With two billion more people living in urban areas by 2050 (Bai et al., 2018), urbanization poses unprecedented challenges. Economic, social, and cultural promises associated with urbanization are counteracted by the depletion or degradation of natural resources, biodiversity, and ecosystem services (WBGU, 2016). These degradation trends harm human health and wellbeing (Sarkar and Webster, 2017). Moreover, cities are significant drivers of climate change (Revi et al., 2014) and are particularly vulnerable to its effects, including heat waves, droughts, hurricanes, and flooding (Baccini et al., 2007). This resulted in more than 20,000 additional heat-related death cases in German cities in the period 2018–2020 (Winklmayr et al., 2022). Current challenges are to increase the capacity of urban areas to deal with a changing environment, mitigate ecological impacts, enhance resource efficiency, human health, social inclusiveness, and equality, and harness the productivity of local economies and value-added activities (United Nations, 2017).

Many studies have suggested that turning urban environments into biodiversity hotspots might improve the quality of urban life and its resilience against extreme weather events and other stressors. This would enhance the wellbeing and health of people living in urban areas (Dearborn and Kark, 2010; Faeth et al., 2011; Kazemi et al., 2011; Guilland et al., 2018; Rega-Brodsky et al., 2022). In the past decades, several concepts were developed for an improved green infrastructure (Pauleit et al., 2020) that could enhance urban biodiversity, e.g., Biodiversity Sensitive Urban Design (Kirk et al., 2021), and Animal Aided Design (Weisser and Hauck, 2017). This includes implementing novel types of buildings and infrastructure with more natural elements, as done in Singapore (Rowe and Hee, 2019). In addition to adapting entire cities to environmental challenges, solutions at the local level of urban districts, streets, or building blocks have been discussed and implemented. These concepts can provide synergies for people and the environment. However, often they may need more socioeconomic acceptance due to the high costs of their implementation. A potential solution is combining different urban functions as green-blue-grey infrastructure also considered nature-based solutions (Hoang and Fenner, 2016). For example, Sustainable Urban Drainage Systems (SUDS) are infiltration sites that combine the function of dewatering with other functions such as decontamination, microclimate regulation, and habitat for biodiversity (Fletcher et al., 2015) (Figure 1).

Contemporary SUDS have largely been designed from a technical perspective as infiltration systems without fully considering the potential of such systems to benefit biodiversity or human recreation (Zhou, 2014; Zhang and Chui, 2019), or by developing functional plant communities with more efficient ecological functions including water cycle regulation (Rojas-Botero et al., 2021; Rojas-Botero et al., 2023a). This would generate positive feedback loops for the urban environment, including mitigating climate-related issues (Monberg et al., 2019; Monberg et al., 2018). Thus, the implementation of SUDS can provide multiple ESS, that range from supporting biodiversity (Filazzola et al., 2019), increasing human wellbeing (Coutts and Hahn, 2015), and reducing the urban heat island effect (Norton et al., 2015). The conversion of SUDS into those of multifunctional green infrastructure could reduce land-use conflicts between built infrastructure and biodiversity hotspots, as infiltration systems are already present in some urban areas, albeit often poorly designed and managed. In this perspective paper, the term “multifunctionality” generally refers to the ecological and environmental functions that are provided by SUDS, while the potential social functions (educational value, recreation, etc.) are not addressed in detail.

Despite the obvious advantages of SUDS, we still need a critical understanding of their techno-ecological basis that hinders further engineering of such elements. Thus, more research is necessary to identify optimal design parameters, especially under flooding and drought. Improved SUDS should take up large amounts of stormwater, remove pollutants, prevent erosion, be drought- and flood-tolerant, cope with de-icing salts, and integrate attractive vegetation with low maintenance costs. Thus, the challenge is to combine stormwater treatment and urban drainage with principles of ecological restoration while implementing the latest knowledge in soil science, microbiome research, and vegetation ecology. Principles of restoration include self-supporting populations of native species, normal ecological functions, ecosystem resilience, and landscape connectivity (Holl, 2020). These aims require an in-depth understanding of which plants tolerate the site conditions at SUDS, including stress caused by extreme redox conditions, biocides, and heavy metals, and how these plants can be supported by the specific soil physical and chemical properties and the soil microbiome. Well-designed plant communities will induce positive feedback to the soil by improving carbon stocks as well as the biodiversity of soil biota. The following sections present the key characteristics of SUDS, and discuss how they can be advanced by performing integrative and interdisciplinary research.

From the engineering perspective, SUDS have two main functions: de-watering and purification (Dierkes et al., 2015). Appropriate de-watering is important for the infiltration of stormwater runoff so that SUDS do not have too large of storage volumes. According to German regulations (DWA-A 138-1, 2024), for example, the hydraulic conductivity of the vegetated soil zone of infiltration swales must be in the range of 5·10⁻⁵ m/s to 1·10⁻⁵ m/s. This can be achieved if the mass fraction of organic carbon is max. 4%, and the clay and silt content is <20%. To maintain the infiltration capacity, the soil zone must be covered with vegetation. The purification performance such as pollutant retention and decomposition (Horstmeyer et al., 2016; Deeb et al., 2020; Rommel et al., 2019; Ekka et al., 2021; Galster and Helmreich, 2022; Adhikari et al., 2023) varies depending on the nature of the contaminants and the intensity of the upload charge on the stormwater runoff captured by the SUDS. Contaminants such as nutrients, pathogens, metals, biocides (pesticides, herbicides, etc.), and hydrocarbons can have a significant negative impact on SUDS functions, and the effect of their interaction with other contaminants is still not well understood (Peng et al., 2024). In addition, the effect of emerging contaminants like per- and polyfluoroalkyl substances and others on SUDS performance is not fully identified (Sultan et al., 2024).

Although soil is inherently capable of buffering against pollution, overloading the system in the urban environment threatens some soil functions (Li et al., 2018) and spreading de-icing salt can stress plants and negatively influence the performance of SUDS (Denich et al., 2013). Implementing large-scale SUDS programs requires filling the gap in knowledge and research questions regarding how the engineering soil system and associated biotic interactions (microorganisms and plants) can be optimized for maximum contaminant buffering while ensuring the required functions of stormwater storage and plant community growth.

The functionality of soils as a growth medium and habitat, i.e., supplying water and nutrients, and dealing with contaminants (Lehmann et al., 2020), are intimately linked to the formation and structure of soil aggregates (Totsche et al., 2018). Soil structural stability is a prerequisite for SUDS functions since it guarantees the regulation of water retention and improves infiltration capacity. Soil structure is also essential for other SUDS functions, including gaseous exchange, nutrient dynamics, root growth, and erosion susceptibility. It also provides a habitat for diverse soil organisms and regulates their activity. Improved soil structure can enhance soil remediation of contaminants for improved plant quality and water-holding capacity to support diverse plant communities. The formation and stability of soil aggregate structure strongly depend on the continuous supply of organic matter from plant residues and rhizodeposits, and their turnover by the soil microbial communities. Previous experiments showed a high potential for artificial and immature soils to sequester carbon (Pronk et al., 2017). Therefore, the type of vegetation used and its root system, as well as plant litter, control residue input into the soil and thus decomposition and the formation of soil organic matter. Mowing and removal of the mown materials negatively affect soil development.

The soil organic-mineral composition and the thickness of the topsoil layers play a central role as they largely determine the sorption kinetics and capacities for pollutants (Rommel et al., 2019). Charcoal is a potential approach to improve the capacities of soils to handle different kinds of contaminants (Spahr et al., 2022). In urban environments, high-carbon amendments, such as lawn clippings and compost, are common and cheap, potentially boosting soil carbon and structure (Egerer et al., 2018). Yet, it needs clarification on how varying amounts and types of organic amendments can improve carbon sequestration and binding of water and soil contaminants, and how these benefits support multifunctional SUDS implementation.

Finally, nutrient availability in soils strongly impacts plant development as well as the structure and function of the soil microbiome. Fertilizer addition improves the activity of soil microbiota and triggers plant biomass development but reduces above- and below-ground biodiversity. On the other hand, low nutrient concentrations reduce microbial activity and plant growth. Thus, nutrient management must fit the actual demands of plants and microbiota.

Soils are a hotspot for microbial diversity (Mguni et al., 2016). This enormous diversity forms the basis for many ESS that soils can provide, including carbon sequestration, degradation or fixation of pollutants, safeguarding drinking water resources, and supporting plant growth (Berg et al., 2020). The diversity of bacteria, archaea, fungi, and other eukaryotes determines the resilience of soils. However, like many other biota, a substantial decline in functional microbial richness has been observed in the past decades due to soil degradation followed by a loss of soil-based ecosystem functions (Felipe-Lucia et al., 2020). Recent studies indicate that reduced microbial diversity in the environment drives several common diseases, including infections and allergies (Foesel et al., 2019). At the same time, an increase of microbes that carry antibiotic resistance genes increased due to co-selection of these traits in the presence of other pollutants, which is a risk factor for human health (Seiler and Berendonk, 2012).

Thus, strategies are needed to stabilize the soil microbiome and to maintain functional potentials that improve environmental and human health. In urban environments, such systems are strongly needed, as sealing, compaction, salt, animal feces, microplastic, and pollution of soils induce a substantial decline in functional microbial activities. At the same time, invasive pathogens might increase or the number of antibiotic-resistant microbiota rises (Bodus et al., 2024). In addition, poor soil qualities regarding nutrient availability and soil structuring often interfere with strategies to promote microbial diversity in urban soils.

Approaches to improve biodiversity in soils are well known from restoration ecology and agroecology. In restoration ecology, legumes are often used as pioneer plants, which will enhance nutrient contents of soils (nitrogen, phosphorous) and foster with their rooting network soil structure development (Pihlap et al., 2019). This can induce positive feedback loops for the diversity of the soil microbiome and its functionality (Vuko et al., 2020). In agriculture, soil biodiversity can be enhanced by intercropping, in which two plants with contrasting properties are grown simultaneously (Mwakilili et al., 2021). These approaches are often supported by selected bio inocula, which provide certain functionalities essential for the soil microbiome to benefit the growth of the crop plants (Ptaszek et al., 2023). Translating this knowledge to develop SUDS could mean transplanting a highly diverse microflora from natural grassland to urban soils. This would support beneficial microbiota and their colonization of the soil by designed plant mixtures, which provide the needed niches for the inoculated bacteria, fungi archaea, and protists.

A current challenge in SUDS is to develop plant communities that provide better soil functions, while tolerating chemical (contamination, pharmaceuticals, metals, etc.) and physical stressors (temperature, high water saturation, etc.). Plant communities at SUDS have to be selected to provide at least some of the ESS mentioned by Monberg et al. (2018); Monberg et al. (2019) and Papuga et al. (2022), and innovative plantings are based on the theory of community assembly and trait-based restoration (Laughlin and Laughlin, 2013; Eben et al., 2024b; Eben et al., 2024a). Plant communities should be designed using ecological criteria that combine water management, urban soil development, pollutant immobilization, and biodiversity maintenance, thus boosting synergies of ESS within SUDS. The design must be based on the regional species pool (Bauer et al., 2023). In an urban environment, it can also include non-native species that cope with the extreme site conditions of SUDS. However, the latter must be considered with great caution to avoid biological invasions that do not support principles of ecological restoration (Holl, 2020). Future mixtures should balance life forms, plant traits, and phylogenetic diversity (Yannelli et al., 2017). In some cases, the interaction between various urban stressors could negatively affect plant communities and soil biodiversity (Rojas-Botero et al., 2023b). Thus, the selected plant communities must be tested under heat waves, drought, waterlogging, and pollutants to develop the best practical recommendations.

Moreover, there are functional linkages between above- and belowground processes, and vegetation structure and composition that may modify belowground functioning, including microbial communities, nutrient cycling, and soil composition (Bardgett et al., 2014; Wardle et al., 2004). One of the main insights of biodiversity research is that increasing species richness in a plant community increases overall functioning with respect to many processes (Weisser et al., 2017). Thus, by combining different plant species the added properties of the community may increase the overall provisioning of ecosystem service. For example, plant species richness and composition influence the water balance of herbaceous plant communities through several mechanisms (Leimer et al., 2014). Plant community composition, for example, affects earthworm communities in experimental plant communities, which in turn influence plant species traits and soil water infiltration (Fischer et al., 2014; Fischer et al., 2015). These interactions can together have a strong effect on how plant communities respond to extreme events, whereby plant species richness can stabilize ecosystem functions (e.g., Isbell et al., 2015; Huang et al., 2024). In addition, further plant species may colonize SUDS as so-called spontaneous vegetation. Whether such species that can live under the conditions of SUDS further increase functionality needs further investigation. Thus, when developing novel plant communities for SUDS, the species-specific root systems affecting belowground carbon storage, water retention, and interactions with microorganisms that degrade some pollutants must be considered (Rojas-Botero et al., 2023a), along with the emerging properties of the resulting plant communities. Extreme scenarios, such as flooding or drought, as well as the stability of the plant community over time should also be examined.

However, there is no guarantee that other components of the belowground soil communities will follow suit because faunal diversity is poorly understood in SUDS soils. Thus, besides plant selection, soil components could directly affect soil diversity; for instance, adding topsoil and encouraging connectivity between SUDS will increase faunal richness (Deeb et al., 2020). Indeed, the way SUDS are built affects the functional microbiome (Deeb et al., 2020). The combination of landscape design, selected plants, location, and the desired ratio of the mineral-organic mixture in the soil (Deeb et al., 2020; Deeb et al., 2017) is expected to influence microorganism activity in SUDS. Finally, the landscape connectivity of urban green and blue infrastructure must be considered to realize the full biodiversity potential of SUDS, as this could influence species colonization as well as the ability of SUDS to act as a stepping stone habitat in the landscape.

An integrated approach to understand and to create evidence-based multifunctional SUDS should be the aim of interdisciplinary studies. The soil, microbiome, and plants must be considered as functional components of SUDS. Thus, approaches derived from systems biology are needed to promote positive feedback loops, encouraging functionalities of SUDS, including increased biodiversity. To understand the factors that drive SUDS multifunctionality, we must investigate and test how different combinations of soil and plants can influence functions including infiltration, water storage, and carbon sequestration. This knowledge informs how SUDS can be engineered to optimize such tasks in the urban environment while promoting biodiversity and aesthetics (Backhaus and Fryd, 2013). Specifically, the engineering of SUDS needs to integrate various functions that are tailored to the location of where the installation is feasible and recommended to achieve the best balance, e.g., of organic adsorption to minerals, and how to apply them sequentially as soil layers. This can then predict the development of soil structure, including soil aggregation processes and soil porosity, which can facilitate habitat installation while increasing carbon storage. In addition, plant communities with various root systems should be selected to increase water retention capacity, soil stability, and faunal activity.

Furthermore, it requires working with urban planners and policymakers to adapt and integrate SUDS within the respective city landscape, as green infrastructure may require working with laws, regulations, and city ordinances that enable or hinder their implementation (Dhakal and Chevalier, 2017). Cross-sectoral and well-coordinated governance, availability of resources (e.g., human, financial, data), stakeholder awareness, socially inclusive and just planning approaches, and reliable evidence of the long-term costs and benefits, are key factors for the establishment of multifunctional SUDS.

Future research should test how soil composition (e.g., different ratios of organic mineral components), along with specific plant communities (combinations of native and non-native plants of various functional traits), influence the soil microbiome and the ability of the SUDS to filter pollutants. In addition to regulating and purifying water, future experiments should evaluate other soil functions, such as carbon and water storage. One could, for example, use locally excavated mineral materials from urban construction work and organic materials such as biochar and compost, taking a circular economy approach in the urban environment as they are locally available. The materials are then mixed in different ratios and organized into specific horizons to maximize promising ecosystem services and increase the sustainability of SUDS. These can provide nutrients that promote certain combinations of plant and microbial communities, while soil structural elements support pollutant removal.