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  1. Abstract Aim The microbial metabolic quotient (MMQ; mg CO 2 ‐C/mg MBC/h), defined as the amount of microbial CO 2 respired (MR; mg CO 2 ‐C/kg soil/h) per unit of microbial biomass C (MBC; mg C/kg soil), is a key parameter for understanding the microbial regulation of the carbon (C) cycle, including soil C sequestration. Here, we experimentally tested hypotheses about the individual and interactive effects of multiple nutrient addition (nitrogen + phosphorus + potassium + micronutrients) and herbivore exclusion on MR, MBC and MMQ across 23 sites (five continents). Our sites encompassed a wide range of edaphoclimatic conditions; thus, we assessed which edaphoclimatic variables affected MMQ the most and how they interacted with our treatments. Location Australia, Asia, Europe, North/South America. Time period 2015–2016. Major taxa Soil microbes. Methods Soils were collected from plots with established experimental treatments. MR was assessed in a 5‐week laboratory incubation without glucose addition, MBC via substrate‐induced respiration. MMQ was calculated as MR/MBC and corrected for soil temperatures (MMQsoil). Using linear mixed effects models (LMMs) and structural equation models (SEMs), we analysed how edaphoclimatic characteristics and treatments interactively affected MMQsoil. Results MMQsoil was higher in locations with higher mean annual temperature, lower water holding capacity and lower soil organic C concentration, but did not respond to our treatments across sites as neither MR nor MBC changed. We attributed this relative homeostasis to our treatments to the modulating influence of edaphoclimatic variables. For example, herbivore exclusion, regardless of fertilization, led to greater MMQsoil only at sites with lower soil organic C (< 1.7%). Main conclusions Our results pinpoint the main variables related to MMQsoil across grasslands and emphasize the importance of the local edaphoclimatic conditions in controlling the response of the C cycle to anthropogenic stressors. By testing hypotheses about MMQsoil across global edaphoclimatic gradients, this work also helps to align the conflicting results of prior studies. 
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    Free, publicly-accessible full text available June 1, 2024
  2. null (Ed.)
    Arthropod herbivores cause substantial economic costs that drive an increasing need to develop environmentally sustainable approaches to herbivore control. Increasing plant diversity is expected to limit herbivory by altering plant-herbivore and predator-herbivore interactions, but the simultaneous influence of these interactions on herbivore impacts remains unexplored. We compiled 487 arthropod food webs in two long-running grassland biodiversity experiments in Europe and North America to investigate whether and how increasing plant diversity can reduce the impacts of herbivores on plants. We show that plants lose just under half as much energy to arthropod herbivores when in high-diversity mixtures versus monocultures and reveal that plant diversity decreases effects of herbivores on plants by simultaneously benefiting predators and reducing average herbivore food quality. These findings demonstrate that conserving plant diversity is crucial for maintaining interactions in food webs that provide natural control of herbivore pests. 
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  3. null (Ed.)
    Abstract Human activities are transforming grassland biomass via changing climate, elemental nutrients, and herbivory. Theory predicts that food-limited herbivores will consume any additional biomass stimulated by nutrient inputs (‘consumer-controlled’). Alternatively, nutrient supply is predicted to increase biomass where herbivores alter community composition or are limited by factors other than food (‘resource-controlled’). Using an experiment replicated in 58 grasslands spanning six continents, we show that nutrient addition and vertebrate herbivore exclusion each caused sustained increases in aboveground live biomass over a decade, but consumer control was weak. However, at sites with high vertebrate grazing intensity or domestic livestock, herbivores consumed the additional fertilization-induced biomass, supporting the consumer-controlled prediction. Herbivores most effectively reduced the additional live biomass at sites with low precipitation or high ambient soil nitrogen. Overall, these experimental results suggest that grassland biomass will outstrip wild herbivore control as human activities increase elemental nutrient supply, with widespread consequences for grazing and fire risk. 
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  4. Abstract

    The diversity of primary producers strongly affects the structure and diversity of species assemblages at other trophic levels. However, limited knowledge exists of how plant diversity effects at small spatial scales propagate to consumer communities at larger spatial scales. We assessed arthropod community β and γ‐diversity in response to experimentally manipulated plant community richness in two long‐term grassland biodiversity experiments (Jena, Germany and Cedar Creek, USA) replicated over two years. We calculated arthropod species turnover among all plot combinations (β‐diversity), and accumulated number of arthropod species occurring on (1) all pairwise plot combinations and (2) 40 randomly selected six‐plot combinations (γ‐diversity). The components of arthropod diversity were tested against two measures of plant diversity, namely average plant α‐diversity () and the average difference in plant α‐diversity between plots (ΔPSR). Whereas points to the overall importance of plant α‐diversity for arthropod community turnover and diversity on a larger scale, ΔPSR represents the role of habitat heterogeneity. We demonstrate that arthropod γ‐diversity is supported by high, homogeneous plant α‐diversity, despite lower arthropod β‐diversity among high‐ compared to low‐diversity plant communities. We also show that, in six‐plot combinations, average plant α‐diversity has a positive influence on arthropod γ‐diversity only when homogeneity in plant α‐diversity is also high. Varying heterogeneity in six‐plot combinations showed that combinations consisting solely of plots with an intermediate level of plant α‐diversity support a higher number of arthropod species compared to combinations that contain a mix of high‐ and low‐diversity plots. In fact, equal levels of arthropod diversity were found for six‐plot combinations with only intermediate or high plant α‐diversity, due to saturating benefits of local and larger‐scale plant diversity for higher trophic levels. Our results, alongside those of recent observational studies, strongly suggest that maintaining high α‐diversity in plant communities is important for conserving multiple components of arthropod diversity. As arthropods carry out a range of essential ecosystem functions, such as pollination and natural pest‐control, our findings provide crucial insight for effective planning of human‐dominated landscapes to maximize both ecological and economic benefits in grassland systems.

     
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  5. Abstract

    Soil stores approximately twice as much carbon as the atmosphere and fluctuations in the size of the soil carbon pool directly influence climate conditions. We used the Nutrient Network global change experiment to examine how anthropogenic nutrient enrichment might influence grassland soil carbon storage at a global scale. In isolation, enrichment of nitrogen and phosphorous had minimal impacts on soil carbon storage. However, when these nutrients were added in combination with potassium and micronutrients, soil carbon stocks changed considerably, with an average increase of 0.04 KgCm−2 year−1(standard deviation 0.18 KgCm−2 year−1). These effects did not correlate with changes in primary productivity, suggesting that soil carbon decomposition may have been restricted. Although nutrient enrichment caused soil carbon gains most dry, sandy regions, considerable absolute losses of soil carbon may occur in high‐latitude regions that store the majority of the world's soil carbon. These mechanistic insights into the sensitivity of grassland carbon stocks to nutrient enrichment can facilitate biochemical modelling efforts to project carbon cycling under future climate scenarios.

     
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