Effects of char additions to soil on microbial community composition and CO2 emissions

We’ve posted a pre-print version of our paper, Dynamics of microbial community composition and soil organic carbon mineralization in soil following addition of pyrogenic and fresh organic matter, at bioRxiv. The paper is currently under review at ISME J. I’m really pleased to have it submitted, since it is the final piece of my Ph.D. research. (The other part of Chapter 4, the 3-part stable isotope partitioning method, was recently published in Nature Communications.)

Briefly, we added either charred or fresh corn stover to a field soil, and then monitored CO2 emissions and microbial communities over 12 weeks. There are several elements to the paper that I think are pretty cool, but I’ll just focus on what is perhaps my favourite figure from the paper here:


This figure represents the response of the microbial community to 350°C charred corn stover (i.e., “pyrogenic organic matter”, or PyOM) on the x-axis, and fresh stover on the y-axis. The scale indicates whether the microbes increased or decreased in relative abundance (“log2-fold change”), as compared to the soil with no organic matter additions, where 0 indicates no change, +1 indicates a doubling in relative abundance, +2 indicates a quadrupling in relative abundance, and so forth. Each point indicates a single microbial “operational taxonomic unit” (OTU) (which you can think of as something like a species, as Robert Edgar discusses here), coloured by the phylum to which they belong – you can see that Proteobacteria and Bacteroidetes are among the strongest responders. The bars on the sides indicate the relative density of the OTUs – you can see that most are grouped around 0, indicating that most OTUs don’t change very much. The black line indicates a 1:1 relationship, where the response to the PyOM is the same as the response to the stover. Any OTU above this line responds more to stover, and any OTU below responds more to PyOM. Thus, the OTUs in the blue-shaded circle respond mostly to stover, those in the pink circle respond to PyOM, but not to stover, and those in the purple circle respond strongly to both. We compare the responses 12 days after the PyOM or stover were added (left panel), and 82 days after (right panel). You can see that some microbes respond immediately to the stover additions, while the response to PyOM takes longer to emerge.

For me, the most exciting about this data is what we can infer from how the OTUs respond to each amendment in isolation vs. together. For example, 70% of the OTUs that respond to PyOM by day 12 also responded to stover. These microbes might be responsible for decomposing the easily-mineralizable C found in PyOM – while much of PyOM is difficult to decompose, there is a fraction of it that is rapidly mineralized to CO2. I am also very interested in the OTUs that respond to PyOM, but not to stover. For example, 7 OTUs from the phylum Gemmatiomnadetes show this response, and 2 of them are among the top 10 most abundant PyOM-responders. These microbes are likely responding to something unique about the PyOM – e.g., its polyaromatic C compounds or its effect on soil pH (although PyOM only increased pH by ~0.75). Focusing on these types of microbes in future studies could help us better understand the effects of PyOM additions on soil C cycling.