Bacterial Growth Habits in the Soil Critical Zone and Importance to Biophysical Processes.

In the soil critical zone, bacteria do not continuously coat particle surfaces and thus bacteria on particles and in intervening pores must exist in spatially-segregated patches.  But what, physically, defines a microbial patch?  Viewed from the microscale, what are the soil bacterial habitats?  High resolution imaging provides some clues. Conceivable bacterial growth habits include: 1) free-living at the air-water interface, 2) free-living in pore water, 3) attached to particle surfaces, 4) attached to particle surfaces and embedded in extracellular polymeric substances (EPS), and 5) attached to organic carbon deposits on particle surfaces and embedded in EPS.  In all cases, a water film is expected to coat the outer layers.  Water films will vary in thickness with varying soil matric water potential at the microscale, which will feedback to alter microhabitats.  The characteristics and evidence for each of the habitats is discussed.  Each has consequences to local bacterial physiology and mass transfer, and thus each influences local nutrient processing.  The possible consequences to soil nutrient processing observed at the larger scale are discussed and an attempt is made to quantitatively demonstrate the relevance of microscale bacterial habitats to larger scale observations.