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ABSTRACT Biodiversity promotes ecosystem productivity and stability, positive impacts that often strengthen over time. But ongoing global changes such as rising atmospheric carbon dioxide (CO₂) levels and anthropogenic nitrogen (N) deposition may modulate the impact of biodiversity on ecosystem productivity and stability over time. Using a quarter‐century grassland biodiversity‐global change experiment we show that diversity increasingly enhanced productivity over time irrespective of global change treatments. In contrast, the positive influence of diversity on ecosystem stability strengthened over time under ambient conditions but weakened to varying degrees under global change treatments, largely driven by a greater reduction in species asynchrony under global changes. Thus, over 25 years, CO₂and N enrichment gradually eroded some of the positive effects of biodiversity on ecosystem stability. As elevated CO₂, N eutrophication, and biodiversity loss increasingly co‐occur in grasslands globally, our results raise concerns about their potential joint detrimental effects on long‐term grassland stability. 

Abstract Anthropogenic activities add more reactive nitrogen (N) to the environment than all natural sources combined, and the fate of this N is of environmental concern. If N that is deposited on terrestrial ecosystems through atmospheric deposition is retained in plant tissues or soil organic matter, it could stimulate carbon (C) storage in plant biomass or soils. However, added N also could increase soil inorganic N concentrations and leaching, potentially polluting watersheds, particularly in areas with low-N soils and/or a high propensity for leaching, such as sandy or arid areas. Here, we assessed N allocation and retention across a 13-year experimental N addition gradient in a temperate grassland. We found that N accumulation decreased significantly at mid- to high levels of N addition compared to the Control, such that ecosystem N pools were equivalent across a 10 g m⁻² year⁻¹range of annual N addition rates (0–10 g N m⁻² year⁻¹), which spans most of the global range of N deposition. Nitrogen addition increased plant tissue percent N, but the total pool of N did not increase because of reduced plant biomass, particularly in roots. Nitrogen addition also increased soil inorganic N concentrations. Our results indicate that N addition is unlikely to increase grassland N pools, particularly in sandy or low-fertility ecosystems with a high potential for leaching because high application rates lead to N saturation, and additional inputs are lost. 

Abstract Litter decomposition is one of the largest carbon (C) fluxes in terrestrial ecosystems and links aboveground biomass to soil C pools. In grasslands, decomposition drivers have received substantial attention but the role of grassland herbivores in influencing decay rates is often ignored despite their potentially large effects on standing biomass and nutrient cycling. Recent work has demonstrated that nutrient addition increases early-stage decay and suppresses late-stage decay. Mammalian herbivores can mediate the effects of nutrient supply on biomass, suggesting herbivores may alter the effects of nutrients on decomposition, though this is largely unknown. We examined how herbivory mediates the effects of nutrient supply on long-term decomposition across 19 grassland sites of the Nutrient Network distributed experiment. At each site, a full-factorial experiment of combined nitrogen (N), phosphorus (P), and micronutrient (K) enrichment (‘control’ or ‘ + NPK’) and mammalian herbivore (> ~ 50 g) exclusion (‘unfenced’ or ‘fenced’) was carried out in a randomized block design. We hypothesized that nutrient effects on litter decomposition would be strongest where herbivores caused the greatest reductions in aboveground plant biomass (i.e., at sites with more intense herbivory). After accounting for wide variation in decomposition rates across sites, we found that, within sites, elevated nutrients increased early-stage decay and suppressed late-stage decay. In contrast, neither herbivore exclusion (i.e., fencing) nor site level changes in aboveground biomass due to herbivory altered the nutrient effects on decomposition rates. Across grasslands, our results indicate that elevated nutrient supply modifies litter decomposition rates independent of herbivore impacts. 

Measuring the chemical traits of leaf litter is important for understanding plants’ influence on nutrient cycles, including through nutrient resorption and litter decomposition, but conventional leaf trait measurements are often destructive and labor-intensive. Here, we develop and evaluate the performance of partial least-squares regression models that use reflectance spectra of intact or ground leaves to estimate leaf litter traits, including carbon and nitrogen concentration, carbon fractions, and leaf mass per area (LMA). Our analyses included more than 300 samples of senesced foliage from 11 species of temperate trees, including both needleleaf and broadleaf species. Across all samples, we could predict each trait with moderate-to-high accuracy from both intact-leaf litter spectra (validation R² = 0.543–0.941; %root mean squared error (RMSE) = 7.49–18.5) and ground-leaf litter spectra (validation R² = 0.491–0.946; %RMSE = 7.00–19.5). Notably, intact-leaf spectra yielded better predictions of LMA. Our results support the feasibility of building models to estimate multiple chemical traits from leaf litter of a range of species. In particular, intact-leaf spectral models allow non-destructive trait estimation in a matter of seconds, which could enable researchers to measure the same leaves over time in studies of nutrient resorption. 

Abstract Plant disease often increases with N, decreases with CO₂, and increases as biodiversity is lost (i.e., the dilution effect). Additionally, all these factors can indirectly alter disease by changing host biomass and hence density-dependent disease transmission. Yet over long periods of time as communities undergo compositional changes, these biomass-mediated pathways might fade, intensify, or even reverse in direction. Using a field experiment that has manipulated N, CO₂, and species richness for over 20 years, we compared severity of a specialist rust fungus (Puccinia andropogonis) on its grass host (Andropogon gerardii) shortly after the experiment began (1999) and twenty years later (2019). Between these two sampling periods, two decades apart, we found that disease severity consistently increased with N and decreased with CO₂. However, the relationship between diversity and disease reversed from a dilution effect in 1999 (more severe disease in monocultures) to an amplification effect in 2019 (more severe disease in mixtures). The best explanation for this reversal centered on host density (i.e., aboveground biomass), which was initially highest in monoculture, but became highest in mixtures two decades later. Thus, the diversity-disease pattern reversed, but disease consistently increased with host biomass. These results highlight the consistency of N and CO₂as drivers of plant disease in the Anthropocene and emphasize the critical role of host biomass—despite potentially variable effects of diversity—for relationships between biodiversity and disease.