Cecilia Palmborg

Ecosystem effects of biodiversity manipulations in European grasslands.

We present a multisite analysis of the relationship between plant diversity and ecosystem functioning within the European BIODEPTH network of plant-diversity manipulation experiments. We report results of the analysis of 11 variables addressing several aspects of key ecosystem processes like biomass production, resource use (space, light, and nitrogen), and decomposition, measured across three years in plots of varying plant species richness at eight different European grassland field sites. Differences among sites explained substantial and significant amounts of the variation of most of the ecosystem processes examined. However, against this background of geographic variation, all the aspects of plant diversity and composition we examined (i.e., both numbers and types of species and functional groups) produced significant, mostly positive impacts on ecosystem processes. Analyses using the additive partitioning method revealed that complementarity effects (greater net yields than predicted from monocultures due to resource partitioning, positive interactions, etc.) were stronger and more consistent than selection effects (the covariance between monoculture yield and change in yield in mixtures) caused by dominance of species with particular traits. In general, communities with a higher diversity of species and functional groups were more productive and utilized resources more completely by intercepting more light, taking up more nitrogen, and occupying more of the available space. Diversity had significant effects through both increased vegetation cover and greater nitrogen retention by plants when this resource was more abundant through N2 fixation by legumes. However, additional positive diversity effects remained even after controlling for differences in vegetation cover and for the presence of legumes in communities. Diversity effects were stronger on above- than belowground processes. In particular, clear diversity effects on decomposition were only observed at one of the eight sites. The ecosystem effects of plant diversity also varied between sites and years. In general, diversity effects were lowest in the first year and stronger later in the experiment, indicating that they were not transitional due to community establishment. These analyses of our complete ecosystem process data set largely reinforce our previous results, and those from comparable biodiversity experiments, and extend the generality of diversity–ecosystem functioning relationships to multiple sites, years, and processes.

The role of plant diversity and composition for nitrate leaching in grasslands

The relationship between plant diversity and nitrate leaching into groundwater was investigated in a mid-European semi-natural grassland ecosystem. An experimental approach was used to directly manipulate plant diversity in the field, while holding other environmental factors constant. Species loss was simulated by establishing grassland communities of 16, 8, 4, 2, 1, and 0 plant species, composed of 3, 2, or 1 functional groups (grasses, legumes, and non-legume herbs). Every diversity treatment was replicated with several different species mixtures. Nitrate leaching was determined by continuous extraction of soil solution below the rooting zone and modeling of seepage rates. The concentration of nitrate in the soil solution was highly variable within each level of diversity. In bare ground plots and several low-diversity mixtures containing legumes, nitrate concentrations were higher than the official European Union threshold value for drinking water of 50 mg/L, with maximum values of up to 350 mg/L measured in Trifolium pratense monocultures. Total annual loss of nitrate was unaffected by the number of plant species or functional groups, but it was highly dependent on the specific species composition of the communities, and plots with legumes lost significantly more nitrate than plots without them. Aboveground biomass had no influence on nitrate loss, whereas leaching was negatively correlated with increasing root biomass. The abundance of legumes within a community, litter decomposition rates, and net nitrification were all positively correlated with total nitrate loss. However, in those communities containing legumes, leaching decreased with increasing diversity, because higher species richness led to a reduction in legume dominance, to a reduced nitrate supply through nitrification, and to a complementary uptake of nitrate by grasses and non-leguminous herbs. Based on these results, we expect that increasing the diversity of non-leguminous species or functional groups would reduce the risk of nitrate leaching in low-diversity grass–clover mixtures of ley-farming systems, while allowing for a more efficient exploitation of the beneficial fertilization effect provided by legumes. Corresponding Editor: M. Loreau.

Long‐term effects of plant diversity and composition on soil nematode communities in model grasslands

An important component of plant-soil feedbacks is how plant species identity anddiversity influence soil organism communities. We examine the effects of grassland plant species growing alone and together up to a richness of 12 species on nematode diversity and feeding group composition, eight years after the establishment of experimental grassland plots at the BIODEPTH site in northern Sweden. This is a substantially longer time than most other experimental studies of plant effects on soil fauna. We address the hypotheses that (la) higher species or functional diversity of plants increases nematode diversity, as well as influences nematode community composition. Alternatively, (1b) individual plant species traits are most important for nematode diversity and community composition. (2) Plant effects on soil organisms will decrease with increasing number of trophic links between plants and soil fauna. Plant species identity was often more important than plant diversity for nematode community composition, supporting hypothesis 1b. There was a weak positive relation between plant and nematode richness;which could be attributed to the presence of the legume Trifolium pratense, but also to some other plant species, suggesting a selection or sampling effect. Several plant species in different functional groups affected nematode community composition. For example, we found that legumes increased bacterial-feeding nematodes, most notably r-selected Rhabditida, while fungal-feeding nematodes were enhanced by forbs. Other bacterial feeders and obligate root feeders were positively related to grasses. Plant effects were usually stronger on plant-, bacterial- and fungal-feeding nematodes than on omnivores/predators, which supports hypothesis 2. Our study suggests that plant identity has stronger effects than plant diversity on nematode community composition, but when comparing our results with similar previous studies the effects of particular plant species appear to vary. We also found that more productive plant species affected bacterial-feeding nematodes more than fungal feeders. Moreover, we observed stronger effects the fewer the number of trophic links there were between a nematode feeding group and plants. Although we found clear effects of plants on soil nematodes, these were probably not large enough to result in strong and persistent plant-soil-organism-plant feedback loops.

Modelling microbial activity and biomass in forest soil with substrate quality measured using near infrared reflectance spectroscopy

Soil respiration rates before and during glucose decomposition in samples from the A0 horizon of podzolized forest soils were monitored with computerized equipment. Near infrared reflectance (NIR) spectra of the organic matter composition of the same samples were used to model the variation in basal respiration rate, substrate induced respiration (SIR) and lag time. Basal respiration rate was determined for intact sample cores, after sieving the samples, and after adjustment of the water content to 2.5 times the organic matter content. Multivariate regression with the Partial Least Squares algorithm was used in the modelling. Before sieving, 81.8% of the variation in basal respiration could be modelled by 15 NIR wavelengths measured on freeze-dried and milled soil. After sieving, 93.0–97.7% of the variation was explained by the same 15 wavelengths, which also explained 88.4% of the variation in SIR. 93.1% of the variation in lag time could be modelled by 83 NIR wavelengths measured on soil with a water content of 2.5 times the organic matter content.

Can complementarity in water use help to explain diversity–productivity relationships in experimental grassland plots?

Positive diversity–productivity relationships have repeatedly been found in experimental grassland plots, but mechanistic explanations are still under debate. We tested whether complementarity for the exploitation of the soil water resource helps to explain these relationships. In the dry summer of 2003, evapotranspiration (ET) was assessed at the Swedish BIODEPTH site using two different approaches: snapshot measurements of canopy surface temperature and simulation of time-accumulated ET by means of a soil water balance model. More diverse plots were characterized by lower surface temperatures and higher accumulated ET. Transgressive overyielding tests revealed that ET in polycultures was higher than in the best-performing monocultures, but this pattern was reversed at high degrees of water stress. Our results indicate that a more complete exploitation of soil water by more diverse grassland systems is on the one hand likely to be a driver for their increased biomass production, but on the other hand causes the more diverse communities to be affected earlier by drought. Nevertheless, the results also suggest that productivity may (at least partially) be maintained under dry conditions due to the higher likelihood of including drought-tolerant species in the more diverse communities.

Plant diversity effects on grassland productivity are robust to both nutrient enrichment and drought

Global change drivers are rapidly altering resource availability and biodiversity. While there is consensus that greater biodiversity increases the functioning of ecosystems, the extent to which biodiversity buffers ecosystem productivity in response to changes in resource availability remains unclear. We use data from 16 grassland experiments across North America and Europe that manipulated plant species richness and one of two essential resources—soil nutrients or water—to assess the direction and strength of the interaction between plant diversity and resource alteration on above-ground productivity and net biodiversity, complementarity, and selection effects. Despite strong increases in productivity with nutrient addition and decreases in productivity with drought, we found that resource alterations did not alter biodiversity–ecosystem functioning relationships. Our results suggest that these relationships are largely determined by increases in complementarity effects along plant species richness gradients. Although nutrient addition reduced complementarity effects at high diversity, this appears to be due to high biomass in monocultures under nutrient enrichment. Our results indicate that diversity and the complementarity of species are important regulators of grassland ecosystem productivity, regardless of changes in other drivers of ecosystem function.

Partitioning the variation of microbial measurements in forest soils into heavy metal and substrate quality dependent parts by use of near infrared spectroscopy and multivariate statistics

Combinations of different multivariate statistical methods were used to separate the variation in microbial parameters into one heavy metal dependent part, and one organic matter quality dependent part (measured by Near Infrared (NIR) spectroscopy). The microbial variation caused by differences in the organic matter quality was removed from the original parameters. The new parameters were called heavy metal related. This operation increased the significance of the correlations between microbial activity and the logarithm of the (copper + zinc) content. For basal respiration rate it increased from P(2 tail = 0.077 to P < 0.001 and for SIR from P = 0.068 to P < 0.001. For lag time and specific growth rate after glucose addition, no relevant models of the organic matter quality dependent part of the respiration could be made. The advantages and disadvantages of using combinations of Marked Variable Projection (MVP), Partial Least Squares (PLS) regression, Principal Components Analysis (PCA), Multiple Linear Regression (MLR) and different cross validation techniques are discussed.

Discrimination against 15N in three N2-fixing Trifolium species as influenced by Rhizobium strain and plant age

Abstract When estimating N2 fixation with the 15N natural abundance method, the 15N abundance in plants grown with N2 in air as only N source (B), must be known in order to account for discrimination against 15N during N2 fixation. Trifolium hybridum L., T. pratense L. and T. repens L. were grown without N in the nutrient solution in a greenhouse. The aim was to establish B values in these species in symbioses with Scandinavian Rhizobium genotypes, and to test the hypothesis that B is related to N2 fixation efficiency and plant age. Choice of Rhizobium strain significantly influenced B in shoots of T. hybridum and T. repens, and N content in all Trifolium species. B in shoots was partly correlated to amounts of N2 fixed, while plant age only had a marginal effect. Soil suspension inoculum or single strain inocula gave very similar B in shoots of T. pratense.

Multivariate modelling of soil microbial variables in forest soil contaminated by heavy metals using wet chemical analyses and pyrolysis GC/MS.

Abstract Microbial activity, biomass and community patterns were measured in the forest soils surrounding a large smelter emitting heavy metals and sulphur in northern Sweden. The chemical background to the high variation of the microbial variables in mor samples with a content of Cu+Zn between 100 and 1000 μ gg −1 organic matter was investigated. Soil respiration rate was modelled using different combinations of wet chemistry data and pyrolysis. A combination of carbohydrate data, humus fractionation data and physicochemical data explained 86% of the variance in soil respiration rate. Pyrolysis data explained less variance and was more difficult to interpret. Phospholipid fatty acid (PLFA) patterns in the soils were also analysed. A multivariate (MVP-PLS) model of basal respiration rate, substrate induced respiration (SIR), lag time after glucose addition and the scores from the first component of a principal components analysis (PCA) of 33 phospholipid fatty acids (PLFAcomp1) was made. The explained variance for basal respiration rate was 75%, for SIR 85%, for lag time 52% and for PLFAcomp1 82%. Respiration rate and SIR were negatively correlated to the amount of soil organic matter in the mor layer (SOM m −2 ), nitrogen content and humic acids and positively correlated to glucans and humins. The content of base cations and pH were positively correlated to respiration rate and SIR and negatively correlated to lag time. The phospholipid fatty acid patterns (PLFAcomp1) showed that fungal fatty acid patterns dominated in shallow mor layers with higher cellulose content, while gram-positive bacteria were more abundant in thicker mor layers with more humic acids and a higher nitrogen content. The variation in the microbial variables was partitioned into a heavy metal dependent and an organic matter quality dependent part, which increased the correlation between heavy metals and the microbial variables.

Ecological Stoichiometry and Density Responses of Plant-Arthropod Communities on Cormorant Nesting Islands

Seabirds deposit large amounts of nutrient rich guano on their nesting islands. The increased nutrient availability strongly affects plants and consumers. Consumer response differs among taxonomic groups, but mechanisms causing these differences are poorly understood. Ecological stoichiometry might provide tools to understand these mechanisms. ES suggests that nutrient rich taxa are more likely to be nutrient limited than nutrient poorer taxa and are more favored under nutrient enrichment. Here, we quantified differences in the elemental composition of soil, plants, and consumers between islands with and without nesting cormorant colonies and tested predictions made based on ES by relating the elemental composition and the eventual mismatch between consumer and resource stoichiometry to observed density differences among the island categories. We found that nesting cormorants radically changed the soil nutrient content and thereby indirectly plant nutrient content and resource quality to herbivores. In contrast, consumers showed only small differences in their elemental composition among the island categories. While we cannot evaluate the cause of the apparent homeostasis of invertebrates without additional data, we can conclude that from the perspective of the next trophic level, there is no difference in diet quality (in terms of N and P content) between island categories. Thus, bottom-up effects seemed mainly be mediated via changes in resource quantity not quality. Despite a large potential trophic mismatch we were unable to observe any relation between the invertebrate stoichiometry and their density response to nesting cormorant colonies. We conclude that in our system stoichiometry is not a useful predictor of arthropod responses to variation in resource nutrient content. Furthermore, we found no strong evidence that resource quality was a prime determinant of invertebrate densities. Other factors like resource quantity, habitat structure and species interactions might be more important or masked stoichiometric effects.