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Riverine export of carbon (C) is an important part of the global C cycle; however, most riverine C budgets focus on individual forms of C and fail to comprehensively measure both organic and inorganic C species in concert. To address this knowledge gap, we conducted high frequency sampling of multiple C forms, including dissolved organic C (DOC), inorganic carbon (as alkalinity), particulate organic C (POC), coarse particulate organic C (CPOC), and invertebrate biomass C across the main run-off season in a predominantly rain-fed watershed in Southeast Alaska. Streamwater concentrations were used to model daily watershed C export from May through October. Concentration and modeled yield data indicated that DOC was the primary form of riverine C export (8708 kg C/km2), except during low flow periods when alkalinity (3125 kg C/km2) was the dominant form of C export. Relative to DOC and alkalinity, export of particulate organic C (POC: 992 kg C/km2; CPOC: 313 kg C/km2) and invertebrates (40 kg C/km2) was small, but these forms of organic matter could disproportionately impact downstream food webs because of their higher quality, assessed via C to nitrogen ratios. These seasonal and flow driven changes to C form and export likely provide subsidies to downstream and nearshore ecosystems such that predicted shifts in regional hydroclimate could substantially impact C transfer and incorporation into aquatic food webs. [ABSTRACT FROM AUTHOR]

Methane (CH4) emissions from wetland ecosystems are controlled by redox conditions in the soil, which are currently underrepresented in Earth system models. Plant-mediated radial oxygen loss (ROL) can increase soil O2 availability, affect local redox conditions, and cause heterogeneous distribution of redox-sensitive chemical species at the root scale, which would affect CH4 emissions integrated over larger scales. In this study, we used a subsurface geochemical simulator (PFLOTRAN) to quantify the effects of incorporating either spatially homogeneous ROL or more complex heterogeneous ROL on model predictions of porewater solute concentration depth profiles (dissolved organic carbon, methane, sulfate, sulfide) and column integrated CH4 fluxes for a tidal coastal wetland. From the heterogeneous ROL simulation, we obtained 18% higher column averaged CH4 concentration at the rooting zone but 5% lower total CH4 flux compared to simulations of the homogeneous ROL or without ROL. This difference is because lower CH4 concentrations occurred in the same rhizosphere volume that was directly connected with plant-mediated transport of CH4 from the rooting zone to the atmosphere. Sensitivity analysis indicated that the impacts of heterogeneous ROL on model predictions of porewater oxygen and sulfide concentrations will be more important under conditions of higher ROL fluxes or more heterogeneous root distribution (lower root densities). Despite the small impact on predicted CH4 emissions, the simulated ROL drastically reduced porewater concentrations of sulfide, an effective phytotoxin, indicating that incorporating ROL combined with sulfur cycling into ecosystem models could potentially improve predictions of plant productivity in coastal wetland ecosystems. [ABSTRACT FROM AUTHOR]

Salt marshes can attenuate nutrient pollution and store large amounts of 'blue carbon' in their soils, however, the value of sequestered carbon may be partially offset by nitrous oxide (N2O) emissions. Global climate and land use changes result in higher temperatures and inputs of reactive nitrogen (Nr) into coastal zones. Here, we investigated the combined effects of elevated temperature (ambient + 5℃) and Nr (double ambient concentrations) on nitrogen processing in marsh soils from two climatic regions (Quebec, Canada and Louisiana, U.S.) with two vegetation types, Sporobolus alterniflorus (= Spartina alterniflora) and Sporobolus pumilus (= Spartina patens), using 24-h laboratory incubation experiments. Potential N2O fluxes increased from minor sinks to major sources following elevated treatments across all four marsh sites. One day of potential N2O emissions under elevated treatments (representing either long-term sea surface warming or short-term ocean heatwaves effects on coastal marsh soil temperatures alongside pulses of N loading) offset 15–60% of the potential annual ambient N2O sink, depending on marsh site and vegetation type. Rates of potential denitrification were generally higher in high latitude than in low latitude marsh soils under ambient treatments, with low ratios of N2O:N2 indicating complete denitrification in high latitude marsh soils. Under elevated temperature and Nr treatments, potential denitrification was lower in high latitude soil but higher in low latitude soil as compared to ambient conditions, with incomplete denitrification observed except in Louisiana S. pumilus. Overall, our findings suggest that a combined increase in temperature and Nr has the potential to reduce salt marsh greenhouse gas (GHG) sinks under future global change scenarios. [ABSTRACT FROM AUTHOR]

Nutrient limitation of tree growth can intensify when nutrients are lost to forest harvest, creating challenges for forest growth and sustainability. Forest harvest accelerates nutrient loss by removing nutrient-containing biomass and by increasing nutrient leaching, shaping patterns of nutrient depletion that cause long-term shifts in nutrient limitation. Nitrogen most frequently limits tree growth, but where nitrogen is abundant, nutrient limitation often shifts to phosphorus and base cations, depending on soil mineralogy. We used the process-based biogeochemical model NutsFor to evaluate how multiple elements can limit long-term forest growth via interactions between soil nitrogen (low vs. high nitrogen) and soil mineralogy (sedimentary vs. basaltic bedrock). Simulations were run for 525 years with 40-year harvest intervals for managed Douglas-fir forests of the Oregon Coast Range. In low nitrogen sites, nutrient limitation switched after several centuries from nitrogen to phosphorus, as cycles of forest growth and harvest depleted soil organic phosphorus pools. In contrast, high nitrogen sites displayed severe base cation depletion and reduced tree growth within only one to two rotations, with sedimentary bedrock sites limited by calcium and basaltic sites by both calcium and potassium. Harvesting stimulated the largest fractional losses of nitrogen and potassium across all simulations, and additionally of calcium in high nitrogen sites. These multi-element simulations of interactions among harvesting, soil nitrogen, and bedrock type provide a set of testable predictions to guide monitoring and changes in management aimed at sustaining long-term forest productivity across a wide range of soil biogeochemical conditions. [ABSTRACT FROM AUTHOR]

In alpine zones, soil microbial biomass and activity are strongly dependent on the seasonal snow cover. Current models assume that microbial biomass reaches an annual peak in winter under the insulating snowpack with a subsequent sharp decline during snowmelt. In this study, we investigated the seasonal dynamics of the soil microbial biomass in the Central Alps, where usually early snowfall buffers winter soil temperatures. We conducted a large-scale survey in three mountains around Davos (Switzerland) along altitudinal gradients from approximately 1900 to 2800 m above sea level. Using a space-for-time approach during snowmelt, soil samples were taken (1) under, (2) at the edge of, and (3) one meter away from remaining snow patches. One additional sample per site was taken in summer to further evaluate the seasonal dynamics. In total, 184 soil samples from 46 different sites were analyzed. We measured microbial biomass C and N, enzymatic activity and dissolved C and N. We observed an increase of microbial biomass and dissolved C during and immediately after snowmelt, as well as an increase from spring to summer. We suggest that the absence of soil freezing in winter and the growing amounts of dissolved C supported a continued growth, without a sudden collapse of the microbial biomass. Our results underline the importance of the insulating effect of the seasonal snow cover for the microbial dynamics. Global warming is modifying the timing and abundance of the seasonal snow cover, and our results will help to refine models for the dynamics of soil microbes in alpine ecosystems. [ABSTRACT FROM AUTHOR]

The Eastern James Bay has been undergoing major shifts in its physical, chemical, and ecological functioning over the past decades, driven by a combination of climate and landscape changes, and human intervention that includes the damming of major regional rivers. Rivers play a role in the functioning of the Bay, delivering not only freshwater and key materials but also transmitting environmental and climatic signals. Here we present a study of the spatial variability of riverine export fluxes and yields of freshwater, dissolved organic carbon (DOC), total suspended solids, total nitrogen, and total phosphorus of 18 major rivers flowing into Eastern James Bay, in the boreal region of Québec (CA). We characterized discharge patterns and identified and modeled the main landscape drivers of riverine material concentrations in these watersheds. We then combined these to model the contribution of unsampled watersheds to generate a regional budget of riverine freshwater and material export to the Bay. The peak of water export occurs in spring for free-flowing rivers, whereas for the dammed river La Grande, which accounts for half of the freshwater inputs, the peak discharge was shifted to winter due to high energy demands. The large gradient of watershed areas, discharge and environmental conditions resulted in the wide range of material concentrations across these boreal rivers, and we show that overall, the James Bay is a hot spot of DOC loading to the entire Hudson Bay System. We further reconstructed past (pre-damming) riverine export and compared this with current, and potential future scenarios, and we demonstrate that damming and climate change are impacting the patterns of water and material export of these boreal rivers to the Eastern James Bay. [ABSTRACT FROM AUTHOR]

Microbial Fe(III) reduction significantly influences the fate of various elements and contaminants. Previous research has employed different Fe(III)-OM complexes and ferrihydrite to study Fe(III)-reduction-related biogeochemistry processes. However, the effects of adding specific Fe(III)-OM complexes and ferrihydrite on the Fe(III)-reducing bacterial community, Fe(III)-reducing kinetics, and Fe(III)-related functional genes remain largely unexplored. This study applied microcosm experiments and metagenomic analysis of lake sediments with and without amendments of ferrihydrite, Fe(III)-citrate, or Fe(III)-EDTA. Results showed that sediments amended with Fe(III)-citrate and Fe(III)-EDTA exhibited faster Fe(III) reduction rates and more significant changes in bacterial community structures compared to those amended with ferrihydrite. Geobacter and Clostridium were enriched in the sediments amended with Fe(III)-EDTA and Fe(III)-citrate, respectively. Despite a slower reduction rate and lack of enrichment of specific Fe(III)-reducing bacteria, ferrihydrite still led to an increase in the copy numbers of genes related to Fe(III) reduction and iron assimilation in the metagenomes, suggesting an increase in these capacities. These results suggest that introducing various Fe(III)-OM complexes and ferrihydrite into the environment would result in differences in not only Fe(III) reduction rates and Fe(III)-reducing bacterial communities but also in iron-related functional genes. Meanwhile, variations in Fe(III) reduction rates and Fe(III)-reducing bacterial communities do not necessarily correlate with changes in the abundances of functional genes relevant to Fe(III) reduction and iron assimilation in the metagenomes. These results provide a better understanding of the adaptive mechanisms of Fe(III)-reducing bacteria in different environmental systems. [ABSTRACT FROM AUTHOR]

This document is a correction notice for an article titled "Simulated plant-mediated oxygen input has strong impacts on fine-scale porewater biogeochemistry and weak impacts on integrated methane fluxes in coastal wetlands" published in the journal Biogeochemistry. The correction addresses an error in Table 2, where an arrow symbol was mistakenly replaced by the symbol "à". The online version of the paper has been updated to correct this error. The correction notice also includes the names of the authors of the original article. Springer Nature, the publisher, remains neutral regarding jurisdictional claims and institutional affiliations. [Extracted from the article]

Fe biogeochemistry is associated with important ecosystem services provided by mangrove forests, including carbon sequestration and the retention of potentially toxic elements. The biogeochemical processes controlling Fe fate in mangroves are naturally affected by the soil geochemical environment, which controls Fe dynamics. However, ongoing climate changes and the associated extreme weather events may drastically affect the biogeochemistry of this important micronutrient for both terrestrial and oceanic environments. Therefore, this study aimed to evaluate how massive mangrove mortality after an extreme weather event altered the Fe dynamics in mangrove soils. The results show a significant decrease in soil carbon stock in the dead mangrove forests (25 kg m−2), as compared with the undisturbed forests (37 kg m−2). In addition, we observed a substantial Fe loss (greater than 50% of soil Fe forms, i.e., 17,000 mg kg−1) in the dead mangrove soils, which was associated with pyrite (9000 mg kg−1) and low crystallinity Fe oxyhydroxides (2400 mg kg−1). These impacts led to a decrease in the pyritization in soils, which resulted in a loss of 170 tons of Fe from 500 ha of dead mangrove forests within one year. Thus, the pyritization process may critically compromise a mangrove forests' ability to immobilize pollutants (e.g., metals) and sequester carbon in the long term, thereby altering their ability to provide these ecosystem services. Overall, our results revealed that the Fe biogeochemical cycle of mangrove forests is very sensitive to future climate change scenarios and increased extreme weather events. [ABSTRACT FROM AUTHOR]

Cryoconite holes host active microbial communities despite their extreme physical conditions. In the McMurdo Dry Valleys of Antarctica, these perennially cold, mini-ecosystems form ice lids that can persist for many years thereby isolating the cryoconite from nutrient and carbon inputs. Despite much recent work on cryoconite holes in Antarctica, little is known about nutrient dynamics and limitations in these ice-enclosed ecosystems. We used multiple biogeochemical approaches, including stable isotope signatures (δ15N and δ13C), nutrients concentrations (C, N, P), and enzyme activities, to evaluate what nutrients are likely limiting to biological activity in cryoconite hole sediments on Taylor, Canada, and Commonwealth glaciers in Taylor Valley, one of the McMurdo Dry Valleys. Nutrient concentrations (C, N, and P) varied in accordance with previous studies showing that the most inland of the three glaciers (Taylor Glacier) is the most oligotrophic. C-to-N ratios of Canada and Commonwealth cryoconite-hole sediments were close to the global mean for biologically-active sediments and soils, whereas Taylor Glacier cryoconite deviated from the global mean and were similar to the high C:N ratios seen in Taylor Valley soils. C and N stable isotope signatures on Commonwealth and Canada glaciers are congruent with values for efficient C and N fixation by nostocalean cyanobacteria, combined with higher levels of denitrification on Canada Glacier. In contrast, stable isotope signatures on the more oligotrophic Taylor Glacier are reflective of atmospheric deposition of N and C, or N inputs from nearby soils. Enzyme stoichiometric approaches further support extreme nutrient limitation on Taylor Glacier and indicate that P is the ultimate limiting nutrient across all three glaciers. Extremely high DIN-to-phosphate ratios also indicate P limitation across all three glaciers with Commonwealth Glacier being less severely P-limited than the other two glaciers. At a broader scale, this work provides a comprehensive framework for understanding how biogeochemical cycling of C, N and P vary across nutrient and climatic gradients in the cryobiosphere, and point towards the need for experimental work to test the relative controls of climate, microbes, and nutrients on biogeochemistry of cryoconite holes and other ecosystems of the cryosphere. [ABSTRACT FROM AUTHOR]

The biogeochemistry of dissolved selenium (Se) was investigated over 3 years (2015–2017) in the subtropical, warm monomictic and meso-eutrophic Lake Kinneret (Sea of Galilee, Israel). We monitored seasonal variation and vertical distribution of dissolved total Se (T.Se), inorganic oxyanions (Se(IV) & Se(VI)), reduced Se fraction (Red.Se), organic (Org.Se) and volatile Se compounds. T.Se varied between ~ 100 and 160 ng L−1 with Red.Se comprising 40–80% of the Se inventory, and Se(VI) dominating over Se(IV) most of the time. The variation in T.Se and species correlated with winter holomixis vs. summer fall stratification periods. The annual cycle includes: (a) increase of T.Se from fall/winter to spring, representing increased allochthonous Se input flux, along with Se recycling via holomixis; (b) decrease of T.Se from spring to the end of the year, representing the diminishing Se inputs and the evolving output fluxes to the lake's bottom and to the atmosphere. Org.Se variations are directly associated with Chlorophyll-a and primary production attesting for the significant role of phytoplankton activity in the Se cycle. An important Se output flux comprises spring to summer Se uptake by phytoplankton and further volatile compounds production and volatilization accounting for ~ 10% of estimated Se input. The similarity of total dissolved Se concentrations in this work and from mid 1990s attests for long-term stability of the Se inventory. The TVSe concentrations in lacustrine systems being similar to that of estuary systems, the biological role of phytoplankton and eventually the degradation of organic material may produce similar fluxes of volatile Se to the atmosphere. [ABSTRACT FROM AUTHOR]

The biogeochemical model, PnET-BGC, was modified and parameterized using field data from an experimental whole-tree harvest of watershed (W5) in 1983–1984 at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, USA. The model simulated the hydrology, biomass accumulation, and soil solution and stream water chemistry responses to forest cutting. The parameterized model was then applied to other experimentally cut watersheds at the HBEF; including a devegetation experiment (W2; devegetation and herbicide treatment) and a commercial strip-cut (W4) to evaluate the ability of the model to depict ecosystem responses to a range of cutting regimes. Revisions of algorithms of PnET-BGC improved model performance in predicting short- and long-term dynamics of major elements following various approaches to forest cutting. Despite some initial differences in species composition and biomass accumulation rates among the cut watersheds, simulations of total forest biomass for all three treated watersheds (W2, W4 and W5) were consistent with expectations based on the growth trajectory of a second-growth, reference watershed (W6) at the HBEF. The modified two-soil-layer PnET-BGC captured the immediate increase in stream concentrations of NO3−, Ca2+, Mg2+ and Na+ as well as enhanced adsorption of SO42− following cuttings and indicated a greater response for the devegetation and the whole-tree harvest treatments than the sequential strip-cut of W4. Simulations indicated intense NO3− leaching with the devegetation and herbicide treatment and consequent accelerated decline in soil base saturation and a slower recovery pattern during forest regrowth by the end of the simulation period (2100) compared to the other treatments. [ABSTRACT FROM AUTHOR]

Iron is one of the most abundant elements on the planet and a micronutrient for most organisms. In many coastal regions worldwide, mangrove forests affect the dynamics and mobility of different elements to the oceans especially through their soils. The biogeochemistry of mangrove soils responds to numerous factors that vary within different spatial and time scales. In this sense, seasonality can be crucial in determining the role of these ecosystems towards the iron biogeochemical cycle. Thus, the main goal of the present study was to assess the effects of contrasting climatic conditions on iron biogeochemistry in different mangrove forests along the Brazilian coast. We studied the soils (n = 435) of 14 different mangrove forests distributed along two contrasting climate regions: the semi-arid Northeast and the humid Southeast coasts of Brazil. In the SE region, water surplus and lower temperatures in both seasons did not cause significant changes in iron concentrations (wet season: 216 µmol g−1; dry season: 230 µmol g−1) where oxyhydroxides and pyrite immobilize iron in the mangrove soils. In contrast, in the semi-arid mangroves, a marked water deficit during the dry season caused both pyrite oxidation and iron reprecipitation as oxyhydroxides. Contrarily, in the rainy season, the establishment of a suboxic environment (Eh ~ + 100 mV) favored significant iron losses via iron reduction. We conclude that seasonality can affect the maintenance and cycling of iron in mangrove soils, which may function as important sources of this element for adjacent ecosystems. [ABSTRACT FROM AUTHOR]

Global environmental change alters the production, terrestrial export, and photodegradation of organic carbon in northern lakes. Sedimentary biogeochemical records can provide a unique means to understand the nature of these changes over long time scales, where observational data fall short. We deployed in situ experiments on two shallow subarctic lakes with contrasting light regimes; a clear tundra lake and a dark woodland lake, to first investigate the photochemical transformation of carbon and nitrogen elemental (C/N ratio) and isotope (δ13C, δ15N) composition in lake water particulate organic matter (POM) for downcore inferences. We then explored elemental, isotopic, and spectral (inferred lake water total organic carbon [TOC] and sediment chlorophyll a [CHLa]) fingerprints in the lake sediments to trace changes in aquatic production, terrestrial inputs and photodegradation before and after profound human impacts on the global carbon cycle prompted by industrialization. POM pool in both lakes displayed tentative evidence of UV photoreactivity, reflected as increasing δ13C and decreasing C/N values. Through time, the tundra lake sediments traced subtle shifts in primary production, while the woodland lake carried signals of changing terrestrial contributions, indicating shifts in terrestrial carbon export but possibly also photodegradation rates. Under global human impact, both lakes irrespective of their distinct carbon regimes displayed evidence of increased productivity but no conspicuous signs of increased terrestrial influence. Overall, sediment biogeochemistry can integrate a wealth of information on carbon regulation in northern lakes, while our results also point to the importance of considering the entire spectrum of photobiogeochemical fingerprints in sedimentary studies. [ABSTRACT FROM AUTHOR]

Freeze–thaw (FT) events exert a great physiological stress on the soil microbial community and thus significantly impact soil biogeochemical processes. Studies often show ambiguous and contradicting results, because a multitude of environmental factors affect biogeochemical responses to FT. Thus, a better understanding of the factors driving and regulating microbial responses to FT events is required. Soil chronosequences allow more focused comparisons among soils with initially similar start conditions. We therefore exposed four soils with contrasting organic carbon contents and opposing soil age (i.e., years after restoration) from a postmining agricultural chronosequence to three consecutive FT events and evaluated soil biochgeoemical responses after thawing. The major microbial biomass carbon losses occurred after the first FT event, while microbial biomass N decreased more steadily with subsequent FT cycles. This led to an immediate and lasting decoupling of microbial biomass carbon:nitrogen stoichiometry. After the first FT event, basal respiration and the metabolic quotient (i.e., respiration per microbial biomass unit) were above pre-freezing values and thereafter decreased with subsequent FT cycles, demonstrating initially high dissimilatory carbon losses and less and less microbial metabolic activity with each iterative FT cycle. As a consequence, dissolved organic carbon and total dissolved nitrogen increased in soil solution after the first FT event, while a substantial part of the liberated nitrogen was likely lost through gaseous emissions. Overall, high-carbon soils were more vulnerable to microbial biomass losses than low-carbon soils. Surprisingly, soil age explained more variation in soil chemical and microbial responses than soil organic carbon content. Further studies are needed to dissect the factors associated with soil age and its influence on soil biochemical responses to FT events. [ABSTRACT FROM AUTHOR]

Potassium presents a conundrum for biogeochemists. Potassium is cycled wastefully at the plant level, but it appears to be conserved in the nutrient budgets of entire ecosystems, where it sometimes limits net primary productivity. An increasing demand for K fertilizer may accompany the expansion of agriculture into highly weathered tropical soils, where limited supplies of K may control the distribution and productivity of natural vegetation. However, the molar ratios of 4.6 for N/K in herbaceous plants, 17.3 in current global fertilizer applications, and 0.33 in the commodity price of fertilizer suggest that N is vastly overused and K is vastly over-priced in modern agriculture. [ABSTRACT FROM AUTHOR]

As an illustration of the usefulness of the concept of water-agro-food systems, this paper describes the mechanisms behind the nitrogen cascade from agricultural soils to the coastal sea rivers through aquifers, riparian wetlands, rivers and streams and the estuary, for the case study of the Seine river watershed and its receiving coastal sea. It is stressed that the structure of the agro-food system, i.e. the way food production and trade, as well as consumption and waste management are organized, is the main determinant of the quality of ground- and surface water as well as of aquatic ecosystem functioning. [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]

This document is a correction notice for an article titled "Topsoil removal for Sphagnum establishment on rewetted agricultural bogs" published in the journal Biogeochemistry. The correction addresses an error in Figure 3, where the initial value of "n" was incorrectly stated as 100 and has been corrected to 20. The correction notice also includes a statement from the publisher, Springer Nature, emphasizing their neutrality regarding jurisdictional claims and institutional affiliations. The authors of the original article are listed as Sannimari A. Käärmelahti, Christian Fritz, Gabrielle R. Quadra, Maider Erize Gardoki, Greta Gaudig, Matthias Krebs, and Ralph J. M. Temmink. [Extracted from the article]

The perhumid coastal temperate rainforest (PCTR) of northwestern North America is projected to become warmer and wetter in coming decades, with largely unquantified implications for the magnitude and speciation of riverine nitrogen (N) and phosphorus (P) export from PCTR ecosystems. We collected streamwater at weekly to monthly intervals for a year and intensively during two multi-day storms (one each in summer and the autumn rainy season) from streams draining three of the most common landcover types in southeast Alaska (poor fen, forested wetland and upland forest). Our goal was to investigate how seasonal and episodic (stormflows) changes in runoff influence the magnitude and species of dissolved N and P exported from PCTR watersheds. Riverine yields of total dissolved N and P ranged from 238 to 406 kg km2 year− 1 for N and 11 to 17 kg km2 year− 1 for P and were dominated by organic nutrient forms. Yields of N and P showed a varied response to runoff, with both hydrologic transport and source limitation of nutrient yields observed across the landcover types. During stormflows, log transformed ratios of dissolved inorganic N to soluble reactive P decreased from prestorm levels of ~ 1.0 to 1.5 to less than 0.3 during peak flow at all sites, illustrating that storms induce ephemeral changes in inorganic nutrient export and stoichiometry. Our findings highlight the pulsed nature of dissolved N and P export from PCTR watersheds suggesting that future changes in the seasonality and intensity of precipitation may influence the flow of terrestrial nutrients to marine ecosystems. [ABSTRACT FROM AUTHOR]

Global change is predicted to have a marked impact on freshwater ecosystems in the High Arctic, including temperature increase, enhanced precipitation, permafrost degradation and increased vegetation cover. These changes in river catchments can alter flow regime, solute transport to streams and substantially affect stream ecosystem functioning. The objective of this study was to evaluate changes in stream functioning in a high-Arctic stream in relation to changes in discharge, and runoff flow path. We measured environmental factors, biofilm structure, nutrient uptake rates and metabolism. We studied three reaches in a headwater stream in NE Greenland with different catchment characteristics in early and late summer (during two different years) to evaluate the potential influence of environmental change on Arctic stream ecosystem functioning. Highest nutrient uptake, primary production and ecosystem respiration was found in late summer showing that streams are more efficient at retaining nutrients and have higher autotrophic production, likely due to less impact of snowmelt, and lower discharge increasing the surface to volume ratio between streambed and water column. Nutrient uptake rates in late summer from high-Arctic tundra streams were comparable to uptake rates in temperate pristine streams, likely due to no shading by bank vegetation and longer days in the high-Arctic summer compared to temperate streams. Overall, the results of this study aids in the endeavor of predicting how climate-derived changes will affect in-stream nutrient uptake and metabolism in high-Arctic streams. The results suggest that their capacity to transport, cycle and retain carbon and nutrient may increase if the importance of soil water flow paths for streams also increase, thus with effect to stream trophic relations and solute export to coastal areas. [ABSTRACT FROM AUTHOR]

Dimethylsulfoniopropionate (DMSP) is a ubiquitous organic sulfur compound that underpins sulfur cycling in the marine environment and is the precursor to the climatically active gas dimethylsulfide (DMS). Modelling studies have identified the Southern Ocean as a DMS hot spot during summer, yet except for the bloom forming haptophyte Phaeocystis, little is known about sulfur production by other important members of the marine microbial community. Here, we measured DMSP concentrations and DMSP lyase activity (DLA), with corresponding carbon, nitrogen and Chl a content, in 15 species of Antarctic phototrophic phytoplankton (14 microalgae species and one cyanobacterium) and one phagotrophic flagellate. We found that 11 of the 16 species were able to produce DMSP and eight possess DLA. DMSP content ranged from 0.06 to 73 fmol cell−1 and estimated DMSP production rates ranged from 0.008 to 12.42 fmol cell−1 day−1. As expected, Phaeocystis was amongst the highest producers, however, contrary to expectation DMSP concentrations were high in several pennate diatom species, with intracellular concentrations between 1.85 and 46.6 mM. Here we present the first evidence that the cyanobacterium Synechococcus may be a DMSP producer, with the potential to contribute significantly to the DMSP pool. This study has provided the first analysis of DMSP production and DLA in a suite of phototrophic and phagotrophic species isolated from Antarctica, revealing the variability in DMSP concentrations across multiple strains and within genera and delivered new evidence for potential DLA in diatoms. [ABSTRACT FROM AUTHOR]

The transport of dissolved organic matter (DOM) across the land-ocean-aquatic-continuum (LOAC), from freshwater to the ocean, is an important yet poorly understood component of the global carbon budget. Exploring and quantifying this flux is a significant challenge given the complexities of DOM cycling across these contrasting environments. We developed a new model, UniDOM, that unifies concepts, state variables and parameterisations of DOM turnover across the LOAC. Terrigenous DOM is divided into two pools, T1 (strongly-UV-absorbing) and T2 (non- or weakly-UV-absorbing), that exhibit contrasting responses to microbial consumption, photooxidation and flocculation. Data are presented to show that these pools are amenable to routine measurement based on specific UV absorbance (SUVA). In addition, an autochtonous DOM pool is defined to account for aquatic DOM production. A novel aspect of UniDOM is that rates of photooxidation and microbial turnover are parameterised as an inverse function of DOM age. Model results, which indicate that ~ 5% of the DOM originating in streams may penetrate into the open ocean, are sensitive to this parameterisation, as well as rates assigned to turnover of freshly-produced DOM. The predicted contribution of flocculation to DOM turnover is remarkably low, although a mechanistic representation of this process in UniDOM was considered unachievable because of the complexities involved. Our work highlights the need for ongoing research into the mechanistic understanding and rates of photooxidation, microbial consumption and flocculation of DOM across the different environments of the LOAC, along with the development of models based on unified concepts and parameterisations. [ABSTRACT FROM AUTHOR]

Concentrations of dissolved silicon (DSi) in the ocean have the potential to impact the drawdown of atmospheric CO2 because diatoms, which account for 43% of marine primary productivity, have an absolute requirement for DSi. Rivers are the main source of Si to the oceans, and despite the importance of rivers to the marine Si budget and CO2 drawdown, the controls of river DSi flux are not fully resolved. To help understand the controls of river DSi export, we investigated the relationships between river DSi yield and various environmental parameters for watersheds in western Canada, including climate, land cover, lithology, slope, and soil. Statistical analyses indicated that land cover and climate are the best predictors of DSi yield in western Canada. The land cover classes with the highest correlation to DSi yield were mixed forest, savanna, and wetland. Our results indicate that future climate change and shifting vegetation distribution have the potential to impact DSi export from continents to oceans, which has implications for the marine Si budget and atmospheric CO2 drawdown. [ABSTRACT FROM AUTHOR]

River ecosystem dynamics are strongly regulated by the surrounding watershed. The availability and sources of energy and nutrient resources that drive these systems are controlled by topography, climate, geology, and position in the watershed. Here we examined particulate and dissolved organic matter (POM and DOM, respectively) and nutrient concentrations across an entire watershed during multiple flow regimes in an unregulated, low gradient river with extensive floodplain forests. The objectives of the study were to: (1) determine the influence of watershed position and floodplain connectivity on POM, DOM, and nutrient concentrations; (2) examine the influence of flow variability on POM, DOM, and nutrient concentrations; and (3) develop an empirical rating curve to predict POM, DOM, and nutrient transport and flux. We sampled POM, DOM, and nutrient concentrations (ammonia, nitrate, nitrite and soluble reactive phosphorus) at ten sites across the Sipsey River watershed that varied in their degree of connectivity to the floodplain over a two-year period. Both watershed position and flow regime influenced POM, DOM, and nutrient concentrations. In particular, a large floodplain swamp in the middle of the watershed, the Sipsey Swamp, strongly controlled the relative availability of particulate and dissolved materials in the water. In the headwaters, there was a greater proportion of particulate material in suspension relative to dissolved carbon. While in the downstream reaches, both within and downstream of the Sipsey Swamp, DOC accompanied by greater molecular mass and more aromatic DOM was in greater quantity than particulate materials at high flows. Nutrient concentrations in the stream water tended to decline through the floodplain swamp across all flow conditions and tended to be lower in high flows. We demonstrate that floodplains can disrupt the upstream-downstream continuum by supplying a large quantity of allochthonous organic matter. Using long-term data we estimated the total annual flux of DOC and nitrate to range between 1221-6500 and 24-35 tonnes per year, respectively, between 2007 and 2017 with the highest flux rates occurring during high flow periods. Our study shows the complex dynamics of a natural floodplain river system and generally supports the flood pulse concept by highlighting the importance of wetland complexes and floodplain connectivity on material and nutrient transport. Description of organic matter and nutrient dynamics in natural low gradient rivers is critical to understanding production of organisms, food webs and ecosystem processes in the face of climate and land use changes. [ABSTRACT FROM AUTHOR]

As global sea-levels rise, low-lying coastal lands are subject to shallow coastal flooding and saltwater intrusion, affecting the productivity of farmlands worldwide. Soil biogeochemistry can be dramatically altered as saltwater intrudes agricultural fields. We selected three farm fields in Somerset Co., Maryland affected by saltwater intrusion and established transects from the ditch bank to the center of the cropped field and collected soils (to ~ 140 cm) at five points along this transect. The three fields in this study have different soil types, are located along different tributaries in the county, and receive different fertilizer rates, yet they all showed similar biogeochemical responses to saltwater intrusion. We found an increase in electrical conductivity and concentrations of chloride, sulfate, and forms of phosphorus (P) from the center of the field (low) to the ditch banks (high). As inundation increased, the structure of iron (Fe) changed from crystalline to non-crystalline forms, possibly due to dissolution under saturated conditions. Near the edges of the fields, the formation of organometallic complexes was positively associated with increases in soil carbon and organic soil P concentrations. Compared to areas of the fields where crops were actively growing, soil P concentrations are 2-3 higher on field edges, suggesting that saltwater intrusion may be transporting P to the edges of agricultural fields. These field edges are frequently saturated, thus reduction of Fe could lead to P release into solution potentially harming water quality. As climate change pushes saltwater further inland, it is important to understand the biogeochemical consequences for ecosystems up- and downstream. Understanding the how fractions of P move and change across fields affected by saltwater intrusion will be crucial for planning current and future management of coastal agricultural lands. [ABSTRACT FROM AUTHOR]

The conversion of tropical forests to other land use forms is known to affect biogeochemical processes, but its influence on asymbiotic nitrogen fixation (ANF) remains poorly understood. In this study, we investigate patterns of soil ANF and its biogeochemical controls across multiple land uses and topographic positions in the Brazilian Atlantic Forest. We report ANF rates derived from observational and manipulative laboratory experiments using 15N-labeled dinitrogen (15N2) incubations of soils from primary forest, secondary forest, eucalypt plantations and pastures sampled at three hillslope positions (summit, backslope, and footslope). Our field observations revealed significant interactions between land use and topography on ANF rates, which among 14 measured soil parameters were primarily correlated with total nitrogen and available iron (explaining 69% of ANF variance). Complex non-linear relationships were observed between ANF and (mineral and organic) nutrient pools in forest soils, whereas simple positive linear relationships were found between ANF and total soil nitrogen in managed pastures and plantations. Consistent with this observation, experiments with nitrogen and phosphorus addition did not affect ANF rates in pasture or plantation soils, possibly as a result of previous fertilization and homogenization of plant and microbial communities, but variable positive and negative ANF responses were measured in primary and secondary forest soils depending on hillslope position. These findings show that ANF can be simultaneously affected by multiple variables that are sensitive to environmental conditions and demonstrate the need for considering land-use change and topography to improve predictions of terrestrial biogeochemical cycles. [ABSTRACT FROM AUTHOR]

Although many climate predictions suggest that the frequency and intensity of large storm events might increase in the coming decades, few studies document the full impact of such events along their path. Here, we synthesize information on the impact of Hurricane Irene (formed August 21 2011) and Tropical Storm Lee (formed August 30, 2011) on erosion and sediment transport, lake metabolism, riparian hydrology and biogeochemistry, and stream water quality, from North Carolina to Maine. In almost all cases, these storms generated unprecedented changes in water quality (concentrations, loads), from tenfold increases in DOC and 100-fold increases in POC in Maryland, to 100-fold increases in TSS concentrations in Pennsylvania. Overbank flooding and up to 200-year streamflow events were recorded in New York and Vermont. In many cases, particulate loads (e.g. POC, PP, TSS) occurring during Irene and Lee represented more than 30% of the annual load. The dominance of particulate exports over solutes during Irene and Lee is consistent with the mobilization of normally immobile sediment pools, and massive erosion as reported at many locations across the Northeastern US. Several studies reported long lasting (> 1 year) effects of Irene and Lee on cyanobacterial blooms, erosion, or stream suspended sediment concentrations. However, this review also highlighted the lack of a consistent strategy in terms of methods, and measured water quality parameters. This strongly hinders our ability to fully assess the large-scale impact of such events on our environment, and ultimately their impact on our economy and society. [ABSTRACT FROM AUTHOR]

Recent studies have highlighted the composition and complexity of dissolved organic matter (DOM) in glacial environments. Climate-induced changes to glacier runoff are projected to be an important source of DOM to coastal ecosystems. Photochemical and microbial (termed photo-biochemical) degradation of DOM would determine its fate on the glacier surface and in recipient coastal ecosystems. In order to understand the molecular imprints of photo-biochemical alteration of DOM, in situ field experiments were conducted over a period of 35 days in a coastal Antarctic site and DOM molecularly characterised using ultrahigh-resolution mass spectrometry. We show that the biogeochemistry of DOM is highly complex and intimately connected with microbial and photochemical processes operating individually or in combination. Photo-biochemical processes resulted in shifts in the nitrogen, sulfur, and phosphorous content of the DOM. These processes are also an important mechanism for transforming refractory DOM, like dissolved black carbon and carboxylic rich alicyclic molecules from the snow surface. This study is unique, as it provides new molecular-level information on compounds that comprise the photo- and bio-labile, photo- and bio-refractory, as well as photo- and bio-produced fractions of the supraglacial DOM pool. These insights into the interactions between microbes, light, and specific components of the DOM pool highlight the need for studies focused on the biogeochemistry of supraglacial carbon and its response to a changing climate. [ABSTRACT FROM AUTHOR]

The toxicity of arsenic (As) towards life on Earth is apparent in the dense distribution of genes associated with As detoxification across the tree of life. The ability to defend against As is particularly vital for survival in As-rich shallow submarine hydrothermal ecosystems along the Hellenic Volcanic Arc (HVA), where life is exposed to hydrothermal fluids containing up to 3000 times more As than present in seawater. We propose that the removal of dissolved As and phosphorus (P) by sulfide and Fe(III)(oxyhydr)oxide minerals during sediment-seawater interaction, produces nutrient-deficient porewaters containing < 2.0 ppb P. The porewater arsenite-As(III) to arsenate-As(V) ratios, combined with sulfide concentration in the sediment and/or porewater, suggest a hydrothermally-induced seafloor redox gradient. This gradient overlaps with changing high affinity phosphate uptake gene abundance. High affinity phosphate uptake and As cycling genes are depleted in the sulfide-rich settings, relative to the more oxidizing habitats where mainly Fe(III)(oxyhydr)oxides are precipitated. In addition, a habitat-wide low As-respiring and As-oxidizing gene content relative to As resistance gene richness, suggests that As detoxification is prioritized over metabolic As cycling in the sediments. Collectively, the data point to redox control on Fe and S mineralization as a decisive factor in the regulation of high affinity phosphate uptake and As cycling gene content in shallow submarine hydrothermal ecosystems along the HVA. [ABSTRACT FROM AUTHOR]

In contrast to substantial studies and established knowledge of aluminum (Al) effects (mainly toxicity) on freshwater organisms and terrestrial plants, and even on human health, only a few studies of Al effects on marine organisms have been reported, and our understanding of the role of Al in marine biogeochemistry is limited. In this paper, we review the results of both field and laboratory experiments on the effects of Al on marine organisms, including Al toxicity to marine phytoplankton and the beneficial effects of Al on marine phytoplankton growth, and we discuss possible links of Al to the biological pump and the global carbon cycle. We propose a revised Iron (Fe) Hypothesis, i.e., the Fe-Al Hypothesis that introduces the idea that Al as well as Fe play an important role in the glacial-interglacial change in atmospheric CO2 concentrations and climate change. We propose that Al could not only facilitate Fe utilization, dissolved organic phosphorus utilization and nitrogen fixation by marine phytoplankton, enhancing phytoplankton biomass and carbon fixation in the upper oceans, but also reduce the decomposition and decay of biogenic matter. As a result, Al allows potentially more carbon to be exported and sequestered in the ocean depths through the biological pump. We also propose that Al binds to superoxide to form an Al-superoxide complex, which could catalyze the reduction of Fe(III) to Fe(II) and thus facilitate Fe utilization by marine phytoplankton and other microbes. Further ocean fertilization experiments with Fe and Al are suggested, to clarify the role of Al in the stimulation of phytoplankton growth and carbon sequestration in the ocean depths. [ABSTRACT FROM AUTHOR]

The Pantanal is the largest wetland in the world and yet little is known about the variability in carbon (C) dynamics across its flood seasons. We examined the effect of inundation on the C cycle in the 2013-2015 flood cycles illustrated by dissolved CO2, CH4, organic C (DOC) concentration measurements, and optical properties of dissolved organic matter (DOM) evaluated by absorbance and fluorescence spectroscopy with parallel factor analysis (PARAFAC). During the 2015 flood cycle, pCO2 varied between 5973 and 14,292 μatm, with pCH4 concentrations ranging between 2956 and 51,675 μatm respectively, with high temporal variability for both gases. The supersaturation of CO2 and CH4 in relation to the atmospheric equilibrium caused the system to behave as a net source of CO2 and CH4 to the atmosphere with evasion rates of 320 mg CO2 m−2 d−1 and 20 mg CH4 m−2 d−1, respectively. Mean DOC concentration was 7.0 ± 0.4 mg L−1 and did not differ between flood cycles. Higher concentrations of DOC were measured at the start (rising floodwaters) and at the end (receding floodwaters) of flood cycles, while lower DOC concentrations were observed during the peak flood. The PARAFAC analysis indicated the presence of five DOM components: humic (C1 and C2) and fulvic type material (C3) showed the highest relative abundance (68.5% of the total PARAFAC component fluorescence), as well as protein-like material (C4 and C5) derived from microorganisms. Our measured diffusive flux levels were below the range of emissions found for wetlands and floodplains for CO2, but were slightly higher for CH4 relative to other studies in lakes and seasonally flooded areas of the Pantanal. The large variations in concentrations of CO2, CH4 and DOC and the optical properties of DOM during the course of each flood cycle suggest a close relationship between carbon and water cycles in this tropical wetland. [ABSTRACT FROM AUTHOR]

Sediment porewater nutrients often occur at concentrations that are orders of magnitude higher than nutrients in overlying waters, and accordingly may subsidise growth of benthic macroalgal mats in estuarine ecosystems. The relative contribution of porewater nutrients is expected to be particularly important for macroalgae entrained in intertidal mudflat sediments, where access to water column nutrients is tidally constrained. In this study, filamentous Gracilaria chilensis thalli were simultaneously exposed to sediment and overlying water nutrient sources, labelled using 15N tracers (15NH4+ or 15NO3−) during a 5-day experiment. Dissolved inorganic N (DIN) uptake from porewater and overlying water accounted for 33 and 52%, respectively, of the N estimated as necessary to support the growth of G. chilensis, despite the two-fold lower DIN concentration of the overlying water and its periodic availability (8 h day−1). Of the total N assimilated by the plants, ~ 15% could not be accounted for, supporting the acquisition of other N forms in order to meet demand. We also found that regardless of background NH4+:NO3− ratios (i.e. 1:3 in overlying water and 12:1 in porewater), plants accumulated 15NH4+ significantly more readily than 15NO3−, indicating a preference for NH4+. This ability to utilise multiple sources and species of N relatively rapidly may partly explain the competitive success of entrained macroalgae relative to non-entrained species and historically abundant seagrass beds in these environments. These results underscore the significance of both internal nutrient loading and external inputs as important in sustaining opportunistic macroalgal blooms in shallow estuaries. [ABSTRACT FROM AUTHOR]

Warming may increase the extent and intensity of insect defoliations within Arctic ecosystems. A thorough understanding of the implications of this for litter decomposition is essential to make predictions of soil-atmosphere carbon (C) feedbacks. Soil nitrogen (N) and C cycles naturally are interlinked, but we lack a detailed understanding of how insect herbivores impact these cycles. In a laboratory microcosm study, we investigated the growth responses of heterotrophic soil fungi and bacteria as well as C and N mineralisation to simulated defoliator outbreaks (frass addition), long-term increased insect herbivory (litter addition at higher background N-level) and non-outbreak conditions (litter addition only) in soils from a Subarctic birch forest. Larger amounts of the added organic matter were mineralised in the outbreak simulations compared to a normal year; yet, the fungal and bacterial growth rates and biomass were not significantly different. In the simulation of long-term increased herbivory, less litter C was respired per unit mineralised N (C:N of mineralisation decreased to 20 ± 1 from 38 ± 3 for pure litter), which suggests a directed microbial mining for N-rich substrates. This was accompanied by higher fungal dominance relative to bacteria and lower total microbial biomass. In conclusion, while a higher fraction of foliar C will be respired by insects and microbes during outbreak years, predicted long-term increases in herbivory linked to climate change may facilitate soil C-accumulation, as less foliar C is respired per unit mineralised N. Further work elucidating animal-plant-soil interactions is needed to improve model predictions of C-sink capacity in high latitude forest ecosystems. [ABSTRACT FROM AUTHOR]

Improved quantification of the factors controlling soil organic matter (SOM) stabilization at continental to global scales is needed to inform projections of the largest actively cycling terrestrial carbon pool on Earth, and its response to environmental change. Biogeochemical models rely almost exclusively on clay content to modify rates of SOM turnover and fluxes of climate-active CO2 to the atmosphere. Emerging conceptual understanding, however, suggests other soil physicochemical properties may predict SOM stabilization better than clay content. We addressed this discrepancy by synthesizing data from over 5,500 soil profiles spanning continental scale environmental gradients. Here, we demonstrate that other physicochemical parameters are much stronger predictors of SOM content, with clay content having relatively little explanatory power. We show that exchangeable calcium strongly predicted SOM content in water-limited, alkaline soils, whereas with increasing moisture availability and acidity, iron- and aluminum-oxyhydroxides emerged as better predictors, demonstrating that the relative importance of SOM stabilization mechanisms scales with climate and acidity. These results highlight the urgent need to modify biogeochemical models to better reflect the role of soil physicochemical properties in SOM cycling. [ABSTRACT FROM AUTHOR]

The stream-groundwater interface (SGI) is thought to be an important location within stream networks for dissolved organic carbon (DOC) processing (e.g., degradation, removal), since it is considered a hotspot for microbial activity and biogeochemical reactions. This research is one of the first attempts to collect and assess DOC conditions at the SGI across a stream network-an entire third-order, lowland watershed in Michigan, USA. We present an initial exploration of this unique data set and highlight some of the challenges when working at these scales. Overall, our results show that SGI DOC conditions are complex at the network scale and do not conform to predictions based upon previous point- and small-scale studies. We found no strong pattern of DOC removal within the SGI at the network scale even after using chloride and temperature as natural tracers to differentiate between hydrological processes and biogeochemical reactions influencing DOC cycling. Instead, trends in DOC quantity and molecular qualities suggest that potential biotic reactions, including aerobic microbial respiration, had an influence on DOC concentrations at only some of the sites, while physical mixing of ground and stream surface waters appears to explain the majority of the observed changes in DOC concentrations at other sites. In addition, results show that neither SGI sites indicating DOC removal nor DOC molecular quality shifts measured correlated with stream order. It did reveal that DOC variability across surface water, groundwater, and the SGI locations was consistently greatest in the shallow sediments of the SGI, demonstrating that the SGI is a systematically distinct location for DOC conditions in the watershed. Our empirical stream network-scale SGI data are some of the first that are compatible with recently developed process-based, network-scale, SGI models. Our results indicate that these process-based models may not accurately represent SGI exchange of lowland, groundwater discharge-dominated streams like the one in this study. Lastly, this study shows that new methods are needed to achieve the goal of making and linking SGI observations to network-scale biogeochemical processes and theory. To help develop these methods we provide a discussion of the approach we used (i.e., 'lessons-learned') that might become the basis for systematic data analysis of SGI porewaters in future, network-wide biogeochemical studies of the SGI. [ABSTRACT FROM AUTHOR]

Tree root exudation (TRE) of water soluble organic carbon (WSOC) is an important but under-assessed component of net primary production, and is thought to strongly influence rhizosphere biogeochemistry. Riparian systems in particular are often viewed as biogeochemical hot spots fueled partially by root exudate WSOC. However, TRE rates have not been previously reported for these systems. The δC signatures of exudates may provide important insights into plant physiology and inform isotope-based methods to identify sources of soil CO fluxes, but this information is also generally lacking. In the present study, root exudate WSOC was collected in situ to assess both net exudation rates and exudate δC values in a temperate riparian forest. Net TRE rates were found to be most strongly related to a combination of tree species, root characteristics and net ecosystem exchange (Adj. R = 0.73; p < 0.001). In contrast, exudate δC values were correlated to time-lagged vapor pressure deficit (Adj. R = 0.21; p < 0.05) and air temperature (Adj. R = 0.43; p < 0.05), suggesting a rapid transfer of photosynthate from the canopy to the rhizosphere. Extrapolation of mean net TRE rates (13 µmol C g root day) from a root mass basis to the entire sampling area suggests that TRE may account for as much as 3% of net annual C uptake and represents an important input of organic matter to riparian soils. Our findings of predictable TRE rates and exudate δC values in the present study suggest that future studies examining δC values of different plant components, soil organic matter and respired soil CO would benefit by accounting for the impact of root exudates. [ABSTRACT FROM AUTHOR]

Carbon cycling associated with biological soil crusts, which occupy interspaces between vascular plants in drylands globally, may be an important part of the coupled climate-carbon cycle of the Earth system. A major challenge to understanding CO fluxes in these systems is that much of the biotic and biogeochemical activity occurs in the upper few mm of the soil surface layer (i.e., the 'mantle of fertility'), which exhibits highly dynamic and difficult to measure temperature and moisture fluctuations. Here, we report a multi-sensor approach to simultaneously measuring temperature and moisture of this biocrust surface layer (0-2 mm), and the deeper soil profile, concurrent with automated measurement of surface soil CO effluxes. Our results illuminate robust relationships between biocrust water content and field CO pulses that have previously been difficult to detect and explain. All observed CO pulses over the measurement period corresponded to surface wetting events, including when the wetting events did not penetrate into the soil below the biocrust layer (0-2 mm). The variability of temperature and moisture of the biocrust surface layer was much greater than even in the 0-5 cm layer of the soil beneath the biocrust, or deeper in the soil profile. We therefore suggest that coupling surface measurements of biocrust moisture and temperature to automated CO flux measurements may greatly improve our understanding of the climatic sensitivity of carbon cycling in biocrusted interspaces in our study region, and that this method may be globally relevant and applicable. [ABSTRACT FROM AUTHOR]

Fundamental changes in seawater carbonate chemistry and sea surface temperatures associated with the ocean uptake of anthropogenic CO are accelerating, but investigations of the susceptibility of biogeochemical processes to the simultaneous occurrence of multiple components of climate change are uncommon. Here, we quantify how concurrent changes in enhanced temperature and atmospheric pCO, coupled with an associated shift in macrofaunal community structure and behavior (sediment particle reworking and bioirrigation), modify net carbon and nutrient concentrations (NH-N, NO-N, PO-P) in representative shelf sea sediment habitats (mud, sandy-mud, muddy-sand and sand) of the Celtic Sea. We show that net concentrations of organic carbon, nitrogen and phosphate are, irrespective of sediment type, largely unaffected by a simultaneous increase in temperature and atmospheric pCO. However, our analyses also reveal that a reduction in macrofaunal species richness and total abundance occurs under future environmental conditions, varies across a gradient of cohesive to non-cohesive sediments, and negatively moderates biogeochemical processes, in particular nitrification. Our findings indicate that future environmental conditions are unlikely to have strong direct effects on biogeochemical processes but, particularly in muddy sands, the abundance, activity, composition and functional role of invertebrate communities are likely to be altered in ways that will be sufficient to regulate the function of the microbial community and the availability of nutrients in shelf sea waters. [ABSTRACT FROM AUTHOR]

Results from a 1D setup of the European Regional Seas Ecosystem Model (ERSEM) biogeochemical model were compared with new observations collected under the UK Shelf Seas Biogeochemistry (SSB) programme to assess model performance and clarify elements of shelf-sea benthic biogeochemistry and carbon cycling. Observations from two contrasting sites (muddy and sandy) in the Celtic Sea in otherwise comparable hydrographic conditions were considered, with the focus on the benthic system. A standard model parameterisation with site-specific light and nutrient adjustments was used, along with modifications to the within-seabed diffusivity to accommodate the modelling of permeable (sandy) sediments. Differences between modelled and observed quantities of organic carbon in the bed were interpreted to suggest that a large part (>90%) of the observed benthic organic carbon is biologically relatively inactive. Evidence on the rate at which this inactive fraction is produced will constitute important information to quantify offshore carbon sequestration. Total oxygen uptake and oxic layer depths were within the range of the measured values. Modelled depth average pore water concentrations of ammonium, phosphate and silicate were typically 5-20% of observed values at the muddy site due to an underestimate of concentrations associated with the deeper sediment layers. Model agreement for these nutrients was better at the sandy site, which had lower pore water concentrations, especially deeper in the sediment. Comparison of pore water nitrate with observations had added uncertainty, as the results from process studies at the sites indicated the dominance of the anammox pathway for nitrogen removal; a pathway that is not included in the model. Macrofaunal biomasses were overestimated, although a model run with increased macrofaunal background mortality rates decreased macrofaunal biomass and improved agreement with observations. The decrease in macrofaunal biomass was compensated by an increase in meiofaunal biomass such that total oxygen demand remained within the observed range. The permeable sediment modification reproduced some of the observed behaviour of oxygen penetration depth at the sandy site. It is suggested that future development in ERSEM benthic modelling should focus on: (1) mixing and degradation rates of benthic organic matter, (2) validation of benthic faunal biomass against large scale spatial datasets, (3) incorporation of anammox in the benthic nitrogen cycle, and (4) further developments to represent permeable sediment processes. [ABSTRACT FROM AUTHOR]

In this study we examine pools of carbon (C), nitrogen (N), and phosphorus (P) in the Baltic Sea, both simulated and reconstructed from observations. We further quantify key fluxes in the C, N, and P cycling. Our calculations include pelagic reservoirs as well as the storage in the active sediment layer, which allows a complete coverage of the overall C, N, and P cycling on a system-scale. A striking property of C versus N and P cycling is that while the external supplies of total N and P (TN and TP) are largely balanced by internal removal processes, the total carbon (TC) supply is mainly compensated by a net export out of the system. In other words, external inputs of TN and TP are, in contrast to TC, rather efficiently filtered within the Baltic Sea. Further, there is a net export of TN and TP out of the system, but a net import of dissolved inorganic N and P (DIN and DIP). There is on the contrary a net export of both the organic and inorganic fractions of TC. While the pelagic pools of TC and TP are dominated by inorganic compounds, TN largely consists of organic N because allochthonous organic N is poorly degradable. There are however large basin-wise differences in C, N, and P elemental ratios as well as in inorganic versus organic fractions. These differences reflect both the differing ratios in external loads and differing oxygen conditions determining the redox-dependent fluxes of DIN and DIP. [ABSTRACT FROM AUTHOR]