Effect of soil heterogeneity on gross nitrogen mineralization measured by 15N-pool dilution techniques

Pool dilution techniques, where the soil ammonium pool is labeled with (NH4+)-N-15, are commonly used to estimate gross N mineralization rates in soil. To estimate the rates unbiased, it is assumed that the (NH4+)-N-15 is distributed homogenously in ambient (NH4+)-N-14 pool of the soil. However, completely homogeneous distribution of (NH4+)-N-15 may commonly not be feasible. The objective of this paper was to examine the importance of the spatial distribution of (NH4+)-N-14 and (NH4+)-N-15 on the measured gross N mineralization rate. In a 2-day incubation experiment, gross N mineralization rates were measured in soil, where different distributions of (NH4+)-N-14 and (NH4+)-N-15 were combined. Generally, distribution of (NH4+)-N-15 to 50% of the soil volume did not alter the measured gross mineralization rate however more heterogeneous distribution caused the rate to be underestimated. Certain combinations of (NH4+)-N-14 and (NH4+)-N-15 distributions caused the rate to be overestimated. Based on the experimental results, we developed a 2-dimensional model array of soil compartments, to estimate the gross N mineralization and gross NH4+ consumption rates in local microsites in the soil. If one of the nitrogen-isotopes was more abundant in a compartment with high NW-concentration, and the other nitrogen-isotope was more abundant in a compartment with low NH4+-concentration, the nitrogen-isotopes would have different apparent bioavailability, hence the gross N mineralization rate would be erroneously estimated. On the other hand, in soil where all compartments had low NH4+-concentrations, heterogeneous distribution of (NH4+)-N-14, (NH4+)-N-15 and microbial activity did not influence the measured gross N mineralization rate significantly.