Nitrogen to phosphorus ratios of two understory plant species in response to nitrogen and phosphorus addition in tropical forest of southern China北大核心CSCD

The secondary tropical forests in southern China have suffered from frequent human disturbance and increasing high N deposition. In order to explore the nutrient limitation status in secondary tropical forests of South China, this 3-year field experiment of nitrogen (+N) and phosphorus (+P) addition investigated nitrogen (N) and phosphorus (P) concentrations of the aboveground tissue (leaf and branch) of two widely distributed understory native species Clerodendrum cyrtophyllum and Uvaria microcarpa in a secondary tropical forest of South China. The results showed that: 1) the N and P concentrations of the two species were significantly different (P < 0.001); N and P concentrations of different tissues in the same species were different; N&P addition greatly affected N and P concentrations in branch rather than new leaf and older leaf. 2) +N treatment had no significant effect on N or P concentrations of either species, but significantly decreased N:P ratios (P = 0.001), at the level of 9% for C. cyrtophyllum and 50% for U. microcarpa, respectively. 3) +P treatment had no significant effect on tissue N concentrations, but significantly increased plant P concentrations (P < 0.001), at 54% for C. cyrtophyllum and 88% for U. microcarpa, respectively; +P treatment significantly decreased plant N:P ratios (P < 0.001), at 28% and 60%, respectively. 4) The alterations of P concentrations of two species had significantly negative correlations with N:P alterations under +N/+P treatment (P < 0.001), suggesting that the alteration of P concentrations in plant tissue was the major driver for N:P alteration. Our results show that N and P addition would affect tissue N and P concentrations of the two species, with +P treatment having relatively greater effect on nutrient concentrations than +N treatment; the branch is more sensitive than new or older leaf in response to nutrient addition. Therefore, P availability may be the limiting factor for plant growth in the tropical forests.     

Not just the usual suspects: insect herbivore populations and communities are associated with multiple plant nutrients

The relationship between plant nutrient content and insect herbivore populations and community structure has long interested ecologists. Insect herbivores require multiple nutrients, but ecologists have focused mostly on nitrogen (an estimate of plant protein content), and more recently phosphorus (P); other nutrients have received little attention. Here we document nutrient variation in grass and forb samples from grassland habitats in central Nebraska using an elemental approach; in total we measured foliar concentrations of 12 elements (N and P, plus S, B, Ca, Mg, Na, K, Zn, Fe, Mn, and Cu). We detected significant variability among sites for N, P, Mg, Na, K, and Cu. We next used a model selection approach to explore how this nutritional variation and plant biomass correlate with grasshopper densities (collectively and at the feeding-guild level), and principal component analysis to explore nutrient correlations with grasshopper community species composition. When all grasshoppers were pooled, densities varied among sites, but only P was associated with abundance of the elements shown to vary between sites. Different responses occurred at the feeding-guild level. For grass specialists, densities were associated with N, plus P, Mg, and Na. For forb specialists, N and P were often associated with density, but associations with Na and K were also observed. Finally, mixed-feeder abundance was strongly associated with biomass, and to a lesser extent P, Mg, Na, and Cu. At the community level, B, Ca, Zn, and Cu, plus biomass, explained > 30% of species composition variation. Our results confirm the positive association of N and P with insect herbivore populations, while suggesting a potential role for Mg, Na, and K. They also demonstrate the importance of exploring effects at the feeding-guild level. We hope our data motivate ecologists to think beyond N and P when considering plant nutrient effects on insect herbivores, and make a call for studies to examine functional responses of insect herbivores to dietary manipulation of Mg, Na, and K. Finally, our results demonstrate correlations between variation in nutrients and species assemblages, but factors not linked to plant nutrient quality or biomass likely explain most of the observed variation.     

Plant polyphenols – implications of different sampling,storage and sample processing in biodiversity-ecosystem functioning experiments

Plant polyphenols are involved in important ecosystem processes and may affect nutrient cycling. Previous investigations have demonstrated detrimental effects of suboptimal sample treatment on the quantity of extractable plant polyphenols. We compared leaf polyphenol concentrations of 20 tree species from East China in two sample sets collected under different conditions: (a) according to established protocols and stored more than three years, (b) under conditions optimised for leaf polyphenols. We investigated the variance brought about by suboptimal sample handling as compared to the variance caused by the taxonomic range of species. Family-affiliation explained the largest proportion of variance, whereas sample handling had only minor effects. Reducing the taxonomic range increased the impact of differences in sample handling. Additionally, we showed that the concentrations of leaf polyphenols were phylogenetically more conserved than other leaf traits. Non-metric-multi-dimensional scaling revealed similar ordination patterns for leaf polyphenol concentrations in both sample sets with both ordinations being closely correlated. Finally, we computed separate ordinations including an extended set of leaf traits and found that both analyses led to similar ecological conclusions. Consequently, in studies comprising a wide taxonomic range, the adverse effects of suboptimal sample handling may be overridden by the variation brought about by phylogeny.     

Controls over foliar N:P ratios in tropical rain forests

Correlations between foliar nutrient concentrations and soil nutrient availability have been found in multiple ecosystems. These relationships have led to the use of foliar nutrients as an index of nutrient status and to the prediction of broadscale patterns in ecosystem processes. More recently, a growing interest in ecological stoichiometry has fueled multiple analyses of foliar nitrogen:phosphorus (N:P) ratios within and across ecosystems. These studies have observed that N:P values are generally elevated in tropical forests when compared to higher latitude ecosystems, adding weight to a common belief that tropical forests are generally N rich and P poor. However, while these broad generalizations may have merit, their simplicity masks the enormous environmental heterogeneity that exists within the tropics; such variation includes large ranges in soil fertility and climate, as well as the highest plant species diversity of any biome. Here we present original data on foliar N and P concentrations from 150 mature canopy tree species in Costa Rica and Brazil, and combine those data with a comprehensive new literature synthesis to explore the major sources of variation in foliar N:P values within the tropics. We found no relationship between N:P ratios and either latitude or mean annual precipitation within the tropics alone. There is, however, evidence of seasonal controls; in our Costa Rica sites, foliar N:P values differed by 25% between wet and dry seasons. The N:P ratios do vary with soil P availability and/or soil order, but there is substantial overlap across coarse divisions in soil type, and perhaps the most striking feature of the data set is variation at the species level. Taken as a whole, our results imply that the dominant influence on foliar N:P ratios in the tropics is species variability and that, unlike marine systems and perhaps many other terrestrial biomes, the N:P stoichiometry of tropical forests is not well constrained. Thus any use of N:P ratios in the tropics to infer larger-scale ecosystem processes must comprehensively account for the diversity of any given site and recognize the broad range in nutrient requirements, even at the local scale.     

Effects of river otter activity on terrestrial plants in trophically altered Yellowstone Lake

Animals that deposit aquatically derived nutrients on terrestrial landscapes link food webs and affect a variety of in situ processes. This phenomenon, however, is poorly documented in freshwater habitats, especially where species introductions have drastically changed an ecosystem's trophic structure. In this study, we used stable isotopes to document water-to-land nutrient transport by river otters (Lontra canadensis) around Yellowstone Lake, an ecosystem recently altered by nonnative species invasions. We then investigated the effects of otter fertilization on plant growth and prevalence at latrine (scent-marking) sites and evaluated how the recent changes to the lake's food web could influence these plant responses. Values of delta15N were higher on latrines compared to non-latrine sites in five of seven sample plant taxa. Additionally, latrine grasses had higher percentage N than those from non-latrines. Foliar delta15N positively related to fecal deposition rate for some plants, indicating that increased otter scent-marking led to a rise in these N values. Logistic regression models indicated that otters selected for well-shaded latrines with access to foraging. Atypical latrines, misclassified as non-latrines by the regression models, had values of delta15N similar to correctly classified latrines, suggesting that site effects alone cannot explain elevated N values at otter latrine sites. No difference in plant diversity or percent cover of N-fixing taxa occurred between latrine and nonlatrine sites, though specific genera did differ between site types. Measurements of shoot lengths indicated increased growth of some latrine currants (Ribes sp.). In Yellowstone Lake, a twofold reduction in otter numbers could result in an even greater decline in nutrient deposition at latrines, as otters may become less social in a system with decreased prey availability. Our results highlight the role of animals in linking aquatic and terrestrial habitats in inland freshwater systems and suggest that ongoing changes in the trophic structure of Yellowstone Lake could have unexpected ramifications well beyond the lake itself.     

The Nutrient Status of Some Graminaceous Species in Britain. 1. Molinia Caerulea (L.) Moench

Above-ground biomass biomass of Molinia caerulea from 36 sites throughout Britain was analysed for concentrations of elements (N, P, K, Ca, Mg, Fe, Mn, Cu, Zn and Na), with matching analyses of soil nutrients, together with pH and LOI. Between-year and seasonal variation in the nutrients was also studied at one of the sites. the grass was found on acid sites ranging between pH 3.1 and 5.8. the soil humus content varied widely, and there were significant correlations between that and elements (N, P, K, Ca, Mg and Na) in the soil. Concentrations of calcium, magnesium, zinc and manganese in the plant material were significantly correlated with extractable levels in the soils. Between year variation in the plant nutrient concentrations at one site was of a similar order to variation between the sites. Generally there was less variation in nitrogen, phosphorus and potassium tissue levels than in calcium, magnesium, zinc and copper. Iron and sodium were the most variable. Tissue concentrations of nitrogen, phosphorus, potassium and copper declined throughout the growing season, whereas magnesium, calcium and iron built up until August/September after which a decline set in as the leaves senesced. the results are discussed in relation to the importance of standardizing the time of sampling, in comparison with concentrations of tissue levels in other plants and the growth strategy of this deciduous grass.     

The Nutrient Status of Some Graminaceous Species in Britain. 1. Molinia Caerulea (L.) Moench

Above-ground biomass biomass of Molinia caerulea from 36 sites throughout Britain was analysed for concentrations of elements (N, P, K, Ca, Mg, Fe, Mn, Cu, Zn and Na), with matching analyses of soil nutrients, together with pH and LOI. Between-year and seasonal variation in the nutrients was also studied at one of the sites. the grass was found on acid sites ranging between pH 3.1 and 5.8. the soil humus content varied widely, and there were significant correlations between that and elements (N, P, K, Ca, Mg and Na) in the soil. Concentrations of calcium, magnesium, zinc and manganese in the plant material were significantly correlated with extractable levels in the soils. Between year variation in the plant nutrient concentrations at one site was of a similar order to variation between the sites. Generally there was less variation in nitrogen, phosphorus and potassium tissue levels than in calcium, magnesium, zinc and copper. Iron and sodium were the most variable. Tissue concentrations of nitrogen, phosphorus, potassium and copper declined throughout the growing season, whereas magnesium, calcium and iron built up until August/September after which a decline set in as the leaves senesced. the results are discussed in relation to the importance of standardizing the time of sampling, in comparison with concentrations of tissue levels in other plants and the growth strategy of this deciduous grass.     

Nitrogen- versus phosphorus-limited growth and sources of nutrients for coral reef macroalgae

Recent investigations of nutrient-limited productivity in coral reef macroalgae have led to the conclusion that phosphorus, rather than nitrogen, is the primary limiting nutrient. In this study, comparison of the dissolved inorganic nitrogen:phosphorus ratio in the water column of Kaneohe Bay, Hawaii, with tissue nitrogen:phosphorus ratios in macroalgae from Kaneohe Bay suggested that nitrogen, rather than phosphorus, generally limits productivity in this system. Results of nutrient-enrichment experiments in a flow-through culture system indicated that inorganic nitrogen limited the growth rates of 8 out of 9 macroalgae species tested. In 6 of the species tested, specific growth rates of thalli cultured in unenriched seawater from the Kaneohe Bay water column were zero or negative after 12 d. These results suggest that, in order to persist in low-nutrient coral reef systems, some macroalgae require high rates of nutrient advection or access to benthic nutrient sources in addition to nutrients in the overlying water column. Nutrient concentrations in water samples collected from the microenvironments inhabited or created by macroalgae were compared to nutrient concentrations in the overlying water column. On protected reef flats, inorganic nitrogen concentrations within dense mats of Gracilaria salicornia and Kappaphycus alvarezii, and inorganic nitrogen and phosphate concentrations in sediment porewater near the rhizophytic algae Caulerpa racemosa and C. sertularioides were significantly higher than in the water column. The sediments associated with these mat-forming and rhizophytic species appear to function as localized nutrient sources, making sustained growth possible despite the oligotrophic water column. In wave-exposed habitats such as the Kaneohe Bay Barrier Reef flat, water motion is higher than at protected sites, sediment nutrient concentrations are low, and zones of high nutrient concentrations do not develop near or beneath macroalgae, including dense Sargassum echinocarpum canopies. Under these conditions, macroalgae evidently depend on rapid advection of low-nutrient water from the water column, rather than benthic nutrient sources, to sustain growth. Received: 1 December 1997 / Accepted: 9 July 1998     

Traits, not origin, explain impacts of plants on larval amphibians

Managing habitats for the benefit of native fauna is a priority for many government and private agencies. Often, these agencies view nonnative plants as a threat to wildlife habitat, and they seek to control or eradicate nonnative plant populations. However, little is known about how nonnative plant invasions impact native fauna, and it is unclear whether managing these plants actually improves habitat quality for resident animals. Here, we compared the impacts of native and nonnative wetland plants on three species of native larval amphibians; we also examined whether plant traits explain the observed impacts. Specifically, we measured plant litter quality (carbon : nitrogen : phosphorus ratios, and percentages of lignin and soluble phenolics) and biomass, along with a suite of environmental conditions known to affect larval amphibians (hydroperiod, temperature, dissolved oxygen, and pH). Hydroperiod and plant traits, notably soluble phenolics, litter C:N ratio, and litter N:P ratio, impacted the likelihood that animals metamorphosed, the number of animals that metamorphosed, and the length of larval period. As hydroperiod decreased, the likelihood that amphibians achieved metamorphosis and the percentage of tadpoles that successfully metamorphosed also decreased. Increases in soluble phenolics, litter N:P ratio, and litter C:N ratio decreased the likelihood that tadpoles achieved metamorphosis, decreased the percentage of tadpoles metamorphosing, decreased metamorph production (total metamorph biomass), and increased the length of larval period. Interestingly, we found no difference in metamorphosis rates and length of larval period between habitats dominated by native and nonnative plants. Our findings have important implications for habitat management. We suggest that to improve habitats for native fauna, managers should focus on assembling a plant community with desirable traits rather than focusing only on plant origin.     

Extending the leaf economics spectrum to decomposition: evidence from a tropical forest

I investigated the relationship between leaf physiological traits and decomposition of leaf litter for 35 plant species of contrasting growth forms from a lowland tropical forest in Panama to determine whether leaf traits could be used to predict decomposition. Decomposition rate (k) was correlated with specific leaf area (SLA), leaf nitrogen (N), phosphorus (P), and potassium (K) across all species. Photosynthetic rate per unit mass (Amass) was not correlated with k, but structural equation modeling showed support for a causal model with significant indirect effects of Amass on k through SLA, N, and P, but not K. The results indicate that the decomposability of leaf tissue in this tropical forest is related to a global spectrum of leaf economics that varies from thin, easily decomposable leaves with high nutrient concentrations and high photosynthetic rates to thick, relatively recalcitrant leaves with greater physical toughness and defenses and low photosynthetic rates. If this pattern is robust across biomes, then selection for suites of traits that maximize photosynthetic carbon gain over the lifetime of the leaf may be used to predict the effects of plant species on leaf litter decomposition, thus placing the ecosystem process of decomposition in an evolutionary context.     

Biomonitoring von Stickstoffimmissionen

Background and aim

Air pollution caused by oxidized and reduced nitrogen is distributed over wide areas of Europe at a high level. As an alternative or complement to physical measurements and modelling calculations, biomonitoring with plants provides techniques to assess amounts and effects of pollution from oxidized and reduced nitrogen compounds (depositions and concentrations). Many of the previously implemented techniques are based on well-proven standardised methods, e.g. documented in VDI guidelines, modified more or less for a biomonitoring of atmospheric nitrogen pollution. This paper gives a review of the techniques for a biomonitoring of atmospheric nitrogen pollution, their possibilities as well as their limitations.

Main features

-diversity of the ground vegetation

-nitrogen accumulation in vascular plants

-exposure of vascular plants

-mapping of epiphytic lichens, bryophytes and algae

-nitrogen accumulation in lichens and bryophytes

-exposure of lichens and bryophytes

Results and Discussion

Important response parameters are nitrogen concentrations in plant tissue (shoot, needle, leave) and biodiversity of plant species. These responses of vascular plants in many cases are influenced by other local varying conditions, in particular the soil. The exposure of vascular plants over a short period provides a standardised quantification of the total atmospheric nitrogen input in a model ecosystem. The enrichment of nitrogen in the plant tissue of bryophytes and lichens from the ground vegetation is closely correlated with the amount of nitrogen deposition. The diversity of epiphytic lichens and the response of exposed Hypogymnia physodes is more sensitive to ammonia than to nitrous oxide, whereas with the diversity of epiphytic bryophytes or the abundance of algae no significant correlation with atmospheric N pollution were found.

Conclusions

Some bioindication techniques provide a causeeffect related, partly standardised biomonitoring of nitrogen pollution. In some cases varying experiences with some biomonitors like the biodiversity of the ground vegetation, exposure of lichens, moss bags and grass cultures stress the need for more research in this subject.     

Urbanization and nutrient retention in freshwater riparian wetlands.

Urbanization can degrade water quality and alter watershed hydrology, with profound effects on the structure and function of both riparian wetlands (RWs) and aquatic ecosystems downstream. We used freshwater RWs in Fairfax County, Virginia, USA, as a model system to examine: (1) the effects of increasing urbanization (indexed by the percentage of impervious surface cover [%ISC] in the surrounding watershed) on nitrogen (N) and phosphorus (P) concentrations in surface soils and plant tissues, soil P saturation, and soil iron (Fe) chemistry; and (2) relationships between RW soil and plant nutrient chemistries vs. the physical and biotic integrity of adjacent streams. Soil total P and NaOH-extractable P (representing P bound to aluminum [Al] and Fe hydrous oxides) varied significantly but nonlinearly with % ISC (r2 = 0.69 and 0.57, respectively); a similar pattern was found for soil P saturation but not for soil total N. Relationships were best described by second-order polynomial equations. Riparian wetlands appear to receive greater P loads in moderately (8.6-13.3% ISC) than in highly (25.1-29.1% ISC) urbanized watersheds. These observations are consistent with alterations in watershed hydrology that occur with increasing urbanization, directing water and nutrient flows away from natural RWs. Significant increases in total and crystalline soil Fe (r2 = 0.57 and 0.53, respectively) and decreases in relative soil Fe crystallinity with increasing %ISC suggest the mobilization and deposition of terrestrial sediments in RWs, likely due to construction activities in the surrounding watershed. Increases in RW plant tissue nutrient concentrations and %ISC in the surrounding watershed were negatively correlated with standard indices of the physical and biotic integrity of adjacent streams. In combination, these data suggest that nutrient and sediment inputs associated with urbanization and storm-water management are important variables that affect wetland ecosystem services, such as water quality improvement, in urbanizing landscapes.     

The floristics and conservation status of sand-dune communities in Wales

An inventory of the vascular plants, bryophytes, fungi and terricolous lichens recorded within the different sand-dune communities in Wales has been compiled and a summary of the importance of these different taxonomic groups to the ecology and conservation of sand dunes is provided. The total floristic assemblage amounted to 945 species, with vascular plants representing 439 of these. Fungi unexpectedly formed the next most important group with ca. 289 species, followed by bryophytes (171 species) and terricolous lichens (66 species). However, very few of these species are confined to sand dunes. Of the vascular plants only ca. 7 % (32 species) could be classed as either wholly dependent or strongly associated with sand dunes, whereas up to 13% (22 species) of the bryophytes fell within this category. Of the fungi only 4% (10 species) appear to be restricted to sand dunes, but none of the lichen species could be classed as being either limited to, or strongly associated with, this habitat. Over 9% (91 species) of the total flora are considered to be rare, scarce or endangered within the UK or Europe, and ca. 8% of the vascular plant species are considered to be endemic or near-endemic to Europe. The inventory therefore not only provides an insight in the overall plant diversity of dune systems in Wales, it also gives an indication of the proportion of species that are under threat, and the numbers of species that are more-or-less totally dependent on sand dunes. In the discussion various relevant sand-dune management issues are addressed.     

Intertidal community structure and oceanographic patterns around Santa Cruz Island, CA, USA

Recent studies suggest that nearshore oceanographic conditions can have important effects on the structure of benthic communities. On Santa Cruz Island (SCI), CA, USA there is a persistent difference in mean annual sea surface temperature (SST) around the island due to its location at the confluence of opposing cold and warm ocean current systems. Over the course of a 4-year study (1997–2001) seawater nutrient and chl-a concentrations, algal tissue C:N ratios, recruitment and growth of filter-feeders (barnacles and mussels), and intertidal community structure were measured at six intertidal sites around the island. There were strong associations between remotely sensed SST and patterns of community structure. Macrophyte abundance was highest at sites with persistently low SST, while recruitment, abundance, and growth of filter-feeding invertebrates were strongly, positively correlated with SST. The cold-water sites were associated with higher nutrient concentrations and lower algal C:N ratios, particularly in the winter months. Values of chl-a were generally low and variable among sites, and were not correlated with the predominant SST gradient. Recruitment of barnacles and mussels was positively correlated with adult abundance across all sites. While detailed experimental studies are needed to further evaluate the mechanisms underlying community dynamics, these results indicate that the confluence of cold- and warm-water masses around SCI may determine the contrasting patterns of intertidal community structure.     

Modelling above-ground herbage mass for a wide range of grassland community types

Whereas it is recognized that management of plant diversity can be the key to reconciling production and environmental aims, most grassland models are tailored for high-value grass species. We proposed to adapt a mono-specific grass model to take into account specific features of species-rich permanent grasslands, especially over the reproductive phase. To this end, we used the concept of plant functional type (PFT), i.e. the grouping of plant species according to plant traits determined by the response of plant species to different management practices (land use and fertilization) and characterizing of agronomic properties of the corresponding species. In the model, weather and nutrient availability act upon rates of biophysical processes (radiation capture and use, plant senescence). These rates are modified over times due to PFT-specific parameters determined experimentally which represent the different strategies of plant species regarding growth. The integration of these parameters into the model made it possible to predict herbage biomass accumulation rate under different management practices for a wide range of plant communities differing in their PFT composition. The model was evaluated in two steps, first by analyzing separately the effects of PFT and an indicator of nutrient availability on herbage accumulation and then by conducting a sensitivity analysis. It was validated using two independent datasets; a cutting experiment running over the whole growing season to examine the consistency of the model outputs under different cutting regimes, and a monitoring of meadows and pastures in spring over a whole growth cycle to assess the model’s ability to reproduce growth curves. Although a good fit was observed between the simulated and observed data, the few discrepancies noticed between field data and predicted values were attributed mainly to the potential presence of non-grass species. More specifically, we noticed that nutrient (mainly nitrogen) availability is the main driver of plant growth rate, and that PFT determines the times at which this rate changes in relation to the phenological characteristics of species present. We concluded that integration of the PFT concept into the initial mono-specific growth model is especially suited to evaluating the consequences of management practices on species-rich permanent grasslands to meet feed production targets.     

Gastropod herbivory in response to elevated CO2 and N addition impacts plant community composition

In this study, the influence of elevated carbon dioxide (CO2) and nitrogen (N) deposition on gastropod herbivory was investigated for six annual species in a California annual grassland community. These experimentally simulated global changes increased availability of important resources for plant growth, leading to the hypothesis that species with the most positive growth and foliar nutrient responses would experience the greatest increase in herbivory. Counter to the expectations, shifts in tissue N and growth rates caused by N deposition did not predict shifts in herbivore consumption rates. N deposition increased seedling N concentrations and growth rates but did not increase herbivore consumption overall, or for any individual species. Elevated CO2 did not influence growth rates nor have a statistically significant influence on seedling N concentrations. Elevated CO2 at ambient N levels caused a decline in the number of seedlings consumed, but the interaction between CO2 and N addition differed among species. The results of this study indicate that shifting patterns of herbivory will likely influence species composition as environmental conditions change in the future; however, a simple trade-off between shifting growth rates and palatability is not evident.     

Light-mediated thresholds in stream-water nutrient composition in a river network

The elemental composition of solutes transported by rivers reflects combined influences of surrounding watersheds and transformations within stream networks, yet comparatively little is known about downstream changes in effects of watershed loading vs. in-channel processes. In the forested watershed of a river under a mediterranean hydrologic regime, we examined the influence of longitudinal changes in environmental conditions on water-column nutrient composition during summer base flow across a network of sites ranging from strongly heterotrophic headwater streams to larger, more autotrophic sites downstream. Small streams (0.1-10 km2 watershed area) had longitudinally similar nutrient concentration and composition with low (approximately 2) dissolved nitrogen (N) to phosphorus (P) ratios. Abrupt deviations from this pattern were observed in larger streams with watershed areas > 100 km2 where insolation and algal abundance and production rapidly increased. Downstream, phosphorus and silica concentrations decreased by > 50% compared to headwater streams, and dissolved organic carbon and nitrogen increased by approximately 3-6 times. Decreasing dissolved P and increasing dissolved N raised stream-water N:P to 46 at the most downstream sites, suggesting a transition from N limitation in headwaters to potential P limitation in larger channels. We hypothesize that these changes were mediated by increasing algal photosynthesis and N fixation by benthic algal assemblages, which, in response to increasing light availability, strongly altered stream-water nutrient concentration and stoichiometry in larger streams and rivers.     

The Nutrient Status of Some Graminaceous Species in Britain. 2. Deschampsia Flexuosa (L.) Trin

Above-ground biomass of Deschampsia flexuosa from 40 sites throughout Britain was analysed for concentrations of elements (N, P, K, Ca, Mg, Fe, Mn, Cu, Zn and Na), with matching analyses of nutrient levels in the soils, together with pH and LOI. Seasonal variation in the nutrients was also studied in detail at one of the sites. the grass was found on acid sites ranging between pH 3.1 and 4.7. the soil humus content varied widely, and there were significant correlations between that and elements (N, P, K, Ca, Mg, Zn and Na) in the soil. Concentrations of zinc and manganese in the plant material were the only elements that significantly correlated with extractable levels in the soils. Tissue concentrations of nitrogen, phosphorus, and potassium declined throughout the growing season, whereas the other elements showed a tendency to build up until the end of September. Calcium, magnesium and manganese concentrations then declined as growth ceased. the results are compared with above-ground concentrations of minerals in other plants and the growth strategy of this perennial grass.     

Nutrient limitation in soils exhibiting differing nitrogen availabilities: what lies beyond nitrogen saturation?

The nature of nutrient limitation in large areas of temperate forest may be changing due to human activities. As N availability in these forests increases, other nutrients could increasingly constrain productivity and other ecosystem processes. To determine the nature of nutrient limitation (N, P, and Ca) in forest soils exhibiting differing N availability, we conducted three field studies in the Fernow Experimental Forest, West Virginia, USA. The first used a ubiquitous herbaceous species, Viola rotundifolia, to compare indices of N availability to the activity of root-associated phosphomonoesterase (PME) activity at two spatial scales. The second study used fertilized, root in-growth cores to assess the extent of N, P, and Ca limitation. Finally, we measured the root-associated PME activity of V. rotundifolia growing in experimental plots that have received various combinations of nutrient additions and harvest treatments. For entire watersheds, stream water nitrate concentrations were positively related to PME activities (R2 = 0.986). For small plots, PME activities were positively associated with soil nitrate availability (R2 = 0.425), and to a lesser extent with the leaf N concentrations (R2 = 0.291). Root growth into microsites fertilized with P was greater than growth into microsites fertilized with either N or Ca, especially in watersheds with high N availability. Experimental additions of N increased the root-associated PME activity of V. rotundifolia, supporting the causality of the relationship between N availability and PME activity. Collectively, our results indicate that, as N availability increases, P becomes increasingly limiting at the sites examined. Understanding how nutrient limitations change during N saturation should improve ecosystem models and better inform our attempts to mitigate any undesired effects.     

The importance of groundwater discharge for plant species number in riparian zones

Riparian zones are hotspots of plant species richness in temperate and boreal biomes. The phenomenon is believed to be caused primarily by river-related processes, and upland influences on riparian zones have received relatively little attention. We investigated the importance of discharge of groundwater derived from uplands on riparian patterns in vascular plant species composition. We found that groundwater discharge areas in riparian zones were 36-209% more species rich than non-discharge areas, depending on spatial scale (1-50 m wide transects from annual high-water levels to summer low-water levels) and river (one free-flowing and one regulated). Higher nitrogen availability and less drought stress during low river stages are suggested as the major causes for the higher species diversity in discharge areas. Riparian zones lacking groundwater discharge lost more species following water-level regulation than did discharge areas. This indicates that groundwater discharge areas are more resistant to regulation because both individual plants and plant populations may grow larger in discharge areas. These results demonstrate that riparian zones are controlled by water and nutrient input from upland parts of catchments in ways that have been overlooked despite more than three decades of research into linkages between stream ecosystems and their valleys.