RECOVERY OF N-15 LABELED UREA AS AFFECTED BY FIXATION OF AMMONIUM BY CLAY-MINERALS

A greenhouse experiment was conducted with Lolium perenne to determine whether ammonium (NH4+-N) fixation by clay minerals can increase the recovery of nitrogen following application of N-15 labelled urea. A si lty loam subsoil, B-1 horizon from an Alfisol derived from loess, pH ( CaCl2) 7.9, was chosen for the experiment. The NH4+-N fixation capacit y was altered by varying the distribution of potassium (K) in the uppe r 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 30 0 md K kg(-1) soil, respectively. The soil in the lower layer of the K -3 treatment was not supplied with K. The recovery of N-15 from applie d urea (N-15 uptake of Lolium perenne plus residual soil N-15(1)) was 86.1% in the K-0 treatment and 75.2%, 69.1% and 69.6% in K-1-, K-2- an d K-3 treatments, respectively, showing that N-15 losses were smallest in the K-0 treatment. Two weeks after applying N-15 labelled urea the amounts of nonexchangeable (NH4+)-N-15-N in the upper soil layer were significantly higher in the K-0 treatment than in the treatments with K application. Apparently, NH4+-N fixation by clay minerals can reduc e NH3 volatilization after urea fertilization, if the amount of exchan geable K is low.