Effect of microbial nitrogen immobilization during the growth period on the availability of nitrogen fertilizer for winter cereals

Pot and field experiments were conducted to determine microbial immobilization of N fertilizer during growth periods of winter wheat and winter barley. In a pot experiment with winter wheat, Ca((NO3)-N-15)(2) was applied at tillering [Zadok's growth stage (GS) 25)], stem elongation (GS 31) and ear emergence (GS 49). Rates of 100 mg N pot(-1), 200 mg N pot(-1) or 300 mg N pot(-1) were applied at each N application date. At crop maturity, N-15-labelled fertilizer N immobilization was highest at the highest N rate (3 x 300 mg N pot(-1)). For each N-rate treatment about 50% of the total immobilized fertilizer N was immobilized from the first N dressing, and 30% and 20% of the total N-15 immobilized was derived from the second and third applications, respectively. In field trials with winter wheat (three sites) and winter barley tone site) N was applied at the same growth stages as for the pot trial. N was also applied to fallow plots, but only at GS 25. N which was not recovered (neither in crops nor in soil mineral N pools) was considered to represent net immobilized N. A clear effect of N rate (51-255 kg N ha(-1)) on net N immobilization was not found. The highest net N immobilization was found for the period between GS 25 (March) and GS 31 (late April) which amounted to 54-97% of the total net N immobilized at harvest (July/August). At GS 31, non-recovered N was found to be of similar magnitude for cropped and fallow plots, indicating that C from roots did not affect net N immobilization. Microbial biomass N (N-mic) was determined for cropped plots at GS 31. Although N-mic tended to be higher in fertilized than in unfertilized plots, fertilizer-induced increases in N-mic and net N immobilization were poorly correlated. It can be concluded that microbial immobilization of fertilizer N is particularly high after the first N application when crop growth and N uptake are low.