EcoPress

Interview: Francesca Cotrufo on a new framework, Microbial Efficiency – Matrix Stabilization

EcoPress Interview: Dr. Francesca Cotrufo on a new framework, Microbial Efficiency – Matrix Stabilization (MEMS), that links plant litter decomposition and soil organic matter stabilization.  Check out the video for the full interview and the text for highlights.

Welcome to EcoPress Interview!  Today’s guest is Dr. Francesca Cotrufo of Colorado State University.  Please tell us about yourself. 

I’m Francesca Cotrufo, and I’m a professor here at the Department of Soil and Crop Sciences and a senior scientist at the Natural Resources Ecology Lab.  I consider myself a soil ecologist, and I’ve worked the majority of my career on litter decomposition and soil organic matter formation processes under different management strategies and how they are impacted by climate change.  I also use stable isotope techniques to understand carbon and nitrogen dynamics between plants and soils.

You recently published a paper on formation of stable soil organic matter.  Why is soil organic matter important?

Soil organic matter is crucial to all of us. It provides some of the most important ecosystem services, from food production, detoxification, cycling of nutrients, to aiding water percolation and the formation of ground water. Soil organic matter is one of the key resources in terrestrial ecosystems that help support our growing population.  Understanding soil organic matter dynamics is important, how it’s formed and how it decomposes, especially in light of climate change because soil organic matter is one of the most important yet vulnerable stocks of carbon.  If we want to mitigate climate change in any way, we have to manage our soil to increase the content of soil organic matter rather than to decrease it.

Soil organic matter is crucial to all of us. It provides some of the most important ecosystem services, from food production, detoxification, cycling of nutrients, to aiding water percolation and the formation of ground water.

What is the current state of the field of soil organic matter?

It’s a very exciting field.  Many breakthrough results and perspectives have been published in the past few years. We now know that a lot of the old views on humification are incorrect. We now know that microbes control the processing of plant inputs below ground, and that microbial products together with plant chemicals form macro-structures that get stabilized onto soil minerals.  I think working in this field is really exciting because we are now in a much better position to understand soil organic matter dynamics and to move even further into effective management strategies.

Your paper proposes a new framework to better understand soil organic matter dynamics.  Please explain.

The framework, Microbial Efficiency – Matrix Stabilization (MEMS), unifies our view of litter decomposition and microbial processes with soil mineral chemistry, and suggests that we shouldn’t study any of them in isolation.  In particular, MEMS focuses on microbes as the actor in the decomposition of plant material.  We used to think that whatever material microbes didn’t decompose was left behind and would accumulate.  Molecules like lignin are more chemically recalcitrant to microbial decomposition, and would be left behind to constitute the bulk of soil organic matter. But recent studies show no evidence for selective preservation of lignin in soil.  Lignin gets degraded alongside all the other chemical components of plant material.  Microbes can access almost every chemical component of plant material. This is why MEMS stresses not the speed of degradation but the fate of the product of microbial decomposition.  Whether alfalfa degrades in a few months or pine needles degrade in 10 – 15 years isn’t fundamentally important.  What is more important is how much of that initial alfalfa and how much of that initial pine stays in the soil 100 years later.  Microbes take a long time to decompose lignin, but once they do most of the lignin gets respired as CO2 and very little stays in soil as organic matter. What stays in soil has a higher chance of stabilizing on to minerals, which offers longer-term protection (from degradation).

Microbial Efficiency – Matrix Stabilization (MEMS) unifies our view of litter decomposition and microbial processes with soil mineral chemistry, and suggests that we shouldn’t study any of them in isolation.

Why the name MEMS?

MEMS stands for Microbial Efficiency – Matrix Stabilization.  We have debated whether microbial efficiency was more important than microbial allocation.  Efficiency determines how much of the initial carbon material gets respired as CO2 and how much stays in soil as microbial biomass or products, but allocation determines what proportion of the microbial products is fatty acids, proteins, etc, which influences stabilization.  But we had to pick one.

As recently pointed out by Josh Schimel, microbial efficiency and allocation determine the fate of plant carbon, but the story doesn’t end there.  The microbial products stay in soil for a long time only if they somehow get stabilized.  At that point, soil mineral chemistry matters more than where the carbon came from.  For organic matter to stay in soil for centuries, it needs to be stabilized, and it gets stabilized if it interacts with soil minerals (which is sometimes called soil matrix).  Microbes move carbon from plant into soil, but the carbon will not stay without stabilizing as aggregates or chemically bonding onto mineral surfaces.

Microbial Efficiency determines how much of the initial carbon material gets respired as CO2 and how much stays in soil as microbial biomass or products.  What stays in soil gets stabilized if it interacts with soil minerals.

MEMS predicts more stable soil organic matter from fast decomposing litter than slower decomposing and lower quality litter.  Why?

In soil, organic matter can be stabilized by the soil matrix (minerals) or accumulate as free organic matter, or what we call coarse organic matter or light fraction.  In a boreal pine forest, for example, a lot of organic matter in soil is not stabilized but is in the humus layer made of light fraction.  The humus layer is partly decomposed because the litter is recalcitrant and the climate is cold, which makes decomposition progress slowly.  In ecosystems like this, litter accumulates but remains unstabilized and vulnerable.  This organic matter would burn in a fire very easily.  It would decompose fast if management strategy changes.  In contrast, more of the labile litter gets stabilized in the organic matter pool.  High quality labile litter gets used more efficiently by microbes and becomes microbial products, which in turn gets really stabilized in the soil matrix.

Those of us working in natural ecosystems think that the more recalcitrant the litter, the more soil organic matter there is.  But in agriculture, you plant legumes if you want more soil organic matter.  This has been long practiced, for example, by ancient cultures in China where farmers planted corn and legumes together not only to increase nitrogen in soil but also to sequester more carbon into soil.

How does MEMS advance our understanding of soil organic matter?

I hope MEMS bring together soil scientists of different sub-disciplines.  I’ve worked on litter decomposition but have been annoyed that we mostly put litter bags out to study mass loss but didn’t care where the lost mass went.  Those of us working on soil organic matter formation didn’t care where the soil organic matter came from either.  Microbial ecologists trying to understand microbial dynamics tended to study them in isolation as well.  With MEMS, we wanted to provide an invitation to everyone in the field to consider carbon cycling as a continuum from litter input to stabilization and to address questions within the framework rather than focusing on one component only.  I believe we can advance our understanding of soil organic matter if we use MEMS in modeling, designing experiments, or setting up collaborations.

With MEMS, we wanted to provide an invitation to everyone in the field to consider carbon cycling as a continuum from litter input to stabilization and to address questions within the framework rather than focusing on one component only.

Any room for plant scientists in MEMS framework?

MEMS does not address one important aspect of soil organic matter dynamics: continuous fresh input of carbon from plants, such as root exudates.  MEMS mostly focuses on litter decomposition, both above and below ground, but mostly on structural plant material.  This is an area of active research, and it is important to understand the role of roots, root dynamics, and root exudates in controlling microbial community and decomposition and therefore microbial resource use efficiency.

Thank you very much for joining us.

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