DENITRIFICATION CAPACITY AND DENITRIFICATION POTENTIAL OF SUBSURFACE SOILS

Although numerous studies of denitrification in surface soils have been reported, few attempts have been made to study denitrifying activity in subsurface soils. We collected samples of four Iowa soil profiles to a depth of 3 m and measured their population of denitrifying bacteria and their capacity and potential for denitrification. Their denitrification capacity was assessed from their ability to reduce nitrate when incubated anaerobically (helium atmosphere) at 30-degrees-C for 72 hours after treatment with nitrate, and their denitrification potential was assessed from their corresponding ability when incubated anaerobically after treatment with both nitrate and organic carbon (as glucose). We found that the denitrification potentials of the subsurface samples studied greatly exceeded their denitrification capacities and that whereas both the population of denitrifying bacteria and the denitrification capacity of the samples decreased appreciably with depth in the profile, the denitrification potential of the samples did not decrease with depth. These findings indicate that the slow rate of denitrification in Iowa subsoils is not due to a lack of denitrifying microorganisms but to a lack of organic carbon that can by utilized by these microorganisms for reduction of nitrate.