Wednesday 5th December is World Soil Day 2018.

This soil celebration gives me the opportunity to highlight great papers related to soil published in Journal of Ecology this year.

While the role of soil in shaping plant community assembly, composition and diversity has long be recognized, the mechanisms involved to explain these effects are not always so clear. New developments in soil research, particularly soil microbes with new screening methods for soil microbial communities, has helped us achieve a better grasp of what is happening below-ground and how soil can influence the plants that grow above-ground. 

Soil microbial communities have been shown to strongly impact competition between plants. For example, it can alter conspecific and congeneric competition as demonstrated by Siefert et al. (2018) on three different Trifolium species. They found that the presence of microbes can shift competition to facilitation between congeners and reduce competition between conspecific individuals, thus promoting coexistence in multiple ways simultaneously (Figure 1).

Figure 1. Soil microbes can influence coexistence in multiple ways simultaneously. Summary of overall effects of soil microbes on plant-soil feedbacks and plant-plant interactions in relation to observations of field co-occurrence (Figure 5 in Siefert et at. 2018).

Plant competition is thus strongly influenced by soil microbial community and associated plant-soil feedback but a study by Bezemer et al. (2018) shows that competition between plants can also drive plant-soil feedback effect with varying impacts depending on plant age. In a successive three phases plant growth experiment, the authors found that the biomass of one plant species was influenced by soil legacy effects of the first growth phase, while the biomass of another plant species was affected by soil legacy effects of the second growth phase, thus highlighting the temporal dynamic of plant-soil feedback.

Figure 2. Scenarios for the different novel plant–soil biota interactions that could occur along an elevation gradient following asynchronous migration of plants and soil biota to higher elevation with changing climate. (Figure 1 in Cardinaux et al. 2018).

Plant-soil feedback can have strong impact on plant-plant interactions, and it is particularly the case when plants shift their ranges and create novel interactions between plant competitors, or when plants invade existing communities. In a greenhouse experiment, Cardinaux et al. (2018) investigated different scenarios of climate change-induced altitudinal shifts in plant and soil communities in the Swiss Alps (Figure 2). Overall, they found that low-elevation plant species tended to exclude high-elevation species independently of the soil biota origin. However, one low-elevation grass, Poa trivialis, was only able to invade communities of high-elevation Poa alpina in the presence of a low-elevation soil biota, showing the importance of synchronicity in plant and soil shifts to successfully impact novel competitive interactions. 

In Australia, Roux et al. (2018) also showed that invasive acacia species can strongly modify soil abiotic conditions and impact the community composition of mutualistic rhizobia, with evidence for acacias to have a homogenizing effect within and between invaded sites, which positively fed back to the acacia growth. Mutualism between plants and rhizobia has also been investigated by Keller & Lau (2018), with the aim of determining when these interactions may be the most important to communities. Findings of their study show that the presence of rhizobia consistently reduced the diversity of native plant community, particularly in the presence of the legume Chamaecrista fasciculata that was highly dependent on rhizobia, and when nitrogen was most limiting. 

Figure 3. Soil microcosm design used Jacobs et al. (2018) to determine the effects of litter type and soil origin on carbon losses.

Moving on to mycorrhiza, another type of soil mutualist, Zhu et al. (2018) found that mycorrhizal associations were related to terrestrial ecosystem properties. More particularly, they demonstrated that increasing ectomycorrhizal dominance in soil was associated with higher soil C:N, mostly due to a reduction in soil N rather than changes in soil C. To go deeper in the understanding of mycorrhizal impacts on soil properties, Jacobs et al. (2018) tested the impact of tree litter type (root and/or shoot) and soil origin (AM, arbuscular mycorrhizal or ECM, ectomycorrhizal-dominated soil) on forest carbon losses (Figure 3). They found that in AM soils, elevated C losses were driven by roots accelerating leaf decay, while in ECM soils, losses resulted from roots and leaves accelerating the decay of soil organic matter (SOM), thus highlighting the need to integrate these different processes to better estimate the magnitude and source of C losses from soils. 

Last but not least, a study by Wooliver et al. (2018) published in the November issue confirms the important role of soil fungi, particularly in mediating plant responses to nitrogen enrichment induced by fossil fuel combustion and agricultural fertilizer practices. In this study, the authors found that eucalypt responses to N enrichment was dependent on both soil fungi and phylogenetic lineages of the eucalypt species. Indeed, conspecific-conditioned soil fungi enhanced growth responses to N enrichment for plants within one lineage (lineage 1) but depressed growth responses to N enrichment for plants within another lineage (lineage 2). Overall, findings of this study emphasize the importance of plant evolutionary history and can help predict which plants will win or lose under future N deposition scenario.

To obtain more information about these papers, please go look at the full versions and enjoy learning about soils and their inhabitants. There is much more interesting findings to discover in soils, so let’s continue putting our hands in the dirt and exploring the unknown below our feet.

Happy World Soil Day 2018! 

Pierre Mariotte, Blog Editor and Associate Editor of Journal of Ecology