SHORT-TERM TRANSFORMATION OF APPLIED (NO3-)-N-15 IN PRAIRIE AND CULTIVATED SOILS

Studies were undertaken to trace plant uptake and microbial transformation of applied NO3- on paired prairie and cultivated soils. Microplots (225 cm(2)) in prairie and cultivated soils within the same soil map unit were treated with 26 mg (NO3-)-N-15-N (22.36 atom% N-15) as KNO3. The KNO3 was applied as a fine mist to the soil surface with an atomizer. Plant tissue was removed and soil was excavated to a depth of 15 cm from the microplots after 24 and 72 h. Surface soils were assayed for total Kjeldahl N, and atom% N-15 in total Kjeldahl N (including NO3-), NH4+, and NO3-. The subsurface soil, plant roots, and plant shoots were analyzed for total Kjeldahl N and atom% N-15. After 72 h about 75-83% of the applied N-15-NO3- remained as NO3 in the cultivated soil, whereas only 22 - 30% remained as NO3- in the prairie soil. At 24 h much of the N-15 was found in the organic N pool in both prairie and cultivated soils, but by 72 h plant roots contained the greatest percentage of the immobilized (NO3)-N-15 in the prairie soils.