Microbes explain the messiness in decomposition data

Decomposition is a central process in the carbon cycle. Its response to a changing environment is projected to magnify anthropogenic climate change. We test the theory underlying these projections, and find that decomposition theory needs substantive revision.

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Nov 13, 2017
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The paper in Nature Ecology & Evolution is here.

Science is conveyed as careful, thoughtful work that incrementally builds on top of facts that have themselves, been carefully and thoughtfully worked out. But to slightly subvert the lyrics of John Lennon, “[Science] is what happens while you are busy making other plans.” And when it happens, it can challenge what you thought you knew.

My plans were disrupted in 2013. I was making graphs of data that were collected as part of a study looking at ants and termites across the east coast of the United States. We were interested in how these soil animals might affect the breakdown rate of dead wood, a major carbon store and important habitat in forests. The data looked awful: random scatter all across the page, as though someone had splashed ink onto it from a great height. 

What I had expected to see were neat little clusters of points which, when joined across the page, would show a straight and steeply upward trending line. The lowest position of the line should have corresponded with our coldest forest site in the northeast U.S. Our warmest site in Florida should have represented the highest position. My first thought was that we had screwed up the methods, meaning the data were useless. After all, they contradicted what all ecosystem ecologists like me knew: temperature is a dominant control on the rate of carbon cycle processes such as decomposition.

This knowledge as to the importance of temperature control is used for making projections about how ecosystems will respond to and influence climate change. But as I dug into the data, it became clear that they were sound. The ‘random’ pattern was explained and became a clean line when fungi were considered. Temperature was important, but much less so than the microbiology.

I remember rushing to prepare these new data for a talk at the Ecological Society of America annual meeting in Minnesota. As I finished, a hand shot up in the front row to ask whether I thought my findings were specific to wood. The point being that we knew that for the decomposition of plant leaf litter, temperature was all-important and microbes mattered little, so presumably our general theory was intact. 

That question planted the seed for the current paper. Ciska Veen – a scientist at the Netherlands Institute of Ecology – made it grow. I was on sabbatical at the Institute in 2015. Together we talked through the ideas and designed the work. This time, we deliberately set up the study to ensure the resulting data were as ‘messy’ as possible. And then measured the microbes – in addition to expected controls – to see if they would explain the mess the same way they did for wood. The study became a reality because of the collaborative spirit of friends and colleagues arrayed from northern Sweden down to southern France, each willing to run an experimental site.

Much of my past work, and the work of many of the co-authors, relied on the validity of the general theory that decomposition is primarily controlled by physical climate and leaf chemistry. Our results chip away at the foundation of this knowledge. By doing so I would like to think we’re on a path to replacing it with a new general theory. A theory founded on knowledge that for controls on carbon cycle processes, biology matters as much as physics and chemistry. 

The paper in Nature Ecology & Evolution is here: http://go.nature.com/2zyPtFJ 

Go to the profile of Mark A. Bradford

Mark A. Bradford

Professor, Yale University

Soil, Ecology, Carbon, Agriculture, Microbes, Forests

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