Effects of altered precipitation on insect community composition and structure in a meadow steppe

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Tradeoffs and thresholds in the effects of nitrogen addition on biodiversity and ecosystem functioning: evidence from inner Mongolia Grasslands

Nitrogen (N) deposition is widely considered an environmental problem that leads to biodiversity loss and reduced ecosystem resilience; but, N fertilization has also been used as a management tool for enhancing primary production and ground cover, thereby promoting the restoration of degraded lands. However, empirical evaluation of these contrasting impacts is lacking. We tested the dual effects of N enrichment on biodiversity and ecosystem functioning at different organizational levels (i.e., plant species, functional groups, and community) by adding N at 0, 1.75, 5.25, 10.5, 17.5, and 28.0 g N m^?2 yr^?1 for four years in two contrasting field sites in Inner Mongolia: an undisturbed mature grassland and a nearby degraded grassland of the same type. N addition had both quantitatively and qualitatively different effects on the two communities. In the mature community, N addition led to a large reduction in species richness, accompanied by increased dominance of early successional annuals and loss of perennial grasses and forbs at all N input rates. In the degraded community, however, N addition increased the productivity and dominance of perennial rhizomatous grasses, with only a slight reduction in species richness and no significant change in annual abundance. The mature grassland was much more sensitive to N‐induced changes in community structure, likely as a result of higher soil moisture accentuating limitation by N alone. Our findings suggest that the critical threshold for N‐induced species loss to mature Eurasian grasslands is below 1.75 g N m^?2 yr^?1, and that changes in aboveground biomass, species richness, and plant functional group composition to both mature and degraded ecosystems saturate at N addition rates of approximately 10.5 g N m^?2 yr^?1. This work highlights the tradeoffs that exist in assessing the total impact of N deposition on ecosystem function.     

Plant community responses to nitrogen addition and increased precipitation: the importance of water availability and species traits

Global nitrogen (N) enrichment and changing precipitation regimes are likely to alter plant community structure and composition, with consequent influences on biodiversity and ecosystem functioning. Responses of plant community structure and composition to N addition and increased precipitation were examined in a temperate steppe in northern China. Increased precipitation and N addition stimulated and suppressed community species richness, respectively, across 6 years (2005–2010) of the manipulative experiment. N addition and increased precipitation significantly altered plant community structure and composition at functional groups levels. The significant relationship between species richness and soil moisture (SM) suggests that plant community structure is mediated by water under changing environmental conditions. In addition, plant height played an important role in affecting the responses of plant communities to N addition, and the effects of increased precipitation on plant community were dependent on species rooting depth. Our results highlight the importance and complexity of both abiotic (SM) and biotic factors (species traits) in structuring plant community under changing environmental scenarios. These findings indicate that knowledge of species traits can contribute to mechanistic understanding and projection of vegetation dynamics in response to future environmental change.     

Dominant species maintain ecosystem function with non-random species loss

Loss of species caused by widespread stressors, such as drought and fragmentation, is likely to be non‐random depending on species abundance in the community. We experimentally reduced the number of rare and uncommon plant species while independently reducing only the abundance of dominant grass species in intact, native grassland. This allowed us to simulate a non‐random pattern of species loss, based on species abundances, from communities shaped by natural ecological interactions and characterized by uneven species abundance distributions. Over two growing seasons, total above‐ground net primary productivity (ANPP) declined with reductions in abundance of the dominant species but was unaffected by a threefold decline in richness of less common species. In contrast, productivity of the remaining rare and uncommon species decreased with declining richness, in part due to loss of complementary interactions among these species. However, increased production of the dominant grasses offset the negative effects of species loss. We conclude that the dominant species, as controllers of ecosystem function, can provide short‐term resistance to reductions in ecosystem function when species loss is nonrandom. However, the concurrent loss of complementary interactions among rare and uncommon species, the most diverse component of communities, may contribute to additional species loss and portends erosion of ecosystem function in the long term.     

The unseen invaders: introduced earthworms as drivers of change in plant communities in North American forests (a meta‐analysis)

Globally, biological invasions can have strong impacts on biodiversity as well as ecosystem functioning. While less conspicuous than introduced aboveground organisms, introduced belowground organisms may have similarly strong effects. Here, we synthesize for the first time the impacts of introduced earthworms on plant diversity and community composition in North American forests. We conducted a meta‐analysis using a total of 645 observations to quantify mean effect sizes of associations between introduced earthworm communities and plant diversity, cover of plant functional groups, and cover of native and non‐native plants. We found that plant diversity significantly declined with increasing richness of introduced earthworm ecological groups. While plant species richness or evenness did not change with earthworm invasion, our results indicate clear changes in plant community composition: cover of graminoids and non‐native plant species significantly increased, and cover of native plant species (of all functional groups) tended to decrease, with increasing earthworm biomass. Overall, these findings support the hypothesis that introduced earthworms facilitate particular plant species adapted to the abiotic conditions of earthworm‐invaded forests. Further, our study provides evidence that introduced earthworms are associated with declines in plant diversity in North American forests. Changing plant functional composition in these forests may have long‐lasting effects on ecosystem functioning.     

Mycorrhizal suppression and phosphorus addition influence the stability of plant community composition and function in a temperate steppe

Nutrient enrichment can reduce ecosystem stability, typically measured as temporal stability of a single function, e.g. plant productivity. Moreover, nutrient enrichment can alter plant–soil interactions (e.g. mycorrhizal symbiosis) that determine plant community composition and productivity. Thus, it is likely that nutrient enrichment and interactions between plants and their soil communities co-determine the stability in plant community composition and productivity. Yet our understanding as to how nutrient enrichment affects multiple facets of ecosystem stability, such as functional and compositional stability, and the role of above–belowground interactions are still lacking. We tested how mycorrhizal suppression and phosphorus (P) addition influenced multiple facets of ecosystem stability in a three-year field study in a temperate steppe. Here we focused on the functional and compositional stability of plant community; functional stability is the temporal community variance in primary productivity; compositional stability is represented by compositional resistance, turnover, species extinction and invasion. Community variance was partitioned into population variance defined as community productivity weighted average of the species temporal variance in performance, and species synchrony defined as the degree of temporal positive covariation among species. Compared to treatments with mycorrhizal suppression, the intact AM fungal communities reduced community variance in primary productivity by reducing species synchrony at high levels of P addition. Species synchrony and population variance were linearly associated with community variance with the intact AM fungal communities, while these relationships were decoupled or weakened by mycorrhizal suppression. The intact AM fungal communities promoted the compositional resistance of plant communities by reducing compositional turnover, but this effect was suppressed by P addition. P addition increased the number of species extinctions and thus promoted compositional turnover. Our study shows P addition and AM fungal communities can jointly and independently modify the various components of ecosystem stability in terms of plant community productivity and composition.     

A role for the sampling effect in invaded ecosystems

Species loss and invasion of exotic species are two components of global biodiversity change that are expected to influence ecosystem functioning. Yet how they interact in natural settings remains unclear. Experiments have revealed two major mechanisms for the observed increase in primary productivity with plant species richness. Plant productivity may rise with species richness due to the increased amount of resources used by more diverse communities (niche complementarity) or through the increased probability of including a highly productive, dominant species in the community (sampling effect). Current evidence suggests that niche complementarity is the most relevant mechanism, whereas the sampling effect would only play a minor and transient role in natural systems. In turn, exotic species can invade by using untapped resources or because they possess a fitness advantage over resident species allowing them to dominate the community. We argue that the sampling effect can be a significant biodiversity mechanism in ecosystems invaded by dominant exotic species, and that the effect can be persistent even after decades of succession. We illustrate this idea by analyzing tree species richness–productivity relationships in a subtropical montane forest (NW Argentina) heavily invaded by Ligustrum lucidum, an evergreen tree from Asia. We found that the forest biomass increased along a natural gradient of tree species richness whether invaded by L. lucidum or not. Consistent with the sampling effect, L. lucidum invasion tripled total tree biomass irrespective of species richness, and monocultures of L. lucidum were more productive than any of the most species‐rich, uninvaded communities. Hence, the sampling effect may not be restricted to randomly assembled, synthetic communities. We emphasize that studying invaded ecosystems may provide novel insights on the mechanisms underlying the effect of biodiversity on ecosystem function.     

Drivers of productivity and its temporal stability in a tropical tree diversity experiment

There is increasing evidence that mixed‐species forests can provide multiple ecosystem services at a higher level than their monospecific counterparts. However, most studies concerning tree diversity and ecosystem functioning relationships use data from forest inventories (under noncontrolled conditions) or from very young plantation experiments. Here, we investigated temporal dynamics of diversity–productivity relationships and diversity–stability relationships in the oldest tropical tree diversity experiment. Sardinilla was established in Panama in 2001, with 22 plots that form a gradient in native tree species richness of one‐, two‐, three‐ and five‐species communities. Using annual data describing tree diameters and heights, we calculated basal area increment as the proxy of tree productivity. We combined tree neighbourhood‐ and community‐level analyses and tested the effects of both species diversity and structural diversity on productivity and its temporal stability. General patterns were consistent across both scales indicating that tree–tree interactions in neighbourhoods drive observed diversity effects. From 2006 to 2016, mean overyielding (higher productivity in mixtures than in monocultures) was 25%–30% in two‐ and three‐species mixtures and 50% in five‐species stands. Tree neighbourhood diversity enhanced community productivity but the effect of species diversity was stronger and increased over time, whereas the effect of structural diversity declined. Temporal stability of community productivity increased with species diversity via two principle mechanisms: asynchronous responses of species to environmental variability and overyielding. Overyielding in mixtures was highest during a strong El Niño‐related drought. Overall, positive diversity–productivity and diversity–stability relationships predominated, with the highest productivity and stability at the highest levels of diversity. These results provide new insights into mixing effects in diverse, tropical plantations and highlight the importance of analyses of temporal dynamics for our understanding of the complex relationships between diversity, productivity and stability. Under climate change, mixed‐species forests may provide both high levels and high stability of production.     

Biodiversity–productivity relationship in ponds: Community and metacommunity patterns along time and environmental gradients

Primary production correlates with diversity in various ways. These patterns may result from the interaction of various mechanisms related to the environmental context and the spatial and temporal scale of analysis. However, empirical evidence on diversity‐productivity patterns typically considers single temporal and spatial scales, and does not include the effect of environmental variables. In a metacommunity of macrophytes in ephemeral ponds, we analysed the diversity‐productivity relationship patterns in the field, the importance of the environmental variables of pond size and heterogeneity on such relationship, and the variation of these patterns at local (community level) and landscape scales (metacommunity level) across 52 ponds on twelve occasions, over five years (2005–2009). Combining all sampling dates, there were 377 ponds and 1954 sample‐unit observations. Vegetation biomass was used as a proxy for productivity, and biodiversity was represented by species richness, evenness, and their interaction. Environmental variables comprised pond area, depth and internal heterogeneity. Productivity and species richness were not directly related at the metacommunity level, and were positively related at the community level. Taking environmental variables into account revealed positive species richness‐productivity relationships at the metacommunity level and positive quadratic relationships at the community level. Productivity showed both positive and negative linear and nonlinear relationships with the size and heterogeneity of ponds. We found a weak relationship between productivity and evenness. The identity of variables associated with productivity changed between spatial scales and through time. The pattern of relationships between productivity and diversity depends on spatial scale and environmental context, and changes idiosyncratically through time within the same ecosystem. Thus, the diversity‐productivity relationship is not only a property of the study system, but also a consequence of environmental variations and the temporal and spatial scale of analysis.     

Richness, structure and functioning in metazoan parasite communities

Ecosystem functioning, characterized by components such as productivity and stability, has been extensively linked with diversity in recent years, mainly in plant ecology. The aim of our study was thus to quantify general relationships between diversity, community structure and ecosystem functions in metazoan parasite communities. We used data on parasite communities from 15 species of marine fish hosts from coastal Chile. The volumetric abundance (volume of all parasite species per individual host, in mm³) was used as a surrogate for productivity. Species diversity was measured using both species richness and evenness, while community structure was estimated using the co‐occurrence indices V‐ratio, C‐score and a new C‐score_s index standardized for the number of host replicates. After correcting for fish size, 47% of host species show no relationship, 13% show a hump shaped curve and 40% show positive monotonic relationships between productivity and parasite richness across all host individuals in a sample. We obtained a logarithmically decreasing relationship between evenness and productivity for all fish species, and propose a ‘dominance‐resistance’ hypothesis based on immunity to explain this pattern. The stability of the parasite community, measured as the coefficient of variation in productivity among individual hosts, was strongly and positively related to mean species richness across the 15 host species. The C‐score_s index, based on the number of checkerboard units in the host‐parasite presence/absence matrix, increases linearly with mean productivity across the 15 host species, suggesting that parasite communities tend to be more structured when they are more productive. This is the likely reason why linear relationships between richness and productivity were not observed consistently in all fish species. Parasite communities provide some clear patterns for the diversity–ecosystem functioning debate in ecology, although other factors, such as the history of community assembly, may also influence these patterns.     

Herbivore species richness, composition and community structure mediate predator richness effects and top-down control of herbivore biomass

Changes in predator species richness can have important consequences for ecosystem functioning at multiple trophic levels, but these effects are variable and depend on the ecological context in addition to the properties of predators themselves. Here, we report an experimental study to test how species identity, community attributes, and community structure at the herbivore level moderate the effects of predator richness on ecosystem functioning. Using mesocosms containing predatory insects and aphid prey, we independently manipulated species richness at both predator and herbivore trophic levels. Community structure was also manipulated by changing the distribution of herbivore species across two plant species. Predator species richness and herbivore species richness were found to negatively interact to influence predator biomass accumulation, an effect which is hypothesised to be due to the breakdown of functional complementarity among predators in species-rich herbivore assemblages. The strength of predator suppression of herbivore biomass decreased as herbivore species richness and distribution across host plants increased, and positive predator richness effects on herbivore biomass suppression were only observed in herbivore assemblages of relatively low productivity. In summary, the study shows that the species richness, productivity and host plant distribution of prey communities can all moderate the general influence of predators and the emergence of predator species richness effects on ecosystem functioning.     

A gradient of nutrient enrichment reveals nonlinear impacts of fertilization on Arctic plant diversity and ecosystem function

Rapid environmental change at high latitudes is predicted to greatly alter the diversity, structure, and function of plant communities, resulting in changes in the pools and fluxes of nutrients. In Arctic tundra, increased nitrogen (N) and phosphorus (P) availability accompanying warming is known to impact plant diversity and ecosystem function; however, to date, most studies examining Arctic nutrient enrichment focus on the impact of relatively large (>25x estimated naturally occurring N enrichment) doses of nutrients on plant community composition and net primary productivity. To understand the impacts of Arctic nutrient enrichment, we examined plant community composition and the capacity for ecosystem function (net ecosystem exchange, ecosystem respiration, and gross primary production) across a gradient of experimental N and P addition expected to more closely approximate warming‐induced fertilization. In addition, we compared our measured ecosystem CO₂ flux data to a widely used Arctic ecosystem exchange model to investigate the ability to predict the capacity for CO₂ exchange with nutrient addition. We observed declines in abundance‐weighted plant diversity at low levels of nutrient enrichment, but species richness and the capacity for ecosystem carbon uptake did not change until the highest level of fertilization. When we compared our measured data to the model, we found that the model explained roughly 30%–50% of the variance in the observed data, depending on the flux variable, and the relationship weakened at high levels of enrichment. Our results suggest that while a relatively small amount of nutrient enrichment impacts plant diversity, only relatively large levels of fertilization—over an order of magnitude or more than warming‐induced rates—significantly alter the capacity for tundra CO₂ exchange. Overall, our findings highlight the value of measuring and modeling the impacts of a nutrient enrichment gradient, as warming‐related nutrient availability may impact ecosystems differently than single‐level fertilization experiments.     

Small mammal herbivores mediate the effects of soil nitrogen and invertebrate herbivores on grassland diversity

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From richer to poorer: successful invasion by freshwater fishes depends on species richness of donor and recipient basins

Evidence for the theory of biotic resistance is equivocal, with experiments often finding a negative relationship between invasion success and native species richness, and large‐scale comparative studies finding a positive relationship. Biotic resistance derives from local species interactions, yet global and regional studies often analyze data at coarse spatial grains. In addition, differences in competitive environments across regions may confound tests of biotic resistance based solely on native species richness of the invaded community. Using global and regional data sets for fishes in river and stream reaches, we ask two questions: (1) does a negative relationship exist between native and non‐native species richness and (2) do non‐native species originate from higher diversity systems. A negative relationship between native and non‐native species richness in local assemblages was found at the global scale, while regional patterns revealed the opposite trend. At both spatial scales, however, nearly all non‐native species originated from river basins with higher native species richness than the basin of the invaded community. Together, these findings imply that coevolved ecological interactions in species‐rich systems inhibit establishment of generalist non‐native species from less diverse communities. Consideration of both the ecological and evolutionary aspects of community assembly is critical to understanding invasion patterns. Distinct evolutionary histories in different regions strongly influence invasion of intact communities that are relatively unimpacted by human actions, and may explain the conflicting relationship between native and non‐native species richness found at different spatial scales.     

The Influence of Historical Land Use and Water Availability on Grassland Restoration

The ecological role of historical land use has rarely been explored in the context of grassland restoration. We conducted a 4‐year field experiment in a steppe and an old field in Inner Mongolia in northern China to examine the influence of historical land use and water availability on ecosystem restoration. Species richness, evenness, and plant cover were higher in the steppe than in the old field. The steppe was more temporally stable compared with the old field in terms of species richness, evenness, plant density, and cover. Water addition increased peak aboveground biomass, belowground net primary productivity, species richness, plant density, and cover in both the steppe and the old field. Water addition also enhanced the stability of ecosystems and the restoration of grassland. Our findings suggested that historical land use determines community structure and influences the process of grassland restoration. Converting grasslands to farmland in semiarid areas can cause the long‐term loss of biodiversity and instability of ecosystem with consequent impacts on ecosystem services. The amendment of limited resources is an effective practice to increase the success of ecosystem restoration.     

Multiple nutrients and herbivores interact to govern diversity,productivity, composition,and infection in a successional grassland

In spite of increasing awareness that interactions between herbivory and the supply rates of multiple nutrients control biodiversity, ecosystem functions and ecosystem services in ecological communities, few experimental studies have concurrently examined the independent and joint effects of multiple nutrients and mammalian consumers on these responses in natural systems. Here we quantify the independent and interactive effects of multiple concurrent changes to resources and consumers in an invaded annual grassland community in California. In a two‐year study using thirty‐seven 400‐m² plots, we examine interactions among four nutrient treatments (N, P, K and micronutrients) and a keystone herbivore (pocket gopher Thomomys bottae) on four plant community outcomes: 1) plant diversity, 2) functional group composition, 3) net biomass production, an important ecosystem function, and 4) infection risk by a group of viral pathogens shared by crop and non‐crop grasses (barley and cereal yellow dwarf viruses), an important regulating ecosystem service. We found that grassland biodiversity and infection risk were controlled by nutrient identity and supply ratio whereas nutrients interacted strongly with consumers to control grassland composition and net primary productivity. The most important insights arising from this multi‐factor experiment are that net biomass production increased with phosphorus or nitrogen supply; however, when gophers were present, nitrogen caused no net effect on biomass production. In addition, infection risk was driven by phosphorus, nitrogen and micronutrient supply. Infection in a sentinel host increased strongly with the addition of micronutrients or phosphorus; however, infection declined with increasing N/P supply ratio, indicating stoichiometric control of infection risk. Finally, in spite of manipulating multiple factors, plant species richness declined with nitrogen, alone. The importance of higher‐order interactions demonstrates that a multi‐factor approach is critical for effective predictions in a world in which anthropogenic activities are simultaneously changing herbivore abundance and the relative supply of many nutrients.     

Species and genotype diversity drive community and ecosystem properties in experimental microcosms

Species diversity is important to ecosystems because of the increased probability of including species that are strong interactors and/or because multiple-species communities are more efficient at using resources due to synergisms and resource partitioning. Genetic diversity also contributes to ecosystem function through effects on primary productivity, community structure and resilience, and modulating energy and nutrient fluxes. Lacking are studies investigating the relationship between ecosystem function and diversity where hierarchical levels of biological diversity are systematically varied during experimentation. In this experiment, we manipulated both species and genotypic diversity of two Daphnia species in microcosms initially seeded with Chlamydomonas and measured community- and ecosystem-level properties to determine which level of diversity was most important for explaining variation in the property. Our results show that species diversity alters bacterial community composition while high genotypic diversity reduces bacterial richness and primary productivity. In addition, the highest levels of genotypic and species richness appear to increase community and ecosystem stability. These findings reveal that species and genotypic diversity are significant drivers of community and ecosystem properties and stability.     

Quantifying levels of biological invasion: towards the objective classification of invaded and invasible ecosystems

Biological invasions are a global phenomenon that threatens biodiversity, and few, if any, ecosystems are free from alien species. The outcome of human‐mediated introductions is affected by the invasiveness of species and invasibility of ecosystems, but research has primarily focused on defining, characterizing and identifying invasive species; ecosystem invasibility has received much less attention. A prerequisite for characterizing invasibility is the ability to compare levels of invasion across ecosystems. In this paper, we aim to identify the best way to quantify the level of invasion by nonnative animals and plants by reviewing the advantages and disadvantages of different metrics. We explore how interpretation and choice of these measures can depend on the objective of a study or management intervention. Based on our review, we recommend two invasion indices and illustrate their use by applying them to two case studies. Relative alien species richness and relative alien species abundance indicate the contribution that alien species make to a community. They are easy to measure, can be applied to various taxa, are independent of scale and are comparable across regions and ecosystems, and historical data are often available. The relationship between relative alien richness and abundance can indicate the presence of dominant alien species and the trajectory of invasion over time, and can highlight ecosystems and sites that are heavily invaded or especially susceptible to invasion. Splitting species into functional groups and examining invasion patterns of transformer species may be particularly instructive for gauging effects of alien invasion on ecosystem structure and function. Establishing standard, transparent ways to define and quantify invasion level will facilitate meaningful comparisons among studies, ecosystem types and regions. It is essential for progress in ecology and will help guide ecosystem restoration and management.     

Harvester ants modify seed rain using nest vegetation and granivory

1. In annual plant communities, the seed bank determines each generation's potential emergence and any factor that influences it could alter community structure. Harvester ants are common seed predators that may cause seed sources and seed rain composition to vary. 2. In semi‐arid shrubland of the Negev desert of Israel, we hypothesised that the positive effects of dense vegetation on nests of the harvester ant Messor ebeninus Sant. would outweigh reduction by granivory. 3. To test whether nests were seed sources, we removed vegetation on 10 of 20 sampled nests prior to seeding. We tested the interaction between seed sources and granivory by locating seed traps at three locations: nests, foraging trails, and unvisited areas. Trapped seeds were recorded during spring and summer dispersal. Plant recruitment from the altered seed bank was recorded the following spring. 4. During April, seeds were more abundant near nests than the other locations and at uncut compared with cut nests. Foraging trails had fewer species and higher equitability compared with nests or unvisited areas. May and June to August had highest species richness at nests, but richness of cut and uncut nests were similar. Ordination of plant recruitment showed slightly lower vegetation community variation by diminished species in the cut seed bank. 5. While intact nest vegetation initially had high abundance of locally deposited seeds, it apparently blocked wind‐dispersed seeds later in the summer. Conversely, granivory reduced seed species richness only on trails and decreased dominance structure by preferential removal of some species.     

The potential bias of nitrogen deposition effects on primary productivity and biodiversity

Atmospheric nitrogen (N) deposition is composed of both inorganic nitrogen (IN) and organic nitrogen (ON), and these sources of N may exhibit different impacts on ecosystems. However, our understanding of the impacts of N deposition is largely based on experimental gradients of INs or more rarely ONs. Thus, the effects of N deposition on ecosystem productivity and biodiversity may be biased. We explored the differential impacts of N addition with different IN:ON ratios (0:10, 3:7, 5:5, 7:3, and 10:0) on aboveground net primary productivity (ANPP) of plant community and plant diversity in a typical temperate grassland with a long-term N addition experiment. Soil pH, litter biomass, soil IN concentration, and light penetration were measured to examine the potential mechanisms underlying species loss with N addition. Our results showed that N addition significantly increased plant community ANPP by 68.33%–105.50% and reduced species richness by 16.20%–37.99%. The IN:ON ratios showed no significant effects on plant community ANPP. However, IN-induced species richness loss was about 2.34 times of ON-induced richness loss. Soil pH was positively related to species richness, and they exhibited very similar response patterns to IN:ON ratios. It implies that soil acidification accounts for the different magnitudes of species loss with IN and ON additions. Overall, our study suggests that it might be reasonable to evaluate the effects of N deposition on plant community ANPP with either IN or ON addition. However, the evaluation of N deposition on biodiversity might be overestimated if only IN is added or underestimated if only ON is added.