Preprints
https://doi.org/10.5194/egusphere-2023-2753
https://doi.org/10.5194/egusphere-2023-2753
12 Dec 2023
 | 12 Dec 2023

Errors associated with calculating the gross nitrification rates in forested catchments using the triple oxygen isotopic composition (Δ17O) of stream nitrate

Weitian Ding, Urumu Tsunogai, and Fumiko Nakagawa

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.

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Weitian Ding, Urumu Tsunogai, and Fumiko Nakagawa

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-2753', Anonymous Referee #1, 11 Jan 2024
    • AC1: 'Reply on RC1', Weitian Ding, 12 Mar 2024
  • RC2: 'Comment on egusphere-2023-2753', Anonymous Referee #2, 13 Feb 2024
    • AC2: 'Reply on RC2', Weitian Ding, 12 Mar 2024
Weitian Ding, Urumu Tsunogai, and Fumiko Nakagawa
Weitian Ding, Urumu Tsunogai, and Fumiko Nakagawa

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Short summary
Past studies have used the Δ17O of stream nitrate to estimate the GNR in each forested catchment by approximating the Δ17O value of soil nitrate to be equal to that of stream nitrate. Based on formulaic inference and simulated calculation of measured data, we found that this approximation resulted in an overestimated GNR. Therefore, it is essential to clarify and verify the Δ17O NO3 values in forested soils and streams before applying the Δ17O values of stream NO3 to GNR estimation.