Jm. Stark et Mk. Firestone, MECHANISMS FOR SOIL-MOISTURE EFFECTS ON ACTIVITY OF NITRIFYING BACTERIA, Applied and environmental microbiology, 61(1), 1995, pp. 218-221
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.