Tag Archives: Carbon

PSE journal club: Vegetation exerts a greater control on litter decomposition than climate warming in peatlands

A bright, blustery day in England’s north Pennines. Fluffy cotton grass heads bob and bounce on the breeze. On the side of a hill, a strange array of hexagonal, knee-high structures glint and sparkle. Four figures, hunched against the wind, move methodically along jaunty wooden boardwalks, which rest on the blanket bog, crouching at each hexagon in turn. Welcome to Moor House National Nature Reserve, the site of an experiment designed to investigate how a warmer climate will affect the speed with which plant litter is recycled back into the soil, and ultimately the atmosphere.

One of the inherent difficulties associated with upland experiments.

One of the inherent difficulties associated with upland experiments.

If you’re wondering about the shift in tone in the opening paragraph of this #psejclub post compared to some of the others, it’s because I was there. I helped to set up and sample the aforementioned experiment, which is located a few hundred metres away from where I did my PhD fieldwork, in exchange for help with my own work, so I have to admit to having a degree of personal bias! I do think that the work is of general interest, though, and conveys some important findings about litter decomposition in peatlands that will help us to build a picture of how these key processes might change as plant communities shift in response to climate change.

The paper, currently a preprint in ESA Ecology, describes an experiment that uses a combination of open-top, passive warming chambers and plant removal treatments to investigate how the presence of certain plant functional types and warmer temperatures affect rates of litter decomposition. The authors used litter bags, filled with the litter of each plant functional type and buried in plots beneath the plant removal and warming treatments. In all, there were eight treatments (combinations of graminoid, shrub and bryophyte removal, a bare plot and a control with no plants removed), replicated over four blocks, with half the plots warmed by the passive chambers.

So how do peatland processes respond to warming?

So how do peatland processes respond to warming?

The main finding of the paper is that presence or absence of plant functional groups had a stronger effect on peatland litter decomposition than the warming of approximately 1°C achieved by the passive chambers. Removing the shrubs from the peatland resulted in faster decomposition of graminoid and bryophyte litter, after two years. Litter identity was also important – in the first year of the experiment this was the main factor controlling rates of litter decomposition, with bryophyte litter decomposing most slowly, followed by shrubs and graminoids. After two years, the live plants present in the plot (i.e. presence of shrubs) were more important than the litter identity. Warming affected the composition of the bacterial community, while the fungi responded more strongly to the presence of shrubs.

While these results are compelling, they should be taken with caution, as the authors suggest: since the duration of the experiment was two years, further interesting effects resulting from the decomposition of shrub and bryophyte litter, which happens more slowly than in graminoids, might not have been captured. After four or five years, the decomposition of more recalcitrant litter could reveal more interesting effects. The same is true for the warming treatment: given a longer study period, the effect of 1°C warming on plant litter decomposition might become more important. It is, however, easy to write ‘more long-term experiments needed!’ while, in reality, the amount of effort required to maintain the plant removal treatments and warming chambers in the harsh upland environment of the Pennines represents a considerable hurdle. And one has to motivate one’s volunteers to see past the inherent absurdity of weeding a moorland!

Overall then, what’s the message? Dead or alive, whether you’re graminoid, shrub or bryophyte seems to exert much stronger control on litter decomposition rates in peatlands than temperature. While warming doesn’t have much of an effect on plant decomposition, it does affect the bacterial community in the peat, which might have important implications for graminoid decomposition, since bacteria are well-equipped to munch through labile substrates. Fungi respond when you take away the shrubs, which provide the more recalcitrant litter they’re specialised for dealing with, and might therefore moderate the response of the peatland carbon cycle to warming. Given a longer time period, more effects may emerge from this experiment.

I’m interested to know about the responses of decomposition processes in other ecosystems to warming and plant functional group removal. In grasslands, for instance, what happens when you increase the ambient temperature and remove the nitrogen fixers? Of course, these sorts of studies require a sufficiently long period to become stable and start producing results. In a time of apparently perpetually shrinking budgets, what’s next for long-term field experiments in ecology and biogeochemistry? What are the current barriers to their deployment, or do we in fact have enough to answer our more pertinent questions? Let me know what you think – get involved by commenting on this post, and posting on Twitter and Facebook using the #psejclub tag. I’m looking forward to hearing from you!

Seasonal not annual rainfall determines grassland biomass response to carbon dioxide

After a brief hiatus, journal club is back and this time we’re discussing a paper by Hovenden et al from Nature in May exploring the interaction between carbon dioxide and rainfall on plant biomass.

At this point it seems that we ecologists have a reasonably good idea of what effect many environmental variables – like water, temperature, and carbon dioxide – have on certain ecosystem parameters, though always with a few caveats and exceptions thrown in to keep it interesting.  However, our understanding of how these variables interact and the effects of these interactions, especially over various temporal and spatial scales, is still pretty woeful.

For example, we know that plants need water to grow, and when there isn’t enough water they stop growing – very straightforward. Plants also need CO2 to grow, and in general higher CO2 levels lead to higher plant biomass.  This is because increased CO2 allows for higher rates of photosynthesis and greater water use efficiency.  Due to the greater water use efficiency, we also expect that the effect of elevated CO2 (eCO2) on plant growth will be greater when water scarce.  Basically, with higher CO2­, plants can photosynthesize more per unit available water, so will be able to grow more before the water runs out compared to plants grown at lower CO2 levels.

As with so many things in ecology, what we predict is exactly what we see … except when we don’t.  If the relationship between water availability, eCO2 and plant biomass is so straightforward, biomass responses to eCO2 would always be positive and we would see the strongest responses in the driest years.  I bet you can see where this is going…

TasFACE ring

Photo credit: TasFACE website

Hovenden et al looked at data from a nine year FACE experiment in Tasmania (TasFACE) and found that the eCO2 effect was far from consistent across years.  Some years there was no discernable eCO2 effect on biomass, some years it was positive (like we’d expect) and one year it was actually strongly negative; and these responses were not correlated with annual rainfall or soil water availability.

Instead, Hovenden et al found that the biomass responses to eCO2 were strongly correlated with seasonal rainfall variability.  Higher rainfall in the summer resulted in a positive effect of eCO­2 on biomass, as we would expect.  Summer rain at the site tends to come in short, sharp bursts, so the increased water use efficiency would allow the plants to maintain growth for longer between rain events.  However, increased rain during the spring and autumn were correlated with a negative effect of eCO2 on biomass.  During these cooler, wetter periods plants don’t grow as much and it is likely that increased rain would leach nutrients from the soil.  This was supported by a strong negative relationship between spring rain and soil nitrogen availability.

It seems probable that such a relationship between seasonal rainfall and eCO2 effects on biomass could be seen throughout temperate and seasonally wet systems, and that this could have big implications for global carbon models.  It also highlights the importance of looking beyond plants to fully understand the mechanisms that drive responses to climate change.

I would love to see similar analyses of other FACE datasets to see if these trends are replicated in other systems.  It’s an important finding, but opens up lots of other interesting questions: How does vegetation type or soil type effect the relationship between seasonal rainfall and eCO2 effects on biomass?  Does seasonal temperature variability affect the relationship significantly?  What about increased nitrogen pollution or fertilisation – would increased nitrogen deposition overturn the negative relationship between high spring/autumn rain and the eCO2 effect on biomass?

As always, we’d love to hear what you think about the paper.  Is it the best paper you’ve ever read or do you think it contains some fundamental flaw? Does it raise interesting questions or link well with something else you’ve read recently?  Would you use similar methods or could you propose a better protocol?  Let us know in the comments or on twitter with hashtag #psejclub!

Finally, don’t forget about our joint meeting with the Plant Environmental Physiology group coming up in October.  All the details, including links for registration and abstract submission, are available here.  It’s going to great!

#psejclub discusses: plants, fungi, competition and carbon storage

It’s nearly two weeks since Franciska wrote the first post for our journal club (#psejclub), about a paper published in Nature, entitled ‘Mycorrhiza-mediated competition between plants and decomposers drives soil carbon storage‘. Her thoughts on the paper prompted an interesting discussion, which you can catch up on here.

Keep your eyes peeled for a new #psejclub post in the next couple of days!

Mycorrhiza-mediated competition between plants and decomposers drives soil carbon storage

– 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.