The impact of nitrogen deposition on acid grasslands in the Atlantic region of Europe

A novel method to estimate the response of habitat types to nitrogen deposition

Increasing nitrogen depositions adversely affect European landscapes, including habitats within the Natura2000 network. Critical loads for nitrogen deposition have been established to quantify the loss of habitat quality. When the nitrogen deposition rises above a habitat-specific critical load, the quality of the focal habitat is expected to be negatively influenced. Here, we investigate how the quality of habitat types is affected beyond the critical load. We calculated response curves for 60 terrestrial habitat types in the Netherlands to the estimated nitrogen deposition (EMEP-data). The curves for habitat types are based on the occurrence of their characteristic plant species in North-Western Europe (plot data from the European Vegetation Archive). The estimated response curves were corrected for soil type, mean annual temperature and annual precipitation. Evaluation was carried out by expert judgement, and by comparison with gradient deposition field studies. For 39 habitats the response to nitrogen deposition was judged to be reliable by five experts, while out of the 41 habitat types for which field studies were available, 25 showed a good agreement. Some of the curves showed a steep decline in quality and some a more gradual decline with increasing nitrogen deposition. We compared the response curves with both the empirical and modelled critical loads. For 41 curves, we found a decline already starting below the critical load.

Effects of nitrogen addition on species composition and diversity of early spring herbs in a Korean pine plantation

Under the background of global nitrogen deposition, temperate forest ecosystems are suffering increasing threats, and species diversity is gradually decreasing. In this study, nitrogen addition experiments were conducted on Korean pine (Pinus koraiensis) plantations in Northeast China to explore the effect of long-term nitrogen addition on herb species diversity to test the following hypothesis: long-term nitrogen addition further reduced plant species diversity by affecting plant growth, which may be due to soil acidification caused by excessive nitrogen addition. Experimental nitrogen addition was conducted from 2014 to 2021, and the nitrogen treatment levels were as follows: N0 (control treatment, 0/(kg N ha-1 year-1)), N20 (low nitrogen treatment, 20/(kg N ha-1 year-1)), N40 (medium nitrogen treatment, 40/(kg N ha-1 year-1)) and N80 (high nitrogen treatment, 80/(kg N ha-1 year-1)). A herb community survey was conducted in the region from 2015 to 2021. The results showed that long-term nitrogen addition decreased soil pH, changed the species and composition of herbaceous plants, and decreased the species diversity of understory herbaceous plants. With the increase in nitrogen application years, middle- and high-nitrogen treatments significantly reduced the diversity of early-spring flowering herbs and early-spring foliating herbs, and their diversity decreased with the decrease in soil pH, indicating that soil acidification caused by long-term nitrogen addition may lead to the decrease of plant diversity. However, for early-spring growing herbs, adequate nitrogen addition may promote their growth. Our results show that plants have evolved different life-history strategies based on their adaptation mechanisms to the environment, and different life-history strategies have different responses to long-term nitrogen addition. However, for most plants, long-term nitrogen application will have a negative impact on the growth and diversity of herbs in temperate forests.

A review on moss nitrogen and isotope signatures evidence for atmospheric nitrogen deposition

Moss nitrogen (N) concentration and isotopic composition (δ¹⁵N) values can reveal a better understanding of atmospheric N deposition patterns. Here, we summarize the moss N content and δ¹⁵N signatures using data compiled from 104 papers. Based on the dataset, we summarize the models for assessing the level and reduced (NH_x): oxidised compounds (NO_x) ratio of atmospheric N deposition. Results showed a historical increase in N concentration and ¹⁵N depletion of specimen mosses close to anthropogenic N sources from intensive animal production and agricultural activities (NH_x emission) since the 1800s. However, an increase of moss N with a less negative ¹⁵N observed in the last three decades could be due to a substantial fossil fuel combustion contributed NO_x emission. Spatially, N deposition in Europe decreased due to successful control actions, but Asia has become a hotspot for NH_x emission from agriculture. The present results highlight the importance of moss N and δ¹⁵N values for estimating atmospheric N deposition patterns at spatio-temporal trends.

Effects of nitrogen addition on vegetation and soil and its linkages to plant diversity and productivity in a semi-arid steppe

Nitrogen (N) deposition and fertilization, which represent key sources of N input in many terrestrial ecosystems, influence all levels of the ecosystem and involve complex mechanisms. Quantitative and modelling approaches can be used to understand this complexity. In this study, we carried out in situ N addition experiments in a Stipa krylovii steppe in northern China. We evaluated the effects of N addition on plant diversity and productivity under two scenarios (fertilization and simulated increased N deposition) using a structural equation model (SEM). N addition had direct effects on community weighted means (CWM) of plant functional traits and soil properties but had indirect effects on community structure. The changes in community structure and soil properties caused by N addition decreased plant diversity, whereas productivity remained relatively stable and was mainly controlled by changes in community structure. The changes in soil properties and plant diversity caused by N addition had little effect on productivity or soil pH. We conclude that the changes in plant diversity and productivity with increased N input in the S. krylovii steppe were mainly due to differences in growth responses of different species to increased N and the resulting community responses, such as changes in community structure. The results of the present study provide a theoretical basis for grassland management and conservation in the wake of global environmental change.

Atmospheric nitrogen deposition: A review of quantification methods and its spatial pattern derived from the global monitoring networks

Atmospheric nitrogen (N) deposition is a vital component of the global N cycle. Excessive N deposition on the Earth's surface has adverse impacts on ecosystems and humans. Quantification of atmospheric N deposition is indispensable for assessing and addressing N deposition-induced environmental issues. In the present review, we firstly summarized the current methods applied to quantify N deposition (wet, dry, and total N deposition), their advantages and major limitations. Secondly, we illustrated the long-term N deposition monitoring networks worldwide and the results attained via such long-term monitoring. Results show that China faces heavier N deposition than the United States, European countries, and other countries in East Asia. Next, we proposed a framework for estimating the atmospheric wet and dry N deposition using a combined method of surface monitoring, modeling, and satellite remote sensing. Finally, we put forth the critical research challenges and future directions of the atmospheric N deposition. CAPSULE: A review of quantification methods and the global data on nitrogen deposition and a systematic framework was proposed for quantifying nitrogen deposition.

Global patterns and controlling factors of soil nitrification rate

Soil nitrification, an important pathway of nitrogen transformation in ecosystems, produces soil nitrate that influences net primary productivity, while the by-product of nitrification, nitrous oxide, is a significant greenhouse gas. Although there have been many studies addressing the microbiology, physiology, and impacting environment factors of soil nitrification at local scales, there are very few studies on soil nitrification rate over large scales. We conducted a global synthesis on the patterns and controlling factors of soil nitrification rate normalized at 25°C by compiling 3,140 observations from 186 published articles across terrestrial ecosystems. Soil nitrification rate tended to decrease with increasing latitude, especially in the Northern Hemisphere, and varied largely with ecosystem types. The soil nitrification rate significantly increased with mean annual temperature (MAT), soil nitrogen content, microbial biomass carbon and nitrogen, soil ammonium, and soil pH, but decreased with soil carbon:nitrogen and carbon:nitrogen of microbial biomass. The total soil nitrogen content contributed the most to the variations of global soil nitrification rate (total coefficient = 0.29) in structural equation models. The microbial biomass nitrogen (MBN; total coefficient = 0.19) was nearly of equivalent importance relative to MAT (total coefficient = 0.25) and soil pH (total coefficient = 0.24) in determining soil nitrification rate, while soil nitrogen and pH influenced soil nitrification via changing soil MBN. Moreover, the emission of soil nitrous oxide was positively related to soil nitrification rate at a global scale. This synthesis will advance our current understanding on the mechanisms underlying large-scale variations of soil nitrification and benefit the biogeochemical models in simulating global nitrogen cycling.

Ammonia emission abatement does not fully control reduced forms of nitrogen deposition

Human activities and population growth have increased the natural burden of reactive nitrogen (N) in the environment. Excessive N deposition on Earth's surface leads to adverse feedbacks on ecosystems and humans. Similar to that of air pollution, emission control is recognized as an efficient means to control acid deposition. Control of nitrogen oxides (NOx = NO + NO2) emissions has led to reduction in deposition of oxidized nitrogen (NOy, the sum of all oxidized nitrogen species, except nitrous oxide [N2O]). Reduced forms of nitrogen (NHx = ammonia [NH3] + ammonium [NH4+]) deposition have, otherwise, increased, offsetting the benefit of reduction in NOy deposition. Stringent control of NH3 emissions is being considered. In this study, we assess the response of N deposition to N emission control on continental regions. We show that significant reduction of NHx deposition is unlikely to be achieved at the early stages of implementing NH3 emission abatement. Per-unit NH3 emission abatement is shown to result in only 60-80% reduction in NHx deposition, which is significantly lower than the demonstrated 80-120% benefit of controlling NOx emissions on NOy deposition. This 60-80% effectiveness of NHx deposition reduction per unit NH3 emission abatement reflects, in part, the effects of simultaneous reductions in NOx and SO2 emissions.

Multiyear Measurements on Δ17O of Stream Nitrate Indicate High Nitrate Production in a Temperate Forest

Nitrification is a crucial step in ecosystem nitrogen (N) cycling, but scaling up from plot-based measurements of gross nitrification to catchments is difficult. Here, we employed a newly developed method in which the oxygen isotope anomaly (Δ¹⁷O) of nitrate (NO₃^-) is used as a natural tracer to quantify in situ catchment-scale gross nitrification rate (GNR) for a temperate forest from 2014 to 2017 in northeastern China. The annual GNR ranged from 71 to 120 kg N ha^-1 yr^-1 (average 94 ± 10 kg N ha^-1 yr^-1) over the 4 years in this forest. This result and high stream NO₃^- loss (4.2-8.9 kg N ha^-1 yr^-1) suggest that the forested catchment may have been N-saturated. At the catchment scale, the total N output of 10.7 kg N ha^-1 yr^-1, via leaching and gaseous losses, accounts for 56% of the N input from bulk precipitation (19.2 kg N ha^-1 yr^-1). This result indicates that the forested catchment is still retaining a large fraction of N from atmospheric deposition. Our study suggests that estimating in situ catchment-scale GNR over several years when combined with other conventional flux estimates can facilitate the understanding of N biogeochemical cycling and changes in the ecosystem N status.

Anthropogenic global shifts in biospheric N and P concentrations and ratios and their impacts on biodiversity, ecosystem productivity, food security, and human health

The availability of carbon (C) from high levels of atmospheric carbon dioxide (CO2 ) and anthropogenic release of nitrogen (N) is increasing, but these increases are not paralleled by increases in levels of phosphorus (P). The current unstoppable changes in the stoichiometries of C and N relative to P have no historical precedent. We describe changes in P and N fluxes over the last five decades that have led to asymmetrical increases in P and N inputs to the biosphere. We identified widespread and rapid changes in N:P ratios in air, soil, water, and organisms and important consequences to the structure, function, and biodiversity of ecosystems. A mass-balance approach found that the combined limited availability of P and N was likely to reduce C storage by natural ecosystems during the remainder of the 21st Century, and projected crop yields of the Millennium Ecosystem Assessment indicated an increase in nutrient deficiency in developing regions if access to P fertilizer is limited. Imbalances of the N:P ratio would likely negatively affect human health, food security, and global economic and geopolitical stability, with feedbacks and synergistic effects on drivers of global environmental change, such as increasing levels of CO2 , climatic warming, and increasing pollution. We summarize potential solutions for avoiding the negative impacts of global imbalances of N:P ratios on the environment, biodiversity, climate change, food security, and human health.

Biomonitoring freshwater FISH farms by measuring nitrogen concentrations and the δ15N signal in living and devitalized moss transplants

The trophic balance of freshwater aquaculture activities has traditionally been monitored by chemical analysis of water; however, the parameters measured are usually characterized by high temporal variability. Aquatic mosses can be used as biomonitors as they integrate both continuous and episodic contamination events. Here we report, for the first time, a method for monitoring N enrichment in the surroundings of fish farms by measuring the N content and isotopic signal (δ¹⁵N) of transplanted living and devitalized specimens of the aquatic moss Fontinalis antipyretica. For this purpose, moss samples ("moss bags") were exposed at increasing distances (10, 100, 300 and 1000 m) up- and downstream of the effluent discharge points of four trout farms, for 10 and 30 days. The low natural (background) variability in δ¹⁵N in upstream samples enabled detection of outlier values, caused by aquaculture discharges, at distances of 10 and 100 m downstream, especially in devitalized moss and after 10 days of exposure. However, the unexpectedly low N contents of moss samples exposed close to the discharge points complicates interpretation of the high levels of N forms detected by conventional physicochemical analysis of water. Although the mechanisms that modify N parameters in moss tissues were not clear, measurement of the isotopic signal δ¹⁵N in devitalized moss exposed for 10 days proved useful for monitoring the N pollution associated with intensive freshwater aquaculture.

Decline in atmospheric sulphur deposition and changes in climate are the major drivers of long-term change in grassland plant communities in Scotland

The predicted long lag time between a decrease in atmospheric deposition and a measured response in vegetation has generally excluded the investigation of vegetation recovery from the impacts of atmospheric deposition. However, policy-makers require such evidence to assess whether policy decisions to reduce emissions will have a positive impact on habitats. Here we have shown that 40 years after the peak of SO_x emissions, decreases in SO_x are related to significant changes in species richness and cover in Scottish Calcareous, Mestrophic, Nardus and Wet grasslands. Using a survey of vegetation plots across Scotland, first carried out between 1958 and 1987 and resurveyed between 2012 and 2014, we test whether temporal changes in species richness and cover of bryophytes, Cyperaceae, forbs, Poaceae, and Juncaceae can be explained by changes in sulphur and nitrogen deposition, climate and/or grazing intensity, and whether these patterns differ between six grassland habitats: Acid, Calcareous, Lolium, Nardus, Mesotrophic and Wet grasslands. The results indicate that Calcareous, Mesotrophic, Nardus and Wet grasslands in Scotland are starting to recover from the UK peak of SO_x deposition in the 1970's. A decline in the cover of grasses, an increase in cover of bryophytes and forbs and the development of a more diverse sward (a reversal of the impacts of increased SO_x) was related to decreased SO_x deposition. However there was no evidence of a recovery from SO_x deposition in the Acid or Lolium grasslands. Despite a decline in NO_x deposition between the two surveys we found no evidence of a reversal of the impacts of increased N deposition. The climate also changed significantly between the two surveys, becoming warmer and wetter. This change in climate was related to significant changes in both the cover and species richness of bryophytes, Cyperaceae, forbs, Poaceae and Juncaceae but the changes differed between habitats.

Context-dependent environmental quality standards of soil nitrate for terrestrial plant communities

Environmental quality standards (EQS) specify the maximum permissible concentration or level of a specific environmental stressor. Here, a procedure is proposed to derive EQS that are specific to a representative species pool and conditional on confounding environmental factors. To illustrate the procedure, a dataset was used with plant species richness observations of grasslands and forests and accompanying soil nitrate-N and pH measurements collected from 981 sampling sites in the Netherlands. Species richness was related to soil nitrate-N and pH with quantile regression allowing for interaction effects. The resulting regression models were used to derive EQS for nitrate conditional on pH, quantified as the nitrate-N concentrations at a specific pH level corresponding with a species richness equal to 95% of the species pool, for both grasslands and forest communities. The EQS varied between 1.8 mg/kg nitrate-N at pH 9-65 mg/kg nitrate-N at pH 4. EQS for forests and grasslands were similar, but EQS based on Red List species richness were considerably lower (more stringent) than those based on overall species richness, particularly at high pH levels. The results indicate that both natural background pH conditions and Red List species are important factors to consider in the derivation of EQS for soil nitrate-N for terrestrial ecosystems.

Vegetation community change in Atlantic oak woodlands along a nitrogen deposition gradient

Atlantic old sessile oak woodlands are of high conservation importance in Europe, listed in the European Union (EU) Habitats Directive Annex I, and known for their rich bryophyte communities. Their conservation status ranges from unfavourable to bad across their known distribution, which is predominantly within the UK and Ireland, but also extends into Iberia and Brittany. The objectives of this study were to determine if nitrogen (N) deposition, a known driver of terrestrial biodiversity loss, was a significant predictor of community composition in old sessile oak woodlands (i.e., EU Habitats Directive Annex I class: 91A0), and to identify significant changes in individual plant species and community-level abundance (i.e., change points) along an N deposition gradient. Relevé data from 260 Irish oak woodland plots were evaluated using Canonical Correspondence Analysis (CCA) and Threshold Indicator Taxa ANalysis (TITAN). Nitrogen deposition accounted for 14% of the explainable variation in the dataset (inertia = 0.069, p < 0.005). A community scale change point of 13.2 kg N ha(-1) yr(-1) was indicated by TITAN, which falls within the current recommended critical load (CL) range for acidophilous Quercus-dominated (oak) woodlands (10-15 kg N ha(-1) yr(-1)). The results suggest that the current CL is sufficient for maintaining a core group of indicator species in old sessile oak woodlands, but many nutrient sensitive species may disappear even at the CL range minimum.

Physiological Responses of Two Epiphytic Bryophytes to Nitrogen, Phosphorus and Sulfur Addition in a Subtropical Montane Cloud Forest

Atmospheric depositions pose significant threats to biodiversity and ecosystem function. However, the underlying physiological mechanisms are not well understood, and few studies have considered the combined effects and interactions of multiple pollutants. This in situ study explored the physiological responses of two epiphytic bryophytes to combined addition of nitrogen, phosphorus and sulfur. We investigated the electrical conductivity (EC), total chlorophyll concentration (Chl), nutrient stoichiometry and chlorophyll fluorescence signals in a subtropical montane cloud forest in south-west China. The results showed that enhanced fertilizer additions imposed detrimental effects on bryophytes, and the combined enrichment of simulated fertilization exerted limited synergistic effects in their natural environments. On the whole, EC, Chl, the effective quantum yield of photosystem II (ΦPSII) and photochemical quenching (qP) were the more reliable indicators of increased artificial fertilization. However, conclusions on nutrient stoichiometry should be drawn cautiously concerning the saturation uptake and nutrient interactions in bryophytes. Finally, we discuss the limitations of prevailing fertilization experiments and emphasize the importance of long-term data available for future investigations.

Evaluation of the use of moss transplants (Pseudoscleropodium purum) for biomonitoring different forms of air pollutant nitrogen compounds

We investigated whether three different types of moss transplants (devitalized moss bags with and without cover and auto-irrigated moss transplants) are suitable for use as biomonitors of the deposition of oxidised and/or reduced forms of N. For this purpose, we determined whether the concentration of atmospheric NO2 was related to the % N, δ(15)N and the activity of the enzyme biomarkers phosphomonoesterase (PME) and nitrate reductase (NR) in the tissues of moss transplants. We exposed the transplants in 5 different environments of Galicia (NW Spain) and Cataluña (NE Spain): industrial environments, urban and periurban environments, the surroundings of a cattle farm and in a monitoring site included in the sampling network of the European Monitoring Programme. The results showed that the moss in the auto-irrigated transplants was able of incorporating the N in its tissues because it was metabolically active, whereas in devitalized moss bags transplants, moss simply intercepts physically the N compounds that reached it in particulate or gaseous form. In addition, this devitalization could limit the capacity of moss to capture gaseous compounds (i.e. reduced N) and to reduce the oxidised compounds that reach the specimens. These findings indicate that devitalized moss transplants cannot be used to monitor either oxidised or reduced N compounds, whereas transplants of metabolically active moss can be used for this purpose. Finally, the NR and PME biomarkers should be used with caution because of the high variability in their activities and the limits of quantification should be evaluated in each case.

Management practices regulate the response of Moso bamboo foliar stoichiometry to nitrogen deposition

Moso bamboo, well known for its high growth rate, is being subjected to increasing amounts of nitrogen deposition. However, how anthropogenic management practices regulate the effects of N deposition on Moso bamboo stoichiometry remains poorly understood. We observed the effects of two years of simulated N deposition (30, 60 and 90 kg N ha(-1)yr(-1)) on the foliar stoichiometry of Moso bamboo plantations under conventional management (CM) and intensive management (IM). Young bamboo had significantly greater foliar N and P concentrations and N:P ratios than mature plants (P < 0.05). IM significantly increased the foliar N concentrations of young bamboo and P concentrations of mature bamboo but decreased mature bamboo foliar N:P ratios (P < 0.05). Nitrogen increased foliar N and P concentrations in IM bamboo plantations, but the positive effects were diminished when the addition rate exceeded 60 kg N ha(-1)yr(-1). Nitrogen increased foliar N concentrations but aggravated P deficiency in CM bamboo plantations. The positive effects of N deposition on foliar stoichiometry were influenced by management practices and bamboo growth stage. The effects of N deposition on foliar stoichiometry combined with anthropogenic management practices can influence ecosystem production, decomposition, and subsequent N and P cycles in Moso bamboo plantations.

Relevance of canopy drip for the accumulation of nitrogen in moss used as biomonitors for atmospheric nitrogen deposition in Europe

High atmospheric deposition of nitrogen (N) impacts functions and structures of N limited ecosystems. Due to filtering and related canopy drip effects forests are particularly exposed to N deposition. Up to now, this was proved by many studies using technical deposition samplers but there are only some few studies analysing the canopy drip effect on the accumulation of N in moss and related small scale atmospheric deposition patterns. Therefore, we investigated N deposition and related accumulation of N in forests and in (neighbouring) open fields by use of moss sampled across seven European countries. Sampling and chemical analyses were conducted according to the experimental protocol of the European Moss Survey. The ratios between the measured N content in moss sampled inside and outside of forests were computed and used to calculate estimates for non-sampled sites. Potentially influencing environmental factors were integrated in order to detect their relationships to the N content in moss. The overall average N content measured in moss was 20.0mgg(-1) inside and 11.9mgg(-1) outside of forests with highest N values in Germany inside of forests. Explaining more than 70% of the variance, the multivariate analyses confirmed that the sampling site category (site with/without canopy drip) showed the strongest correlation with the N content in moss. Spatial variances due to enhanced dry deposition in vegetation stands should be considered in future monitoring and modelling of atmospheric N deposition.

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.

Biogeochemical indicators of elevated nitrogen deposition in semiarid Mediterranean ecosystems

Nitrogen (N) deposition has doubled the natural N inputs received by ecosystems through biological N fixation and is currently a global problem that is affecting the Mediterranean regions. We evaluated the existing relationships between increased atmospheric N deposition and biogeochemical indicators related to soil chemical factors and cryptogam species across semiarid central, southern, and eastern Spain. The cryptogam species studied were the biocrust-forming species Pleurochaete squarrosa (moss) and Cladonia foliacea (lichen). Sampling sites were chosen in Quercus coccifera (kermes oak) shrublands and Pinus halepensis (Aleppo pine) forests to cover a range of inorganic N deposition representative of the levels found in the Iberian Peninsula (between 4.4 and 8.1 kg N ha(-1) year(-1)). We extended the ambient N deposition gradient by including experimental plots to which N had been added for 3 years at rates of 10, 20, and 50 kg N ha(-1) year(-1). Overall, N deposition (extant plus simulated) increased soil inorganic N availability and caused soil acidification. Nitrogen deposition increased phosphomonoesterase (PME) enzyme activity and PME/nitrate reductase (NR) ratio in both species, whereas the NR activity was reduced only in the moss. Responses of PME and NR activities were attributed to an induced N to phosphorus imbalance and to N saturation, respectively. When only considering the ambient N deposition, soil organic C and N contents were positively related to N deposition, a response driven by pine forests. The PME/NR ratios of the moss were better predictors of N deposition rates than PME or NR activities alone in shrublands, whereas no correlation between N deposition and the lichen physiology was observed. We conclude that integrative physiological measurements, such as PME/NR ratios, measured on sensitive species such as P. squarrosa, can provide useful data for national-scale biomonitoring programs, whereas soil acidification and soil C and N storage could be useful as additional corroborating ecosystem indicators of chronic N pollution.

Influence of canopy drip on the indicative N, S and δ(15)N content in moss Hypnum cupressiforme

Samples of Hypnum cupressiforme were collected at two types of site in forest areas: within the forest stand and within forest openings, and analyzed for N and S concentrations and δ(15)N. Mosses sampled within forest openings reflect the atmospheric N deposition; however, no influence of throughfall N deposition on the N in the moss that was sampled within the forest stand was found, nor was any influence of S deposition on the S in the moss found. For the N and S concentrations in the mosses sampled within forest openings, the within-site variability was comparable to the between-site variability, and for the δ(15)N, the within-site variability was lower than the between-site. The results showed that a short distance (<1 m) between the sampling location and the nearest tree canopy increases the N in the moss, and significantly higher values are found in mosses sampled in areas within the forest stand.

Does prescribed burning result in biotic homogenization of coastal heathlands?

Biotic homogenization due to replacement of native biodiversity by widespread generalist species has been demonstrated in a number of ecosystems and taxonomic groups worldwide, causing growing conservation concern. Human disturbance is a key driver of biotic homogenization, suggesting potential conservation challenges in seminatural ecosystems, where anthropogenic disturbances such as grazing and burning are necessary for maintaining ecological dynamics and functioning. We test whether prescribed burning results in biotic homogenization in the coastal heathlands of north-western Europe, a seminatural landscape where extensive grazing and burning has constituted the traditional land-use practice over the past 6000 years. We compare the beta-diversity before and after fire at three ecological scales: within local vegetation patches, between wet and dry heathland patches within landscapes, and along a 470 km bioclimatic gradient. Within local patches, we found no evidence of homogenization after fire; species richness increased, and the species that entered the burnt Calluna stands were not widespread specialists but native grasses and herbs characteristic of the heathland system. At the landscapes scale, we saw a weak homogenization as wet and dry heathland patches become more compositionally similar after fire. This was because of a decrease in habitat-specific species unique to either wet or dry habitats and postfire colonization by a set of heathland specialists that established in both habitat types. Along the bioclimatic gradient, species that increased after fire generally had more specific environmental requirements and narrower geographical distributions than the prefire flora, resulting in a biotic 'heterogenisation' after fire. Our study demonstrates that human disturbance does not necessarily cause biotic homogenization, but that continuation of traditional land-use practices can instead be crucial for the maintenance of the diversity and ecological function of a seminatural ecosystem. The species that established after prescribed burning were heathland specialists with relatively narrow geographical ranges.

Independent effects of warming and nitrogen addition on plant phenology in the Inner Mongolian steppe

BACKGROUND AND AIMS

Phenology is one of most sensitive traits of plants in response to regional climate warming. Better understanding of the interactive effects between warming and other environmental change factors, such as increasing atmosphere nitrogen (N) deposition, is critical for projection of future plant phenology.

METHODS

A 4-year field experiment manipulating temperature and N has been conducted in a temperate steppe in northern China. Phenology, including flowering and fruiting date as well as reproductive duration, of eight plant species was monitored and calculated from 2006 to 2009.

KEY RESULTS

Across all the species and years, warming significantly advanced flowering and fruiting time by 0·64 and 0·72 d per season, respectively, which were mainly driven by the earliest species (Potentilla acaulis). Although N addition showed no impact on phenological times across the eight species, it significantly delayed flowering time of Heteropappus altaicus and fruiting time of Agropyron cristatum. The responses of flowering and fruiting times to warming or N addition are coupled, leading to no response of reproductive duration to warming or N addition for most species. Warming shortened reproductive duration of Potentilla bifurca but extended that of Allium bidentatum, whereas N addition shortened that of A. bidentatum. No interactive effect between warming and N addition was found on any phenological event. Such additive effects could be ascribed to the species-specific responses of plant phenology to warming and N addition.

CONCLUSIONS

The results suggest that the warming response of plant phenology is larger in earlier than later flowering species in temperate grassland systems. The effects of warming and N addition on plant phenology are independent of each other. These findings can help to better understand and predict the response of plant phenology to climate warming concurrent with other global change driving factors.

The global nitrogen cycle in the twenty-first century

Global nitrogen fixation contributes 413 Tg of reactive nitrogen (Nr) to terrestrial and marine ecosystems annually of which anthropogenic activities are responsible for half, 210 Tg N. The majority of the transformations of anthropogenic Nr are on land (240 Tg N yr(-1)) within soils and vegetation where reduced Nr contributes most of the input through the use of fertilizer nitrogen in agriculture. Leakages from the use of fertilizer Nr contribute to nitrate (NO3(-)) in drainage waters from agricultural land and emissions of trace Nr compounds to the atmosphere. Emissions, mainly of ammonia (NH3) from land together with combustion related emissions of nitrogen oxides (NOx), contribute 100 Tg N yr(-1) to the atmosphere, which are transported between countries and processed within the atmosphere, generating secondary pollutants, including ozone and other photochemical oxidants and aerosols, especially ammonium nitrate (NH4NO3) and ammonium sulfate (NH4)2SO4. Leaching and riverine transport of NO3 contribute 40-70 Tg N yr(-1) to coastal waters and the open ocean, which together with the 30 Tg input to oceans from atmospheric deposition combine with marine biological nitrogen fixation (140 Tg N yr(-1)) to double the ocean processing of Nr. Some of the marine Nr is buried in sediments, the remainder being denitrified back to the atmosphere as N2 or N2O. The marine processing is of a similar magnitude to that in terrestrial soils and vegetation, but has a larger fraction of natural origin. The lifetime of Nr in the atmosphere, with the exception of N2O, is only a few weeks, while in terrestrial ecosystems, with the exception of peatlands (where it can be 10(2)-10(3) years), the lifetime is a few decades. In the ocean, the lifetime of Nr is less well known but seems to be longer than in terrestrial ecosystems and may represent an important long-term source of N2O that will respond very slowly to control measures on the sources of Nr from which it is produced.

On the use of epigaeic mosses to biomonitor atmospheric deposition of nitrogen

In this study, we investigated whether the terrestrial moss Pseudoscleropodium purum can be used to biomonitor atmospheric deposition of nitrogen (N). For this purpose, we first determined whether there are any interspecific differences in the concentrations of total N and δ(15)N between the two species of terrestrial moss most commonly used in biomonitoring studies, P. purum and Hypnum cupressiforme. Second, we determined the spatial distribution of N and δ(15)N at small and large scales: (1) by analysis of 165 samples from the surroundings of an aluminium smelter and (2) by analysis of 149 samples from sites forming part of a regular 15 × 15-km sampling network in Galicia (northwest Spain). We did not find any interspecific differences in either total N or δ(15)N. Analysis of δ(15)N enabled us to identify large-scale spatial patterns of distribution that were congruent with the location of the main N emission sources (unlike the analysis of total N). However, we did not identify any such patterns for the small-scale source of N emission studied. The results show that analysis of δ(15)N has an advantage compared with the analysis of total N in that it provides information about the source of N rather than about the amount of N received. Furthermore, isotope discrimination appears to occur, with the bryophytes preferentially accumulating the N(14) isotope. Although this amplifies the signal of reduced forms, it is not problematical for determining spatial-distribution patterns.

Addressing the Impact of Atmospheric Nitrogen Deposition on Western European Grasslands

There is a growing evidence base demonstrating that atmospheric nitrogen deposition presents a threat to biodiversity and ecosystem function in acid grasslands in Western Europe. Here, we report the findings of a workshop held for European policy makers to assess the perceived importance of reactive nitrogen deposition for grassland conservation, identify areas for policy development in Europe and assess the potential for managing and mitigating the impacts of nitrogen deposition. The importance of nitrogen as a pollutant is already recognized in European legislation, but there is little emphasis in policy on the evaluation of changes in biodiversity due to nitrogen. We assess the potential value of using typical species, as defined in the European Union Habitats Directive, for determining the impact of nitrogen deposition on acid grasslands. Although some species could potentially be used as indicators of nitrogen deposition, many of the typical species do not respond strongly to nitrogen deposition and are unlikely to be useful for identifying impact on an individual site. We also discuss potential mitigation measures and novel ways in which emissions from agriculture could be reduced.