1 INTRODUCTION
It has been estimated that up to 40% of the planet’s land is degraded, directly affecting the global population and contributing to ongoing biodiversity losses (United Nations Convention to Combat Desertification, 2022). In response, substantial efforts have been invested to halt and reverse this trend through ecological restoration, spurred on by large-scale global efforts including the Bonn Challenge, aiming to restore 350 million hectares of degraded land by 2030 (with 210 million hectares already pledged for restoration; www.bonnchallenge.org/progress), and the UN Decade on Ecosystem Restoration (www.decadeonrestoration.org). Such restoration efforts are imperative to ensuring that Earth’s biodiversity and ecosystems persist into the future, and placing biodiversity at the heart of ecosystem restoration will contribute to strengthening the relationship between people and nature (Mori, 2020).
1.1 Soil biology and associated functions – A primer
Plants are critical to terrestrial ecosystems, providing ecosystem carbon inputs through net primary production. A large proportion of the plant-assimilated carbon enters the soil as root exudates or litter, which fuels the soil food web. Soil biota regulates the breakdown of this organic matter, releasing essential nutrients that enable plant growth (Chapin et al., 1979). This cycling of plant-assimilated carbon and nutrients released by the soil biota is fundamental to the long-term functioning of ecosystems (Kopittke et al., 2022; Nielsen et al., 2015). In addition, the soil microbes and invertebrates influence plant community dynamics through several mechanisms (Figure 2). For example, many free-living and root-associated microbes can enhance nutrient availability, nutrient mineralisation and solubilisation and nutrient uptake, thereby enhancing plant growth (van der Heijden et al., 2008). Furthermore, both free-living and plant-associated microbes contribute through indirect mechanisms, including by influencing soil aggregation and structure, producing plant growth-promoting compounds and pathogens modifying plant community dynamics (Bardgett & van der Putten, 2014). The larger soil fauna regulate soil processes via three main pathways: 1) microbial grazers and detritivores contribute to nutrient mineralisation, which generally enhances nutrient availability and plant growth (Trap et al., 2016); 2) root herbivores regulate plant growth, competition and carbon and nutrient cycling (De Deyn et al., 2003; Rasmann et al., 2011); and 3) ecosystem engineers modify soil structure and properties, thereby regulating soil processes and plant growth (Barrios, 2007; Nielsen, 2019; Snyder & Hendrix, 2008). Several papers have reviewed the role of soil and its biology in restoration (e.g. Auclerc et al., 2022; Callaham et al., 2008; Nolan et al., 2021; Rawat et al., 2023), including studies emphasising the roles of aboveground–belowground linkages (Kardol & Wardle, 2010) and plant–soil feedbacks (van der Putten et al., 2013). Together, this literature strongly supports the value of explicitly considering soil biology and its function.
Ecological restoration often encounters barriers caused by past human or natural disturbances. Among these, soil-borne barriers – rarely considered in restoration contexts – are critical, as changes in soil biology and functions influence restoration outcomes. For example, soil-borne barriers can slow recovery via inappropriate nutrient cycles, a lack of plant-beneficial microbes, or the presence of pests and pathogens. In addition to physical and chemical degradation, soil degradation can manifest in contrasting forms, ranging from enriched conditions, with excessive nutrient contents and altered biological communities, to depleted nutrient conditions, with low biological activity. Such contrasting types of degradation require different types of restoration interventions to achieve target conditions (Figure 3). Where soil biological resources have been depleted, interventions should initially foster the colonisation and development of soil biological communities by encouraging biological activity, biomass accumulation and functional diversity. This can be achieved by, for example, adding organic matter, planting fast-growing species to increase carbon inputs, using nitrogen-fixing plants to increase nitrogen availability and increasing plant diversity to provide a range of resources (Franklin et al., 2025; Kardol & Wardle, 2010).Once soil communities begin to establish, adaptive management can guide suitable recovery trajectories towards target ecosystem conditions. Conversely, restoring nutrient-enriched sites can be particularly challenging due to plant–soil feedbacks, where high nutrient levels tend to favour fast-growing plants, including many invasive species. These plants are problematic because they often have traits, such as high growth rates and litter with high nitrogen content, which stimulate soil carbon inputs and enhance nutrient cycling (van Kleunen et al., 2010), further reinforcing their dominance and slowing ecosystem recovery. While these plant traits may be desirable and deliberately used in heavily nutrient-depleted sites to encourage soil development, they can be problematic in nutrient-enriched sites by perpetuating soil biota that maintain elevated nutrient levels and fluxes that favours exotics over natives. Removing invasive plant species, harvesting biomass and introducing plants with traits that reduce microbial activity (e.g., those producing recalcitrant leaf litter and high carbon to nitrogen ratios), favour specific soil biota (e.g., nitrogen-fixing and mycorrhizal fungi plants) and reduce excess nutrients from soil (e.g., phytomining) can decrease nutrient availability and shift soil biological communities (see Raupp et al., 2024 for detail).
