By: Noah Fierer
Some of you may have seen this paper that came out recently in Nature Plants by Wubs et al. “Soil inoculation steers restoration of terrestrial ecosystems”. As a soil ecologist – this paper brings joy to my heart. It was great to read this paper as it demonstrates the likely importance of soils in guiding the trajectory of plant community development. I’ll avoid summarizing the entire paper, but essentially they showed that depending on the nature of the soil inoculum (removing topsoil from plots and then amending the scraped plots with either soil from a dry heathland or a grassland), they could alter the outcome of plant community development. A barren plot inoculated with ‘native’ soil from a heathland became more like a heathland over time and vice versa.
This is an impressive study. The authors coupled both field and mesocosm studies with a broad array of soil and vegetation analyses to effectively demonstrate how soil properties are relevant to the trajectory and outcome of a restoration effort. If you want to successfully restore an ecosystem, it is not just a matter of planting in some seeds from the plants of interest and hoping that the intended plant community will develop. It is never a good idea to ignore soil.
Like any good scientific study, this study raises more questions than it answers. It remains unclear what biotic and/or abiotic factors are responsible for the patterns they observe (a point the authors were careful to acknowledge). As much as I love the bacteria, fungi, nematodes, and other fauna that live in soil and as much as I’d love to admit that they truly do rule the terrestrial world, there are multiple possible explanations for the patterns they observe. In particular, it is possible that the patterns they observe have nothing to do with the different soil inocula having different biota, but rather the patterns could be driven by differences in the abiotic properties of the soil inocula. Let me explain. The two ‘donor’ soils added to the barren plots were very distinct (see their Supplementary Table S5). Perhaps not surprisingly, this led to dramatic shifts in the abiotic properties of the soils in the experimental plots. For example, the plots amended with the heathland soil had 30% more organic C and 30% more available phosphorus than the plots amended with the grassland ‘donor’ soil. Do these changes in the abiotic properties of the soil explain the patterns observed? Perhaps. There is a large body of literature showing that different plant species often prefer soils with different edaphic properties. We have to be careful to ascribe the observed changes in plant community development to changes in the soil biota alone.
This paper has interesting parallels with the growing body of literature on fecal transplants (reviewed here and here). In both cases the goal is to restore an ecosystem and push it towards a desired direction. Fecal microbial transplants can often be effective in altering human health and gut microbial communities, but, as pointed out in a recent paper by Bojanova and Bordenstein – when a fecal transplant is performed, we are not just transplanting the living microbes from the feces of a healthy donor, we are also transplanting organic material (including metabolites and many dead cells). Thus, whether we are talking about restoring plant communities or restoring gut communities, we have to be very careful when ascribing the success (or failure) of the restoration effort entirely to the transfer of living organisms. Fecal transplants and soil transplants are clearly an important component of restoration efforts, but we need to specifically identify why the restoration efforts, are (or are not) successful. This is not going to be trivial, but it is an important next step.