Fate of Nitrogen‐15 in a Long‐Term Nitrogen Rate Study: I. Interactions with Soil Nitrogen

A better understanding of how N management practices affect transformations and movement of fertilizer N may lead to more efficient N management. The objectives of this work were to determine how long‐term N fertilizer history in a continuous corn ( Zea mays L.) production system affects (i) movement of fertilizer N through the soil profile and (ii) cycling of fertilizer N between available and nonavailable soil forms. Nitrogen‐15‐labeled ammonium nitrate ( 15 NH 4 15 NO 3 ) was applied at 0, 67, 134, 201, or 268 kg N ha −1 to subplots of long‐term N rate plots. Twenty to 55% of labeled N was converted into either organic or clay‐fixed forms during the first growing season, with the percentage decreasing with increasing N application rate. Significantly more N was released from nonavailable forms in plots where the historical N application rate had exceeded the long‐term optimum (186 kg ha −1 ) than in plots that received lower rates. Little fertilizer‐derived N leached from the profile during the first growing season, but losses did occur during the off‐season and subsequent growing season when N application rate was higher than the optimum. It was concluded that a history of excessive N application may decrease response of subsequent crops to fertilizer N due to greater release from nonavailable N forms, most likely as a result of increased mineralization of crop residues and recently formed soil organic N.