Matrix Effects and Measuring Microbial Responses to Xenobiotics in Soil.

Compartmentalization due to tortuous pore space promotes great diversity and functional redundancy in unsaturated soils.  It is difficult to simultaneously expose discontiguous pore space (and organisms therein) to the same substance, which creates some challenges in measuring microbial responses to xenobiotics added to soil.  Moreover, the physicochemical environments among soil compartments likely vary profoundly, and dominant organisms or their functions may also vary accordingly.  Advances in environmental microbiology have stimulated interest in attempting to link microbial phylogeny with function in terrestrial environments. For example, the use of stable isotopes to label phylogenetically informative bio-molecules, referred to as stable isotope probing (SIP), has potential for identifying a microbial population active in a particular process in which C or N is assimilated into the biomass of the target population. SIP is one of the few techniques available that can be used effectively to identify active players in a particular soil process.  Example data will be presented to illustrate matrix effects, including sorption, diffusion (gas and liquid phase), coexistence of oxic and anoxic compartments in unsaturated soil, and the role of water content in microbial access to substrates.  Progress in working with the soil matrix to understand its impact on microbial diversity and function is clearly needed.  Ability to readily measure (and link to microbial data) the pore environment would greatly empower a researcher attempting to interpret molecular microbiology data from soil.