Abstract
Pool dilution techniques, where the soil ammonium pool is labeled with 15NH4 +, are commonly used to estimate gross N mineralization rates in soil. To estimate the rates unbiased, it is assumed that the 15NH4 + is distributed homogenously in ambient 14NH4 + pool of the soil. However, completely homogeneous distribution of 15NH4 + may commonly not be feasible. The objective of this paper was to examine the importance of the spatial distribution of 14NH4 + and 15NH4 + 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 14NH4 + and 15NH4 + were combined. Generally, distribution of 15NH4 + 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 14NH4 + and 15NH4 + 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 NH4 +-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 14NH4 +, 15NH4 + and microbial activity did not influence the measured gross N mineralization rate significantly.
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Luxhøi, J., Nielsen, N. & Jensen, L. Effect of soil heterogeneity on gross nitrogen mineralization measured by 15N-pool dilution techniques. Plant and Soil 262, 263–275 (2004). https://doi.org/10.1023/B:PLSO.0000037043.26369.9b
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DOI: https://doi.org/10.1023/B:PLSO.0000037043.26369.9b