A comparative study of nitrogen forms in a tallgrass prairie and agricultural field.

Abstract

A study of nitrogen forms in a an tallgrass prairie and adjacent agricultural field was conducted. Both sites were located on the same Irwin silty clay loam soil series. Samples were taken from each site every 2-4 weeks and analyzed using the microdiffusion procedure of Keeney and Nelson (1982). A nitrification potential experiment was designed to minimize the disturbance to the system. When an abundance of ammonium (50 ppm) was added, nitrate production increased in both sites. One and one-half weeks after the beginning of this experiment, the Nitrosomonas population was examined and found to be 1000 times greater than the previous reading in both sites. With applied ammonium (and increased numbers of nitrifying bacteria), even the prairie produced a good deal of nitrate. Results from the microdiffusion tests showed ammonium to be the dominant nitrogen form in the tallgrass prairie throughout the year, however, a substantial amount of nitrate (not less than 25 %of the total) was present at every sampling. The summer samples showed nitrate accounting for 47 %of the total mineral nitrogen present in the prairie. The agricultural field showed more fluctuation, with ammonium dominating winter and spring months and nitrate in summer and fall. The data suggest a correlation between soil conditions and nitrogen form present. Saturated soil (with low oxygen levels) contained a high ammonium concentration in both sites, while drier, better oxygenated soils (as in summer) had a higher nitrate content. The microdiffusion tests and nitrification potential experiment suggest that in heavy clay soils, a dominance of ammonium may be due to increased soil water content and not to allelopathic effects.