N-15 PARTITIONING BETWEEN INORGANIC AND ORGANIC N FRACTIONS IN SOIL AMENDED WITH (NH4NO3)-N-15 OR (NH4NO3)-N-15 IN POT EXPERIMENTS

A pot experiment with soil (Cambisol) was carried out to investigate t he fate of N-15 (supplied as (NH4NO3)-N-15 or (NH4NO3)-N-15) during an experimental period of 56 days. The soil was either left unsown and i ncubated at 25 degrees C or sown with grass (Lolium perenne) and culti vated in a greenhouse. Soil samples were taken at intervals of 7 or 14 days and immediatedly frozen at -18 degrees C. After thawing at 4 deg rees C the samples were dried at 40 degrees C for 24 hours, extracted with 0.01 M CaCl2 for inorganic N (N-inorg) and organic N (N-org) and analyzed for the N-15 labelling of these fractions. A method for the p reparation of extractable N fractions for mass spectrometry by steam d istillation was developed and used to separate the inorganic and extra ctable organic N fraction. Additionally, N-15 in total soil N (N-t) wa s determined at day 42 and 56 to analyze whether N-15 had been incorpo rated into nonextractable N fractions. The results showed that, within the experimental period, the incorporation of N-15 into N-org is negl igible. Furthermore, there were no indications that this extractable N -org fraction had mediated N flux from inorganic N to nonextractable o rganic N pools. N-15 losses in the range of 20-30% of the amount suppl ied were noted.