Tobias Ceulemans

Soil phosphorus constrains biodiversity across European grasslands

Nutrient pollution presents a serious threat to biodiversity conservation. In terrestrial ecosystems, the deleterious effects of nitrogen pollution are increasingly understood and several mitigating environmental policies have been developed. Compared to nitrogen, the effects of increased phosphorus have received far less attention, although some studies have indicated that phosphorus pollution may be detrimental for biodiversity as well. On the basis of a dataset covering 501 grassland plots throughout Europe, we demonstrate that, independent of the level of atmospheric nitrogen deposition and soil acidity, plant species richness was consistently negatively related to soil phosphorus. We also identified thresholds in soil phosphorus above which biodiversity appears to remain at a constant low level. Our results indicate that nutrient management policies biased toward reducing nitrogen pollution will fail to preserve biodiversity. As soil phosphorus is known to be extremely persistent and we found no evidence for a critical threshold below which no environmental harm is expected, we suggest that agro-environmental schemes should include grasslands that are permanently free from phosphorus fertilization.

Increasing Soil Nutrient Loads of European Semi-natural Grasslands Strongly Alter Plant Functional Diversity Independently of Species Loss

Anthropogenically increased input of nitrogen (N) and phosphorous (P) have led to a severe reduction of plant species richness in European semi-natural grasslands. Although it is well established that this species loss is not trait neutral, a thorough analysis of the effects of nutrient addition on trait based functional diversity and functional composition, independently of species loss, is lacking so far. We compiled data on the plant species abundance (relevé’s) of 279 Nardus grasslands from nine European countries, across a gradient of soil N and P content. Functional diversity (Petchy and Gaston’s FDc, weighted FDc and quadratic entropy) and mean trait composition were calculated for each relevé, based on 21 functional traits. Differences in functional diversity and functional composition were related to differences in soil N, atmospheric N deposition, soil P and pH, while controlling for geographic location and species richness. All functional diversity measures decreased with increasing soil N, with wFDc also decreased by soil P, independent of species loss. This was accompanied by clear shifts in functional trait composition, associated with shifts from below-ground competition for nutrients to above-ground competition for light. This resulted in a decrease in insect-pollinated therophytes and chamaephytes and an increase in long-lived, clonal graminoids and hemicryptophytes under increasing soil N and P. These functional community changes can be expected to alter both ecosystem functioning and service provisioning of the studied grasslands. Our research emphasizes the importance of a reduction of both N and P emission throughout Europe for sustainable conservation of these communities.

Phosphorus resource partitioning shapes phosphorus acquisition and plant species abundance in grasslands

Species diversity is commonly hypothesized to result from trade-offs for different limiting resources, providing separate niches for coexisting species1–4. As soil nutrients occur in multiple chemical forms, plant differences in acquisition of the same element derived from different compounds may represent unique niche dimensions5,6. Because plant productivity of ecosystems is often limited by phosphorus7, and because plants have evolved diverse adaptations to acquire soil phosphorus6,8, a promising yet untested hypothesis is phosphorus resource partitioning6,9,10. Here, we provided two different chemical forms of phosphorus to sown grassland mesocosms to investigate phosphorus acquisition of eight plant species that are common in European grasslands, and to identify subsequent patterns of plant abundance. For the first time, we show that the relative abundance of grassland plant species can be influenced by soil phosphorus forms, as higher abundance was linked to higher acquisition of a specific form of phosphorus. These results were supported by a subsequent isotope dilution experiment using intact grassland sods that were treated with different inorganic or organic phosphorus forms. Here, 5 out of 14 species showed greater phosphorus acquisition in the inorganic phosphorus treatment, and 4 in the organic phosphorus treatments. Furthermore, for the species used in both experiments we found similar acquisition patterns. Our results support the hypothesis of phosphorus resource partitioning and may provide a new mechanistic framework to explain high plant diversity in phosphorus-poor ecosystems6,11–13. As world biodiversity hotspots are almost invariably related to phosphorus limitation8,11,12, our results may thus also be key to understanding biodiversity loss in an era of ever-increasing nutrient enrichment14.

Experimental fertilization increases amino acid content in floral nectar, fruit set and degree of selfing in the orchid Gymnadenia conopsea

Floral traits have evolved to maximize reproductive success by attracting pollinators and facilitating pollination. Highly attractive floral traits may, however, also increase the degree of self-pollination, which could become detrimental for plant fitness through inbreeding depression. Floral nectar is a trait that is known to strongly mediate pollinator attraction and plant reproductive success, but the particular role of the nectar amino acid (AA) composition is poorly understood. Therefore, we experimentally manipulated the nectar AA composition and abundance of the Lepidoptera-pollinated orchid Gymnadenia conopsea through soil fertilization, and we quantified AA content and AA composition through high performance anion exchange chromatography with pulsed amperometric detection. Mixed models were then used to evaluate differences in pollinia removal, fruit set, seed set and degree of selfing between fertilized and control individuals. Selfing rates were estimated using microsatellite markers. We found that fertilized individuals had a significantly higher nectar AA content and an altered AA composition, whereas plant height, number of flowers, nectar volume and sugar concentration remained unchanged. Fertilized individuals also had significantly more pollinia removed and a higher fruit set, whereas control plants that did not receive the fertilization treatment had significantly fewer selfed seeds, and more viable seeds. Although we cannot exclude a role of changes in floral scent following the fertilization treatment, our results strongly suggest a relation among nectar AA composition, fruiting success and selfing rates. Our results also indicate potential consequences of nutrient pollution for plant reproductive success, through the induced changes in nectar AA composition.

Application of slow-release phosphorus fertilizers increases arbuscular mycorrhizal fungal diversity in the roots of apple trees

AimsArbuscular mycorrhizal fungi (AMF) play a key role in the functioning of agricultural ecosystems. Therefore, understanding how the application of fertilizers, a common management practice, affects AMF communities is of major importance. Here we aimed to: (i) experimentally test whether different amounts and forms of phosphorus (P) fertilizer affect AMF diversity and community composition associated with the roots of apple trees (Malus domestica); (ii) identify differences in tolerance to P fertilization between AMF taxa.MethodsWe used 454-pyrosequencing of the small subunit rRNA gene amplicons to quantify AMF diversity and community composition in root samples obtained from a three year field experiment, with two inorganic, three slow-release P fertilization and one control treatment.ResultsThe slow-release fertilizer treatments showed significantly higher AMF richness and differed in community composition compared to the inorganic fertilizer treatments. The distribution of AMF OTUs showed a significantly nested pattern. Additionally, AMF communities in the inorganic fertilizer treatments were a subset of the communities in the slow-release fertilizer treatments.ConclusionsWe demonstrate that application of slow-release fertilizers promoted AMF diversity in the roots of cultivated apple trees in comparison to the other treatments. The application of inorganic fertilizers elevated levels of plant-available P in the soil and selected only a small subset of abundant AMF, resulting in a lower AMF diversity. This may result in AMF communities dominated by inferior AMF mutualists.

Dispersal constraints for the conservation of the grassland herb Thymus pulegioides L. in a highly fragmented agricultural landscape

Species-rich grassland communities are one of the most important habitats for biodiversity and of high conservation priority in Europe. Restoration actions are mainly focused on the improvement of abiotic conditions, such as nutrient depletion techniques, and are generally based on the assumption that the target community will re-establish at the restored site when the target species exist in the neighborhood. Information on the contemporary seed-dispersal range is therefore crucial to develop effective conservation measures. Here, we investigated the contemporary long-distance seed dispersal and genetic structure of the grassland herb Thymus pulegioides in an intensively managed agricultural landscape in Flanders (Northern Belgium). Assignment tests based on amplified fragment length polymorphisms revealed very low levels of effective seed dispersal between populations although seed availability and seed viability was not a limiting factor. The process of fragmentation has resulted in a high population differentiation and without further incoming gene flow the remnant populations are prone to further genetic erosion and perhaps extinction. Our findings illustrate that restoring suitable abiotic habitat conditions in the neighborhood of existing populations does likely not guarantee colonization for this grassland specialist. For the survival of the species, existing populations should be functionally connected and seed addition may be necessary for successful conservation to overcome dispersal-limitation.

Nutrient enrichment is associated with altered nectar and pollen chemical composition in Succisa pratensis Moench and increased larval mortality of its pollinator Bombus terrestris L

Pollinators are declining worldwide and possible underlying causes include disease, invasive pest species and large scale land use changes resulting in habitat loss and degradation. One particular cause of habitat degradation is the increased inflow of nutrients due to anthropogenic combustion processes and large scale application of agricultural fertilizers. This nutrient pollution has been shown to affect pollinators through the loss of nectar and pollen-providing plant species. However, it may also affect pollinators through altering the nectar and pollen chemical composition of plant species, hence influencing pollinator food quality. Here, we experimentally investigated the effect of nutrient enrichment on amino acid and sugar composition of nectar and pollen in the grassland plant Sucissa pratensis, and the subsequent colony size and larval mortality of the pollinating bumblebee Bombus terrestris. We found less of the essential amino acids glycine and arginine in the pollen of fertilized plants, and more arginine, ornithine and threonine in the pollen of control plants. Nectar glucose and pollen fructose levels were lower in fertilized plants as compared to control plants. Furthermore, bumblebee colonies visiting fertilized plants showed more dead larvae than colonies visiting control plants. Our results suggest that the fitness of bumblebees can be negatively affected by changes in their food quality following nutrient pollution. If similar patterns hold for other plant and pollinator species, this may have far reaching implications for the maintenance of pollination ecosystem services, as nutrient pollution continues to rise worldwide.

The role of above-ground competition and nitrogen vs. phosphorus enrichment in seedling survival of common European plant species of semi-natural grasslands

Anthropogenic activities have severely altered fluxes of nitrogen and phosphorus in ecosystems worldwide. In grasslands, subsequent negative effects are commonly attributed to competitive exclusion of plant species following increased above-ground biomass production. However, some studies have shown that this does not fully account for nutrient enrichment effects, questioning whether lowering competition by reducing grassland productivity through mowing or herbivory can mitigate the environmental impact of nutrient pollution. Furthermore, few studies so far discriminate between nitrogen and phosphorus pollution. We performed a full factorial experiment in greenhouse mesocosms combining nitrogen and phosphorus addition with two clipping regimes designed to relax above-ground competition. Next, we studied the survival and growth of seedlings of eight common European grassland species and found that five out of eight species showed higher survival under the clipping regime with the lowest above-ground competition. Phosphorus addition negatively affected seven plant species and nitrogen addition negatively affected four plant species. Importantly, the negative effects of nutrient addition and higher above-ground competition were independent of each other for all but one species. Our results suggest that at any given level of soil nutrients, relaxation of above-ground competition allows for higher seedling survival in grasslands. At the same time, even at low levels of above-ground competition, nutrient enrichment negatively affects survival as compared to nutrient-poor conditions. Therefore, although maintaining low above-ground competition appears essential for species’ recruitment, for instance through mowing or herbivory, these management efforts are likely to be insufficient and we conclude that environmental policies aimed to reduce both excess nitrogen and particularly phosphorus inputs are also necessary.

Mycorrhizal fungi show regular community compositions in natural ecosystems

Dissimilarity overlap curve analysis has shown that ‘universality’ is a common feature in many complex microbial communities, suggesting that the same taxa interact in a similar manner when shared between communities. We present evidence that arbuscular mycorrhizal fungi, common plant root symbionts, show universal community compositions in natural ecosystems and that this pattern is conserved even at larger spatial scales. However, universality was not detected in agricultural ecosystems potentially implying that agricultural symbiont communities are formed in a different manner.

Variation in ectomycorrhizal fungal communities associated with Silver linden (Tilia tomentosa) within and across urban areas

Trees in urban areas face harsh environmental conditions. Ectomycorrhizal fungi (EcM) form a symbiosis with many tree species and provide a range of benefits to their host through their extraradical hyphal network. Although our understanding of the environmental drivers and large scale geographical variation of EcM communities in natural ecosystems is growing, our knowledge of EcM communities within and across urban areas is still limited. Here, we characterized EcM communities using Illumina miseq sequencing on 175 root samples of the urban tree Tilia tomentosa from three European cities, namely Leuven (Belgium), Strasbourg (France) and Porto (Portugal). We found strong differences in EcM richness and community composition between cities. Soil acidity, organic matter and moisture content were significantly associated with EcM community composition. In agreement, the explained variability in EcM communities was mostly attributed to general soil characteristics, whereas very little variation was explained by city and heavy metal pollution. Overall, our results suggest that EcM communities in urban areas are significantly associated with soil characteristics, while heavy metal pollution and biogeography had little or no impact. These findings deliver new insights into EcM distribution patterns in urban areas and contribute to specific inoculation strategies to improve urban tree vitality.