Errors associated with calculating the gross nitrification rates in forested catchments using the triple oxygen isotopic composition (Δ<sup>17</sup>O) of stream nitrate

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<jats:p>Abstract. A novel method for quantifying the gross nitrification rate (GNR) in each forested catchment using the triple oxygen isotopic composition (Δ17O) of stream nitrate eluted from the catchment has been proposed and applied in several recent studies. However, the equations used in the calculations include the approximation that the Δ17O value of nitrate metabolized through either assimilation or denitrification within the forested soil is equal to the Δ17O value of nitrate in the stream. The GNR estimated from the Δ17O value of stream nitrate was more than six times the actual GNR in our simulated calculation for a forested catchment where the nitrate in the soil exhibited Δ17O values larger than those in the stream while showing a decreasing trend with increasing depths until that of stream nitrate at the bottom. As most of the reported soil nitrate in forested catchments from past studies showed Δ17O values higher than those of the stream nitrate eluted from each catchment, we concluded that the GNR estimated from the Δ17O value of stream nitrate in the forested catchments was, to some extent, an overestimate of the actual GNR. </jats:p>

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