The science of plant decomposition and why it matters

Newswise — How plant material breaks down may not sound particularly interesting, but this process has important implications for the climate and the health of the environment.

Each year, Earth’s plants drop about 100 billion tons organic material, depositing vast amounts of leaves, sticks, bark and pollen into the environment. Much of it ends up in waterways where it decomposes, perpetuating a cycle that shuttles carbon back and forth between the environment and living things.

But human activities appear to be disrupting that vital process, according to new research by Jennifer Follstad Shah, an ecosystems ecologist and associate professor in the University of Utah’s School of Environment, Society & Sustainability.

(The College of Social and Behavioral Science this month formed the school by merging its Department of Geography and Environmental Studies program.)

She teamed with several other U.S.-based ecologists to analyze data from more than 500 freshwater streams and riparian areas around the world, including Utah’s Provo and Logan rivers and Red Butte Creek. They discovered that decomposition rates are increasing the most where human activity is impacting landscapes. Streams cover only 3% of Earth’s landmass, yet these aquatic ecosystems play an outsized role in the global carbon cycle, according to Follstad Shah.

What follows is a Q&A with Follstad Shah about her team’s findings, reported recently in the journal Science.

Why is it important to understand the decomposition rates of this plant material?

Organic matter is the base of detrital food webs. It’s the primary resource for stream and river macroinvertebrates that in turn feed higher trophic levels, like fish, which humans rely upon for recreation and food production. But it’s also important to understand decomposition because it is mediated by both microbes and macro invertebrates. The fate of organic matter differs depending on which taxonomic group is contributing most to the process, with consequences for the balance of gaseous carbon emissions versus the formation of fine particulate organic matter that can be stored or transported downstream.

There seems to be important climate implications here. Could you reflect on that.

Water temperature responds to air temperature. However, this relationship may vary from place to place depending on the degree of shading that you find over streams and rivers or the degree of groundwater inputs to these systems, which can buffer temperature. In general, though, as air temperatures increase, water temperatures also increase.

And decomposition rates increase if you’ve got warmer temperatures?

Exactly. That’s why temperature was the factor that contributed the most to our explanatory model of decomposition for cotton strip decay.

That brings us to the worldwide research collaborative called CELLDEX Consortium that gathered the study’s field data using these canvas strips. Tell us about that.

CELLDEX stands for Cellulose Decomposition Experiment. This project used crowdsourcing amongst a network of colleagues, most of whom know each other to some extent from attending similar conferences. Scott Tiegs from Oakland University,  Dave Costello from Kent State University, and Krista Capps from the University of Georgia sent out canvas strips, like you would find in sailing cloth, to 150 colleagues around the world. These are standard substrates that have a similar chemical composition, similar size, and so you’re reducing a lot of the variation that you find in organic matter in the natural world. It’s a really simple experiment. You put these strips of canvas out in the environment, both in riparian and stream ecosystems, leave them there for a specific amount of time, collect them, dry them out, and then measure the tensile strength, or the amount of force required to punch a hole through the canvas. Decay rate was calculated as the difference in tensile strength before and after the strips were deployed and the incubation time.

And that gave you a kind of proxy measurement of decomposition rates?

Exactly.

Your team looked at field data from 514 streams all over the world. What did you learn?

We learned that there is a signature of biomes when it comes to decomposition of these cellulose strips. The average rate of breakdown of these strips, as well as the variation around that mean differed amongst biomes. And because these cellulose strips are similar in size and chemical composition, that variation is really a signature of environmental factors that differ between these biomes.

We found that the top three factors that predict the breakdown of the cellulose strips globally is temperature and the availability of nitrogen and phosphorus. There were about a hundred different factors entered into this model, and only a fraction were significant predictor variables, but those were the top three.

You suspect that these decomposition rates are speeding up, and the higher rates of decomposition coincided with areas that had the most anthropogenic impact.

Correct. We know temperature is increasing because of human activities related to elevated CO2 in the atmosphere, but also due to land use changes, such as the clearing of riparian areas for agriculture and urbanization, which reduces shading of these streams and rivers. Water extraction also reduces the volume of water in streams and rivers, allowing them to heat more rapidly. And when it comes to nutrients, humans have doubled the amount of nitrogen in the biosphere due to the Haber-Bosch process, and also have increased phosphorus availability due to mining, both of which enter streams and rivers from the runoff of fertilizers or in effluent from storm drains and wastewater treatment plants.

Is there anything you want to add?

The study of leaves breaking down in streams and rivers might sound really mundane. It might not excite a lot of people, but I hope the next time somebody goes by a stream or river and takes a look at the trees around them and the leaves falling into these systems, they remember what an important service the process of decompositionprovides, not only for the fish that folks like to catch, but also for regulating the carbon cycle.

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The study, titled “Human activities shape global patterns of decomposition rates in rivers,” appeared May 30 in Science. Follstad Shah’s co-authors include Krista Capps and John Paul Schmidt, University of Georgia; Scott Tiegs, Oakland University; David Costello, Kent State University; Chris Patrick, Virginia Institute of Marine Science; and Carri LeRoy, Evergreen State College. Funding for the research came from the Ecuadoran National Science Foundation and the U.S. National Science Foundation.