Plant diversity enhances ecosystem responses to elevated CO2 and nitrogen deposition

Effects of biochar on the ecological performance of a subtropical landfill

Landfills commonly occupy large areas of land that may be ecologically important. Ecological restoration of landfill cover is a necessary approach to rebuild sustainable habitats. However, unfavourable soil conditions and invasion by exotic plants in certain regions hinder the restoration. In this study, the effects of biochar as a soil amendment on the restoration of a landfill cover were investigated under field condition. Topsoils of a landfill cover in the subtropical region (Shenzhen, China) were mixed with 0, 5 and 10% (v/v) of biochar. Soil pH, electronic conductivity, organic matter, total organic carbon, water content, total N and total P were enhanced by biochar amendment. After nine months of self-succession, plant productivity, species richness and diversity were enhanced by biochar. The structures of soil bacterial and arbuscular mycorrhizal (AM) fungal communities were changed, and species richness and diversity were moderately promoted. Enhanced plant growth and diversity were probably attributed to a number of enhanced bacterial functions related to nutrient cycling including aerobic ammonia oxidation, aerobic nitrite oxidation, nitrification, sulphur respiration, nitrate respiration, nitrogen respiration, ureolysis, chemoheterotrophy and fermentation. The higher abundances of bacteria Streptomyces sp. and Pseudomonas sp. in biochar treatments potentially enhanced the AM fungal diversity. The bacterial diversity was more related to the soil properties, especially pH, than AM fungi. Continuous monitoring is necessary to track the changes of species composition and ecological functions over time. This is the first comprehensive study on the effects of biochar on the ecological performance of a man-made ecosystem. In addition to agricultural application, biochar can be used for restoring degraded lands.

Plant diversity maintains multiple soil functions in future environments

Biodiversity increases ecosystem functions underpinning a suite of services valued by society, including services provided by soils. To test whether, and how, future environments alter the relationship between biodiversity and multiple ecosystem functions, we measured grassland plant diversity effects on single soil functions and ecosystem multifunctionality, and compared relationships in four environments: ambient conditions, elevated atmospheric CO₂, enriched N supply, and elevated CO₂ and N in combination. Our results showed that plant diversity increased three out of four soil functions and, consequently, ecosystem multifunctionality. Remarkably, biodiversity-ecosystem function relationships were similarly significant under current and future environmental conditions, yet weaker with enriched N supply. Structural equation models revealed that plant diversity enhanced ecosystem multifunctionality by increasing plant community functional diversity, and the even provision of multiple functions. Conserving local plant diversity is therefore a robust strategy to maintain multiple valuable ecosystem services in both present and future environmental conditions.

Functional groups differ in trait means, but not in trait plasticity to species richness in local grassland communities

Despite growing interest in incorporating intraspecific variation of functional traits in community-level studies, it remains unclear whether species classified into functional groups based on interspecific trait differences are similar regarding their variation in trait expression in response to varying plant diversity and composition in local communities. In a large biodiversity experiment (Jena Experiment) designed on a trait-based a priori definition of functional groups (grasses, legumes, small herbs, tall herbs), we studied means, extent of variation (coefficient of variation across communities) and plasticity to increased plant diversity (slopes over a logarithmic species richness ranging from 1, 2, 4, 8 and 16 to 60 species) for nine functional traits. Species means and extent of variation in traits related to nitrogen (N) acquisition and N use differed among functional groups and were more similar in phylogenetically closely related species than expected by chance. Species in the same functional group showed a weak phylogenetic signal and varied widely in means and extent of variation in traits related to shoot architecture and to a smaller extent in leaf traits related to carbon acquisition. This indicated that functional groups were less distinguishable in light than in nitrogen acquisition strategies. The direction and degree of trait plasticity to increasing species richness did not show a phylogenetic signal and were not different among functional groups, but varied largely among species within functional groups. Correlation structures in trait means, extent of trait variation and trait plasticity revealed functional tradeoffs in the acquisition of nitrogen and light across species. While correlations between trait means and extent of trait variation varied from trait to trait (positive, negative or unrelated), trait means and trait plasticity were mostly unrelated. Our results suggest that the concept of functional groups is viable, but context-specific trait measurements are required to improve our understanding about the functional significance of intraspecific trait variation and interspecific trait differences in local plant communities.

Comment on "Unexpected reversal of C3 versus C4 grass response to elevated CO2 during a 20-year field experiment"

Reich et al (Reports, 20 April 2018, p. 317) assert that the responses of C₃ and C₄ grass biomass to elevated CO₂ "challenge the current C₃-C₄ [elevated CO₂] paradigm," but these responses can be explained by the natural history of the experimental plants and soils without challenging this paradigm.

Biodiversity effects on ecosystem functioning respond unimodally to environmental stress

Understanding how biodiversity (B) affects ecosystem functioning (EF) is essential for assessing the consequences of ongoing biodiversity changes. An increasing number of studies, however, show that environmental conditions affect the shape of BEF relationships. Here, we first use a game-theoretic community model to reveal that a unimodal response of the BEF slope can be expected along environmental stress gradients, but also how the ecological mechanisms underlying this response may vary depending on how stress affects species interactions. Next, we analysed a global dataset of 44 experiments that crossed biodiversity with environmental conditions. Confirming our main model prediction, the effect of biodiversity on ecosystem functioning tends to be greater at intermediate levels of environmental stress, but varies among studies corresponding to differences in stress-effects on species interactions. Together, these results suggest that increases in stress from ongoing global environmental changes may amplify the consequences of biodiversity changes.

Divergent Responses of the Diazotrophic Microbiome to Elevated CO2 in Two Rice Cultivars

The species-specific responses of plant growth to elevated atmospheric CO2 concentration (eCO2) could lead to N limitation and potentially influence the sustainability of ecosystem. Questions remain unanswered with regards to the response of soil N2-fixing community to eCO2 when developing high-yielding agroecosystem to dampen the future rate of increase in CO2 levels and associated climate warming. This study demonstrates the divergent eCO2 influences on the paddy diazotrophic community between weak- and strong-responsive rice cultivars. In response to eCO2, the diazotrophic abundance increased more for the strong-responsive cultivar treatments than for the weak-responsive ones. Only the strong-responsive cultivars decreased the alpha diversity and separated the composition of diazotrophic communities in response to eCO2. The topological indices of the ecological networks further highlighted the different co-occurrence patterns of the diazotrophic microbiome in rice cultivars under eCO2. Strong-responsive cultivars destabilized the diazotrophic community by complicating and centralizing the co-occurrence network as well as by shifting the hub species from Bradyrhizobium to Dechloromonas in response to eCO2. On the contrary, the network pattern of the weak-responsive cultivars was simplified and decentralized in response to eCO2, with the hub species shifting from Halorhodospira under aCO2 to Sideroxydans under eCO2. Collectively, the above information indicates that the strong-responsive cultivars could potentially undermine the belowground ecosystem from the diazotrophs perspective in response to eCO2. This information highlights that more attention should be paid to the stability of the belowground ecosystem when developing agricultural strategies to adapt prospective climatic scenarios by growing high-yielding crop cultivars under eCO2.

Effects of increased temperature on plant communities depend on landscape location and precipitation

Global climate change is affecting and will continue to affect ecosystems worldwide. Specifically, temperature and precipitation are both expected to shift globally, and their separate and interactive effects will likely affect ecosystems differentially depending on current temperature, precipitation regimes, and other biotic and environmental factors. It is not currently understood how the effects of increasing temperature on plant communities may depend on either precipitation or where communities lie on soil moisture gradients. Such knowledge would play a crucial role in increasing our predictive ability for future effects of climate change in different systems. To this end, we conducted a multi-factor global change experiment at two locations, differing in temperature, moisture, aspect, and plant community composition, on the same slope in the northern Mongolian steppe. The natural differences in temperature and moisture between locations served as a point of comparison for the experimental manipulations of temperature and precipitation. We conducted two separate experiments, one examining the effect of climate manipulation via open-top chambers (OTCs) across the two different slope locations, the other a factorial OTC by watering experiment at one of the two locations. By combining these experiments, we were able to assess how OTCs impact plant productivity and diversity across a natural and manipulated range of soil moisture. We found that warming effects were context dependent, with the greatest negative impacts of warming on diversity in the warmer, drier upper slope location and in the unwatered plots. Our study is an important step in understanding how global change will affect ecosystems across multiple scales and locations.

Long-term effects of species loss on community properties across contrasting ecosystems

Biodiversity loss can heavily affect the functioning of ecosystems, and improving our understanding of how ecosystems respond to biodiversity decline is one of the main challenges in ecology^1-4. Several important aspects of the longer-term effects of biodiversity loss on ecosystems remain unresolved, including how these effects depend on environmental context^5-7. Here we analyse data from an across-ecosystem biodiversity manipulation experiment that, to our knowledge, represents the world's longest-running experiment of this type. This experiment has been set up on 30 lake islands in Sweden that vary considerably in productivity and soil fertility owing to differences in fire history^8,9. We tested the effects of environmental context on how plant species loss affected two fundamental community attributes-plant community biomass and temporal variability-over 20 years. In contrast to findings from artificially assembled communities^10-12, we found that the effects of species loss on community biomass decreased over time; this decrease was strongest on the least productive and least fertile islands. Species loss generally also increased temporal variability, and these effects were greatest on the most productive and most fertile islands. Our findings highlight that the ecosystem-level consequences of biodiversity loss are not constant across ecosystems and that understanding and forecasting these consequences necessitates taking into account the overarching role of environmental context.

A multivariate test of disease risk reveals conditions leading to disease amplification

Theory predicts that increasing biodiversity will dilute the risk of infectious diseases under certain conditions and will amplify disease risk under others. Yet, few empirical studies demonstrate amplification. This contrast may occur because few studies have considered the multivariate nature of disease risk, which includes richness and abundance of parasites with different transmission modes. By combining a multivariate statistical model developed for biodiversity-ecosystem-multifunctionality with an extensive field manipulation of host (plant) richness, composition and resource supply to hosts, we reveal that (i) host richness alone could not explain most changes in disease risk, and (ii) shifting host composition allowed disease amplification, depending on parasite transmission mode. Specifically, as predicted from theory, the effect of host diversity on parasite abundance differed for microbes (more density-dependent transmission) and insects (more frequency-dependent transmission). Host diversity did not influence microbial parasite abundance, but nearly doubled insect parasite abundance, and this amplification effect was attributable to variation in host composition. Parasite richness was reduced by resource addition, but only in species-rich host communities. Overall, this study demonstrates that multiple drivers, related to both host community and parasite characteristics, can influence disease risk. Furthermore, it provides a framework for evaluating multivariate disease risk in other systems.

Elevated tropospheric CO2 and O3 concentrations impair organic pollutant removal from grassland soil

The concentrations of tropospheric CO₂ and O₃ have been rising due to human activities. These rising concentrations may have strong impacts on soil functions as changes in plant physiology may lead to altered plant-soil interactions. Here, the effects of eCO₂ and eO₃ on the removal of polycyclic aromatic hydrocarbon (PAH) pollutants in grassland soil were studied. Both elevated CO₂ and O₃ concentrations decreased PAH removal with lowest removal rates at elevated CO₂ and elevated O₃ concentrations. This effect was linked to a shift in soil microbial community structure by structural equation modeling. Elevated CO₂ and O₃ concentrations reduced the abundance of gram-positive bacteria, which were tightly linked to soil enzyme production and PAH degradation. Although plant diversity did not buffer CO₂ and O₃ effects, certain soil microbial communities and functions were affected by plant communities, indicating the potential for longer-term phytoremediation approaches. Results of this study show that elevated CO₂ and O₃ concentrations may compromise the ability of soils to degrade organic pollutants. On the other hand, the present study also indicates that the targeted assembly of plant communities may be a promising tool to shape soil microbial communities for the degradation of organic pollutants in a changing world.

Quantifying effects of biodiversity on ecosystem functioning across times and places

Biodiversity loss decreases ecosystem functioning at the local scales at which species interact, but it remains unclear how biodiversity loss affects ecosystem functioning at the larger scales of space and time that are most relevant to biodiversity conservation and policy. Theory predicts that additional insurance effects of biodiversity on ecosystem functioning could emerge across time and space if species respond asynchronously to environmental variation and if species become increasingly dominant when and where they are most productive. Even if only a few dominant species maintain ecosystem functioning within a particular time and place, ecosystem functioning may be enhanced by many different species across many times and places (β-diversity). Here, we develop and apply a new approach to estimate these previously unquantified insurance effects of biodiversity on ecosystem functioning that arise due to species turnover across times and places. In a long-term (18-year) grassland plant diversity experiment, we find that total insurance effects are positive in sign and substantial in magnitude, amounting to 19% of the net biodiversity effect, mostly due to temporal insurance effects. Species loss can therefore reduce ecosystem functioning both locally and by eliminating species that would otherwise enhance ecosystem functioning across temporally fluctuating and spatially heterogeneous environments.

Toward a methodical framework for comprehensively assessing forest multifunctionality

Biodiversity-ecosystem functioning (BEF) research has extended its scope from communities that are short-lived or reshape their structure annually to structurally complex forest ecosystems. The establishment of tree diversity experiments poses specific methodological challenges for assessing the multiple functions provided by forest ecosystems. In particular, methodological inconsistencies and nonstandardized protocols impede the analysis of multifunctionality within, and comparability across the increasing number of tree diversity experiments. By providing an overview on key methods currently applied in one of the largest forest biodiversity experiments, we show how methods differing in scale and simplicity can be combined to retrieve consistent data allowing novel insights into forest ecosystem functioning. Furthermore, we discuss and develop recommendations for the integration and transferability of diverse methodical approaches to present and future forest biodiversity experiments. We identified four principles that should guide basic decisions concerning method selection for tree diversity experiments and forest BEF research: (1) method selection should be directed toward maximizing data density to increase the number of measured variables in each plot. (2) Methods should cover all relevant scales of the experiment to consider scale dependencies of biodiversity effects. (3) The same variable should be evaluated with the same method across space and time for adequate larger-scale and longer-time data analysis and to reduce errors due to changing measurement protocols. (4) Standardized, practical and rapid methods for assessing biodiversity and ecosystem functions should be promoted to increase comparability among forest BEF experiments. We demonstrate that currently available methods provide us with a sophisticated toolbox to improve a synergistic understanding of forest multifunctionality. However, these methods require further adjustment to the specific requirements of structurally complex and long-lived forest ecosystems. By applying methods connecting relevant scales, trophic levels, and above- and belowground ecosystem compartments, knowledge gain from large tree diversity experiments can be optimized.

Disentangling species and functional group richness effects on soil N cycling in a grassland ecosystem

Species richness (SR) and functional group richness (FGR) are often confounded in both observational and experimental field studies of biodiversity and ecosystem function. This precludes discernment of their separate influences on ecosystem processes, including nitrogen (N) cycling, and how those influences might be moderated by global change factors. In a 17-year field study of grassland species, we used two full factorial experiments to independently vary SR (one or four species, with FGR = 1) and FGR (1-4 groups, with SR = 4) to assess SR and FGR effects on ecosystem N cycling and its response to elevated carbon dioxide (CO₂ ) and N addition. We hypothesized that increased plant diversity (either SR or FGR) and elevated CO₂ would enhance plant N pools because of greater plant N uptake, but decrease soil N cycling rates because of greater soil carbon inputs and microbial N immobilization. In partial support of these hypotheses, increasing SR or FGR (holding the other constant) enhanced total plant N pools and decreased soil nitrate pools, largely through higher root biomass, and increasing FGR strongly reduced mineralization rates, because of lower root N concentrations. In contrast, increasing SR (holding FGR constant and despite increasing total plant C and N pools) did not alter root N concentrations or net N mineralization rates. Elevated CO₂ had minimal effects on plant and soil N metrics and their responses to plant diversity, whereas enriched N increased plant and soil N pools, but not soil N fluxes. These results show that functional diversity had additional effects on both plant N pools and rates of soil N cycling that were independent of those of species richness.

Competitive network determines the direction of the diversity-function relationship

The structure of the competitive network is an important driver of biodiversity and coexistence in natural communities. In addition to determining which species survive, the nature and intensity of competitive interactions within the network also affect the growth, productivity, and abundances of those individuals that persist. As such, the competitive network structure may likewise play an important role in determining community-level functioning by capturing the net costs of competition. Here, using an experimental system comprising 18 wood decay basidiomycete fungi, we test this possibility by quantifying the links among competitive network structure, species diversity, and community function. We show that species diversity alone has negligible impacts on community functioning, but that diversity interacts with two key properties of the competitive network-competitive intransitivity and average competitive ability-to ultimately shape biomass production, respiration, and carbon use efficiency. Most notably, highly intransitive communities comprising weak competitors exhibited a positive diversity-function relationship, whereas weakly intransitive communities comprising strong competitors exhibited a negative relationship. These findings demonstrate that competitive network structure can be an important determinant of community-level functioning, capturing a gradient from weakly to strongly competitive communities. Our research suggests that the competitive network may therefore act as a unifying link between diversity and function, providing key insight as to how and when losses in biodiversity will impact ecosystem function.

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.

Linking the influence and dependence of people on biodiversity across scales

Biodiversity enhances many of nature's benefits to people, including the regulation of climate and the production of wood in forests, livestock forage in grasslands and fish in aquatic ecosystems. Yet people are now driving the sixth mass extinction event in Earth's history. Human dependence and influence on biodiversity have mainly been studied separately and at contrasting scales of space and time, but new multiscale knowledge is beginning to link these relationships. Biodiversity loss substantially diminishes several ecosystem services by altering ecosystem functioning and stability, especially at the large temporal and spatial scales that are most relevant for policy and conservation.

Linking primary producer diversity and food quality effects on herbivores: A biochemical perspective

Biodiversity can strongly influence trophic interactions. The nutritional quality of prey communities and how it is related to the prey diversity is suspected to be a major driver of biodiversity effects. As consumer growth can be co-limited by the supply of several biochemical components, biochemically diverse prey communities should promote consumer growth. Yet, there is no clear consensus on how prey specific diversity is linked to community biochemical diversity since previous studies have considered only single nutritional quality traits. Here, we demonstrate that phytoplankton biochemical traits (fatty acids and sterols) can to a large extent explain Daphnia magna growth and its apparent dependence on phytoplankton species diversity. We find strong correlative evidence between phytoplankton species diversity, biochemical diversity, and growth. The relationship between species diversity and growth was partially explained by the fact that in many communities Daphnia was co-limited by long chained polyunsaturated fatty acids and sterols, which was driven by different prey taxa. We suggest that biochemical diversity is a good proxy for the presence of high food quality taxa, and a careful consideration of the distribution of the different biochemical traits among species is necessary before concluding about causal links between species diversity and consumer performance.

A global trait-based approach to estimate leaf nitrogen functional allocation from observations

Nitrogen is one of the most important nutrients for plant growth and a major constituent of proteins that regulate photosynthetic and respiratory processes. However, a comprehensive global analysis of nitrogen allocation in leaves for major processes with respect to different plant functional types (PFTs) is currently lacking. This study integrated observations from global databases with photosynthesis and respiration models to determine plant-functional-type-specific allocation patterns of leaf nitrogen for photosynthesis (Rubisco, electron transport, light absorption) and respiration (growth and maintenance), and by difference from observed total leaf nitrogen, an unexplained "residual" nitrogen pool. Based on our analysis, crops partition the largest fraction of nitrogen to photosynthesis (57%) and respiration (5%) followed by herbaceous plants (44% and 4%). Tropical broadleaf evergreen trees partition the least to photosynthesis (25%) and respiration (2%) followed by needle-leaved evergreen trees (28% and 3%). In trees (especially needle-leaved evergreen and tropical broadleaf evergreen trees) a large fraction (70% and 73%, respectively) of nitrogen was not explained by photosynthetic or respiratory functions. Compared to crops and herbaceous plants, this large residual pool is hypothesized to emerge from larger investments in cell wall proteins, lipids, amino acids, nucleic acid, CO₂ fixation proteins (other than Rubisco), secondary compounds, and other proteins. Our estimates are different from previous studies due to differences in methodology and assumptions used in deriving nitrogen allocation estimates. Unlike previous studies, we integrate and infer nitrogen allocation estimates across multiple PFTs, and report substantial differences in nitrogen allocation across different PFTs. The resulting pattern of nitrogen allocation provides insights on mechanisms that operate at a cellular scale within leaves, and can be integrated with ecosystem models to derive emergent properties of ecosystem productivity at local, regional, and global scales.

Rainfall variability counteracts N addition by promoting invasive Lonicera maackii and extending phenology in prairie

Although encroaching woody plants have reduced the global extent of grasslands, continuing increases in soil nitrogen availability could slow this trend by favoring resident herbaceous species. At the same time, projected increases in rainfall variability could promote woody encroachment by aligning spatiotemporal patterns of soil moisture availability with the needs of woody species. We evaluated the responses of two deciduous woody species to these simulated environmental changes by planting seedlings of Quercus palustris and Lonicera maackii into tallgrass prairie communities grown under a factorial combination of increased rainfall variability and nitrogen addition. Lonicera maackii growth was reduced 20% by nitrogen addition, and increased rainfall variability led to 33% larger seedlings, despite greater competition for light and soil resources. In contrast, Q. palustris growth showed little response to either treatment. Increased rainfall variability allowed both species to retain their leaves for an additional 6.5 d in autumn, potentially in response to wetter end-of-season shallow soils. Our findings suggest increases in rainfall variability will counteract the inhibitory effect of nitrogen deposition on growth of L. maackii, extend autumn phenology, and promote the encroachment of some woody species into grasslands.

Experimental fire increases soil carbon dioxide efflux in a grassland long-term multifactor global change experiment

Numerous studies have demonstrated that soil respiration rates increase under experimental warming, although the long-term, multiyear dynamics of this feedback are not well constrained. Less is known about the effects of single, punctuated events in combination with other longer-duration anthropogenic influences on the dynamics of soil carbon (C) loss. In 2012 and 2013, we assessed the effects of decadal-scale anthropogenic global change - warming, increased nitrogen (N) deposition, elevated carbon dioxide (CO₂ ), and increased precipitation - on soil respiration rates in an annual-dominated Mediterranean grassland. We also investigated how controlled fire and an artificial wet-up event, in combination with exposure to the longer-duration anthropogenic global change factors, influenced the dynamics of C cycling in this system. Decade-duration surface soil warming (1-2 °C) had no effect on soil respiration rates, while +N addition and elevated CO₂ concentrations increased growing-season soil CO₂ efflux rates by increasing annual aboveground net primary production (NPP) and belowground fine root production, respectively. Low-intensity experimental fire significantly elevated soil CO₂ efflux rates in the next growing season. Based on mixed-effects modeling and structural equation modeling, low-intensity fire increased growing-season soil respiration rates through a combination of three mechanisms: large increases in soil temperature (3-5 °C), significant increases in fine root production, and elevated aboveground NPP. Our study shows that in ecosystems where soil respiration has acclimated to moderate warming, further increases in soil temperature can stimulate greater soil CO₂ efflux. We also demonstrate that punctuated short-duration events such as fire can influence soil C dynamics with implications for both the parameterization of earth system models (ESMs) and the implementation of climate change mitigation policies that involve land-sector C accounting.

The economic value of grassland species for carbon storage

Carbon storage by ecosystems is valuable for climate protection. Biodiversity conservation may help increase carbon storage, but the value of this influence has been difficult to assess. We use plant, soil, and ecosystem carbon storage data from two grassland biodiversity experiments to show that greater species richness increases economic value: Increasing species richness from 1 to 10 had twice the economic value of increasing species richness from 1 to 2. The marginal value of each additional species declined as species accumulated, reflecting the nonlinear relationship between species richness and plant biomass production. Our demonstration of the economic value of biodiversity for enhancing carbon storage provides a foundation for assessing the value of biodiversity for decisions about land management. Combining carbon storage with other ecosystem services affected by biodiversity may well enhance the economic arguments for conservation even further.

Food webs obscure the strength of plant diversity effects on primary productivity

Plant diversity experiments generally find that increased diversity causes increased productivity; however, primary productivity is typically measured in the presence of a diverse food web, including pathogens, mutualists and herbivores. If food web impacts on productivity vary with plant diversity, as predicted by both theoretical and empirical studies, estimates of the effect of plant diversity on productivity may be biased. We experimentally removed arthropods, foliar fungi and soil fungi from the longest-running plant diversity experiment. We found that fungi and arthropods removed a constant, large proportion of biomass leading to a greater reduction of total biomass in high diversity plots. As a result, the effect of diversity on measured plant productivity was much higher in the absence of fungi and arthropods. Thus, diversity increases productivity more than reported in previous studies that did not control for the effects of heterotrophic consumption.

Global-change drivers of ecosystem functioning modulated by natural variability and saturating responses

Humans are altering global environment at an unprecedented rate through changes in biodiversity, climate, nitrogen cycle, and land use. To address their effects on ecosystem functioning, experiments most frequently explore one driver at a time and control as many confounding factors as possible. Yet, which driver exerts the largest influence on ecosystem functioning and whether their relative importance changes among systems remain unclear. We analyzed experiments in the Patagonian steppe that evaluated the aboveground net primary production (ANPP) response to manipulated gradients of species richness, precipitation, temperature, nitrogen fertilization (N), and grazing intensity. We compared the effect on ANPP relative to ambient conditions considering intensity and direction of manipulations for each driver. The ranking of responses to drivers with comparable manipulation intensity was as follows: biodiversity>grazing>precipitation>N. For a similar intensity of manipulation, the effect of biodiversity loss was 4.0, 3.6, and 1.5, times larger than N deposition, decreased precipitation, and increased grazing intensity. We interpreted our results considering two hypotheses. First, the response of ANPP to changes in precipitation and biodiversity is saturating, so we expected larger effects when the driver was reduced, relative to ambient conditions, than when it was increased. Experimental manipulations that reduced ambient levels had larger effects than those that increased them. Second, the sensitivity of ANPP to each driver is inversely related to the natural variability of the driver. In Patagonia, the ranking of natural variability of drivers is as follows: precipitation>grazing>temperature>biodiversity>N. So, in general, the ecosystem was most sensitive to drivers that varied the least. Comparable results from Cedar Creek (MN) support both hypotheses and suggest that sensitivity to drivers varies among ecosystem types. Given the importance of understanding ecosystem sensitivity to predict global-change impacts, it is necessary to design new experiments located in regions with contrasting natural variability and that include the full range of drivers.

Plants are less negatively affected by flooding when growing in species-rich plant communities

Flooding is expected to increase in frequency and severity in the future. The ecological consequences of flooding are the combined result of species-specific plant traits and ecological context. However, the majority of past flooding research has focused on individual model species under highly controlled conditions. An early summer flooding event in a grassland biodiversity experiment in Jena, Germany, provided the opportunity to assess flooding responses of 60 grassland species in monocultures and 16-species mixtures. We examined plant biomass, species-specific traits (plant height, specific leaf area (SLA), root aerenchyma, starch content) and soil porosity. We found that, on average, plant species were less negatively affected by the flood when grown in higher-diversity plots in July 2013. By September 2013, grasses were unaffected by the flood regardless of plant diversity, and legumes were severely negatively affected regardless of plant diversity. Plants with greater SLA and more root aerenchyma performed better in September. Soil porosity was higher in higher-diversity plots and had a positive effect on plant performance. As floods become more frequent and severe in the future, growing flood-sensitive plants in higher-diversity communities and in soil with greater soil aeration may attenuate the most negative effects of flooding.

Nutrient constraints on terrestrial carbon fixation: The role of nitrogen

Carbon dioxide (CO₂) concentrations in the earth's atmosphere are projected to rise from current levels near 400ppm to over 700ppm by the end of the 21st century. Projections over this time frame must take into account the increases in total net primary production (NPP) expected from terrestrial plants, which result from elevated CO₂ (eCO₂) and have the potential to mitigate the impact of anthropogenic CO₂ emissions. However, a growing body of evidence indicates that limitations in soil nutrients, particularly nitrogen (N), the soil nutrient most limiting to plant growth, may greatly constrain future carbon fixation. Here, we review recent studies about the relationships between soil N supply, plant N nutrition, and carbon fixation in higher plants under eCO₂, highlighting key discoveries made in the field, particularly from free-air CO₂ enrichment (FACE) technology, and relate these findings to physiological and ecological mechanisms.

Per capita interactions and stress tolerance drive stress-induced changes in biodiversity effects on ecosystem functions

Environmental stress changes the relationship between biodiversity and ecosystem functions, but the underlying mechanisms are poorly understood. Because species interactions shape biodiversity-ecosystem functioning relationships, changes in per capita interactions under stress (as predicted by the stress gradient hypothesis) can be an important driver of stress-induced changes in these relationships. To test this hypothesis, we measure productivity in microalgae communities along a diversity and herbicide gradient. On the basis of additive partitioning and a mechanistic community model, we demonstrate that changes in per capita interactions do not explain effects of herbicide stress on the biodiversity-productivity relationship. Instead, assuming that the per capita interactions remain unaffected by stress, causing species densities to only change through differences in stress tolerance, suffices to predict the stress-induced changes in the biodiversity-productivity relationship and community composition. We discuss how our findings set the stage for developing theory on how environmental stress changes biodiversity effects on ecosystem functions.

Adaptation to elevated CO2 in different biodiversity contexts

In the absence of migration, species persistence depends on adaption to a changing environment, but whether and how adaptation to global change is altered by community diversity is not understood. Community diversity may prevent, enhance or alter how species adapt to changing conditions by influencing population sizes, genetic diversity and/or the fitness landscape experienced by focal species. We tested the impact of community diversity on adaptation by performing a reciprocal transplant experiment on grasses that evolved for 14 years under ambient and elevated CO2, in communities of low or high species richness. Using biomass as a fitness proxy, we find evidence for local adaptation to elevated CO2, but only for plants assayed in a community of similar diversity to the one experienced during the period of selection. Our results indicate that the biological community shapes the very nature of the fitness landscape within which species evolve in response to elevated CO2.

Resource Availability Alters Biodiversity Effects in Experimental Grass-Forb Mixtures

Numerous experiments, mostly performed in particular environments, have shown positive diversity-productivity relationships. Although the complementary use of resources is discussed as an important mechanism explaining diversity effects, less is known about how resource availability controls the strength of diversity effects and how this response depends on the functional composition of plant communities. We studied aboveground biomass production in experimental monocultures, two- and four-species mixtures assembled from two independent pools of four perennial grassland species, each representing two functional groups (grasses, forbs) and two growth statures (small, tall), and exposed to different combinations of light and nutrient availability. On average, shade led to a decrease in aboveground biomass production of 24% while fertilization increased biomass production by 36%. Mixtures were on average more productive than expected from their monocultures (relative yield total, RYT>1) and showed positive net diversity effects (NE: +34% biomass increase; mixture minus mean monoculture biomass). Both trait-independent complementarity effects (TICE: +21%) and dominance effects (DE: +12%) positively contributed to net diversity effects, while trait-dependent complementarity effects were minor (TDCE: +1%). Shading did not alter diversity effects and overyielding. Fertilization decreased RYT and the proportion of biomass gain through TICE and TDCE, while DE increased. Diversity effects did not increase with species richness and were independent of functional group or growth stature composition. Trait-based analyses showed that the dominance of species with root and leaf traits related to resource conservation increased TICE. Traits indicating the tolerance of shade showed positive relationships with TDCE. Large DE were associated with the dominance of species with tall growth and low diversity in leaf nitrogen concentrations. Our field experiment shows that positive diversity effects are possible in grass-forb mixtures irrespective of differences in light availability, but that the chance for the complementary use of resources increases when nutrients are not available at excess.

The diversity-disease relationship: evidence for and criticisms of the dilution effect

The dilution effect, that high host species diversity can reduce disease risk, has attracted much attention in the context of global biodiversity decline and increasing disease emergence. Recent studies have criticized the generality of the dilution effect and argued that it only occurs under certain circumstances. Nevertheless, evidence for the existence of a dilution effect was reported in about 80% of the studies that addressed the diversity-disease relationship, and a recent meta-analysis found that the dilution effect is widespread. We here review supporting and critical studies, point out the causes underlying the current disputes. The dilution is expected to be strong when the competent host species tend to remain when species diversity declines, characterized as a negative relationship between species' reservoir competence and local extinction risk. We here conclude that most studies support a negative competence-extinction relationship. We then synthesize the current knowledge on how the diversity-disease relationship can be modified by particular species in community, by the scales of analyses, and by the disease risk measures. We also highlight the complex role of habitat fragmentation in the diversity-disease relationship from epidemiological, evolutionary and ecological perspectives, and construct a synthetic framework integrating these three perspectives. We suggest that future studies should test the diversity-disease relationship across different scales and consider the multiple effects of landscape fragmentation.

The Diversity and Co-occurrence Patterns of N₂-Fixing Communities in a CO₂-Enriched Grassland Ecosystem

Diazotrophs are the major organismal group responsible for atmospheric nitrogen (N2) fixation in natural ecosystems. The extensive diversity and structure of N2-fixing communities in grassland ecosystems and their responses to increasing atmospheric CO2 remain to be further explored. Through pyrosequencing of nifH gene amplicons and extraction of nifH genes from shotgun metagenomes, coupled with co-occurrence ecological network analysis approaches, we comprehensively analyzed the diazotrophic community in a grassland ecosystem exposed to elevated CO2 (eCO2) for 12 years. Long-term eCO2 increased the abundance of nifH genes but did not change the overall nifH diversity and diazotrophic community structure. Taxonomic and phylogenetic analysis of amplified nifH sequences suggested a high diversity of nifH genes in the soil ecosystem, the majority belonging to nifH clusters I and II. Co-occurrence ecological network analysis identified different co-occurrence patterns for different groups of diazotrophs, such as Azospirillum/Actinobacteria, Mesorhizobium/Conexibacter, and Bradyrhizobium/Acidobacteria. This indicated a potential attraction of non-N2-fixers by diazotrophs in the soil ecosystem. Interestingly, more complex co-occurrence patterns were found for free-living diazotrophs than commonly known symbiotic diazotrophs, which is consistent with the physical isolation nature of symbiotic diazotrophs from the environment by root nodules. The study provides novel insights into our understanding of the microbial ecology of soil diazotrophs in natural ecosystems.

Long-term nitrogen deposition linked to reduced water use efficiency in forests with low phosphorus availability

The impact of long-term nitrogen (N) deposition is under-studied in phosphorus (P)-limited subtropical forests. We exploited historically collected herbarium specimens to investigate potential physiological responses of trees in three subtropical forests representing an urban-to-rural gradient, across which N deposition has probably varied over the past six decades. We measured foliar [N] and [P] and stable carbon (δ(13) C), oxygen (δ(18) O) and nitrogen (δ(15) N) isotopic compositions in tissue from herbarium specimens of plant species collected from 1947 to 2014. Foliar [N] and N : P increased, and δ(15) N and [P] decreased in the two forests close to urban centers. Consistent with recent studies demonstrating that N deposition in the region is (15) N-depleted, these data suggest that the increased foliar [N] and N : P, and decreased [P], may be attributable to atmospheric deposition and associated enhancement of P limitation. Estimates of intrinsic water use efficiency calculated from foliar δ(13) C decreased by c. 30% from the 1950s to 2014, contrasting with multiple studies investigating similar parameters in N-limited forests. This effect may reflect decreased photosynthesis, as suggested by a conceptual model of foliar δ(13) C and δ(18) O. Long-term N deposition may exacerbate P limitation and mitigate projected increases in carbon stocks driven by elevated CO2 in forests on P-limited soils.

Complementarity among four highly productive grassland species depends on resource availability

Positive species richness-productivity relationships are common in biodiversity experiments, but how resource availability modifies biodiversity effects in grass-legume mixtures composed of highly productive species is yet to be explicitly tested. We addressed this question by choosing two grasses (Arrhenatherum elatius and Dactylis glomerata) and two legumes (Medicago × varia and Onobrychis viciifolia) which are highly productive in monocultures and dominant in mixtures (the Jena Experiment). We established monocultures, all possible two- and three-species mixtures, and the four-species mixture under three different resource supply conditions (control, fertilization, and shading). Compared to the control, community biomass production decreased under shading (-56 %) and increased under fertilization (+12 %). Net diversity effects (i.e., mixture minus mean monoculture biomass) were positive in the control and under shading (on average +15 and +72 %, respectively) and negative under fertilization (-10 %). Positive complementarity effects in the control suggested resource partitioning and facilitation of growth through symbiotic N2 fixation by legumes. Positive complementarity effects under shading indicated that resource partitioning is also possible when growth is carbon-limited. Negative complementarity effects under fertilization suggested that external nutrient supply depressed facilitative grass-legume interactions due to increased competition for light. Selection effects, which quantify the dominance of species with particularly high monoculture biomasses in the mixture, were generally small compared to complementarity effects, and indicated that these species had comparable competitive strengths in the mixture. Our study shows that resource availability has a strong impact on the occurrence of positive diversity effects among tall and highly productive grass and legume species.

Elevated carbon dioxide accelerates the spatial turnover of soil microbial communities

Although elevated CO2 (eCO2 ) significantly affects the α-diversity, composition, function, interaction and dynamics of soil microbial communities at the local scale, little is known about eCO2 impacts on the geographic distribution of micro-organisms regionally or globally. Here, we examined the β-diversity of 110 soil microbial communities across six free air CO2 enrichment (FACE) experimental sites using a high-throughput functional gene array. The β-diversity of soil microbial communities was significantly (P < 0.05) correlated with geographic distance under both CO2 conditions, but declined significantly (P < 0.05) faster at eCO2 with a slope of -0.0250 than at ambient CO2 (aCO2 ) with a slope of -0.0231 although it varied within each individual site, indicating that the spatial turnover rate of soil microbial communities was accelerated under eCO2 at a larger geographic scale (e.g. regionally). Both distance and soil properties significantly (P < 0.05) contributed to the observed microbial β-diversity. This study provides new hypotheses for further understanding their assembly mechanisms that may be especially important as global CO2 continues to increase.

Shifting grassland plant community structure drives positive interactive effects of warming and diversity on aboveground net primary productivity

Ecosystems worldwide are increasingly impacted by multiple drivers of environmental change, including climate warming and loss of biodiversity. We show, using a long-term factorial experiment, that plant diversity loss alters the effects of warming on productivity. Aboveground primary productivity was increased by both high plant diversity and warming, and, in concert, warming (≈1.5 °C average above and belowground warming over the growing season) and diversity caused a greater than additive increase in aboveground productivity. The aboveground warming effects increased over time, particularly at higher levels of diversity, perhaps because of warming-induced increases in legume and C4 bunch grass abundances, and facilitative feedbacks of these species on productivity. Moreover, higher plant diversity was associated with the amelioration of warming-induced environmental conditions. This led to cooler temperatures, decreased vapor pressure deficit, and increased surface soil moisture in higher diversity communities. Root biomass (0-30 cm) was likewise consistently greater at higher plant diversity and was greater with warming in monocultures and at intermediate diversity, but at high diversity warming had no detectable effect. This may be because warming increased the abundance of legumes, which have lower root : shoot ratios than the other types of plants. In addition, legumes increase soil nitrogen (N) supply, which could make N less limiting to other species and potentially decrease their investment in roots. The negative warming × diversity interaction on root mass led to an overall negative interactive effect of these two global change factors on the sum of above and belowground biomass, and thus likely on total plant carbon stores. In total, plant diversity increased the effect of warming on aboveground net productivity and moderated the effect on root mass. These divergent effects suggest that warming and changes in plant diversity are likely to have both interactive and divergent impacts on various aspects of ecosystem functioning.

Interspecific competition in phytoplankton drives the availability of essential mineral and biochemical nutrients

The underlying mechanisms and consequences of competition and diversity are central themes in ecology. A higher diversity of primary, producers often results in higher resource use efficiency in aquatic and terrestrial ecosystems. This may result in more food for consumers on one hand, while, on the other hand, it can also result in a decreased food quality for consumers; higher biomass combined with the same availability of the limiting compound directly reduces the dietary proportion of the limiting compound. Here we tested whether and how interspecific competition in phytoplankton communities leads to changes in resource use efficiency and cellular concentrations of nutrients and fatty acids. The measured particulate carbon: phosphorus ratios (C:P) and fatty acid concentrations in the communities were compared to the theoretically expected ratios and concentrations of measurements on simultaneously running monocultures. With interspecific competition, phytoplankton communities had higher concentrations of the monounsaturated fatty acid oleic acid and also much higher concentrations of the ecologically and physiologically relevant long-chain polyunsaturated fatty acid eicosapentaenoic acid than expected concentrations based on monocultures. Such higher availability of essential fatty acids may contribute to the positive relationship between phytoplankton diversity and zooplankton growth, and may compensate limitations by mineral nutrients in higher trophic levels.