The latest issue of Journal of Applied Ecology features as Spotlight on Soil Biota. Here, Martín A. Nuñez (Senior Editor for the journal) and Nahuel Policelli discuss the merits of these papers, specially chosen and grouped together by our Editors.

‘Managing soil health’, ‘considering hidden herbivores’, ‘restoring mycorrhizal fungal diversity’, are just some of the examples that highlight an increase interest on recognizing belowground aspects in applied ecological problems. Under a context of global change and an increasing human population, there is a clear need to learn new ways of improving food security, air and water quality, avoiding biodiversity loss and assuring sustainable ways of production. Some answers to these necessities seem to be below the ground. In this issue of Journal of Applied Ecology, a group of topical research articles shed light on previously-neglected aspects of restoration, with a special emphasis on belowground aspects.

The relationship between global change and belowground biotic processes is not always obvious. Frew et al., for example, experimentally assessed how belowground impacts could be triggered by changes in environmental conditions mediated by changes in plant chemistry. Under increased CO₂ conditions, they demonstrated that belowground herbivores increased their performance and consumed more plant roots biomass. As plant carbon content is predicted to increase due to increased CO₂ concentration in the air, plant nitrogen concentration in the plant is predicted to be diluted. Belowground root chewing insects, which generally have nitrogen limitations in their diets, could increase their consumption to compensate that nitrogen dilution. In this experiment, the application of silicon was effective at decreasing root-herbivore performance and consumption. Silicon-based defenses, an often-ignored functional trait in plants, could potentially have a key role as a sustainable way to protect crops against pathogens and insects under increased CO₂ concentration. Changes in abiotic conditions belowground could also produce a complete change in soil biota. Wu et al. for example, showed how water-table decline facilitates the establishment, population growth and vertical distribution of invasive earthworms, which in turn indirectly reduces soil organic carbon in a peatland area. Carbon emissions from these types of systems could be reduced by preventing further water-table decline and controlling earthworm invasion.

Since belowground biotic interactions are complex and usually context dependent, there is a clear need of long-term, multi-scale experiments. One interesting example is an 8-year mesocosm experiment across three locations in England carried out by Fry et al. They were able to test at the same time the relative role of soil type, intensive or extensive management, nutrient composition, and microbial community in determining colonization order of grassland species. They found that the presence of early-colonizer species was the main factor affecting the establishment of late colonizing grassland species (target of grassland restoration) with a context dependency mediated by soil type. Does this presence implicate strong competition effects or soil conditioning by early colonizers? According to their results, competition is responsible for the reduced establishment of target species, suggesting that priority effects determine restoration success.  Thus, restoration success of grassland may rely primarily on ensuring sufficient niche space available for target species, suppressing competing species with similar traits syndromes.

The recognition that belowground biota composition could improve the establishment of target plant species is crucial in grassland restoration.  In this sense, Koziol et al. showed that arbuscular mycorrhizal fungi (AMF) composition drives aboveground plant composition, with some AMF species combination favoring desirable plant species from a grassland restoration point of view. Although both early and late successional species benefited from AMF inoculation, late successional plants were obligately reliant on beneficial AMF species. Early successional species showed a facultative association with AMF, which could explain their higher success at colonizing areas with disturbed soil communities. One interesting aspect of the study is that not all fungal species would equally be useful for grassland restoration. Diversity per se seems not to be relevant in this case as the removal of a few beneficial AMF species could change the entire community and lead to failure a grassland restoration plan. Which ecological processes determine AMF community assemblages in nature and how they are affected by common practices in grassland would be a key aspect for future studies.

There is vast evidence that soil biota can also mediate in invasion processes. One possible mechanism is transforming allelopathic compounds in less or more toxic chemicals that can in turn favor or disfavor invasive species. Li et al. showed an example in which soil microbes decreased allelopathic effects of an invasive plant by degrading its allelochemicals. Interestingly, this effect may increase with growing invasive species abundance.  The mechanisms by which native soil microbes are able to degrade novel chemicals introduced by the invasive species still needs further research, but their high reproduction rate and their coexistence with similar chemicals in the native habitat could partially explain their success. Allelopathic interactions in nature are complex and context-dependent but allelopathic degrading microbes could work as a sustainable and powerful tool for the restoration of invaded habitats using native species.

In summary, most of the major aspects of global change (increasing carbon dioxide, land use change, biological invasions, and the consequent loss of biodiversity) can be addressed from a belowground perspective. Moreover, all these studies show that is not possible to restore plant communities without considering belowground communities. This is hard evidence that it is time to put belowground processes in the spotlight.

Spotlight on Soil Biota is available in the latest issue of Journal of Applied Ecology.

Find out more about the Spotlight feature.