Discontinuity in the responses of ecosystem processes and multifunctionality to altered soil community composition.

Ecosystem management policies increasingly emphasize provision of multiple, as opposed to single, ecosystem services. Management for such "multifunctionality" has stimulated research into the role that biodiversity plays in providing desired rates of multiple ecosystem processes. Positive effects of biodiversity on indices of multifunctionality are consistently found, primarily because species that are redundant for one ecosystem process under a given set of environmental conditions play a distinct role under different conditions or in the provision of another ecosystem process. Here we show that the positive effects of diversity (specifically community composition) on multifunctionality indices can also arise from a statistical fallacy analogous to Simpson's paradox (where aggregating data obscures causal relationships). We manipulated soil faunal community composition in combination with nitrogen fertilization of model grassland ecosystems and repeatedly measured five ecosystem processes related to plant productivity, carbon storage, and nutrient turnover. We calculated three common multifunctionality indices based on these processes and found that the functional complexity of the soil communities had a consistent positive effect on the indices. However, only two of the five ecosystem processes also responded positively to increasing complexity, whereas the other three responded neutrally or negatively. Furthermore, none of the individual processes responded to both the complexity and the nitrogen manipulations in a manner consistent with the indices. Our data show that multifunctionality indices can obscure relationships that exist between communities and key ecosystem processes, leading us to question their use in advancing theoretical understanding--and in management decisions--about how biodiversity is related to the provision of multiple ecosystem services.

Soil microbial legacies influence plant survival and growth in mine reclamation.

Plants alter soil biological communities, generating ecosystem legacies that affect the performance of successive plants, influencing plant community assembly and successional trajectories. Yet, our understanding of how microbe-mediated soil legacies influence plant establishment is limited for primary successional systems and forest ecosystems, particularly for ectomycorrhizal plants. In a two-phase greenhouse experiment using primary successional mine reclamation materials with or without forest soil additions, we conditioned soil with an early successional shrub with low mycorrhizal dependence (willow, Salix scouleriana) and a later-successional ectomycorrhizal conifer (spruce, Picea engelmannii × glauca). The same plant species and later-successional plants (spruce and/or redcedar, Thuja plicata, a mid- to late-successional arbuscular mycorrhizal conifer) were grown as legacy-phase seedlings in conditioned soils and unconditioned control soils. Legacy effects were evaluated based on seedling survival and biomass, and the abundance and diversity of root fungal symbionts and pathogens. We found negative intraspecific (same-species) soil legacies for willow associated with pathogen accumulation, but neutral to positive intraspecific legacies in spruce associated with increased mycorrhizal fungal colonization and diversity. Our findings support research showing that soil legacy effects vary with plant nutrient acquisition strategy, with plants with low mycorrhizal dependence experiencing negative feedbacks and ectomycorrhizal plants experiencing positive feedbacks. Soil legacy effects of willow on next-stage successional species (spruce and redcedar) were negative, potentially due to allelopathy, while ectomycorrhizal spruce had neutral to negative legacy effects on arbuscular mycorrhizal redcedar, likely due to the trees not associating with compatible mycorrhizae. Thus, positive biological legacies may be limited to scenarios where mycorrhizal-dependent plants grow in soil containing legacies of compatible mycorrhizae. We found that soil legacies influenced plant performance in mine reclamation materials with and without forest soil additions, indicating that initial restoration actions may potentially exert long-term effects on plant community composition, even in primary successional soils with low microbial activity.

Relationship between soil properties and patterns of bacterial beta-diversity across reclaimed and natural boreal forest soils.

Productivity gradients in the boreal forest are largely determined by regional-scale changes in soil conditions, and bacterial communities are likely to respond to these changes. Few studies, however, have examined how variation in specific edaphic properties influences the composition of soil bacterial communities along environmental gradients. We quantified bacterial compositional diversity patterns in ten boreal forest sites of contrasting fertility. Bulk soil (organic and mineral horizons) was sampled from sites representing two extremes of a natural moisture-nutrient gradient and two distinct disturbance types, one barren and the other vegetation-rich. We constructed 16S rRNA gene clone libraries to characterize the bacterial communities under phylogenetic- and species-based frameworks. Using a nucleotide analog to label DNA-synthesizing bacteria, we also assessed the composition of active taxa in disturbed sites. Most sites were dominated by sequences related to the alpha-Proteobacteria, followed by acidobacterial and betaproteobacterial sequences. Non-parametric multivariate regression indicated that pH, which was lowest in the natural sites, explained 34% and 16% of the variability in community structure as determined by phylogenetic-based (UniFrac distances) and species-based (Jaccard similarities) metrics, respectively. Soil pH was also a significant predictor of richness (Chao1) and diversity (Shannon) measures. Within the natural edaphic gradient, soil moisture accounted for 32% of the variance in phylogenetic (but not species) community structure. In the boreal system we studied, bacterial beta-diversity patterns appear to be largely related to "master" variables (e.g., pH, moisture) rather than to observable attributes (e.g., plant cover) leading to regional-scale fertility gradients.

Decoupling the direct and indirect effects of nitrogen deposition on ecosystem function.

Elevated nitrogen (N) inputs into terrestrial ecosystems are causing major changes to the composition and functioning of ecosystems. Understanding these changes is challenging because there are complex interactions between 'direct' effects of N on plant physiology and soil biogeochemistry, and 'indirect' effects caused by changes in plant species composition. By planting high N and low N plant community compositions into high and low N deposition model terrestrial ecosystems we experimentally decoupled direct and indirect effects and quantified their contribution to changes in carbon, N and water cycling. Our results show that direct effects on plant growth dominate ecosystem response to N deposition, although long-term carbon storage is reduced under high N plant-species composition. These findings suggest that direct effects of N deposition on ecosystem function could be relatively strong in comparison with the indirect effects of plant community change.

Effects of management regime and plant species on the enzyme activity and genetic structure of N-fixing, denitrifying and nitrifying bacterial communities in grassland soils.

Management by combined grazing and mowing events is commonly used in grasslands, which influences the activity and composition of soil bacterial communities. Whether observed effects are mediated by management-induced disturbances, or indirectly by changes in the identity of major plant species, is still unknown. To address this issue, we quantified substrate-induced respiration (SIR), and the nitrification, denitrification and free-living N(2)-fixation enzyme activities below grass tufts of three major plant species (Holcus lanatus, Arrhenatherum elatius and Dactylis glomerata) in extensively or intensively managed grasslands. The genetic structures of eubacterial, ammonia oxidizing, nitrate reducing, and free-living N(2)-fixing communities were also characterized by ribosomal intergenic spacer analysis, and denaturing gradient gel electrophoresis (DGGE) or restriction fragment length polymorphism (RFLP) targeting group-specific genes. SIR was not influenced by management and plant species, whereas denitrification enzyme activity was influenced only by plant species, and management-plant species interactions were observed for fixation and nitrification enzyme activities. Changes in nitrification enzyme activity were likely largely explained by the observed changes in ammonium concentration, whereas N availability was not a major factor explaining changes in denitrification and fixation enzyme activities. The structures of eubacterial and free-living N(2)-fixing communities were essentially controlled by management, whereas the diversity of nitrate reducers and ammonia oxidizers depended on both management and plant species. For each functional group, changes in enzyme activity were not correlated or were weakly correlated to overall changes in genetic structure, but around 60% of activity variance was correlated to changes in five RFLP or DGGE bands. Although our conclusions should be tested for other ecosystems and seasons, these results show that predicting microbial changes induced by management in grasslands requires consideration of management-plant species interactions.

Functional biodiversity of microbial communities in the rhizospheres of hybrid larch (Larix eurolepis) and Sitka spruce (Picea sitchensis).

The diversity of microorganisms associated with trees and their different functional capabilities is thought to be a consequence of variation in carbon compounds in the rhizosphere. We used the Biolog(R) system (Biolog Inc., Hayward, CA), a redox-based test, to construct sole carbon source utilization profiles (metabolic fingerprints) of microbial communities from the rhizospheres and rhizoplanes of hybrid larch (Larix eurolepis A. Henry) and Sitka spruce (Picea sitchensis Bong. Carr.) taken from a farm woodland site and two second-rotation plantation forest sites. Canonical variate analysis (CVA) of carbon utilization data differentiated among the microbial communities from the three forest sites, with the greatest discrimination between the farm woodland and the two second-rotation forest sites. Carbohydrates and carboxylic acids were the substrates responsible for this discrimination. Carbon profiles of the microbial communities from the rhizospheres of the two tree species also clustered when evaluated by CVA, as a result of differences in utilization of carboxylic acids and amino acids, suggesting that these tree species differ in the exudates they produce. Isolation and enumeration of organisms confirmed that there were qualitative and quantitative differences in the culturable populations of microorganisms at the different sites and between tree species. We conclude that Biolog is a useful technique for evaluating the functional diversity of microbial communities; however, to interpret the results accurately, they must be assessed in conjunction with the actual carbon substrates available in the particular ecosystem under study.