METHOD TO LABEL SOIL CORES WITH (NH3)-N-15 GAS AS A PREREQUISITE FOR N-15 ISOTOPIC DILUTION AND MEASUREMENT OF GROSS N MINERALIZATION

Application of N-15 isotopic dilution techniques to measure gross N mi neralization require pool to be uniformly enriched with N-15 for accur ate results. Solutions containing N-15 labelled salts have typically b een added to soil which may also be mixed to ensure uniform distributi on. This approach is not ideal for course-textured soils where the add ition of solution can result in enhanced microbial activity and increa sed N mineralization in many circumstances. A technique and equipment were developed to enrich the soil (NH4+)-N-14 pool by injecting N-15 e nriched NH3 gas into intact soil cores. This technique was tested in t wo course-textured Western Australian agricultural soils with low orga nic matter contents. Injection of NH3 enabled gross N mineralization t o be measured without major soil disturbance or change in the soil wat er content and proved suitable for use under field conditions. The gas injector distributed a mixture of N-15-labelled NH3 and air throughou t the surface 0-12 cm of soil cores. The soil (NH4+)-N-14 pool was enr iched with N-15 as the (NH3)-N-15 dissolved to form (NH4+)-N-15. A ser ies of independent tests were made to determine that the injector deli vered a constant amount of N-15 to assess uniformity in the vertical a nd radial distribution of (NH4+)-N-15 throughout soil cores and to exa mine the effect of NH3 concentration on the rate of gross N mineraliza tion and gross NH4+ consumption. The injector was found to apply a con stant amount of (NH4+)-N-15: to replicate sail cores. Distribution of the (NH3)-N-15 within intact soil cores was more uniform in the loamy sand compared with sandy-clay loam. An increase in NH3 concentration u p to four times the proposed working concentration did not significant ly alter the rate of gross N mineralization or NH4+ consumption; diffe r ences in gross N mineralization rates with respect to NH3 concentrat ion in the injected gas typically reflected the variability in the amo unt of microbial biomass-N present within undisturbed soil cores. The injection of small quantities of NH3 in this technique appeared to min imize the effect of NH3 fixation and subsequent release on the measure ment of gross N mineralization. Nevertheless, it is recommended that u sers of this technique examine whether the release of fixed NH3 is lik ely to cause an error in the estimate of gross N mineralization for ot her soils and rates of injection of NH3. (C) 1997 Elsevier Science Ltd .