A method to assess whether 'preferential use' occurs after 15N ammonium addition; implication for the 15N isotope dilution technique
2005
Herrmann, A. | Witter, E. | Katterer, T.
The robustness of the assumption of equilibrium between native and added N during 15N isotope dilution has recently been questioned by Watson et al. (Soil Biol Biochem 32 (2000) 2019-2030). We re-analyzed their raw data using equations that consider the added and native NH4(+) and NO3(-) pools as separate state variables. Gross mineralization rates and first-order rate constants for NH4(+) and NO3(-) consumption were obtained by combining analytical integration of the differential equations with a non-linear fitting procedure. The first-order rate constants for NH4(+) consumption and NO3(-) immobilization for the added NH4(+) and NO3(-) pool were used to estimate gross mineralization rates and first-order rate constants for nitrification of native NH4(+). The latter were 2-4 times lower than the first-order rate constants derived from the added N pool. This discrepancy between first-order rate constants for nitrification implies that one or more process rates estimated for the added N pools cannot be applied to the native N pools. Preferential use of the added N resulted in an overestimation of the gross mineralization by 1.5-2.5-fold, emphasizing the need for critical evaluation of the assumption of equilibrium before gross mineralization rates are calculated.
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