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Abstract Global interest and investment in nature‐based solutions (NbS) are rapidly increasing because of the potential of this approach to concurrently counter biodiversity loss, provide cost‐effective measures for climate change adaptations, and maintain natural processes that underpin human health and wellbeing.Recognition is growing that grasslands in many regions will protect carbon stores more effectively than forests in the warmer, drier, more fire‐prone conditions of the future while also serving as hotspots for biodiversity. Yet grasslands have received less attention for their NbS potential. Despite the wide‐ranging goals of this approach, many investments in nature‐based solutions also have focused narrowly on using plants to meet climate pledges, often without considering plant interactions with herbivores and the abiotic environment that jointly control ecosystem functioning and underpin the success of nature‐based solutions.Here, we review the roles that large and small vertebrate and invertebrate herbivores play in the ability of the world's grasslands to provide nature‐based solutions, with a focus on wild herbivore impacts on biodiversity and carbon storage.Synthesis. Planning for nature‐based solutions with a holistic, ecologically informed view that includes the role of herbivores and their interaction with plants and the environment will allow NbS investments to more likely achieve successful, sustainable outcomes. 

Abstract Grasslands cover approximately a third of the Earth’s land surface and account for about a third of terrestrial carbon storage. Yet, we lack strong predictive models of grassland plant biomass, the primary source of carbon in grasslands. This lack of predictive ability may arise from the assumption of linear relationships between plant biomass and the environment and an underestimation of interactions of environmental variables. Using data from 116 grasslands on six continents, we show unimodal relationships between plant biomass and ecosystem characteristics, such as mean annual precipitation and soil nitrogen. Further, we found that soil nitrogen and plant diversity interacted in their relationships with plant biomass, such that plant diversity and biomass were positively related at low levels of nitrogen and negatively at elevated levels of nitrogen. Our results show that it is critical to account for the interactive and unimodal relationships between plant biomass and several environmental variables to accurately include plant biomass in global vegetation and carbon models. 

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. 

Abstract Forbs (“wildflowers”) are important contributors to grassland biodiversity but are vulnerable to environmental changes. In a factorial experiment at 94 sites on 6 continents, we test the global generality of several broad predictions: (1) Forb cover and richness decline under nutrient enrichment, particularly nitrogen enrichment. (2) Forb cover and richness increase under herbivory by large mammals. (3) Forb richness and cover are less affected by nutrient enrichment and herbivory in more arid climates, because water limitation reduces the impacts of competition with grasses. (4) Forb families will respond differently to nutrient enrichment and mammalian herbivory due to differences in nutrient requirements. We find strong evidence for the first, partial support for the second, no support for the third, and support for the fourth prediction. Our results underscore that anthropogenic nitrogen addition is a major threat to grassland forbs, but grazing under high herbivore intensity can offset these nutrient effects. 

ABSTRACT Accurately representing the relationships between nitrogen supply and photosynthesis is crucial for reliably predicting carbon–nitrogen cycle coupling in Earth System Models (ESMs). Most ESMs assume positive correlations amongst soil nitrogen supply, leaf nitrogen content, and photosynthetic capacity. However, leaf photosynthetic nitrogen demand may influence the leaf nitrogen response to soil nitrogen supply; thus, responses to nitrogen supply are expected to be the largest in environments where demand is the greatest. Using a nutrient addition experiment replicated across 26 sites spanning four continents, we demonstrated that climate variables were stronger predictors of leaf nitrogen content than soil nutrient supply. Leaf nitrogen increased more strongly with soil nitrogen supply in regions with the highest theoretical leaf nitrogen demand, increasing more in colder and drier environments than warmer and wetter environments. Thus, leaf nitrogen responses to nitrogen supply are primarily influenced by climatic gradients in photosynthetic nitrogen demand, an insight that could improve ESM predictions. 

Abstract Background and aimsA synergistic response of aboveground plant biomass production to combined nitrogen (N) and phosphorus (P) addition has been observed in many ecosystems, but the underlying mechanisms and their relative importance are not well known. We aimed at evaluating several mechanisms that could potentially cause the synergistic growth response, such as changes in plant biomass allocation, increased N and P uptake by plants, and enhanced ecosystem nutrient retention. MethodsWe studied five grasslands located in Europe and the USA that are subjected to an element addition experiment composed of four treatments: control (no element addition), N addition, P addition, combined NP addition. ResultsCombined NP addition increased the total plant N stocks by 1.47 times compared to the N treatment, while total plant P stocks were 1.62 times higher in NP than in single P addition. Further, higher N uptake by plants in response to combined NP addition was associated with reduced N losses from the soil (evaluated based on soil δ¹⁵N) compared to N addition alone, indicating a higher ecosystem N retention. In contrast, the synergistic growth response was not associated with significant changes in plant resource allocation. ConclusionsOur results demonstrate that the commonly observed synergistic effect of NP addition on aboveground biomass production in grasslands is caused by enhanced N uptake compared to single N addition, and increased P uptake compared to single P addition, which is associated with a higher N and P retention in the ecosystem. 

Abstract Nutrient availability and grazing are known as main drivers of grassland plant diversity, and increased nutrient availability and long‐term cessation of grazing often decrease local‐scale plant diversity. Experimental tests of mechanisms determining plant diversity focus mainly on vascular plants (VP), whereas non‐vascular plants (NVP, here bryophytes) have been ignored. It is therefore not known how the current models based on VPs predict the rates of total (NVP + VP) losses in plant diversity.Here we used plant community data, including VPs and NVPs, from nine sites in Europe and North America and belonging to the Nutrient Network experiment, to test whether neglecting NVPs leads to biased estimates of plant diversity loss rates. The plant communities were subjected to factorial addition of nitrogen (N), phosphorus (P), potassium with micronutrients (K_+μ), as well as a grazing exclusion combined with multi‐nutrient fertilization (NPK_+μ) treatment.We found that nutrient additions reduced both NVP and VP species richness, but the effects on NVP species richness were on average stronger than on VPs: NVP species richness decreased 67%, while VP species richness decreased 28%, causing their combined richness to decrease 38% in response to multi‐nutrient (NPK_+μ) fertilization. Thus, VP diversity alone underestimated total plant diversity loss by 10 percentage points.Although NVP and VP species diversities similarly declined in response to N and NPK_+μfertilizations, the evenness of NVPs increased and that of VPs remained unchanged. NP, NPK_+μfertilization and NPK_+μfertilization combined with grazing exclusion, associated with decreasing light availability at ground level, led to the strongest loss of NVP species or probability of NVP presence. However, grazing did not generally mitigate the fertilization effects.Synthesis. In nine grassland sites in Europe and North America, nutrient addition caused a larger relative decline in non‐vascular plant (NVP) than vascular plant species richness. Hence, not accounting for NVPs can lead to underestimation of losses in plant diversity in response to continued nutrient pollution of grasslands. 

Ecosystems are experiencing changing global patterns of mean annual precipitation (MAP) and enrichment with multiple nutrients that potentially colimit plant biomass production. In grasslands, mean aboveground plant biomass is closely related to MAP, but how this relationship changes after enrichment with multiple nutrients remains unclear. We hypothesized the global biomass–MAP relationship becomes steeper with an increasing number of added nutrients, with increases in steepness corresponding to the form of interaction among added nutrients and with increased mediation by changes in plant community diversity. We measured aboveground plant biomass production and species diversity in 71 grasslands on six continents representing the global span of grassland MAP, diversity, management, and soils. We fertilized all sites with nitrogen, phosphorus, and potassium with micronutrients in all combinations to identify which nutrients limited biomass at each site. As hypothesized, fertilizing with one, two, or three nutrients progressively steepened the global biomass–MAP relationship. The magnitude of the increase in steepness corresponded to whether sites were not limited by nitrogen or phosphorus, were limited by either one, or were colimited by both in additive, or synergistic forms. Unexpectedly, we found only weak evidence for mediation of biomass–MAP relationships by plant community diversity because relationships of species richness, evenness, and beta diversity to MAP and to biomass were weak or opposing. Site-level properties including baseline biomass production, soils, and management explained little variation in biomass–MAP relationships. These findings reveal multiple nutrient colimitation as a defining feature of the global grassland biomass–MAP relationship.