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 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. The need to better understand soils was motivated by the importance of soil processes in climate and for figuring out “How to feed the soil and the planet?” in the anthropocene – 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) scales, but our observations are typically limited to a much narrower scope given measurement and resource constraints. These issues were elegantly summarized in the recent 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 are relevant to the policies and issues 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 components of the whole system. 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? I walked away from the meeting in awe of the amount of unanswered questions on soil complexity and scale. However, with the increasing technical capability in soil and microbial measurements, and efforts at meetings like this one, made it evident that progress will continue in this area.
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, and perhaps starting with a simple approach like this will help me embrace the system and its questions.
Biologist/architect team Tobi Lyn Schmidt and Mike Bogan created a course linking artists, designers, architects, and biologists from the California College of the Arts (CCA) and Stanford University. I served as a postdoc mentor to help inspire and guide the process of cross-hybridizing biology and design (some examples) with three really talented undergraduate CCA students: Leslie Greene, Sakurako Gibo, and David Lee.
The students were first charged with creating designs to illustrate scientific concepts in my field of research. I challenged them think about the issue of scale with respect to the biogeochemical cycles I study. The processes I investigate occur over a wide range of spatial and temporal scales, which is a challenge for their measurement and interpretation. David focused on a selection of atmospheric trace gases with a wide range of abundances, and that interact with each other through key reactions. In his image, the hydroxyl radical (OH) is illustrated by the white dot from which orange and blue strings respectively represent the path length to molecules of hydrogen (H2) and methane (CH4) in the surrounding space. The density of the strings is representative of the concentration of H2 and CH4 relative to OH. I love the sense of competition in this image. These reduced molecules compete for reaction with OH, and with other trace gases not shown, which helps explain the relatively their long lifetimes of H2 (~2 years) and CH4 (~10 years) in the atmosphere.
The second task for the students was to manipulate a biological system for design or artistic ends. All three students visited the Welander geobiology lab at Stanford and the Berry lab at Carnegie on campus where atmospheric trace gases are measured. For her project, Leslie was interested in manipulating microorganisms to reveal art. Using a combination of strains from the lab and purchased online, Leslie created competitive interactions between organisms and against antibiotics to reveal structures that were both patterned and complex. In the example below, she laid a cross-pattern of Streptomyces ghanaensis and Bacillus subtilis colonies and let them grow and compete. Intriguing features arose, appearing as if the Streptomyces strain grew on top of the Bacillus strain, perhaps antagonistically or not. Leslie overlaid emergent patterns in topology and color from microbial cultures with and without competition to create an amazing image that reveals some very aesthetic order in the systems.
Finally, the students illustrated various concepts related to my work including artistic renditions of Streptomyces colonies and concepts of complexity (see related post). I really love the feel of the image created by Sakurako Gibo showing the atmospheric H2 concentrations that I measured between the ground and top of a measurement tower (y-axis) over the year-long experiment (x-axis) at Harvard Forest as an ephemeral curtain. Higher concentrations of H2 are represented with a deeper intensity of blue. The impact of the soil sink is illustrated by the lightening of the color near the base of the image caused by high rates of soil microbial H2 consumption in summer and fall.