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Visible and near‐infrared (vis–NIR) spectroscopy is a promising technology for the analysis of different soil quality parameters. In this study, we used in‐situ vis–NIR spectroscopy in association with partial least squares regression to predict the total and the mineral (nitrate + ammonium) nitrogen content, the permanganate oxidizable carbon (POXC), as well as the ratio of soil organic carbon‐to‐clay content in different agricultural soils in Switzerland. These parameters can indeed be used as indicators of soil quality in response to agronomic practices. To this goal, a total number of 134 soil samples were used for carbon‐, total nitrogen‐ and clay‐related parameters, whereas 69 soil samples were used for the mineral nitrogen‐related parameters. We found that the partial least squares regression model can successfully predict the total nitrogen and the POXC content as well as the ratio of soil organic carbon‐to‐clay content (ratio of performance to interquartile range, RPIQ > 2.62, R2 > 0.73, Lin's concordance correlation coefficient > 0.83). As concerns the mineral nitrogen, it was not possible to successfully predict this parameter by vis–NIR spectroscopy. By demonstrating the possibility to reliably predict POXC content and the soil organic carbon‐to‐clay ratio, we show that vis–NIR can be also used to analyse soil parameters associated with both the quality of organic carbon and the structural quality of agricultural soils. [ABSTRACT FROM AUTHOR]

In‐field soil spectroscopy represents a promising opportunity for fast soil analysis, allowing the prediction of several soil properties from one spectral reading representing one soil sample. This facilitates data acquisition from large amounts of samples through its rapidity and the absence of required chemical processing. This is of particular interest in agriculture, where the chance to retrieve information from soils directly in the field is very appealing. This review is focused on in‐field visible to near infrared (Vis–NIR) spectroscopy (350–2500 nm), aimed at analysing soils directly in the field through proximal sensing. The main scope was to explore the available knowledge to identify existing gaps limiting the reliability and robustness of in‐field measurement, to foster future research and help transition towards the practical application of this technology. For this purpose, a literature review was performed, and surveyed information encompassed sensor range, carrier platforms in use, sensor type, distance to the soil sample, measurement methodology, measured soil properties and soil management, among many others. From this, we derived a list of tools in use with their spectral measurement properties, including the potential cross‐calibration with soil spectral libraries from laboratory spectroscopy of soil samples and potential measured target soil properties. Different instruments and sensors used to measure at varying wavelength ranges and with different spectral qualities are available for a large range of prices. The most frequently analysed soil properties included soil carbon contents (soil organic carbon, soil organic matter, total carbon), texture (clay, silt, sand), total nitrogen, pH and cation exchange capacity. Future perspectives comprise the implementation of larger databases, including different instruments and cropping systems as well as methodologies combining existing knowledge regarding laboratory spectroscopy with in‐field methods. The authors highlight the need for a broadly accepted measurement protocol for in‐field soil spectroscopy, fostering harmonization and standardization and consequently a more robust application in practice. [ABSTRACT FROM AUTHOR]

The application of visible and near-infrared (vis–NIR) spectroscopy to characterize soil samples has gained growing interest as a fast and cost-effective methodology forsoil fertility assessment.In orderto profit from the full potential of vis–NIR spectroscopy, the acquisition of soil spectra directly in-situ would increase the possibility to obtain data rapidly and at a high spatial and temporal resolution. In the present study, we test and propose the best practice to characterize a set of fertility-related parameters (i.e. texture, organic carbon, pH, cation exchange capacity and major nutrients) of agricultural soils by measuring vis–NIR spectra in the field. To reach this goal, we compare the spectra obtained from different scanning positions with two portable spectrometers, that is, a micro-electromechanical systems (MEMS)-based spectrometer and a research-grade vis–NIR spectrometer. On the basis of 134 soil sampling points, vis–NIR spectra were recorded from: (1) the cutaway side of a soil sample collected with an Edelman auger to a depth of 20 cm, (2) the raw soil surface, as well as (3) the cleaned and smoothed soil surface. Partial least squares regression (PLSR) calibration models were built for the selected soil parameters, scanning positions and different spectral pretreatments for both spectrometers. The model performance was evaluated based on the ratio of performance to interquartile range (RPIQ), the R2 , the root mean squared error (RMSE) and Lin's concordance correlation coefficient (CCC). Overall, the following soil parameters were successfully predicted: clay, sand, pH, organic carbon, cation exchange capacity, total nitrogen and exchangeable magnesium. In contrast, total and exchangeable Ca, K and P, as well as total Mg could not be predicted at a satisfactory level for both the spectrometers. The best scanning position for the successfully calibrated models was along the cutaway sides of the Edelman auger. Although the research-grade spectrometer gave better performance indicators for most of the parameters, the calibrations with the MEMS-based spectrometer still resulted in satisfactory predictions. Based on these findings, the proposed best practice for obtaining in-situ soil vis–NIR scans is to scan along the cutaway sides of a soil core using at least five replicate scans. [ABSTRACT FROM AUTHOR]

In northern peatlands, reduction of Sphagnum dominance in favour of vascular vegetation is likely to influence biogeochemical processes. Such vegetation changes occur as the water table lowers and temperatures rise. To test which of these factors has a significant influence on peatland vegetation, we conducted a 3‐year manipulative field experiment in Linje mire (northern Poland). We manipulated the peatland water table level (wet, intermediate and dry; on average the depth of the water table was 17.4, 21.2 and 25.3 cm respectively), and we used open‐top chambers (OTCs) to create warmer conditions (on average increase of 1.2°C in OTC plots compared to control plots). Peat drying through water table lowering at this local scale had a larger effect than OTC warming treatment per see on Sphagnum mosses and vascular plants. In particular, ericoid shrubs increased with a lower water table level, while Sphagnum decreased. Microclimatic measurements at the plot scale indicated that both water‐level and temperature, represented by heating degree days (HDDs), can have significant effects on the vegetation. In a large‐scale complementary vegetation gradient survey replicated in three peatlands positioned along a transitional oceanic–continental and temperate–boreal (subarctic) gradient (France–Poland–Western Siberia), an increase in ericoid shrubs was marked by an increase in phenols in peat pore water, resulting from higher phenol concentrations in vascular plant biomass. Our results suggest a shift in functioning from a mineral‐N‐driven to a fungi‐mediated organic‐N nutrient acquisition with shrub encroachment. Both ericoid shrub encroachment and higher mean annual temperature in the three sites triggered greater vascular plant biomass and consequently the dominance of decomposers (especially fungi), which led to a feeding community dominated by nematodes. This contributed to lower enzymatic multifunctionality. Our findings illustrate mechanisms by which plants influence ecosystem responses to climate change, through their effect on microbial trophic interactions. [ABSTRACT FROM AUTHOR]

Background: Northern peatlands have accumulated vast amounts of carbon (C) as peat. Warming temperatures may affect peatland C stores by increasing microbial decomposition of ancient peat through enhanced input of labile root exudates by expansion of vascular plants, thereby accelerating atmospheric warming. Aims: We set out to explore how much freshly assimilated C is allocated belowground by vascular plants, and if the above‐ to belowground allocation is affected by temperature and plant functional types. Methods: We traced the C allocation pathways of two dominant plant functional types (i.e., sedges and shrubs) in two peatlands under different temperature regimes by combining selective plant removal in mixed sedge‐shrub vegetation and in situ 13C pulse‐labelling. Aboveground to belowground C allocation as well as the C turnover were assessed by quantifying 13C in plant leaves and soil respiration and by measuring δ13C in dissolved organic C. A depth‐resolved quantification of 13C in the peat soil gave additional insight into belowground C allocation patterns. Results: Temperature did not affect the rate at which 13C was assimilated into shoots, but higher temperature decreased the fraction of assimilated C that was allocated belowground by vascular plants. Sedges assimilated CO2 faster into their shoot biomass (faster depletion in 13C in shoots) and allocated more of the assimilated 13C belowground than shrubs. Conversely, sedges retained this belowground allocated C better than shrubs, leading to lower 13C in soil respiration measured under sedges. Conclusions: Climate induced vascular plant expansion will increase input of fresh assimilates into the peat substantially, even though part of this effect will be offset by reduced above‐ to belowground allocation rates. If shrub density increases relative to sedges, fresh assimilates are more likely to be respired than translocated to roots where they could reach and, potentially mobilize, ancient C stored in deeper peat layers. [ABSTRACT FROM AUTHOR]

Peatlands contain a significant fraction of global soil carbon, but how these reservoirs will respond to the changing climate is still relatively unknown. A global picture of the variations in peat organic matter chemistry will aid our ability to gauge peatland soil response to climate. The goal of this research is to test the hypotheses that (a) peat carbohydrate content, an indicator of soil organic matter reactivity, will increase with latitude and decrease with mean annual temperatures, (b) while peat aromatic content, an indicator of recalcitrance, will vary inversely, and (c) elevation will have a similar effect to latitude. We used Fourier Transform Infrared Spectroscopy to examine variations in the organic matter functional groups of 1034 peat samples collected from 10 to 20, 30–40, and 60–70 cm depths at 165 individual sites across a latitudinal gradient of 79°N–65°S and from elevations of 0–4,773 m. Carbohydrate contents of high latitude peat were significantly greater than peat originating near the equator, while aromatic content showed the opposite trend. For peat from similar latitudes but different elevations, the carbohydrate content was greater and aromatic content was lower at higher elevations. Higher carbohydrate content at higher latitudes indicates a greater potential for mineralization, whereas the chemical composition of low latitude peat is consistent with their apparent relative stability in the face of warmer temperatures. The combination of low carbohydrates and high aromatics at warmer locations near the equator suggests the mineralization of high latitude peat until reaching recalcitrance under a new temperature regime. Plain Language Summary: Peatlands are a large global soil carbon reservoir, containing a quantity of carbon that is equivalent to about half or more of the carbon dioxide in the atmosphere. What will be their fate on a warming planet? Across a latitudinal gradient from 79°N to 65°S, we measured the fraction of the peat made up of carbohydrates, which are easily decomposed, and of aromatics, which are less easily decomposed. We found that peat from high latitudes and high elevations had greater carbohydrate content, while aromatic content showed the opposite trend. Larger carbohydrate content in organic matter indicates greater decomposability, while greater aromatic content indicates lower decomposability. We suggest that this latitude/elevation difference that we observed predicts how high‐latitude and high‐elevation peats may change under warmer conditions. Our work indicates that while a large portion of the carbohydrate fraction in these peats could be lost upon warming, releasing greenhouse gases, a residual fraction will survive and become more aromatic‐rich, making the remainder more resistant to rapid decomposition. Key Points: Peatland soil organic matter stability can be evaluated through the relative abundances of carbohydrate and aromatic content in peatPeat at higher latitudes and elevations has larger carbohydrate and smaller aromatic content relative lower latitudes and elevationsOur results foreshadow a transition of organic matter from higher latitude peatlands to a more recalcitrant form in response to warming [ABSTRACT FROM AUTHOR]

The relative importance of global versus local environmental factors for growth and thus carbon uptake of the bryophyte genus Sphagnum—the main peat‐former and ecosystem engineer in northern peatlands—remains unclear.We measured length growth and net primary production (NPP) of two abundant Sphagnum species across 99 Holarctic peatlands. We tested the importance of previously proposed abiotic and biotic drivers for peatland carbon uptake (climate, N deposition, water table depth and vascular plant cover) on these two responses. Employing structural equation models (SEMs), we explored both indirect and direct effects of drivers on Sphagnum growth.Variation in growth was large, but similar within and between peatlands. Length growth showed a stronger response to predictors than NPP. Moreover, the smaller and denser Sphagnum fuscum growing on hummocks had weaker responses to climatic variation than the larger and looser Sphagnum magellanicum growing in the wetter conditions. Growth decreased with increasing vascular plant cover within a site. Between sites, precipitation and temperature increased growth for S. magellanicum. The SEMs indicate that indirect effects are important. For example, vascular plant cover increased with a deeper water table, increased nitrogen deposition, precipitation and temperature. These factors also influenced Sphagnum growth indirectly by affecting moss shoot density.Synthesis. Our results imply that in a warmer climate, S. magellanicum will increase length growth as long as precipitation is not reduced, while S. fuscum is more resistant to decreased precipitation, but also less able to take advantage of increased precipitation and temperature. Such species‐specific sensitivity to climate may affect competitive outcomes in a changing environment, and potentially the future carbon sink function of peatlands. [ABSTRACT FROM AUTHOR]

Groundwater level is crucial for wetland plant growth and reproduction, but the extent of its effect on plant growth can vary along with changed precipitation and temperature at different seasons. In this context, we investigated the effect of two groundwater levels (10 cm vs. 20 cm depth) on growth and reproductive parameters of Carex cinerascens, a dominant plant species in the Poyang Lake wetland, during three seasons (spring, summer, and autumn) and during two consecutive years (2015 and 2016). Carex cinerascens showed low stem number, height, and individual and population biomass in summer compared to spring and autumn. 10 cm groundwater level was overall more suitable for plant growth resulting in higher stem height and biomass. However, the interactive effect between groundwater level and season clearly demonstrated that the effect of groundwater level on plant growth occurred mainly in autumn. After the withering of the plant population in summer, we observed that C. cinerascens growth recovered in autumn to similar values observed in spring only with 10 cm groundwater level. Consequently, we could deduce that lowering groundwater level in the studied Poyang Lake wetland will negatively impact C. cinerascens regeneration and growth particularly during the second growth cycle occurring in autumn. Additionally, our results showed that, independently of the season and groundwater level, population biomass of C. cinerascens was lower during drier year. Altogether, our findings suggest that water limitation due to both reduction in precipitation and decreased groundwater level during the year can strongly impact plant communities. [ABSTRACT FROM AUTHOR]

Recent studies show that soil eukaryotic diversity is immense and dominated by micro‐organisms. However, it is unclear to what extent the processes that shape the distribution of diversity in plants and animals also apply to micro‐organisms. Major diversification events in multicellular organisms have often been attributed to long‐term climatic and geological processes, but the impact of such processes on protist diversity has received much less attention as their distribution has often been believed to be largely cosmopolitan. Here, we quantified phylogeographical patterns in Hyalosphenia papilio, a large testate amoeba restricted to Holarctic Sphagnum‐dominated peatlands, to test if the current distribution of its genetic diversity can be explained by historical factors or by the current distribution of suitable habitats. Phylogenetic diversity was higher in Western North America, corresponding to the inferred geographical origin of the H. papilio complex, and was lower in Eurasia despite extensive suitable habitats. These results suggest that patterns of phylogenetic diversity and distribution can be explained by the history of Holarctic Sphagnum peatland range expansions and contractions in response to Quaternary glaciations that promoted cladogenetic range evolution, rather than the contemporary distribution of suitable habitats. Species distributions were positively correlated with climatic niche breadth, suggesting that climatic tolerance is key to dispersal ability in H. papilio. This implies that, at least for large and specialized terrestrial micro‐organisms, propagule dispersal is slow enough that historical processes may contribute to their diversification and phylogeographical patterns and may partly explain their very high overall diversity. [ABSTRACT FROM AUTHOR]

Climate warming affects plant physiology through genetic adaptation and phenotypic plasticity, but little is known about how these mechanisms influence ecosystem processes. We used three elevation gradients and a reciprocal transplant experiment to show that temperature causes genetic change in the sedge Eriophorum vaginatum. We demonstrate that plants originating from warmer climate produce fewer secondary compounds, grow faster and accelerate carbon dioxide (CO2) release to the atmosphere. However, warmer climate also caused plasticity in E. vaginatum, inhibiting nitrogen metabolism, photosynthesis and growth and slowing CO2 release into the atmosphere. Genetic differentiation and plasticity in E. vaginatum thus had opposing effects on CO2 fluxes, suggesting that warming over many generations may buffer, or reverse, the short‐term influence of this species over carbon cycle processes. Our findings demonstrate the capacity for plant evolution to impact ecosystem processes, and reveal a further mechanism through which plants will shape ecosystem responses to climate change. [ABSTRACT FROM AUTHOR]

Abstract: Climate change can alter peatland plant community composition by promoting the growth of vascular plants. How such vegetation change affects peatland carbon dynamics remains, however, unclear. In order to assess the effect of vegetation change on carbon uptake and release, we performed a vascular plant‐removal experiment in two Sphagnum‐dominated peatlands that represent contrasting stages of natural vegetation succession along a climatic gradient. Periodic measurements of net ecosystem CO2 exchange revealed that vascular plants play a crucial role in assuring the potential for net carbon uptake, particularly with a warmer climate. The presence of vascular plants, however, also increased ecosystem respiration, and by using the seasonal variation of respired CO2 radiocarbon (bomb‐14C) signature we demonstrate an enhanced heterotrophic decomposition of peat carbon due to rhizosphere priming. The observed rhizosphere priming of peat carbon decomposition was matched by more advanced humification of dissolved organic matter, which remained apparent beyond the plant growing season. Our results underline the relevance of rhizosphere priming in peatlands, especially when assessing the future carbon sink function of peatlands undergoing a shift in vegetation community composition in association with climate change. [ABSTRACT FROM AUTHOR]

Abstract: Ecosystems are increasingly prone to climate extremes, such as drought, with long‐lasting effects on both plant and soil communities and, subsequently, on carbon (C) cycling. However, recent studies underlined the strong variability in ecosystem's response to droughts, raising the issue of nonlinear responses in plant and soil communities. The conundrum is what causes ecosystems to shift in response to drought. Here, we investigated the response of plant and soil fungi to drought of different intensities using a water table gradient in peatlands—a major C sink ecosystem. Using moving window structural equation models, we show that substantial changes in ecosystem respiration, plant and soil fungal communities occurred when the water level fell below a tipping point of −24 cm. As a corollary, ecosystem respiration was the greatest when graminoids and saprotrophic fungi became prevalent as a response to the extreme drought. Graminoids indirectly influenced fungal functional composition and soil enzyme activities through their direct effect on dissolved organic matter quality, while saprotrophic fungi directly influenced soil enzyme activities. In turn, increasing enzyme activities promoted ecosystem respiration. We show that functional transitions in ecosystem respiration critically depend on the degree of response of graminoids and saprotrophic fungi to drought. Our results represent a major advance in understanding the nonlinear nature of ecosystem properties to drought and pave the way towards a truly mechanistic understanding of the effects of drought on ecosystem processes. [ABSTRACT FROM AUTHOR]

Ecological benefits associated with plant growth-promoting rhizobacteria (PGPR) inoculants offer a promising integrated nutrient management option to counteract plant nitrogen (N) deficiency. We performed field experiments to evaluate the effect of integrated N fertilizer regime involving chemical N fertilizer (CNF) and N-enriched compost (NEC), either alone or combined with selected PGPR ( Pseudomonas aeruginosa) on sunflower seed quality, N use efficiency (NUE) and soil fertility during 2014-2015. We found that integrated N biofertilizer application resulted in significantly higher seed oil concentration, fatty acid composition, and harvest index in both cropping years. Greater effects on N yield efficiency (NYE), N use efficiency (NUE), N physiological efficiency (NPE), and photosynthetic N use efficiency (PNUE) were recorded in nitrogen-enriched compost+PGPR inoculant (NECPI) treatment followed by chemical N fertilizer+PGPR inoculant (CNFPI) treatment. Statistically significant differences were observed in linoleic and linolenic acid, NYE, and NUE for treatment × year interaction, thus, suggesting that the integrated N biofertilizer approach facilitates the efficient N use by sunflower for improving yield and seed quality. Moreover, we also found considerable enhancement of soil N fertility after two consecutive cropping years of sunflower. The enhancement of seed quality, N use efficiencies, and soil N fertility through integrated N biofertilizer application emphasizes the importance of balanced crop N nutrition, ensuring sufficient N supply to sunflower with adequate N balance in soil for the next crop. Overall, combination of PGPR with NEC amendment may optimize N uptake efficiency and reduce N fertilizer losses, which is necessarily required for the sustainable sunflower production. [ABSTRACT FROM AUTHOR]

Extreme climate events are predicted to become more frequent and intense. Their ecological impacts, particularly on carbon cycling, can differ in relation to ecosystem sensitivity. Peatlands, being characterized by peat accumulation under waterlogged conditions, can be particularly sensitive to climate extremes if the climate event increases soil oxygenation. However, a mechanistic understanding of peatland responses to persistent climate extremes is still lacking, particularly in terms of aboveground-belowground feedback. Here, we present the results of a transplantation experiment of peat mesocosms from high to low altitude in order to simulate, during 3 years, a mean annual temperature c. 5 °C higher and a mean annual precipitation c. 60% lower. Specifically, we aim at understanding the intensity of changes for a set of biogeochemical processes and their feedback on carbon accumulation. In the transplanted mesocosms, plant productivity showed a species-specific response depending on plant growth forms, with a significant decrease (c. 60%) in peat moss productivity. Soil respiration almost doubled and Q10 halved in the transplanted mesocosms in combination with an increase in activity of soil enzymes. Spectroscopic characterization of peat chemistry in the transplanted mesocosms confirmed the deepening of soil oxygenation which, in turn, stimulated microbial decomposition. After 3 years, soil carbon stock increased only in the control mesocosms whereas a reduction in mean annual carbon accumulation of c. 30% was observed in the transplanted mesocosms. Based on the above information, a structural equation model was built to provide a mechanistic understanding of the causal connections between peat moisture, vegetation response, soil respiration and carbon accumulation. This study identifies, in the feedback between plant and microbial responses, the primary pathways explaining the reduction in carbon accumulation in response to recurring climate extremes in peat soils. [ABSTRACT FROM AUTHOR]

Question A better understanding of the response of Sphagnum mosses and associated vascular plants to climate warming is relevant for predicting the carbon balance of peatlands in a warmer world. Open-top chambers ( OTCs) have been used to investigate the effect on soil biogeochemical processes in peatlands, but little information is available on the effects of OTCs on microclimate conditions and the associated response of the plant community. We aimed to understand how simulated warming and differences in soil moisture affect plant species cover. Location A Sphagnum-dominated peatlands in French Jura. Methods We used OTCs to measure the effect of a near-ground temperature increase (+1.5 °C on average) on vegetation dynamics over five growing seasons (2008-2012) in a Sphagnum-dominated peatland, in two adjacent microhabitats with different hydrological conditions - wet and dry. Microclimatic conditions and plant species abundance were monitored at peak biomass in years 1, 2, 3 and 5 and monthly during the plant growing season in year 5. Results The response to warming differed between vascular plants and bryophytes, as well as among species within these groups, and also varied in relation to soil moisture. Andromeda polifolia abundance responded positively to warming, while Vaccinium oxycoccus responded negatively, and Eriophorum vaginatum showed a high resistance. Conclusion Depth of rooting of vascular plants appeared to control the response in plant abundance, while moss abundance depended on various other interacting factors, such as shading by the vascular plant community, precipitation and soil moisture. [ABSTRACT FROM AUTHOR]

The ongoing expansion of shrub cover in response to climate change represents a unique opportunity to explore the link between soil microbial communities and vegetation changes. This link is particularly important in peatlands where shrub expansion is expected to feed back negatively on the carbon sink capacity of these ecosystems., Microbial community structure and function were measured seasonally in four peatlands located along an altitude gradient representing a natural gradient of climate and associated vascular plant abundance., We show that increased soil temperature and reduced water content are associated with greater vascular plant biomass, in particular that of ericoids, and that this, in turn, is correlated with greater microbial biomass. More specifically, microbial community structure is characterized by an increasing dominance of fungi over bacteria with improved soil oxygenation. We also found that the carbon and nitrogen stoichiometry of microbial biomass differs in relation to soil microbial community structure and that this is ultimately associated with a different investment in extracellular enzymatic activity., Our findings highlight the fact that the determination of the structural identity of microbial communities can help to explain the biogeochemical dynamics of organic matter and provide a better understanding of ecosystem response to environmental changes. [ABSTRACT FROM AUTHOR]

Peatlands are important sinks for atmospheric carbon (C), yet the role of plant functional types (PFTs) for C sequestration under climatic perturbations is still unclear. A plant-removal experiment was used to study the importance of vascular PFTs for the net ecosystem C02 exchange (NEE) during (i.e., resistance) and after (i.e., recovery) an experimental drought. The removal of PFTs caused a decrease of NEE, but the rate differed between microhabitats (i.e., hummocks and lawns) and the type of PFTs. Ericoid removal had a large effect on NEE in hummocks, while the graminoids played a major role in the lawns. The removal of PFTs did not affect the resistance or the recovery after the experimental drought. We argue that the response of Sphagnum mosses (the only PFT present in all treatments) to drought is dominant over that of coexisting PFTs. However, we observed that the moment in time when the system switched from C sink to C source during the drought was controlled by the vascular PFTs. In the light of climate change, the shifts in species composition or even the loss of certain PFTs are expected to strongly affect the future C dynamics in response to environmental stress. [ABSTRACT FROM AUTHOR]

Bogs are globally important sinks of atmospheric carbon ( C) due to the accumulation of partially decomposed litter that forms peat. Because bogs receive their nutrients from the atmosphere, the world-wide increase of nitrogen ( N) deposition is expected to affect litter decomposition and, ultimately, the rate of C accumulation. However, the mechanism of such biogeochemical alteration remains unclear and quantification of the effect of N addition on litter accumulation has yet to be done. Here, we show that 7 years of N addition to a bog decreased the C : N ratio, increased the bacterial biomass and stimulated the activity of hydrolytic and oxidative enzymes in surface peat. Furthermore, N addition modified nutrient limitation of microbes during litter decomposition so that phosphorus became a primary limiting nutrient. Alteration of N release from decomposing litter affected bog water chemistry and the competitive balance between peat-forming mosses and vascular plants. We estimate that deposition of about 4 g N m−2 yr−1 will cause a mean annual reduction of fresh litter C accumulation of about 40 g m−2 primarily as a consequence of decreased litter production from peat-forming mosses. Our findings show that N deposition interacts with both above and below ground components of biodiversity to threaten the ability of bogs to act as N-sinks, which may offset the positive effects of N on C accumulation seen in other ecosystems. [ABSTRACT FROM AUTHOR]

Heat waves, which are projected to be more frequent and intense in a warmer climate, could become a serious threat to plants that rely on water surplus availability, such as bryophytes. Here, I take the advantage of the European summer 2003 climate anomaly to assess the impact of an extreme heat wave on peat mosses of the genus Sphagnum, a group of bryophytes forming the bulk of living and dead biomass in peatlands. With this aim, 20 selected bogs in the Italian Alps were checked for Sphagnum survival in the years following the heat wave. Over the study area, the period May–September 2003 was characterized by higher mean monthly air temperature (13.5 °C) and lower mean monthly precipitation (87 mm) compared with normal climatic conditions (11.5 °C and 117 mm, respectively) so that the heat wave coincided with a drought spell. As a consequence of the unusual water stress, I documented an increased mortality of peat mosses forming high hummocks. In particular, at habitat scale, the distribution of desiccated peat mosses was restricted to the hummock face receiving the greatest amount of solar irradiation. However, at regional scale, the present study identified a climatic threshold, simply defined by the ratio of precipitation to temperature ( P : T), which triggered an irreversible desiccation of peat mosses when mean monthly P : T dropped below 6.5 (mm : °C) during May–September 2003. The absence of any sign of recovery after 4 years since the drought must be seen as a harbinger of the deleterious effects of extreme heat waves on organisms not adapted to cope with abrupt climate anomaly. [ABSTRACT FROM AUTHOR]

• Nitrogen and phosphorus were added experimentally in a bog in the southern Alps. It was hypothesized that alleviating nutrient limitation will increase vascular plant cover. As a consequence, more carbon will be fixed through higher rates of net ecosystem CO2 exchange (NEE). • The vascular cover did increase at the expense of Sphagnum mosses. However, such vegetation changes were largely independent of the treatment and were probably triggered by an exceptional heatwave in summer 2003. • Contrary to the tested hypothesis, NEE was unaffected by the nutrient treatments but was strongly influenced by temperature and water-table depth. In particular, ecosystem respiration in the hot summer of 2003 increased dramatically, presumably owing to enhanced heterotrophic respiration in an increased oxic peat layer. • At the end of the experiment, the Sphagnum cover decreased significantly in the nitrogen-fertilized treatment at hummock microhabitats. In the long term, this will imply a proportionally greater accumulation of vascular litter, more easily decomposable than the recalcitrant Sphagnum litter. As a result, rates of carbon fixation may decrease because of stimulated respiration. [ABSTRACT FROM AUTHOR]

Increasing rates of atmospheric nitrogen (N) deposition may reduce growth and accelerate decomposition of Sphagnum mosses in bogs. Sphagnum growth and rates of Sphagnum litter decomposition may also vary because of climate change as both processes are controlled by climatic factors. The initial purpose of this study was to assess if growth and litter decomposition of hummock and lawn Sphagnum species varied with increasing N input in a factorial mid-term (2002–2005) experiment of N and phosphorus (P) addition, in a bog on the southern Alps of Italy. However, as the experimental period was characterized by an exceptional heat wave in summer 2003, we also explored the interacting effects of fertilization and strongly varying climate on growth and decomposition rates of Sphagnum. The heat wave implied strong dehydration of the upper Sphagnum layer even if precipitation in summer 2003 did not differ appreciably from the overall mean. Sphagnum production was somewhat depressed by high levels (3 g m−2 yr−1) of N addition without concomitant addition of P presumably because of nutrient imbalance in the tissues, but production rates were much lower than the overall means in 2003, when no effect of nutrient addition could be observed. Adding N at high level also increased the potential decay of Sphagnum litter. Higher CO2 emission from N-fertilized litter was due to amelioration of litter chemistry showing lower C/N quotients in the N-fertilized treatments. Rates of CO2 emission from incubated litter also were more strongly affected by water content than by nutrient status, with practically no CO2 emission detected when litter was dry. We conclude that higher rates of atmospheric N availability input may depress Sphagnum growth because of P, and presumably potassium, (co-)limitation. Higher N availability is also expected to promote potential decay of Sphagnum litter by ameliorating litter chemistry. However, both effects are less pronounced if the growing Sphagnum apex and the underlying senescing tissues dry out. [ABSTRACT FROM AUTHOR]

Alteration of the global nitrogen (N) cycle because of human-enhanced N fixation is a major concern particularly for those ecosystems that are nutrient poor by nature. BecauseSphagnum-dominated mires are exclusively fed by wet and dry atmospheric deposition, they are assumed to be very sensitive to increased atmospheric N input. We assessed the consequences of increased atmospheric N deposition on total N concentration, N retention ability, andδ15N isotopic signature ofSphagnumplants collected in 16 ombrotrophic mires across 11 European countries. The mires spanned a gradient of atmospheric N deposition from about 0.1 up to about 2 g m−2 yr−1. Mean N concentration inSphagnumcapitula was about 6 mg g−1 in less polluted mires and about 13 mg g−1 in highly N-polluted mires. The relative difference in N concentration between capitulum and stem decreased with increasing atmospheric N deposition, suggesting a possible metabolic mechanism that reduces excessive N accumulation in the capitulum.Sphagnumplants showed lower rates of N absorption under increasing atmospheric N deposition, indicating N saturation inSphagnumtissues. The latter probably is related to a shift from N-limited conditions to limitation by other nutrients. The capacity of theSphagnumlayer to filter atmospheric N deposition decreased exponentially along the depositional gradient resulting in enrichment of the mire pore water with inorganic N forms (i.e., NO3−+NH4+).Sphagnumplants hadδ15N signatures ranging from about−8‰ to about−3‰. The isotopic signatures were rather related to the ratio of reduced to oxidized N forms in atmospheric deposition than to total amount of atmospheric N deposition, indicating thatδ15N signature ofSphagnumplants can be used as an integrated measure ofδ15N signature of atmospheric precipitation. Indeed, mires located in areas characterized by greater emissions of NH3 (i.e., mainly affected by agricultural activities) hadSphagnumplants with a lowerδ15N signature compared with mires located in areas dominated by NO x emissions (i.e., mainly affected by industrial activities). [ABSTRACT FROM AUTHOR]

• We studied the effects of increasing levels of atmospheric nitrogen (N) deposition on nutrient limitation of ombrotrophic Sphagnum plants. • Fifteen mires in 11 European countries were selected across a natural gradient of bulk atmospheric N deposition from 0.1 to 2 g/m2 year−1. Nutritional constraints were assessed based on nutrient ratios of N, phosphorus (P), and potassium (K) in Sphagnum plants collected in hummocks (i.e. relatively drier microhabitats) and in lawns (i.e. relatively wetter microhabitats). • Nutrient ratios in Sphagnum plants increased steeply at low atmospheric N input, but above a threshold of N deposition of c. 1 g/m2 year−1 the N : P and N : K ratios tended to saturation. Increasing atmospheric N deposition was also accompanied by a reduced retention of Ca and Mg in Sphagnum plants and a decreased stem volumetric density in hummock Sphagnum plants. • We suggest a critical load of N deposition in Europe of 1 g/m2 year−1 above which Sphagnum plants change from being N-limited to be K + P colimited, at N : P > 30 and N : K > 3. New Phytologist (2004) 163: 609–616 © New Phytologist (2004) doi: 10.1111/j.1469-8137.2004.01154.x [ABSTRACT FROM AUTHOR]

Question: Do soil water content and/or soil nitrogen (N) content and/or soil phosphorus (P) content affect the biomass of Vaccinium myrtillus and V. vitis-idaea in a sub-alpine heath? Location: Dolomites, northern Italy, 1800 m a.s.l. Methods: We determined above-ground and below-ground biomass of the shrubs at three sites, each on a different substrate type. At each site, we determined soil N- and P- contents. We also determined leaf water potential (ψ1), N- and P-concentrations in plant tissues and litter, as well as δ13C and δ15N in mature leaves. Results: V. myrtillus biomass was highest at the silicate site, V. vitis-idaea biomass was highest at the carbonate site. Both shrubs had low biomass at the peat site, possibly due to a toxic effect of waterlogging in wet soils. For both species, pre-dawn ψ1 indicated optimal hydration and midday ψ1 did not show any sign of water stress. Water use efficiency (WUE) did not differ among sites for any species. Whole-plant nutrient concentrations showed that, with increasing biomass, N was diluted in V. myrtillus tissues while P was diluted in V. vitis- idaea tissues. Foliar N-concentration was higher overall for V. myrtillus. Foliar P-concentration in V. myrtillus peaked at the silicate site. Foliar N : P ratios suggested that V. myrtillus was primarily P-limited and V. vitis-idaea primarily N-limited. Conclusions: Water content affected the distribution of the two shrubs in a similar way, higher P-availability in the soil enhanced V. myrtillus rather than V. vitis-idaea. [ABSTRACT FROM AUTHOR]

Summary 1 We assessed the role of climatic conditions and the effects of different, long-term atmospheric depositions in controlling the mineral and nutrient contents in pore-water, surface peat and in living Sphagna at a boreo-nemoral mire in Sweden and an alpine mire in Italy. 2 The terrestrial contribution of Ca2+ , Mg2+ and &formmu0; in bulk precipitation was much greater at the Italian mire, in accordance with the different bedrock in the region and the higher level of atmospheric pollution. 3 At both mires, the contribution of bulk precipitation to the concentration of major ions in mire pore-water was much greater in the ombrotrophic than in the minerotrophic part, because of the raised morphology of the mires, which limited the inflow of mineral soil water to the margins. The only ions strongly depleted in mire pore-water compared with precipitation were K+ , &formmu1; and &formmu2; and these were therefore limiting to plant growth. 4 Higher &formmu3; concentration in pore-water at the Swedish mire, which experienced lower atmospheric inputs of sulphate, was probably caused by oxidative processes during a long dry period in the summer before sampling. 5 Higher rates of &formmu4;, &formmu5;, as well as &formmu6; atmospheric inputs at the Italian mire were reflected in significantly higher N and, partly, S concentrations in ombrotrophic Sphagna. Higher &formmu7; concentration in pore-water at the Italian mire was associated with a lower N retention coefficient of the ombrotrophic Sphagnum plants, suggesting a reduced nitrogen filtering ability of the moss layer. [ABSTRACT FROM AUTHOR]

Periodic measurements of gas-exchange rates and determinations of foliar N and P concentrations were used for evaluating instantaneous water-use efficiency and photosynthetic nutrient-use efficiency in two co-existing dwarf shrubs of different growth form (V. myrtillus, deciduous, and V. vitis-idaea, evergreen) in a subalpine heath in the southern Alps of Italy. Those data were compared with cumulative assessments of water-use efficiency and photosynthetic nutrient-use efficiency obtained by measuring leaf carbon isotope discrimination in leaf tissues and by estimating nutrient resorption from senescing leaves. V. myrtillus presented higher dry-weight based rates of net photosynthesis (Aweight) compared to V. vitis-idaea. Aweight was positively correlated with foliar-nutrient status and intercellular-to-ambient gradient in CO2 concentrations. Aweight was, furthermore, negatively correlated with leaf specific mass. Instantaneous photosynthetic nutrient-use efficiency did not differ between the two species but the percentages of N and P pools resorbed from senescing leaves were somewhat higher in the deciduous species. The evergreen species showed lower P concentrations in senescing leaves which indicated a higher proficiency in resorbing phosphorus compared to the deciduous species. In addition, the evergreen species achieved a higher carbon gain per unit foliar N and P, due to a longer mean residence time of both nutrients. The two species did not differ from each other with respect to both instantaneous and long-term water-use efficiency. This was consistent with the climatic pattern, showing no sign of water deficiency through the growing season. Current-year V. vitis-idaea leaves had a significantly higher δ13C compared to previous-year leaves, possibly mirroring a long term acclimation of evergreen leaves, as far as they age, to the habitat conditions in the understory where evergreen species are usually confi... [ABSTRACT FROM AUTHOR]

Background: Northern peatlands have accumulated vast amounts of carbon (C) as peat. Warming temperatures may affect peatland C stores by increasing microbial decomposition of ancient peat through enhanced input of labile root exudates by expansion of vascular plants, thereby accelerating atmospheric warming. Aims: We set out to explore how much freshly assimilated C is allocated belowground by vascular plants, and if the above‐ to belowground allocation is affected by temperature and plant functional types. Methods: We traced the C allocation pathways of two dominant plant functional types (i.e., sedges and shrubs) in two peatlands under different temperature regimes by combining selective plant removal in mixed sedge‐shrub vegetation and in situ 13C pulse‐labelling. Aboveground to belowground C allocation as well as the C turnover were assessed by quantifying 13C in plant leaves and soil respiration and by measuring δ13C in dissolved organic C. A depth‐resolved quantification of 13C in the peat soil gave additional insight into belowground C allocation patterns. Results: Temperature did not affect the rate at which 13C was assimilated into shoots, but higher temperature decreased the fraction of assimilated C that was allocated belowground by vascular plants. Sedges assimilated CO2 faster into their shoot biomass (faster depletion in 13C in shoots) and allocated more of the assimilated 13C belowground than shrubs. Conversely, sedges retained this belowground allocated C better than shrubs, leading to lower 13C in soil respiration measured under sedges. Conclusions: Climate induced vascular plant expansion will increase input of fresh assimilates into the peat substantially, even though part of this effect will be offset by reduced above‐ to belowground allocation rates. If shrub density increases relative to sedges, fresh assimilates are more likely to be respired than translocated to roots where they could reach and, potentially mobilize, ancient C stored in deeper peat layers. [ABSTRACT FROM AUTHOR]