Denitrifying bacteria stratify above fragipans

Fragipans are impermeable layers in soil that affect more than 950,000 ha o f land in Kentucky and create seasonally perched water tables chat may prom ote denitrifier growth and activity. We used denitrification enzyme activit y assays and Most Probable Number (MPN) estimates of soil and water samples to investigate whether a fragipan soil in south-central Kentucky developed bacteria populations with increased denitrification capacity. Preliminary studies showed that up to 5.0 x 10(4) denitrifiers mL(-1) were present in p erched water above the fragipans, Nitrous oxide concentrations in peizomete rs were as high as 8.3 ppm and suggested that the denitrifiers were active throughout the year, Cover crops suppressed the denitrifier populations in perched water almost tenfold, and decreased N2O concentrations by 8%, Compa red with unamended soil, denitrifiers were more numerous at all depths when manure was added. Subsequent experiments showed that there were up to 8.6 x 10(3) denitrifiers g(-1) soil immediately above the fragipan and fewer th an 4.6 x 10(2) within it. An analysis of denitrifier populations by depth i ndicated that their numbers increased in soil immediately above the fragipa n, Denitrifier growth was greater than that of other heterotrophic bacteria . The ratio of MPN denitrifiers to total culturable heterotrophic bacteria plated at each depth increased 64% immediately above the fragipan, We concl ude that the fragipan in this soil selected for denitrifiers, that the deni trifiers were affected by the management techniques applied at the soil sur face, such as cover crop use and manure application, and that this environm ent has the potential to reduce NO3- leaching beyond the plant rooting zone in fragipan soils.