Denitrification in a Tallgrass Prairie Landscape

We characterized factors controlling denitrification and quantified rates of N gas production by this process in a tallgrass prairie landscape in central Kansas. The experimental design included three land use classes (unburned, annually burned, and an- nually burned and grazed) in factorial combination with three slope positions (summit, back-slope, toe-slope), plus a cultivated site in a toe-slope position (10 sites total). Deni- trification was measured using an acetylene-based soil core technique four times in 1987, once in early 1988, and six times in 1989. Cores were incubated under field-moist conditions and after amendment with water or water plus nitrate. Microbial biomass and nitrification and denitrification enzyme activities were also measured. Denitrification was higher (P < .05) in unburned sites than in burned, burned and grazed, and cultivated sites in both 1987/1988 and 1989. The cultivated site consistently had low rates of denitrification relative to the native prairie sites, even when water and nitrate were added. Levels of microbial biomass C and nitrification and denitrification enzyme activities were an order of magnitude lower in the cultivated site than in the native prairie sites. Denitrification rates were highest in the early spring of 1987 and were low at all other times. Although temporal patterns of activity were generally related to patterns of soil moisture, water additions did not stimulate activity in ungrazed prairie soils. Water plus nitrate additions consistently gave significant increases in activity. The results are consistent with previous research that has found that unburned prairie is wetter and has higher concentrations of NO3- in soil solution than burned sites. In certain years, denitrification may be significant to site fertility, landscape water quality, and atmospheric chemistry in the tallgrass prairie region.