A global synthesis reveals biodiversity loss as a major driver of ecosystem change

Biodiversity responses to ongoing climate and atmospheric changes will affect both ecosystem processes and the delivery of ecosystem goods and services. Combined effects of co-occurring global changes on diversity, however, are poorly understood. We examined plant diversity responses in a California annual grassland to manipulations of four global environmental changes, singly and in combination: elevated CO2, warming, precipitation, and nitrogen deposition. After 3 years, elevated CO2 and nitrogen deposition each reduced plant diversity, whereas elevated precipitation increased it and warming had no significant effect. Diversity responses to both single and combined global change treatments were driven overwhelmingly by gains and losses of forb species, which make up most of the native plant diversity in California grasslands. Diversity responses across treatments also showed no consistent relationship to net primary production responses, illustrating that the diversity effects of these environmental changes could not be explained simply by changes in productivity. In two- to four-way combinations, simulated global changes did not interact in any of their effects on diversity. Our results show that climate and atmospheric changes can rapidly alter biological diversity, with combined effects that, at least in some settings, are simple, additive combinations of single-factor effects.

Land use intensification is associated with loss of biodiversity and altered ecosystem functioning. Until now most studies on the relationship between biodiversity and ecosystem functioning focused on random loss of species, while loss of rare species that usually are the first to disappear received less attention. Here we test if the effect of rare microbial species loss on plant productivity depends on the origin of the microbial soil community. Soils were sampled from three land use types at two farms. Microbial communities with increasing loss of rare species were created by inoculating sterilized soils with serially diluted soil suspensions. After 8 months of incubation, the effects of the different soil communities on abiotic soil properties, soil processes, microbial community composition, and plant productivity was measured. Dilution treatments resulted in increasing species loss, which was in relation to abundance of bacteria in the original field soil, without affecting most of the other soil parameters and processes. Microbial species loss affected plant biomass positively, negatively or not at all, depending on soil origin, but not on land use history. Even within fields the effects of dilution on plant biomass varied between replicates, suggesting heterogeneity in microbial community composition. The effects of medium and severe species loss on plant biomass were similar, pointing toward a saturating effect of species loss. We conclude that changes in the composition of the soil microbial community, including rare species loss, can affect plant productivity, depending on the composition of the initial microbial community. Future work on the relation between function and species loss effects should address this variation by including multiple sampling origins.

Global environmental changes and biological invasions are important environmental problems. A better understanding of their interactions will help us improve management for plant invasion control under global change context. Mikania micrantha and Chromolaena odorata are two of the most notorious invasive weeds in southern China. They have caused serious damage to semi-natural forests, pastures, crops and plantations. In this study, they were grown under elevated and ambient CO2 (700 ± 50 and 380 ± 50 μmol mol−1, respectively) in open-top chambers in south China botanical garden and treated with fertilizers in the form of soluble nitrogen (N), phosphorus (P), N + P (NP), and with tap water as control. Elevated CO2 had significant positive effects on most variables for M. micrantha, and N addition had positive effects on most variables for C. odorata. Elevated CO2 and N or NP addition increased Rubisco content, photosynthetic rates and photosynthetic N use efficiency of both weeds, which improved their resource capture ability and resource utilization efficiency, and improved their growth capacity. SDS-insoluble protein decreased in response to elevated CO2 in both weeds indicating a decrease in their mechanical defense when CO2 elevated. P addition decreased the quantity of N invested in the photosynthetic apparatus under ambient CO2 in both weeds, but this effect was reduced under elevated CO2, suggesting that high soil P will not benefit for the growth of the two weeds under current CO2 levels but less so if CO2 levels continue to rise. From the results we predict that the invasion potential of both weeds will decrease in the high P and low N area at present CO2 concentration, but increase in the high N deposition area, especially with elevated CO2.

The capacity of forest ecosystems to sequester C in the soil relies on the net balance between litter production above, as well as, below ground, and decomposition processes. Nitrogen mineralization and its availability for plant growth and microbial activity often control the speed of both processes. Litter production, decomposition and N mineralization are strongly interdependent. Thus, their responses to global environmental changes (i.e. elevated CO2, climate, N deposition, etc.) cannot be fully understood if they are studied in isolation. In the present experiment, we investigated litter fall, litter decomposition and N dynamics in decomposing litter of three Populus spp., in the second and third growing season of a short rotation coppice under FACE. Elevated CO2 did not affect annual litter production but slightly retarded litter fall in the third growing season. In all species, elevated CO2 lowered N concentration, resulting in a reduction of N input to the soil via litter fall, but did not affect lignin concentrations. Litter decomposition was studied in bags incubated in situ both in control and FACE plots. Litter lost between 15% and 18% of the original mass during the eight months of field incubation. On average, litter produced under elevated CO2 attained higher residual mass than control litter. On the other end, when litter was incubated in FACE plots it exhibited higher decay rates. These responses were strongly species‐specific. All litter increased their N content during decomposition, indicating immobilization of N from external sources. Independent of the initial quality, litter incubated on FACE soils immobilized less N, possibly as a result of lower N availability in the soil. Indeed, our results refer to a short‐term decomposition experiment. However, according to a longer‐term model extrapolation of our results, we anticipate that in Mediterranean climate, under elevated atmospheric CO2, soil organic C pool of forest ecosystems may initially display faster turnover, but soil N availability will eventually limit the process.

Relatively stable metal(loid) levels in surface soils of a semiarid Inner Mongolia steppe under multiple environmental change factors

Humans are altering the composition of biological communities through a variety of activities that increase rates of species invasions and species extinctions, at all scales, from local to global. These changes in components of the Earth's biodiversity cause concern for ethical and aesthetic reasons, but they also have a strong potential to alter ecosystem properties and the goods and services they provide to humanity. Ecological experiments, observations, and theoretical developments show that ecosystem properties depend greatly on biodiversity in terms of the functional characteristics of organisms present in the ecosystem and the distribution and abundance of those organisms over space and time. Species effects act in concert with the effects of climate, resource availability, and disturbance regimes in influencing ecosystem properties. Human activities can modify all of the above factors; here we focus on modification of these biotic controls. The scientific community has come to a broad consensus on many aspects of the re- lationship between biodiversity and ecosystem functioning, including many points relevant to management of ecosystems. Further progress will require integration of knowledge about biotic and abiotic controls on ecosystem properties, how ecological communities are struc- tured, and the forces driving species extinctions and invasions. To strengthen links to policy and management, we also need to integrate our ecological knowledge with understanding of the social and economic constraints of potential management practices. Understanding this complexity, while taking strong steps to minimize current losses of species, is necessary for responsible management of Earth's ecosystems and the diverse biota they contain.

Reproduction is an important life history trait that strongly affects dynamics of plant populations. Although it has been well documented that elevated carbon dioxide (CO2) in the atmosphere greatly enhances biomass production in plants, the overall effect of elevated CO2 on reproductive allocation (RA), i.e., the proportion of biomass allocated to reproductive structures, is little understood. We combined meta-analysis with graphical vector analysis to examine the overall effect of elevated CO2 on RA and how other environmental factors, such as low nutrients, drought and elevated atmospheric ozone (O3), interacted with elevated CO2 in affecting RA in herbaceous plants. Averaged across all species of different functional groups and environmental conditions, elevated CO2 had little effect on RA (-0.9%). RA in plants of different reproductive strategies and functional groups, however, differed in response to elevated CO2. For example, RA in iteroparous wild species decreased by 8%, while RA in iteroparous crops increased significantly (+14%) at elevated CO2. RA was unaffected by CO2 in plants grown with no stress or in low-nutrient soils. RA decreased at elevated CO2 and elevated O3, but increased in response to elevated CO2 in drought-stressed plants, suggesting that elevated CO2 could ameliorate the adverse effect of drought on crop production to some extent. Our results demonstrate that elevated CO2 and other global environmental changes have the potential to greatly alter plant community composition through differential effects on RA of different plant species and thus affect the dynamics of natural and agricultural ecosystems in the future.

Aims The influence of elevated atmospheric CO2 concentration and nitrogen(N) addition on soil carbon pool is one of the foci among international ecological research communities. The changes of soil carbon pool induced by atmospheric CO2 concentration and/or N deposition will lead to changes in atmospheric carbon pool and thus the global climate change. However, few studies have been carried out in the subtropical China. Our objective was to understand the effect of elevated CO2 concentration and N addition on soil carbon stability in south subtropical experimental forests. Methods Experimental forest ecosystems were constructed in open top chambers. Six native tree species in southern China were planted in these experimental forest ecosystems. The species were exposed to elevated CO2 and N addition in the open top chambers beginning in May 2005. The four treatments were: elevated CO2 and high N addition(CN), elevated CO2 and ambient N deposition(CC), high N addition and ambient CO2(NN), and ambient CO2 and ambient N deposition(CK). The elevated CO2 was(700 ± 20) μmol·mol–1. The total amount of added NH4NO3-N was 100 kg N·hm–2·a–1. In January 2010, soil samples were collected from the open top chambers and then relevant variables were measured. Important findings Elevated CO2 concentration and N addition(CN) effectively increased the soil total organic carbon in different soil layers, among which the increases in the lower soil layers(5–60 cm) were statistically significant. Different components of the active organic carbon pool differed in the responses to treatments. The differences in microbial biomass carbon were significant in the 0–5 cm, 5–10 cm and 10–20 cm soil layers amongthe treatments, and the readily oxidized carbon showed significant responses to the elevated CO2 concentration and N addition treatments in the 10–20 cm and 20–40 cm soil layers, while there was no significant difference in the dissolved organic carbon among different treatments in all the soil layers. The responses of carbon in different aggregate fractions differed among the treatments. The carbon in the 250–2 000 μm aggregates was significantly different among treatments in the 20–40 cm and 40–60 cm soil layers. The carbon in the 53–250 μm aggregates was susceptible to treatments in the 40–60 cm soil layer as both the CC and NN treatments facilitated the chronic carbon accumulation in deeper soil layers, especially under the CN treatment. Carbon in the 53 μm fraction showed significant differences among treatments in deeper soil layers(10–20 cm, 20–40 cm and 40–60 cm). In conclusion, elevated CO2 concentration and N addition increased soil organic carbon in the experimental forest ecosystems, and facilitated the accumulation of carbon in micro-aggregates and silt-clay fraction in deep soil layers, thus strengthened the stability of soil organic carbon pool.

The control of soil nitrogen (N) availability under elevated atmospheric CO2 is central to predicting changes in ecosystem carbon (C) storage and primary productivity. The effects of elevated CO2 on belowground processes have so far attracted limited research and they are assumed to be controlled by indirect effects through changes in plant physiology and chemistry. In this study, we investigated the effects of a 4‐year exposure to elevated CO2 (ambient + 400 µmol mol−1) in open top chambers under Scots pine (Pinus sylvestris L) seedlings on soil microbial processes of nitrification and denitrification. Potential denitrification (DP) and potential N2O emissions were significantly higher in soils from the elevated CO2 treatment, probably regulated indirectly by the changes in soil conditions (increased pH, C availability and NO3– production). Net N mineralization was mainly accounted for by nitrate production. Nitrate production was significantly larger for soil from the elevated CO2 treatment in the field when incubated in the laboratory under elevated CO2 (increase of 100%), but there was no effect when incubated under ambient CO2. Net nitrate production of the soil originating from the ambient CO2 treatment in the field was not influenced by laboratory incubation conditions. These results indicate that a direct effect of elevated atmospheric CO2 on soil microbial processes might take place. We hypothesize that physiological adaptation or selection of nitrifiers could occur under elevated CO2 through higher soil CO2 concentrations. Alternatively, lower microbial NH4 assimilation under elevated CO2 might explain the higher net nitrification. We conclude that elevated atmospheric CO2 has a major direct effect on the soil microbial processes of nitrification and denitrification despite generally higher soil CO2 concentrations compared to atmospheric concentrations.

Species loss can result in secondary extinctions and changes in ecosystem functions at distant trophic levels. Such effects of species loss are predicted to be affected by both the number of species lost within a trophic level (horizontal diversity) and the number of trophic levels lost (vertical diversity). We experimentally manipulated horizontal and vertical diversity within an aquatic insect community, and examined responses throughout the food web. Horizontal and vertical diversity both impacted ciliates: reduction of detritivorous insect diversity resulted in secondary extinctions and decreased density of ciliates, but only when an insect predator was simultaneously absent. Horizontal and vertical diversity differed in their effect on other foodweb processes, including detrital processing, predator growth, and densities of rotifers, flagellates and flatworms. These results caution that foodweb effects of multitrophic species loss may not be reliably predicted from manipulations of just one dimension of diversity.

Forests exchange large amounts of CO2 with the atmosphere and can influence and be influenced by atmospheric CO2. There has been a recent proliferation of literature on the effects of atmospheric CO2 on forest trees. More than 300 studies of trees on five different continents have been published in the last five years. These include an increasing number of field studies with a long‐term focus and involving CO2×stress or environment interactions. The recent data on long‐term effects of elevated atmospheric CO2 on trees indicate a potential for a persistent enhancement of tree growth for several years, although the only relevant long‐term datasets currently available are for juvenile trees.The current literature indicates a significantly larger average long‐term biomass increment under elevated CO2 for conifers (130%) than for deciduous trees (49%) in studies not involving stress components. However, stimulation of photosynthesis by elevated CO2 in long‐term studies was similar for conifers (62%) and deciduous trees (53%). Recent studies indicate that elevated CO2 causes a more persistent stimulation of biomass increment and photosynthesis than previously expected. Results of seedling studies, however, might not be applicable to other stages of tree development because of complications of age‐dependent and size‐dependent shifts in physiology and carbon allocation, which are accelerated by elevated CO2. In addition, there are many possible avenues to down‐regulation, making the predicted canopy CO2 exchange and growth of mature trees and forests in a CO2‐rich atmosphere uncertain. Although, physiological down‐regulation of photosynthetic rates has been documented in field situations, it is rarely large enough to offset entirely photosynthetic gains in elevated CO2. A persistent growth stimulation of individual mature trees has been demonstrated although this effect is more uncertain in trees in natural stands.Resource interactions can both constrain tree responses to elevated CO2 and be altered by them. Although drought can reduce gas‐exchange rates and offset the benefits of elevated CO2, even in well watered trees, stomatal conductance is remarkably less responsive to elevated CO2 than in herbaceous species. Stomata of a number of tree species have been demonstrated to be unresponsive to elevated CO2. We conclude that positive effects of CO2 on leaf area can be at least as important in determining canopy transpiration as negative, direct effects of CO2 on stomatal aperture. With respect to nutrition, elevated CO2 has the potential to alter tree–soil interactions that might influence future changes in ecosystem productivity. There is continued evidence that in most cases nutrient limitations diminish growth and photosynthetic responses to elevated CO2 at least to some degree, and that elevated CO2 can accelerate the appearance of nutrient limitations with increasing time of treatment. In many studies, tree biomass responses to CO2 are artefacts in the sense that they are merely responses to CO2‐induced changes in internal nutritional status of the tree.There are numerous interactions between CO2 and factors of the biotic and abiotic environment. The importance of increasing atmospheric CO2 concentrations for productivity is likely to be overestimated if these are not taken into account. Many interactions, however, are simply additive rather than synergistic or antagonistic. This appears to hold true for many parameters under elevated CO2 in combination with temperature, elevated O3, and other atmospheric pollutants. However, there is currently little evidence that elevated CO2 will counteract O3 damage. When the foliage content of C, mineral nutrients and secondary metabolites is altered by elevated CO2, tree×insect interactions are modified. In most trees, mycorrhizal interactions might be less important for direct effects of CO2 than for alleviating general nutrient deficiencies.Since many responses to elevated CO2 and their interactions with stress show considerable variability among species/genotypes, one principal research need is for comparative studies of a large variety of woody species and ecosystems under realistic conditions. We still need more long‐term experiments on mature trees and stands to address critical scaling issues likely to advance our understanding of responses to elevated CO2 at different stages of forest development and their interactions with climate and environment. The only tools available at present for coping with the consequences of rising CO2 are management of resources and selection of genotypes suitable for the future climate and environment. CONTENTS Summary 396 I. Introduction 396 II. Influence of experimental approach 398 III. Growth and development 403 IV. Impact of water relations 405 V. Photosynthesis and respiration 410 VI. Foliar chemical composition 415 VII. Interaction with mineral nutrition 417 VIII. Interactions with temperature 419 IX. Interactions with air pollution, u.v.‐B and salt 420 X. Biotic interactions 422 XI. The Ecosystem 423 XII. Future research needs and unresolved questions 429 Acknowledgements 429

Elevated atmospheric carbon dioxide (CO2) has the potential to alter soil carbon (C) and nitrogen (N) cycling in arid ecosystems through changes in net primary productivity. However, an associated feedback exists because any sustained increases in plant productivity will depend upon the continued availability of soil N. We took soils from under the canopies of major shrubs, grasses, and plant interspaces in a Mojave Desert ecosystem exposed to elevated atmospheric CO2 and incubated them in the laboratory with amendments of labile C and N to determine if elevated CO2 altered the mechanistic controls of soil C and N on microbial N cycling. Net ammonification increased under shrubs exposed to elevated CO2, while net nitrification decreased. Elevated CO2 treatments exhibited greater fluxes of N2O–N under Lycium spp., but not other microsites. The proportion of microbial/extractable organic N increased under shrubs exposed to elevated CO2. Heterotrophic N2‐fixation and C mineralization increased with C addition, while denitrification enzyme activity and N2O–N fluxes increased when C and N were added in combination. Laboratory results demonstrated the potential for elevated CO2 to affect soil N cycling under shrubs and supports the hypothesis that energy limited microbes may increase net inorganic N cycling rates as the amount of soil‐available C increases under elevated CO2. The effect of CO2 enrichment on N‐cycling processes is mediated by its effect on the plants, particularly shrubs. The potential for elevated atmospheric CO2 to lead to accumulation of NH4+ under shrubs and the subsequent volatilization of NH3 may result in greater losses of N from this system, leading to changes in the form and amount of plant‐available inorganic N. This introduces the potential for a negative feedback mechanism that could act to constrain the degree to which plants can increase productivity in the face of elevated atmospheric CO2.

Effects of elevated O3 and CO2 on the relative contribution of carbohydrates to soil organic matter in an agricultural soil

As global change continues to intensify, the mode and rate of nitrogen input from the atmosphere to grassland ecosystems had changed dramatically. Firstly, we conducted a systematic analysis of the literature on the topic of nitrogen deposition impacts over the past 30 years using a bibliometric analysis. A systematic review of the global research status, publication patterns, research hotspots and important literature. We found a large number of publications in the Chinese region, and mainly focuses on the field of microorganisms. Secondly, we used a meta-analysis to focus on microbial changes using the Chinese grassland ecosystem as an example. The results show that the research on nitrogen deposition in grassland ecosystems shows an exponential development trend, and the authors and research institutions of the publications are mainly concentrated in China, North America, and Western Europe. The keyword clustering results showed 11 important themes labeled climate change, elevated CO2, species richness and diversity, etc. in these studies. The burst keyword analysis indicated that temperature sensitivity, microbial communities, etc. are the key research directions. The results of the meta-analysis found that nitrogen addition decreased soil microbial diversity, and different ecosystems may respond differently. Treatment time, nitrogen addition rate, external environmental conditions, and pH had major effects on microbial alpha diversity and biomass. The loss of microbial diversity and the reduction of biomass with nitrogen fertilizer addition will alter ecosystem functioning, with dramatic impacts on global climate change. The results of the study will help researchers to further understand the subject and have a deep understanding of research hotspots, which are of great value to future scientific research.

Temperate forests across Europe and eastern North America have become denser since the 1950s due to less intensive forest management and global environmental changes such as nitrogen deposition and climate warming. Denser tree canopies result in lower light availability at the forest floor. This shade may buffer the effects of nitrogen deposition and climate warming on understorey plant communities.We conducted an innovative in situ field experiment to study the responses of co‐occurring soil microbial and understorey plant communities to nitrogen addition, enhanced light availability and experimental warming in a full‐factorial design.We determined the effects of multiple environmental drivers and their interactions on the soil microbial and understorey plant communities, and assessed to what extent the soil microbial and understorey plant communities covary.High light led to lower biomass of the soil microbes (analysed by phospholipid fatty acids), but the soil microbial structure, i.e. the ratio of fungal biomass to bacterial biomass, was not affected by light availability. The composition of the soil bacterial community (analysed by high‐throughput sequencing) was affected by both light availability and warming (and their interaction), but not by nitrogen addition. Yet, the number of unique operational taxonomic units was higher in plots with nitrogen addition, and there were significant interactive effects of light and nitrogen addition. Light availability also determined the composition of the plant community; no effects of nitrogen addition and warming were observed. The soil bacterial and plant communities were co‐structured, and light availability explained a large part of the variance of this co‐structure.We provide robust evidence for the key role of light in affecting both the soil microbial and plant communities in forest understoreys. Our results advocate for more multifactor global change experiments that investigate the mechanism underlying the (in) direct effects of light on the plant–soil continuum in forests.Aplain language summaryis available for this article.