Effects of N-fertilization on shoots and roots of rape (Brassica napus L.) and consequences for the soil matric potential

Abstract

The effect of increasing rates of 15N-labelled Ca(NO3)2 (N0 = no N application, N300 = 300 mg N/pot; N600 = 600 mg N/pot; N900 = 900 mg N/pot) on recovery of fertilizer N in winter wheat plants and soil (total soil N, soil microbial biomass N [Nmic], extractable organic N [Norg]) and on N mineralization (NMsoil) was investigated at milk-ripe growth stage in a pot experiment. The N rates were equally split at tillering, stem elongation and ear emergence. Fertilizer N recovered in crops increased with increasing N rates (N300: 223.5 mg N/pot [74.5% of applied fertilizer N], N600: 445.6 mg N/pot [74.3%], N900: 722.1 mg N/pot [80.2%]). NMsoil slightly increased from N0 (43.8 mg N/pot) to N900 (75.6 mg N/pot) indicating that N application enhanced availability of soil-derived N for the plants. However, in fertilized treatments NMsoil is balanced by immobilization and losses (non-recovered fertilizer N). Therefore the effective soil N mineralization is indicated by apparent net N mineralization (ANNM = NMsoil — fertilizer N immobilization — lost fertilizer N). Fertilizer N immobilization in soil increased from N300 (38.7 mg N/pot) to N600 (60.7 mg N/pot) and N900 (65.5 mg N/pot). Lost fertilizer N increased from N300 (14.8 mg N/pot) to N600 (56.7 mg N/pot) and N900 (62.1 mg N/pot). As a consequence negative ANNM values were calculated at N600 and N900. Due to the small differences between N600 and N900 fertilizer N immobilization and lost fertilizer N did not increase linearly with increasing N rates, i.e. both processes were limited by factors other than N rate. Only 5.6—7.4% of the immobilized fertilizer N was recovered in Norg and 5.4—9.3% in Nmic soil pools. It is assumed that most of the immobilized fertilizer N was in non-extractable organic N forms. Nmic and Norg were weak indicators for the extent of fertilizer N immobilization.