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Previous studies showed that bark cover at early-decay stage had profound control on the invertebrate assemblages of bark and wood, with possible consequence for the decomposition process. However, previous experimental designs could not disentangle how bark versus wood traits affect the invertebrate assemblage process in bark and/or wood separately because wood traits of different tree species may vary independently from bark traits. Furthermore, we do not know whether such tree species-specific bark trait effects are still influential at mid-decay stage. To unravel whether and how bark and wood traits influence invertebrate communities in tree logs at mid-decay stage, we introduce reciprocal bark transplantation within pairs of different tree species as a new method. We applied this method to two pairs of phylogenetically contrasting species of gymnosperms (pair I: Araucaria araucana and Cryptomeria japonica, pair II: Picea abies and Thuja plicata) and another gymnosperm (Chamaecyparis lawsoniana) set as disturbance control to test for potential bark manipulation artefacts on invertebrate community composition. Our bark exchange experiment revealed that both bark and wood host abundant and divergent subsets of invertebrates on mid-decay logs of different tree species. We further documented that the invertebrate community composition was predominantly shaped by the traits of host tissue per se, while also being significantly but less strongly affected by the traits of the other tissue, that is, the adjacent bark or wood. Our results indicated that bark trait effects faded with time and how long bark trait effects persist greatly depends on bark thickness. Synthesis. Our study suggests that maintaining deadwood heterogeneity related to variation between tree species and bark versus wood, is important for nursing a large biodiversity of invertebrates. Combined with bark removal methodology, our bark exchange method can be further extended to more decay stages and more forest biomes to track bark trait effects and bark induced priority effects on deadwood decomposition, and its associated invertebrate and microbial communities.

Sustainable ecosystem management relies on our ability to predict changes in plant diversity and to understand the underlying mechanisms. Empirical evidence demonstrates that abundance- and functional-based processes simultaneously explain the loss of plant diversity in response to human activities. Recently, a novel indicator based on percent cover (CoverD) and maximum height (HeightD) of the dominant plant species – space resource utilization (SRUD) – has proven to give robust and better predictions of plant diversity dynamics than community biomass. Whether the superior predictive ability of SRUD is due to its capacity to simultaneously capture abundance- and functional-based processes remains unknown. Here, we tested this hypothesis by quantifying mechanistic links between changes in SRUD and biodiversity in response to nutrients and herbivores. Furthermore, we assessed the relative contribution of dominant, intermediate and rare species to reduced density of individuals by combining null model analysis with field experiments. We found that SRUD successfully captured changes in ground-level light availability and changes in the number of individuals to predict plant diversity dynamics, and each of CoverD and HeightD partly and independently contributed to both processes. Comparative results from null model analysis and field experiments confirmed that individual losses of dominant, intermediate and rare species followed non-random processes. Specifically, compared with random loss process, rare species lost proportionally more individuals and thus disproportionately contributed to species loss, while dominant and intermediate species lost less. Our results demonstrate that SRUD captures both abundance- and functional-based processes thus explaining why SRUD provides more accurate predictions of changes in species diversity. Given that rare species can play an important role in shaping community structure, resisting against invasion, impacting higher trophic levels and providing multiple ecosystem functions, reducing the SRU of dominant species could alleviate the risk of exclusion of rare species by mitigating abundance- and functional-based competition processes.

Peat decomposition in managed peatlands is responsible for a decrease of 0.52 GtC yr−1 in global carbon stock and is strongly linked to drainage to improve the agricultural bearing capacity, which increases aeration of the soil. Microbial aerobic decomposition is responsible for the bulk of the net CO2 emission from the soil and could be reduced by wetting efforts or minimizing drainage. However, the effects of rewetting efforts on microbial respiration rate are largely unknown. In this study, we aimed to obtain more process-based understanding of these rewetting effects on peat decomposition by integrating high-quality field measurements and literature relationships with an advanced hydrological modelling approach where soil moisture and temperature are centralized as the main drivers for peat decomposition. In 2020 and 2021, two dairy farming peatlands, where subsoil irrigation and drainage (SSI) was tested against a control situation, were continuously monitored for CO2 fluxes, groundwater table, soil moisture and soil temperature. After successfully representing field hydrology and carbon dynamic measurements within our process-based model, we further explored the effects of rewetting under different weather conditions, water management strategies (raising ditchwater levels and SSI) and hydrological seepage settings. To represent peat carbon dynamics we introduced a methodology to estimate potential aerobic microbial respiration rate, based on potential respiration rate curves for soil temperature and water-filled pore space (WFPS). Measurements show that rewetting with SSI resulted in higher summer groundwater levels, soil temperatures and WFPS. SSI reduced the net ecosystem carbon balance (NECB) by 1.58 ± 0.56 kg CO2 m−2 yr−1 (83 ± 25 %) and 0.66 ± 0.62 kg CO2 m−2 yr−1 (28 ± 15 %) for Assendelft and Vlist respectively in 2020. SSI had a negligible effect in 2021 for both research locations, due to more precipitation, lower temperatures and different SSI management (in Assendelft) as compared to 2020. Simulated rewetting effects were in agreement with measured rewetting effects. Model simulations indicate that raising ditchwater levels always reduces peat respiration rates. Furthermore, we found that the application of SSI (i) reduces yearly peat respiration rates in a dry year and/or with downward hydrological fluxes and (ii) increases peat respiration rates in a wet year and/or when upward groundwater seepage is present. Moreover, combining SSI with high ditchwater levels or pressurizing SSI systems will further reduce peat respiration rates. We show that our process-based approach based on temperature and WFPS soil conditions to determine NECB represents observed variance to a greater extent than empirical relationships that involve average groundwater level observations or simulations. Therefore, we recommend using this kind of approach to estimate the effectiveness of rewetting. When this is not possible, we recommend using mean summer groundwater level instead of mean annual groundwater level as a proxy to estimate NECB. Such relations between mean groundwater levels and NECB are prone to underestimating NECB for SSI parcels.

Peat decomposition in managed peatlands is responsible for a decrease of 0.52 GtC yr−1 in global carbon stock and is strongly linked to drainage to improve the agricultural bearing capacity, which increases aeration of the soil. Microbial aerobic decomposition is responsible for the bulk of the net CO2 emission from the soil and could be reduced by wetting efforts or minimizing drainage. However, the effects of rewetting efforts on microbial respiration rate are largely unknown. We aimed to obtain more insight in these rewetting effects and measured them for 1 year for two dairy farming peatlands where submerged drainage subsurface irrigation (SDSI) was tested against a control situation. With a modelling approach, we explored the effects of rewetting under different weather conditions, water management strategies (raising ditch water levels and SDSI) and hydrological settings. We introduced a methodology to estimate potential aerobic microbial respiration rate as measure for peat decomposition in managed peatlands, based on potential respiration rate curves for soil temperature and water filled pore space (WFPS). Rewetting with SDSI resulted in higher summer groundwater levels, soil temperatures and WFPS. SDSI reduced net ecosystem production (NEP) with 1.27 ± 0.39 kg CO2 m−2 yr−1 (83 %) and 0.78 ± 0.37 kg CO2 m−2 yr−1 (35 %) for Assendelft and Vlist respectively. With the process based modelling approach we found that raising ditch water levels always reduces peat respiration rates. Furthermore, we found that the application of SDSI reduces yearly peat respiration rates in environments in a dry year and/or with downward hydrological fluxes, and increases peat respiration rates in a wet year and/or when upward groundwater fluxes are present. Moreover, combining SDSI with high ditch water levels or pressurizing SDSI systems will further reduce peat respiration rates. We highly recommend to use a process-based approach based on temperature and WFPS soil conditions to determine effectivities of rewetting efforts over empirical relationships between average groundwater level and NEP. Such a more process based approach allows to distinguish between groundwater levels raised by SDSI and ditch water levels. When this is not possible, we recommend using mean summer groundwater level instead of mean annual groundwater level as a proxy to estimate NEP. Such relations between mean groundwater levels and NEP need to be corrected for situations with SDSI.

Dead wood availability and the variability in dead wood quality, i.e. tree species and decay stages, are often low in managed forests, which negatively affects biodiversity of invertebrate species. Leaving more (coarse) dead wood can increase invertebrate richness, but it remains unclear how many and which combinations of tree taxa and decay stages are required to optimize niche heterogeneity in managed forests. We investigated the diversity of the main arthropod groups associated with dead wood, i.e. millipedes, centipedes, isopods and beetles, through the first four years of decomposition of logs of twenty common temperate tree species placed in the “common garden” experiment LOGLIFE. We hypothesized that (1) invertebrate richness for combinations of a given number of tree species would be promoted by mixing both tree species and decay period and that (2) invertebrate richness increases up to a saturation point with more tree species at different decay stages added. We also hypothesized that (3) an increase in phylogenetic distance among the tree species in combinations would promote their overall invertebrate diversity. We found that the better combinations, in terms of invertebrate richness, after one and two years of decay, but not after four years, consisted of a mix of gymnosperms and angiosperms, indicating that variation in tree species is especially important during the initial decomposition period. The best combinations in terms of invertebrate richness consisted of at least one tree species from each decay period, indicating that also variation in the decay stage of the tree is important to promote invertebrate diversity. We observed that at least four wood types were required to approach the 95% saturation point for species richness. The third hypothesis, that dissimilarity in phylogenetic position could be a predictive tool for increasing invertebrate richness in combinations of tree species, was not supported by our results. Thus, in order to maintain diversity of dead wood invertebrates in forests we recommend not only to provide richness in tree species, but also to plant particular combinations of trees (preferably angiosperm-gymnosperm combinations) that differ in the invertebrate communities they typically host and to temporally spread the logging of trees. This way the logging residues cover different resources and habitats at each moment in time, which is likely to result in a large diversity of dead wood invertebrates.

Peat soils are an important carbon stock in the global carbon cycle containing more than two third of the atmospheric carbon amount (600 GtC of 760 GtC) despite their relatively small landmass of 3% worldwide. Drainage of peatlands contributes significantly to the enhanced global warming, as it allows oxygen to intrude the soil, intensifying aerobic microbial decomposition associated with carbon dioxide emission. Water management strategies that result in a raise in (summer) groundwater tables can have the opposite effect. These measures, such as raising the surface water level and/or the application of submerged drain subsurface irrigation systems, are already being applied. However, the outcome of these strategies remains debated and is still largely to be tested. We aim to explore the potential effects of these water management strategies on reducing GHG emission in peatlands.We simulated the effects of several water management strategies on potential aerobic peat decomposition in a managed Dutch grassland on sedge peat under various hydrological and climatological conditions. To estimate potential microbial activity in the unsaturated zone two main drivers, temperature and water filled pore space (WFPS) were used. We found that increasing ditch water levels yields a decrease in potential aerobic peat decomposition independent of summer drought, hydrological regime and peat hydrological conductivity. Furthermore, we found that submerged drainage-irrigation systems tend to establish a stable moist zone relatively close to the warm soil surface in which potential microbial activity can remain high over the complete summer period. Due to these stable conditions, we expect peat decomposition in this layer to be high, possibly counteracting the effects of decreased aeration depth due to higher water tables. Submerged drainage-irrigation systems generally decrease potential microbial activity in environments with downward flow, but increase the activity in environments with upward flow. Increased benefits of the submerged systems are found for dry years, with high surface water levels and/or decreasing hydrological conductivity of the intact peat.

A central paradigm in comparative ecology is that species sort out along a global economic strategy spectrum, ranging from slow to fast growth. Many studies evaluated plant strategy spectra for leaf traits, b u t few studies evaluated stem strategy spectra using a comprehensive set of anatomical, chemical and morphological traits, addressing key stem functions of different stem compartments (inner wood, outer wood and bark). This study evaluates how stem traits vary in the wood and bark of temperate tree species, and whether a slow-fast growth strategy spectrum exists and what traits make up this plant strategy spectrum. For 14 temperate gymnosperm and angiosperm species, 20 traits belonging to six key stem functions were measured for three stem compartments. Both across and within gymnosperms and angiosperms, a slow-fast stem strategy spectrum is found. Gymnosperms have slow traits and showed converging stem strategies because of their uniform tracheids. Angiosperms have fast traits and showed diverging stem strategies because of a wider array of tissues (vessels, parenchyma and fibers) and vessel size and arrangements (ring-porous versus diffuse porous). Gymnosperms showed a main trade-off between hydraulic efficiency and safety, and angiosperms showed a main trade-off between ‘slow’ diffuse porous species and ‘fast’ ring porous species. The slow traits of gymnosperms allow for a high hydraulic safety, an evergreen leaf habit and steady but slow growth makes them successful in unproductive habitats whereas the fast traits of angiosperms allow for high conductivity, a deciduous leaf habit and fast growth which makes them successful in productive habitats.

Soil degradation represents a pressing worldwide problem that is being accelerated by processes of erosion, depletion of soil organic matter, soil compaction, acidification, salinization, and drought. Soil microorganisms can influence soil aggregation via a range of mechanisms such as the production of exopolysaccharides and other extracellular matrix polymers such those involved in biofilm formation. In this study, we south to use bacteria harboring specific traits to enhance soil aggregation. To this end, 120 bacterial strains were isolated from an experiment field under drought conditions and tested for their ability to grow under drought, salinity tolerance, rapid growth, biofilm, and exopolysacharides production. Based upon this trait assessment, 24 strains were further tested at two moisture levels for their ability to impact soil structure after 8 weeks of incubation at 25ºC. The mean weight diameter (MWD) of water-stable aggregates and carbohydrates were determined for treated soils. Three strains were shown to impact soil aggregate properties at the higher moisture content: one affiliated with Bacillus niacini, one affiliated with Paenarthrobacter nitroguajacolicus and one of unclear classification. The first of these strains also affected soil structure at the lower moisture level. This B. niacini strain also increased the carbohydrate content of the soil, as did two other strains, related to B. wiedmannii and B. aryabhattai, respectively. However, no positive correlation was observed between the MWD and the production of carbohydrates in soil. Our results suggest that soil inoculation with specific microbial strains can improve soil structure.

Dead tree trunks have significant ecosystem functions related to biodiversity and biogeochemical cycles. When lying on the soil surface, they are colonized by an array of invertebrate fauna, but what determines their community composition is still unclear. We apply community assembly theory to colonization of tree logs by invertebrates. During early decomposition, the attached bark is critically important as an environment filter for community assembly through habitat provision. Specifically, we hypothesized that the more dissimilar bark traits were between tree species, the more their faunal community compositions would differ. We tested this hypothesis by investigating the effects of bark traits on the invertebrate communities in the early-decomposing logs of 11 common, temperate tree species placed in the ‘common garden’ experiment LOGLIFE. Bark traits included bark looseness, fissure index, outer bark thickness, ratio of inner to outer bark thickness, punch resistance, water storage capacity and bark pH. The predominant faunal groups studied were Annelida, Isopoda, Chilopoda, Diplopoda, Diptera and Coleoptera. Our results showed (i) strong interspecific differences in bark traits, (ii) that bark traits related to environmental buffering had profound effects on the abundance of specific invertebrate groups, and (iii) the higher the overall bark trait dissimilarity between tree species, the more dissimilar these tree species were in faunal community composition, and the higher was the joint invertebrate family richness. A suite of bark traits together has fundamental afterlife effects on invertebrate community assembly, strongly filtering the colonizing invertebrates in early-decomposing logs, driving variation in their community composition and diversity. Our findings indicate that bark trait dissimilarity among tree species in forest stands is likely a better indicator of early-phase dead trunk fauna diversity than tree species diversity per se. A lay summary is available for this article.

Worldwide, forests have absorbed around 30% of global anthropogenic emissions of carbon dioxide (CO2) annually, thereby acting as important carbon (C) sinks. It is proposed that leaving large fragments of dead wood, coarse woody debris (CWD), in forest ecosystems may contribute to the forest C sink strength. CWD may take years to centuries to degrade completely, and non-respired C from CWD may enter the forest soil directly or in the form of dissolved organic C. Although aboveground decomposition of CWD has been studied frequently, little is known about the relative size, composition and fate of different C fluxes from CWD to soils under various substrate-specific and environmental conditions. Thus, the exact contribution of C from CWD to C sequestration within forest soils is poorly understood and quantified, although understanding CWD degradation and stabilization processes is essential for effective forest C sink management. This review aims at providing insight into these processes on the interface of forest ecology and soil science, and identifies knowledge gaps that are critical to our understanding of the effects of CWD on the forest soil C sink. It may be seen as a “call-to-action” crossing disciplinary boundaries, which proposes the use of compound-specific analytical studies and manipulation studies to elucidate C fluxes from CWD. Carbon fluxes from decaying CWD can vary considerably due to interspecific and intraspecific differences in composition and different environmental conditions. These variations in C fluxes need to be studied in detail and related to recent advances in soil C sequestration research. Outcomes of this review show that the presence of CWD may enhance the abundance and diversity of the microbial community and constitute additional fluxes of C into the mineral soil by augmented leaching of dissolved organic carbon (DOC). Leached DOC and residues from organic matter (OM) from later decay stages have been shown to be relatively enriched in complex and microbial-derived compounds, which may also be true for CWD-derived OM. Emerging knowledge on soil C stabilization indicates that such complex compounds may be sorbed preferentially to the mineral soil. Moreover, increased abundance and diversity of decomposer organisms may increase the amount of substrate C being diverted into microbial biomass, which may contribute to stable C pools in the forest soil.

Facilitation between species is thought to be a key mechanism in community assembly and diversity, as certain species create microhabitats for others. A profound characteristic of forest ecosystems is a large amount of dead wood which is colofiised by a vast array of invertebrate species. Bark beetles (Scolytinae) feed and breed on dying or dead trees, puncturing holes into the bark and engraving inner bark and outer wood with their galleries. These holes and galleries might facilitate other invertebrates by providing access to the inner bark for shelter, feeding and reproduction. We tested this hypothesised facilitative interaction during the early decomposition phase of coarse woody debris of spruce (Picea abies L Karst.) incubated in two environmentally contrasting forest sites in the 'common garden' experiment LOGLIFE. We sampled invertebrates in 25 cm diameter logs at different degrees of colonisation by bark beetles. Our results indicated that (1) bark beetles facilitated the entrance into spruce logs of other invertebrates with body width that matched the size of bark-beetle holes, and (2) the abundance of invertebrates was often positively related to the proportional surface area of inner bark consumed by bark beetles, but more so in the nutrient-rich site than in the nutrient poor site. This study provides the first quantitative test of facilitation between invertebrate clades in dead wood communities. Including facilitative interaction in community assembly studies may change some predictions about relationships between tree functional traits and invertebrate diversity and will lead to a more accurate and comprehensive understanding of dead wood communities. (C) 2016 Elsevier Ltd. All rights reserved.

Riparian zones along streams and other small wetlands perform important ecosystem services in agricultural landscapes. They improve water quality by retaining nutrients and sediment from through-flowing water, sequester carbon and attenuate flood peaks at high discharge. At the same time, they have been shown to enhance biodiversity at the landscape as well as regional scale. However, there is concern about loss of biodiversity and enhanced greenhouse gas emissions where wetlands have become overloaded with nutrient-rich surface water or groundwater. This paper discusses conditions for the best performance of riparian zones and other small wetlands in agricultural landscapes in terms of nutrient and greenhouse gas retention and biodiversity. High nitrous oxide emissions have been identified in riparian zones used for N removal and could be a concern which would diminish their overall environmental benefit. However, it has been shown in riparian zones in the Rhine and Scheldt catchment that nitrous oxide emissions mainly occur when soil pH values are lower than 5. Such conditions occur mostly in stream headwater areas only where sandy sediments have a low buffering capacity, whereas higher-order streams and their riparian zones often have fine silty clay or loam sediments with higher pH values, supporting effective denitrification with low nitrous oxide emission. The question whether the nutrient attenuation service comes at the expense of biodiversity losses should be answered differently for existing, (semi-)natural wetlands and for new wetlands created in the agricultural landscape. Existing wetlands are quite vulnerable for nutrient loading, depending on their natural nutrient richness. Oligotrophic and mesotrophic wetlands such as bogs and fens are negatively affected by nutrient loadings > 4 g N m −2 y −1 and 0.5 g P m −2 y −1 , which lead to loss of diversity and characteristic species. Wetlands in riparian zones are naturally more nutrient-rich and can tolerate much higher nutrient loadings without consequences for ecosystem health and diversity, their loading limits are 50 g N m −2 y −1 and 4 g P m −2 y −1 . For nitrogen the relatively high limit is explained by significant denitrification activities in wetlands, transforming nitrate to mostly inert elemental nitrogen gas. There are also indications that only one of the major nutrients needs to be in short supply (i.e. below the limits indicated) in order to avoid major eutrophication problems. In agricultural areas where large numbers of wetlands have been created for N removal and biodiversity enhancement, the experience of this ‘dual purpose’ are generally positive, particularly in landscapes where not much of the original wetland resource remains.

Microbes catalyze all major geochemical cycles on earth. However, the role of microbial traits and community composition in biogeochemical cycles is still poorly understood mainly due to the inability to assess the community members that are actually performing biogeochemical conversions in complex environmental samples. Here we applied a polyphasic approach to assess the role of microbial community composition in modulating methane emission from a riparian floodplain. We show that the dynamics and intensity of methane consumption in riparian wetlands coincide with relative abundance and activity of specific subgroups of methane-oxidizing bacteria (MOB), which can be considered as a minor component of the microbial community in this ecosystem. Microarray-based community composition analyses demonstrated linear relationships of MOB diversity parameters and in vitro methane consumption. Incubations using intact cores in combination with stable isotope labeling of lipids and proteins corroborated the correlative evidence from in vitro incubations demonstrating γ-proteobacterial MOB subgroups to be responsible for methane oxidation. The results obtained within the riparian flooding gradient collectively demonstrate that niche partitioning of MOB within a community comprised of a very limited amount of active species modulates methane consumption and emission from this wetland. The implications of the results obtained for biodiversity–ecosystem functioning are discussed with special reference to the role of spatial and temporal heterogeneity and functional redundancy. Keywords: biodiversity–ecosystem functioning; methane oxidation; stable isotope labeling; proteomics; wetlands

1.Changes in the balance between soil carbon storage and release can significantly amplify or attenuate global warming. Although a lot of progress has been made in determining potential drivers of carbon release through large-scale decomposition experiments, climate predictions are still hampered by data limitation at a global scale as a result of high effort and measurement costs of comparative litter decomposition studies. 2.We introduce an innovative, cost-effective, well-standardised method to gather data on decomposition rate and litter stabilisation using commercially available tea bags as standardised test kits. By using two tea types with contrasting decomposability, we can construct a decomposition curve using a single measurement in time. The acquired Tea Bag Index (TBI) consists of two parameters describing decomposition rate (k) and litter stabilisation factor (S). 3.The method was tested for its sensitivity and robustness in contrasting ecosystems and biomes, confirming that the TBI is sensitive enough to discriminate between these systems. Within an ecosystem, TBI is responsive to differences in abiotic circumstances such as soil temperature and moisture content. The collected k and S values are in accordance with expectations based on decomposition process literature. They are therefore interpretable within the current knowledge framework. 4.Tea Bag Index is a unique, multifunctional method requiring few resources and minimal prior knowledge. The standardisation and simplicity of the method make it possible to collect comparable, globally distributed data through crowdsourcing. TBI can further provide an excellent decomposition reference and has the potential to increase reliability of soil carbon flux estimates based on extrapolations of decomposition data.

For decades, the conversion of organic nitrogen to dinitrogen gas by heterotrophic bacteria, termed heterotrophic denitrification, was assumed to be the main pathway of nitrogen loss in natural ecosystems. Recently, however, autotrophic bacteria have been shown to oxidize ammonium in the absence of oxygen, yielding dinitrogen gas. This process, termed anammox, accounts for over 50% of nitrogen loss in marine ecosystems1–5. However, the significance of anammox in freshwater ecosystems has remained uncertain 6,7. Here, we use molecular and isotopic techniques to monitor anammox activity in sediments sampled fromlake riparian zones in North China. Laboratory incubations in the presence of ammonium or nitrate—at concentrations equivalent to no more than 10% of those detected in situ—yielded some of the highest potential anammox activities reported for natural environments to date. Potential rates of anammox peaked in sediments sampled from the interface between the land and the water, as did the abundance of annamox bacteria. Scaling our findings up to the entire lake system, we estimate that interfacial anammox hotspots account for the loss of 103 gNm-2 yr-1 from this lake region, and around one fifth of the nitrogen lost from the land–water interface.

This study assessed the risk of decomposition-driven soil subsidence in drained peat soils in the Netherlands, contrasting in peat origin and current land use. In a full factorial design, fen peat and bog peat were sampled from sites in use for nature conservation and for dairy farming, which contrast in history of drainage and fertilisation. In these four peat types, which frequently occur in the Netherlands, the microbial activity and respiration dynamics were studied in samples from superficial oxic peat layers by measuring Substrate Induced Respiration (SIR) and Substrate Induced Growth Response (SIGR). Total and active microbial biomass, microbial growth potential and potential exo-enzyme activities were determined in unamended samples and after nitrogen and/or glucose amendments. Remarkably, peat origin and land use did not affect basal respiration rates. In contrast, land use affected microbial biomass and potential growth rates as they were quadrupled in dairy meadows compared to nature reserves. This may be attributable to the pulses of organic and inorganic fertiliser that are being supplied in agricultural peatlands. Potential activities of oxidative exo-enzymes (phenol oxidase, POX, and phenol peroxidase, POD), in contrast, depended more on peat type, indicating a difference in peat substrate quality. Basal respiration rates and enzyme activities were not related. Phosphorus enrichment was identified as a potential driver of increased peat decomposition. The activity of the oxidative enzyme phenol oxidase and the concentration of phenolic compounds, which are considered to be the main regulators of peat decomposition according to the enzymic latch theory, were not related to respiration rates. It was concluded that decomposition theories like the enzymic latch theory (attributing a main role in the regulation of decomposition to phenolic compounds and phenol oxidase) were not supported by our research in the drained peat soils in the Netherlands.

Dead wood provides a huge terrestrial carbon stock and a habitat to wide-ranging organisms during its decay. Our brief review highlights that, in order to understand environmental change impacts on these functions, we need to quantify the contributions of different interacting biotic and abiotic drivers to wood decomposition. LOGLIFE is a new long-term 'common-garden' experiment to disentangle the effects of species' wood traits and siterelated environmental drivers on wood decomposition dynamics and its associated diversity of microbial and invertebrate communities. This experiment is firmly rooted in pioneering experiments under the directorship of Terry Callaghan at Abisko Research Station, Sweden. LOGLIFE features two contrasting forest sites in the Netherlands, each hosting a similar set of coarse logs and branches of 10 tree species. LOGLIFE welcomes other researchers to test further questions concerning coarse wood decay that will also help to optimise forest management in view of carbon sequestration and biodiversity conservation. Copyright © Royal Swedish Academy of Sciences 2012.

Denitrification has long been regarded as the only pathway for terrestrial nitrogen (N) loss to the atmosphere. Here we demonstrate that large-scale anaerobic ammonium oxidation (anammox), an overlooked N loss process alternative to denitrification which bypasses nitrous oxide (N2O), is ubiquitous in inland waters of China and contributes significantly to N loss. Anammox rates in aquatic systems show different levels (1.0–975.9 μmol N m−2 h−1, n = 256) with hotspots occurring at oxic-anoxic interfaces and harboring distinct biogeochemical and biogeographical features. Extrapolation of these results to the China-national level shows that anammox could contribute about 2.0 Tg N yr−1, which equals averagely 11.4% of the total N loss from China’s inland waters. Our results indicate that a significant amount of the nitrogen lost from inland waters bypasses denitrification, which is important for constructing more accurate climate models and may significantly reduce potential N2O emission risk at a large scale.

Quantification of harmful nitrous oxide (N(2)O) emissions from soils is essential for mitigation measures. An important N(2)O producing and reducing process in soils is denitrification, which shows deceased rates at low pH. No clear relationship between N(2)O emissions and soil pH has yet been established because also the relative contribution of N(2)O as the denitrification end product decreases with pH. Our aim was to show the net effect of soil pH on N(2)O production and emission. Therefore, experiments were designed to investigate the effects of pH on NO(3)(-) reduction, N(2)O production and reduction and N(2) production in incubations with pH values set between 4 and 7. Furthermore, field measurements of soil pH and N(2)O emissions were carried out. In incubations, NO(3)(-) reduction and N(2) production rates increased with pH and net N(2)O production rate was highest at pH 5. N(2)O reduction to N(2) was halted until NO(3)(-) was depleted at low pH values, resulting in a built up of N(2)O. As a consequence, N(2)O:N(2) production ratio decreased exponentially with pH. N(2)O reduction appeared therefore more important than N(2)O production in explaining net N(2)O production rates. In the field, a negative exponential relationship for soil pH against N(2)O emissions was observed. Soil pH could therefore be used as a predictive tool for average N(2)O emissions in the studied ecosystem. The occurrence of low pH spots may explain N(2)O emission hotspot occurrence. Future studies should focus on the mechanism behind small scale soil pH variability and the effect of manipulating the pH of soils.

Soil denitrification is a major source of nitrous oxide emission that causes ozone depletion and global warming. Low soil pH influences the relative amount of N₂O produced and consumed by denitrification. Furthermore, denitrification is strongly inhibited in pure cultures of denitrifying microorganisms below pH 5. Soils, however, have been shown to denitrify at pH values as low as pH 3. Here we used a continuous bioreactor to investigate the possibility of significant denitrification at low pH under controlled conditions with soil microorganisms and naturally available electron donors. Significant NO₃⁻ and N₂O reduction were observed for 3 months without the addition of any external electron donor. Batch incubations with the enriched biomass showed that low pH as well as low electron donor availability promoted the relative abundance of N₂O as denitrification end-product. Molecular analysis of the enriched biomass revealed that a Rhodanobacter-like bacterium dominated the community in 16S rRNA gene libraries as well as in FISH microscopy during the highest denitrification activity in the reactor. We conclude that denitrification at pH 4 with natural electron donors is possible and that a Rhodanobacter species may be one of the microorganisms involved in acidic denitrification in soils.

Chronically nitrate-loaded riparian buffer zones show high N(2)O emissions. Often, a large part of the N(2)O is emitted from small surface areas, resulting in high spatial variability in these buffer zones. These small surface areas with high N(2)O emissions (hotspots) need to be investigated to generate knowledge on the factors governing N(2)O emissions. In this study the N(2)O emission variability was investigated at different spatial scales. Therefore N(2)O emissions from three 32 m(2) grids were determined in summer and winter. Spatial variation and total emission were determined on three different scales (0.3 m(2), 0.018 m(2) and 0.0013 m(2)) at plots with different levels of N(2)O emissions. Spatial variation was high at all scales determined and highest at the smallest scale. To test possible factors inducing small scale hotspots, soil samples were collected for slurry incubation to determine responses to increased electron donor/acceptor availability. Acetate addition did increase N(2)O production, but nitrate addition failed to increase total denitrification or net N(2)O production. N(2)O production was similar in all soil slurries, independent of their origin from high or low emission soils, indicating that environmental conditions (including physical factors like gas diffusion) rather than microbial community composition governed N(2)O emission rates.

Spatial variability in hydrological flowpaths and nitrate-removal processes complicates the overall assessment of riparian buffer zone functioning in terms of water quality improvement as well as enhancement of the greenhouse effect by N2O emissions. In this study, we evaluated denitrification and nitrous oxide emission in winter and summer along two groundwater flowpaths in a nitrate-loaded forested riparian buffer zone and related the variability in these processes to controlling soil factors. Denitrification and emissions of N2O were measured using flux chambers and incubation experiments. In winter, N2O emissions were significantly higher (12.4 mg N m−2 d−1) along the flowpath with high nitrate removal compared with the flowpath with low nitrate removal (2.58 mg N m−2 d−1). In summer a reverse pattern was observed, with higher N2O emissions (13.6 mg N m−2 d−1) from the flowpath with low nitrate-removal efficiencies. Distinct spatial patterns of denitrification and N2O emission were observed along the high nitrate-removal transect compared to no clear pattern along the low nitrate-removal transect, where denitrification activity was very low. Results from this study indicate that spots with high nitrate-removal efficiency also contribute significantly to an increased N2O emission from riparian zones. Furthermore, we conclude that high variability in N2O:N2 ratio and weak relationships with environmental conditions limit the value of this ratio as a proxy to evaluate the environmental consequences of riparian buffer zones.

Knowledge of the spatial distribution of wetlands within landscapes is needed for a number of different purposes as non-point pollution control, aquatic life, flood protection or sediment traps or sources. The purpose of this paper was to propose and to test a parsimonious method for predicting wetlands in small catchments across a wide range of geological, topographic and climatic conditions. A climato-topographic index has been derived from the topographic index—taking into account the local slope and the upslope drainage area, first proposed by Beven and Kirkby [Hydrol. Sci. Bull. 24 (1979) 43]. This new index takes into account the effective rainfall, in addition to topographic factors, in order to achieve a prediction of wetland occurrence for places with different rainy conditions. The two indexes were compared. Six study areas were selected representing a wide range of climate, geology and geomorphology across Europe. The wetland area ranges from 1 to 30% of the total area for the different catchments, and the annual effective rainfall from 67 to 592 mm. The climato-topographic index seems able to predict the structure and general areal extent of wetlands without any local calibration of the model, except where the permeability of the soil surface is important and very different from the other sites considered. The climato-topographic index gives a classification of wetland occurrence similar to the field data. The exact location of the wetland is often poorly predicted, but this is mostly due to the current poor quality of the Digital Terrain Model. Thus, there is currently a technical limitation to the use of the approach advocated here.

Ammonium/ammonia is the sole energy substrate of ammonia oxidizers, and is also an essential nitrogen source for other microorganisms. Ammonia oxidizers therefore must compete with other soil microorganisms such as methane-oxidizing bacteria (MOB) in terrestrial ecosystems when ammonium concentrations are limiting. Here we report on the interactions between nitrifying communities dominated by ammonia-oxidizing archaea (AOA) and Nitrospira-like nitrite-oxidizing bacteria (NOB), and communities of MOB in controlled microcosm experiments with two levels of ammonium and methane availability. We observed strong stimulatory effects of elevated ammonium concentration on the processes of nitrification and methane oxidation as well as on the abundances of autotrophically growing nitrifiers. However, the key players in nitrification and methane oxidation, identified by stable-isotope labeling using (13)CO2 and (13)CH4, were the same under both ammonium levels, namely type 1.1a AOA, sublineage I and II Nitrospira-like NOB and Methylomicrobium-/Methylosarcina-like MOB, respectively. Ammonia-oxidizing bacteria were nearly absent, and ammonia oxidation could almost exclusively be attributed to AOA. Interestingly, although AOA functional gene abundance increased 10-fold during incubation, there was very limited evidence of autotrophic growth, suggesting a partly mixotrophic lifestyle. Furthermore, autotrophic growth of AOA and NOB was inhibited by active MOB at both ammonium levels. Our results suggest the existence of a previously overlooked competition for nitrogen between nitrifiers and methane oxidizers in soil, thus linking two of the most important biogeochemical cycles in nature.

In recent decades, notably in large parts of the Western World, many waterfowl populations have increased because of increased food availability. Not uncommonly, eutrophication in these areas may occur perhaps not only because of increased dropping loads but also because nutrients are more readily released from droppings than from fresh and decomposing plant material. We conducted an experiment to study this effect comparing the decay rate and nutrient release of fresh grass leaves with that of droppings in water at 10, 20 and 30 degrees C. Using a two-component exponential decay model, allowing distinguishing between an easily decomposable (labile) and recalcitrant fraction of the material, we found that the labile carbon (C) fraction of droppings decomposed faster than that of grass leaves, whereas the recalcitrant C decomposition rate was similar to that of grass leaves. Higher temperature increased C decomposition rates of the labile fractions. No temperature dependence was observed for the recalcitrant fractions. Surprisingly, grass leaves and droppings did not differ in the amounts of nitrogen (N) and phosphorus (P) released in the first 90 days, i.e. more than 60% of N and 40% of P were released into the water column. Continuous mineralization of N and P predominated during decomposition of grass leaves, whereas a massive initial leaching was followed by immobilization and later mineralization during decomposition of droppings. All differences during decomposition of the litter types were attributed to the stoichiometric requirement of decomposers and their temperature sensitivity. Copyright (C) 2013 John Wiley & Sons, Ltd.

The effects of temperature on N mineralization were studied in two organic surface horizons (LF and H) of soil from a boreal forest. The soil was incubated at 5 °C and 15 °C after adding 15 N and gross N fluxes were calculated using a numerical simulation model. The model was calibrated on microbial C and N, basal respiration, and KCl-extractable NH4+, NO3−, 15NH4+ and 15 NO3−. In the LF layer, increased temperature resulted in a faster turnover of all N pools. In both layers net N mineralization did not increase at elevated temperature because both gross NH4+ mineralization and NH4+ immobilization increased. In the H layer, however, both gross NH4+ mineralization and NH4+ immobilization were lower at 15 °C than at 5 °C and the model predicted a decrease in microbial turnover rate at higher temperature although measured microbial activity was higher. The decrease in gross N fluxes in spite of increased microbial activity in the H layer at elevated temperature may have been caused by uptake of organic N. The model predicted a decrease in pool size of labile organic matter and microbial biomass at elevated temperature whereas the amount of refractory organic matter increased. Temperature averaged microbial C/N ratio was 14.7 in the LF layer suggesting a fungi-dominated decomposer community whereas it was 7.3 in the H layer, probably due to predominance of bacteria. Respiration and microbial C were difficult to fit using the model if the microbial C/N ratio was kept constant with time. A separate 15N-enrichment study with the addition of glucose showed that glucose was metabolized faster in the LF than in the H layer. In both layers, decomposition of organic matter appeared to be limited by C availability.

Many surface waters in The Netherlands are polluted with nutrients from agricultural land. Riparian buffer zones have been shown to be very valuable in reducing non-point source pollution from agricultural land to streams. The nitrogen removal in two riparian zones vegetated with alder thicket and grass respectively and the contribution of denitrification activity in nitrate removal were investigated. In the period from September 1996 to June 1997, nitrogen transport and denitrification activity in the headwaters of a small stream, the Hazelbeek in Twente, was measured at 11 dates. At all sampling dates we found clear spatial differences in groundwater quality. High nitrate concentrations (>40 mg N l−1) were measured in groundwater tubes placed at the boundary of the agricultural land (maize) and the riparian forest, and lower nitrate concentrations were measured close to the stream (0.1–2 mg N l−1). Stream water quality measurements indicated that (sub) surface flow from the maize field strongly contributed to the nitrogen load in the stream. Results suggest that nitrate concentrations in groundwater decreased by 95% when it flowed through the riparian buffer zone. Denitrification rates measured in the top soil (0–30 cm) of the buffer zones varied between 9 and 200 kg N ha−1 year−1 in the forested buffer zone and between 1.2 and 32 kg N ha−1 year−1 in the grassland buffer zone. The higher denitrification rates measured in the forest zone compared to the grassland zone could be due to the higher availability of nitrate in the forested soil compared to the grassland zone and/or a higher residence time of the water in the forested zone.

To obtain knowledge about heavy metal kinetics in earthworms (Dendrobaena veneta), we performed accumulation and excretion experiments under laboratory conditions using soils from a heavy metal (Cu, Pb, and Zn)-contaminated site. To determine heavy metal accumulation rate, earthworms were exposed for 1, 2, 3, 7, 14, 28, 56, or 112 d to soil MH (Cu : 815 mg kg -1 , Pb : 340 mg kg -1 , and Zn : 225 mg kg -1 ). After exposure for 28 d to soil LB (Cu 242 mg k -1 , Pb : 109 mg kg -1 , Zn 72 mg kg -1 ) or for 112 d to soil MH, D. veneta were transferred to uncontaminated soil and sampled after 1, 2, 3, 7, 14, 28, or 56 d to determine heavy metal excretion rates. Fourteen days after the transfer to the uncontaminated soil, some earthworms were transferred back to soil LB to determine the accumulation after a short recovery period. The Cu accumulation until Day 56 could be described by a one-compartment model. At Day 112, however, we observed an unexpected further increase in tissue Cu concentration (C Cu ). We observed a significant increase in C Zn only at Day 112. Excretion of Cu could be described excellently by a two compartment model, the half-life times being t 1/2 ;1 0.36 d, and t 1/2 ;2 37 d. Excretion of Pb could be described with a one-compartment model. After a fast initial excretion (t 1/2 0.70 d), Pb excretion stagnated. The Cu accumulation after a 14-d recovery period appeared to be significantly less than the initial accumulation. The Pb accumulation appeared to be slower after recovery than before.

Background and Aims: Alterations in snow cover driven by climate change may impact ecosystem functioning, including biogeochemistry and soil (microbial) processes. We elucidated the effects of snow cover manipulation (SCM) on above-and belowground processes in a temperate peatland. Methods: In a Swiss mountain-peatland we manipulated snow cover (addition, removal and control), and assessed the effects on Andromeda polifolia root enzyme activity, soil microbial community structure, and leaf tissue and soil biogeochemistry. Results: Reduced snow cover produced warmer soils in our experiment while increased snow cover kept soil temperatures close-to-freezing. SCM had a major influence on the microbial community, and prolonged ‘close-to-freezing' temperatures caused a shift in microbial communities toward fungal dominance. Soil temperature largely explained soil microbial structure, while other descriptors such as root enzyme activity and pore-water chemistry interacted less with the soil microbial communities. Conclusions: We envisage that SCM-driven changes in the microbial community composition could lead to substantial changes in trophic fluxes and associated ecosystem processes. Hence, we need to improve our understanding on the impact of frost and freeze-thaw cycles on the microbial food web and its implications for peatland ecosystem processes in a changing climate; in particular for the fate of the sequestered carbon

Rewetting of agriculturally used peatlands has been proposed as a measure to stop soil subsidence, conserve peat and rehabilitate ecosystem functioning. Unintended consequences might involve nutrient release and changes in the greenhouse gas (GHG) balance towards CH4-dominated emission. To investigate the risks and benefits of rewetting, we subjected soil columns from drained peat- and clay-covered peatlands to different water level treatments: permanently low, permanently inundated and fluctuating (first inundated, then drained). Surface water and soil pore water chemistry, soil-extractable nutrients and greenhouse gas fluxes were measured throughout the experiment. Permanent inundation released large amounts of nutrients into pore water, especially phosphorus (up to 11.7 mg P-PO4 l−1) and ammonium (4.8 mg N-NH4 l−1). Phosphorus release was larger in peat than in clay soil, presumably due to the larger pool of iron-bound phosphorus in peat. Furthermore, substantial amounts of phosphorus and potassium were exported from the soil matrix to the surface water, risking the pollution of local species-rich (semi-)aquatic ecosystems. Rewetting of both clay and peat soil reduced CO2 emissions. CH4 emissions increased, but, in contrast to the expectations, the fluxes were relatively low. Calculations showed that rewetting reduced net cumulative GHG emissions expressed as CO2 equivalents.

Microbial communities display a variety of biogeographical patterns mainly driven by large-scale environmental gradients. Here, we analysed the spatial distribution of methane-oxidizing bacteria (MOB) and methane oxidation in a strongly fluctuating environment. We investigated whether the spatial variability of the MOB community can be explained by an environmental gradient and whether this changes with different plot sizes. We applied a pmoA-specific microarray to detect MOB, measured methane oxidation, methane emissions and soil properties. All variables were measured in a 10 × 10 m, 1 × 1 m and 20 × 20 cm plot and interpreted using a geostatistical approach. Methane oxidation as well as MOB displayed spatial patterns reflected in the underlying flooding gradient. Overlapping and contrasting spatial patterns for type I and type II MOB suggested different ecological life strategies. With smaller plot size, the environmental gradient could not explain the variability in the data and local factors became more important. In conclusion, environmental gradients can generally explain variability in microbial spatial patterns; however, we think that this does not contribute to a mechanistic explanation for microbial diversity because the relevant scales for microorganisms are much smaller than those normally measured.

International audience; Emissions and deposition of ammonia and nitrogen oxides have strongly increased since the 1950s. This has led to significant changes in the nitrogen (N) cycle, vegetation composition and plant diversity in many ecosystems of high conservation value in Europe. As a consequence of different regional pollution levels and of the increased importance of reduced N in the near future, determining the effect of different forms of N is an important task for understanding the consequences of atmospheric N inputs. We have initiated three replicated N addition experiments in species-rich, acidic grasslands spanning a climatic gradient in the Atlantic biogeographic region of Europe in Norway, Wales and France at sites with low levels of pollution. N was added in two doses (0 and 70 kg N ha−1 year−1 above background) and in three forms (oxidised N, reduced N and a 50–50 combination). After 2.5 years of N additions, the effects of these treatments on plant biomass, plant nutritional status, soil pH and soil nutrient availability were determined. Impacts of the N additions were observed within the 2.5-year research period. In some cases, the first signs of differential effects of N form could also be demonstrated. In the French site, for example, grass biomass was significantly increased by the oxidised N treatments but decreased by the reduced N treatments. In the Norwegian site, the reduced N treatments significantly reduced soil pH, whereas oxidised N did not. Effects on nutrient availability were also observed. These experiments will be continued to elucidate the longer term impacts of N deposition on these grasslands.

Mangrove forests are sites of intense carbon and nutrient cycling, which result in soil carbon sequestration on a global scale. Currently, mangrove forests receive increasing quantities of exogenous nutrients due to coastal development. The present paper quantifies the effects of nutrient loading on microbial growth rates and the mineralisation of soil organic carbon (SOC) in two mangrove soils contrasting in carbon content. An increase in SOC mineralisation rates would lead to the loss of historically sequestered carbon and an enhanced CO2 release from these mangrove soils. In an incubation experiment we enriched soils from Avicennia and Rhizophora mangrove forests bordering the Red Sea with different combinations of nitrogen, phosphorus and glucose to mimic the effects of wastewater influx. We measured microbial growth rates as well as carbon mineralisation rates in the natural situation and after enrichment. The results show that microbial growth is energy limited in both soils, with nitrogen as a secondary limitation. Nitrogen amendment increased the rate at which labile organic carbon was decomposed, while it decreased SOC mineralisation rates. Such an inhibitory effect on SOC mineralisation was not found for phosphorus enrichment. Our data confirm the negative effect of nitrogen enrichment on the mineralisation of recalcitrant carbon compounds found in other systems. Based on our results it is not to be expected that nutrient enrichment by itself will cause degradation of historically sequestered soil organic carbon in nitrogen limited mangrove forests.

In river basins, soils, groundwater, riparian zones and floodplains, streams, rivers, lakes and reservoirs act as successive filters in which the hydrology, ecology and biogeochemical processing are strongly coupled and together act to retain a significant fraction of the nutrients transported. This paper compares existing river ecology concepts with current approaches to describe river biogeochemistry, and assesses the value of these concepts and approaches for understanding the impacts of interacting global change disturbances on river biogeochemistry. Through merging perspectives, concepts, and modeling techniques, we propose integrated model approaches that encompass both aquatic and terrestrial components in heterogeneous landscapes. In this model framework, existing ecological and biogeochemical concepts are extended with a balanced approach for assessing nutrient and sediment delivery, on the one hand, and nutrient in-stream retention on the other hand.

Measurements along a hill-slope vegetation gradient in nutrient-poor grasslands from acid grasslands via matgrass swards to calcareous grasslands showed increased ammonium to nitrate ratios in the matgrass swards. These results generated the research question whether there might be a difference in nitrification activity or nitrifying community composition between the different zones in this hill-slope gradient. In each of the vegetation types along the gradient, soil samples were taken in five grassland nature reserves. Potential nitrification rates have been determined as an indication of the size of the active ammonia-oxidising microbial communities. Additionally, the dominant ammonia-oxidising sequences related to the β-Proteobacteria have been determined by a Polymerase Chain Reaction (PCR) based on the 16S rRNA gene in combination with Denaturing Gradient Gel Electrophoresis (DGGE) at one of the nature reserves. Compared to the top and lower zones of the vegetation gradient (i.e. acid grasslands and calcareous grasslands, respectively), potential nitrification rates were clearly repressed in the middle, matgrass swards zone. In contrast to the differences in potential nitrification activities observed in one of the nature reserves (Bemelerberg), no differences in dominant ammonia-oxidising sequences were observed at this location. One sequence belonging to cluster 3 of the Nitrosospira lineage appeared to be dominant among the sequences belonging to the ammonia-oxidising species of the β-Proteobacteria in all vegetation zones. Nitrification was apparently inhibited by the vegetation, whereas no shift in nitrifier populations could be shown. The possible role of repressed nitrification in the decline of this vegetation type is discussed.

Questions: Which nutrient(s) limit(s) vegetation productivity in Calthion grasslands? Is phosphorus release a bottleneck for restoration of species-rich Calthion grasslands on rewetted dairy meadows? Location: Three species-rich Calthion grasslands in the Western Peat District in the Netherlands. Methods: We conducted a field fertilization experiment with nitrogen (N), phosphorus (P) and potassium (K) in three existing Calthion grasslands to evaluate the potential for restoration on rewetted dairy meadows. Responses of above-ground biomass, tissue nutrient concentrations and nutrient ratios were determined after 2 yr of fertilization. Results: Biomass increased with fertilization with Nonly and K-only but did not react to P-only additions. Comparisons of tissue nutrient concentrations and nutrient ratios also gave indications of N and K limitation. Conclusions: The strong P release expected after rewetting should not necessarily interfere with restoration of Calthion communities on rewetted dairy meadows. It is concluded that for successful restoration management measures should focus on reducing N and/or K availability. Potassium might be an overlooked bottleneck in the restoration of species-rich grasslands.

1. Denitrification in floodplain soils is one of the main biological processes emitting and reducing nitrous oxide, a greenhouse gas, and the main process responsible for the buffering capacity of riparian zones against diffuse nitrate pollution. 2. The aim of this study was to measure denitrification rates under a wide range of current climatic conditions and hydrological regimes in Europe (from latitude 64°N to latitude 42°N and from longitude 2°W to longitude 25°E), in order to determine the response patterns of this microbial process under different climatic and hydrological conditions, and to identify denitrification proxies robust enough to be used at the European scale. 3. Denitrification activity was significant in all the floodplain soils studied whatever the latitude. However, we found an increase in rates of an order of magnitude from high to mid latitudes. Maximum rates (above 30 g N m?2 month?1) were measured in the maritime conditions of the Trent floodplain. These rates are similar to mineralisation rates measured in alluvial soils and of the same order of magnitude as the amount of N stored in herbaceous plants in alluvial soils. 4. We used Multivariate Adaptative Regression Splines to relate the response variable denitrification with five relevant predictors, namely soil moisture, temperature, silt plus clay, nitrate content and herbaceous plant biomass. 5. Soil moisture, temperature, and nitrate were the three main control variables of microbial denitrification in alluvial soils in decreasing order of importance. 6. The model developed for denitrification with interaction effects outperformed a pure additive model. Soil moisture was involved in all interactions, emphasising its importance in predicting denitrification. 7. These results are discussed in the context of scenarios for future change in European hydrological regimes.

18 páginas, 6 tablas, 9 figuras., Plant uptake and denitrification are considered to be the most important processes responsible for N retention and mitigation in riparian buffers. In many riparian buffers, however, nutrients taken up by plants remain in the system only temporarily and may be gradually released by mineralization later. Still, plants increase the residence time of nutrients considerably by reducing their mobility. We investigated the importance of plant N uptake and N immobilization in litter for N retention in riparian buffers. Nitrogen uptake in vegetation and N dynamics in litter were measured over a two-year period in a range of forested and herbaceous riparian buffers along a climatic gradient in Europe, receiving different loadings of N-enriched groundwater. Plant production, nitrogen uptake, and N retention were significantly higher in the forested buffer sites compared to the herbaceous buffer sites. However, in herbaceous buffers, periodic harvesting of herbaceous biomass contributed considerably to the N retention. No relationship between lateral N loading and plant productivity or N uptake was observed; this indicated that plant growth was not N-limited. In the winter period, decaying leaf litter had a small but significant role in N retention in a majority of the riparian ecosystems studied. Moreover, no responses to the climatic gradient were found. Generally, we can state that annual N retention in the vegetation and litter compartment is substantial, making up 13–99% of the total N mitigation., This study arose from the European program: Nitrogen Control by Landscape Structures in Agricultural Environments, NICOLAS, which was supported by an European grant from the Environment and Climate Program no. ENV4-CT-97-0395.

Water quality in many stream catchments and river basins is severely impacted by nutrient enrichment as a result of agriculture. Water-resource managers worldwide are considering the potential role of riparian zones and floodplain wetlands in improving stream-water quality, as there is evidence at the site scale that such wetlands are efficient at removing nutrients from through-flowing water. However, recent studies have highlighted disadvantages of such use of wetlands, including emissions of greenhouse gases and losses of biodiversity that result from prolonged nutrient loading. Here, we discuss the water purification function of wetlands at the site and catchment scale and suggest ways in which these disadvantages could be overcome.

International audience; Riparian zones have long been considered as nitrate sinks in landscapes. Yet, riparian zones are also known to be very productive ecosystems with a high rate of nitrogen cycling. A key factor regulating processes in the N cycle in these zones is groundwater table fluctuation, which controls aerobic/anaerobic conditions in the soil. Nitrification and denitrification, key processes regulating plant productivity and nitrogen buffering capacities are strictly aerobic and anaerobic processes, respectively. In this study we compared the effects of these factors on the nitrogen cycling in riparian zones under different climatic conditions and N loading at the European scale. No significant differences in nitrification and denitrification rates were found either between climatic regions or between vegetation types. On the other hand, water table elevation turned out to be the prime determinant of the N dynamics and its end product. Three consistent water table thresholds were identified. In sites where the water table level is within -10 cm of the soil surface, ammonification is the main process and ammonium accumulates in the topsoils. Average water tables between -10 and -30 cm favour denitrification and therefore reduce the nitrogen availability in soils. In drier sites, that is, water table level below -30 cm, nitrate accumulates as a result of high net nitrification. At these latter sites, denitrification only occurs in fine textured soils probably triggered by rainfall events. Such a threshold could be used to provide a proxy to translate the consequences of stream flow regime change to nitrogen cycling in riparian zones and consequently, to potential changes in nitrogen mitigation.

Soil nitrification is a key process in regulating the relative availability of the various inorganic N forms to plants. In the current study, we investigated the effect of different plant species on numbers of ammoniaoxidizing microbial cells by measuring the potential nitrification activity (PAA). Soil from matgrass sward and from calcareous grassland was collected in the field and four characteristic plant species of each vegetation type were cultivated from seed. These plant species grew for 4 months in the two soil types. After those 4 months, PAA were significantly higher in calcareous soil compared to the matgrass sward soil and the presence of matgrass sward species had significantly decreased PAA in this soil. In soils from matgrass stands, PAA were much lower, and no effect of the different plant species could be detected. Plant biomass of the calcareous grassland species was overall positively correlated with PAA, whereas for matgrass plant species a negative trend was found. We conclude that matgrass sward plant species had a clear repressing effect on the potential ammonia-oxidizing activity in calcareous grassland soil within this 4-month growth experiment. The observed repression of PAA is in accordance with earlier field observations of PAA in the different vegetation zones, where repressed PAA and significant higher ammonium to nitrate ratios were observed in the matgrass sward vegetation compared to the other vegetation zones. Major findings of this study indicate that plant species can influence the nitrification potential of their habitats, i.e. certain species can repress nitrification potential and others can increase nitrification potential of their habitats.

International audience; We evaluated nitrogen (N) removal efficiency by ri-parian buffers at 14 sites scattered throughout seven European countries subject to a wide range of climatic conditions. The sites also had a wide range of nitrate inputs, soil characteristics, and vegetation types. Dissolved forms of N in groundwater and associated hydrological parameters were measured at all sites; these data were used to calculate nitrate removal by the riparian buffers. Nitrate removal rates (expressed as the difference between the input and output nitrate concentration in relation to the width of the riparian zone) were mainly positive, ranging from 5% m 1 to 30% m 1 , except for a few sites where the values were close to zero. Average N removal rates were similar for herbaceous (4.43% m 1) and forested (4.21% m 1) sites. Nitrogen removal efficiency was not affected by climatic variation between sites, and no significant seasonal pattern was detected. When nitrate inputs were low, a very large range of nitrate removal efficiencies was found both in the forested and in the nonforested sites. However, sites receiving nitrate inputs above 5 mg N L 1 showed an exponential negative decay of nitrate removal efficiency (nitrate removal efficiency 33.6 e 0.11 NO 3 input , r 2 0.33, P 0.001). Hydraulic gradient was also negatively related to nitrate removal (r 0.27, P 0.05) at these sites. On the basis of this intersite comparison, we conclude that the removal of nitrate by biological mechanisms (for example, denitrification, plant uptake) in the riparian areas is related more closely to nitrate load and hydraulic gradient than to climatic parameters.