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

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.

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

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.

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.

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.

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 decomposition provides, not only for the fish that folks like to catch, but also for regulating the carbon cycle.