Soils are complex systems, in which physical, geochemical and biological processes interact in aggregate structures situated in dynamically shifting air- and water-filled spaces. It is both difficult to adequately sample soil properties and to model processes related to those soil measurements. These challenges were discussed in a stimulating three-day conference on Complex Soils Systems in Berkeley a few weeks ago. Attendees came from an incredible diversity of backgrounds with a common interest in tackling issues in soil science. An overarching concern was to better understand soils to know “How to feed the soil and the planet?” in the anthropocene, which was a question posed early on by Professor John Crawford.
Issues of scale were brought up explicitly or were evident implicitly in many of the presentations. Namely, that relevant processes in biogeochemical cycles occur over a wide range of spatial (nano- to mega-meter) and temporal (seconds to millennia), but our observations are typically limited to a much narrower range given measurement and resource constraints. These issues were elegantly summarized in the article “Digging Into the World Beneath Our Feet: Bridging Across Scales in the Age of Global Change” by Hinckley, Wieder, Fierer and Paul in Eos, Transactions American Geophysical Union 95 (11), 96-97. In a real sense, the scale issue presents problems when societal decisions regarding soil sustainability and ecosystem services are made using data and models derived from different (often smaller) spatial scales than the policies themselves.
One illustration of the concept of a spatially complex soil system is illustrated with the figure below by California College of the Arts (CCA) student Sakurako Gibo. The image depicts a theoretical assemblage of soil microbes with different morphologies (for instance round spores versus string-like mycelia). In the second figure, the complex system is “pulled apart” into bins that might represent the effect of a sampling strategy that subsamples the whole system or measurements that only count certain members of the communities. The information about the original complex assemblage and connections is not retained, and as a result, data and rules based off of the binned samples may be different from the case in the real intact community.
What to do? Well, let’s just say I walked away from the meeting knowing that there are plenty of unanswered questions on soil complexity and scale. With the increasing technical capability in soil and microbial measurements, and communication at meetings like this one, we’ll get there soon.
I’ll end with another neat set of figures produced by CCA student Leslie Greene who illustrated an emergent pattern of predicted H2 consumption (o) based on the availability of H2 (•) from the atmosphere (distributed) and from N2-fixing root nodules (gray filled circles). She created the pattern of H2 consumption based on one rule, soil moisture had to be above 10% and below 50%, as indicated by the concentric rings around water-logged soil sites (red filled circles). From this simple scheme, an irregular pattern emerges of the location where H2 consumption occurs. When faced with the complexity of soil, it is easy to feel paralyzed (for me anyway), and perhaps starting with a simple approach like this will help me embrace the system and its questions.
Thank you for the BioDesign course organizers at California College of the Arts (Tobi Lyn Schmidt and Mike Bogan)!