1 INTRODUCTION

Soil biodiversity, which accounts for an astonishing 59% of Earth′s species diversity (Anthony et al., 2023), has a significant impact on ecosystem functioning, including primary productivity and stability (Cardinale et al., 2012; Mori et al., 2018), with far-reaching implications for the environment and human well-being (Bardgett & van der Putten, 2014; Guerra et al., 2020; Nielsen et al., 2015). In terrestrial systems, biodiversity is critical to essential ecological functions like agricultural production, soil health, water and air quality, all of which have a direct impact on human welfare (Figure 1) (Cardinale et al., 2012; Mori et al., 2021). These processes are inextricably linked to the diversity and distribution of plant, animal and microbial species across ecosystems. However, the current urgency stems from the alarming decline of biodiversity around the world, which is being driven by both direct and indirect drivers of change such as climate change, pollution, conflicts and epidemics (Butchart et al., 2010; IPBES, 2019). The decline affects not only species extinction, but also loss of genetic and functional diversity at community, landscape and global scales (Almeida-Rocha et al., 2020; Díaz et al., 2015; IPBES, 2019; Isbell et al., 2017). Biodiversity decline generates serious ecological concerns regarding the stability and productivity of managed and natural ecosystems. As biodiversity loss accelerates around the world, understanding the importance of biodiversity and biotic interactions in defining ecosystem functioning becomes increasingly important.

Positive relationships between aboveground biodiversity and ecosystem functioning are well established in terrestrial and aquatic ecosystems (Maestre, Castillo-Monroy, et al., 2012; Nielsen et al., 2011; Reich et al., 2012). Recent research found a similar link between belowground diversity and ecosystem functioning (Delgado-Baquerizo et al., 2020). An improved understanding of BEF is recognized for both managed (e.g., agriculture) and natural ecosystems. Translation of this knowledge can have major socioeconomic and environmental benefits. In natural ecosystems, increased plant diversity has been linked to higher primary productivity and carbon sequestration. For example, growing research indicates that multiple-species cropping, or mixed-pasture systems are more productive than monocultures (Li et al., 2023). Similarly, higher soil biodiversity is linked to increased agricultural productivity because higher soil biodiversity plays a crucial role by contributing to key soil functions such as nutrient cycling and carbon storage, which can promote increased nutrient availability for crops and foster protection against crop diseases (Bach et al., 2020; Delgado-Baquerizo, Powell, et al., 2017; Delgado-Baquerizo et al., 2020; Nazaries et al., 2013). There is growing evidence linking both above- and belowground diversity directly to human health (Berg et al., 2017; Singh, Yan, et al., 2023; Yan et al., 2022). Furthermore, there is growing evidence to support the idea that biotic interactions between above- and belowground biodiversity eventually influence ecosystem functions and stability (Eisenhauer, 2012; Gonzalez et al., 2020; Jing et al., 2015; Wang et al., 2021). However significant knowledge gaps exist, limiting our capacity to establish management and conservation policies that may be utilized to improve ecosystem functioning in both managed and natural ecosystems. This review aims to shed light on the complex interplay between biodiversity, biotic interactions and ecosystem functions, with a special emphasis on the critical role of belowground dynamics in generating larger ecological relationships. In doing so, it reveals critical knowledge gaps, research goals to fill those gaps, and potential future paths.

2 CONCEPTS AND THEORIES UNDERPINNING BIODIVERSITY-ECOSYSTEM FUNCTIONING (BEF) RELATIONSHIPS

Biodiversity refers to the variety and variability of life forms in an ecosystem, including genetic, species and ecosystem diversity, which collectively shape ecosystem structure, functioning and resilience. Ecosystem functions include a wide range of processes that are critical for stability and productivity, including nutrient cycling, primary production, carbon sequestration and pest regulation (Čapek et al., 2018; Romero et al., 2023; Trivedi et al., 2019; Wang et al., 2021; Yang et al., 2019). The interplay between biodiversity and ecosystem functions is a cornerstone of ecological theory, that has gained significance in the last 25 years as concerns about continued biodiversity loss have grown. This has given rise to research subfield within ecology focused on BEF (Box 1). Understanding ecosystem functioning has been a key focus of BEF research due to its importance in providing services, maintaining integrity and fostering human well-being (Figure 1).

BEF research has evolved from early observations to a more holistic perspective (Figure 2), connecting both above- and belowground biodiversity and biotic interactions to ecosystem functioning. However, our understanding of the direction and shape of BEF relationships remains limited for most ecosystems and communities. The shape of BEF relations varies depending on ecosystems and communities (Nielsen et al., 2011), with trajectories broadly classified into four categories (Figure 3): linear (Cardinale et al., 2007; Tilman et al., 2001), functionally redundant (Reich et al., 2012) nonredundant (Delgado-Baquerizo, Giaramida, et al., 2016) and idiosyncratic (Naeem et al., 2021). Such trajectories reflect the varying roles species play within ecosystems. For examples, a linear BEF relationship gains ground when each species contributes unique traits that augment ecosystem processes (Naeem & Wright, 2003). If multiple species share similar functional traits, then a saturating (redundancy) relationship will be reached with increasing diversity (Yachi & Loreau, 1999), because the chance of adding a species possessing a trait not already found in the community becomes progressively smaller as species richness increases. An idiosyncratic relationship materializes from trait-related disparities or biotic interactions influencing functional rates (Hooper et al., 2002; O′Connor & Crowe, 2005). In the extreme case of a nonredundant relationship, even minimal species loss leads to an immediate catastrophic effect on ecosystem functions (Cardinale et al., 2011). Previously, it has been reported that general functions may follow a different shape (i.e., saturating) than specialized functions (i.e., nonredundant). However, it′s notable that these assumptions have evolved with new insights revealing a linear relationship strengthening over time (Reich et al., 2012; Vogel et al., 2019). Recent evidence indicates that BEF relationships are context dependent. For instance, the shape of the relationship may vary between ecosystem types (Ratcliffe et al., 2017), or land-use types (Felipe-Lucia et al., 2020) and management practices (Gonzalez et al., 2020). Nevertheless, the extent to which BEF is context dependant is not well understood.

One of the key factors that may influence the shape and strength of BEF relationships is complementarity, which refers to the positive effects of species diversity on ecosystem functioning through niche differentiation, resource partitioning, or facilitation (Brooker et al., 2021). Complementarity can occur at different levels of biological organization, such as genetic, functional, or trophic (Frank, 2022). Complementarity can also affect multiple ecosystem functions simultaneously, leading to multifunctional complementarity (Hagan et al., 2021). Complementarity can be measured by comparing the observed ecosystem functioning with the expected functioning based on the average performance of individual species or functional groups (Gamfeldt et al., 2008). Complementarity is often considered as a proxy for the mechanisms underlying the BEF relationships, as it captures the net effects of species interactions on ecosystem processes (Eisenhauer, 2012; Hagan et al., 2021; Oram et al., 2018). Therefore, incorporating complementarity into BEF research can help reveal the complex and context-dependent nature of the relationship between biodiversity and ecosystem functioning.

3 SOIL BIODIVERSITY: EXTENDING BEF THEORIES BELOWGROUND

Belowground biodiversity, involving soil biotic interactions, is as important as aboveground biodiversity for ecosystem productivity and stability (Delgado-Baquerizo et al., 2020; Eisenhauer et al., 2022, 2024; Jayaramaiah et al., 2023; Reiss et al., 2009; Yang et al., 2018). Experimental studies have revealed diverse microbial BEF relationships, impacting functions from decomposition to detoxification. This belowground microbial world intricately shapes aboveground biodiversity and ecosystem functions, emphasizing its critical role (Delgado-Baquerizo, Maestre, et al., 2016; Wang et al., 2019). While many BEF theories have been developed and tested in aboveground ecosystems, recent research emphasizes the need to extend these frameworks to belowground interactions (Bardgett & van der Putten, 2014; Delgado-Baquerizo et al., 2019; Martins et al., 2023). Soil biodiversity accounts for most of the ecosystem diversity on planet Earth (Anthony et al., 2023). Soil biodiversity, encompassing a diverse array of microorganisms, including bacteria and fungi, along with viruses, protists, fauna and flora, can be considered a cornerstone in terrestrial ecosystems. These components collectively drive essential ecosystem functions, including primary productivity, nutrient cycling and climate regulation (Bardgett & van der Putten, 2014; Delgado-Baquerizo, Eldridge, et al., 2017). For instance, bacteria and fungi are critical to biogeochemical cycling of macro and micronutrients enabling nutrient availability to plants, while mycorrhizal fungi facilitate nutrient uptake (van Der Heijden, Verkade, et al., 2008). Soil viruses regulate microbial populations and affect nutrient cycling dynamics (Williamson et al., 2017), and protists influence microbial community structure and nutrient availability through grazing on microorganisms (Geisen et al., 2019). Soil fauna, including earthworms and nematodes, enhance soil structure, regulate microbial populations and influence nutrient cycling (Eisenhauer & Powell, 2017). Plant roots, in symbiosis with mycorrhizal fungi, provide organic matter and exudates and impact soil microbial communities (Smith & Read, 2008), and the synergies between mycorrhizal fungi and soil microbial communities enhances the acquisition of nitrogen by plants (Hestrin et al., 2019).

Soil biodiversity, a critical component of overall biodiversity, has been shown to significantly impact ecosystem functions (Bardgett & van der Putten, 2014). For instance, investigations into the soil microbial community′s role in nutrient cycling have revealed how microbial diversity enhances nutrient availability to plants, thereby positively influencing primary production and overall ecosystem productivity (Wagg et al., 2014). Additionally, studies on mycorrhizal associations have elucidated their role in nutrient uptake and transfer between plants, highlighting the intricate belowground web of interactions that shape ecosystem processes (Bardgett & van der Putten, 2014). Despite significant advancements in our understanding, the intricate interplay between microbial diversity and ecosystem functioning remains somewhat unknown.

4 SOIL MICROBIAL DIVERSITY AND ECOSYSTEM FUNCTIONING

Understanding the mechanisms and magnitude of the impact of soil biodiversity on ecosystem functioning is a fundamental goal for soil ecologists. Previous studies in natural systems provides some insights, but recent advances in sequencing technology, -omics and in-situ measurements of ecosystem processes have allowed researchers to directly connect microbial diversity to ecosystem functioning more effectively (Delgado-Baquerizo, Maestre, et al., 2016; Maron et al., 2018; Trivedi et al., 2019; Wagg et al., 2021). These advancements have revealed the critical role of soil microbial communities in maintaining multiple ecosystem functions and services (multifunctionality). The BEF relationship, particularly concerning belowground soil microbial diversity, encompasses a wide array of critical functions essential for ecosystem functions and stability. For example, microbial communities play pivotal roles in nutrient cycling, such as the mineralisation of carbon, nitrogen and phosphorus, which are fundamental processes supporting plant growth and productivity (van der Heijden, Bardgett, et al., 2008). Recent research has highlighted microbial competition for phosphorus as a limiting factor in the CO₂ response of mature forests, illustrating the intricate balance between microbial nutrient acquisition strategies and ecosystem carbon dynamics (Jiang et al., 2024). Soil microbiomes also contribute significantly to disease suppression through mechanisms such as antibiosis and competition for resources, thereby influencing plant health and resilience (Singh, Delgado-Baquerizo, et al., 2023). Recent studies emphasize the importance of both microbial richness and composition, suggesting that they independently drive multiple ecosystem functions related to nutrient cycling and organic matter decomposition (Delgado-Baquerizo, Trivedi, et al., 2017). These findings underscore the critical role of microbial diversity in driving multiple ecosystem functions in terrestrial ecosystems (Liu et al., 2017; Trivedi et al., 2019; Wagg et al., 2014). Furthermore, research has revealed a strong connection between the responses of microbial community composition to global change drivers and the resistance of ecosystem multifunctionality in drylands worldwide (Delgado-Baquerizo, Eldridge, et al., 2017; Ye et al., 2019). This highlights the importance of maintaining soil microbial diversity for ecosystem resilience in the face of global environmental changes. Empirical evidence suggests that a loss of microbial and faunal diversity can reduce multifunctionality, impacting services such as climate regulation, soil health and food production by terrestrial ecosystems (Delgado-Baquerizo, Maestre, et al., 2016). As our understanding of the complex interactions between soil microbial diversity and ecosystem functioning grows, it becomes increasingly clear that preserving and managing soil biodiversity is crucial for maintaining the health and productivity of both natural and managed ecosystems.

Despite these established roles and mechanisms of microbial interactions, the relationship between microbial diversity and ecosystem functions continues to be a subject of ongoing debate. Early experimental studies with controlled microbial richness levels indicated that changes in species number can affect ecosystem functioning (Bell et al., 2005; van der Heijden, Bardgett, et al., 2008). Further studies that successfully manipulated microbial communities to explore their relationship with ecosystem functioning, initially hypothesized high functional redundancy driven by the enormous diversity residing in soi microbial communities (Allison & Martiny, 2008). However, increasing evidence suggests a wide range of shapes for microbial biodiversity-ecosystem functioning relationships, which might depend on the level of specialisation of the function taken into consideration (Bell et al., 2005; Peter et al., 2011; Ylla et al., 2013). For example, while functional redundancy among soil microbes is thought to be common, certain functions, like pesticide degradation, are restricted to specialized functional groups. Consequently, even a moderate loss of diversity can impact crucial specialized functions, indicating a lack of functional redundancy in the microbial BEF relationship (Delgado-Baquerizo, Giaramida, et al., 2016; Singh et al., 2014). However, some specialized groups, such as methanotrophs, display a positive linear relationship between microbial diversity and functioning (Schnyder et al., 2018). Likewise, a strong positive association was found between microbial composition and functional genes regulating soil respiration (Nazaries et al., 2013; Trivedi, Delgado-Baquerizo, Trivedi, et al., 2016). While empirical evidence suggests varying shapes for the biodiversity-ecosystem functioning relationship, our understanding remains incomplete, particularly regarding context dependency and the influence of environmental factors like climate or land-use on soil biodiversity and ecosystem functioning.

Recent research has underscored the central role of microbial diversity in driving soil ecosystem functioning. Wagg et al. (2019) highlight how manipulating microbial diversity significantly impacts multifunctionality in grassland ecosystems, particularly in nutrient cycling processes. Similarly, Domeignoz-Horta et al. (2020) demonstrate the influence of microbial diversity on carbon use efficiency (CUE) in soil, with abiotic factors like temperature and moisture modulating this relationship. Moreover, He et al. (2021) reveal seasonal variations in microbial growth in response to climate change, emphasizing the importance of considering temporal dynamics in understanding soil carbon fate. Additionally, Maron et al. (2018) stress the critical role of microbial diversity in organic matter decomposition, a fundamental process supporting various ecosystem services provided by soil ecosystems. Together, recent research underscores the importance of microbial diversity in maintaining soil ecosystem stability and functioning, underscoring the need for further investigation into specific microbial community characteristics to predict ecosystem responses to global change accurately (Fan et al., 2023; Wagg et al., 2021; Osburn et al., 2023). Collectively, these studies accentuate the intricate connections between microbial diversity, ecosystem functioning, emphasizing the importance of comprehensive approaches to address global changes effectively.

Recent evidence also points to the critical importance of soil fauna in shaping the aboveground biodiversity and functioning of terrestrial ecosystem (Bardgett & van der Putten, 2014; Delgado-Baquerizo et al., 2019; Liu et al., 2017; Trivedi, Delgado-Baquerizo, Anderson, et al., 2016). For example, earthworms play a role in nutrient cycling (Edwards & Arancon, 2022), via their burrowing activities, which facilitate water movement and create hotspots for microbial growth, enriching microbial biomass and metabolic activity. The accelerated nutrient cycling processes driven by earthworms and other soil fauna have cascading effects on soil fertility and plant nutrient uptake, ultimately shaping aboveground biodiversity (Zhang et al., 2023). The enhanced soil health facilitated by soil fauna contributes to increased crop yields, reduced reliance on synthetic fertilizers, and improved soil resilience against environmental stressors. While they contribute positively to nutrient cycling, soil structure and organic matter decomposition, it′s essential to recognize their potential negative impacts. For instance, invasive earthworms can affect some plant functional traits that could impact resource uptake, competition and plant species composition (Thouvenot et al., 2021). Plant-feeding and plant-pathogenic nematodes can cause substantial crop losses in agriculture (Rahman Khan & Khan, 2023). Recognizing both positive and negative aspects of soil fauna is crucial for a holistic understanding of ecosystem functioning. They play a complex role in ecosystems and has profound implications for both natural ecosystems and sustainable agriculture practices, and their impact depends on context, diversity and interactions with other biota. Future studies should explore functional traits, assess long-term effects, and quantify ecosystem services provided by these fascinating organisms.

5 LINKING ABOVE- AND BELOWGROUND DIVERSITY TO ECOSYSTEM FUNCTIONING

The inseparable connection between above- and belowground diversity is well established, positively influencing ecosystem functioning (Bardgett & Wardle, 2010; Eisenhauer, 2012). While plants channel energy via photosynthesis, microbial activities belowground drive decomposition, nutrient cycling and energy transfer, underscoring their interdependence (Reese et al., 2018; Wardle et al., 2004). Plant communities determine the type and quality of organic matter input to the soil; similarly, soil communities determine which plant community establishes well via symbiosis, pathogenicity and predation. This interplay between above- and belowground biodiversity, often mediated by plant communities, is pivotal in shaping ecosystem dynamics. For instance, research has revealed that invasive annual plant species can have varying impacts on above- and belowground plant communities in deciduous forests, leading to alterations in root biomass and changes in plant species richness and composition (Gaggini et al., 2019). Over time, a growing correlation has been observed between above- and belowground community composition, indicating an increasing connectedness between soil biota and plants (Wubs et al., 2019). Interestingly, temporal changes in aboveground plant diversity do not consistently mirror changes in belowground biodiversity during ecosystem development, challenging the assumption of synchronous patterns (Delgado-Baquerizo et al., 2019). These findings raise questions regarding existing frameworks linking below-ground biodiversity with above-ground plant productivity (Mezeli et al., 2020). In urban environments, the positive impact of native plants on biodiversity is evident, supporting their prioritization in urban green initiatives (Berthon et al., 2021). Dominant plant species emerge as influential drivers of belowground diversity, shaping interactions with understory communities and microbial assemblages (Wang et al., 2019). It should be noted that recent studies have shown a mismatch in the relationship between plant diversity and soil microbial diversity in several biomes and spatial scales (Cameron et al., 2019; Guerra et al., 2022). Conflicting results suggest that this relationship is not uniform and may be influenced by specific environmental contexts (Liu et al., 2020). For instance, soil carbon content across global biomes plays a role in shaping the microbial diversity–biomass relationship (Bastida et al., 2021). Such finding has implication for conservation approached as well as for BEF. It is likely that in soil biodiversity hotspots, soil organism contributions to ecosystem functions are higher than plants but such knowledge is not available.

There is compelling evidence that the intricate connections between above- and belowground components significantly influence ecosystem functioning, and suggest that these biotic interactions regulate nutrient dynamics, microbial communities and overall ecosystem performance (Bardgett & Wardle, 2010; Wagg et al., 2014; Weisser et al., 2017). However, despite recognizing their importance, the relationships between above- and belowground communities remain insufficiently explored, hindering our understanding of the consequences arising from simultaneous shifts in both above- and belowground biodiversity. This knowledge gap highlights the need for further work to better understand BEF relationships to identify key mechanisms driving ecosystem processes and services (Ball et al., 2009; Blouin et al., 2005; Delgado-Baquerizo et al., 2018a, 2018c; Eisenhauer et al., 2017; IPBES, 2019; Jing et al., 2015; Steinauer et al., 2016; Wardle et al., 2004). Recent studies emphasize the need to delve into the intricate interactions between species diversity, functional traits, and ecosystem functioning (Åkesson et al., 2021; Gross et al., 2017). In ecosystem dynamics, microbial traits like nitrogen fixation, cellulose degradation and biofilm formation are crucial. Plant traits such as specific leaf area (SLA), leaf nitrogen content and nitrogen fixation in legumes are also important. They affect how plants grow, use resources and form relationships with microbes, influencing overall ecosystem health. These traits play essential roles in ecosystem functioning and are critical for understanding plant-microbe interactions and soil functions (Delgado-Baquerizo et al., 2018b). By addressing these gaps and gaining mechanistic insights into the linkages between above- and belowground processes, we can enhance our ability to predict and manage ecosystem responses to global changes. This knowledge is crucial for informed decision-making in conservation, sustainable land use practices, and ecosystem restoration efforts.

An essential yet often overlooked component in bridging the gap between above- and belowground diversity lies with plant-soil feedbacks (PSF) (Png et al., 2023; van der Putten et al., 2016). These feedbacks represent the dynamic interactions between plant species and the soil microbial community exerting profound effects on both above- and belowground components of terrestrial ecosystems. Through these feedback mechanisms, plants influence the soil microbial community and, reciprocally, the soil microbes shape the growth, fitness and composition of plant communities (Cortois et al., 2016; Harrison & Bardgett, 2010). The impact of PSF extends beyond individual plant–soil interactions; it forms a crucial link in the intricate web connecting above- and belowground biodiversity. Positive PSF, where plants enhance the soil environment for their own species, can contribute to the maintenance of aboveground plant diversity. Conversely, negative PSFs can lead to shifts in plant community composition, favouring the persistence of rare species alongside dominant ones (Goossens et al., 2023). PSF influences ecosystem productivity and stability through complex interactions. For instance, diverse soil microbiomes emit volatiles that trigger phenotypic variations in plants, enhancing their resilience to environmental stressors and ultimately improving productivity (Nadarajah & Abdul Rahman, 2023). Similarly, diverse plant species secrete varying root exudates that shape soil microbiome composition, which in turn influences nutrient availability and plant health (Ali & Glick, 2024; Seitz et al., 2022). These interactions illustrate how aboveground biodiversity can indirectly impact belowground processes and vice versa, forming feedback loops that regulate ecosystem functioning. Moreover, the presence of diverse plant communities can enhance soil structure and stability, promoting water infiltration and nutrient retention, which are critical for maintaining ecosystem services such as water regulation and soil fertility (Pedrinho et al., 2024). Plants influencing nutrient availability through PSF can have cascading effects on aboveground plant growth and belowground microbial communities. For example, nitrogen-fixing plants may enhance soil nitrogen levels, benefiting both their own growth and that of neighbouring species (Cortois et al., 2016). PSF influences the composition and diversity of soil microbial communities, where most competitive plant species can shape the soil bacterial community to its advantage (Hortal et al., 2017), which, in turn, can affect nutrient cycling and plant health. Mycorrhizal associations, a common example of PSF, showcase the intricate linkages between above- and belowground communities (Hestrin et al., 2019). Therefore, understanding PSF is pivotal for unravelling the complexities of above- and belowground diversity′s joint impact on ecosystem functioning. PSF acts as a mediator, influencing nutrient dynamics, shaping microbial communities and ultimately determining the performance of terrestrial ecosystems (Png et al., 2023). By incorporating PSF into the broader framework of BEF relationships, we gain deeper insights into the consequences of simultaneous shifts in above- and belowground biodiversity on ecosystem functions (De Long et al., 2023; Eisenhauer, 2012; Martins et al., 2023; Thakur et al., 2021). Future research directions should focus on investigating long-term PSF dynamics in natural ecosystems, exploring the role of PSF in ecosystem resilience to climate change, developing models that integrate PSF into BEF predictions, and examining the potential of manipulating PSF for ecological restoration. Further research into these interactions will contribute to a more holistic understanding of ecosystem dynamics, aiding in the formulation of effective conservation and management strategies.

6 IMPACTS OF GLOBAL CHANGE ON BIODIVERSITY AND ECOSYSTEM FUNCTIONING DYNAMICS

A comprehensive understanding of the BEF relationships is crucially important to link above- and belowground communities to better predict the consequences of declining biodiversity mediated by global change drivers and maintain ecosystem functions and services (Reese et al., 2018; van der Plas, 2019). On a global scale, climate change exerts both direct and indirect effects on biodiversity and ecosystem functioning, presenting complex challenges for ecosystems worldwide (Bowker et al., 2013; Maestre, Quero, et al., 2012b; Ramirez et al., 2018; Trivedi, Delgado-Baquerizo, Trivedi, et al., 2016; Valencia et al., 2018). For example, climatic conditions directly influence ecosystem functions by altering temperature and precipitation patterns (Hawkins, 2001), and accelerating the activity and interactions among consumers, decomposers and microbes (Wardle et al., 2004; Wolters et al., 2000). Rising temperatures can significantly affect the physiology, distribution and behaviour of numerous species, driving shifts in their geographic ranges and hence impacts ecosystem function they provide. For example, certain species may thrive in warmer conditions, while others may face habitat loss due to changing climate zones (Hillebrand & Matthiessen, 2009; Kirwan et al., 2007). Additionally, extreme weather events, intensified by climate change, such as hurricanes, droughts and wildfires, can result in immediate and devastating consequences for BEF (Cameron et al., 2019; Guerra et al., 2022).

Indirect effects of climate change further compound the challenges faced by biodiversity and ecosystem functioning. These indirect impacts often stem from shifts in species distributions and ecological interactions (Bardgett & Wardle, 2010). For instance, shifts in the timing of plant phenology induced by climate change can have significant implication for belowground interactions (Piao et al., 2019). Alterations in the timing of plant growth, flowering, and fruiting can disrupt the synchronization between plant nutrient demands and the activities of soil microbial communities (Hagedorn et al., 2016). When plants and soil microbes are no longer in sync, it can impact nutrient cycling processes, such as nitrogen mineralization and phosphorus availability, which are vital for plant growth (Kuzyakov & Blagodatskaya, 2015). This can lead to suboptimal nutrient acquisition by the plant, potentially affecting its growth and overall health, which, in turn, can have cascading effects on the entire ecosystem (Bardgett & van der Putten, 2014; Delgado-Baquerizo, Eldridge, et al., 2017; Wardle et al., 2004). Hence, it is important to develop a mechanistic and quantitative knowledge for the relative importance of above- and belowground diversity in regulating soil functions is crucial, particularly in the context of climate change (Eisenhauer et al., 2024; Trivedi et al., 2019).

Several models and empirical studies have been proposed to elucidate the complex relationships between biodiversity and ecosystem functioning. Loreau (1998) introduced one of the foundational mechanistic models, emphasizing how species interactions and functional traits drive ecosystem processes. This model highlighted the necessity of incorporating both species complementarity and selection effects to predict ecosystem performance. Further work from Tilman et al. (1997), Hooper et al. (2005), and Barry et al. (2021), underscored the importance of considering spatial and temporal scales in BEF studies, a factor often overlooked in earlier models. Recently, modelling attempts of linking ecological models with socioeconomic factors to offer a comprehensive framework for evaluating ecosystem services Fulford et al. (2020) predicting functions based on network theory (Hou et al., 2023) have advanced the discipline. By integrating mechanistic insights with quantitative frameworks, they offer valuable tools for understanding how biodiversity influences soil functions in a changing environment. However, these models need further modification to account for microbial dynamics and its impacts on ecosystem functions.

Notably, the extent of above- and belowground species interactions depend on the scale considered (Cameron et al., 2019). Latitude has long been recognized as a determinant of species diversity, with a general trend of declining diversity as one moves toward higher latitudes (Hawkins, 2001). However, this relationship exhibits variation when considering above- and belowground species (Hillebrand & Matthiessen, 2009; Kirwan et al., 2007; Tilman et al., 2001). For example, new evidence indicates that at least 25% of sites globally have mismatch between above and belowground biodiversity (Guerra et al., 2022) indicating that aboveground biodiversity is not necessarily a reliable proxy for belowground biodiversity at large spatial scales. At local scales however, studies suggest that the connection between above- and belowground biodiversity primarily operates through plant communities (Kirwan et al., 2007). Thus, the loss of aboveground biodiversity can disrupt above- and belowground interactions, contingent upon which specific species are lost (Bardgett & Wardle, 2010). In cases of declining species diversity, species-poor systems tend to suffer more profoundly than species-rich ones (Pennekamp et al., 2018). Overall, above- and belowground interactions can significantly alter primary productivity and plant diversity, but we have little understanding of the consequences of simultaneously shifts in the biodiversity of both above- and belowground on soil functions. Negative consequences of global change drivers can only be prevented or counteracted by accurate predictions for dynamics in above- and belowground biodiversity, and sustainable provision of ecosystem functions and services (Reese et al., 2018; van der Plas, 2019).

7 CURRENT GAPS IN THE KNOWLEDGE AND PRIORITIES FOR FUTURE RESEARCH

Despite the implied functional connection between above- and belowground biodiversity (Jayaramaiah et al., 2023; Jing et al., 2015; Martins et al., 2023; Ramirez et al., 2018; Reese et al., 2018; Soliveres et al., 2016; Wagg et al., 2014) the vast majority of studies have focused only on the aboveground communities, due to the obvious advantage of being able to observe, manipulate and quanitfy. In perticular, there is a very limitted understanding of biotic interactions and implications of global change drivers on soil functions. In the last 10 years increasing eveidence indicate microbial diversity is also directly associated with key soil functions from local to global scales (Delgado-Baquerizo et al., 2020; Jayaramaiah et al., 2022; Trivedi et al., 2019; Wagg et al., 2014). While the last decade has witnessed a growing body of evidence supporting the direct association between microbial diversity and key soil functions at various scales, it is imperative to consider counterarguments that bring nuance to this relationship. Critics emphasize the presence of confounding factors that may influence observed correlations, such as other environmental variables, land management practices, or climate conditions (Mendes et al., 2015; Pedrinho et al., 2024; Wagg et al., 2021; Xue et al., 2022). In addition, recent reports discusses the impacts of multiple global change factors on soil functions and soil biodiversity (Rillig et al., 2019), that increasing the number of simultaneous global change factors caused increasing directional changes in soil properties, soil processes and soil communities. Furthermore, some studies report threshold effects, suggesting that beyond a certain point, additional species in soil biodiversity may not significantly enhance soil function (Delgado-Baquerizo, Maestre, et al., 2016; Yuan et al., 2020). The notion of functional redundancy within soil communities (Delgado-Baquerizo, Giaramida, et al., 2016; Li et al., 2021; Louca et al., 2018) suggests that it can stabilize community function, provide ecological insurance across habitats, and increase ecosystem resilience by compensating for the loss of specific species. Scale dependency is a recurrent theme, with many studies highlighting potential variations in the soil biodiversity-soil function relationships at different spatial scales (Averill et al., 2021; Gonzalez et al., 2020; Thompson et al., 2018). A few other studies bring attention to the temporal dynamics, suggesting that the short-term benefits of soil biodiversity may not necessarily translate into long-term effects (Carini et al., 2020; Shade & Gilbert, 2015; Wagg et al., 2018). Lastly, some recent studies have highlighted that the relationship between soil biodiversity and soil functions can be highly context-dependent (Delgado-Baquerizo, Maestre, et al., 2016; Hu et al., 2021; Jia et al., 2023). For example, in specific ecosystems or under certain conditions, soil biodiversity may not consistently exhibit a strong correlation with soil functions (Delgado-Baquerizo et al., 2020; Fan et al., 2023; Ratcliffe et al., 2017), emphasizing the need of considering the factors that mediate these relationship (e.g., soil type, land use, climate). Incorporating these nuanced perspectives and addressing these counterarguments is essential for a comprehensive understanding of the complex interplay between microbial diversity and soil functions in diverse ecosystems.

Evidence from manipulative experiments remains inconsistent in supporting soil biodiversity and soil function relationships in some cases (Delgado-Baquerizo, Maestre, et al., 2016; Guerra et al., 2021; Luo et al., 2018; Maron et al., 2018; Rillig et al., 2019; Wagg et al., 2019; Yang et al., 2022). This inconsistency is partly due to the complex and context-dependent nature of these relationships. Considering the ecological and economic importance of ecosystem functions, it is crucial to understand impacts of simultaneous loss in soil and plant biodiversity on multiple microbe-mediated soil functions under future climatic conditions. Studies on reconstructed communities offer controlled settings to model biodiversity-ecosystem functioning (BEF) relationships and provide insights into the mechanisms driving these dynamics (Banitz et al., 2020; Wu et al., 2023). Previous research highlighted how competitive interactions among species can lead to negative BEF relationships, emphasizing the need to consider species interactions and competitive dynamics (D'Andrea et al., 2024). Moreover, studies have shown that soil biodiversity positively influences multiple ecosystem functions and that climate modulates the relationships between above- and belowground biodiversity and ecosystem multifunctionality (Jing et al., 2015; Wagg et al., 2014). These reconstructed community studies help clarify the conditions under which biodiversity enhances or reduces ecosystem functioning, aiding in resolving inconsistencies observed in manipulative experiments. However, studies exploring the implications of disturbance caused by global change drivers on ecosystem functions mainly focused either on plant or soil microbial diversity, overlooking biotic interactions between above- and belowground communities (Figure 4). This limits our understanding of the direct and indirect impacts of changing environments (such as drought, pollution) and biodiversity loss on terrestrial ecosystems. This critical lack of knowledge hampers our ability to accurately predict and manage emerging agricultural and ecological consequences of global change drivers and protect natural resources for the sustainability of ecosystem functioning under changing environments. To fill these critical knowledge gaps, ecologists have scope to develop a trait-based framework using new technological advances in combination with contemporary molecular and -omics approaches and ecophysiology approaches, to forge new understandings of BEF relationships and to generate systematic principles of ecology.

8 FUTURE DIRECTIONS: CHARTING A PATH FORWARD IN BEF RESEARCH

As the knowledge of soil biodiversity and how it affects ecosystem functioning continues to unfold, development of a framework to address key knowledge gaps in soil BEF is required. Therefore, research efforts should prioritize investigations into specific aspects of belowground interactions. For instance, understanding the role of keystone species and their effects on soil processes can provide valuable insights into the maintenance of ecosystem stability, which is essential for ecosystem effective ecosystem management and conservation (Bardgett & van der Putten, 2014; Shang et al., 2023). Additionally, unravelling the complex interactions within soil communities and their functional redundancies can enhance our comprehension of soil biodiversity′s contributions to ecosystem resilience, which is crucial for the sustainability of ecosystems in the face of environmental changes and disturbances (Delgado-Baquerizo, Eldridge, et al., 2017; Eisenhauer & Türke, 2018; van der Putten et al., 2016; Wagg et al., 2014). This will require interdisciplinary approaches to be implemented by integrating ecological theories with experimental design and predictive modelling, which can gain a more comprehensive understanding of ecosystems and their responses to environmental changes (Malik et al., 2019; Schmitz et al., 2010; Suding et al., 2015). By fostering interdisciplinary partnerships, researchers can tackle multifaceted questions and generate holistic insights into belowground interactions (Bardgett & van der Putten, 2014).

From a practical perspective, quantitative understanding of these biotic interactions especially focused on BEF research remains challenging. In particular, (a) plants and microbes differentially contribute to specific functions for example, a simple metric such as plant diversity is likely to be a poor predictor of soil processes (e.g., N mineralization) that are mainly carried out by soil organsims, (b) plant and soil communities can respond independently to similar abiotic properties or differ in spatial and temporal scales at which they function, (c) theoretical and experimental frameworks that address BEF-oriented plant-microbe interactions are limited, (d) different measures of biodiversity, such as species, functional and phylogenetic diversity, can capture different aspects of this concept, but each measure has its own strengths and weaknesses, and no single measure can fully represent the diversity of life and its effects on ecosystem function (Cadotte et al., 2011; Hatfield et al., 2018; Magurran, 2021). Therefore, the choice of diversity measure can profoundly influence our interpretation of BEF relationships, either independently or in conjunction with one another (Cadotte et al., 2011; Hector & Bagchi, 2007; Heijden et al., 2007; Thompson et al., 2018). Combining experiments and observations with multiple measures of biodiversity will be essential for understanding the intricate relationships between different components of biodiversity and ecosystem functioning (Hector et al., 2007; Heijden et al., 2013). In particular, by deliberately manipulating variables and conditions, we can understand the cause and effect, and intricate mechanisms that drive ecosystem functioning. Further, advancements in cutting-edge methodologies such as multiomics endeavours, and stable isotope investigations have the potential to expand our understanding of microbial functions and metabolic interactions (Kong et al., 2019; Nam et al., 2023), but also shed light on the intricate dialogues between plants and their microbial companions (Baldrian, 2019; Mishra et al., 2022).

Recent long-term studies emphasize the significance of continuous monitoring and long-term experiments in capturing the dynamic nature of soil biodiversity and ecosystem functions. The establishment of research networks and platforms that enable ongoing data collection across diverse ecosystems can provide invaluable insights into how soil communities respond to environmental changes over extended periods. Additionally, evidence from long-term grassland experiments suggests that biodiversity-stability relationships become increasingly robust over time (Wagg et al., 2022), highlighting the need for sustained research endeavours to unravel the intricacies of soil communities and their roles in maintaining ecosystem stability and resilience. Long-term experiments, such as those investigating the impacts of various land management practices on soil biodiversity and functioning, can yield important data for informing sustainable land use planning and policy development. Artificial intelligence and machine learning provide opportunities in deciphering intricate patterns concealed within vast data sets. As BEF research reaches beyond local ecosystem and reaches into global biomes, future trajectories must adopt a scale-conscious perspective that will bridge microcosms and macrosystems (Reich et al., 2012). Overall, the path ahead requires embracing collaboration across disciplines, uniting ecologists, microbiologists, geneticists and data scientists. Integrating field observations, molecular analyses, artificial intelligence and modelling that also encompass scale and temporal aspects can help to unravel the complexity of the BEF relationships (Anthony et al., 2023; Eisenhauer, 2012).

9 CONCLUSION

In the quest to understand the complex relationship between biodiversity, biotic interactions and ecosystem functioning, new research has shed light on the functions of soil communities, plant-microbe interactions and aboveground-belowground linkages. The combination of observational and experimental investigations has highlighted the vital role of biodiversity in essential ecosystem services such as nutrient cycling, carbon sequestration and plant productivity. These findings collectively emphasize the urgent need to integrate soil biodiversity considerations into broader conservation and management strategies. However, effective integration will necessitate efforts to close knowledge gaps and explore new horizons. Incorporating interdisciplinary approaches, advanced research methodologies, and using the knowledge gained from global programs such as the Global Initiative for Sustainable Agriculture (Singh et al., 2020). Long-term monitoring initiatives like SoilBON (Guerra et al., 2022) combined with manipulative works embedded in strong theoretical framework, researchers stand on the threshold of unlocking new dimensions of biotic interactions. This holistic approach will not only advance our scientific understanding but also guide practical applications for sustainable agriculture, land management and conservation efforts in the face of global change. As we explore BEF relationships further, it becomes increasingly evident that the synergy between biodiversity and biotic interactions is crucial for understanding the mysteries of sustainable ecosystems. This knowledge holds the potential for a more ecologically resilient and sustainable future.

AUTHOR CONTRIBUTIONS

The idea was conceptualized by Ramesha H. Jayaramaiah, Brajesh K. Singh, Eleonora Egidi and Catriona A. Macdonald. The first draft was written by Ramesha H. Jayaramaiah with subsequent revisions by Eleonora Egidi, Catriona A. Macdonald and Brajesh K. Singh. All authors contributed to the final manuscript.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

ETHICS STATEMENT

This material is the authors′ own original work, which has not been previously published elsewhere, and not currently being considered for publication elsewhere.