WRS student Robert Dietz studies patterns and drivers of carbon storage in Minnesota lakes

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“Our research illustrates that land use activity is the principal driver of both regional and temporal variability in sediment accumulation, which then profoundly influences carbon burial,” says Dietz.

For decades, scientists have been researching the cycling and sequestration of carbon in forests and other terrestrial ecosystems. More recently, the process of carbon burial in lakes and other inland waters has caught the attention of water and climate researchers.  

As part of his doctoral research, Water Resource Sciences PhD candidate Robert Dietz has brought a Minnesota focus to the research in his exploration of the long-term historical relationships between land use and carbon burial in 116 Minnesota lakes spanning multiple ecoregions.

For data, Dietz relied upon core samples dating back to 1860 from the archives of the Saint Croix Watershed Research Station. The lakes he surveyed represented three major ecoregions of Minnesota: the boreal forests of northern and eastern Minnesota; the north central hardwood forests and mixed-use Twin Cities metropolitan area; and the agriculture-dominated south and western Minnesota.

The study, Patterns and drivers of change in organic carbon burial across a diverse landscape: Insights from 116 Minnesota lakes, was co-authored by Daniel Engstrom and N. Johnson Anderson and recently published in the journal Global Biogeochemical Cycles.

Dietz found that since Euro-American settlement, ecoregion-specific median carbon burial rates have increased substantially, by 2.2 times in northern forested lakes, 3.2 times in southern agricultural lakes, and 2.5 times in central hardwood forest lakes.  The highest burial rates occur in lakes characterized by high nutrient and/or mineral sedimentation associated with agricultural lakes, with the greatest increase in carbon burial occurring prior to the widespread adoption of industrial fertilizers, during an era of land clearance and farmland expansion.

Furthermore, northern boreal lakes, impacted by historical logging in the 1800s, bury carbon at rates three times lower than agricultural lakes and exhibit much smaller increases in carbon burial. In agricultural lakes, the greatest change in carbon burial occurred during the early period of land clearance and initial cultivation rather than during the post-1950 era of heavy commercial fertilizer use, raising questions about the importance of eutrophication as a driver of carbon burial.

“Our research illustrates that land use activity is the principal driver of both regional and temporal variability in sediment accumulation, which then profoundly influences carbon burial,” says Dietz.

“Nutrients are important in stimulating the growth of algae and thus carbon, but the key ingredient in carbon burial is sediment,” he says. “For carbon burial to increase substantially, it would seem there also needs to be an increase in the sediment load.”

Dietz calculates that on an annual basis, modern carbon burial rates in Minnesota lakes amount to only 1.5 percent of statewide carbon emissions from fossil fuel combustion.  “The amount of carbon that humans pump into the atmosphere is astronomical, so the amount buried in lakes can only seem paltry by comparison,” he says.  “But it’s an important component of inland carbon storage over century-to millennial timescales.”

After graduating this spring, Dietz aims to pursue a science policy fellowship and afterward contemplate a career that bridges scientific and political arenas. “There’s a strong need for translators of science on a variety of levels.” He adds with a smile, “But I also don’t want to stop doing science.”