Comparison of the difference method and 15N technique for studying the fate of nitrogen from plant residues in soil

Abstract

We compared the dynamics of net mineralization of nitrogen (N) derived from white clover material (Ndfc) as measured by the difference and the ¹⁵N methods in a pot experiment with a sandy loam (15°C and pF 2.4) planted with Italian ryegrass. On day 22, mineralized Ndfc (soil mineral N plus plant N uptake) was 5.8% and 1.3% of added N for the ¹⁵N and the difference methods, respectively. The discrepancy was reduced 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%, respectively. The results obtained by the two methods were linearly correlated (r = 0.987) and, on average, did not differ significantly. Nevertheless, the different temporal patterns led to appreciably different parameter values as estimated by fitting of a reparameterized Richards model. 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 amendment led to a reduction in plant growth and uptake of Ndfs, most likely because of allelopathy, while mineral Ndfs did not increase correspondingly. 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 been reimmobilized. Towards the end of the experiment, clover-induced Ndfs 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 strongly suggested that this increase was caused by a real stimulation of humus N mineralization by clover amendment rather than by isotope displacement or pool substitution.