds because of non-point source loading is a commonly documented phenomenon, the temporal dynamics of urban streamwater solute chemistry and its relationship to development patterns is less well understood. We continuously monitored streamflow, temperature, and conductivity as well as bimonthly variation in the solute chemistry of 24 watershed outlets in the Raleigh-Durham-Chapel Hill metropolitan area in North Carolina, USA for two years. Watersheds were selected to have similar proportions of development (10–36% development) while spanning the full measured range in road and stormwater pipe density for urban watersheds in this metropolitan area. We found remarkable temporal variation in the chemical regimes of the urban streams draining this set of watersheds, despite their similar proportion of development. For multiple major ions (SO42−, Cl−, NO3−), bimonthly concentrations varied tenfold across the study streams and temporal variation in streamwater chemical composition increased across watershed road and stormwater pipe density gradients. Total temporal variation in baseflow and stormflow dissolved ions, as measured by the coefficient of variation of specific conductance, was highly correlated with road density after accounting for underlying geology (R2 > 0.60). Using structural equation modeling, we found that subsurface piping mediates the relationship between roads and stream chemistry. In watersheds with high stormwater pipe density, streamwater ionic strength was low at baseflow while high and highly variable during event flow. This 'flashy' chemical signal in those watersheds, mirrors their more responsive hydrographs. In contrast, in our urban watersheds with the lowest pipe density, baseflow ionic strengths were higher, and event flow ionic strengths were lower and less variable. These results suggest that the extent to which pavement is drained by pipes creates a tradeoff between routing urban salts directly to streams via pipes versus indirectly to streams via loading to groundwater. Results of this study demonstrate the importance of considering altered temporal chemical variability, not just elevated solute concentrations, as a key feature of the impact of urbanization on streams. Future urban development design which minimizes the extent of roads and stormwater pipes in watersheds through denser development may be an effective strategy to abate impacts on downstream water quality. [ABSTRACT FROM AUTHOR]

While much research over the past 30 years has focused on the deleterious effects of excess N on forests and associated aquatic ecosystems, recent declines in atmospheric N deposition and unexplained declines in N export from these ecosystems have raised new concerns about N oligotrophication, limitations of forest productivity, and the capacity for forests to respond dynamically to disturbance and environmental change. Here we show multiple data streams from long-term ecological research at the Hubbard Brook Experimental Forest in New Hampshire, USA suggesting that N oligotrophication in forest soils is driven by increased carbon flow from the atmosphere through soils that stimulates microbial immobilization of N and decreases available N for plants. Decreased available N in soils can result in increased N resorption by trees, which reduces litterfall N input to soils, further limiting available N supply and leading to further declines in soil N availability. Moreover, N oligotrophication has been likely exacerbated by changes in climate that increase the length of the growing season and decrease production of available N by mineralization during both winter and spring. These results suggest a need to re-evaluate the nature and extent of N cycling in temperate forests and assess how changing conditions will influence forest ecosystem response to multiple, dynamic stresses of global environmental change. [ABSTRACT FROM AUTHOR]

Redox transitions induced by seasonal changes in water column O2 concentration can have important effects on solutes exchange across the sediment-water interface in systems polluted with acid mine drainage (AMD), thus influencing natural attenuation and bioremediation processes. The effect of such transitions was studied in a mesocosm experiment with water and sediment cores from an acidic reservoir (El Sancho, SW Spain). Rates of aerobic organic matter mineralization and oxidation of reduced inorganic compounds increased under oxic conditions (OX). Anaerobic process, like Fe(III) and sulfate reduction, also increased due to higher O2 availability and penetration depth in the sediment, resulting in higher regeneration rates of their corresponding anaerobic e− acceptors. The contribution of the different processes to oxygen uptake changed considerably over time. pH decreased due to the precipitation of schwertmannite and the release of H+ from the sediment, favouring the dissolution of Al-hydroxides and hydroxysulfates at the sediment surface. The increase in dissolved Al was the main contributor to water column acidity during OX. Changes in organic matter degradation rates and co-precipitation and dissolution of dissolved organic carbon and nitrogen with redox-sensitive Fe(III) compounds affected considerably C and N cycling at the sediment-water interface during redox transitions. The release of NO2− and NO3− during the hypoxic period could be attributed to ammonium oxidation coupled to ferric iron reduction (Feammox). Considering the multiple effects of redox transitions at the sediment-water interface is critical for the successful outcome of natural attenuation and bioremediation of AMD impacted aquatic environments. [ABSTRACT FROM AUTHOR]

Increased nitrogen (N) deposition rates were simulated for 20 years (1995-2015) by sprinkling rain water enriched with NHNO (+22 kg ha year N) in a small headwater catchment within a spruce ( Picea abies) forest at Alptal (central Switzerland). The added N was labelled with NHNO during the first year. A control catchment was labelled in years 6 and 8. NO leaching doubled in the N-addition catchment during the first year, resulting almost entirely from loss of N-labelled NO . The proportion of added N leached as NO -N reached 1/3 after 5 years, then remained stable for the next 10 years, at 6.6-10 kg ha year compared with 1.4-2.9 kg ha year in the control catchment. In each catchment, half of the large trees were girdled during the 15th year and felled 1 year later to assess their role in soil N retention. Girdling and felling resulted in a strong increase in NO -N leaching from the N-addition catchment (up to 19 kg ha year). The increase was significantly smaller in the control (up to 4 kg ha year). NO leached after tree girdling contained only 0.18% of the originally applied tracer in the control and none detectable in the N-addition catchment. The increased leaching was thus mainly from new N and due to reduced N immobilisation by trees rather than to increased N mineralisation and nitrification in the soil. Dissolved organic N leaching (average 6.1 kg ha year), in contrast, was unaffected by the treatment. [ABSTRACT FROM AUTHOR]

As urban ecosystems are known to be phosphorus (P)-rich environments, the sources of this nutrient and its biogeochemistry within human-dominated landscapes remain in need of study. Specifically, very little is known about how different P species vary within and among urban aquatic environments. In this study, we examined the phosphorus dynamics in urban stormwater ponds that are embedded in residential landscapes. Water samples were collected from stormwater ponds located in southern Ontario, Canada during the summer of 2012. We measured several P types: total P; TP, particulate P; PP, and dissolved inorganic P; DIP as well as two classes of DOP (phosphomonoesters (MP) and phosphodiesters (DP)) in the surface water samples. In most ponds, PP in outflowing water were found at concentrations higher (up to 12 times) than those measured in incoming stormwater. With respect to DOP, DP increased and became more abundant in outflowing compared to inflowing waters while MP concentrations decreased within ponds. The magnitude of these spatial changes from pond inlets to outlets was strongly linked to recent periods of rainfall and the hydrological condition of pond inflows. Elevated MP was found adjacent to inlets especially during periods of more frequent rainfall, which indicates that MP is derived from terrestrial sources and delivered by stormwater. In contrast, DP production was more related to internal biological activity in ponds as its concentrations increased during relatively dry conditions with low stormwater flow. Our results demonstrate that stormwater ponds have significant influence on the quantity and types of P moving through these highly human-modified environments. [ABSTRACT FROM AUTHOR]

The influence of built urban infrastructure on stream chemistry was quantified throughout the drainage network of the tropical Río Piedras watershed, San Juan metropolitan area, Puerto Rico. Urbanization and failing domestic wastewater infrastructure appeared to drive changes in surface water chemistry throughout the watershed. Mean baseflow concentrations of chloride (Cl), ammonium (NH), dissolved organic carbon (DOC), dissolved organic nitrogen (DON), and phosphate (PO) all increased with urban infrastructure, while nitrate (NO) and dissolved oxygen (DO) decreased. These patterns in stream chemistry suggest that sewage effluent from failing or illegally connected sewer pipes has a major impact on surface water quality. Concentrations of Cl, DO, and NH in stream water were most strongly related to sewer pipe volume, demonstrating the tight connection between urban infrastructure and stream chemistry. The loading and transformation of NO and NH were modeled along the river network and NH loading rates from the landscape were strongly related to urban infrastructure, whereas NO loading rates showed only weak relationships, highlighting the importance for incorporating NH dynamics into river network models in urban environments. Water quality appears to be severely impacted by sewage in this tropical basin, despite large investments in built infrastructure. The high temperatures in the Río Piedras exacerbate water quality problems by reducing saturation DO levels in streams, and intense rainstorms tax the ability of built infrastructure to adequately manage overland flows. These problems are likely typical of much of the urbanized humid tropics. [ABSTRACT FROM AUTHOR]

Rapid increases in human population and land transformation in arid and semi-arid regions are altering water, carbon (C) and nitrogen (N) cycles, yet little is known about how urban ephemeral stream channels in these regions affect biogeochemistry and trace gas fluxes. To address these knowledge gaps, we measured carbon dioxide (CO), nitrous oxide (NO), and methane (CH) before and after soil wetting in 16 ephemeral stream channels that vary in soil texture and organic matter in Tucson, AZ. Fluxes of CO and NO immediately following wetting were among the highest ever published (up to 1,588 mg C m h and 3,121 μg N m h). Mean post-wetting CO and NO fluxes were significantly higher in the loam and sandy loam channels (286 and 194 mg C m h; 168 and 187 μg N m h) than in the sand channels (45 mg C m h and 7 μg N m h). Factor analyses show that the effect of soil moisture, soil C and soil N on trace gas fluxes varied with soil texture. In the coarser sandy sites, trace gas fluxes were primarily controlled by soil moisture via physical displacement of soil gases and by organic soil C and N limitations on biotic processes. In the finer sandy loam sites trace gas fluxes and N-processing were primarily limited by soil moisture, soil organic C and soil N resources. In the loam sites, finer soil texture and higher soil organic C and N enhance soil moisture retention allowing for more biologically favorable antecedent conditions. Variable redox states appeared to develop in the finer textured soils resulting in wide ranging trace gas flux rates following wetting. These findings indicate that urban ephemeral channels are biogeochemical hotspots that can have a profound impact on urban C and N biogeochemical cycling pathways and subsequently alter the quality of localized water resources. [ABSTRACT FROM AUTHOR]

The article presents research on urban biogeochemistry, with a focus on the issue of eutrophication of urban surface waters due to excess nitrogen (N) and phosphorus (P) in water quality management. Topics include the influences of stormflow and baseflow pathways on nitrogen and phosphorus export from an urban watershed, the flow of water moving through storm drains in non-stormy conditions, and proposals for the prevention of the contamination of shallow groundwater.

This overview and synthesis paper focuses on the evolution of urban biogeochemical cycles across time. We synthesize empirical data and review existing literature, including papers in this special issue, and we propose the concept of 'urban evolution.' The built environment often changes quickly in response to human activities, thus contributing to an urban evolution that affects structure, function, and ecosystem services of human settlements over time. Depending upon management, these changes can result in rapid losses of ecosystem functions/services or progress towards restoration. We explore urban evolution through empirical examples such as: (1) land development and nitrogen inputs within a metropolitan region over half a century; (2) watershed drainage by different forms of stormwater management over decades; (3) human-accelerated weathering in urbanized watersheds over decades; and (4) global salinization of freshwater across urbanizing landscapes over a century. We also synthesize concepts relevant to studying urban evolution of infrastructure and ecosystems including: (1) urban watersheds have challenged our whole notion of the 'watershed approach' due to complex hydrologic boundaries and flow paths over time; (2) the urban hydrologic cycle evolves due to changing infrastructure and human water use over time; (3) the importance of extending research beyond individual sites using an urban watershed approach over space and time; (4) salinization as a universal tracer of watershed urbanization over time; (5) human-accelerated weathering of concrete and construction materials contributing to an 'urban karst' over time; (6) human alteration of the carbon cycle in urban watersheds over time; and (7) detecting distinct biogeochemical signatures across cities globally over time. Our synthesis and this special issue suggest that urban biogeochemical cycles have exerted a major influence on the elemental composition of the Earth's surface from local to global scales. A new global research agenda is needed to track the evolution of urban biogeochemical cycles as land development proceeds and infrastructure/management changes so we can better evaluate potential losses in ecosystem services, set realistic watershed and river restoration goals, and formulate effective environmental policy for Earth's growing urban population. [ABSTRACT FROM AUTHOR]

Concentrations of nitrogen (N) in surface waters reflect the export of different organic and inorganic forms from terrestrial environments and the modification of these resources within aquatic habitats. We evaluated the relative influence of terrestrial ecosystem state factors, anthropogenic gradients, and aquatic habitat variables on patterns of N concentration in streams and rivers across Sweden. We analyzed data from 115 national monitoring stations distributed along a 1,300 km latitudinal gradient, draining catchments that differed by more than 10 °C in mean annual temperature (MAT), and more than five orders of magnitude in area. Regional trends in total organic nitrogen (TON) and carbon:nitrogen (C:N) were closely linked to broad-scale gradients in state factors (e.g., MAT), reflecting the importance of long-term ecosystem development on terrestrial organic matter accrual and export. In contrast, trends in nitrate (NO), the dominant form of inorganic N, were largely unrelated to state factors, but instead were closely connected to gradients related to anthropogenic inputs (e.g., agricultural cover). Despite large differences in drainage size and cover by lakes and wetlands among sites, these descriptors of the aquatic environment had little influence on spatial patterns of N chemistry. The temporal variability in N concentrations also differed between forms: inorganic N was strongly seasonal, with peaks during dormant periods that underscore biotic control over terrestrial losses of limiting resources. Organic N showed comparatively weaker seasonality, but summertime increases suggest temperature-driven patterns of soil TON production and export-temporal signals which were modified by variables that govern water residence time within catchments. Unique combinations of regional predictors reflect basic differences in the cycling of organic versus inorganic N and highlight variation in the sensitivity of these different N forms to environmental changes that directly alter inputs of resources, or indirectly modify terrestrial ecosystems through shifts in species composition, rates of forest productivity, soil development, and hydrologic routing. [ABSTRACT FROM AUTHOR]

We compared carbon (C), nitrogen (N), and phosphorus (P) concentrations in atmospheric deposition, runoff, and soils with microbial respiration [dehydrogenase (DHA)] and ecoenzyme activity (EEA) in an ombrotrophic bog and a minerotrophic fen to investigate the environmental drivers of biogeochemical cycling in peatlands at the Marcell Experimental Forest in northern Minnesota, USA. Ecoenzymatic stoichiometry was used to construct models for C use efficiency (CUE) and decomposition (M), and these were used to model respiration (R). Our goals were to determine the relative C, N, and P limitations on microbial processes and organic matter decomposition, and to identify environmental constraints on ecoenzymatic processes. Mean annual water, C, and P yields were greater in the fen, while N yields were similar in both the bog and fen. Soil chemistry differed between the bog and fen, and both watersheds exhibited significant differences among soil horizons. DHA and EEA differed by watersheds and soil horizons, CUE, M, and R differed only by soil horizons. C, N, or P limitations indicated by EEA stoichiometry were confirmed with orthogonal regressions of ecoenzyme pairs and enzyme vector analyses, and indicated greater N and P limitation in the bog than in the fen, with an overall tendency toward P-limitation in both the bog and fen. Ecoenzymatic stoichiometry, microbial respiration, and organic matter decomposition were responsive to resource availability and the environmental drivers of microbial metabolism, including those related to global climate changes. [ABSTRACT FROM AUTHOR]

Heathlands and grasslands occur in montane regions, naturally or due to anthropogenic land-use. These are typically nutrient-poor but exposure to elevated nitrogen deposition and intensive livestock grazing causes large-scale ecological change. We studied the long-term implications of grazing removal on soil and drainage water biogeochemistry and the implications for nitrogen cycling in 50-year replicated grazing exclosures on a montane grassland exposed to high rates of ambient nitrogen deposition. Evidence of 'ecosystem recovery' represented by successional change from graminoid to shrub-dominance after cessation of grazing was not reflected in the soil biogeochemistry. Cessation of grazing had a negative impact, with increased soil extractable and soil solution nitrate concentrations; an apparent shift towards a more nitrogen-rich, bacterially dominated microbial community; and the acidification of soils and leachate. The increase in nitrate leaching appears to have been counterbalanced by a decrease in dissolved organic nitrogen leaching, approximately maintaining the overall nitrogen balance of the system, whilst apparently altering ecosystem functioning. High rates of organic matter cycling and inorganic nitrogen uptake in grazed grassland may have sustained ecosystem N limitation under elevated nitrogen deposition. Grazing removal caused long-term over-supply of nitrogen from mineralisation of enriched organic matter, exacerbated by continued high nitrogen deposition, exceeding the uptake demand of heath vegetation and resulting in nitrification and nitrate leaching. This disequilibrium between vegetation and soil following grazing removal has implications for restoration after periods of intensive grazing. Grazing may not simply leave a legacy of nutrient enrichment but its cessation may trigger nitrogen saturation and soil and freshwater eutrophication and acidification which counteract the immediate benefits of natural vegetation recovery. Long term, nitrogen saturation of abandoned grasslands is likely to reduce ecosystem resilience to invasion by nitrophilous species, pathogen attack and vulnerability to environmental pressures such as climate change. We conclude that partial and/or phased reduction in grazing levels may permit the more synchronised recovery of soils and vegetation, thereby avoiding imbalances between nitrogen supply and nitrogen demand and detrimental ecological effects. [ABSTRACT FROM AUTHOR]

Industrial emissions of SO 2 and NO x, resulting in the formation and deposition of sulfuric and nitric acids, affect the health of both terrestrial and aquatic ecosystems. Since the mid-late 20th century, legislation to control acid rain precursors in both Europe and the US has led to significant declines in both SO 4–S and H + in precipitation and streams. However, several authors noted that declines in streamwater SO 4–S did not result in stoichiometric reductions in stream H +, and suggested that observed reductions in base cation inputs in precipitation could lessen the effect of air pollution control on improving stream pH. We examined long-term precipitation chemistry (1978–2010) from nearly 30 sites in the US and Europe that are variably affected by acid deposition and that have a variety of industrial and land-use histories to (1) quantify trends in SO 4–S, H +, NH 4–N, Ca, and NO 3–N, (2) assess stoichiometry between H + and SO 4–S before and after 1990, and (3) examine regional synchrony of trends. We expected that although the overall efforts of developed countries to reduce air pollution and acid rain by the mid-late 20th century would tend to synchronize precipitation chemistry among regions, geographically varied patterns of fossil fuel use and pollution control measures would produce important asynchronies among European countries and the United States. We also expected that control of particulate versus gaseous emission, along with trends in NH 3 emissions, would be the two most significant factors affecting the stoichiometry between SO 4–S and H +. Relationships among H +, SO 4–S, NH 4–N, and cations differed markedly between the US and Europe. Controlling for SO 4–S levels, H + in precipitation was significantly lower in Europe than in the US, because (1) alkaline dust loading from the Sahara/Sahel was greater in Europe than the US, and (2) emission of NH 3, which neutralizes acidity upon conversion to NH 4+, is generally significantly higher in Europe than in the US. Trends in SO 4–S and H + in precipitation were close to stoichometric in the US throughout the period of record, but not in Europe, especially eastern Europe. Ca in precipitation declined significantly before, but not after 1990 in most of the US, but Ca declined in eastern Europe even after 1990. SO 4–S in precipitation was only weakly related to fossil fuel consumption. The stoichiometry of SO 4–S and H + may be explained in part by emission controls, which varied over time and among regions. Control of particulate emissions reduces alkaline particles that neutralize acid precursors as well as S-containing particulates, reducing SO 4–S and Ca more steeply than H +, consistent with trends in the northeastern US and Europe before 1990. In contrast, control of gaseous SO 2 emissions results in a stoichiometric relationship between SO 4–S and H +, consistent with trends in the US and many western European countries, especially after 1991. However, in many European countries, declining NH 3 emissions contributed to the lack of stoichiometry between SO 4–S and H +.Recent reductions in NO x emissions have also contributed to declines in H + in precipitation. Future changes in precipitation acidity are likely to depend on multiple factors including trends in NO x and NH 3 emission controls, naturally occurring dust, and fossil fuel use, with significant implications for the health of both terrestrial and aquatic ecosystems. [ABSTRACT FROM AUTHOR]

Elevated and chronic nitrogen (N) deposition to N-limited terrestrial ecosystems can lead to 'N saturation', with resultant ecosystem damage and leaching of nitrate (NO) to surface waters. Present-day N deposition, however, is often a poor predictor of NO leaching, and the pathway of the ecosystem transition from N-limited to N-saturated remains incompletely understood. The dynamics of N cycling are intimately linked to the associated carbon (C) and sulphur (S) cycles. We hypothesize that N saturation is associated with shifts in the microbial community, manifest by a decrease in the fungi-to-bacteria ratio and a transition from N to C limitation. Three mechanisms could lead to lower amount of bioavailable dissolved organic C (DOC) for the microbial community and to C limitation of N-rich systems: (1) Increased abundance of N for plant uptake, causing lower C allocation to plant roots; (2) chemical suppression of DOC solubility by soil acidification; and (3) enhanced mineralisation of DOC due to increased abundance of electron acceptors in the form of $${{\text{SO}}_{ 4}}^{ 2-}$$ and NO in anoxic soil micro-sites. Here we consider each of these mechanisms, the extent to which their hypothesised impacts are consistent with observations from intensively-monitored sites, and the potential to improve biogeochemical models by incorporating mechanistic links to the C and S cycles. [ABSTRACT FROM AUTHOR]

The transport and deposition of anthropogenic nitrogen (N) to downwind ecosystems is significant and can be a dominant source of new N to many watersheds. Bacterially mediated denitrification in lake sediments may ameliorate the effects of N loading by permanently removing such inputs. We measured denitrification in sediments collected from lakes in the Colorado Rocky Mountains (USA) receiving elevated (5-8 kg N ha y) or low (<2 kg N ha y) inputs of atmospheric N deposition. The nitrate (NO) concentration was significantly greater in high-deposition lakes (11.3 μmol l) compared to low-deposition lakes (3.3 μmol l). Background denitrification was positively related to NO concentrations and we estimate that the sampled lakes are capable of removing a significant portion of N inputs via sediment denitrification. We also conducted a dose-response experiment to determine whether chronic N loading has altered sediment denitrification capacity. Under Michaelis-Menten kinetics, the maximum denitrification rate and half-saturation NO concentration did not differ between deposition regions and were 765 μmol N m h and 293 μmol l NO, respectively, for all lakes. We enumerated the abundances of nitrate- and nitrite-reducing bacteria and found no difference between high- and low-deposition lakes. The abundance of these bacteria was related to available light and bulk sediment resources. Our findings support a growing body of evidence that lakes play an important role in N removal and, furthermore, suggest that current levels of N deposition have not altered the abundance of denitrifying bacteria or saturated the capacity for sediment denitrification. [ABSTRACT FROM AUTHOR]

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 h) were up to two orders of magnitude higher than rates measured in isolated wetlands (2-231 μmol N m h), suggesting a strong regulation of the process by nitrate availability. These rates were also significantly higher in summer (9-1,888 μmol N m h) than in winter (2-365 μmol N m h). Denitrification supported by water column nitrate (D) accounted for 60-100% of total denitrification (Dtot); denitrification coupled to nitrification (D) 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. [ABSTRACT FROM AUTHOR]

Understanding and predicting the responses of plant communities to multiple overlapping disturbances remains a challenging task. Hurricane Wilma represents a large, yet infrequent type of disturbance that was superimposed on an existing disturbance gradient of time since fire. We examined disturbance and recovery patterns in response to these overlapping disturbances by measuring how canopy structure, fine roots, mycorrhizae, and soil nitrogen dynamics, varied along a fire chronosequence in the 2 years after Hurricane Wilma. Hurricane damage increased canopy openness in all seral stages. In the early-seral stage, canopy openness returned to pre-hurricane conditions within 2 years, whereas canopy openness in the late-seral stage remained significantly higher throughout the study. We observed no significant change in root length density in the early- and mid-seral stages. However, in the late-seral stage, root length density was significantly reduced immediately after the hurricane and remained so 2 years after the hurricane. In the late-seral stage, we also observed a significant reduction in percent soil nitrogen and a significant increase in soil nitrogen isotopic composition (δ15N) values, indicating a loss of soil nitrogen. In contrast, in the early- and mid-seral stages, there were no significant changes in percent nitrogen or soil δ15N values. Results from this study suggest that forest fire disturbance history influences responses to hurricane damage. Moreover, feedbacks between aboveground and belowground processes have the potential to influence forest recovery. [ABSTRACT FROM AUTHOR]

Seasonal and diurnal variations of in-stream NO3-N concentration oscillations were studied through high-frequency measurements of streamwater’s physical, chemical parameters (in-stream NO3-N concentration, water temperature, dissolved oxygen, pH) and hydrometeorological variables (stream discharge, solar radiation) under hydrologically stable conditions. The study was carried out in 2006, within the 42 km2 forested Padež stream watershed in the southwestern part of Slovenia, which is characterized by distinctive hydrogeological settings (flysch) and climate conditions (transitional area between the Mediterranean and continental climate). Fine temporal data resolution (15-min interval) enabled identification of the factors responsible for seasonal variability in the diurnal pattern of the streamwater NO3-N concentrations versus seasonal and diurnal behavior of meteorological and other water chemistry constituents. The observed seasonal variability of in-stream NO3-N daily oscillations indicates the important role of primary production uptake, particularly during seasons when deciduous vegetation is dormant and light levels in the stream are high. Highest daily NO3-N concentration amplitudes (0.3 mg/l-N) and daily changes in the NO3-N flux (0.4–0.5 g/s-N) were observed in spring; the NO3-N concentration oscillations in summer showed a considerably smaller effect of the in-stream uptake (maximum NO3-N daily concentration amplitude 0.1 mg/l-N; daily change in the NO3-N flux 0.02 g/s-N). Seasonal shifts in the timing of daily maximum (up to 6 h) and minimum NO3-N concentrations (between 1 and 3 h) provided some additional indications of seasonal changes in the in-stream primary production uptake and its relation to the terrestrial component of the forested watershed. [ABSTRACT FROM AUTHOR]

Different phosphorus fractions and metal element composition of surficial sediments were measured on three occasions in 2005 and 2006 along a transect between Nyanza Gulf and offshore Lake Victoria, in order to assess the potential for sediments to contribute to the water column P concentrations in Lake Victoria. Total phosphorus (TP), apatite phosphorus (AP), inorganic phosphorus (IP) and organic phosphorus (OP) increased in sediments along the gulf towards the main lake while the non-apatite inorganic phosphorus (NAIP) increases were less defined. The longitudinal gradient of sediment TP and its fractions in Nyanza Gulf is a result of high rates of terrigenous input and resuspension and transport of the light, phosphorus rich inorganic and organic matter towards the main lake. TP in the sediment ranged from 812.7 to 1,738 mg/kg dry weight (DW) and was highest in the Rusinga Channel, the exchange zone between the gulf and the main lake. AP was the most important TP fraction, contributing between 35 and 57.3% of TP. Ca content in the sediment was strongly associated with TP and AP in the sediment (r2 = 0.92 and 0.98, respectively) in the gulf and the channel, indicating the importance of apatite in controlling P availability in these zones. In the gulf and the Rusinga Channel, the less bioavailable apatite phosphorus dominated, whereas in the deeper main lake OP was the major fraction illustrating the importance of anaerobic release of P from sediments and acceleration of internal P loading in the main lake. [ABSTRACT FROM AUTHOR]

Atmospheric deposition contributes a large fraction of the annual nitrogen (N) input to the basin of the Susquehanna River, a river that provides two-thirds of the annual N load to the Chesapeake Bay. Yet, there are few measurements of the retention of atmospheric N in the Upper Susquehanna’s forested headwaters. We characterized the amount, form (nitrate, ammonium, and dissolved organic nitrogen), isotopic composition (δ15N- and δ18O-nitrate), and seasonality of stream N over 2 years for 7–13 catchments. We expected high rates of N retention and seasonal nitrate patterns typical of other seasonally snow-covered catchments: dormant season maxima and growing season minima. Coarse estimates of N export indicated high rates of inorganic N retention (>95%), yet streams had unexpected seasonal nitrate patterns, with summer peaks (14–96 μmol L−1), October crashes (<1 μmol L−1), and modest rebounds during the dormant season (<1–20 μmol L−1). Stream δ18O-nitrate values indicated microbial nitrification as the primary source of stream nitrate, although snowmelt or other atmospheric source contributed up to 47% of stream nitrate in some March samples. The autumn nitrate crash coincided with leaffall, likely due to in-stream heterotrophic uptake of N. Hypothesized sources of the summer nitrate peaks include: delayed release of nitrate previously flushed to groundwater, weathering of geologic N, and summer increases in net nitrate production. Measurements of shale δ15N and soil-, well-, and streamwater nitrate within one catchment point toward a summer increase in soil net nitrification as the driver of this pattern. Rather than seasonal plant demand, processes governing the seasonal production, retention, and transport of nitrate in soils may drive nitrate seasonality in this and many other systems. [ABSTRACT FROM AUTHOR]

Denitrification, the anaerobic reduction of nitrogen oxides to nitrogenous gases, is an extremely challenging process to measure and model. Much of this challenge arises from the fact that small areas (hotspots) and brief periods (hot moments) frequently account for a high percentage of the denitrification activity that occurs in both terrestrial and aquatic ecosystems. In this paper, we describe the prospects for incorporating hotspot and hot moment phenomena into denitrification models in terrestrial soils, the interface between terrestrial and aquatic ecosystems, and in aquatic ecosystems. Our analysis suggests that while our data needs are strongest for hot moments, the greatest modeling challenges are for hotspots. Given the increasing availability of high temporal frequency climate data, models are promising tools for evaluating the importance of hot moments such as freeze-thaw cycles and drying/rewetting events. Spatial hotspots are less tractable due to our inability to get high resolution spatial approximations of denitrification drivers such as carbon substrate. Investigators need to consider the types of hotspots and hot moments that might be occurring at small, medium, and large spatial scales in the particular ecosystem type they are working in before starting a study or developing a new model. New experimental design and heterogeneity quantification tools can then be applied from the outset and will result in better quantification and more robust and widely applicable denitrification models. [ABSTRACT FROM AUTHOR]

Evidence for abiotic immobilization of nitrogen (N) in soil is accumulating, but remains controversial. Identifying the fate of N from atmospheric deposition is important for understanding the N cycle of forest ecosystems. We studied soils of two Abies pinsapo fir forests under Mediterranean climate seasonality in southern Spain—one with low N availability and the other with symptoms of N saturation. We hypothesized that biotic and abiotic immobilization of nitrate (NO3 −) would be lower in soils under these forests compared to more mesic temperate forests, and that the N saturated stand would have the lowest rates of NO3 − immobilization. Live and autoclaved soils were incubated with added 15NO3 − (10 μg N g−1 dry soil; 99% enriched) for 24 h, and the label was recovered as total dissolved-N, NO3 −, ammonium (NH4 +), or dissolved organic-N (DON). To evaluate concerns about possible iron interference in analysis of NO3 − concentrations, both flow injection analysis (FIA) and ion chromatography (IC) were applied to water extracts, soluble iron was measured in both water and salt extracts, and standard additions of NO3 − to salt extracts were analyzed. Good agreement between FIA and IC analysis, low concentrations of soluble Fe, and 100% (±3%) recovery of NO3 − standard additions all pointed to absence of an interference problem for NO3 − quantification. On average, 85% of the added 15NO3 − label was recovered as 15NO3 −, which supports our hypothesis that rates of immobilization were generally low in these soils. A small amount (mean = 0.06 μg N g−1 dry soil) was recovered as 15NH4 + in live soils and none in sterilized soils. Mean recovery as DO15N ranged from 0.6 to 1.5 μg N g−1 dry soil, with no statistically significant effect of sterilization or soil type, indicating that this was an abiotic process that occurred at similar rates in both soils. These results demonstrate a detectable, but modest rate of abiotic immobilization of NO3 − to DON, supporting our first hypothesis. These mineral soils may not have adequate carbon availability to support the regeneration of reducing microsites needed for high rates of NO3 − reduction. Our second hypothesis regarding lower expected abiotic immobilization in soils from the N-saturated site was not supported. The rates of N deposition in this region may not be high enough to have swamped the capacity for soil NO3 − immobilization, even in the stand showing some symptoms of N saturation. A growing body of evidence suggests that soil abiotic NO3 − immobilization is common, but that rates are influenced by a combination of factors, including the presence of plentiful available carbon, reduced minerals in anaerobic microsites and adequate NO3 − supply. [ABSTRACT FROM AUTHOR]

Dissolved organic carbon (DOC) concentrations have risen in upland waters across large areas of Europe and North America. Two proposed drivers of these increases are (1) deposition of atmospheric pollutant nitrogen (N) with consequent effects on plant and decomposer carbon dynamics, and (2) soil recovery from acidification associated with decreasing sulphur deposition. Examination of 12 European and North American field N addition experiments showed inconsistent (positive, neutral, and negative) responses of DOC to N addition. However, responses were linked to the form of N added and to resulting changes in soil acidity. Sodium nitrate additions consistently increased DOC, whereas ammonium salts additions usually decreased DOC. Leachate chemistry was used to calculate an index of “ANC forcing” of the effect of fertilization on the acid-base balance, which showed that DOC increased in response to all de-acidifying N additions, and decreased in response to all but three acidifying N additions. Exceptions occurred at two sites where N additions caused tree mortality, and one experiment located on an older, unglaciated soil with high anion adsorption capacity. We conclude that collectively these experiments do not provide clear support for the role of N deposition as the sole driver of rising DOC, but are largely consistent with an acidity-change mechanism. It is however possible that the unintended effect of acidity change on DOC mobility masks genuine effects of experimental N enrichment on DOC production and degradation. We suggest that there is a need, more generally, for interpretation of N manipulation experiments to take account of the effects that experimentally-induced changes in acidity, rather than elevated N per se, may have on ecosystem biogeochemistry. [ABSTRACT FROM AUTHOR]

Terrestrial desert ecosystems are strongly structured by the distribution of plants, which concentrate resources and create islands of fertility relative to interplant spaces. Atmospheric nitrogen (N) deposition resulting from urbanization has the potential to change those spatial patterns via resource inputs, resulting in more homogeneous soil resource availability. We sampled soils at 12 desert remnant sites around Phoenix, Arizona along a model-predicted gradient in N deposition to determine the degree to which deposition has altered spatial patterns in soil resource availability and microbial activity. Soil microbial biomass and abundance were not influenced by atmospheric N deposition. Instead, plant islands remained strong organizers of soil microbial processes. These islands of fertility exhibited elevated pools of resources, microbial abundance, and activity relative to interspaces. In both plant islands and interspaces, soil moisture and soil N concentrations predicted microbial biomass and abundance. Following experimental wetting, carbon dioxide (CO2) flux from soil of interspaces was positively correlated with N deposition, whereas in plant islands, soil CO2 flux was positively correlated with soil moisture content and soil organic matter. Soil CO2 flux in both patch types showed rapid and short-lived responses to precipitation, demonstrating the brief time scales during which soil biota may process deposited materials. Although we observed patterns consistent with N limitation of microbes in interspaces, we conclude that atmospheric N deposition likely accumulates in soils because microbes are primarily limited by water and secondarily by carbon or nitrogen. Soil microbial uptake of atmospherically deposited N likely occurs only during sparse and infrequent rainfall. [ABSTRACT FROM AUTHOR]

Storages and cycling of total mercury (HgT), methylmercury (MeHg), and Hg0 are described for Spring Lake, a small bog lake in the Marcell Experimental Forest in north-central Minnesota. We quantified photoredox transformations, MeHg photolysis, burial to the sediments, and internal and external loadings of HgT and MeHg. Atmospheric deposition was the main input of HgT; MeHg was supplied by a combination of atmospheric, near-shore wetland, and biotic (methylation) sources. HgT outputs were dominated by burial (67%), and Hg0 evasion accounted for 26% of HgT outputs. The watershed of Spring Lake is small (3.7× lake surface area), and accordingly, bog and upland runoff were minor contributors to both HgT and MeHg inputs. Wet deposition was ∼9% of total MeHg input, and other external inputs (runoff, sediment porewater) provided only an additional 7%, indicating that internal production of MeHg was occurring in the lake. Photolysis of MeHg, measured in the field and laboratory, removed ∼3× the lake mass of MeHg (20 mg) annually, and was the dominant sink for MeHg. Residence times of MeHg and HgT in the lake were 48 and 61 days, respectively, during the open-water season, compared with only 8 days for the residence time of MeHg on settling particles (seston). Photoreduction of Hg2+ to Hg0 was greater than the reverse reaction (Hg0 photooxidation), and the residence time of Hg0 in the photic zone was short (hours). Data from this study show active cycling of all the measured species of mercury (HgT, MeHg, and Hg0) and the importance of MeHg photolysis and photo-redox processes. [ABSTRACT FROM AUTHOR]

Dissolved organic carbon (DOC) and total and inorganic nitrogen and phosphorus concentrations were determined over 3 years in headwater streams draining two adjacent catchments. The catchments are currently under different land use; pasture/grazing vs plantation forestry. The objectives of the work were to quantify C and nutrient export from these landuses and elucidate the factors regulating export. In both catchments, stream water dissolved inorganic nutrient concentrations exhibited strong seasonal variations. Concentrations were highest during runoff events in late summer and autumn and rapidly declined as discharge increased during winter and spring. The annual variation of stream water N and P concentrations indicated that these nutrients accumulated in the catchments during dry summer periods and were flushed to the streams during autumn storm events. By contrast, stream water DOC concentrations did not exhibit seasonal variation. Higher DOC and NO3 − concentrations were observed in the stream of the forest catchment, reflecting greater input and subsequent breakdown of leaf-litter in the forest catchment. Annual export of DOC was lower from the forested catchment due to the reduced discharge from this catchment. In contrast however, annual export of nitrate was higher from the forest catchment suggesting that there was an additional NO3 − source or reduction of a NO3 − sink. We hypothesize that the denitrification capacity of the forested catchment has been significantly reduced as a consequence of increased evapotranspiration and subsequent decrease in streamflow and associated reduction in the near stream saturated area. [ABSTRACT FROM AUTHOR]

The fate and transport of watershed-derived ammonium in a tidal freshwater marsh fringing the nutrient rich Scheldt River, Belgium, was quantified in a whole ecosystem 15N labeling experiment. In late summer (September) we added 15N-NH to the flood water entering a 3477 m² tidal freshwater marsh area, and traced the ammonium processing and retention in four subsequent tide cycles. In this paper we present the results for the water-phase components of the marsh system and compare them to a similar experiment conducted in spring/early summer (May). Changes in concentration and isotopic enrichment of NO3- NO2-, N2O, N2, NH4+ and suspended particulate nitrogen (SPN) were measured in concert with a mass balance study. All analyzed N-pools were labeled, and 49% of the added 15NH4+ was retained or transformed. The most important pool for 15N was nitrate, accounting for 17% of 15N-transformation. N2, N2O and SPN accounted for 2.4, 0.02 and 1.4%, respectively. The temporal and spatial patterns of 15N transformation in the water phase component of the system were remarkably similar to those observed in May, indicating good reproducibility of the whole ecosystem labeling approach, but the absolute ammonium transformation rate was 3 times higher in May. While the marsh surface area was crucial for nitrification in May this was less pronounced in September. Denitrification, on the other hand, appeared more important in September compared to May. [ABSTRACT FROM AUTHOR]

Analyzes the inorganic nitrogen (N) yield and retention in high-elevation watersheds of the Sierra Nevada and Rocky Mountains. Landscape characteristics of the high elevation watersheds; Nitrogen chemistry and fluxes; Correlations and probabilities between N fluxes, N retention, nitrate concentrations and catchment landscape features for high elevation watersheds.

Presents results from a snow manipulation experiment in the northern hardwood forest ecosystem at the Hubbard Brook Experimental Forest in the White Mountains of New Hampshire. Importance of snow as an insulator of forest soils; Changes in fine root and microbial mortality due to soil freezing; Indirect effects of climate change on the structure and function of ecosystems.

Evaluates the biogeochemistry of nitrogen (N) for three forest ecosystems in the Adirondack Mountains in New York. Relationships between atmospheric N deposition and forest ecosystem biogeochemistry; Effect of elevated N inputs on forest ecosystems; Relation of the patterns in N abundance to N concentrations in vegetation.

Evaluates the nitrogen dynamics within the Arbutus Lake watershed in the Adirondack Mountains of New York State. Total dissolved nitrogen flux; Differences between the factors that control dissolved organic nitrogen; Biotic controls in the growing season.

The S cycle in the water column of a small, soft-water lake was studied for 9 years as part of an experimental study of the effects of acid rain on lakes. The two basins of the lake were artificially separated, and one basin was experimentally acidified with sulfuric acid while the other served as a reference or control. Spatial and seasonal patterns of sulfate uptake by plankton (53-70 mmol m-2 yr-1), deposition of sulfur to sediments in settling seston (53 mmol m-2 yr-1), and sulfate diffusion (0-39 mmol m-2 yr-1) into sediments were examined. Measurements of inputs (12-108 mmol m-2 yr-1) andoutputs (5.5-25 mmol m-2 yr-1) allowed construction of a mass balance that was then compared with rates of S accumulation in sediments cores (10-28 mmol m-2 yr-1)and measured fluxes of S into the sediments. Because of the low SO42- concentrations (Mu mole L-1) in the lake, annual uptake by plankton (53-70 mmol m-2 yr-1) represented a large fraction (>50%) of the SO42- inventory in the lake. Despite this large flux through theplankton, only small seasonal fluctuations in SO42- concentrations (Mu mole L-1) were observed; rapid mineralization of organic matter (half-life <3 months) prevented sulfate depletion in the water column. The turnover time for sulfate in thewater column is only 1.4 yr; much less than the 11-yr turnover time of a conservative ion in this seepage lake. Sulfate diffusion into and reduction in the sediments (0-160 5mole m-2 d-1) caused SO42- depletion in the hypolimnion. Modeling of seasonal changes in lake-water SO42- concentrations indicated that only 30-50% of the diffusive flux of sulfate to the sediments was permanently incorporated in solid phases, an [ABSTRACT FROM AUTHOR]

The accumulation of total mercury (HgT) and methylmercury(MeHg) was evaluated in sediments of Tivoli South Bay, a freshwater tidal mudflat wetland in the Hudson River National Estuarine ResearchReserve system. HgT concentrations in sediment cores weremeasured to evaluate the spatial variability of HgT deposition, and to establish a chronology of HgT accumulation. Cores taken from the northern, middle, and southern sections of the bay had similar distribution patterns and concentrations of HgT, suggesting a common source of HgT throughout the bay. Sediment concentrations ranged from 190 to 1040 ng Hg g-1, 2 to 10 times greater than concentrations expected in sediments from non-anthropogenic sources. HgT deposition rates were similar in different regions of the bay, and increased from 200 ng Hg cm-2 yr-1in the 1930s to a maximum of 300 ng Hg cm-2 yr-1 in the 1960s. Deposition rates have steadily declined since the 1970s and are currently at 80 ng Hg cm-2 yr-1. Transport of HgT by tidal waters from the Hudson River is likely the main source of HgT in the bay. Distribution patterns and absolute concentrations of MeHg in sediment cores were similar throughout the bay, with concentrations ranging from 0.43 to 2.95 ng g-1. Maxima in MeHg concentration profiles occurred just below the sediment-water interface and at a depth of 30 cm. The maximum at 30 cm was coincident with maximumHgT concentrations. MeHg concentrations in suspended particulate matter (SPM) from the Hudson River suggest that MeHg in the bay could be derived from riverine SPM rather than formed in situ. [ABSTRACT FROM AUTHOR]

Cumulative effects of atmospheric N deposition may increase N exportfrom watersheds and contribute to the acidification of surface waters, but natural factors (such as forest productivity and soil drainage) that affect forest N cycling can also control watershed N export. To identify factors that are related to stream-water export of N, elevational gradients in atmospheric deposition and natural processes were evaluated in a steep, first-order watershed in the Catskill Mountains of New York, from 1991 to 1994. Atmospheric deposition of SO42-, and probably N, increased with increasing elevation within this watershed. Stream-water concentrations of SO42- increased with increasing elevation throughout the year, whereas stream-water concentrations of NO3- decreased with increasing elevation during the winter and spring snowmelt period, and showed no relation with elevation during the growing season or the fall. Annual export of N in stream water for the overall watershed equaled 12% to 17% of the total atmospheric input on the basis of two methods of estimation. This percentage decreased withincreasing elevation, from about 25% in the lowest subwatershed to 7% in the highest subwatershed; a probable result of an upslope increase in the thickness of the surface organic horizon, attributable to an elevational gradient in temperature that slows decomposition rates at upper elevations. Balsam fir stands, more prevalent at upper elevations than lower elevations, may also affect the gradient of subwatershed N export by altering nitrification rates in the soil. Variationsin climate and vegetation must be considered to determine how futuretrends in atmospheric deposition will effect watershed export of nitrogen. [ABSTRACT FROM AUTHOR]

Seven Wisconsin rivers with contrasting, relatively homogeneous watershed composition were selected to assess the factors controlling mercury transport. Together, these watersheds allow comparisons of wetland, forest, urban and agricultural land-uses. Each site was sampled nine times between September 1993 and September 1994 to establish seasonal signatures and transport processes of total mercury (Hg-T) and methylmercury (MeHg). Our results clearly show that land use and land cover strongly influence mercury transport processes. Under base-flowconditions, unfiltered MeHg yield varies by a factor of sixteen (12-195 mg km-2 d-1), and increases with the fraction of wetland area in the watershed. Elevated mercury yields during high flow are particle-phase associated in agricultural sites, but filtered-phase associated in wetland sites. Methylmercury represented less than 5% of total mercury mobilized during the spring thaw across all watersheds. Autumn MeHg yield was generally 11-15% of Hg-T in wetland influenced watersheds, though a maximum of 51% was observed. In some cases, single high-flow events may dominate the annual export of mercury from a watershed. For example, one high-flow event on the agricultural Rattlesnake Creek had the largest Hg-T and MeHg yield in the study (107 and 2.32 mg km-2 d-1, respectively). The mass of mercury transported downstream by this single event was an order of magnitude larger than the eight other (non-event) sampling dates combined. These results underscore the importance of watershed characteristics and seasonal events on the fate of mercury in fresh water rivers. [ABSTRACT FROM AUTHOR]

The biogeochemistry of Hg was evaluated in a small wetland in the Adirondack region of New York. Concentrations of total Hg (HgT) in streamwater draining the wetland showed little temporal variation. The annual areal watershed flux of HgT (2.2 mu g/m2-yr) was considerably smaller than regional inputs of atmospheric deposition of HgT, indicating that the terrestrial environment is a net sink for atmospheric deposition of HgT. Drainage inputs of HgT were conservatively transported through the beaver impoundment. The annual flux of total methylmercury(CH3 HgT+) was greater than literature values of atmospheric deposition suggesting that the watershed isa net source of CH3 HgT+. Stream concentrations of CH3 HgT+ increased during low-flow summer conditions in a riparian wetland, and particularly at the outlet of the beaver impoundment. Net production of CH3 HgT+ occurred in the beaver impoundment (0.45 mu g/m2-yr). Rates of net methylation for the beaver impoundment were comparable to values reported in the literature for wetlands. [ABSTRACT FROM AUTHOR]

Methylmercury (MeHg) concentrations and production rates were examined along with sulfur biogeochemistry in Everglades sediments in March, July and December, 1995, as part of a large, multi-investigator study, the Aquatic Cycling of Mercury in the Everglades (ACME) project. The sites examined constitute a trophic gradient, generated from agricultural runoff, across the Everglades Nutrient Removal (ENR) Area, which is a re-constructed wetland, and Water Conservation Areas (WCA) 2A, 2B and 3 in the northern Everglades. MeHg concentrations and %MeHg (MeHg as a percent of total Hg) were lowest in the more eutrophic areas and highest in the more pristine areas in the south. MeHg concentrations ranged from <0.1 ng gdw-1 sediment in the ENR to 5 ng gdw-1 in WCA3 sediments; and MeHg constituted <0.2% of total Hg (HgT) in ENR, but up to about 2% in two sites in WCA2B and WCA3. Methylation rates in surficial sediments, estimatedusing tracer-level injections of 203Hg (II) sediment cores, ranged from 0 to 0.12 d-1, or about 1 to 10 ng g-1 d-1 when the per day values are multiplied by the ambient total Hg concentration. Methylation was generally maximal at or within centimeters of the sediment surface, and was never observed in water overlying cores. The spatial pattern of MeHg production generally matched that of MeHg concentration. The coincident distributions of MeHg and its production suggest that in situ production controlsconcentration, and that MeHg concentration can be used as an analog for MeHg production. In addition, the spatial pattern of MeHg in Everglades sediments matches that in biota. suggesting that MeHg bioaccumulation may be predominantly a function of the de novo methylation rate in surficial sediments. Sulfate concentrations in surficial pore waters (up to 400 mu m), microbial sulfate-reduction rates (up to 800 nm cc-1 d-1) and resultant pore water sulfide con [ABSTRACT FROM AUTHOR]

Models of the effects of atmospheric N deposition in forested watersheds have not adequately accounted for the effects of aquatic and near-stream processes on the concentrations and loads of NO3- in surface waters. This study compared the relative effects of aquatic and near-stream processes with those from the terrestrial ecosystem on the retention and transport of NO3- in two contrasting stream reaches of the Neversink River, a forested watershed in the Catskill Mountains of New York that receives among the highest load of atmospheric N deposition in the northeastern United States. Stream water samples were collected every two hours and ground-water and tributary samples were collected daily at base flow conditions during four 48-hour periods from April to October 1992, and NO3- mass balances were calculated for eachsite. Results indicated diurnal variations in stream NO3- concentrations in both reaches during all four sampling periods; this is consistent with uptake of NO3- by photoautotrophs during daylight hours. Mass-balance results revealed significant stream reach losses of NO3- at both sites during all sampling periods. The diurnal variations in NO3- concentrations and the retention of NO3- relative to terrestrial contributions to the stream reaches were greater downstream than upstream because physical factors such as the head gradients of inflowing ground water and the organic matter content of sediment are more favorable to uptake and denitrification downstream. The mass retention of NO3-increased as the mean 48-hr stream discharge increased at each site,indicating that the responsible processes are dependent on NO3- supply. Low stream temperatures during the April sampling period, however, probably reduced the rate of retention processes [ABSTRACT FROM AUTHOR]

A 12-month study was conducted to measure the concentrations of dissolved organic matter (DOC, TDN, TDP) in four sites within a 119 km long reach of the Ohio River, near Louisville, KY. In this study we test whether specific geomorphological and biological factors influencedvariations in dissolved organic matter. Concentrations of DOC in theriver averaged ~~ 1200 mu mol/L, and varied by nearly two orders of magnitude seasonally (mean DOC during base flow ~~ 620 mu mol/L). Peak periods for DOC at all sites were during April--May. The site nearest a navigation dam (deeper, lower current velocities) had significantly lower concentrations of TDN and greater C:N ratios than upstream sites. The largest tributary entering this reach (Kentucky River) hadno significant effect on levels of DOM in the main river, despite having significantly greater concentrations of TDN and lower levels of DOC during most months of the year. Concentrations of DOC, TDN, and TDP were not significantly different in littoral and pelagic habitats at all sites studied, suggesting little floodplain influence on DOM in this constricted-channel section of the Ohio River. C:N ratios of DOM in the Ohio were significantly different among seasons; C:N exceeded or equaled Redfield ratios in summer and fall (6 to 10), but were below Redfield (1.8 to 3.0) during winter and spring. Regression models suggest that total phytoplankton densities and flow conditions arethe two most important factors regulating DOM in this very large river. [ABSTRACT FROM AUTHOR]

Rates of in situ sulfate reduction (SRR) in planted and unplanted rice field soil were measured by the 35SO42--radiotrac er method using soil microcosms. The concentration of35SO42- decreased exponentially with time. However, time course experiments indicated that incubation times of 10--30 min were appropriate for measurements of SRR using a single time point in routine assays. Unplanted microcosms showed high SRR of 177 nmol cm-3 d-1 in the uppermost centimeter where average sulfate concentrations were < 33 mu M. Fine scaledmeasurements (1 mm resolution) localized highest SRR (< 100 nmol cm- 3 d-1) at the oxic/anoxic interface at 2--5 mmdepth. In planted rice field soil, SRR of <310 nmol cm-3 d-1 were observed at 0--2 cm depth. Sulfate reduction rates were determined at a millimeter-scale with distance to a two dimensional root compartment. The SRR was highest at 0--1.5 mm distance to the root layer with rates up to 500 nmol cm-3 d-1, indicating a high stimulation potential of the rice roots. SRR seemed to be mainly dependent on the in situ sulfate porewater concentrations. At the soil surface of unplanted microcosms sulfate concentration decreased from < 150 mu M to < 10 mu M within the first 8 mm of depth. In planted microcosms sulfate concentration varied from 87--99mu M sulfate at the 0--3 mm distance to the root layer to 48-- 62 muM sulfate at a root distance > 4 mm from the roots. The depth distribution of inorganic sulfur compounds was determined for planted and unplanted rice field soil. Sulfate, acid volatile sulfide (AVS) and chromium reducible sulfide (CRS) were up to 20 fold higher in planted than in unplanted microcosms. CRS was the major 3 insoluble sulfur fraction with concentrations > 1.7 mu mol cm-3. Organic sulfur accounted for 25--46% of the total sulfur present (269 mu g/g dw) in an [ABSTRACT FROM AUTHOR]

Intact rich fen soil cores with controlled water levels near the surface were installed in an open greenhouse. To simulate short term summer drought, water levels were lowered (20 cm) after two weeks in half of the cores (experimental cores) and remained near the surface in the other half (blanks). After two more weeks, the water levels were brought back to the surface in the experimental cores and remained there for another two weeks. In the blanks, reduction and alkalinization of the top peat layer occurred. In the experimental soil cores oxidation and acidification started within one week after drawdown. An indication for a drought induced rise in soluble reactive phosphorus has been found. The velocity of the acidification process illustrates the dynamic nature of the hydrochemical conditions in fen soils duringdrought. The processes controlling the acid/base status of rich fen,the effect of drought induced acidification on P availability and the significance for the vegetation are discussed. [ABSTRACT FROM AUTHOR]

Concentrations of dissolved organic carbon (DOC) and color were measured as a function of time in enclosures and lakes at the Experimental Lakes Area, to calculate their net loss rates. Loss rates in enclosures were first order for both DOC and color, with half-times for loss of 166 and 122 d, respectively. Thus, the colored, light-attenuating component of the DOC pool is lost from water more rapidly than is bulk DOC. Loss rates in lakes, calculated from a steady state model, were similar to values for color in enclosures, but for DOC in lakes were four times slower than in enclosures. In lakes, loss rate for DOCincreased rapidly with decreasing water residence time (tau{sub}w{end}) but was independent of tau{sub}w{end} when it was greater than 3 years. In lakes, the loss rate for color was independent of water residence time. The difference in losses of DOC and color between lakes and enclosures could be from release of low-color DOC from sediments. [ABSTRACT FROM AUTHOR]

Reductions in the global stratospheric ozone layer are thought to beincreasing the amount of ultraviolet B (UV-B) radiation reaching theplanet's surface and may be affecting the chemistry of dissolved organic carbon (DOC) in surface waters. We studied the abundance of chromophores in DOC collected in four different aquatic environments in Southwestern Nova Scotia using {sup}13{end}C nuclear magnetic resonance ({sup}13{end}C NMR) spectroscopy. We showed a clear seasonal pattern in the distribution of structural carbon related to light-sensitivechromophores. There seemed to be little variation in the UV-B related chemical structure of DOC between lakes and streams, though water from a bog pool showed large differences from the other samples. Thesepatterns of potential UV-B reactivity tend to be dampened however, by variations in DOC concentrations which also occur seasonally. [ABSTRACT FROM AUTHOR]