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One of the less studied components of carbon cycling that could improve our understanding of how and how strongly Amazonian ecosystems act as sinks or sources of carbon is the amount that is carried downstream by rivers. In this paper, we show that a headwater river can carry from 25 to 130 % of the reported sink for Amazonian forests, therefore not being negligible for ecosystem-level carbon budgets. Based on monthly measurements from May 2004 to April 2005 of the upper Purus River, southwestern Amazonia, we found that: water pH, dissolved oxygen, specific electrical conductivity, and dissolved inorganic carbon (DIC) were inversely related to water discharge and precipitation; pCO was directly and strongly related to discharge and precipitation, and to a lesser extent to pH and dissolved oxygen; and dissolved organic carbon (DOC) was not related to any measured variable. Annual flux of dissolved carbon (DIC + DOC) at the sampling site was estimated as 604 ± 55 Gg C a. More than 75 % was in the form of bicarbonate, with the remainder as CO and DOC. This amount is equivalent to 0.15 ± 0.01 Mg C ha a in the upstream drainage basin, which is on the same order of magnitude as terrestrial carbon fixation. [ABSTRACT FROM AUTHOR]

This paper shows that the sub-surface light regime in the offshore North Sea varies spatially and seasonally between different ecohydrodynamic regions, which is likely to have important implications for primary production and carbon and nutrient fluxes in different areas of the North Sea. Measurements of downward irradiance were collected using different instruments (i.e. water column-profiling instruments, semi-autonomous moorings, and remote sensing) at three ecohydrodynamically distinct sites in the North Sea: in the southern Bight (SB), at the Oyster Grounds (OG) and north of the Dogger Bank (ND). The ND site was the deepest, and had the lowest and least variable light attenuation coefficients (mean K(PAR) = 0.11 m). The onset of the phytoplankton spring bloom was earlier than at the other two sites. In summer, ND had low K(PAR) ~ 0.07 m and light penetration was shifted towards blue-green wavelengths (490-560 nm), with water itself being one of the strongest contributors to overall attenuation. In contrast, the SB site was characterised by the highest and most variable values of K(PAR) (mean = 0.54 m), comparable to near-coastal waters, and the spring bloom started almost a month later than at the ND site. The vertical variability of the attenuation coefficient and the strong PAR attenuation in the blue region of the spectrum were the result of higher concentrations of phytoplankton, CDOM and SPM, due to riverine inputs, shallow depth and strong tidal mixing. The OG site showed intermediate conditions between the ND and SB sites with a mean K(PAR) = 0.23 m, and deepest penetration of irradiance in the green region of the spectrum at 560 nm. The implications of these results for phytoplankton growth and ecosystem modelling are discussed. [ABSTRACT FROM AUTHOR]

Microbial communities can potentially mediate feedbacks between global change and ecosystem function, owing to their sensitivity to environmental change and their control over critical biogeochemical processes. Numerous ecosystem models have been developed to predict global change effects, but most do not consider microbial mechanisms in detail. In this idea paper, we examine the extent to which incorporation of microbial ecology into ecosystem models improves predictions of carbon (C) dynamics under warming, changes in precipitation regime, and anthropogenic nitrogen (N) enrichment. We focus on three cases in which this approach might be especially valuable: temporal dynamics in microbial responses to environmental change, variation in ecological function within microbial communities, and N effects on microbial activity. Four microbially-based models have addressed these scenarios. In each case, predictions of the microbial-based models differ-sometimes substantially-from comparable conventional models. However, validation and parameterization of model performance is challenging. We recommend that the development of microbial-based models must occur in conjunction with the development of theoretical frameworks that predict the temporal responses of microbial communities, the phylogenetic distribution of microbial functions, and the response of microbes to N enrichment. [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]

This paper presents an integrated analysis of organic carbon (C) pools in soils and vegetation, within-ecosystem fluxes and net ecosystem exchange (NEE) in three 40-year old Norway spruce stands along a north-south climatic gradient in Sweden, measured 2001–2004. A process-orientated ecosystem model (CoupModel), previously parameterised on a regional dataset, was used for the analysis. Pools of soil organic carbon (SOC) and tree growth rates were highest at the southernmost site (1.6 and 2.0-fold, respectively). Tree litter production (litterfall and root litter) was also highest in the south, with about half coming from fine roots (<1 mm) at all sites. However, when the litter input from the forest floor vegetation was included, the difference in total litter input rate between the sites almost disappeared (190–233 g C m−2 year−1). We propose that a higher N deposition and N availability in the south result in a slower turnover of soil organic matter than in the north. This effect seems to overshadow the effect of temperature. At the southern site, 19% of the total litter input to the O horizon was leached to the mineral soil as dissolved organic carbon, while at the two northern sites the corresponding figure was approx. 9%. The CoupModel accurately described general C cycling behaviour in these ecosystems, reproducing the differences between north and south. The simulated changes in SOC pools during the measurement period were small, ranging from −8 g C m−2 year−1 in the north to +9 g C m−2 year−1 in the south. In contrast, NEE and tree growth measurements at the northernmost site suggest that the soil lost about 90 g C m−2 year−1. [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]

Chlorine is a minor constituent of most rocks and a minor (although essential) element in plants, but it cycles rapidly through the hydrosphere and atmosphere. In forest ecosystem studies, chloride ion (Cl−) is often thought to be conservative in the sense that the sources and sinks within the ecosystem are assumed negligible compared to inputs and outputs. As such, Cl− is often used as a conservative tracer to assess sources and transformations of other ions. In this paper we summarize research on chloride over the course of 36 years (1964–2000) at the Hubbard Brook Experimental Forest (HBEF) in central New Hampshire, USA. Evidence presented here suggests that in the 1960s and 1970s the dominant source of atmospheric Cl− deposition was from pollutant sources, probably coal burning. In the 1970s the Cl− inputs in bulk deposition declined, and the lower Cl− deposition in the last two decades is dominated by marine sources. Between 1964 and 2000 there was no significant trend in Cl− export in stream flow, thus the net hydrologic flux (NHF = bulk deposition inputs − streamflow outputs) has changed over this period. Early in the record the NHF was on average positive, indicating net retention of Cl− within the system, but since about 1980 the NHF has been consistently negative, indicating an unmeasured input or source within the ecosystem. Dry deposition can account for at least part of that unmeasured source, and it appears that release of Cl− from mineralization of soil organic matter (SOM) may also play an important role. We believe that accumulation of Cl− in vegetation during the 1960s and 1970s offset the unmeasured source and resulted in net ecosystem retention. Accumulation of vegetative biomass has ceased since about 1982, leading to the apparent net export (negative NHF) since that time. Although we have no direct measurements of Cl− accumulation in vegetation, our estimates suggest that an aggrading forest could sequester about 32 mol Cl ha−1 year−1, or about a third of the annual average bulk deposition flux to this ecosystem. Experimental additions of Cl− to the forest floor cause increases in Cl− concentration in foliage, throughfall, and soil solution. Manipulations of vegetation also affect the Cl− cycle. Harvesting or devegetation of watersheds causes an increase in the Cl− concentration and flux in stream water for several years after the disturbance. This period of release is followed by a period of reaccumulation of Cl− that may last more than 15 years. In this respect, the behavior of Cl− after disturbance parallels that of NO3−, for which export increases after disturbance due to reduced plant nitrogen uptake and mineralization of nitrogen from detritus, rather than SO42−, for which export decreases after disturbance due to pH-dependent adsorption onto mineral soils. The interannual pattern of Cl− export from the system primarily reflects the atmospheric inputs, but the net retention and cycling of Cl− within the system appears to be largely under biological, rather than geochemical, control. [ABSTRACT FROM AUTHOR]

This paper describes detailed budgets of water, Cl−, dissolved Si and both inorganic and organic forms of nitrogen and phosphorus for two small glacier basins in Arctic Svalbard (Midre Lovénbreen and AustreBrøggerbreen). Rates of nutrient deposition are modest, dominated by inorganic nitrogen and episodically enhanced by extreme events. Hence deposition rates are also variable, ranging from 20 to 72 kg NO3-N km−2 a−1 and 10–37 kg NH4-N km−2 a−1 over just two consecutive years. Deposition of dissolved organic and particulate forms of nitrogen (DONand PN respectively) also appears significant and therefore requires further investigation (3–8 kg DON-N km−2 and 7–26 kg PN-N km−2 during winter – no summer data are available). Evidence for microbially mediated nutrient cycling within the glacial system is clear in the nutrient budgets, as is the release of large phosphorus, Si and organic/particulate nitrogen fluxes by subglacial erosion. The latter is entirely dependent upon the presence of subglacial drainage, promoting silicate mineral dissolution and the erosion of largely unweathered apatite. The large DON and PN fluxes are surprising and may relate to young organic nitrogen associated with microbial life within the glaciers. This is because wide spread assimilation of NH4+ and perhaps even nitrification occurs on the glacier surface, most likely within abundant cryoconite holes. Further microbial activity also occurs at the glacier bed, where denitrification and sulphate reduction is now known to take place. Thus a two component ‘glacial ecosystem’ is proposed that is highly sensitive to climate change. [ABSTRACT FROM AUTHOR]

Nitrogen dynamics in semi-natural environments is crucial for the development and ecological stability of these systems. The present paper shows the results of the reinvestigation of a 15N-tracer experiment, which was established in the Grossglockner massif in Austria at 2300 m a.s.l. in 1974/1975. We show that large quantities of nitrogen introduced by a single pulse labelling (amounting to approximately 1.7% of the nitrogen in the system) into an alpine grassland remain in the soil–plant system, with only 55% being lost during 27–28 years. In the first 10 cm of the four investigated soil profiles 40% of 15N was recovered, being mainly bound in organic forms. A simple site specific model was established on the basis of the results considering a biological, residual and labile N-pool, the latter being the source for N-losses. By the model a long mean residence time close to 100 years was derived for the remaining 15N. [ABSTRACT FROM AUTHOR]

Presents the writing guidelines for various biochemistry authors in Ithaca, New York. Creation of articles that deals with the geochemical control of the structure and function of the ecosystems; Efforts of the publishers to encourage the writers to submit a Web-enable online manuscript; Need of providing keywords to the articles.

The biogeochemical input-output fluxes of two forested catchments with contrasting levels of atmospheric deposition were investigated in Germany. This paper focuses on the effects of recent changes in atmospheric inputs on the chemical composition in the soil solution and stream. The catchment 'Schluchsee' (Black Forest; SW Germany) is characterized by relatively low atmospheric inputs whereas 'Rotherdbach' (Ore Mountains; E Germany) received significant amounts of acid deposition (mainly originating from SO2 emissions) until recent years. Both sites reveal decreases in H+ and S deposition during the 1990s. This pattern is typical when compared to trends in Europe. In response to the reduced S deposition, soil solution and streamwater SO42- concentrations decreased significantly. A net release of SO42- (output > input) was observed at both sites due to the release of S previously stored in the soil. The level of N deposition was more or less constant at both sites. At Schluchsee, NO3- concentration in streamwater remained more or less unchanged, whereas a decrease at Rotherdbach was observed. A recovery from acidification was found in seepage water as indicated by increasing acid neutralizing capacity (ANC). Streamwater ANC increased only in the permanently acidified Rotherdbach. No change of ANC was observed in the Schluchsee stream, which was characterized by episodic acidification during high-flow conditions. Nevertheless, the key factor controlling the recovery from surface water acidification was the type, amount and distribution of stored S pools in the ecosystem. Thus, time series analysis of long-term data of input-output chemistry can be a valuable instrument in order to improve the understanding of linked terrestrial-aquatic systems and give useful clues for modeling efforts. [ABSTRACT FROM AUTHOR]

Ocean acidification is one of the most dramatic effects of the massive atmospheric release of anthropogenic carbon dioxide (CO2) that has occurred since the Industrial Revolution, although its effects on marine ecosystems are not well understood. Submarine volcanic hydrothermal fields have geochemical conditions that provide opportunities to characterise the effects of elevated levels of seawater CO2 on marine life in the field. Here, we review the geochemical aspects of shallow marine CO2-rich seeps worldwide, focusing on both gas composition and water chemistry. We then describe the geochemical effects of volcanic CO2 seepage on the overlying seawater column. We also present new geochemical data and the first synthesis of marine biological community changes from one of the best-studied marine CO2 seep sites in the world (off Vulcano Island, Sicily). In areas of intense bubbling, extremely high levels of pCO2 (> 10,000 μatm) result in low seawater pH (< 6) and undersaturation of aragonite and calcite in an area devoid of calcified organisms such as shelled molluscs and hard corals. Around 100–400 m away from the Vulcano seeps the geochemistry of the seawater becomes analogous to future ocean acidification conditions with dissolved carbon dioxide levels falling from 900 to 420 μatm as seawater pH rises from 7.6 to 8.0. Calcified species such as coralline algae and sea urchins fare increasingly well as sessile communities shift from domination by a few resilient species (such as uncalcified algae and polychaetes) to a diverse and complex community (including abundant calcified algae and sea urchins) as the seawater returns to ambient levels of CO2. Laboratory advances in our understanding of species sensitivity to high CO2 and low pH seawater, reveal how marine organisms react to simulated ocean acidification conditions (e.g., using energetic trade-offs for calcification, reproduction, growth and survival). Research at volcanic marine seeps, such as those off Vulcano, highlight consistent ecosystem responses to rising levels of seawater CO2, with the simplification of food webs, losses in functional diversity and reduced provisioning of goods and services for humans. [ABSTRACT FROM AUTHOR]

Although aquatic ecosystems are a major carbon reservoir, how their carbon dynamics will respond to increasing concentrations of atmospheric CO is not well understood. The availability of essential nutrients has the potential to modify carbon fluxes under elevated CO by altering carbon processing and storage in the biota. Here, we describe a semi-continuous culture experiment with natural phytoplankton and bacteria assemblages designed to investigate (1) how carbon dynamics in aquatic ecosystems respond to continuously elevated atmospheric CO, and (2) whether carbon fluxes resulting from elevated CO are modified by changes in inorganic nitrogen and phosphorus availability. Our results showed that elevated CO led to significant increases in photosynthetic carbon uptake, despite a decrease in the algal chlorophyll a concentrations and no significant change in total algal biovolume. This enhancement of inorganic carbon uptake was accompanied by a significant increase in dissolved organic carbon (DOC) production. Concurrent increases in the C/N and C/P ratios of dissolved organic matter also suggested that DOC stability increased. Nutrient availability, especially nitrogen availability, had strong effects on inorganic carbon uptake and biomass carbon pools. In contrast, CO-enhanced DOC production was not significantly affected by varying concentrations of inorganic nitrogen and phosphorus. Our study underscores the importance of DOC as a potential carbon sink in aquatic ecosystems. The observed responses to changes in CO and nutrient availability could have important implications for long-term carbon cycling in aquatic ecosystems, and highlight the potential buffering capacity of aquatic ecosystems to future environmental change. [ABSTRACT FROM AUTHOR]

Biological nitrogen (N) fixation is the primary source of 'new' N to unmanaged ecosystems, and recent analyses suggest that terrestrial N inputs via free-living N fixation may be more important than previously assumed. This may be particularly true in some tropical rain forests, where free-living fixation could outpace symbiotic N fixation to represent the dominant source of new N inputs. However, our understanding of the controls over free-living N fixation in tropical rain forests remains poor, which directly constrains our ability to predict how N cycling will respond to changing environmental conditions. Although both phosphorus (P) and molybdenum (Mo) availability have been shown to limit free-living N fixation rates in the tropics, few studies have simultaneously explored P versus Mo limitation or the potential importance of P × Mo interactions. Here, an archived set of foliar, litter, and soil samples from a Costa Rican tropical rain forest provided an opportunity to simultaneously assess the relative strength of P versus Mo relationships with free-living N fixation rates. We also conducted a short-term, full-factorial (P × Mo) litter incubation experiment to directly assess nutrient limitation, allowing us to explore P and Mo controls over free-living N fixation rates using both observational and experimental approaches. We previously showed that N fixation rates were positively correlated with P concentrations in all substrates and, using the archived samples, we now show that Mo concentrations correlated with N fixation only in canopy leaves (where total Mo concentrations were extremely low). Likewise, fertilization with P alone (and not Mo) stimulated leaf litter N fixation rates. Thus, our results suggest that P availability dominantly controls free-living N fixation at this site, and when taken with data from other studies, our results suggest that attempts to identify 'the nutrient' that limits N fixation in 'the tropics' may be misguided. Rather, nutrient controls over free-living N fixation appear to be more nuanced-and the true nature of nutrient limitation to N fixation likely varies over a variety of scales across the vast tropical rain forest biome. [ABSTRACT FROM AUTHOR]

Dissolved organic matter (DOM) is a critical phase in terrestrial carbon and nutrient cycling forming the basis of many ecosystem functions, yet the primary drivers controlling its flux from organic horizons and resultant chemical composition remain only partially understood. We studied dissolved organic matter production and chemistry from organic soil horizons across a 4.1 My old well-constrained chronosequence in Hawaii. Controlled soil column irrigation and leaching experiments were conducted on field moist organic soil horizons to quantify microbial activity, DOM production and chemistry. Both microbial activity (defined as CO production per unit substrate C) and DOM production were found to be lowest in the youngest (0.3 ky) and oldest (4.1 My) sites of the chronosequence, where nutrients (N and P respectively) were most limiting. By contrast, DOM production and microbial activity was greatest at the intermediate-aged (20-350 ky) sites where nutrients were least limiting, unrelated to the mass of organic matter found in the organic horizons. While differences in production rates were found, C NMR spectroscopic results indicated that there was a convergence of chemistry from the solid to the dissolved phase at all sites. In particular, all DOM samples were found to have a high proportion of aromatic acids. With supporting data from a diverse range of ecosystems, we postulate that chemical homogenization of DOM relative to source material is a common feature of many ecosystems due to two microbially mediated processes: (1) similar extracellular enzymatic oxidation conferring solubility to a subset of degradation products; and (2) the rapid selective consumption of the more labile organic compounds in the soil solution. [ABSTRACT FROM AUTHOR]

Solute chemistry and stable isotope tracers of NO were used to assess bacterial NO production and denitrification in a High Arctic glacial ecosystem during 2009. Changes in the NO concentration and the δO-NO in all the proglacial streams revealed that up to 95 % of total NO was most likely bacterially-derived during low flow conditions towards the end of the summer (day of year 250). However, overlapping ranges of δN values for snow NH, soil organic matter, cryoconite debris and geological nitrogen in host rocks mean that neither the preferred substrate(s), nor the pathway (i.e. nitrification or simple mineralisation) can be discerned. The most plausible explanation for the bacterial production of NO is nitrification in snowmelt-fed flowpaths through avalanche fans that flank the glacier and along subglacial drainage pathways at the glacier bed. Interestingly, there was no evidence for denitrification in subglacial outflow, which is contrary to earlier research at this site. Instead, increases in the δN-NO of up to 20 ‰ downstream of the glacier margin, suggests that denitrification in the glacier forefield and/or the sediments that flank it was most discernable during 2009. Our observations therefore suggest that poorly understood temporal variations in the mixing ratio of nitrifying and denitrifying flowpaths occur in this glacial ecosystem. [ABSTRACT FROM AUTHOR]

A conceptual model of the main carbon and nitrogen flows through pelagic and benthic food webs was used to identify the key biogeochemical processes representing ecosystem functioning, and to select indicators of each of these processes. A combined fieldwork and modelling approach was used to provide the data required to evaluate the indicators in terms of their suitability for assessing and managing the impacts of climate change and demersal trawling. Four of our 16 proposed indicators (phytoplankton production and productivity, near-bed oxygen concentrations and oxygen penetration of the seabed) met the majority of criteria we used for evaluating indicators. Five indicators (depth of anoxic sediment, zoobenthos biomass, production, productivity and bioturbation potential) did not comply with sufficient criteria to be considered as good indicators. Six of our proposed indicators (zooplankton biomass, size structure, production and productivity; ecosystem productivity; ecosystem balance) could not be assessed for sensitivity and specificity using our models, and therefore need to be addressed in future work aimed at improving both the models and the fieldwork. Our results indicate that evaluation of indicators is difficult, because of the number and variety of human pressures which need to be considered in reality, and the interactions between these pressures and the ecosystem components which they affect. The challenge will be to establish if there are indeed any indicators which are able to meet the majority of criteria for good indicators in holistic ecosystem-based assessments. [ABSTRACT FROM AUTHOR]

Dissolved silica (DSi) is an important nutrient in aquatic ecosystems. Increased DSi retention within the fluvial system due to damming and eutrophication has led to a decrease in DSi exports to coastal waters, which can have severe consequences for coastal areas where ecosystem functioning depends on fluvial DSi inputs. The analysis of fluvial DSi fluxes and DSi retention at regional to global scales is thus an important research topic. This study explores the possibility to empirically assess regional DSi retention based on a spatially explicit estimation of DSi mobilization and fluvial DSi fluxes calculated from hydrochemical monitoring data. The uncertainty of DSi retention rates (r) estimated for particular rivers is high. Nevertheless, for the St. Lawrence River (r = 91 %) and the Mississippi River (r = 13 %) the estimated DSi retention rates are reasonable and are supported by literature values. The variety of sources of the uncertainty in the DSi retention assessment is discussed. [ABSTRACT FROM AUTHOR]

Post-fire coarse woody debris can represent a valuable nutrient reservoir for a regenerating ecosystem, helping to prevent soil fertility losses after a wildfire. However, there is scarce information on its effect on soil nutrient cycling and availability. We established three study sites along an altitudinal gradient in a burnt pine forest (SE Spain). At each site we determined: (1) decomposition rates and nutrient dynamics in charred logs left on the ground, 2 and 4 years after the fire, and (2) available nutrients in the soil and in the microbial fraction below charred logs and in bare soil areas. Despite the relatively slow decay rates in this Mediterranean climate (ca. 10 % of dry weight lost after 4 years), N and P were progressively released by logs, accounting for ca. 40 and 65 % of the initial content respectively after 4 years. This implies that the total aboveground biomass of the burnt forest released around 20 kg ha of N and 2 kg ha of P during this period. The presence of post fire coarse woody debris consistently increased soil organic matter by around 18 %, total C and N by 42 and 26 %, respectively, dissolved organic C and N by 47 %, available inorganic P by 68 %, and microbial biomass and nitrogen by some 36 and 48 %, respectively. By contrast, soil bulk density decreased by ca. 18 % under logs compared to bare areas. Thus, the fire-killed wood was useful in the recovery of soil fertility and nutrient availability. Leaving the post-fire woody debris on site can enhance the biogeochemical sustainability, microbiological processes and soil ecological functioning. The detrimental effect of post-fire salvage logging on soil fertility should be therefore considered when making management decisions. [ABSTRACT FROM AUTHOR]

Dissolved Organic Matter (DOM) is a fundamental component of the aquatic carbon cycle and a key driver of the biogeochemical interactions between terrestrial and aquatic ecosystems. The origin, properties and role of DOM are increasingly characterised in lakes, rivers and streams, but little is known about DOM characteristics in ephemeral washes, which are the most common water flows in Mediterranean landscapes. Here, we examine the patterns in the optical properties of DOM in ephemeral washes draining a small catchment in the island of Menorca, Western Mediterranean. We used concentration data (dissolved organic carbon and nitrogen) and several spectroscopic descriptors (SUVA, absorption coefficient at 440 nm, fluorescence index, and excitation-emission fluorescence matrices) to assess changes in DOM concentrations and quality at both seasonal and spatial scales. Two periods were clearly distinguished in the DOM properties: autumn and winter-spring. In autumn, which includes the first flows of the hydrological year, DOM showed an aromatic character and was spatially homogenous over the catchment. In winter-spring, DOM was smaller and recently produced, and a considerable spatial heterogeneity was observed in all descriptors. The variability in DOM concentrations and quality was driven by hydromorphology and by the landscape features of the catchment, but the influence of these drivers on DOM properties changed along the hydrological year. In autumn, hydromorphology was the main factor determining DOM properties, whereas in winter-spring the land uses in the watershed highly determined the observed differences in DOM quality between subcatchments. [ABSTRACT FROM AUTHOR]

Wetland ecosystems have a high carbon storage potential as a result of high primary productivity and low decomposition rates dictated by water saturation. In the herbaceous wetlands of the Paraná River Delta, drainage and afforestation with poplars represents one of the dominant land uses. We explored the effects of these interventions on the volume and carbon storage of the young sedimentary soils of the region. At three sites we identified paired stands occupying similar landscape positions and soil types but subject to natural flooding and covered by natural herbaceous communities or drainage and flood control by dikes and covered by poplar plantations established 12, 17 and 19 years ago. Soil sampling at these sites revealed a reduction of the litter compartment (−86 %) and decreasing volume and porosity of its underlying mineral layer (0-10 cm in the wetland reduced to 0-4 cm in the plantation). Our comparisons of carbon storage accounted for these volumetric shifts by using accumulated mineral mass rather than depth as a reference, showing that tree plantations gained in the mineral soil (22 Mg C ha) almost as much as what they lost in the litter. These gains were particularly large at intermediate depths (4-43 cm in the plantations) were soil porosity remained unaffected and C was raised by 64 % explained by (1) the pulse of inputs from overlaying litter and organic layers subject to rapid decomposition and mobilization after drainage and (2) root colonization, since tree plantations had 75 % of their fine root biomass at these intermediate soil depths, whereas roots in the wetlands did not explore the mineral soil profile and were completely confined to the organic layer. A neutral C balance following wetland drainage and afforestation resulted from the opposing effects of aeration, favoring decomposition in the organic layer, root colonization and organic matter stabilization, favoring its accumulation in the mineral soil. [ABSTRACT FROM AUTHOR]

Soil microbes regulate the transfer of carbon (C) from ecosystems to the atmosphere and in doing so influence feedbacks between terrestrial ecosystems and global climate change. Fire is one element of global change that may influence soil microbial communities and, in turn, their contribution to the C dynamics of ecosystems. In order to improve our understanding of how fire influences belowground communities, we conducted a meta-analysis of 42 published microbial responses to fire. We hypothesized that microbial biomass as a whole, and fungal biomass specifically, would be altered following fires. Across all studies, fire reduced microbial abundance by an average of 33.2% and fungal abundance by an average of 47.6%. However, microbial responses to fire differed significantly among biomes and fire types. For example, microbial biomass declined following fires in boreal and temperate forests but not following grasslands fires. In addition, wildfires lead to a greater reduction in microbial biomass than prescribed burns. These differences are likely attributable to differences in fire severity among biomes and fire types. Changes in microbial abundance were significantly correlated with changes in soil CO emissions. Altogether, these results suggest that fires may significantly decrease microbial abundance, with corresponding consequences for soil CO emissions. [ABSTRACT FROM AUTHOR]

As the earth system changes in response to human activities, a critical objective is to predict how biogeochemical process rates (e.g. nitrification, decomposition) and ecosystem function (e.g. net ecosystem productivity) will change under future conditions. A particular challenge is that the microbial communities that drive many of these processes are capable of adapting to environmental change in ways that alter ecosystem functioning. Despite evidence that microbes can adapt to temperature, precipitation regimes, and redox fluctuations, microbial communities are typically not optimally adapted to their local environment. For example, temperature optima for growth and enzyme activity are often greater than in situ temperatures in their environment. Here we discuss fundamental constraints on microbial adaptation and suggest specific environments where microbial adaptation to climate change (or lack thereof) is most likely to alter ecosystem functioning. Our framework is based on two principal assumptions. First, there are fundamental ecological trade-offs in microbial community traits that occur across environmental gradients (in time and space). These trade-offs result in shifting of microbial function (e.g. ability to take up resources at low temperature) in response to adaptation of another trait (e.g. limiting maintenance respiration at high temperature). Second, the mechanism and level of microbial community adaptation to changing environmental parameters is a function of the potential rate of change in community composition relative to the rate of environmental change. Together, this framework provides a basis for developing testable predictions about how the rate and degree of microbial adaptation to climate change will alter biogeochemical processes in aquatic and terrestrial ecosystems across the planet. [ABSTRACT FROM AUTHOR]

Climate models project that precipitation patterns will likely intensify in the future, resulting in increased duration of droughts and increased frequency of large soil rewetting events, which are stressful to the microorganisms that drive soil biogeochemical cycling. Historical conditions can affect contemporary microbial responses to environmental factors through the persistence of abiotic changes or through the selection of a more tolerant microbial community. We examined how a history of intensified rainfall would alter microbial functional response to drying and rewetting events, whether this historical legacy was mediated through altered microbial community composition, and how long community and functional legacies persisted under similar conditions. We collected soils from a long-term field manipulation (Rainfall Manipulation Plot Study) in Kansas, USA, where rainfall variability was experimentally amplified. We measured respiration, microbial biomass, fungal:bacterial ratios and bacterial community composition after collecting soils from the field experiment, and after subjecting them to a series of drying-rewetting pulses in the lab. Although rainfall history affected respiration and microbial biomass, the differences between field treatments did not persist throughout our 115-day drying-rewetting incubation. However, soils accustomed to more extreme rainfall did change less in response to lab moisture pulses. In contrast, bacterial community composition did not differ between rainfall manipulation treatments, but became more dissimilar in response to drying-rewetting pulses depending on their previous field conditions. Our results suggest that environmental history can affect contemporary rates of biogeochemical processes both through changes in abiotic drivers and through changes in microbial community structure. However, the extremity of the disturbance and the mechanism through which historical legacies occur may influence how long they persist, which determines the importance of these effects for biogeochemical cycling. [ABSTRACT FROM AUTHOR]

Dissolved organic matter (DOM) influences inland water ecosystems through its light absorbing qualities. We investigated how pH affects light absorption by DOM with pH manipulation experiments and with data from two lake surveys. We hypothesized that: (1) light absorption and photobleaching of DOM would increase with increasing pH, and (2) as a result of photobleaching, molar absorption (i.e. light absorbance at 440 nm/dissolved organic carbon concentration) would decrease among lakes with increasing pH. In experiments with filtered lake water both initial light absorption and photobleaching rates increased at higher (i.e. more basic) pH along with a concomitant shift in the size of DOM toward larger colloidal materials measured by dynamic light scattering (DLS). Both scanning electron microscopy (SEM) and atom force microscopy (AFM) revealed large colloidal to particulate-sized organic matter in alkaline relative to acidic treatments. In the lake surveys, molar absorption coefficients were negatively related to pH across gradients similar to the experiments. Our results are consistent with a conceptual model in which at low pH DOM polymers and colloids are condensed limiting exposure of chromophores to light; at higher pH, polymers and colloids are expanded exposing chromophores to light resulting in greater initial light absorption and faster photobleaching. Hence, water transparency, which is significantly controlled by DOM, is sensitive to environmental changes that influence the pH and chemical composition of inland waters. [ABSTRACT FROM AUTHOR]

Increasing leaching losses of carbon from soils due to accelerated weathering and increasing concentrations of dissolved carbon as a result of intensified soil respiration are suspected to provide a negative feedback on rising atmospheric CO concentrations. We tested this hypothesis by studying concentrations of dissolved carbon and groundwater recharge at the Braunschweig free air carbon dioxide enrichment (FACE) experiment under winter wheat and winter barley. Dissolved carbon concentrations under elevated atmospheric CO and ambient conditions were rather similar and not consistently higher under FACE. An analysis of δC signatures suggested that dissolved organic and inorganic carbon contained 9-29% (DOC) and 26-49% (DIC) of 'new' carbon originating from CO added to the FACE rings. Dissolved inorganic carbon additionally contained 15-42% of carbonate-derived C. A 15% reduction in evapotranspiration under elevated CO increased groundwater recharge by 60 mm or 55%, which was the main driver for an observed 81% increase in dissolved carbon leaching from 2.7 to 4.9 g C m year at 90 cm depth. Our results suggest that future changes of dissolved carbon leaching losses will be mainly governed by changes in climate and groundwater recharge and to a lesser extent by increasing dissolved carbon concentrations. [ABSTRACT FROM AUTHOR]

Atmospheric nitrogen (N) deposition increasingly impacts remote ecosystems. At high altitudes, snow is a key carrier of water and nutrients from the atmosphere to the soil. Medium-sized subalpine grassland terraces are characteristic of agricultural landscapes in the French Alps and influence spatial and temporal snow pack variables. At the Lautaret Pass, we investigated snow and soil characteristics along mesotopographic gradients across the terraces before and during snowmelt. Total N concentrations in the snowpack did not vary spatially and were dominated by organic N forms either brought by dry deposition trapped by the snow, or due to snow-microbial immobilization and turnover. As expected, snowpack depth, total N deposited with snow and snowmelt followed the terrace toposequence; more snow-N accumulated towards the bank over longer periods. However, direct effects of snow-N on soil-N cycling seem unlikely since the amount of nitrogen released into the soil from the snowpack was very small relative to soil-N pools and N mineralization rates. Nevertheless, some snow-N reached the soil at thaw where it underwent biotic and abiotic processes. In situ soil-N mineralization rates did not vary along the terrace toposequence but soil-N cycling was indirectly affected by the snowpack. Indeed, N mineralization responded to the snowmelt dynamic via induced temporal changes in soil characteristics (i.e. moisture and T°) which cascaded down to affect N-related microbial activities and soil pH. Soil-NH and DON accumulated towards the bank during snowmelt while soil-NO followed a pulse-release pattern. At the end of the snowmelt season, organic substrate limitation might be accountable for the decrease in N mineralization in general, and in NH production in particular. Possibly, during snowmelt, other biotic or abiotic processes (nitrification, denitrification, plant uptake, leaching) were involved in the transformation and transfer of snow and soil-N pools. Finally, subalpine soils at the Lautaret Pass during snowmelt experienced strong biotic and abiotic changes and switched between a source and a sink of N. [ABSTRACT FROM AUTHOR]

The efficacy of higher plants at mining Si from primary and secondary minerals in terrestrial ecosystems is now recognized as an important weathering mechanism. Grassland ecosystems are a particularly large reservoir of biogenic silica and are thus likely to be a key regulator of Si mobilization. Herein, we examine the effects of parent material (basaltic and granitic rocks) on the range and variability of biogenic silica pools in grass-dominated ecosystems along two precipitation gradients of Kruger National Park, South Africa. Four soil pedons and adjacent dominant plant species were characterized for biogenic silica content. Our results indicate that although soils derived from basalt had less total Si and dissolved Si than soils derived from granite, a greater proportion of the total Si was made up of biogenically derived silica. In general, plants and soils overlying basaltic versus granitic parent material stored greater quantities of biogenic silica and had longer turnover times of the biogenic silica pool in soils. Additionally, the relative abundance of biogenic silica was greater at the drier sites along the precipitation gradient regardless of parent material. These results suggest that the biogeochemical cycling of Si is strongly influenced by parent material and the hydrologic controls parent material imparts on soils. While soils derived from both basalt and granite are strongly regulated by biologic uptake, the former is a 'tighter' system with less loss of Si than the latter which, although more dependent on biogenic silica dissolution, has greater losses of total Si. Lithologic discontinuities span beyond grasslands and are predicted to also influence biogenic silica cycling in other ecosystems. [ABSTRACT FROM AUTHOR]

Phosphorus (P) availability in estuaries may increase with increasing salinity because sulfate from sea salt supports production of sulfide in sediments, which combines with iron (Fe) making it less available to sequester P. Increased P availability with increased salinity may promote the generally observed switch from P limitation of primary production in freshwater ecosystems to nitrogen (N) limitation in coastal marine waters. To investigate this hypothesis, we analyzed pore water from sediment cores collected along the salinity gradients of four Chesapeake Bay estuaries (the Patuxent, Potomac, Choptank, and Bush Rivers) with watersheds differing in land cover and physiography. At salinities of 1-4 in each estuary, abrupt decreases in pore water Fe concentrations coincided with increases in sulfate depletion and PO concentrations. Peaks in water column PO concentrations also occur at about the same position along the salinity gradient of each estuary. Increases in pore water PO concentration with increasing salinity led to distinct shifts in molar NH:PO ratios from >16 (the Redfield ratio characteristic of phytoplankton N:P) in the freshwater cores to <16 in the cores with salinities >1 to 4, suggesting that release of PO from Fe where sediments are first deposited in sulfate-rich waters could promote the commonly observed switch from P limitation in freshwater to N limitation in mesohaline waters. Finding this pattern at similar salinities in four estuaries with such different watersheds suggests that it may be a fundamental characteristic of estuaries generally. [ABSTRACT FROM AUTHOR]

Small streams that drain agricultural landscapes have come under close scrutiny as potentially significant indirect sources of greenhouse gases (GHGs) to the atmosphere. By exploring the stream-ground water connection in three dimensional space (horizontally and vertically beneath the stream channel, and longitudinally along the stream corridor) our results show (1) ground water can be a significant source of greenhouse gases to streams draining agricultural watersheds with concentrations in excess of atmospheric equilibrium by 221 μmol C L carbon dioxide, 0.64 μmol C L methane, and 0.65 μmol N L nitrous oxide (NO); (2) changes in the stream-ground water connection can create seemingly erratic patterns in GHG concentrations over short longitudinal distances (order of meters); (3) soil-stream interfaces are hotspots for denitrification and methanogenesis; however, no significant NO production was observed at such an interface under a riparian forest; and (4) nitrate (NO) and NO can be preserved as electron acceptors in oxic ground waters draining agriculture landscapes; hence, soil nitrification was the major source of NO to stream water, with a legacy in ground water dating back to the 1960s; NO tracked the seepage of NO into surface waters. In this study, we demonstrate the utility of detailed measurements of multiple trace gases towards revealing spatial and temporal patterns of surface-ground water interactions and biogeochemistry across several small baseflow-dominated stream ecosystems in central Wisconsin, USA. [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]

The South Atlantic Bight (SAB) has been a focus for the study of continental shelf ecosystem respiration during the past two decades. However, two questions concerning respiration in this area have yet to be answered. First, why do previous estimates of respiration in the SAB exceed measured carbon fixation rates by almost an order of magnitude? Second, considering that bacteria are responsible for most of the pelagic community respiration in the SAB, why is respiration almost uniform from the coastline to the shelf break, while bacterial production estimates decrease offshore? This study addresses these critical questions by presenting new pelagic community respiration data that were collected across the entire width of the continental shelf off Georgia, USA from June 2003 to May 2006. The respiration was calculated as in vitro changes of dissolved oxygen and dissolved inorganic carbon concentrations during deck incubations. The measured respiration rates ranged from 0.3(±0.1) to 21.2(±1.4) mmol m−3 day−1. They followed a clear seasonal pattern, being lowest over the entire shelf in winter and reaching maxima in summer. Summertime respiration rates were highest on the inner shelf and decreased with distance offshore. Consistent with this trend, bacterial abundance measurements taken during the sampling month of July 2005 followed a pattern of seaward decline. The SAB organic carbon fluxes calculated from the respiration data are close to the estimates for primary production, which resolves a long-standing mystery regarding perceived carbon imbalance in the SAB. [ABSTRACT FROM AUTHOR]

We describe the climatology, hydrology and biogeochemistry of an extreme nitrogen deposition event that occurred in the highly glacierised environment of the European High Arctic during June 1999. Meteorological analysis, three-dimensional air mass trajectories and a 3D transport model show that blocking high pressures over Scandinavia and the rapid advection of western European pollution toward Svalbard were sufficient to cause the most concentrated (1.15 ppm NO3–N and 1.20 ppm NH4–N), high magnitude (total 26 mm and up to 2.4 mm h−1 at 30 m above sea level) nitrogen deposition event on record in this sensitive, high Arctic environment (78.91° N, 11.93° E). Since the event occurred when much of the catchment remained frozen or under snow cover, microbial utilisation of nitrogen within snowpacks and perennially unfrozen subglacial sediments, rather than soils, were mostly responsible for reducing N export. The rainfall event occurred long before the annual subglacial outburst flood and so prolonged (ca. 10 day) water storage at the glacier bed further enhanced the microbial assimilation. When the subglacial outburst eventually occurred, high runoff and concentrations of NO3 − (but not NH4 +) returned in the downstream rivers. Assimilation accounted for between 53 and 72% of the total inorganic nitrogen deposited during the event, but the annual NO3 − and NH4 + runoff yields were still enhanced by up to 5 and 40 times respectively. Episodic atmospheric inputs of reactive nitrogen can therefore directly influence the biogeochemical functioning of High Arctic catchments, even when microbial activity takes place beneath a glacier at a time when terrestrial soil ecosystems remain frozen and unresponsive. [ABSTRACT FROM AUTHOR]

Grassland ecosystems are an important terrestrial component of the global biogeochemical silicon cycle. Although the structure and ecological functioning of grasslands are strongly influenced by fire and grazing, the role of these key ecological drivers in the production and storage of silicon represents a significant knowledge gap, particularly since they are being altered worldwide by human activities. We evaluated the effects of fire and grazing on the range and variability of plant derived biogenic silica stored in plant biomass and soils by sampling plants and soils from long-term experimental plots with known fire and grazing histories. Overall, plants and soils from grazed sites in the South African ecosystems had up to 76 and 54% greater biogenic silica totals (kg ha−1), respectively, than grazed North American sites. In North American soils, the combination of grazing and annual fire resulted in the greatest abundance of biogenic silica, whereas South African soils had the highest biogenic silica content where grazed regardless of burn frequency. These results as well as those that show greater Si concentrations in grazed South African plants indicate that South African plants and soils responded somewhat differently to fire and grazing with respect to silicon cycling, which may be linked to differences in the evolutionary history and in the grazer diversity and grazing intensity of these ecosystems. We conclude that although fire and grazing (as interactive and/or independent factors) do not affect the concentration of Si taken up by plants, they do promote increased silicon storage in aboveground biomass and soil as a result of directly affecting other site factors such as aboveground net primary productivity. Therefore, as management practices, fire and grazing have important implications for assessing global change impacts on the terrestrial biogeochemical cycling of silicon. [ABSTRACT FROM AUTHOR]

Dissolved organic carbon (DOC) in streams draining hydrologically modified and intensively farmed watersheds has not been well examined, despite the importance of these watersheds to water quality issues and the potential of agricultural soils to sequester carbon. We investigated the dynamics of DOC for 14 months during 2006 and 2007 in 6 headwater streams in a heavily agricultural and tile-drained landscape in the midwestern US. We also monitored total dissolved nitrogen (TDN) in the streams and tile drains. The concentrations of DOC in the streams and tile drains ranged from approximately 1–6 mg L−1, while concentrations of TDN, the composition of which averaged >94% nitrate, ranged from <1 to >10 mg L−1. Tile drains transported both DOC and TDN to the streams, but tile inputs of dissolved N were diluted by stream water, whereas DOC concentrations were generally greater in the streams than in tile drains. Filamentous algae were dense during summer base flow periods, but did not appear to contribute to the bulk DOC pool in the streams, based on diel monitoring. Short-term laboratory assays indicated that DOC in the streams was of low bioavailability, although DOC from tile drains in summer had bioavailability of 27%. We suggest that these nutrient-rich agricultural streams are well-suited for examining how increased inputs of DOC, a potential result of carbon sequestration in agricultural soils, could influence ecosystem processes. [ABSTRACT FROM AUTHOR]

In the prolonged absence of major disturbances, ecosystems may enter a stage of retrogression, which is characterized by decreased ecosystem process rates both above and belowground, and often reduced availability of phosphorus (P). Disturbance through wildfire can increase soil P losses through leaching or erosion, but in the long-term absence of fire, soil P could potentially become increasingly bound in more stable forms that are less available to microbes. We studied forms of P and microbial respiration kinetics in the humus layer of a group of islands that vary considerably in wildfire frequency (40–5,300 years since last fire), and which are known to enter retrogression in the prolonged absence of fire. We found a decrease in labile P with decreasing fire frequency but no change in total P. Soil microorganisms responded more strongly to N than to P addition, and microbial biomass N:P ratios remained unchanged across the gradient. However, the concentration of labile P was the best predictor of microbial respiration responses across the islands, and this provides some evidence that declining access to P could contribute to the decline in soil microbial activity during retrogression. Our results show that even though N is arguably the main limiting nutrient during retrogression in this chronosequence, long term absence of fire also causes a decline in P availability which negatively affects microbial activity. This in turn could potentially impair microbially driven processes such as decomposition and mineralization and further contribute to the reduced availability of soil nutrients during retrogression. [ABSTRACT FROM AUTHOR]

We performed surveys of nitrogen (N2)-fixation in three oligotrophic lake-stream systems in the Sawtooth Mountains of central Idaho to address two questions: (1) Which habitat types within linked lake-stream systems (lake pelagic, lake benthic, and stream) exhibit the highest rates of N2 fixation?, and (2) How does N2 fixation compare to the hydrologic flux of nitrogen? A seasonal survey showed that N2 fixation in a single lake and its outlet stream peaked in late summer, when hydrologic N fluxes were lowest. Benthic lake N2-fixation rates by epiphytes were highest at mid-lake depths, where their percent cover was highest, while rates by epipelon were greatest at shallow lake depths. Pelagic N2 fixation was below detection. Stream N2-fixation rates were greatest on rock substrates and in the lake outlet stream. These patterns were supported by a baseflow survey (late July) in three lake-stream ecosystems which confirmed that N2-fixation rates peaked in the lake benthos at shallow depths and on rock substrates in outlet streams. Scaling N2-fixation rates to whole lake and stream areas revealed that N2 fixation could exceed the nitrate, and sometimes the total dissolved nitrogen flux during baseflow in lakes and outlet streams. Despite low rates, total N2-fixation contributions (kg/day) from lakes were greater because they had far larger surface areas than the stream environments. Fixed nitrogen contributions from stream outlets were also relatively high because of high N2-fixation rates and despite low surface areas. This study suggests that N2 fixation could be a seasonally important nitrogen source to nutrient deficient subalpine lake-stream ecosystems. In addition, the frequency and location of lakes could control N2-fixation contributions to watersheds by providing a large area for within-lake N2 fixation, and creating conditions favorable for N2 fixation in outlet streams. [ABSTRACT FROM AUTHOR]

Nitrate has long been thought to be chemically unreactive in soil. This view was challenged by the report of an apparently abiotic process whereby nitrate (NO3 −) is incorporated into organic compounds (Dail et al . ). In Colman et al . (), we examined how common this process might be by testing for it in 45 soils collected from across a range of ecosystem types. We found no evidence of this process occurring in any of the soils, but found evidence of an analytical artifact that creates the appearance of incorporation. We suggested that prior evidence of this process might be due in part or in total to this analytical artifact. Davidson et al . (), however, challenged our results and conclusions, suggesting that we failed to observe the abiotic incorporation because we eliminated the anaerobic microsites they argue are necessary for the process. We address the criticisms, and show that they actually raise questions about the robustness of the only study to have reported abiotic NO3 − incorporation in sterile soils. We argue that this area of research needs new artifact-free experiments if the controversy is going to be resolved. [ABSTRACT FROM AUTHOR]

Rapid exchange of stream water and groundwater in streambeds creates hotspots of biogeochemical cycling of redox-sensitive solutes. Although stream–groundwater interaction can be increased through stream restoration, there are few detailed studies of the increased heterogeneity of water and solute fluxes through the streambed and associated patterns of biogeochemical processes around stream restoration structures. In this study, we examined the seasonal patterns of water and solute fluxes through the streambed around a stream restoration structure to relate patterns of water flux through the streambed to morphology of the channel and biogeochemical processes occurring in the bed. We characterized different biogeochemical zones in the streambed using principal component analysis (PCA) and examined the change in spatial patterns of these zones during different seasons. The PCA results show that two principal components summarized 83% of the variance in the original data set. Streambed pore water was characterized as oxic (indicating production of nitrate), anoxic (indicating sulfate, iron and manganese reduction), or stream-like (indicating there was minimal change in the stream water chemistry in the bed). Regardless of season of the year, anoxic zones were predominantly located upstream of the structure, in a low-velocity pool, and oxic zones were predominantly located downstream of the structure, in a turbulent riffle. We expect structures that span the full channel, are impermeable, and permanent, such as those installed in natural channel design restoration will similarly impact biogeochemical processing in the streambed. The installation of these types of restoration structures may be a way to increase the degree of biogeochemical cycling in stream ecosystems. [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]

For soil carbon to be effectively sequestered beyond a timescale of a few decades, this carbon must become incorporated into passive reservoirs or greater depths, yet the actual mechanisms by which this occurs is at best poorly known. In this study, we quantified the magnitude of dissolved organic carbon (DOC) leaching and subsequent retention in soils of a coniferous forest and a coastal prairie ecosystem. Despite small annual losses of DOC relative to respiratory losses, DOC leaching plays a significant role in transporting C from surface horizons and stabilizing it within the mineral soil. We found that DOC movement into the mineral soil constitutes 22% of the annual C inputs below 40 cm in a coniferous forest, whereas only 2% of the C inputs below 20 cm in a prairie soil could be accounted for by this process. In line with these C input estimates, we calculated advective transport velocities of 1.05 and 0.45 mm year−1 for the forested and prairie sites, respectively. Radiocarbon measurements of field-collected DOC interpreted with a basic transport-turnover model indicated that DOC which was transported and subsequently absorbed had a mean residence time of 90–150 years. Given these residence times, the process of DOC movement and retention is responsible for 20% of the total mineral soil C stock to 1 m in the forest soil and 9% in the prairie soil. These results provide quantitative data confirming differences in C cycles in forests and grasslands, and suggest the need for incorporating a better mechanistic understanding of soil C transport, storage and turnover processes into both local and regional C cycle models. [ABSTRACT FROM AUTHOR]

In this study, we examined changes in isotopic (13C and 14C) and spectroscopic (UV and 13C NMR) properties of dissolved organic carbon (DOC) in relation to soil organic matter (SOM) to elucidate the sources and sinks of DOC as water percolates through the soils of two contrasting upland coastal California ecosystems—a redwood forest and a coastal prairie. Despite differences in the distribution of C stocks and litter chemistry at these two sites, we found similar shifts in DOC chemistry with soil depth. DOC concentrations dropped rapidly with increasing depth, with an accompanying decrease in the C:N ratio, an increase in the δ13C value and an decrease in specific UV adsorption. In the grassland soil, Δ14C values declined from current atmospheric values (+70‰) in the surface horizon to −75‰ at 100 cm. In the redwood soil, the Δ14C value of 111‰ in O horizon leachates was indicative of OM with a residence time of 8–10 yrs, with a decrease in Δ14C values to −80‰ at 100 cm. Solid-state CP/MAS 13C NMR spectra were generally most similar to highly humified OM, with a general decrease in the relative abundance of aromatic compounds and an increase in the alkyl C/O-alkyl C ratio with increasing depth. All of these trends are consistent with the shifts in SOM properties with increasing depth, which are interpreted to mean a shift from fresh plant material to older, highly altered OM. In this Mediterranean climate, we found distinct seasonal shifts in the quantity and composition of DOC found in soil solution during the winter rainy period that was also consistent with a shift from recent labile substrates to older, highly altered OM. These results fit in with a growing body of literature suggesting that the source of much of the DOC within mineral soils is the local soil OM, and the 14C data, in particular, indicate that DOC at depth is not simply the fraction of surficial leachates that have not been adsorbed or decomposed. Rather, exchange reactions with a portion of the more stabilized SOM pool exert the strongest control on both the concentration and composition of DOC found in these soils. [ABSTRACT FROM AUTHOR]

A simulation study based on recent regional climate scenarios for Sweden investigated possible changes in carbon (C) dynamics and net ecosystem exchange (NEE) of Swedish Norway spruce forest ecosystems. Four sites, representative of well-drained soils in four regions, were included. Stand development was simulated for a 100-year rotation period using a coupled model describing abiotic and biotic processes in the soil-plant-atmosphere system. Two IPCC climate change scenarios, corresponding to a mean annual temperature increase of about 2°C (A2) or 3°C (B2) from the reference period 1961–1990 to a new period 2061–2090, were considered. Annual maximum snow depth decreased with the increase in air temperature, whereas maximum soil frost depth and mean annual soil temperature showed only small changes, especially for the sites in northern Sweden. Simulations suggested that in the warmer climate, gross primary production (GPP) increased by 24–32% in northern Sweden and by 32–43% in the south. In the north, the increase was related to the combined effect of air and soil temperature extending the growing season, whereas in the south it was mainly governed by increased N availability due to increased soil temperature. NEE increased by about 20% (A2) or 25% (B2) at all sites, more or less solely due to increased accumulation of C in the tree biomass (including harvest residues), since changes in soil C were small compared with the current climate. Both light use efficiency and water use efficiency were improved in the future climate scenarios, despite increases in atmospheric CO2 not being considered. [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]

This study examined the nitrogen (N) dynamics of a black spruce (Picea mariana (Mill.) BSP)-dominated chronosequence in Manitoba, Canada. The seven sites studied each contained separate well- and poorly drained stands, originated from stand-killing wildfires, and were between 3 and 151 years old. Our goals were to (i) measure total N concentration ([N]) of all biomass components and major soil horizons; (ii) compare N content and select vegetation N cycle processes among the stands; and (iii) examine relationships between ecosystem C and N cycling for these stands. Vegetation [N] varied significantly by tissue type, species, soil drainage, and stand age; woody debris [N] increased with decay state and decreased with debris size. Soil [N] declined with horizon depth but did not vary with stand age. Total (live + dead) biomass N content ranged from 18.4 to 99.7 g N m-2 in the well-drained stands and 37.8-154.6 g N m-2 in the poorly drained stands. Mean soil N content (380.6 g N m-2) was unaffected by stand age. Annual vegetation N requirement (5.9 and 8.4 g N m-2 yr-1 in the middle-aged well- and poorly drained stands, respectively) was dominated by trees and fine roots in the well-drained stands, and bryophytes in the poorly drained stands. Fraction N retranslocated was significantly higher in deciduous than evergreen tree species, and in older than younger stands. Nitrogen use efficiency (NUE) was significantly lower in bryophytes than in trees, and in deciduous than in evergreen trees. Tree NUE increased with stand age, but overall stand NUE was roughly constant (∼150 g g-1 N) across the entire chronosequence. [ABSTRACT FROM AUTHOR]

N saturation induced by atmospheric N deposition can have serious consequences for forest health in many regions. In order to evaluate whether foliar $$\delta^{15}\hbox{N}$$ may be a robust, regional-scale measure of the onset of N saturation in forest ecosystems, we assembled a large dataset on atmospheric N deposition, foliar and root $$\delta^{15}\hbox{N}$$ and N concentration, soil C:N, mineralization and nitrification. The dataset included sites in northeastern North America, Colorado, Alaska, southern Chile and Europe. Local drivers of N cycling (net nitrification and mineralization, and forest floor and soil C:N) were more closely coupled with foliar $$\delta^{15}\hbox{N}$$ than the regional driver of N deposition. Foliar $$\delta^{15}\hbox{N}$$ increased non-linearly with nitrification:mineralization ratio and decreased with forest floor C:N. Foliar $$\delta^{15}\hbox{N}$$ was more strongly related to nitrification rates than was foliar N concentration, but concentration was more strongly correlated with N deposition. Root $$\delta^{15}\hbox{N}$$ was more tightly coupled to forest floor properties than was foliar $$\delta^{15}\hbox{N}$$ . We observed a pattern of decreasing foliar $$\delta^{15}\hbox{N}$$ values across the following species: American beech>yellow birch>sugar maple. Other factors that affected foliar $$\delta^{15}\hbox{N}$$ included species composition and climate. Relationships between foliar $$\delta^{15}\hbox{N}$$ and soil variables were stronger when analyzed on a species by species basis than when many species were lumped. European sites showed distinct patterns of lower foliar $$\delta^{15}\hbox{N}$$ , due to the importance of ammonium deposition in this region. Our results suggest that examining $$\delta^{15}\hbox{N}$$ values of foliage may improve understanding of how forests respond to the cascading effects of N deposition. [ABSTRACT FROM AUTHOR]

In forest ecosystems the single largest respiratory flux influencing net ecosystem productivity (NEP) is the total soil CO2 efflux; however, it is difficult to make measurements of this flux that are accurate at the ecosystem scale. We examined patterns of soil CO2 efflux using five different methods: auto-chambers, portable gas analyzers, eddy covariance along and two models parameterized with the observed data. The relation between soil temperature and soil moisture with soil CO2 effluxes are also investigated, both inter-annually and seasonally, using these observations/results. Soil respiration rates ( R soil) are greatest during the growing season when soil temperatures are between 15 and 25 °C, but some soil CO2 efflux occurs throughout the year. Measured soil respiration was sensitive to soil temperature, particularly during the spring and fall. All measurement methods produced similar annual estimates. Depending on the time of the year, the eddy covariance (flux tower) estimate for ecosystem respiration is similar to or slightly lower than estimates of annual soil CO2 efflux from the other methods. As the eddy covariance estimate includes foliar and stem respiration which the other methods do not; it was expected to be larger (perhaps 15–30%). The auto-chamber system continuously measuring soil CO2 efflux rates provides a level of temporal resolution that permits investigation of short- to longer term influences of factors on these efflux rates. The expense of building and maintaining an auto chamber system may not be necessary for those researchers interested in estimating R soil annually, but auto-chambers do allow the capture of data from all seasons needed for model parameterization. [ABSTRACT FROM AUTHOR]

MBL-MEL, a simple model of ecosystem biogeochemistry, is amended and applied to plant and soil C, 14N and 15N data for the summers of 2001–2003 from Brandalpynten, a maritime high Arctic site on Svalbard following the application of 15N (99 atom%) as 15NO3-N at or 15NH4-N at concentrations of 1 or 5 kg N ha−1. Variants of this Parent model are also developed to incorporate: temperature dependence into equations describing nutrient fluxes (Temp model); cryptogams (Cryp model); both features combined (CrypTemp model). Goodness-of-fit (GOF) statistics suggest that the addition of temperature-dependence improves the utility of models with and without cryptogams: the residual weighted sums of squares per data point were 5.69, 3.91, 4.31 and 3.93 for the Parent, Temp, Cryp and CrypTemp models respectively. The application of model selection criteria confirm that the addition of temperature-dependence also improves model generalisability. Across all models, the principal discrepancies between observation and prediction are associated with the inorganic soil 15N pool. We conclude that models in which fluxes are described using simple equations that can be augmented to include additional complexity, are an ideal starting point from which the relationship between GOF and model generalisability can be assessed. [ABSTRACT FROM AUTHOR]