Jenny's thought experiment revisited: Plant biodiversity and soil development

George N. Furey and David Tilman, Department of Ecology, Evolution, and Behavior, University of Minnesota, Saint Paul, MN, Nic Jelinski and William Effertz, Department of Soil, Water and Climate, University of Minnesota, Saint Paul, MN, David Tilman, Bren School of Environmental Science & Management, University of California Santa Barbara, Santa Barbara, CA

Background/Question/Methods
Writing in Ecology in 1958, Hans Jenny set out a thought experiment to test the biotic role in soil development. He envisioned that by holding the other state factors constant, climate, parent material and topography one could test how plants caused soils to develop through time. Here we present results from a long-term plant biodiversity experiment to examine the role that plant biodiversity plays in soil development.

We collected soil morphology and soil color data in plots from the Biodiversity II experiment at the Cedar Creek Ecosystem Science Reserve. In each of four bare ground plots, eight monoculture plots, and eight sixteen species plots, a 1-meter core was taken. Each core was described by genetic horizon and classified taxonomically. For each core, we additionally took 21 samples at 5 cm increments from 0-100 cm and measured soil color using a handheld colorimeter (Nix Pro). We applied an integrated darkness index and compared color measurements to soil organic carbon concentrations and combined both the observational soil descriptions and measured soil color to test how plant biodiversity impacted soil morphology.
Results/Conclusions
We found consistent results across both soil descriptions by genetic horizon and measured soil color by depth increments that plots with greater plant biodiversity had both a darker soil and deeper “A” horizons. The soils of this site are sandy (~90 % sand) with limited horizon development and typically classified as Entisols. Our morphological descriptions found that the bare ground, monoculture, and 16-species plots had average A horizon thicknesses of 40cm, 42cm, and 53cm, respectively. A horizons were darker in 16-species plots relative to bare ground plots in the top 20 cm. The morphological changes reflect the 30% greater amounts of soil carbon found in 16-species plots compared to monocultures. In producing greater amounts of root biomass, the high diversity plots added more organic matter at deeper depths causing darkening of the topsoil and deepening of A horizons.

Following 23-years of soil development, we found that plots with greater plant biodiversity were more likely to have darker, thicker topsoils. As Jenny hypothesized, experimentally changing plant community composition caused divergent soil development. Our results suggest that plant biodiversity may be a crucial component of soil development that led to the fertile soils found beneath the world's grasslands. These findings have implications for interpreting soil morphological properties in the context of soil health in agroecosystems.