Plant-soil feedbacks (PSFs) refer to the ability of a given plant to alter soil abiotic or biotic conditions in ways that modify the growth of a plant subsequently growing in the same soil. Over the last three decades, PSFs have been increasingly recognized as important drivers of plant community assembly and through this ecosystem functioning (Bever 2003; van der Putten et al. 2016). PSFs are considered positive and negative when plants modify soils in ways that respectively enhance or reduce the performance of an individual of the same species subsequently grown in the same soil (Bever et al. 1997). There is now evidence that PSFs influence plant community structure and dynamics, plant succession and invasion processes (Bonanomi et al. 2005; Kardol et al. 2006; Klironomos 2002; Revilla et al. 2013). Moreover, there is also growing evidence that climatic and environmental factors influence plant-soil biotic interactions causing shifts in PSFs with implications for the future of plant population dynamics (Bardgett et al. 2018; Weidner et al. 2015; Wubs & Bezemer 2018). Accordingly, several recent studies have shown climate change induced shifts in PSFs related to both direct effects on the plant and indirectly effects driven by changes in the composition and activity of soil biota (Crawford & Hawkes 2020; Snyder & Harmon-Threatt 2019; Xi et al. 2018). However, our knowledge about how climate change drivers alter PSFs, and the potential implications for plant community dynamics, remains limited (van der Putten et al. 2016; Pugnaire et al. 2019).
Plant-soil feedbacks are induced via both abiotic and biotic pathways (Fig. 1). For instance, individual plants can increase their relative belowground carbon (C) allocation (Fig. 1, Pathway 1) which can enhance interactions with soil microbes, such as N-fixing bacteria and mycorrhizal fungi (Fig. 1, Pathway 4), resulting in a positive effect on plant biomass production through increased uptake of nitrogen (N) and phosphorus (P), respectively (van der Putten et al. 2016). Such beneficial associations promote positive feedbacks that may extend beyond the lifetime of an individual through a build-up of beneficial organisms (van der Putten et al. 2016; Revillini et al. 2016). Positive plant-soil biotic interactions may be particularly important in stressful environments, including under nutrient or water limited conditions (Lagueux et al. 2021; Rutten & Gómez-Aparicio 2018). Furthermore, climatic, and environmental conditions influence plant physiological and morphological traits (Fig. 1, Pathway 2) in ways that can affect ecosystem functions, such as litter decomposition and nutrient cycling (Fig. 1, Pathway 3), resulting in subsequent changes in plant performance due to changes in nutrient availability. For example, both heatwaves and drought have been shown to result in the production of more recalcitrant litter (i.e., increased C:N ratios of leaf litter, higher lignin concentration; Almagro et al. 2015) which is likely to slow down decomposition and reduce nutrient availability resulting in negative PSFs (Fig. 1). Moreover, plants are also exposed to pests and pathogens belowground (Fig. 1, Pathway 4) that negatively affects plant growth. For example, the accumulation of root pathogens can reduce the growth of individual plants as well as individuals subsequently growing in the same soil if they are sensitive to those specific pathogens causing negative PSFs (Bezemer et al. 2013; Domínguez-Begines et al. 2021). Similar effects are likely to occur aboveground in response to herbivory, but the broader effects of plant-herbivore interactions on PSFs are not well known and are difficult to predict given the greater mobility of herbivores (Heinze et al. 2020).
Despite a growing body of literature, it is not yet clear how climate factors, such as warming and drought, alter the magnitude and direction of PSFs (Fig. 2). There are three main non-mutually exclusive hypotheses about the drivers of climate effects on plant-soil feedbacks: 1) Change in climate modify plant inputs to soil via litter quantity and quality and root exudation influencing nutrient cycling; 2) Climate change either reduces or enhances the effect of species-specific pests and pathogens; and 3) Climate change disrupts or strengthens the interaction with beneficial microbes. In all cases, the outcome will depend on the climate change factor in question and the level of change (van der Putten et al. 2016). Warming generally enhances decomposition and metabolic rates of microbes (Fig. 2, Pathway 1a) which increases the availability of soil nutrients, thus promoting positive PSFs. However, the potential role of warming on PSFs is less clear in the context of beneficial microbes, such as arbuscular mycorrhizal fungi (AMF), as warming can increase (Rasmussen et al. 2020; Rillig et al. 2002) and decrease (Wilson et al. 2016; Zhang et al. 2021) AMF colonization. Indeed, warming may reduce beneficial microbial colonization (Fig. 2, Pathway 2a) if the plants have access to essential resources thus promoting negative PSFs. However, warming may result in water and nutrient limitation, which may promote root fungal colonization in order to get access to nutrients thus promoting positive PSFs (Rasmussen et al. 2020). Furthermore, warming generally promotes the density and activity of root-feeding soil fauna and increases root pathogen infection (Fig. 2, Pathway 3a) resulting in negative PSFs (Lu et al. 2015; Staddon et al. 2003; Wilson et al. 2016). By contrast, reduced rainfall or drought generally reduces litter decomposition and root exudation by decreasing the activity of decomposers and metabolic rates (Fig. 2, Pathway 1b), respectively, thus promoting negative PSFs. On the other hand, drought could increase mycorrhizal root colonization as the plant allocates more C belowground for nutrient access promoting positive PSFs (Fig. 2, Pathway 2b). Furthermore, drought may reduce pest and pathogen abundance, resulting in relatively more positive PSFs (Fig. 2, Pathway 3b) (Cheng et al. 2016). The role of aboveground herbivory is known to promote negative PSFs due to changes in leaf chemistry, such as shoot nitrogen concentration (Heinze et al. 2020); however, few studies have assessed the role of herbivory under manipulated climates; thus, herbivory is not considered in this meta-analysis (Fig. 2, Pathway 4).
Several general patterns regarding the occurrence of PSFs have been revealed. PSFs known to be species specific but appear to differ consistently across plant functional groups (Fry et al. 2018; Heinze et al. 2017; Kulmatiski et al. 2008). A growing body of literature has found that grasses and forbs generally show negative PSFs while woody species show neutral PSFs (Chung & Rudgers 2016; Cortois et al. 2016). The first meta-analysis assessing PSFs showed that they are predominantly negative at the species level and positive, or at least less negative, at the community level (Kulmatiski et al. 2008). PSFs have similarly been shown to depend on plant life cycle characteristics (annual and perennial) and species origin (native and non-native). Current literature suggests that native species may show negative PSFs due to increased pests and pathogens densities (Callaway et al. 2004; Meisner et al. 2013; van der Putten 2002). On the other hand, non-native species may escape from species-specific soil-borne pathogens in a foreign environment thus reducing negative plant-soil biotic interactions and promoting positive PSFs (Callaway et al. 2004; Meisner et al. 2013; Perkins & Nowak 2013; van der Putten 2002; Zhang et al. 2019). Species with annual life cycles tend to produce more negative PSFs relative to species with longer life cycles (i.e., perennial); however, the mechanisms underlying this pattern are still unknown (Kulmatiski et al. 2008). It is possible that long lived perennial species, such as shrubs and trees, might invest more in plant defence making them less sensitive to pest and pathogens than short lived species (Kulmatiski et al. 2008). How climate change influence these patterns is still unknown (Pugnaire et al. 2019).
Here, our aim was to assess the impact of elevated temperature (henceforth ‘warming’, ET) and reduced rainfall (henceforth ‘drought’, DT) on the magnitude and direction of PSFs. There were too few studies on other climate drivers, such as elevated CO2, for these to be included in the meta-analysis. Synthesizing existing data will enhance our understanding of how warming and drought might modify PSFs and help predict the role of PSFs in shaping plant community dynamics in response to a changing climate. We specifically asked:
a) Do PSFs shift in response to warming and drought?
b) Do warming and drought effects on PSFs differ across plant functional types?
c) Do warming and drought effects on PSFs differ between annual and perennial species and native and non-native species?
d) Are PSFs dependent on experimental conditions (greenhouse/laboratory versus field)? And, lastly,
e) Do PSFs differ between species growing conditions (monoculture and mixture)?