Recovery of 15 N Labelled Urea as Affected by Fixation of Ammonium by Clay Minerals

A greenhouse experiment was conducted with Lolium perenne to determine whether ammonium (NH 4 + ‐N) fixation by clay minerals can increase the recovery of nitrogen following application of 15 N labelled urea. A silty loam subsoil, B t horizon from an Alfisol derived from loess, pH (CaCl 2 ) 7.9, was chosen for the experiment. The NH 4 + ‐N fixation capacity was altered by varying the distribution of potassium (K) in the upper and lower soil layer. In the K 0 treatment (control), the upper soil layer fertilized with urea was not supplied with K, whereas the lower soil layer was fertilized with 300 mg K kg −1 soil. In the K 1 − , K 2 − and K 3 treatment the upper soil was supplied with 100‐, 200‐ and 300 mg K kg −1 soil, respectively. The soil in the lower layer of the K 3 treatment was not supplied with K. The recovery of 15 N from applied urea ( 15 N uptake of Lolium perenne plus residual soil 15 N t ) was 86.1% in the K 0 treatment and 75.2%, 69.1% and 69.6% in K 1‐ , K 2‐ and K 3 treatments, respectively, showing that 15 N losses were smallest in the K 0 treatment. Two weeks after applying 15 N labelled urea the amounts of nonexchangeable 15 NH 4 + ‐N in the upper soil layer were significantly higher in the K 0 treatment than in the treatments with K application. Apparently, NH 4 + ‐N fixation by clay minerals can reduce NH 3 volatilization after urea fertilization, if the amount of exchangeable K is low.