COMPARISON OF THE DIFFERENCE METHOD AND N-15 TECHNIQUE FOR STUDYING THE FATE OF NITROGEN FROM PLANT RESIDUES IN SOIL

We compared the dynamics of net mineralization of nitrogen (N) derived from white clover material (Ndfc) as measured by the difference and t he N-15 methods in a pot experiment with a sandy loam (15 degrees C an d pF 2.4) planted with Italian ryegrass. On day 22, mineralized Ndfc ( soil mineral N plus plant N uptake war 5.8% and 1.3% of added N for th e N-15 and the difference methods. respectively. The discrepancy was r educed on day 43. On day 64, the relationship was reversed, and on day 98 the values given by the two methods were 22.8% and 29.5%, respecti vely. The results obtained by the two methods were linearly correlated (r = 0.987) and, on average, did not differ significantly. Neverthele ss. the different temporal patterns led to appreciably different param eter values as estimated by fitting of a reparameterized Richards mode l. On day 22, clover amendment reduced mineralized N derived from soil (Ndfs) by 3.4 mg N pot(-1). The reason for this was that the clover a mendment led to a reduction in plant growth and uptake of Ndfs, most l ikely because of allelopathy, while mineral Ndfs: did not increase cor respondingly. Clover-induced Ndfs in the microbial biomass of 5.1 mg N pot(-1) suggested that the mineral Ndfs not taken up by plants had be en reimmobilized. Towards the end of the experiment, clover-induced Nd fs in the biomass declined to 1.5 mg N pot(-1), while mineralized Ndfs due to clover amendment increased to 5.1 mg N pot(-1). The results st rongly suggested that this increase was caused by a real stimulation o f humus Pi mineralization by clover amendment rather than by isotope d isplacement or pool substitution.