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.