Ideas and perspectives: Errors associated with the gross nitrification rates in forested catchments calculated from the triple oxygen isotopic composition (Δ17O) of stream nitrate
Abstract. A novel method to quantify 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 used in recent studies. However, the equations used in the calculations assumed homogeneous Δ17O values of nitrate being metabolized through either assimilation or denitrification within the forested soil layers without particular discussions. The GNR estimated from the Δ17O of stream nitrate using the equations was more than six times the actual GNR in our simulated calculation for a forested catchment where the Δ17O values of nitrate being metabolized in the soil were heterogeneous and showed a decreasing trend with increasing depths. Therefore, we should verify that the Δ17O values of nitrate being metabolized are homogeneous in forested soils or estimate the possible range of errors using Δ17O of stream nitrate to estimate the GNR.