Biogeochemistry During Long-Term Soil Development

Well-constrained carbon:nitrogen:phosphorus (C:N:P) ratios in planktonic biomass have greatly advanced our understanding of biological processes and nutrient cycling in marine ecosystems, and have motivated ecologists to search for similar patterns in terrestrial ecosystems.  Recent research by our group and others suggests similar organization in the soil microbial biomass, and this observation provides another potentially useful tool for assessing the nature of nutrient limitation on land. In addition, human activities are driving profound changes in the inputs and abundance of C, N and P globally, and we are interested in how changes in the relatively availability element affects biogeochemical cycles in terrestrial ecosystems.

We are studying the relationship between element abundance and ecosystem processes by utilizing both short (< 100 years following glacial retreat) and long (4000 ky in the Hawaiian Islands) chronosequences. Recently deglaciated soils appear completely devoid of life, as they are often barren, rocky, and lacking vascular plant cover.  However, primary succession in these "young" soils begins immediately following glacial retreat with the colonization of microbial communities. Although soil microbes catalyze a vast suite of biogeochemical reactions, we lack a basic understanding of how these communities assemble and change over the course of soil development.  Along with collaborators from the University of Colorado, we are examining the drivers and mechanisms of biotic succession and ecosystem development across proglacial chronosequences in the North Cascades (Washington State; left), the Peruvian Andes, and in southeast Alaska.

At these three core sites, we are investigating two separate, but related questions: First, are patterns in the structure and function of developing soil microbial communities regulated by shifts in soil elemental ratios through time (C:N:P), and how do these patterns  and controls differ across geographically disparate sites? One ongoing project involves altering the stoichiometry of soil nutrient pools in the most recently exposed soils at each site using field-based nutrient manipulations.  We hypothesize that patterns in soil nutrient pools and stoichiometries will relate to shifts in microbial activity in ways that are similar across sites, however these shift may be site specific as they relate to the phylogenetics of microbial communities.

Next, we are addressing questions about the relationship between soil microbial community structure and function change during early soil development, and how these dynamics change following plant establishment?  To answer this question, we are using both SSU rRNA-based pyrosequencing analysis and community level physiological profiling (CLPP) to assess relationships between soil microbial community structure and function in space and time.

Finally, in collaboration with Dr. Sasha Reed, we are investigating the effects of nutrient availability on belowground ecosystems processes. For example, in spite of evidence that N strongly limits aboveground productivity at a young site in Hawaii (Thurston), our data shows that P (and not N) availability limits the decomposition of soluble carbon. Thus, we we are using a suite of resource manipulation experiments to address the generality and implications of these results. This research culminated in funding of an NCEAS workshop addressing nutrient limitation in the tropics that is currently underway.