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Estuaries have been recognized as one of the major sources of greenhouse gases (GHGs) in aquatic systems; yet we still lack insights into the impact of both anthropogenic and natural factors on the dynamics of GHG emissions. Here, we assessed the spatiotemporal dynamics and underlying drivers of the GHG emissions from the Scheldt Estuary with a focus on the effects of salinity gradient, water pollution, and land use types, together with their interaction. Overall, we found a negative impact of salinity on carbon dioxide (CO 2) and nitrous oxide (N 2 O) emissions which can be due to the decrease of both salinity and water quality when moving upstream. Stronger impact of water pollution on the GHG emissions was found at the freshwater sites upstream compared to saline sites downstream. In particular, when water quality of the sites reduced from good, mainly located in the mouth and surrounded by arable sites, to polluted, mainly located in the upstream and surrounded by urban sites, CO 2 emissions from the sites doubled while N 2 O emissions tripled. Similarly, the effects of water pollution on methane (CH 4) emissions became much stronger in the freshwater sites compared to the saline sites. These decreasing effects from upstream to the mouth were associated with the increase in urbanization as sites surrounded by urban areas released on average almost two times more CO 2 and N 2 O than sites surrounded by nature and industry areas. Applied machine learning methods also revealed that, in addition to salinity effects, nutrient and organic enrichment stimulated the GHG emissions from the Scheldt Estuary. These findings highlight the importance of the interaction between salinity, water pollution, and land use in order to understand their influences on GHG emissions from dynamic estuarine systems. [Display omitted] • Salinity, water pollution and land use play a crucial role in estuary emissions. • Significant differences of estuarine CO 2 and N 2 O emissions across the salinity classes. • Water pollution can double estuarine CO 2 emissions and triple their N 2 O emissions. • Moving downstream, the emissions from urban polluted sites decreased substantially. • Nutrient and organic enrichment stimulated estuarine GHG emissions. [ABSTRACT FROM AUTHOR]

Atmospheric CO 2 concentration and nitrogen (N) deposition have been altered by anthropogenic activity and they affect global biogeochemical cycles. It is still not clear how these environmental changes influence the storage and cycling of organic matter (OM) in soils, although this plays a key role in the biogeochemistry of terrestrial ecosystems. Here, we used n -alkanes as biomarkers for plant-derived OM in specific soil fractions. We investigated the effect of elevated CO 2 concentration and increased N deposition on the molecular and isotopic (δ 13 C) composition of n -alkanes in above-ground and below-ground tree biomass (beech and spruce) and soil density fractions. The n -alkane distribution patterns of spruce needles and root biomass were well reflected in those of particulate soil OM. Long chain (C 27–34 ) n -alkanes were preserved in soil by association with soil minerals, while mid-chain (C 20–27 ) n -alkanes from plant biomass and particulate soil OM were ultimately degraded due to lack of physical protection. Renewal of n -alkanes was lower in roots and spruce needles than in beech leaves. Similar low renewal of n -alkanes in soil reflected the low input of n -alkanes from plant biomass to the soil n -alkane pool. n -Alkane biosynthesis in beech leaves was modified under high N deposition and similar effects were observed for n -alkanes in mineral soil fractions and bulk soil. Here, different biosynthesis of n -alkanes in beech leaves under high N deposition consequently led to a modified input of n -alkanes from plant biomass to soil, with a relatively low proportion of new (experimentally-derived) n -alkanes bound to soil minerals. [ABSTRACT FROM AUTHOR]

Soil microorganisms play a key role in soil organic matter (SOM) dynamics, but little is known about the controls affecting the distribution of microbial biomass and their residues in soil. Here, a forested Cambisol topsoil was incubated with 13C-labeled glycine or beech leaves for 12 weeks prior to sequential density fractionation. The incorporation of the 13C label in amino sugars (AS) was used to gain insight into bacterial and fungal assimilation of the substrates. AS derived from glycine or leaves were compared to total AS to investigate how microbial residues and active communities were distributed among soil density fractions. Bacteria slightly dominated leaf C assimilation, while a pronounced fungal dominance was observed for glycine. The glycine-derived AS and original AS were similarly distributed among the soil density fractions, both peaking in microbial aggregates (1.8-2.4 g cm−3). Leaf-derived AS were mostly found in association with the plant debris (<1.65 g cm−3). The ratios of substrate-derived AS C to substrate-derived C increased with soil fraction density for both glycine and leaves. The same pattern was observed with original AS C to soil fraction C ratios. We concluded that bacteria and fungi were most active where the resource was even though their residues accumulate mostly in microbial aggregates (1.8-2.4 g cm−3). We suggest that such accumulation might be attributed to (1) an increasing stabilization efficiency of microbial residues and (2) the progressive SOM transfer, from plant debris to microbial aggregates (1.8-2.4 g cm−3). [ABSTRACT FROM AUTHOR]

Abstract: Amino sugars are key compounds of microbial cell walls, which have been widely used as biomarker of microbial residues to investigate soil microbial communities and organic residue cycling processes. However, the formation dynamics of amino sugar is not well understood. In this study, two agricultural Luvisols under distinct tillage managements were amended with uniformly 13C-labeled wheat residues of different quality (grain, leaf and root). The isotopic composition of individual amino sugars and CO2 emission were measured over a 21-day incubation period using liquid chromatography–isotope ratio mass spectrometry (LC–IRMS) and trace gas IRMS. Results showed that, the amount of residue derived amino sugars increased exponentially and reached a maximum within days after residue addition. Glucosamine and galactosamine followed different formation kinetics. The maxima of residue derived amino sugars formation ranged from 14 nmol g−1 dry soil for galactosamine (0.8% of the original concentration) to 319 nmol g−1 dry soil for glucosamine (11% of the original concentration). Mean production times of residue derived amino sugars ranged from 2.1 to 9.3 days for glucosamine and galactosamine, respectively. In general, larger amounts of amino sugars were formed at a higher rate with increasing plant residue quality. The microbial community of the no-till soil was better adapted to assimilate low quality plant residues (i.e. leaf and root). All together, the formation dynamics of microbial cell wall components was component-specific and determined by residue quality and soil microbial community. [Copyright &y& Elsevier]

Tidal wetlands are one of the major sources of CH 4 and N 2 O in natural systems to the atmosphere; yet we still lack insights into the impact of their biogeochemical dynamics on the emissions of these greenhouse gases (GHGs). Here, we investigated the CH 4 and N 2 O sources in four tidal wetlands ranging from freshwater to polyhaline with a focus on their production pathways. By using natural abundance isotopes and functional marker genes, we found that salinity level, sediment moisture content, quantity and quality of organic carbon (OC) and nutrients were major drivers of the wetland CH 4 and N 2 O emissions. As the salinity levels decreased in the tidal wetlands, both the labile nature and concentration of nutrients and OC increased. These conditions favored methanogenesis as indicated in the abundance and expression of mcrA and the CH 4 emissions from the freshwater wetland. Conversely, higher salinity depressed organic matter decomposition rates and microbial activities, causing much lower CH 4 production in the saline wetlands. Isotope mapping revealed that denitrification contributed mainly to wetland N 2 O emissions (80–90%), reflected in the strong expression of denitrifier marker genes nirS , nirK , and nosZ and low nosZ :nir ratio. Nitrification played an important role in wetland N 2 O emissions at high NH 4 + and low salinity levels. This condition was the case in the freshwater wetland – the strongest N 2 O emitter – where we found the highest NH 4 + concentrations and the most abundant and expressed nitrifier marker genes amoA AOA and amoA AOB. Methanogen and denitrifier marker genes were more abundant and expressed at the surface layer compared to the subsurface layer, implying the presence of methane and denitrification paradoxes in the tidal wetlands. This study paves a way for the coupling of isotope and functional gene analyses to deeply explore GHG formation pathways and responsible microbial activities in dynamic wetland systems. • Salinity, sediment moisture, organic C and N species affect tidal wetland fluxes. • Low salinity and high labile OC levels favor tidal wetland CH 4 fluxes. • Denitrification is the most important pathway of N 2 O production in tidal wetlands. • Nitrification becomes an important pathway at high NH 4 + and low salinity levels. • Coupling of isotopic and gene analyses to explore tidal wetlands biogeochemistry. [ABSTRACT FROM AUTHOR]

• Variations in fatty acid δ13C values for land uses were assessed at catchment scale. • Highly variable δ13C-FAs values reflected vegetation composition and change. • Paired δ13C values of fatty acids and bulk carbon detected C 4 crops legacy effects. • A priori land use classification for sediment sources was questionable. • Sediment source classification required combining functionally similar land uses. Stable carbon isotope values of fatty acids (>20 C-atoms) (δ13C-FAs) have been increasingly used to apportion sediment sources using isotope mixing models. Understanding the variation of δ13C-FAs within catchment land uses is crucial for correct classification of sediment sources but it has rarely been evaluated. Here, we assessed the variability of δ13C values of FAs (C 22–32) in soils under land uses within a catchment (23 km2) in the mid-hills of Nepal. High δ13C-FA variability in agricultural terraces (−32.8‰ to −20.2‰) and mixed forest (−37.3‰ to −22.3‰) was observed. This is due to differences in inputs of FAs of different chain-lengths with variable δ13C-FAs values by C 3 and C 4 crops as well as from farmyard manure in agricultural terraces. The pine plants succession in the forest and differences in FA preservation explained the observed variation in δ13C-FA values in a mixed forest. A pine forest transect that had undergone land cover change about 40 years ago showed high variability of δ13C-FA values strongly linked to legacy effects of previous C 4 crops. Importantly, soils from the border of a land use and fresh surface deposits mostly increased the within-land use variability of δ13C-FAs values. Overall, δ13C-FA values have low potential to function as a robust tracer for high resolution discrimination of land uses and the validity of a priori sediment source classification is most questionable in catchments that have undergone land cover change. Cluster analysis was promising for identifying and combining functionally similar land uses to define more meaningful sediment sources at the catchment scale. [ABSTRACT FROM AUTHOR]

Leaf-wax n -alkanes are produced by terrestrial plants, and through long-term preservation in sediments their stable hydrogen-isotopic signature (δ2H wax) provides useful information on past hydrological variation for paleoclimate reconstructions. However, gaps remain in our understanding of the relationships between the isotopic signatures of leaf waxes and the plants' source water. In this study, we investigated the influence of plant growth form, habitat and season on the distribution patterns and δ2H wax values of 14 plant species (among which are two grasses, five trees and seven shrubs) sampled during four successive dry and wet seasons in three distinct habitats around Lake Chala in equatorial East Africa. Variation in δ2H wax was analyzed with linear mixed-effect models and compared with the associated values of xylem water (δ2H xylem), leaf water (δ2H leaf) and biosynthetic hydrogen fractionation (ε bio). Our results show that plant growth form was the most important driver of modern-day δ2H wax variability in the study area, and that differences in δ2H wax among habitats to a large extent reflect how each major growth forms is represented in those habitats. Individual plant species appear to express substantial species-specific isotopic fractionation that cannot be attributed to the tested external factors but rather seem to depend on intrinsic (e.g., plant phenological and biosynthesis-related) factors. For the purpose of calibrating δ2H wax signatures against vegetation types, it is thus crucial to analyze representative samples of the plant communities present in the study area. Our results further indicate that paleohydrological studies in regions receiving rain from multiple moisture sources must take into account possible seasonal bias in the δ2H wax signature relative to annual rainfall, due to unequal use of those moisture sources by the plants. Finally, the strong influence of plant growth form on δ2H wax values argues for δ2H wax variation in paleo-records being evaluated in conjunction with independent proxy data on changes in vegetation composition. Differences in n -alkane distribution patterns among trees, shrubs and grasses (e.g., average chain length, carbon preference index and C 31 /(C 29 + C 31) ratio) may provide such proxies, and can be produced from the same leaf-wax n -alkane dataset used to determine δ2H wax. [ABSTRACT FROM AUTHOR]

• N availability, DO & salinity levels, & temperature variation affect N 2 O emissions. • Net N 2 O emissions during winter and summer were comparable in the Scheldt. • Soil N, manure and sewage are possible sources of NO 3 − in the Scheldt. • Denitrification is the most important pathway of N 2 O production in the estuaries. • Importance of combining isotope analyses to gain insights into estuarine N cycling. Estuaries are strong sources of N 2 O to the atmosphere; yet we still lack insights into the impact of their biogeochemical dynamics on the emissions of this powerful greenhouse gas. Here, we investigated the spatiotemporal dynamics of the N cycle in an estuary with a focus on the emission mechanisms and pathways of N 2 O. By coupling N 2 O isotopocule analysis and substrate NO 3 − isotope analysis, we found that nutrient availability, oxygen level, salinity gradient and temperature variation were major drivers of the N 2 O emissions from the Scheldt Estuary. In winter, lower temperature and higher O 2 concentration diminished denitrification rates and reduction of N 2 O to N 2 , while both were enhanced in warmer summer, causing higher fraction of reduced N 2 O. As a result, we found comparable N 2 O fluxes and dissolved concentrations between the two seasons. Decrease in salinity level and increase in NO 3 − concentration accelerated N 2 O production when moving upstream of the estuary where more urbanization and higher NO 3 − from wastewater discharges were found. However, these drivers had no significant effect on the fraction of N 2 O derived by either denitrification or nitrification and/or fungal denitrification since the fractional proportion of these pathways showed no spatiotemporal variations, remaining around 89 % and 11 %, respectively. These findings challenge the conventional notion that N 2 O fluxes are generally higher in summer because of higher denitrification rates while confirming that denitrification is the most important pathway of N 2 O production in the estuaries. Furthermore, our study highlight the importance of combining various isotope analyses to gain in-depth understanding about N 2 O emission pathways and N cycling in dynamic systems like estuaries. [ABSTRACT FROM AUTHOR]

Soil nutrient availability has a strong influence on the fate of soil carbon (C) during microbial decomposition, contributing to Earth's C balance. While nutrient availability itself can impact microbial physiology and C partitioning between biomass and respiration during soil organic matter decomposition, the availability of labile C inputs may mediate the response of microorganisms to nutrient additions. As soil organic matter is decomposed, microorganisms retain or release C, nitrogen (N) or phosphorus (P) to maintain a stoichiometric balance. Although the concept of a microbial stoichiometric homeostasis has previously been proposed, microbial biomass CNP ratios are not static, and this may have very relevant implications for microbial physiological activities. Here, we tested the hypothesis that N, P and potassium (K) nutrient additions impact C cycling in a tropical soil due to microbial stoichiometric constraints to growth and respiration, and that the availability of energy-rich labile organic matter in the soil (i.e. leaf litter) mediates the response to nutrient addition. We incubated tropical soil from French Guiana with a 13 C labeled leaf litter addition and with mineral nutrient additions of +K, +N, +NK, +PK and +NPK for 30 days. We found that litter additions led to a ten-fold increase in microbial respiration and a doubling of microbial biomass C, along with greater microbial N and P content. We found some evidence that P additions increased soil CO 2 fluxes. Additionally, we found microbial biomass CP and NP ratios varied more widely than CN in response to nutrient and organic matter additions, with important implications for the role of microorganisms in C cycling. The addition of litter did not prime soil organic matter decomposition, except in combination with +NK fertilization, indicating possible P-mining of soil organic matter in this P-poor tropical soil. Together, these results point toward an ultimate labile organic substrate limitation of soil microorganisms in this tropical soil, but also indicate a complex interaction between C, N, P and K availability. This highlights the difference between microbial C cycling responses to N, P, or K additions in the tropics and explains why coupled C, N and P cycle modeling efforts cannot rely on strict microbial stoichiometric homeostasis as an underlying assumption. [ABSTRACT FROM AUTHOR]

[Display omitted] • A UV-crosslinkable poly(ɛ-caprolactone) precursor was prepared to serve as biodegradable fertilizer coating. • Fluidized bed technology was exploited as efficient coating method. • Nutrient release dynamics of the coated fertilizer granules showed controlled release up to three months. • An accelerated degradation study showed insight in the degradation behavior of the polymer coating. A photo-crosslinkable biodegradable polymer based on a poly(ɛ-caprolactone) backbone was herein developed to enhance the efficiency of nitrogen uptake from fertilizers. To this end, ammonium and calcium nitrate fertilizers were coated with photo-crosslinkable acrylate-endcapped urethane-based poly(ɛ-caprolactone) through fluidized bed technology. We hypothesized that the coating would enable to control the release time of nitrogen species from the granules for 1-3 months. Fourier transform infrared spectroscopy was used to monitor the reaction progress while chemical structure verification of the acrylate-endcapped urethane-based poly(ɛ-caprolactone) was executed via 1H-NMR spectroscopy. The physical properties and crosslinking efficiency of the coating material were determined through rheological measurements, gel fraction determination and thermogravimetric analysis before and after crosslinking. The data showed that the optimal fluidized bed processing temperature was 29°C. Scanning electron microscopy allowed to evaluate both the coating application and final coating thickness onto the polymer coated fertilizer granules. Three different coating thicknesses were applied onto each granule type. The polymer coated fertilizers were evaluated for their nitrate and ammonium release properties in sand substrate columns on laboratory scale. Osmocote bloom was used as reference throughout the experiments. The resulting cumulative release curves were fitted to a normalized non-linear regression with variable slope and the release was found to be based on a diffusion mechanism. Nitrogen release dynamics were primarily controlled by the thickness of the coating layer. The longest release times were observed for the ammonium nitrate and calcium nitrate granules with thickest coating layers (105 ± 10 μm and 148 ± 12 μm, respectively); 75 % nitrate was released after 75 - 77 and 74 - 75 days, respectively. The release was shown to be in line with the target release time of a commercial fertilizer included as benchmark throughout the release study. The herein developed polymer can be considered promising to serve as coating material for controlled release fertilizer development because of its benefits as compared to Osmocote (i.e., single coating application possible, biodegradable coating, no additional toxic components or solvents and room temperature processing). [ABSTRACT FROM AUTHOR]

Abstract: Soil tillage practices affect the soil microbial community in various ways, with possible consequences for nitrogen (N) losses, plant growth and soil organic carbon (C) sequestration. As microbes affect soil organic matter (SOM) dynamics largely through their activity, their impact may not be deduced from biomass measurements alone. Moreover, residual microbial tissue is thought to facilitate SOM stabilization, and to provide a long term integrated measure of effects on the microorganisms. In this study, we therefore compared the effect of reduced (RT) and conventional tillage (CT) on the biomass, growth rate and residues of the major microbial decomposer groups fungi and bacteria. Soil samples were collected at two depths (0–5 cm and 5–20 cm) from plots in an Irish winter wheat field that were exposed to either conventional or shallow non-inversion tillage for 7 growing seasons. Total soil fungal and bacterial biomasses were estimated using epifluorescence microscopy. To separate between biomass of saprophytic fungi and arbuscular mycorrhizae, samples were analyzed for ergosterol and phospholipid fatty acid (PLFA) biomarkers. Growth rates of saprophytic fungi were determined by [14C]acetate-in-ergosterol incorporation, whereas bacterial growth rates were determined by the incorporation of 3H-leucine in bacterial proteins. Finally, soil contents of fungal and bacterial residues were estimated by quantifying microbial derived amino sugars. Reduced tillage increased the total biomass of both bacteria and fungi in the 0–5 cm soil layer to a similar extent. Both ergosterol and PLFA analyses indicated that RT increased biomass of saprophytic fungi in the 0–5 cm soil layer. In contrast, RT increased the biomass of arbuscular mycorrhizae as well as its contribution to the total fungal biomass across the whole plough layer. Growth rates of both saprotrophic fungi and bacteria on the other hand were not affected by soil tillage, possibly indicating a decreased turnover rate of soil microbial biomass under RT. Moreover, RT did not affect the proportion of microbial residues that were derived from fungi. In summary, our results suggest that RT can promote soil C storage without increasing the role of saprophytic fungi in SOM dynamics relative to that of bacteria. [Copyright &y& Elsevier]

Abstract: A set of 116 structurally very diverse compounds, mainly drugs, was characterized by 1630 molecular descriptors. The biological property modelled in this study was the transdermal permeability coefficient log K p. The main objective was to find a limited set of suitable model compounds for skin penetration studies. The classification and regression trees (CART) approach was applied and the resulting groups were discussed in terms of their role as possible model compounds and their determining descriptors. A second objective was to model transdermal penetration as a function of selected descriptors in quantitative structure–property relationships (QSPR) using a boosted CART (BRT) approach and multiple linear regression (MLR) analysis, where regression models were obtained by stepwise selection of the best descriptors. Evaluation of the standard statistical, as well as descriptor-number dependent, regression quality attributes yielded a maximal 10-dimensional MLR model. The CART and MLR models were subjected to an external validation with a test set of 12 compounds, not included in the original learning set of 104 compounds, to assess the predictive power of the models. [Copyright &y& Elsevier]