Holden Laboratory

Vadose Zone Microbial Ecology

Sedgwick Natural Reserve
Soil horizons at Sedwick sampling site

In our vadose zone microbial ecologyresearch, we work in collaboration with Josh Schimel's lab in EEMB (http://www.lifesci.ucsb.edu/eemb/faculty/schimel/) to study controls on microbial community composition, diversity, and process in surface and deep soils in a California grassland.  Our field sites are in the Sedgwick Natural Reserve (http://sedgwick.nrs.ucsb.edu/) which is part of the overall UC Natural Reserve System (http://nrs.ucop.edu/).  We are generally interested in "who's there" and "what they are doing", but the current thrust in this now decade-long collaboration regards understanding how resource (namely C and N) availability and stress (namely water availability and temperature) together structure microbial communities and populations along the soil profile from surface to deep (1 m or below).  Resource availability and stressful water conditions (defined here as severe desiccation and rapid moisture cycling) decline from surface to deep in the grassland soils that we study.  Microbes cope with stress by investing more energy and nutrition towards survival.  Yet water, as an environmental variable, also strongly controls resource availability by creating solute diffusion paths and reducing air-filled pore space.  In a possible future scenario of a changing climate, soil moisture profiles could change such that wetting/ drying penetrates more deeply into the subsurface.  Under such conditions, microbes may access previously, physically-unavailable C.  We aim to learn how microbes in deep versus surface soils respond to changing wetting and drying regimes, including their population structures, respiration dynamics and physiological impairment.  This work is funded by the National Science Foundation (Ecology Program and Microbial Observatories Program). 

Related is our lab's long term interest in the microscale growth habits of bacteria in surface and deep soils.  We wish to understand the biofilm growth habit in the vadose zone, i.e., its prevalence and biophysical characteristics that affect pollutant biodegradation. While biofilms in wet environments are increasingly recognized as an important growth mode for bacteria, little is known regarding biofilms in dry systems. Our work is towards providing fundamental knowledge of unsaturated biofilm physiology, ecology, chemistry and physics.  This orientation towards biofilms cross cuts our work in all four main thrust areas of our funded research, and is a lens through which we plan and execute experiments, and field sample analysis.