Succession of glacial soils

Sarah Castle, Ph.D. candidate
University of Montana

The world of soils is incredibly diverse and heterogeneous and we are just starting to understand the scope of its complexity. Not only do soils harbor much of the earth’s genetic diversity, but soil environmental conditions can change vastly over distances of only millimeters. The distribution and diversity of soil animals and microorganisms, along with their influence on ecosystem processes, also changes across these micro-distances.

When faced with such complexity, scientists can focus their efforts on relatively simple soil systems to begin to link patterns to processes.

From the tropical Andes of Peru to the icefields of Alaska, glaciers are rapidly melting. As ice melts, we are left with an annually resolved gradient in soil development. Substrates closest to the glacial terminus are the youngest while substrates furthest from the terminus are older. Deglaciated landscapes, with their barren rock and lack of vascular plant cover, often appear to be devoid of life. On the contrary, a growing number of observational studies show that glacial soils, albeit low diversity, are teeming with microscopic organisms that take up residence immediately following the retreat of ice. Looking at how soil biota and the soil environment develop with time in these relatively simple landscapes may help us to unravel the relationships between community structure and ecosystem function that may be otherwise obscured in more complex soil systems.

But before we can link patterns and processes, we must first establish whether or not soil microbial communities undergo succession – the orderly and predictable change in community composition and function with time. In my own work, I am examining microbial communities at glacial sites in both North and South American continents. It appears that young glacial soils host bacterial communities that are very different in terms of structure and function when compared to communities originating from older parts of the landscape. In other words, bacterial communities from distant locations (Peru, Washington, and Alaska) undergo successional change that results in a predictable community composition regardless of site. The graph of points depicts this pattern. Each point indicates a unique soil bacterial community and early communities are much more different than older soil communities. The next step for this work is to understand the drivers of successional change.

We are becoming increasingly aware of the links between the aboveground and belowground biota in ecosystems. Plants have repeatedly been shown to drive microbial activity in soils. Recent work from Shawn Brown and Ari Jumpponen suggests that during early ecosystem succession, the presence of plants can shape soil bacterial communities. As well, studies have demonstrated that soil biota wield a strong influence on the diversity and productivity of plant communities. One relatively unknown question is how the succession of belowground communities is related to the succession of aboveground communities. This is an exciting frontier of research that myself and others are currently working on.

Following a major ecosystem disturbance, soil fauna and microorganisms play a particularly important functional role in soil fertility re-development by driving rock mineral weathering, nutrient recycling, and steadily building up organic matter. The work of Christian Schurig and colleagues from the Damma Glacier in Switzerland highlights this last point and their findings show that in developing systems organic matter largely comes from the cells of dead bacteria and fungi.

Though glacial retreat is one specialized type of ecosystem disturbance, there are many other natural and human caused disturbances that influence microbial communities and their functions. The study of natural gradients may offer us some insight into how to maintain and restore degraded systems.

More information about Sarah Castle’s research can be found here: www.cfc.umt.edu/biogeochemistry

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