MECHANISMS FOR SOIL-MOISTURE EFFECTS ON ACTIVITY OF NITRIFYING BACTERIA

Moisture may limit microbial activity in a wide range of environments including salt water, food, wood, biofilms, and soils. Low water avail ability can inhibit microbial activity by lowering intracellular water potential and thus reducing hydration and activity of enzymes. In sol id matrices, low water content may also reduce microbial activity by r estricting substrate supply. As pores within solid matrices drain and water films coating surfaces become thinner, diffusion path lengths be come more tortuous, and the rate of substrate diffusion to microbial c ells declines. We used two independent techniques to evaluate the rela tive importance of cytoplasmic dehydration versus diffusional limitati ons in controlling rates of nitrification in soil. Nitrification rates in shaken soil slurries, in which NH4+ was maintained at high concent rations and osmotic potential was controlled by the addition of K2SO4, were compared with rates in moist soil incubations, in which substrat e supply was controlled by the addition of NH3 gas. Comparison of resu lts from these techniques demonstrated that diffusional limitation of substrate supply and adverse physiologic effects associated with cell dehydration can explain all of the decline in activity of nitrifying b acteria at low soil water content. However, the relative importance of substrate limitation and dehydration changes at different water poten tials. For the soil-microbial system we worked with, substrate limitat ion was the major inhibiting factor when soil water potentials were gr eater than -0.6 MPa, whereas adverse physiological effects associated with cell dehydration were more inhibiting at water potentials of less than -0.6 MPa.