| Abstract: | 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 15N labelled urea. A silty loam subsoil, Bt horizon from an Alfisol derived from loess, pH (CaCl2) 7.9, was chosen for the experiment. The NH4+-N fixation capacity was altered by varying the distribution of potassium (K) in the upper and lower soil layer. In the K0 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 K1?, K2? and K3 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 K3 treatment was not supplied with K. The recovery of 15N from applied urea (15N uptake of Lolium perenne plus residual soil 15Nt) was 86.1% in the K0 treatment and 75.2%, 69.1% and 69.6% in K1-, K2- and K3 treatments, respectively, showing that 15N losses were smallest in the K0 treatment. Two weeks after applying 15N labelled urea the amounts of nonexchangeable 15NH4+-N in the upper soil layer were significantly higher in the K0 treatment than in the treatments with K application. Apparently, NH4+-N fixation by clay minerals can reduce NH3 volatilization after urea fertilization, if the amount of exchangeable K is low. |
