Latent soil effects of grazing and ammonium deposition on Deschampsia flexuosa tillers inserted and grown in heather moorland soil

Upland heather moorlands support a range of increasingly rare and threatened biota, making them a priority habitat for conservation and restoration. Over-grazing and nitrogen deposition are two of the most important threats to maintaining these heather moorlands, yet there remains a paucity of research into their combined long-term effects. During the summer of 2008, we established an experiment within an existing research site at Pwllpeiran, mid-Wales. The original site, established in 1996, investigated long-term grazing and N deposition treatments (ammonium and nitrate) on heather moorland. Previous findings from the site, following a decade of treatment, suggested no significant, detectable impacts of treatments on soil C:N ratio and the foliar nitrogen of vegetation.The aim of our study was to investigate short- (2008) and long-term (1996–2008) N deposition treatment impacts, in combination with past grazing (1990–2007), on soil nutrient bioavailability. Soil cores were harvested and aboveground vegetation removed. Tillers of the grass Deschampsia flexuosa were planted into these cores which were then reciprocally transplanted back into the field experiment. The N deposition treatment was continued but grazing was excluded. D. flexuosa biomass changes were next assessed and leaf chemistry investigated using the metabolic fingerprinting method Fourier-transform infrared spectroscopy (FTIR) following three months of growth in the field (May–August 2008).Grazing treatment (on its own) had significant negative impacts on aboveground biomass and significant changes in plant chemistry were also revealed through the metabolic fingerprinting method Fourier transform infrared spectroscopy (FTIR). Short-term N deposition treatments during 2008 had no impacts on D. flexuosa growth or chemistry. There were also no detectable latent effects of long-term nitrate treatments on either growth or chemistry of D. flexuosa. However, plants grown in plots that had received long-term treatments of ammonium (NH₄⁺) had significantly lower poly-phenolic contents (revealed by FTIR) than plants grown in either nitrate (NO₃⁻) or control plots, suggesting detectable latent effects of N application in its reduced form. Further work needs to be undertaken to assess the relevance of residual soil nitrogen pools post N deposition and grazing.     

Response of epiphytic bryophytes to simulated N deposition in a subtropical montane cloud forest in southwestern China

A field manipulation experiment was conducted in a subtropical montane cloud forest in southwestern China to determine the possible responses of epiphytic bryophytes to increasing nitrogen (N) deposition from community to physiology level, and to find sensitive epiphytic bryophytes that may be used as indicators for assessing the degree of N pollution. N addition had significantly negative effects on species richness and cover of the epiphytic bryophyte community. Harmful effects of high N loads were recorded for chlorophyll, growth, and vitality of the species tested. The decline of some epiphytic bryophytes may result from detrimental effects on degradation to photosynthetic pigments. Bazzania himalayana (Mitt.) Schiffn., Bazzania ovistipula (Steph.) Mizut., and Homaliodendron flabellatum (Sm.) Fleisch. are candidates in atmospheric nitrogen monitoring. Epiphytic bryophytes in the montane cloud forest are very sensitive to increasing N deposition and often difficult to recover once they have been destroyed, providing early detection of enhanced N pollution for trees or even the whole forest ecosystem. The inference that increasing N pollution may lead to loss of biodiversity is a concern to the developing economy in western China, and should alert the government to the adverse impacts caused by increased industrial pollution during the process of China’s West Development.     

The potential bias of nitrogen deposition effects on primary productivity and biodiversity

Atmospheric nitrogen (N) deposition is composed of both inorganic nitrogen (IN) and organic nitrogen (ON), and these sources of N may exhibit different impacts on ecosystems. However, our understanding of the impacts of N deposition is largely based on experimental gradients of INs or more rarely ONs. Thus, the effects of N deposition on ecosystem productivity and biodiversity may be biased. We explored the differential impacts of N addition with different IN:ON ratios (0:10, 3:7, 5:5, 7:3, and 10:0) on aboveground net primary productivity (ANPP) of plant community and plant diversity in a typical temperate grassland with a long-term N addition experiment. Soil pH, litter biomass, soil IN concentration, and light penetration were measured to examine the potential mechanisms underlying species loss with N addition. Our results showed that N addition significantly increased plant community ANPP by 68.33%–105.50% and reduced species richness by 16.20%–37.99%. The IN:ON ratios showed no significant effects on plant community ANPP. However, IN-induced species richness loss was about 2.34 times of ON-induced richness loss. Soil pH was positively related to species richness, and they exhibited very similar response patterns to IN:ON ratios. It implies that soil acidification accounts for the different magnitudes of species loss with IN and ON additions. Overall, our study suggests that it might be reasonable to evaluate the effects of N deposition on plant community ANPP with either IN or ON addition. However, the evaluation of N deposition on biodiversity might be overestimated if only IN is added or underestimated if only ON is added.     

Patterns of understory diversity in mixed coniferous forests of southern California impacted by air pollution

The forests of the San Bernardino Mountains have been subject to ozone and nitrogen (N) deposition for some 60 years. Much work has been done to assess the impacts of these pollutants on trees, but little is known about how the diverse understory flora has fared. Understory vegetation has declined in diversity in response to elevated N in the eastern U.S. and Europe. Six sites along an ozone and N deposition gradient that had been part of a long-term study on response of plants to air pollution beginning in 1973 were resampled in 2003. Historic ozone data and leaf injury scores confirmed the gradient. Present-day ozone levels were almost half of these, and recent atmospheric N pollution concentrations confirmed the continued air pollution gradient. Both total and extractable soil N were higher in sites on the western end of the gradient closer to the urban source of pollution, pH was lower, and soil carbon (C) and litter were higher. The gradient also had decreasing precipitation and increasing elevation from west to east. However, the dominant tree species were the same across the gradient. Tree basal area increased during the 30-year interval in five of the sites. The two westernmost sites had 30-45% cover divided equally between native and exotic understory herbaceous species, while the other sites had only 3-13% cover dominated by native species. The high production is likely related to higher precipitation at the western sites as well as elevated N. The species richness was in the range of 24 to 30 in four of the sites, but one site of intermediate N deposition had 42 species, while the easternmost, least polluted site had 57 species. These were primarily native species, as no site had more than one to three exotic species. In three of six sites, 20-40% of species were lost between 1973 and 2003, including the two westernmost sites. Two sites with intermediate pollution had little change in total species number over 30 years, and the easternmost site had more species in 2003. The easternmost site is also the driest and has the most sunlight filtering to the forest floor, possibly accounting for the higher species richness. The confounding effects of the precipitation gradient and possibly local disturbances do not show a simple correlation of air pollution with patterns of native and invasive species cover and richness. Nevertheless, the decline of native species and dominance by exotic species in the two westernmost polluted sites is cause for concern that air pollution is affecting the understory vegetation adversely.     

Plant species richness, elevated CO2, and atmospheric nitrogen deposition alter soil microbial community composition and function

We determined soil microbial community composition and function in a field experiment in which plant communities of increasing species richness were exposed to factorial elevated CO₂ and nitrogen (N) deposition treatments. Because elevated CO₂ and N deposition increased plant productivity to a greater extent in more diverse plant assemblages, it is plausible that heterotrophic microbial communities would experience greater substrate availability, potentially increasing microbial activity, and accelerating soil carbon (C) and N cycling. We, therefore, hypothesized that the response of microbial communities to elevated CO₂ and N deposition is contingent on the species richness of plant communities. Microbial community composition was determined by phospholipid fatty acid analysis, and function was measured using the activity of key extracellular enzymes involved in litter decomposition. Higher plant species richness, as a main effect, fostered greater microbial biomass, cellulolytic and chitinolytic capacity, as well as the abundance of saprophytic and arbuscular mycorrhizal (AM) fungi. Moreover, the effect of plant species richness on microbial communities was significantly modified by elevated CO₂ and N deposition. For instance, microbial biomass and fungal abundance increased with greater species richness, but only under combinations of elevated CO₂ and ambient N, or ambient CO₂ and N deposition. Cellobiohydrolase activity increased with higher plant species richness, and this trend was amplified by elevated CO₂. In most cases, the effect of plant species richness remained significant even after accounting for the influence of plant biomass. Taken together, our results demonstrate that plant species richness can directly regulate microbial activity and community composition, and that plant species richness is a significant determinant of microbial response to elevated CO₂ and N deposition. The strong positive effect of plant species richness on cellulolytic capacity and microbial biomass indicate that the rates of soil C cycling may decline with decreasing plant species richness.     

Impact of Land-Use Intensity and Productivity on Bryophyte Diversity in Agricultural Grasslands

While bryophytes greatly contribute to plant diversity of semi-natural grasslands, little is known about the relationships between land-use intensity, productivity, and bryophyte diversity in these habitats. We recorded vascular plant and bryophyte vegetation in 85 agricultural used grasslands in two regions in northern and central Germany and gathered information on land-use intensity. To assess grassland productivity, we harvested aboveground vascular plant biomass and analyzed nutrient concentrations of N, P, K, Ca and Mg. Further we calculated mean Ellenberg indicator values of vascular plant vegetation. We tested for effects of land-use intensity and productivity on total bryophyte species richness and on the species richness of acrocarpous (small & erect) and pleurocarpous (creeping, including liverworts) growth forms separately. Bryophyte species were found in almost all studied grasslands, but species richness differed considerably between study regions in northern Germany (2.8 species per 16 m²) and central Germany (6.4 species per 16 m²) due environmental differences as well as land-use history. Increased fertilizer application, coinciding with high mowing frequency, reduced bryophyte species richness significantly. Accordingly, productivity estimates such as plant biomass and nitrogen concentration were strongly negatively related to bryophyte species richness, although productivity decreased only pleurocarpous species. Ellenberg indicator values for nutrients proved to be useful indicators of species richness and productivity. In conclusion, bryophyte composition was strongly dependent on productivity, with smaller bryophytes that were likely negatively affected by greater competition for light. Intensive land-use, however, can also indirectly decrease bryophyte species richness by promoting grassland productivity. Thus, increasing productivity is likely to cause a loss of bryophyte species and a decrease in species diversity.     

Bryophyte biomass and species richness on the Park Grass Experiment,Rothamsted, UK

The relationships between bryophyte biomass and species richness and soil pH, nutrient applications and vascular plant biomass and species richness were analyzed for the Park Grass Experiment (Rothamsted, UK). The study examined the abundance of bryophytes in relation to long-term fertilizer and lime application and to fertilizer treatments recently being ceased on some plots. The probability of bryophytes being present on a plot increased with increasing soil pH, and on plots at soil pH 3.3–4.5, the lowest values in this experiment, there were virtually no mosses present. Total bryophyte biomass decreased with increasing vascular plant biomass and vascular plant richness. Both bryophyte biomass and species richness showed a curvilinear response to soil pH. Bryophyte biomass was markedly increased on plots where nitrogen (N) fertilization had recently been ceased. The abundance of the common bryophyte species showed individualistic responses to treatments. N had a negative effect on the abundance of Brachythecium rutabulum. Increasing soil pH, and the application of phosphorus (P) and potassium (K) fertilizer together, had a positive effect on Eurhynchium praelongum. This species was also negatively affected by N, but tolerated larger amounts of it (100–150 kg ha^–1 N) than B. rutabulum. An ephemeral moss, Bryum subapiculatum, had a unimodal response to soil pH but showed no response to N, P, K or other explanatory variables.     

Relationships between species richness patterns in deciduous forests at the north Estonian limestone escarpment

Abstract. The flora of the deciduous forests at the base of the north Estonian limestone escarpment is species rich, with an exceptionally high number of rare bryophyte species. Relationships between species richness of bryophyte and herb layers and biotic and environmental conditions were studied, using General Linear Mixed Models. Human disturbance (waste deposit, tree damage etc) was significantly negatively correlated with species richness of both plant layers. Soil nitrogen content was negatively and soil water retention positively correlated with bryophyte species richness, while herb richness was unrelated to soil factors. After eliminating the effects of environment, negative correlations in species richness and cover between the bryophyte and herb layers were discovered on finer scales (1 m²), referring to biotic interactions. This relationship was obscured with the simple correlation analysis. On the other hand, the positive correlation in species pools between the bryophyte and herb layers (0.1 ha) was insignificant. The species pools of both bryophyte and herb layers were significantly positively correlated with the species richness of the tree layer. In summary, bryophyte and herb layer richness responded differently to environmental conditions, but human disturbance significantly decreased the richness of both layers. Due to the uniqueness and small area of these forests we recommend protection and restoration of disturbed sites.     

Lignin signatures show the effects of changes in heather and bracken cover on the composition of organic matter in a moorland soil profile

Bracken (Pteridium aquilinum) is aggressively displacing heather (Calluna vulgaris) on many moorlands in Britain. We investigated the use of lignin derivatives to identify the distribution of soil organic matter (SOM) derived from bracken in moorland soil profiles formed under heather. Phenylpropanoids extracted from recently senesced litters, roots and SOM, using alkaline CuO oxidation, showed distinct signatures for bracken and heather, with vanillyl moieties dominating bracken litter extracts and vanillyl and syringyl dominating heather litter extracts. Ratios of vanillyl and syringyl concentrations characterised the SOM derived from heather and bracken better than the concentrations of the individual moieties. The analysis showed up to a depth of 5 cm under pure bracken cover, and at the interface between heather and bracken, the SOM was largely derived from bracken litter but below that depth SOM was apparently derived from heather. The use of these methods to identify the plant origin of SOM not only enables understanding the effects of changing vegetation cover on organic matter dynamics in moorland soils but could also facilitate management techniques in moorland/heathland restoration which involve the removal of comparatively nutrient-rich SOM derived from bracken. Received: 30 November 1997 / Accepted: 11 April 1998     

Competition between heather and grasses on Scottish moorlands: Interacting effects of nutrient enrichment and grazing regime

Abstract. Considerable losses and degradation of heathlands (in moorlands and lowlands) have been reported across Europe, with Calluna vulgaris (heather) being replaced by other species, often grasses. Increasing atmospheric nitrogen deposition and overgrazing have been suggested as the driving factors behind this change. This possibility was investigated in a study of the interacting effects of nutrient inputs and grazing on heather and three grass species (Nardus stricta, Deschampsia cespitosa and D. flexuosa) in the field, on a moorland in northeastern Scotland. In addition, the interacting effects of increasing nutrients and Calluna canopy height on N. stricta and D. cespitosa were studied using turves in an outdoor experimental area. In the field, fencing had a larger effect than fertilizer on the growth of all species, except for N. stricta, the species most unpalatable to herbivores. Fencing led to an increase in the height of the Calluna canopy, which may reduce light availability for the grasses. In the turf experiment, the height of the Calluna canopy affected the diameter of the grass tussocks and percentage of green matter (i.e. live leaf material), with plants under the more closed Calluna canopies being smaller. This study suggests that the slow‐growing, evergreen Calluna is a more effective competitor than the faster growing grasses when it has a tall, intact canopy, even at increased levels of nutrient supply. However, overgrazing promotes gap formation in the Calluna canopy, providing an opportunity for grasses to take advantage of increased nutrients. Thus the conservation of heather moorlands requires an understanding of the grazing level which allows Calluna to maintain sufficient canopy structure to outcompete grasses for light.     

The Role of Nitrogen Deposition in Widespread Plant Community Change Across Semi-natural Habitats

Experimental studies have shown that deposition of reactive nitrogen is an important driver of plant community change, however, most of these experiments are of short duration with unrealistic treatments, and conducted in regions with elevated ambient deposition. Studies of spatial gradients of pollution can complement experimental data and indicate whether the potential impacts demonstrated by experiments are actually occurring in the ‘real world’. However, targeted surveys exist for only a very few habitats and are not readily comparable. In a coordinated campaign, we determined the species richness and plant community composition of five widespread, semi-natural habitats across Great Britain in sites stratified along gradients of climate and pollution, and related these ecological parameters to major drivers of biodiversity, including climate, pollution deposition, and local edaphic factors. In every habitat, we found reduced species richness and changed species composition associated with higher nitrogen deposition, with remarkable consistency in relative species loss across ecosystem types. Whereas the diversity of mosses, lichens, forbs, and graminoids declines with N deposition in different habitats, the cover of graminoids generally increases. Considered alongside previous experimental studies and survey work, our results provide a compelling argument that nitrogen deposition is a widespread and pervasive threat to terrestrial ecosystems.     

Long-Term Nitrogen Addition Leads to Loss of Species Richness Due to Litter Accumulation and Soil Acidification in a Temperate Steppe

Background

Although community structure and species richness are known to respond to nitrogen fertilization dramatically, little is known about the mechanisms underlying specific species replacement and richness loss. In an experiment in semiarid temperate steppe of China, manipulative N addition with five treatments was conducted to evaluate the effect of N addition on the community structure and species richness.

Methodology/Principal Findings

Species richness and biomass of community in each plot were investigated in a randomly selected quadrat. Root element, available and total phosphorus (AP, TP) in rhizospheric soil, and soil moisture, pH, AP, TP and inorganic N in the soil were measured. The relationship between species richness and the measured factors was analyzed using bivariate correlations and stepwise multiple linear regressions. The two dominant species, a shrub Artemisia frigida and a grass Stipa krylovii, responded differently to N addition such that the former was gradually replaced by the latter. S. krylovii and A. frigida had highly-branched fibrous and un-branched tap root systems, respectively. S. krylovii had higher height than A. frigida in both control and N added plots. These differences may contribute to the observed species replacement. In addition, the analysis on root element and AP contents in rhizospheric soil suggests that different calcium acquisition strategies, and phosphorus and sodium responses of the two species may account for the replacement. Species richness was significantly reduced along the five N addition levels. Our results revealed a significant relationship between species richness and soil pH, litter amount, soil moisture, AP concentration and inorganic N concentration.

Conclusions/Significance

Our results indicate that litter accumulation and soil acidification accounted for 52.3% and 43.3% of the variation in species richness, respectively. These findings would advance our knowledge on the changes in species richness in semiarid temperate steppe of northern China under N deposition scenario.     

The impact of elevated CO2, increased nitrogen availability and biodiversity on plant tissue quality and decomposition

Elevated CO₂, increased nitrogen (N) deposition and increasing species richness can increase net primary productivity (NPP). However, unless there are comparable changes in decomposition, increases in productivity will most likely be unsustainable. Without comparable increases in decomposition nutrients would accumulate in dead organic matter leading to nutrient limitations that could eventually prohibit additional increases in productivity. To address this issue, we measured aboveground plant and litter quality and belowground root quality, as well as decomposition of aboveground litter for one and 2‐year periods using in situ litterbags in response to a three‐way factorial manipulation of CO₂ (ambient vs. 560 ppm), N deposition (ambient vs. the addition of 4 g N m⁻² yr⁻¹) and plant species richness (one, four, nine and 16 species) in experimental grassland plots. Litter chemistry responded to the CO₂, N and plant diversity treatments, but decomposition was much less responsive. Elevated CO₂ induced decreases in % N and % lignin in plant tissues. N addition led to increases in % N and decreases in % lignin. Increasing plant diversity led to decreases in % N and % lignin and an increase in % cellulose. In contrast to the litter chemistry changes, elevated CO₂ had a much lower impact on decomposition and resulted in only a 2.5% decrease in carbon (C) loss. Detectable responses were not observed either to N addition or to species richness. These results suggest that global change factors such as biodiversity loss, elevated CO₂ and N deposition lead to significant changes in tissue quality; however, the response of decomposition is modest. Thus, the observed increases in productivity at higher diversity levels and with elevated CO₂ and N fertilization are not matched by an increase in decomposition rates. This lack of coupled responses between production and decomposition is likely to result in an accumulation of nutrients in the litter pool which will dampen the response of NPP to these factors over time.     

Responses of litter decomposition and nutrient release rate to water and nitrogen addition differed among three plant species dominated in a semi-arid grassland

Background and aims

Precipitation and nitrogen (N) deposition are predicted to increase in northern China. The present paper aimed to better understand how different dominant species in semi-arid grasslands in this region vary in their litter decomposition and nutrient release responses to increases in precipitation and N deposition.

Methods

Above-ground litter of three dominant species (two grasses, Agropyron cristatum and Stipa krylovii, and one forb, Artemisia frigida) was collected from areas without experimental treatments in a semi-arid grassland in Inner Mongolia. Litter decomposition was studied over three years to determine the effects of water and N addition on litter decomposition rate and nutrient dynamics.

Results

Litter mass loss and nutrient release were faster for the forb species than for the two grasses during decomposition. Both water and N addition increased litter mass loss of the grass A. cristatum, while the treatments showed no impacts on that of the forb A. frigida. Supplemental N had time-dependent, positive effects on litter mass loss of the grass S. krylovii. During the three-year decomposition study, the release of N from litter was inhibited by N addition for the three species, and it was promoted by water addition for the two grasses. Across all treatments, N and potassium (K) were released from the litter of all three species, whereas calcium (Ca) was accumulated. Phosphorus (P) and magnesium (Mg) were released from the forb litter but accumulated in the grass litter after three years of decomposition.

Conclusions

Our findings revealed that the litter decomposition response to water and N supplementation differed among dominant plant species in a semi-arid grassland, indicating that changes in dominant plant species induced by projected increases in precipitation and N deposition are likely to affect litter decomposition, nutrient cycling, and further biogeochemical cycles in this grassland. The asynchronous nutrient release of different species’ litter found in the present study highlights the complexity of nutrient replenishment from litter decomposition in the temperate steppe under scenarios of enhancing precipitation and N deposition.

     

Loss of forb diversity in relation to nitrogen deposition in the UK: regional trends and potential controls

In this study we investigate the impact of nitrogen (N) deposition on the diversity of three different vegetation functional groups – forbs, grasses and mosses – using a field survey of acid grasslands across Great Britain. Our aim is to identify the vegetation types that are most vulnerable to enhanced N deposition, and to shed light on the mechanisms that may be driving N‐initiated species changes in the UK. Sixty‐eight randomly selected grasslands belonging to the UK National Vegetation Classification group U4 (Festuca ovina–Agrostis capillaris–Galium saxatile grassland) were studied along a gradient of atmospheric N deposition ranging from 6 to 36 kg N ha^?1 yr^?1. At each site, vegetation was surveyed and samples were taken from the topsoil and subsoil. Aboveground plant material was collected from three species: a forb, grass and moss. Both the species richness and cover of forbs declined strongly with increasing N deposition, from greater than eight species/20% cover per m² quadrat at low levels of N to fewer than two species/5% cover at the highest N deposition levels. Grasses showed a weak but significant decline in species richness, and a trend toward increasing cover with increasing N input. Mosses showed no trends in either species richness or cover. Most of the decline in plant species richness could be accounted for by the level of ammonium deposition. Soil KCl‐extractable ammonium concentration showed a significant positive correlation with N input, but there was no relationship between N deposition and extractable nitrate. In the soil O/A horizon, there was no relationship between N deposition and %N, and only a very weak positive relationship between the level of N deposition and the C : N ratio. Finally, in the vegetation, there was no relationship between N deposition and either shoot tissue N concentration or N : P ratio for any of the three reference species. Combining our regional survey with the results of published N‐addition experiments provides compelling evidence that there has been a significant decline in the species richness and cover of forbs across Great Britain, and that the primary cause is competition due to an increase in the cover of grasses in response to enhanced deposition of reactive N, primarily NH₄⁺.     

模拟氮沉降对两种竹林不同凋落物组分分解过程养分释放的影响

利用原位分解袋法研究了华西雨屏区苦竹(Pleioblastus amarus)和撑绿杂交竹(Bambusa pervariabilis × Dendrocala mopsi)人工林几种凋落物组分在模拟氮沉降下分解过程中养分释放状态,试验周期为2 a。氮沉降水平分别为对照(CK, 0 g · m^-2 · a^-1)、低氮(5 g · m^-2 · a^-1)、中氮(15 g · m^-2 · a^-1)和高氮(30 g · m^-2 · a^-1),每月下旬定量地对各处理施氮(NH₄NO₃)。结果表明,苦竹林和杂交竹林凋落物主要由凋落叶、凋落箨和凋落枝组成,其中凋落叶约占80%;两个竹种凋落物在分解过程中养分元素释放的种间差异主要与初始养分元素含量有关;凋落物养分元素初始含量对元素释放模式和最终净释放率的大小具有重要的决定作用;目前,这两种竹林生态系统土壤氮输入主要以大气氮沉降(8.24 g · m^-2 · a^-1)为主,同时凋落物氮输入(苦竹和杂交竹林分别为1.93,5.07 g · m^-2 · a^-1)也是一个重要途径;模拟氮沉降对苦竹凋落物碳、磷、钾、钙元素和杂交竹凋落物碳、氮、磷、钾、钙、镁元素释放的抑制作用较弱,处理与对照之间元素总释放率差异一般小于10%;氮沉降显著抑制了苦竹林凋落物氮元素释放,减小幅度为19.0%-27.2%,但由于氮沉降增加对土壤肥力的直接改良作用,氮沉降的增加并不会因为凋落物分解速率的降低造成植物生长所需养分供应的减少;从短期来看,在氮沉降继续增加的情况下,该地区这类竹林生态系统的碳吸存能力仍可能会因为N沉降对植物生长的促进作用而增加。     

Plant species richness in a natural Argentinian matorral shrub-land correlates negatively with levels of plant phosphorus

The aim of this study was to ascertain whether there is a relationship between plant species richness and plant-available N, P and water in an environment subject to little anthropogenic disturbance. To accomplish this we studied the vegetation in matorral shrub-lands in northern Patagonia, Argentina. Due to the variation in slope, precipitation and aspect between the sites water status was determined using the ¹²C/¹³C fraction, ??¹³C, to investigate whether this was a confounding factor. The numbers of herb, shrub, liana and tree species were determined at 20 sites along an estimated precipitation gradient. Leaf P and N content and the ??¹³C of Berberis buxifolia were determined, as well as the soil P and N content at the different sites. A negative correlation was found between species richness and Berberis buxifolia foliar P concentration (52% of the species richness variation was accounted for), and a positive correlation was found between plant species richness and Berberis buxifolia foliar N: P ratios (54% of the species richness variation was accounted for). The relationship between species richness and foliar P was seen when all layers of vegetation were included (trees, lianas, shrubs and herbs). Foliar N showed no correlation with species richness, while soil extractable NH₄ showed a weak positive correlation with the number of shrub layer species (lianas, shrubs and trees). The species richness of the shrub layer increased with decreasing values of ??¹³C. Low soil P availability thus affects local species richness in the matorral shrub-lands of Patagonia in Argentina although the growth of vegetation in the area has been shown to be limited by N. We suggest that low P levels increase plant species richness because low soil P concentration is associated with a high P partitioning and high potential for niche separation.     

Differential effects of ammonium and nitrate deposition on fen phanerogams and bryophytes

Question: High atmospheric nitrogen (N) deposition has been shown to affect productivity and species composition of terrestrial ecosystems. This study focused on the differential effects of the two inorganic N forms in atmospheric deposition (i.e. ammonium and nitrate). Methods and location: Nutrient addition experiments were carried out during 4 years in a mesotrophic fen in a low‐deposition area in Ireland. In a factorial design, plots were fertilized with ammonium and/or nitrate, in two doses comparable with 35 and 70 kg N ha^?1 y^?1 and compared with an unfertilized control. Results: Vascular plant biomass as well as bryophyte biomass were not affected by N dose but showed significantly different responses to the N form. In the ammonium‐fertilized plots, vascular plant biomass was higher and moss biomass was lower than the control, while nitrate additions had no effect. Vascular plant species density was high (16 species per 0.49 m²) and was not affected by any of the treatments; bryophyte species density was also high (seven species per 0.04 m²) but showed a significant decrease upon ammonium fertilization. Conclusion: The vulnerability of the mesotrophic vegetation to enhanced atmospheric N deposition depends strongly on the N form. If N would be mainly deposited as NO_x, no detrimental effects on the vegetation will occur. If, however, the deposition is mainly in the form of NH_y, the bryophyte vegetation will be seriously damaged, while the vascular plant vegetation will show an increased biomass production with possible shifts in dominance from Carex and herb species to grasses and shrubs.     

The roles of biotic resistance and nitrogen deposition in regulating non-native understory plant diversity

Understanding the mechanisms that allow for plant invasions is important for both ecologists and land managers, due to both the environmental and economic impacts of native biodiversity losses. We conducted an observational field study in 2008 to examine the relationship between native and non-native forest understory plant species and to investigate the influence of soil nitrogen (N) on plant community richness and diversity. In 2009, we conducted a companion fertilization experiment to investigate how various forms of N deposition (inorganic and organic) influenced native and non-native species richness and diversity. We found that native species richness and diversity were negatively correlated with 1) non-native species richness and diversity and 2) higher total soil inorganic N. In the deposition experiment, adding organic N fertilizers decreased native richness and diversity compared to inorganic N fertilizers. Together, these results indicate that increasing soil N can be detrimental to native species; however, native species richness and diversity may counteract the N-stimulation of non-native species. Furthermore, the negative effects of organic N deposition on native plants may be just as strong, if not stronger, than the effects of inorganic N deposition.     

Changes in species composition of European acid grasslands observed along a gradient of nitrogen deposition

Question: Which environmental variables affect floristic species composition of acid grasslands in the Atlantic biogeographic region of Europe along a gradient of atmospheric N deposition? Location: Transect across the Atlantic biogeographic region of Europe including Ireland, Great Britain, Isle of Man, France, Belgium, The Netherlands, Germany, Norway, Denmark and Sweden. Materials and Methods: In 153 acid grasslands we assessed plant and bryophyte species composition, soil chemistry (pH, base cations, metals, nitrate and ammonium concentrations, total C and N, and Olsen plant available phosphorus), climatic variables, N deposition and S deposition. Ordination and variation partitioning were used to determine the relative importance of different drivers on the species composition of the studied grasslands. Results: Climate, soil and deposition variables explained 24% of the total variation in species composition. Variance partitioning showed that soil variables explained the most variation in the data set and that climate and geographic variables accounted for slightly less variation. Deposition variables (N and S deposition) explained 9.8% of the variation in the ordination. Species positively associated with N deposition included Holcus mollis and Leontodon hispidus. Species negatively associated with N deposition included Agrostis curtisii, Leontodon autumnalis, Campanula rotundifolia and Hylocomium splendens. Conclusion: Although secondary to climate gradients and soil biogeochemistry, and not as strong as for species richness, the impact of N and S deposition on species composition can be detected in acid grasslands, influencing community composition both directly and indirectly, presumably through soil‐mediated effects.