The World's grasslands are under severe threat from on-going degradation, yet they are largely ignored in sustainable development agendas. This degradation is undermining the capacity of grasslands to support biodiversity, ecosystem services, and human wellbeing. In this Perspective, we examine the current state of grasslands worldwide and explore the extent and dominant drivers of global grassland degradation. We identify actions that are critical to the development of socio-ecological solutions to combat degradation and promote restoration of global grasslands. Specifically, we argue that progress can be made by: increasing recognition of grasslands in global policy, developing standardised indicators of grassland degradation, using scientific innovation for effective restoration at regional and landscape scales, and enhancing knowledge transfer and data sharing on restoration experiences. The integration of these strategies into sustainability policy should help to halt grassland degradation and enhance restoration success, and protect the socio-economic, cultural and ecological benefits that grasslands provide.Grasslands, comprising open grassland, grassy shrublands and savannah, cover about 40% of the Earth's surface and some 69% of the world's agricultural land area 1-3 . Not only do they serve as an important global reservoir of biodiversity, including many iconic and endemic species, but also, they provide a wide range of material and non-material benefits to humans and our quality of life. These benefits include a wide range of ecosystem services, such as food production, water supply and regulation, carbon storage and climate mitigation, pollination, and a host of cultural services 1-3 . Despite its importance, grassland degradation is widespread and accelerating in many parts of the world 4-6 with as much as 49% of grassland area worldwide having been degraded to some extent 5,7,8 .Grassland degradation poses an enormous threat to the hundreds of millions of people who rely on grasslands worldwide for food, fuel, fibre and medicinal products, as well as their multiple cultural values 9,10 . In terms of livestock production, the global cost of grassland degradation has been estimated at $6.8 billion 11 , with the impact on human welfare being particularly severe in regions where most the population is below the poverty line Grassland degradation also creates major environmental problems, given that grasslands play a critical role in biodiversity conservation, climate and water regulation, and global biogeochemical cycles 2,4 . For example, the conversion of tropical grassy biomes to arable cropland poses a significant threat to biodiversity given that they have a vertebrate species richness comparable to forests 12 , while the recent expansion of croplands in United States has caused widespread conversion of prairie grasslands, with considerable cost to wildlife 6 . Moreover, the conversion of grasslands to arable cropland and disturbance through overgrazing, fire and invasive species can lead to signif...

The relationship between biodiversity and biomass has been a long standing debate in ecology. Soil biodiversity and biomass are essential drivers of ecosystem functions. However, unlike plant communities, little is known about how the diversity and biomass of soil microbial communities are interlinked across globally distributed biomes, and how variations in this relationship influence ecosystem function. To fill this knowledge gap, we conducted a field survey across global biomes, with contrasting vegetation and climate types. We show that soil carbon (C) content is associated to the microbial diversity–biomass relationship and ratio in soils across global biomes. This ratio provides an integrative index to identify those locations on Earth wherein diversity is much higher compared with biomass and vice versa. The soil microbial diversity-to-biomass ratio peaks in arid environments with low C content, and is very low in C-rich cold environments. Our study further advances that the reductions in soil C content associated with land use intensification and climate change could cause dramatic shifts in the microbial diversity-biomass ratio, with potential consequences for broad soil processes.

Summary The abundance of nitrogen (N)‐fixing plants in ecosystems where phosphorus (P) limits plant productivity poses a paradox because N fixation entails a high P cost. One explanation for this paradox is that the N‐fixing strategy allows greater root phosphatase activity to enhance P acquisition from organic sources, but evidence to support this contention is limited. We measured root phosphomonoesterase (PME) activity of 10 N‐fixing species, including rhizobial legumes and actinorhizal Allocasuarina species, and eight non‐N‐fixing species across a retrogressive soil chronosequence showing a clear shift from N to P limitation of plant growth and representing a strong natural gradient in P availability. Legumes showed greater root PME activity than non‐legumes, with the difference between these two groups increasing markedly as soil P availability declined. By contrast, root PME activity of actinorhizal species was always lower than that of co‐occurring legumes and not different from non‐N‐fixing plants. The difference in root PME activity between legumes and actinorhizal plants was not reflected in a greater or similar reliance on N fixation for N acquisition by actinorhizal species compared to co‐occurring legumes. Synthesis. Our results support the idea that N‐fixing legumes show high root phosphatase activity, especially at low soil P availability, but suggest that this is a phylogenetically conserved trait rather than one directly linked to their N‐fixation capacity.

The importance of soil age as an ecosystem driver across biomes remains largely unresolved. By combining a cross-biome global field survey, including data for 32 soil, plant, and microbial properties in 16 soil chronosequences, with a global meta-analysis, we show that soil age is a significant ecosystem driver, but only accounts for a relatively small proportion of the cross-biome variation in multiple ecosystem properties. Parent material, climate, vegetation and topography predict, collectively, 24 times more variation in ecosystem properties than soil age alone. Soil age is an important local-scale ecosystem driver; however, environmental context, rather than soil age, determines the rates and trajectories of ecosystem development in structure and function across biomes. Our work provides insights into the natural history of terrestrial ecosystems. We propose that, regardless of soil age, changes in the environmental context, such as those associated with global climatic and land-use changes, will have important long-term impacts on the structure and function of terrestrial ecosystems across biomes.

Feedback between plants and soil is an important driver of plant community structure, but it remains unclear whether plant–soil feedback (PSF): (i) reflects changes in biotic or abiotic properties, (ii) depends on environmental context in terms of soil nutrient availability, and (iii) varies among plant functional groups. As soil nutrient availability strongly affects plant distribution and performance, soil chemical properties and plant nutrient‐acquisition strategies might serve as important drivers of PSF. We used soils from young and old stages of a long‐term soil chronosequence to represent sites where productivity is limited by nitrogen (N) and phosphorus (P) availability, respectively. We grew three N‐fixing and three non‐N‐fixing plant species in soils conditioned by co‐occurring conspecific or heterospecific species from each of these two stages. In addition, three soil treatments were used to distinguish biotic and abiotic effects on plant performance, allowing measurements of overall, biotic, and abiotic PSF. In young, N‐poor soils, non‐N‐fixing plants grew better in soils from N‐fixing plants than in their own soils (i.e., negative PSF). However, this difference was not only associated with improved abiotic conditions in soils from N‐fixing plants but also with changes in soil biota. By contrast, no significant PSF was observed for N‐fixing plants grown in young soils. Moreover, we did not observe any significant PSF for either N‐fixing or non‐N‐fixing plants growing in old, P‐impoverished soils. Synthesis. The direction and strength of plant‐soil feedback (PSF) varied among N‐acquisition strategies and soils differing in nutrient availability, with stronger plant‐soil feedback in younger, N‐poor soils compared to older, P‐impoverished soils. Our results highlight the importance of considering soil nutrient availability, plant‐mediated abiotic and biotic soil properties, and plant nutrient‐acquisition strategies when studying plant‐soil feedback, thereby advancing our mechanistic understanding of plant‐soil feedback during long‐term ecosystem development.

Background Plants condition the soil in which they grow, thereby altering the performance of subsequent plants growing in this soil. This phenomenon, known as plant-soil feedback (PSF), has garnered increasing interest. Experiments are moving from single species soil pairings in the glasshouse to community-level field trials. Consequently, our knowledge of the role PSF plays in shaping ecosystem functions has advanced. However, knowledge gaps remain. Scope Here, we explore intrinsic and extrinsic abiotic and biotic drivers of PSF such as maternal effects, plant functional traits, self-DNA, plant-plant competition, herbivory, interactions between soil organisms, temperature, drought, flooding, greenhouse gases, (micro)nutrients, plant-litter-soil feedback and priority effects. These drivers have begun to feature in experiments, thereby increasing our mechanistic understanding of PSF. Nonetheless, many of these topics have received insufficient coverage to determine general principles across larger temporal and spatial scales. Further, conflicting terminology has excluded PSF studies from reviews and meta-analyses. We review terms such as soil sickness, Janzen-Connell hypothesis, soil-related invasive species work, soil legacies, allelopathy and soil-related succession that overlap with PSF but are generally not named as such. Conclusion Holistic experimental designs that consider the continual reciprocal feedback between the extrinsic environment, plants and soil, as well as the unification of terminologies are necessary if we are to realise the full potential of PSF for understanding and steering ecosystem processes. Here, we compile outstanding questions related to PSF research that emphasis the aforementioned topics and suggest ways to incorporate them into future research in order to advance plant-soil ecology.

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