BIOLOGICAL INTERACTIONS BETWEEN SOIL-NITROGEN AND ALKALINE-HYDROLYZING NITROGEN FERTILIZERS

Added N interactions were measured in four soils incubated with N-15-labelled urea or diammonium phosphate. The use of biologically active, gamma-irradiated, or reinoculated gamma-irradiated samples allowed us to separate added N interactions due to chemical and biological processes, and to distinguish real interactions from apparent effects. Real biologically mediated added N interactions were observed in one soil for both fertilizer sources and in three soils amended with urea. These real interactions increased with the N fertilizer rate, but did not differ significantly between N sources. Fertilizer-induced unlabelled organic N in soil extracts declined during incubation in both sterile and non-sterile samples, but the temporal decline was higher in biologically active soil. Changes in fertilizer-induced unlabelled organic N in the extracts of three soils attributed to biological processes were similar to the measured real biologically mediated added N interactions. The results are consistent with the hypothesis that real biologically mediated added N interactions arise from the mineralization of soil organic N solubilized by alkaline-hydrolysing N fertilizers.