Dear Members and Friends of the DMG,
This time, I would like to share a little science in my message. I am writing during the summer break, when there are currently no major developments within the Society to report. Instead, I would like to tell you about plants (!)—the subject of research in our group Surface Geochemistry at the GFZ Helmholtz Centre for Geosciences in Potsdam—and what they have to do with mineralogy. Perhaps some of you are curious about what your DMG President actually works on as a scientist. Let me begin with a question.
What is the largest transfer of dissolved chemical elements at the Earth's surface? For a surface geochemist, the obvious answer would probably be the elements released by rock weathering and transported to the oceans by the world's major rivers. And what controls the magnitude of these transfers? The standard answer would be, first, mountain building and erosion, which are closely linked to weathering, and second, precipitation and the carbonic acid it contains—in other words, climatic controls.
As part of the German Research Foundation (DFG) Priority Programme EarthShape – Earth Surface Shaping by Biota, which I co-coordinate, we investigated another controlling mechanism. Plant growth increases the concentration of carbonic acid in soils, thereby accelerating mineral dissolution. At the same time, higher plants require the essential mineral nutrient phosphorus as well as other vital elements such as potassium and magnesium. Even silicon is important, being stored in leaves as phytoliths in concentrations of up to several percent, where it serves as protection against grazing animals or provides structural support. To obtain these nutrients, plant roots live in symbiosis with fungi—so-called mycorrhizae—which release these elements from minerals in exchange for carbon supplied by the plants. It is one of nature's most elegant partnerships.
The hypothesis therefore seemed straightforward: the more plant growth, the greater the intensity of rock weathering. Within the Priority Programme, we tested this idea along an extraordinary vegetation gradient in Chile. To our surprise, the opposite proved true. Rock weathering does not increase with increasing plant growth. Instead, we discovered just how remarkably efficient plants are at recycling nutrients—a well-known phenomenon in ecology. The more vegetation there is, the more frequently elements are reused. Once they return to the soil in fallen leaves, they are taken up again almost immediately. Globally, an element such as potassium cycles through the terrestrial biomass around twenty times after being released by weathering before it is eventually transported away by rivers. For phosphorus, this number is close to one hundred, and even silicon is recycled about five times. The answer to my opening question is therefore: not rivers—plants drive the largest flux of chemical elements at the Earth's surface.
Why are we interested in this question? First, because plants fractionate the stable isotopes of these elements during uptake. This process is now well established for magnesium, iron, silicon, strontium, and several other metals. We study how plant uptake affects isotope signatures in the weathering zone in order to establish quantitative mass balances—but that is a story for another day. More importantly, we seek to quantify the delicate balance between the living and the geological worlds within the Earth system—a system whose functioning is increasingly threatened by human activities. This is an issue we simply must address, including within the DMG. And finally, one cannot help but admire the remarkably resource-efficient circular economy that natural systems have developed over millions of years—something from which we humans, with our often wasteful use of resources, could learn a great deal. Better yet, we should follow that example as soon as possible.
With my best regards,
Friedhelm von Blanckenburg
