DENITRIFICATION IN SOIL PROFILES BENEATH GRASSLAND AND CULTIVATED SOILS

The denitrification potential of subsoils and aquifers must be charact erized to assess the ultimate fate of soil N. This experiment was cond ucted to study changes in the subsurface distribution and activity of denitrifying bacteria as a result of cultivation. We examined soil pro files for their capacity (N addition) and potential (N + C addition) t o denitrify as well as for denitrifying bacterial numbers in a Reading silt loam (fine, mixed, mesic Typic Argiudoll) soil under grassland a nd cultivation. Denitrifying enzyme activity was undetectable througho ut the subsurface of soil profiles at both sites. Overall, the cultiva ted site had significantly higher NO3- concentrations, denitrifier pop ulations, and denitrification potential. Denitrifying bacteria were st ratified in the vadose zone at the prairie site, with lowest numbers a nd lowest denitrification potential occurring at the interface of the water table. Although C addition enhanced denitrification at this site , the profile was not nearly as limited by C as that of the cultivated site. The results demonstrated that increased N inputs and cultivatio n during approximately 50 yr changed the denitrifying population and d enitrification potential in the vadose and saturated zones of soils.