– added by Franciska de Vries
This paper, by Colin Averill and colleagues, came out as a Letter in Nature almost two weeks ago. It immediately excited me, as the title suggests that in this paper, the authors are revealing the mechanism through which mycorrhizal fungi increase soil carbon storage. Groundbreaking!
I started to read. What the authors did in this paper was compose a global dataset, consisting of observations of soil organic C, N, and clay content (to a depth of one meter) across a range of vegetation types and biomes. They then assigned values of mean annual temperature (MAT), mean annual precipitation (MAP), and net primary productivity (NPP) to each site, using previously published climate interpolations and satellite-based observations. Finally, they assigned a mycorrhizal status to each of the vegetation types, which could be either arbuscular (AM) or ericoid and ecto-mycorrhizal (EEM).
Mycorrhizal status of each site was assigned based on the dominant vegetation present, and knowledge of its mycorrhizal status. Understorey vegetation (in forests) was ignored, and where no vegetation data was present, they used the vegetation description. So, for example, the vegetation description “grassland” was classified as the AM, whereas “mixed coniferous forest” was classified as EEM.
This dataset was then used to explain soil C storage, using soil N and mycorrhizal status as explanatory variables, while at the same time accounting for variation in climate and other soil properties.
Using mixed effects models, the authors found that in ecosystems dominated by EEM fungi, 1.7 times more C was stored per unit N than in AM ecosystems.
For the first time, this study shows that C global cycling does not only depend on abiotic factors like temperature and moisture, but also on soil mycorrhizal status, highlighting the importance of biotic factors in addition to abiotic factors for soil C storage. This finding supports the results from a modelling study by Orwin et al. (2011), who showed that competition for organic N between EEM and decomposer fungi increases soil C storage.
Averill and colleagues conclude that ‘mycorrhizal functional traits are as important a control over decomposition and soil C storage as are soil chemical properties and the physical protection of soil organic matter’, and that ‘the identity and functional traits of soil microbes exert a control over the terrestrial C cycle’.
So, a pretty exciting paper, but does it really do what it promises? When I read the title, I expected a paper reporting on (a range of) mechanistic experiments to prove that mycorrhizal fungi drive soil C storage. But, rather than a mechanistic study, it is an observational study that uses a powerful data set and an advanced modeling approach to show that there is a relationship between mycorrhizal status and soil C storage. However strong their finding is, and although it holds across a range of ecosystems and biomes, their study does not allow for testing a hypothesis and elucidating a mechanism. As Mark Bradford elegantly says in his News and Views article about the paper: “The authors propose, in line with a previous hypothesis, that these richer carbon stores result from competition for nitrogen between EM fungi and freeliving soil microorganisms that feed on organic matter” and: “Pinpointing which mechanism explains Averill and colleagues’ results will require more data and involve challenges common to all large observational data sets, including unobserved variables and spurious cor relations”.
I couldn’t agree more.
However powerful the data set in this paper is, there are several issues that would have to be addressed to come up with a conclusive answer of how and whether mycorrhizal fungi drive soil C storage. First of all, important soil properties that can explain both soil C content and mycorrhizal status, such as soil moisture or pH, haven’t been included in the models. Second, certain vegetation types, such as heathland, which is known to be dominated by ericoid mycorrhizal fungi, are missing. Third, the assigned mycorrhizal type might be occurring under a certain vegetation type because of the quality of the C inputs into the soil, which might itself drive soil C storage; something that the authors do acknowledge in the paper. Finally, Mark Bradford calculated, in his News and Views article, that only when the soil contains more that 3 kg N per square meter, C stores in EEM dominated systems exceeds that of AM dominated systems by 1.3 times.
But, despite these nuances that will need addressing in future studies, this is an important study that proposes an important hypothesis, namely that mycorrhizal type is of pivotal importance in driving soil C storage. By formulating this testable hypothesis and identifying a global relationship between soil biota and soil C storage, this study significantly advances the field of plant-soil interactions. It also highlights that disruptions of links between vegetation and mycorrhizal fungi, as a result of global change, might have far-reaching implications for soil C stocks and thus for the climate mitigation potential of soils.
So, I am curious what other people think about this paper! Do you agree or disagree with my views? How do you think we could go about testing the hypotheses proposed in this paper? Or is this enough evidence already? What do you think about the statistical methods used? Feel free to comment – the aim of this journal club is to stimulate discussion – through replies here, but also on Twitter. If you respond on Twitter, please use the hashtag #psejclub.
With thanks to all members of the Soil and Ecosystem Ecology group of The University of Manchester for inspiring discussions.
Full reference: Averill, C., B. L. Turner, and A. C. Finzi. 2014. Mycorrhiza-mediated competition between plants and decomposers drives soil carbon storage. Nature advance online publication.