Recovery of Anhydrous Ammonia‐Derived Nitrogen‐15 During Three Years of Corn Production in Iowa

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Abstract
Studies were conducted at two locations to determine recovery of anhydrous ammonia‐derived 15N applied with and without nitrapyrin [2‐ehloro‐6‐(trichloromethyI) pyridine] during corn (Zea mays L.) production over a 3‐yr period in which N fertilizers were applied annually at rates commonly used in the Corn Belt. The results showed that 13 to 33% of the labeled N was removed from the plots during grain harvest in the first crop after application. Recovery of labeled N in the whole plants ranged from 29 to 45%, but the stover was returned to the plots. Nitrapyrin increased the amount of N recovered in plant tissue at only one of the six site years studied. Only small percentages (0.3‐1.5%) of the labeled N were recovered in the second and third crops after application, and nitrapyrin had negligible effects on amounts of labeled N recovered by these crops. Analysis of soil samples collected 1 yr after application of labeled N showed that 19 to 23% of this N remained in the soil. Only small portions of this N existed as exchangeable ammonium and nitrate. Most of this N was in the KMI‐N (Kjeldahl minus inorganic‐N) fraction, which includes N from both organic matter and nonexchangeable ammonium. Large portions (47‐94% of the labeled N found in the KMI‐N fraction 1 yr after application were still present 3 yr after application. The results of this study show that a substantial portion (49–64% of the labeled N was lost from the surface 1.5 m of soil during the first year by processes other than plant uptake. A probable explanation as to why the losses reported here are greater than those reported from many comparable studies is that our determinations included the amounts of fertilizer N lost during the fall‐tospring period. These losses must be considered when evaluating the long‐term efficiency of N fertilization practices and the effects of these practices on environmental quality.