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.
Weitian Ding et al.
Status: final response (author comments only)
RC1: 'Comment on bg-2022-236', Anonymous Referee #1, 09 Feb 2023
- AC1: 'Reply on RC1', Weitian Ding, 12 Mar 2023
RC2: 'Comment on bg-2022-236', Joel Bostic, 10 Feb 2023
- AC2: 'Reply on RC2', Weitian Ding, 12 Mar 2023
Weitian Ding et al.
Weitian Ding et al.
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Nitrate is one of important N forms available for microbe and plant. However, it almost impossible to quantify nitrate production rate (nitrifcation) at an ecosystem level, especially for forest ecosystems, where there are very large spatial and temporal variations. A novel method to quantify gross nitrification at an ecosystem level has been originally proposed by Tsunogai et al. (2011, Biogeosciences) by using nitrate triple oxygen isotopes. Herein, we define it as the nitrate oxygen isotope method. Tsunogai et al. had successfully applied this method to quantify gross nitrcation rates for lake systems in Japan. Later, this method was adopted by Fang et al. (2015, PANS), Huang et al. (2019, EST) and Hattori et al. (2019, Sci. Total Environment) to quantify annual gross nitrication rates for forests. The manuscript submitted by Weitian Ding et al. questioned its appliction of this method to forest soils. The authors of this manuscript suggested that the nitrate oxygen method is only suitable for the environment, like lake system, where the distribution of 17O of nitrate is homogeneous. To demonstrate it, the authors calculated gross nitrification rates to be 83.6 kg N/ha.yr when assuming that the distribution of 17O of nitrate is heterogeneous in forest soils from the top to subsoil, while to be 13.0 kg N/ha.yr when assuming that it is homogeneous, respectively, according to results from a Japanese forest reported by Hattori et al. The authos concluded that it should verify that 17O values of nitrate being metabolized are homogeneous in forest soils. The manuscript was clearly written and the topic is within the scopes of Biogeosciences. However, the authors may have misundstood the assumptions for the nitrate oxygen isotope method when applying to forest soils. Thus, the conclusions made by the authors cannnot be supported.
First, it is not necessary to assume that the distribution of 17O of nitrate along the soil profile is homogeneous or heterogeneous when apply the nitrate oxygen isotope method to forest soils. In fact, the assumption of the method is that the plants or microbes access the same nitrate source in forest soil with denitrifiers (Fang et al., 2015, PNAS). This assumpition is not identical to the assumption that the spatial distribution of 17O of nitrate along the soil profile is homogeneous, as demonstrated by the authors (Fig. 2).
Second, it is not correct to assume that the distribution of 17O of nitrate along the soil profile is homogeneous (Fig. 2). 17O has been rarely measured along the soil profile. The one and only study shows a sharp decrease in 17O of nitrate in the top soil and remains relatively constant in the soil from 25 to 95 cm (Hattori et al., 2019, Sci. Total Environment). Thus, the assumption made by the author was not supported the field observation.
Third, I agree that it is the distribution of 17O of nitrate along the soil profile is highly hetrogeneous, as nitrification is dominant in surface soils, and deposited nitrate may enter soil from the forest floor. However, it is not correct to assume that 17O of nitrate decreased linearly with soil depth (Fig. 1). The field observation by Hattori et al did not support this assumption. This may be main reason for unrealstically low gross nitrification rate (13 kg N/ha.yr) as calculated by the authors, in the study forest with modate to high N deposition (16 kg N.ha.yr). Nitrification must be strongly active in this forest, which was supported by high soil nitrate concentrations and a large seasonal varation in 15N and 17O of nitrate (Fig. 3 of Hattori et al.).
In fact, the nitrate oxgen isotope method admit high heterogeneity of soil nitrfication in both spatially and seasonally. And it is difficult and almost impossible to capture these heterogeneities. However, these heterogeneities can be integrated to streamwater. The nitrate oxygen isotope method takes this advantage of it.