Measuring gross nitrogen mineralization, and nitrification by 15 N isotopic pool dilution in intact soil cores

Abstract

SUMMARY: 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 15 NH 4 + or 15 NO 4 − ; gross mineralization rates were calculated from the decline in “N enrichment of the NH: pool during a 24‐h incubation; gross nitrification rates were calculated from the decline in 15 N enrichment of the NO − 3 pool; gross rates of NH 4 + and NO 3 − consumption were calculated from disappearance of the 15 N label. The assumptions required for application of this method to intact cores are evaluated. Sensitivity analysis revealed that homogeneous mixing of added “N 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 15 N and 14 N pool size estimates, In a silt loam soil from a grassland site, abiotic processes consumed over 30% of the added 15 NH 4 + 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 15 N 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 15 N 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.