Effect of increasing rates of N-15-labelled fertilizer on recovery of fertilizer N in plant and soil N pools in a pot experiment with winter wheat

The effect of increasing rates of N-15-labelled Ca(NO3)(2) (N-0 = no N appl ication, N-300 = 300 mg N/pot; N-600 = 600 mg N/pot; N-900 = 900 mg N/pot) on recovery of fertilizer N in winter wheat plants and soil (total soil N, soil microbial biomass N [N-mic], extractable organic N [N-org]) and on N m ineralization (NMsoil) was investigated at milk-ripe growth stage in a pot experiment. The N rates were equally split at tillering, stem elongation an d ear emergence. Fertilizer N recovered in crops increased with increasing N rates (N-300: 2 23.5 mg N/pot [74.5% of applied fertilizer N], N600: 445.6 mg N/pot [74.3%] , N-900: 722.1 mg N/pot [80.2%]). NMsoil slightly increased from N-0 (43.8 mg N/pot) to N-900 (75.6 mg N/pot) indicating that N application enhanced a vailability of soil-derived N for the plants. However, in fertilized treatm ents NMsoil is balanced by immobilization and losses (non-recovered fertili zer N). Therefore the effective soil N mineralization is indicated by appar ent net N mineralization (ANNM = NMsoil - fertilizer N immobilization - los t fertilizer N). Fertilizer N immobilization in soil increased from N-300 ( 38.7 mg N/pot) to N-600 (60.7 mg N/pot) and N-900 (65.5 mg N/pot). Lost fer tilizer N increased from N-300 (14.8 mg N/pot) to N-600 (56.7 mg N/pot) and N-900 (61.1 mg N/pot). As a consequence negative ANNM values were calculat ed at N-600 and N-900. Due to the small differences between N-600 and N-900 fertilizer N immobilization and lost fertilizer N did not increase linearl y with increasing N rates, i.e. both processes were limited by factors othe r than N rate. Only 5.6-7.4% of the immobilized fertilizer N was recovered in N-org and 5.4-9.3% in N-mic soil pools. It is assumed that most of the i mmobilized fertilizer N was in non-extractable organic N forms. N-mic and N -org were weak indicators for the extent of fertilizer N immobilization.