MEASURING GROSS NITROGEN MINERALIZATION, IMMOBILIZATION, AND NITRIFICATION BY N-15 ISOTOPIC POOL DILUTION IN INTACT SOIL CORES

The isotope dilution method for measuring gross rates of N mineralization, immobilization, and nitrification was applied to intact soil cores so that the effects of soil mixing were avoided. Soil cores were injected with solutions of either (NH4+)-N-15 or (NO3-)-N-15; gross mineralization rates were calculated from the decline in N-15 enrichment of the NH4+ pool during a 24-h incubation; gross nitrification rates were calculated from the decline in N-15 enrichment of the NO3- pool; gross rates of NH4+ and NO3- consumption were calculated from disappearance of the N-15 label. The assumptions required for application of this method to intact cores are evaluated. Sensitivity analysis revealed that homogeneous mixing of added N-15 with ambient pools was not a necessary assumption but that bias in distribution of added label, coincident with a non-random distribution of microbial processes, would cause significant errors in rate estimates. Rate estimates were also sensitive to errors in initial N-15 and N-14 pool size estimates. In a silt loam soil from a grassland site, abiotic processes consumed over 30% of the added (NH4+)-N-15 within minutes of adding the label to sterilized soil. Extracting a subset of soil cores at the beginning of an incubation is recommended for obtaining initial pool size,estimates. Gross immobilization is probably stimulated by addition of inorganic N-15 substrate and, therefore, is overestimated by the isotope dilution method. As an alternative method, a non-linear equation is given for calculating the gross immobilization rate from the appearance of N-15 in chloroform-labile microbial biomass; but incomplete extraction of biomass N may result in low estimates. Details of the isotope dilution methodology (injection rates, concentrations, experimental artefacts, etc.) are described and discussed. When care is taken to understand the underlying assumptions and sources of error, the isotope dilution method provides a powerful tool for measuring gross rates of microbial transformations of soil nitrogen in intact soil cores.