Direct uptake of soil nitrogen by mosses

Impact of changing climate on bryophyte contributions to terrestrial water, carbon, and nitrogen cycles

Bryophytes, including the lineages of mosses, liverworts, and hornworts, are the second-largest photoautotroph group on Earth. Recent work across terrestrial ecosystems has highlighted how bryophytes retain and control water, fix substantial amounts of carbon (C), and contribute to nitrogen (N) cycles in forests (boreal, temperate, and tropical), tundra, peatlands, grasslands, and deserts. Understanding how changing climate affects bryophyte contributions to global cycles in different ecosystems is of primary importance. However, because of their small physical size, bryophytes have been largely ignored in research on water, C, and N cycles at global scales. Here, we review the literature on how bryophytes influence global biogeochemical cycles, and we highlight that while some aspects of global change represent critical tipping points for survival, bryophytes may also buffer many ecosystems from change due to their capacity for water, C, and N uptake and storage. However, as the thresholds of resistance of bryophytes to temperature and precipitation regime changes are mostly unknown, it is challenging to predict how long this buffering capacity will remain functional. Furthermore, as ecosystems shift their global distribution in response to changing climate, the size of different bryophyte-influenced biomes will change, resulting in shifts in the magnitude of bryophyte impacts on global ecosystem functions.

Regional air quality study by assessing trace metal atmospheric deposition

This study addresses the atmospheric deposition of trace elements investigated in Albania, Croatia and Macedonia in 2010 as part of the European Moss Study. This study provides data on the concentration of ten metals (Cd, Cr, Cu, Ni, Pb, V, Zn, Fe, Al, and Li) in naturally growing mosses. In general, all concentration data follow a lognormal distribution. Cd, Cr, Ni, and Zn show strong fluctuations in the moss samples from Albania and Macedonia, and Pb, Fe, and Al in the mosses from Croatia. The concentrations of Cd, Cu, and Zn were higher in the samples from Croatia than in those from Albania and Macedonia; the concentrations of Cr, Ni, V, Fe, Al, and Li were higher in the samples from Albania than in those from Croatia and Macedonia; and a higher concentration of Pb was found in the samples from Macedonia. The observed relationship between the concentration of lithogenic elements (Al, Fe, and V) and the air quality index (AQI) confirms that moss species have a high capacity to retain atmospheric deposition particles. The anthropogenic emission sources of these elements from local and long-range transport were considered to be the most important factors affecting air quality in the studied areas.

Seasonal climate drivers of peak NDVI in a series of Arctic peatlands

Changes in plant cover and productivity are important in driving Arctic soil carbon dynamics and sequestration, especially in peatlands. Warming trends in the Arctic are known to have resulted in changes in plant productivity, extent and community composition, but more data are still needed to improve understanding of the complex controls and processes involved. Here we assess plant productivity response to climate variability between 1985 and 2020 by comparing peak growing season NDVI (Normalised Difference Vegetation Index data from Landsat 5 and 7), to seasonal-average weather data (temperature, precipitation and snow-melt timing) in nine locations containing peatlands in high- and low-Arctic regions in Europe and Canada. We find that spring (correlation 0.36 for peat dominant and 0.39 for mosaic; MLR coefficient 0.20 for peat, 0.29 for mosaic), summer (0.47, 0.42; 0.18, 0.17) and preceding-autumn (0.35, 0.25; 0.33, 0.27) temperature are linked to peak growing season NDVI at our sites between 1985 and 2020, whilst spring snow melt timing (0.42, 0.45; 0.25, 0.32) is also important, and growing season water availability is likely site-specific. According to regression trees, a warm preceding autumn (September-October-November) is more important than a warm summer (June-July-August) in predicting the highest peak season productivity in the peat-dominated areas. Mechanisms linked to soil processes may explain the importance of previous-Autumn conditions on productivity. We further find that peak productivity increases in these Arctic peatlands are comparable to those in the surrounding non-peatland-dominant vegetation. Increased productivity in and around Arctic peatlands suggests a potential to increased soil carbon sequestration with future warming, but further work is needed to test whether this is evident in observations of recent peat accumulation and extent.

Ecology and responses to climate change of biocrust-forming mosses in drylands

Interest in understanding the role of biocrusts as ecosystem engineers in drylands has substantially increased during the past two decades. Mosses are a major component of biocrusts and dominate their late successional stages. In general, their impacts on most ecosystem functions are greater than those of early-stage biocrust constituents. However, it is common to find contradictory results regarding how moss interactions with different biotic and abiotic factors affect ecosystem processes. This review aims to (i) describe the adaptations and environmental constraints of biocrust-forming mosses in drylands, (ii) identify their primary ecological roles in these ecosystems, and (iii) synthesize their responses to climate change. We emphasize the importance of interactions between specific functional traits of mosses (e.g. height, radiation reflectance, morphology, and shoot densities) and both the environment (e.g. climate, topography, and soil properties) and other organisms to understand their ecological roles and responses to climate change. We also highlight key areas that should be researched in the future to fill essential gaps in our understanding of the ecology and the responses to ongoing climate change of biocrust-forming mosses. These include a better understanding of intra- and interspecific interactions and mechanisms driving mosses' carbon balance during desiccation-rehydration cycles.

Indirect effects of climate change inhibit N2 fixation associated with the feathermoss Hylocomium splendens in subarctic tundra

Mosses can be responsible for up to 100% of net primary production in arctic and subarctic tundra, and their associations with diazotrophic cyanobacteria have an important role in increasing nitrogen (N) availability in these pristine ecosystems. Predictions about the consequences of climate change in subarctic environments point to increased N mineralization in soil and higher litter deposition due to warming. It is not clear yet how these indirect climate change effects impact moss-cyanobacteria associations and N₂ fixation. This work aimed to evaluate the effects of increased N and litter input on biological N₂ fixation rates associated with the feathermoss Hylocomium splendens from a tundra heath. H. splendens samples were collected near Abisko, northern Sweden, from a field experiment with annual additions of ammonium chloride and dried birch litter and the combination of both for three years. Samples were analyzed for N₂ fixation, cyanobacterial colonization, C and N content and pH. Despite the high N additions, no significant differences in moss N content were found. However, differences between treatments were observed in N₂ fixation rates, cyanobacterial colonization and pH, with the combined ammonium+litter treatment causing a significant reduction in the number of branch-colonizing cyanobacteria and N₂ fixation, and ammonium additions significantly lowering moss pH. A significant, positive relationship was found between N₂ fixation rates, moss colonization by cyanobacteria and pH levels, showing a clear drop in N₂ fixation rates at lower pH levels even if larger cyanobacterial populations were present. These results suggest that increased N availability and litter deposition resulting from climate change not only interferes with N₂ fixation directly, but also acidifies moss microhabitats and reduces the abundance of associated cyanobacteria, which could eventually impact the N cycle in the Subarctic.

The relationship of C and N stable isotopes to high-latitude moss-associated N2 fixation

Moss-associated N₂ fixation by epiphytic microbes is a key biogeochemical process in nutrient-limited high-latitude ecosystems. Abiotic drivers, such as temperature and moisture, and the identity of host mosses are critical sources of variation in N₂ fixation rates. An understanding of the potential interaction between these factors is essential for predicting N inputs as moss communities change with the climate. To further understand the drivers and results of N₂ fixation rate variation, we obtained natural abundance values of C and N isotopes and an associated rate of N₂ fixation with ¹⁵N₂ gas incubations in 34 moss species collected in three regions across Alaska, USA. We hypothesized that δ¹⁵N values would increase toward 0‰ with higher N₂ fixation to reflect the increasing contribution of fixed N₂ in moss biomass. Second, we hypothesized that δ¹³C and N₂ fixation would be positively related, as enriched δ¹³C signatures reflect abiotic conditions favorable to N₂ fixation. We expected that the magnitude of these relationships would vary among types of host mosses, reflecting differences in anatomy and habitat. We found little support for our first hypothesis, with only a modest positive relationship between N₂ fixation rates and δ¹⁵N in a structural equation model. We found a significant positive relationship between δ¹³C and N₂ fixation only in Hypnales, where the probability of N₂ fixation activity reached 95% when δ¹³C values exceeded - 30.4‰. We conclude that moisture and temperature interact strongly with host moss identity in determining the extent to which abiotic conditions impact associated N₂ fixation rates.

Effect of Mosses on physical and chemical properties of soil in temperate forests of Garhwal Himalayas

Mosses are one of the most important and dominant plant communities, especially in the temperate biome, and play a significant role in ecosystem function and dynamics. They influence the water, energy and element cycle due to their unique ecology and physiology. The present study was undertaken in three different temperate forest sites in the Garhwal Himalayas, viz., Triyuginarayan (Kedarnath Wildlife Sanctuary (KWLS)), Chakrata, and Kanasar forest range. The study was focused on understanding the influence of mosses on soil physical properties and nutrient availability. Different physico-chemical properties were analysed under two different substrata, that is, with and without moss cover in two different seasons, viz., monsoon and winter. We observed mosses to influence and alter the physical properties and nutrient status of soil in both seasons. All soil physical and chemical properties, except magnesium, showed significant difference within the substrates, among all the sites and across the two seasons. Besides the soil characteristics underneath the moss vegetation, the study also highlights the diversity of mosses found in the area. Mosses appear to create high nutrient microsites via a high rate of organic matter accumulation and retain nutrients for longer periods thus, maintaining ecosystem stability.

Forest mosses sensitively indicate nitrogen deposition in boreal background areas

Mosses take up nitrogen (N) mainly from precipitation through their surfaces, which makes them competent bioindicators of N deposition. We found positive relationships between the total N concentration (mossN%) of common terrestrial moss species (feather mosses Pleurozium schreberi and Hylocomium splendens, and a group of Dicranum species) and different forms of N deposition in 11-16 coniferous forests with low N deposition load in Finland. The mosses were collected either inside (Dicranum group) or both inside and outside (feather mosses) the forests. Deposition was monitored in situ as bulk deposition (BD) and stand throughfall (TF) and detected for ammonium (NH₄⁺-N), nitrate (NO₃^--N), dissolved organic N (DON), and total N (N_tot, kg ha^-1yr^-1). N_tot deposition was lower in TF than BD indicating that tree canopies absorbed N from deposition in N limited boreal stands. However, mossN% was higher inside than outside the forests. In regression equations, inorganic N in BD predicted best the mossN% in openings, while DON in TF explained most variation of mossN% in forests. An asymptotic form of mossN% vs. TF N_tot curves in forests and free NH₄⁺-N accumulation in tissues in the southern plots suggested mosses were near the N saturation state already at the N_tot deposition level of 3-5 kg ha^-1yr^-1. N leachate from ground litterfall apparently also contributed the N supply of mosses. Our study yielded new information on the sensitivity of boreal mosses to low N deposition and their response to different N forms in canopy TF entering moss layer. The equations predicting the N_tot deposition with mossN% showed a good fit both in forest sites and openings, especially in case of P. schreberi. However, the open site mossN% is a preferable predictor of N deposition in monitoring studies to minimize the effect of tree canopies and N leachate from litterfall on the estimates.

Application of δ15N to trace the impact of penguin guano on terrestrial and aquatic nitrogen cycles in Victoria Land, Ross Sea region, Antarctica

Penguin colonies in Antarctica offer an ideal "natural laboratory" to investigate ecosystem function and the nitrogen (N) cycle. This study assessed the spatial distribution of penguin-derived N from guano and quantitatively assessed its impact on plant N utilization strategies in Victoria Land, Ross Sea region, Antarctica. Soil, moss, and aquatic microbial mats were collected inside and outside an active Adélie penguin (Pygoscelis adeliae) colony and analyzed for δ¹⁵N of total and inorganic nitrogen (NH₄⁺-N and NO₃^--N). The soil total nitrogen (TN), NH₄⁺-N, and NO₃^--N concentrations, as well as their δ¹⁵N values were significantly higher in guano-impacted areas than those in guano-free areas, verifying that guano is an important N source at and near penguin colonies. However, even far from the penguin colonies, soil δ¹⁵N values resembled those in penguin colonies, suggesting strong spatial impacts of penguin-derived N. The moss impacted by guano was more enriched in δ¹⁵N than in guano-free areas. The δ¹⁵N values of NH₄⁺-N and NO₃^--N in soils covered with moss revealed that the moss might prefer inorganic N in the absence of guano, while the dissolved organic N would become an important source for moss growing on ornithogenic soils. Aquatic microbial mat samples near penguin colonies were ¹⁵N-enriched, but ¹⁵N-depleted at upland sites.

Ecological restoration of eroded karst utilizing pioneer moss and vascular plant species with selection based on vegetation diversity and underlying soil chemistry

Soil erosion in karst areas not only destroys the natural environment but also accelerates the decline in land productivity together with the associated increase in poverty for local communities. There are no simple or straightforward answers to controlling soil erosion on karst. Such erosion has become a serious problem in China. This study addresses both the diversity of vegetation (mosses, herbs, shrubs and trees) on karst and the underlying soil chemical characteristics, in order to provide a scientific basis by which suitable plant species can be selected for recovery and restoration of karst degraded by soil erosion. Vegetation diversity and soil chemistry were assessed in areas with five different grades of soil erosion in Guiyang Karst Park, Guizhou, China. Mosses are more tolerant than vascular plants of soil erosion and associated environmental degradation of karst and the order of species diversity was: mosses > herbs > shrubs > trees. Mosses were found to play a major role in ecological restoration of microhabitats on karst. Soil microbial biomass carbon was found to be significantly higher in soil under mosses than in bare soil associated with other plant categories. Mosses were more effective in converting unavailable potassium to available potassium. Vascular plants were found to have a positive effect on total nitrogen fixation and the availability of phosphorus in the soil. Increasing soil degradation was associated with lower levels of total Nitrogen in soil underlying mosses than in soil underlying vascular plants. Thus, based on their different but complementary contributions to soil chemistry, mosses and vascular plants in combination can provide the most practical outcome for the repair and restoration of areas of karst affected by soil erosion. The combination of the moss species, Homomallium plagiangium, Cyrto-hypnum pygmaeuman and Brachythecium perminusculum, the herbs Veronica arvensis and Youngia japonica, and the tree Prunus salicina, are recommended as suitable pioneer plant species to cultivate for use in restoration of regions of karstic soil erosion.

Seasonal variation of secondary metabolites in nine different bryophytes

Bryophytes occur in almost all land ecosystems and contribute to global biogeochemical cycles, ecosystem functioning, and influence vegetation dynamics. As growth and biochemistry of bryophytes are strongly dependent on the season, we analyzed metabolic variation across seasons with regard to ecological characteristics and phylogeny. Using bioinformatics methods, we present an integrative and reproducible approach to connect ecology with biochemistry. Nine different bryophyte species were collected in three composite samples in four seasons. Untargeted liquid chromatography coupled with mass spectrometry (LC/MS) was performed to obtain metabolite profiles. Redundancy analysis, Pearson's correlation, Shannon diversity, and hierarchical clustering were used to determine relationships among species, seasons, ecological characteristics, and hierarchical clustering. Metabolite profiles of Marchantia polymorpha and Fissidens taxifolius which are species with ruderal life strategy (R-selected) showed low seasonal variability, while the profiles of the pleurocarpous mosses and Grimmia pulvinata which have characteristics of a competitive strategy (C-selected) were more variable. Polytrichum strictum and Plagiomnium undulatum had intermediary life strategies. Our study revealed strong species-specific differences in metabolite profiles between the seasons. Life strategies, growth forms, and indicator values for light and soil were among the most important ecological predictors. We demonstrate that untargeted Eco-Metabolomics provide useful biochemical insight that improves our understanding of fundamental ecological strategies.

Foliar heavy metal uptake, toxicity and detoxification in plants: A comparison of foliar and root metal uptake

Anthropologic activities have transformed global biogeochemical cycling of heavy metals by emitting considerable quantities of these metals into the atmosphere from diverse sources. In spite of substantial and progressive developments in industrial processes and techniques to reduce environmental emissions, atmospheric contamination by toxic heavy metals and associated ecological and health risks are still newsworthy. Atmospheric heavy metals may be absorbed via foliar organs of plants after wet or dry deposition of atmospheric fallouts on plant canopy. Unlike root metal transfer, which has been largely studied, little is known about heavy metal uptake by plant leaves from the atmosphere. To the best of our understanding, significant research gaps exist regarding foliar heavy metal uptake. This is the first review regarding biogeochemical behaviour of heavy metals in atmosphere-plant system. The review summarizes the mechanisms involved in foliar heavy metal uptake, transfer, compartmentation, toxicity and in plant detoxification. We have described the biological and environmental factors that affect foliar uptake of heavy metals and compared the biogeochemical behaviour (uptake, translocation, compartmentation, toxicity and detoxification) of heavy metals for root and foliar uptake. The possible health risks associated with the consumption of heavy metal-laced food are also discussed.

Organic nitrogen uptake is a significant contributor to nitrogen economy of subtropical epiphytic bryophytes

Without any root contact with the soil, epiphytic bryophytes must experience and explore poor, patchy, and heterogeneous habitats; while, the nitrogen (N) uptake and use strategies of these organisms remain uncharacterized, which obscures their roles in the N cycle. To investigate the N sources, N preferences, and responses to enhanced N deposition in epiphytic bryophytes, we carried out an in situ manipulation experiment via the (15)N labelling technique in an Asian cloud forest. Epiphytic bryophytes obtained more N from air deposition than from the bark, but the contribution of N from the bark was non-negligible. Glycine accounted for 28.4% to 44.5% of the total N in bryophyte tissue, which implies that organic N might serve as an important N source. Increased N deposition increased the total N uptake, but did not alter the N preference of the epiphytic bryophytes. This study provides sound evidence that epiphytic bryophytes could take up N from the bark and wet deposition in both organic and inorganic N forms. It is thus important to consider organic N and bark N sources, which were usually neglected, when estimating the role of epiphytic bryophytes in N cycling and the impacts of N deposition on epiphytic bryophytes in cloud forests.

Use of lead-210 as a novel tracer for lead (Pb) sources in plants

Lead (Pb) released from anthropogenic sources and stored in environmental repositories can be a potential source for secondary pollution. Here we develop a novel approach for tracking Pb from atmospheric deposition and other sources in the environment using fallout ²¹⁰Pb as a tracer, and apply the method to samples collected from Richmond Park, London, the UK. The mean ratio of ²¹⁰Pb to total Pb in atmospheric depositions collected from a site adjacent to the park during August–October 2012 was 96 Bq mg⁻¹, while the ratio in surface soils from the park was typically an order of magnitude lower. The difference between these values made it possible to trace the source of Pb in the plants. The ²¹⁰Pb/Pb ratios in plants varied from 0 to 34 Bq mg⁻¹ indicating different levels of Pb absorption from the atmosphere. The ratio in mosses had an average value of 22 Bq mg⁻¹. This suggests that only around 20% of the Pb they contain was from direct atmospheric deposition, revealing possible limitations in the use of terrestrial mosses for monitoring atmospheric pollution. As well as tracking sources, variations in the ²¹⁰Pb/Pb ratio can also reveal ways in which Pb is transferred within plants.

Diversification of Nitrogen Sources in Various Tundra Vegetation Types in the High Arctic

Low nitrogen availability in the high Arctic represents a major constraint for plant growth, which limits the tundra capacity for carbon retention and determines tundra vegetation types. The limited terrestrial nitrogen (N) pool in the tundra is augmented significantly by nesting seabirds, such as the planktivorous Little Auk (Alle alle). Therefore, N delivered by these birds may significantly influence the N cycling in the tundra locally and the carbon budget more globally. Moreover, should these birds experience substantial negative environmental pressure associated with climate change, this will adversely influence the tundra N-budget. Hence, assessment of bird-originated N-input to the tundra is important for understanding biological cycles in polar regions. This study analyzed the stable nitrogen composition of the three main N-sources in the High Arctic and in numerous plants that access different N-pools in ten tundra vegetation types in an experimental catchment in Hornsund (Svalbard). The percentage of the total tundra N-pool provided by birds, ranged from 0-21% in Patterned-ground tundra to 100% in Ornithocoprophilous tundra. The total N-pool utilized by tundra plants in the studied catchment was built in 36% by birds, 38% by atmospheric deposition, and 26% by atmospheric N2-fixation. The stable nitrogen isotope mixing mass balance, in contrast to direct methods that measure actual deposition, indicates the ratio between the actual N-loads acquired by plants from different N-sources. Our results enhance our understanding of the importance of different N-sources in the Arctic tundra and the used methodological approach can be applied elsewhere.

Toxicity of fuel-contaminated soil to Antarctic moss and terrestrial algae

Fuel pollution is a significant problem in Antarctica, especially in areas where human activities occur, such as at scientific research stations. Despite this, there is little information on the effects of petroleum hydrocarbons on Antarctic terrestrial biota. The authors demonstrate that the Antarctic mosses Bryum pseudotriquetrum, Schistidium antarctici, and Ceratodon purpureus, and the Antarctic terrestrial alga Prasiola crispa are relatively tolerant to Special Antarctic Blend (SAB) fuel-contaminated soil (measured as total petroleum hydrocarbons). Freshly spiked soils were more toxic to all species than were aged soils containing degraded fuel, as measured by photosynthetic efficiency (variable fluorescence/maximum fluorescence [Fv/Fm]), pigment content, and visual observations. Concentrations that caused 20% inhibition ranged from 16,600 mg/kg to 53,200 mg/kg for freshly spiked soils and from 30,100 mg/kg to 56,200 mg/kg for aged soils. The photosynthetic efficiency of C. purpureus and S. antarctici was significantly inhibited by exposure to freshly spiked soils with lowest-observed-effect concentrations of 27,900 mg/kg and 40,400 mg/kg, respectively. Prasiola crispa was the most sensitive species to freshly spiked soils (Fv/Fm lowest-observed-effect concentration 6700 mg/kg), whereas the Fv/Fm of B. pseudotriquetrum was unaffected by exposure to SAB fuel even at the highest concentration tested (62,900 mg/kg). Standard toxicity test methods developed for nonvascular plants can be used in future risk assessments, and sensitivity data will contribute to the development of remediation targets for petroleum hydrocarbons to guide remediation activities in Antarctica.

Relationship between site-specific nitrogen concentrations in mosses and measured wet bulk atmospheric nitrogen deposition across Europe

To assess the relationship between nitrogen concentrations in mosses and wet bulk nitrogen deposition or concentrations in precipitation, moss tissue and deposition were sampled within a distance of 1 km of each other in seven European countries. Relationships for various forms of nitrogen appeared to be asymptotic, with data for different countries being positioned at different locations along the asymptotic relationship and saturation occurring at a wet bulk nitrogen deposition of ca. 20 kg N ha(-1) yr(-1). The asymptotic behaviour was more pronounced for ammonium-N than nitrate-N, with high ammonium deposition at German sites being most influential in providing evidence of the asymptotic behaviour. Within countries, relationships were only significant for Finland and Switzerland and were more or less linear. The results confirm previous relationships described for modelled total deposition. Nitrogen concentration in mosses can be applied to identify areas at risk of high nitrogen deposition at European scale.

Comparative use of lichens, mosses and tree bark to evaluate nitrogen deposition in Germany

To compare three biomonitoring techniques for assessing nitrogen (N) pollution in Germany, 326 lichen, 153 moss and 187 bark samples were collected from 16 sites of the national N deposition monitoring network. The analysed ranges of N content of all investigated biomonitors (0.32%-4.69%) and the detected δ(15)N values (-15.2‰-1.5‰), made it possible to reveal species specific spatial patterns of N concentrations in biota to indicate atmospheric N deposition in Germany. The comparison with measured and modelled N deposition data shows that particularly lichens are able to reflect the local N deposition originating from agriculture.

Spatial distribution of PAH concentrations and stable isotope signatures (δ13C, δ15N) in mosses from three European areas--characterization by multivariate analysis

Polycyclic aromatic hydrocarbon (PAH) concentrations and N, C stable isotope signatures were determined in mosses Hypnum cupressiforme Hedw. from 61 sites of 3 European regions: Île-de-France (France); Navarra (Spain); the Swiss Plateau and Basel area (Switzerland). Total PAH concentrations of 100-700 ng g(-1), as well as δ(13)C values of -32 to -29‰ and δ(15)N values of -11 to -3‰ were measured. Pearson correlation tests revealed opposite trends between high molecular weight PAH (4-6 aromatic rings) content and δ(13)C values. Partial Least Square regressions explained the very significant correlations (r > 0.91, p < 0.001) between high molecular weight PAH concentrations by local urban land use (<10 km) and environmental factors such as elevation and pluviometry. Finally, specific correlations between heavy metal and PAH concentrations were attributed to industrial emissions in Switzerland and road traffic emissions in Spain.

Ammonium first: natural mosses prefer atmospheric ammonium but vary utilization of dissolved organic nitrogen depending on habitat and nitrogen deposition

Mosses, among all types of terrestrial vegetation, are excellent scavengers of anthropogenic nitrogen (N), but their utilization of dissolved organic N (DON) and their reliance on atmospheric N remain uncharacterized in natural environments, which obscures their roles in N cycles. Natural (15) N abundance of N sources (nitrate (NO(3)(-)), ammonium (NH(4)(+)) and DON in deposition and soil) for epilithic and terricolous mosses was analyzed at sites with different N depositions at Guiyang, China. Moss NO(3)(-) assimilation was inhibited substantially by the high supply of NH(4)(+) and DON. Therefore, contributions of NH(4)(+) and DON to moss N were partitioned using isotopic mass-balance methods. The N contributions averaged 56% and 46% from atmospheric NH(4)(+), and 44% and 17% from atmospheric DON in epilithic and terricolous mosses, respectively. In terricolous mosses, soil NH(4)(+) and soil DON accounted for 16% and 21% of bulk N, which are higher than current estimations obtained using (15) N-labeling methods. Moreover, anthropogenic NH(4)(+) deposition suppressed utilization of DON and soil N because of the preference of moss for NH(4)(+) under elevated NH(4)(+) deposition. These results underscore the dominance of, and preference for, atmospheric NH(4)(+) in moss N utilization, and highlight the importance of considering DON and soil N sources when estimating moss N sequestration and the impacts of N deposition on mosses.

Moss-cyanobacteria associations as biogenic sources of nitrogen in boreal forest ecosystems

The biological fixation of atmospheric nitrogen (N) is a major pathway for available N entering ecosystems. In N-limited boreal forests, a significant amount of N2 is fixed by cyanobacteria living in association with mosses, contributing up to 50% to the total N input. In this review, we synthesize reports on the drivers of N2 fixation in feather moss-cyanobacteria associations to gain a deeper understanding of their role for ecosystem-N-cycling. Nitrogen fixation in moss-cyanobacteria associations is inhibited by N inputs and therefore, significant fixation occurs only in low N-deposition areas. While it has been shown that artificial N additions in the laboratory as well as in the field inhibit N2 fixation in moss-cyanobacteria associations, the type, as well as the amounts of N that enters the system, affect N2 fixation differently. Another major driver of N2 fixation is the moisture status of the cyanobacteria-hosting moss, wherein moist conditions promote N2 fixation. Mosses experience large fluctuations in their hydrological status, undergoing significant natural drying and rewetting cycles over the course of only a few hours, especially in summer, which likely compromises the N input to the system via N2 fixation. Perhaps the most central question, however, that remains unanswered is the fate of the fixed N2 in mosses. The cyanobacteria are likely to leak N, but whether this N is transferred to the soil and if so, at which rates and timescales, is unknown. Despite our increasing understanding of the drivers of N2 fixation, the role moss-cyanobacteria associations play in ecosystem-N-cycling remains unresolved. Further, the relationship mosses and cyanobacteria share is unknown to date and warrants further investigation.

Pitfalls and new mechanisms in moss isotope biomonitoring of atmospheric nitrogen deposition

Moss N isotope (δ(15)N(bulk)) has been used to monitor N deposition, but it remains questionable whether inhibition of nitrate reductase activity (NRA) by reduced dissolved N (RDN) engenders overestimation of RDN in deposition when using moss δ(15)N(bulk). We tested this question by investigation of δ(15)N(bulk) and δ(15)NO(3)(-) in mosses under the dominance of RDN in N depositions of Guiyang, SW China. The δ(15)N(bulk) of mosses on bare rock (-7.9‰) was unable to integrate total dissolved N (TDN) (δ(15)N = -6.3‰), but it reflected δ(15)N-RDN (-7.5‰) exactly. Moreover, δ(15)N-NO(3)(-) in mosses (-1.7‰) resembled that of wet deposition (-1.9‰). These isotopic approximations, together with low isotopic enrichment with moss [NO(3)(-)] variations, suggest the inhibition of moss NRA by RDN. Moreover, isotopic mixing modeling indicated a negligible contribution from NO(3)(-) to moss δ(15)N(bulk) when the RDN/NO(3)(-) reaches 3.8, at which maximum overestimation (21%) of RDN in N deposition can be generated using moss δ(15)N(bulk) as δ(15)N-TDN. Moss δ(15)N-NO(3)(-) can indicate atmospheric NO(3)(-) under distinctly high RDN/NO(3)(-) in deposition, although moss δ(15)N(bulk) can reflect only the RDN therein. These results reveal pitfalls and new mechanisms associated with moss isotope monitoring of N deposition and underscore the importance of biotic N dynamics in biomonitoring studies.

Preliminary insights into δ15N and δ18O of nitrate in natural mosses: a new application of the denitrifier method

Natural mosses have been employed as reactive and accumulative indicators of atmospheric pollutants. Using the denitrifier method, the concentration, δ(15)N and δ(18)O of moss nitrate (NO(3)(-)) were measured to elucidate the sources of NO(3)(-) trapped in natural mosses. Oven drying at 55-70 °C, not lyophilization, was recommended to dry mosses for NO(3)(-) analyses. An investigation from urban to mountain sites in western Tokyo suggested that moss [NO(3)(-)] can respond to NO(3)(-) availability in different habitats. NO(3)(-) in terricolous mosses showed isotopic ratios as close to those of soil NO(3)(-), reflecting the utilization of soil NO(3)(-). Isotopic signatures of NO(3)(-) in corticolous and epilithic mosses elucidated atmospheric NO(3)(-) sources and strength from the urban (vehicle NO(x) emission) to mountain area (wet-deposition NO(3)(-)). However, mechanisms and isotopic effects of moss NO(3)(-) utilization must be further verified to enable the application of moss NO(3)(-) isotopes for source identification.

Nitrogen concentrations in mosses indicate the spatial distribution of atmospheric nitrogen deposition in Europe

In 2005/6, nearly 3000 moss samples from (semi-)natural location across 16 European countries were collected for nitrogen analysis. The lowest total nitrogen concentrations in mosses (<0.8%) were observed in northern Finland and northern UK. The highest concentrations (≥ 1.6%) were found in parts of Belgium, France, Germany, Slovakia, Slovenia and Bulgaria. The asymptotic relationship between the nitrogen concentrations in mosses and EMEP modelled nitrogen deposition (averaged per 50 km × 50 km grid) across Europe showed less scatter when there were at least five moss sampling sites per grid. Factors potentially contributing to the scatter are discussed. In Switzerland, a strong (r(2) = 0.91) linear relationship was found between the total nitrogen concentration in mosses and measured site-specific bulk nitrogen deposition rates. The total nitrogen concentrations in mosses complement deposition measurements, helping to identify areas in Europe at risk from high nitrogen deposition at a high spatial resolution.

Vascular plant removal effects on biological N fixation vary across a boreal forest island gradient

There is currently much interest in understanding how biodiversity loss affects the functioning of ecosystems, but few studies have evaluated how ecosystem processes change in response to one another following biodiversity loss. We focused on a well-described gradient of 30 forested lake islands in northern Sweden, where island size determines the occurrence of lightning-ignited wildfire, which in turn determines successional stage, plant species composition, and productivity. We investigated the effect of biodiversity loss on biological nitrogen fixation by feathermosses through an experiment consisting of factorial removals of three understory shrub species (Vaccinium myrtillis, Vaccinium vitis-idaea, and Empetrum hermaphroditum) and two plant functional groups (shrubs and tree roots). We tested the hypothesis that, following vascular plant species loss, N fixation rates would be impaired by changes in pools or processes that increase extractable soil N, because changes in the supply rate of N to feathermosses should influence their demand for newly fixed N. Further, we hypothesized that the effects of removals on N fixation would depend on environmental context (i.e., island size), because it has been previously demonstrated that the effect of vascular plant species removal on N recycling pools and processes was strongest on productive islands. The data demonstrated that removal of two shrub species (V. vitis-idaea and E. hermaphroditum) negatively aflected the N fixation of Hylocomium splendens, but positively affected Pleurozium schreberi, resulting in unchanged areal N fixation rates. In the functional removal experiment, tree root removal resulted in a significant negative effect on N fixation. The effects of shrub and root removals on N fixation occurred only on small islands and thus were context dependent. This pattern did not correspond to the effect of shrub and root removal treatments on N-recycling pools or processes, which only occurred in response to specific vascular plant removals on large or medium islands. The data thus did not support our hypothesis that N fixation was directly responsive to changes in N-recycling pools or processes caused by vascular plant species removals, but instead highlighted the importance of species-specific interactions and environmental context in determining the manner in which biodiversity loss alters ecosystem processes.

Assessment of atmospheric sulfur with the epilithic moss Haplocladium microphyllum: evidences from tissue sulfur and delta34S analysis

The application of geochemical signals in mosses is more and more popular to investigate the deposition of atmospheric pollutants, but it is unclear whether records of atmospheric sulfur in mosses differ between their diverse habitats. This study aimed to investigate the influence of growing condition on tissue sulfur and delta34S of Haplocladium microphyllum. Epilithic and terricolous mosses in open fields, mosses under different canopy conditions were considered. We found that tissue sulfur and delta34S of mosses under different habitats were not consistent and could not be compared for atmospheric sulfur research with each other even collected at the same site, moss sulfur and delta34S records would be distorted by subsoil and upper canopies in different degrees, which possibly mislead the interpretation of atmospheric sulfur level and sources. Consequently, mosses on open rocks can be used reliably to assess atmospheric-derived sulfur in view of their identical sulfur and delta34S evidences.