AVAILABILITY OF MANURE SLURRY AMMONIUM FO R CORN USING N-15-LABELED (NH4)(2)SO4

The NH4+ fraction of animal manure slurry is often considered to be as available as fertilizer N to a crop; however, immobilization and loss es via denitrification and NH3 volatilization may be higher in manured than in fertilized soil. The apparent N recovery and the N-15 recover y methods were used for corn (Zea mays L.) grown in soil amended with dairy cattle slurry and NH4+ fertilizer to determine the source of the N taken up by corn plants. Manure slurry or (NH4)(2)SO4 fertilizer we re applied at rates equivalent to 100 kg NH4+-N ha(-1) in the greenhou se and the field. In the greenhouse, the apparent NH4(+)(-)N uptake by corn was 76 and 85% with animal manure slurries and NH4+ fertilizer, respectively. In the field, apparent N recovery of NH4+ from dairy cat tle slurry and (NH4)(2)SO4 was 43 anbd 58%, respectively, whereas N-15 recovery from the same treatments was 15 and 29%, respectively. The l ower N-15 recovery values compared with the apparent recovery values s uggest that mineralization-immobilization turnover (MIT) occurred, and that MIT was greater in manured soil than in fertilized soil. A labor atory incubation study showed greater microbial biomass and more N-15 immobilization in soil amended with dairy cattle slurry than in soil a mended with fertilizer.