Co-limitation of plant primary productivity by nitrogen and phosphorus in a species-rich wooded meadow on calcareous soils

High small-scale species richness of calcareous grasslands is generally thought to result from evening of species competitive potentials by limited N availability, because of relatively low herb N/P ratios in these communities. However, P mobility is low in alkaline soils as well. We studied soil chemistry and productivity of herb and moss layers in a very diverse calcareous meadow (up to 76 vascular plant species per m²) to test the hypotheses of a co-limitation of herb productivity by both soil N and P availabilities and moss productivity primarily by P availability. The effect of nutrient supply on productivity was investigated using both a natural productivity gradient as well as fertilization experiments. We observed strong positive correlations of soil P availability and total soil N with the above-ground productivity of herb layer. A long-term fertilization experiment demonstrated that P alone and N and P together increased productivity of vascular species, and that the productivity continuously declined after cessation of fertilization with the effect of previous fertilization occasionally visible even 14 years after treatment termination. A short-term fertilization experiment further demonstrated that N and P when supplied alone increase productivity of vascular plants, suggesting that both elements were limiting. Furthermore, there was a significant interaction between N and P on productivity, indicating that simultaneous N and P supply increased productivity more than separate nutrient additions. Moss productivity was negatively associated with vascular plant productivity. In particular, N addition decreased moss productivity, but moss productivity did not decline in P addition treatments. P requirements of mosses were larger than those of vascular plants. Our data indicate co-limitation of herb productivity by both soil N and P in this highly diverse grassland, while limitation of moss productivity mainly by P. We suggest that N and P co-limitations are common in calcareous diverse grasslands, and may partly explain the extreme small-scale species diversity in these communities.     

Nutrient limitation in species‐rich Calthion grasslands in relation to opportunities for restoration in a peat meadow landscape

Questions: Which nutrient(s) limit(s) vegetation productivity in Calthion grasslands? Is phosphorus release a bottleneck for restoration of species‐rich Calthion grasslands on rewetted dairy meadows? Location: Three species‐rich Calthion grasslands in the Western Peat District in the Netherlands. Methods: We conducted a field fertilization experiment with nitrogen (N), phosphorus (P) and potassium (K) in three existing Calthion grasslands to evaluate the potential for restoration on rewetted dairy meadows. Responses of above‐ground biomass, tissue nutrient concentrations and nutrient ratios were determined after 2 yr of fertilization. Results: Biomass increased with fertilization with N‐only and K‐only but did not react to P‐only additions. Comparisons of tissue nutrient concentrations and nutrient ratios also gave indications of N and K limitation. Conclusions: The strong P release expected after rewetting should not necessarily interfere with restoration of Calthion communities on rewetted dairy meadows. It is concluded that for successful restoration management measures should focus on reducing N and/or K availability. Potassium might be an overlooked bottleneck in the restoration of species‐rich grasslands.     

Nutrient resorption in tropical savanna forests and woodlands of central Brazil

Nutrient limitation in Brazilian savanna (known as cerrado) presumably causes trees to maximize nutrient resorption from senesced leaves to reduce their dependence on nutrient availability. To assess patterns between nutrient resorption and soil fertility, we measured community-level nitrogen (N), phosphorus (P), and potassium (K) concentrations in mature and senesced leaves and soil fertility in the upper 50 cm soil layer in structurally diverse cerrado ecosystems in the Cuiaba Basin (CB) and Pantanal (PAN) of Mato Grosso, Brazil. Foliar nutrient concentration data were used to estimate resorption efficiency and proficiency, and correlation was used to determine whether resorption efficiency and proficiency varied across soil fertility gradients. We found that N and P resorption proficiency (NRP and PRP, respectively) and P resorption efficiency (PRE) increased significantly as total soil N (NRP) and extractable P (PRP and PRE) declined. In contrast, K resorption efficiency (KRE) declined as soil sand content and bulk density increased, which was likely due to a reduction in soil water-holding capacity. Leaf N/P ratios indicate potential N limitation and/or N + P co-limitation for ecosystems in the PAN and P limitation and/or N + P co-limitation for ecosystems in the CB, while trends in leaf N/K ratios indicate possible K or K + P co-limitation for the CB only. Our results illustrate that cerrado forests and woodlands have highly variable nutrient resorption capacities that vary predictably across soil fertility or textural gradients and indicate that cerrado communities have flexible nutrient resorption that can reduce their dependence on soil nutrient availability.     

Balancing positive and negative plant interactions: how mosses structure vascular plant communities

Our understanding of positive and negative plant interactions is primarily based on vascular plants, as is the prediction that facilitative effects dominate in harsh environments. It remains unclear whether this understanding is also applicable to moss–vascular plant interactions, which are likely to be influential in low-temperature environments with extensive moss ground cover such as boreal forest and arctic tundra. In a field experiment in high-arctic tundra, we investigated positive and negative impacts of the moss layer on vascular plants. Ramets of the shrub Salix polaris, herb Bistorta vivipara, grass Alopecurus borealis and rush Luzula confusa were transplanted into plots manipulated to contain bare soil, shallow moss (3 cm) and deep moss (6 cm) and harvested after three growing seasons. The moss layer had both positive and negative impacts upon vascular plant growth, the relative extent of which varied among vascular plant species. Deep moss cover reduced soil temperature and nitrogen availability, and this was reflected in reduced graminoid productivity. Shrub and herb biomass were greatest in shallow moss, where soil moisture also appeared to be highest. The relative importance of the mechanisms by which moss may influence vascular plants, through effects on soil temperature, moisture and nitrogen availability, was investigated in a phytotron growth experiment. Soil temperature, and not nutrient availability, determined Alopecurus growth, whereas Salix only responded to increased temperature if soil nitrogen was also increased. We propose a conceptual model showing the relative importance of positive and negative influences of the moss mat on vascular plants along a gradient of moss depth and illustrate species-specific outcomes. Our findings suggest that, through their strong influence on the soil environment, mat-forming mosses structure the composition of vascular plant communities. Thus, for plant interaction theory to be widely applicable to extreme environments such as the Arctic, growth forms other than vascular plants should be considered.     

Long-term changes in vegetation and soil chemistry in a calcareous and sandy semi-natural grassland

Calcicole plant species are vulnerable to acidification and fertilization, caused by deposition and changes in land use, since they are adapted to nutrient-poor calcareous conditions. In this study we used vegetation data (vascular plants, bryophytes and lichens) from 1964 and 1985 and stored soil samples from 1966 to investigate long-term soil chemistry and vegetation changes in a semi-natural, sandy calcareous grassland in southern Sweden. In the re-investigation in 2008 we found that increased decalcification due to acidification could not be verified. The plant community had changed from stress-tolerant calcareous grassland towards a community promoted by higher nutrient availability. Furthermore, the cover of species indicating calcareous conditions had decreased. A decline in the cover of species adapted to alkaline, phosphorus-poor conditions may be due to increased nutrient availability, but there were also indications that the vegetation had changed due to overgrowth by woody plants. This long-term impoverishment of the plant community highlights the need for appropriate management of calcareous grasslands, in order to limit the nutrients available in the soil and prevent overgrowth by shrubs and trees.     

Alpine plant functional group responses to fertiliser addition depend on abiotic regime and community composition

Background and aims

We ask how productivity responses of alpine plant communities to increased nutrient availability can be predicted from abiotic regime and initial functional type composition.

Methods

We compared four Caucasian alpine plant communities (lichen heath, Festuca varia grassland, Geranium-Hedysarum meadow, snow bed community) forming a toposequence and contrasting in productivity and dominance structure for biomass responses to experimental fertilization (N, P, NP, Ca) and irrigation for 4–5?years.

Results

The dominant plants in more productive communities monopolized added N and P, at the expense of their neighbors. In three out of four communities, N and P fertilizations gave greater aboveground biomass increase than N or P fertilization alone, indicating overall co-limitation of N and P, with N being most limiting. Relative biomass increase in NP treatment was negatively related to biomass in control plots across the four communities. Grasses often responded more vigorously to P, but sedges to N alone. Finally, we present one of the rare examples of a forb showing a strong N or NP response.

Conclusion

Our findings will help improve our ability to predict community composition and biomass dynamics in cool ecosystems subject to changing nutrient availability as induced by climate or land-use changes.     

Multi‐nutrient vs. nitrogen‐only effects on carbon sequestration in grassland soils

Human activities have greatly increased the availability of biologically active forms of nutrients [e.g., nitrogen (N), phosphorous (P), potassium (K), magnesium (Mg)] in many soil ecosystems worldwide. Multi‐nutrient fertilization strongly increases plant productivity but may also alter the storage of carbon (C) in soil, which represents the largest terrestrial pool of organic C. Despite this issue is important from a global change perspective, key questions remain on how the single addition of N or the combination of N with other nutrients might affect C sequestration in human‐managed soils. Here, we use a 19‐year old nutrient addition experiment on a permanent grassland to test for nutrient‐induced effects on soil C sequestration. We show that combined NPKMg additions to permanent grassland have ‘constrained’ soil C sequestration to levels similar to unfertilized plots whereas the single addition of N significantly enhanced soil C stocks (N‐only fertilized soils store, on average, 11 t C ha^?1 more than unfertilized soils). These results were consistent across grazing and liming treatments suggesting that whilst multi‐nutrient additions increase plant productivity, soil C sequestration is increased by N‐only additions. The positive N‐only effect on soil C content was not related to changes in plant species diversity or to the functional composition of the plant community. N‐only fertilized grasslands show, however, increases in total root mass and the accumulation of organic matter detritus in topsoils. Finally, soils receiving any N addition (N only or N in combination with other nutrients) were associated with high N losses. Overall, our results demonstrate that nutrient fertilization remains an important global change driver of ecosystem functioning, which can strongly affect the long‐term sustainability of grassland soil ecosystems (e.g., soils ability to deliver multiple ecosystem services).     

Acidification of sandy grasslands – consequences for plant diversity

Questions: (1) Does soil acidification in calcareous sandy grasslands lead to loss of plant diversity? (2) What is the relationship between the soil content of lime and the plant availability of mineral nitrogen (N) and phosphorus (P) in sandy grasslands? Location: Sandy glaciofluvial deposits in south‐eastern Sweden covered by xeric sand calcareous grasslands (EU habitat directive 6120). Methods: Soil and vegetation were investigated in most of the xeric sand calcareous grasslands in the Scania region (136 sample plots distributed over four or five major areas and about 25 different sites). Environmental variables were recorded at each plot, and soil samples were analysed for exchangeable P and N, as well as limestone content and pH. Data were analysed with regression analysis and canonical correspondence analysis. Results: Plant species richness was highest on weakly acid to slightly alkaline soil; a number of nationally red‐listed species showed a similar pattern. Plant species diversity and number of red‐listed species increased with slope. Where the topsoil had been acidified, limestone was rarely present above a depth of 30 cm. The presence of limestone restricts the availability of soil P, placing a major constraint on primary productivity in sandy soils. Conclusions: Acidification of sandy grasslands leads to reduced abundance of desirable species, although the overall effect is rather weak between pH 5 and pH 9. Slopes are important for high diversity in sandy grasslands. Calcareous soils cannot be restored through shallow ploughing, but deep perturbation could increase the limestone content of the topsoil and favour of target species.     

Determinants of community organization of a South African mesic grassland

Abstract. Question: What is the long‐term influence of nutrient availability, productivity and soil pH on grassland community organization? Location: Ukulinga research farm, KwaZulu‐Natal, South Africa. Methods: The influence of fertilization on soil pH, nitrogen (N) and phosphorus (P) on variation in plant traits, community composition and species richness were examined in a 50‐year grassland fertilization experiment. Results: Averaged over 30 years, above‐ground net primary production (ANPP) was 337, 428 and 518 g.m^‐2 in sites not fertilized, fertilized with N, and fertilized with N plus P respectively. ANPP depended directly on N‐fertilization but not on P‐fertilization or liming, and responded positively to the interaction of N (first limiting nutrient) and P (second limiting nutrient). Short narrow‐leaved grass species —Themeda triandra, Tristachya leucothrix and Setaria nigrirostris— dominated sites of lowest ANPP where N was limiting (unfertilized, P‐fertilized or limed sites). A tall narrow‐leaved species, Eragrostis curvula, dominated sites of intermediate ANPP where P was limiting (N‐fertilized sites). By contrast, a tall broad‐leaved species, Panicum maximum, dominated the most productive sites where neither N nor P were limiting (N‐ and P‐fertilized sites). Certain species responded to liming and type of N‐fertilizer apparently because of their effects on soil pH. N‐fertilization reduced the density of herbaceous dicots (forbs) from 14 (unfertilized) to two (high N, no P, no lime) and five species per m² (high N, no P, limed). This effect was attributed to increased ANPP and a decrease in soil pH from 4.6 (KCl) in unfertilized sites to 3.49 (high N, no lime) and 4.65 (high N and lime). Soil acidification had no effect on grass species richness but influenced the abundance of certain species. Conclusions: Grassland community organization is determined not only by the influence of N availability, but also by the hierarchical interaction of N and P availability, in part through their compounded effect on ANPP, and by individualistic species responses to soil pH.     

Nutrient fluxes via litterfall and leaf litter decomposition vary across a gradient of soil nutrient supply in a lowland tropical rain forest

The extent to which plant communities are determined by resource availability is a central theme in ecosystem science, but patterns of small-scale variation in resource availability are poorly known. Studies of carbon (C) and nutrient cycling provide insights into factors limiting tree growth and forest productivity. To investigate rates of tropical forest litter production and decomposition in relation to nutrient availability and topography in the absence of confounding large-scale variation in climate and altitude we quantified nutrient fluxes via litterfall and leaf litter decomposition within three distinct floristic associations of tropical rain forest growing along a soil fertility gradient at the Sepilok Forest Reserve (SFR), Sabah, Malaysia. The quantity and nutrient content of small litter decreased along a gradient of soil nutrient availability from alluvial forest (most fertile) through sandstone forest to heath forest (least fertile). Temporal variation in litterfall was greatest in the sandstone forest, where the amount of litter was correlated negatively with rainfall in the previous month. Mass loss and N and P release were fastest from alluvial forest litter, and slowest from heath forest litter. All litter types decomposed most rapidly in the alluvial forest. Stand-level N and P use efficiencies (ratios of litter dry mass to nutrient content) were greatest for the heath forest followed by the sandstone ridge, sandstone valley and alluvial forests, respectively. We conclude that nutrient supply limits productivity most in the heath forest and least in the alluvial forest. Nutrient supply limited productivity in sandstone forest, especially on ridge and hill top sites where nutrient limitation may be exacerbated by reduced rates of litter decomposition during dry periods. The fluxes of N and P varied significantly between the different floristic communities at SFR and these differences may contribute to small-scale variation in species composition.     

Responses of moist non-acidic arctic tundra to altered environment: productivity, biomass, and species richness

In arctic Alaska, researchers have manipulated air temperature, light availability, and soil nutrient availability in several tundra communities over the past two decades. These communities responded quite differently to the same manipulations, and species responded individualistically within communities and among sites. For example, moist acidic tundra is primarily nitrogen (N)‐limited, whereas wet sedge tundra is primarily phosphorus (P)‐limited, and the magnitude of growth responses varies across sites within communities. Here we report results of four years of manipulated nutrients (N and/or P) and/or air temperature in an understudied, diverse plant community, moist non‐acidic tussock tundra, in northern Alaska. Our goals were to determine which factors limit above‐ground net primary productivity (ANPP) and biomass, how community composition changes may affect ecosystem attributes, and to compare these results with those from other communities to determine their generality. Although relative abundance of functional groups shifted in several treatments, the only significant change in community‐level ANPP and biomass occurred in plots that received both N and P, driven by an increase in graminoid biomass and production resulting from a positive effect of adding N. There was no difference in community biomass among any other treatments; however, some growth forms and individual species did respond. After four years no one species has come to dominate the treatment plots and species richness has not changed. These results are similar to studies in dry heath, wet sedge, and moist acidic tundra where community biomass had the greatest response to both N and P and warming results were more subtle. Unlike in moist acidic tundra where shrub biomass increased markedly with fertilization, our results suggest that in non‐acidic tundra carbon sequestration in plant biomass will not increase substantially under increased soil nutrient conditions because of the lack of overstory shrub species.     

Development of a diverse epiphyte community in response to phosphorus fertilization

The role of terrestrial soil nutrient supply in determining the composition and productivity of epiphyte communities has been little investigated. In a montane Hawaiian rainforest, we documented dramatic increases in the abundance and species richness of canopy epiphytes in a forest that had been fertilized annually with phosphorus (P) for 15 years; there was no response in forest that had been fertilized with nitrogen (N) or other nutrients. The response of N-fixing lichens to P fertilization was particularly strong, although mosses and non-N-fixing lichens also increased in abundance and diversity. We show that enhancement of canopy P availability is the most likely factor driving the bloom in epiphytes. These results provide strong evidence that terrestrial soil fertility may structure epiphyte communities, and in particular that the abundance of N-fixing lichens – a functionally important epiphyte group – may be particularly sensitive to ecosystem P availability.     

Plant Species Composition and Productivity following Permafrost Thaw and Thermokarst in Alaskan Tundra

Climate warming is expected to have a large impact on plant species composition and productivity in northern latitude ecosystems. Warming can affect vegetation communities directly through temperature effects on plant growth and indirectly through alteration of soil nutrient availability. In addition, warming can cause permafrost to thaw and thermokarst (ground subsidence) to develop, which can alter the structure of the ecosystem by altering hydrological patterns within a site. These multiple direct and indirect effects of permafrost thawing are difficult to simulate in experimental approaches that often manipulate only one or two factors. Here, we used a natural gradient approach with three sites to represent stages in the process of permafrost thawing and thermokarst. We found that vascular plant biomass shifted from graminoid-dominated tundra in the least disturbed site to shrub-dominated tundra at the oldest, most subsided site, whereas the intermediate site was co-dominated by both plant functional groups. Vascular plant productivity patterns followed the changes in biomass, whereas nonvascular moss productivity was especially important in the oldest, most subsided site. The coefficient of variation for soil moisture was higher in the oldest, most subsided site suggesting that in addition to more wet microsites, there were other microsites that were drier. Across all sites, graminoids preferred the cold, dry microsites whereas the moss and shrubs were associated with the warm, moist microsites. Total nitrogen contained in green plant biomass differed across sites, suggesting that there were increases in soil nitrogen availability where permafrost had thawed.     

Nutrient limitation along a productivity gradient in wet meadows

Conservation management in meadows often focuses on reducing soil fertility and consequently community productivity as to promote and sustain species-rich vegetations. The productivity level to which nutrients are limiting growth is, however, unclear, as well as the relationship between productivity and the type of nutrient limitation. We carried out a fertilization experiment with N, P and K in six annually mown meadows along an aerial phytomass gradient (200–650 g m^–2). All meadows were found to be growth-limited by nutrients. Low-productive meadows were N-limited, or N+P co-limited, whereas our higher productive meadows were co-limited by a combination of N, P and/or K. The results from our experiments were compared with the results from 45 other fertilization experiments with N, P and K in grasslands and wetlands (aerial phytomass range 50–1500 g m^–2). Our results were consistent in nitrogen being the most frequent (co)-limiting nutrient, and regarding the equal frequence of occurrence of P (co)-limitation and K (co)-limitation (both in ca. 25–30% of all sites). Co-limitation occurred more often in our sites than in the other experiments. There was no clear relationship between aerial phytomass and type of nutrient limitation, except that K (co)-limitation only occurred at sites with phytomass above 200 g m^–2, and P (co)-limitation below 600 g m^–2. A comparison of productivity and nutrient concentrations in aerial phytomass among two years indicated that the type of nutrient limitation is not a static site characteristic but may vary with dynamic environmental conditions such as soil wetness; drought seems to enhance N-availability which may induce P- and K-limitation.     

Different categories of biodiversity explain productivity variation after fertilization in a Tibetan alpine meadow community

The relationship between productivity and biodiversity has long been an important issue in ecological research. However, in recent decades, most ecologists have primarily focused on species diversity while paying little attention to functional diversity and phylogenetic diversity (PD), especially in alpine meadow communities following fertilization. In this study, a fertilization experiment involving the addition of nitrogen, phosphorus, and a mixture of both was implemented in an alpine meadow on the Tibetan Plateau. Species diversity, functional diversity, and PD were measured, and the responses of these parameters to the variation in productivity were analyzed. We found that the productivity of alpine plant communities was colimited by N and P, with N being the principal and P being the secondary limiting nutrient. Our results supported the prediction of both the mass ratio hypothesis and niche complementarity hypothesis in fertilized communities, but these hypotheses were not mutually exclusive. The combination of different aspects of biodiversity not only provides a crucial tool to explain the variation in productivity and to understand the underlying mechanisms but also plays an important role in predicting the variation in productivity of alpine meadow communities, which are sensitive to nutrient enrichment in the context of global change.     

Nutrient stoichiometry and land use rather than species richness determine plant functional diversity

Plant functional traits reflect individual and community ecological strategies. They allow the detection of directional changes in community dynamics and ecosystemic processes, being an additional tool to assess biodiversity than species richness. Analysis of functional patterns in plant communities provides mechanistic insight into biodiversity alterations due to anthropogenic activity. Although studies have consi‐dered of either anthropogenic management or nutrient availability on functional traits in temperate grasslands, studies combining effects of both drivers are scarce. Here, we assessed the impacts of management intensity (fertilization, mowing, grazing), nutrient stoichiometry (C, N, P, K), and vegetation composition on community‐weighted means (CWMs) and functional diversity (Rao's Q) from seven plant traits in 150 grasslands in three regions in Germany, using data of 6 years. Land use and nutrient stoichiometry accounted for larger proportions of model variance of CWM and Rao's Q than species richness and productivity. Grazing affected all analyzed trait groups; fertilization and mowing only impacted generative traits. Grazing was clearly associated with nutrient retention strategies, that is, investing in durable structures and production of fewer, less variable seed. Phenological variability was increased. Fertilization and mowing decreased seed number/mass variability, indicating competition‐related effects. Impacts of nutrient stoichiometry on trait syndromes varied. Nutrient limitation (large N:P, C:N ratios) promoted species with conservative strategies, that is, investment in durable plant structures rather than fast growth, fewer seed, and delayed flowering onset. In contrast to seed mass, leaf‐economics variability was reduced under P shortage. Species diversity was positively associated with the variability of generative traits. Synthesis. Here, land use, nutrient availability, species richness, and plant functional strategies have been shown to interact complexly, driving community composition, and vegetation responses to management intensity. We suggest that deeper understanding of underlying mechanisms shaping community assembly and biodiversity will require analyzing all these parameters.     

Nitrogen inputs accelerate phosphorus cycling rates across a wide variety of terrestrial ecosystems

? Biologically essential elements--especially nitrogen (N) and phosphorus (P)--constrain plant growth and microbial functioning; however, human activities are drastically altering the magnitude and pattern of such nutrient limitations on land. Here we examine interactions between N and P cycles of P mineralizing enzyme activities (phosphatase enzymes) across a wide variety of terrestrial biomes. ? We synthesized results from 34 separate studies and used meta-analysis to evaluate phosphatase activity with N, P, or N×P fertilization. ? Our results show that N fertilization enhances phosphatase activity, from the tropics to the extra-tropics, both on plant roots and in bulk soils. By contrast, P fertilization strongly suppresses rates of phosphatase activity. ? These results imply that phosphatase enzymes are strongly responsive to changes in local nutrient cycle conditions. We also show that plant phosphatases respond more strongly to fertilization than soil phosphatases. The tight coupling between N and P provides a mechanism for recent observations of N and P co-limitation on land. Moreover, our results suggest that terrestrial plants and microbes can allocate excess N to phosphatase enzymes, thus delaying the onset of single P limitation to plant productivity as can occur via human modifications to the global N cycle.     

Thirty-seven years of soil nitrogen and phosphorus fertility management shapes the structure and function of the soil microbial community in a Brown Chernozem

We tested whether levels of soil available nitrogen (N) and phosphorus (P) control the composition and function of the soil microbial community in a Brown Chernozemic soil on the Canadian Prairie. Soil dissolved organic carbon, N and P, and microbial communities structure (phospholipid fatty acid profile) and function (enzyme activity) were evaluated in the fallow and first wheat (Triticum aestivum L. cv. AC Eatonia) phases of fallow-wheat-wheat rotations where the wheat received soil test recommended rates of mineral N and P fertilizers (+N+P), or where N (?N+P) or P (+N?P) fertilizer use was withheld for 37 years. Differential fertilization modified soil N and P availability, and microbial community structure. Low N level was a major constraint when a rapidly growing wheat crop (heading stage) was drawing on the resource, reducing both plant N uptake and soil microbial biomass-C in ?N+P soils. Available P level in +N?P soils was about half that measured in P-fertilized soils, but P did not limit plant productivity or microbial development at that time. Changes in the microbial community structure seemingly buffered the impact of lower P availability in +N?P soils. Phosphatase activity was not involved, but increased abundance of arbuscular mycorrhizal fungi might be associated with this effect. Low soil N availability explained lower specific denitrification and higher specific nitrogenase activities in ?N+P soil growing wheat. Higher denitrification activity in +N+P soil could be attributed to higher soil C level and fertilization-induced shifts observed in the structure of the soil microbial community. Irrespective of the fertility level of the soil, all microbial communities grew at the relative growth rate of 17% day^?1 in a nutrient limitation assay that revealed no C, N or P limitation in these communities. We conclude that mineral fertilization, which modifies soil available N and P fertility, can be a selective force causing structural and functional shifts in the soil microbial community with a resulting impact on soil quality and nutrient fluxes.     

Bioassays of nutrient limitation in a tropical rain forest soil

Summary Six speices of shrubs and one large herb with contrasting life history patterns were used as bioassays of nutrient availability in a Costa Rican lowland rain forest soil. Growth responses of the herb (Phytolacca rivinoides, Phytolaccaceae) confirmed soil measurements indicating high availability of N and potentially limiting levels of P, K, Mg and Ca. Growth responses of the shrub species (Miconia spp., Melastomataceae and Piper spp., Piperaceae) to a complete nutrient fertilizer were generally less than that of Phytolacca. Lack of a strong shrub response to +P fertilization is probably due to mycorrhizal associations and slower growth rates of woody species. In general, increased growth did not occur at the expense of phenolic production in the leaves. The results emphasize that assessment of specific nutrient limitations to plant growth vary depending on species selected for the bioassay, even among species from the same community.     

Recruitment filtering by a moss layer disadvantages large-seeded grassland species

Calcareous grasslands are rich in biodiversity and thus receive much attention in nature conservation. In such grasslands, the formation of moss layers is perceived as a management problem. However, its impacts on the community level are complex, as not only inhibition but also facilitation of vascular plant recruitment occur. Possible filters of recruitment are shading by mosses, isolation from soil resources and the resulting desiccation. To understand how seed size and shape moderate moss effects, a combined glasshouse and field experiment was conducted in southern Germany. Seeds of 14 species from calcareous grasslands were sown either on top or underneath a moss layer, or on bare soil without moss. We determined the total number of emerged and established seedlings, i.e. the ones that survived until the end of the experiment. Both measures were reduced for most species sown on moss, while mortality was slightly enhanced by moss presence. Seed size explained a significant proportion of the moss effects on plant recruitment. Inhibitive effects on recruitment increased with seed size when seeds were placed on top of moss. When germinating underneath moss, the effect on emergence changed from negative in small-seeded plant species to positive in large-seeded species, but this effect was insignificant for establishment. The positive response in large seeds was probably due to their higher moisture requirements for imbibition, and lower dependency on light for germination. However, moss-seed-interactions were not affected by seed shape. Seedling establishment of plants with large seeds strongly depended on their initial position in moss, while small-seeded species showed no effect. Hence, moss layers constitute a selective recruitment filter by modifying seed penetration and hence germination conditions. This effect could increase beta diversity of vascular plant species in calcareous grasslands, and thus conservation should aim at intermediate and patchy moss abundance.