10:45 AM - 11:00 AM
[AHW27-07] Integrated multi-isotope model to assess nutrient balances in watershed ecosystems: Application of nitrate oxygen isotopes (δ18O and Δ17O)
Keywords:Ecosystem metabolism, Landscape stoichiometry, NO3-Δ17O, NO3-δ18O, Nutrient balance, Reoligotrophication
1. Introduction
Since the Anthropocene, mass production and consumption have led to disturbance of natural biogeochemical cycles of macronutrients, such as phosphorus (P) and nitrogen (N), in watersheds. Nutrient loading has caused serious cultural eutrophication. In order to reduce the nutrient loading, developed countries have installed WWTPs, leading to mitigation of the eutrophication. At present, however, the developed countries are facing with a new social-environmental issue. Some researchers pointed out that recent reduction of fishery production can be attributed to the reoligotrophication, which decreases the ecosystem productivity. In Seto Inland Sea, for instance, a local government recently implemented a legislation to discharge untreated wastewater from WWTPs in order to enhance the ecosystem productivity. However, this manipulation has not yet resulted in the recovery of fishery production, while it poses a risk that the coastal ecosystem may move back to eutrophic state.
In general, N:P ratio is a primary factor to determine ecosystem processes because of its scarcity relative to other macronutrients. According to the theory of ecological stoichiometry, there should exist an optimal nutrient balance to maximize the ecosystem processes. However, no one knows what is the optimum for the whole ecosystem though some studies demonstrated that organismal metabolism is maximized around the Redfield ratio (N/P=16) under microcosm experiments. If we can identify the optimal nutrient balance for a focal ecosystem, the best strategy of watershed management to maximize the ecosystem processes is addition of limiting nutrient or removal of excess nutrient relative to the optimum. According to the concept of landscape stoichiometry in which stoichiometric distribution of watershed varies across a landscape and in turn determines spatial patterns of ecosystem processes (Leroux et al. 2017), we here aim to develop an integrated isotope model to assess the optimal nutrient balance for watershed ecosystems based on in-stream measurements of their N and P metabolism using NO3 and PO4 oxygen isotopes. In my talk, I will introduce an application of δ18O and Δ17O of NO3 for measurement of the N metabolism.
2. Materials & Methods
We selected 22 rivers tributary to Lake Biwa to examine spatial variations in nutrient balances and ecosystem metabolism within the watershed: the study rivers greatly vary in their catchment size and land use pattern. We collected river waters for measurements of nutrient concentrations (TN, TP, nitrate and phosphate) and oxygen isotope ratios (δ18O and Δ17O) of NO3 in the downstream of these rivers during the irrigation period of 2024. Based on GIS data, we first examined how the land use pattern affects N and P loadings and consequently nutrient balances, indicated as TN/TP. Based on the isotope data, we next calculated upward deviation (NO3-δ18Oσ) of river water δ18ONO3 from an isotope mixing line of δ18ONO3 against Δ17ONO3 for atmospheric and remineralised NO3 sources in the study watershed, according to Tsunogai et al. (2016). Assuming that the NO3-δ18Oσ would increase through in-stream NO3 assimilation and denitrification, we used it as an indicator of ecosystem N metabolism.
3. Results & Discussion
Comparative approach revealed that TP increases with the increasing proportional area of rice paddy in catchment, while TN increases with the increasing residential area. As a consequence of NP loadings, TN/TP varied from 6.2 to 63.3 among the tributaries, excepting an outlier (265.4) for a densely populated river. From observed values of Δ17ONO3 on the isotope mixing line, we estimated that almost all NO3 is derived from remineralisation (98.1% on average). As an indicator of N metabolism, NO3-δ18Oσ values varied from 0.9 to 7.3, except for the densely populated river with a value of 13.9 comparable to those in wastewater from WWTPs. The highest NO3-δ18Oσ values were observed around 20 of TN/TP. A quadratic curve fitting against the TN/TP showed that the NO3-δ18Oσ values can reach the highest at 10.7 of TN/TP. The Redfield ratio was within a range between the observed and theoretical maxima, suggesting that there exists an optimal nutrient balance at which ecosystem processes can be maximized through the watershed management.
Since the Anthropocene, mass production and consumption have led to disturbance of natural biogeochemical cycles of macronutrients, such as phosphorus (P) and nitrogen (N), in watersheds. Nutrient loading has caused serious cultural eutrophication. In order to reduce the nutrient loading, developed countries have installed WWTPs, leading to mitigation of the eutrophication. At present, however, the developed countries are facing with a new social-environmental issue. Some researchers pointed out that recent reduction of fishery production can be attributed to the reoligotrophication, which decreases the ecosystem productivity. In Seto Inland Sea, for instance, a local government recently implemented a legislation to discharge untreated wastewater from WWTPs in order to enhance the ecosystem productivity. However, this manipulation has not yet resulted in the recovery of fishery production, while it poses a risk that the coastal ecosystem may move back to eutrophic state.
In general, N:P ratio is a primary factor to determine ecosystem processes because of its scarcity relative to other macronutrients. According to the theory of ecological stoichiometry, there should exist an optimal nutrient balance to maximize the ecosystem processes. However, no one knows what is the optimum for the whole ecosystem though some studies demonstrated that organismal metabolism is maximized around the Redfield ratio (N/P=16) under microcosm experiments. If we can identify the optimal nutrient balance for a focal ecosystem, the best strategy of watershed management to maximize the ecosystem processes is addition of limiting nutrient or removal of excess nutrient relative to the optimum. According to the concept of landscape stoichiometry in which stoichiometric distribution of watershed varies across a landscape and in turn determines spatial patterns of ecosystem processes (Leroux et al. 2017), we here aim to develop an integrated isotope model to assess the optimal nutrient balance for watershed ecosystems based on in-stream measurements of their N and P metabolism using NO3 and PO4 oxygen isotopes. In my talk, I will introduce an application of δ18O and Δ17O of NO3 for measurement of the N metabolism.
2. Materials & Methods
We selected 22 rivers tributary to Lake Biwa to examine spatial variations in nutrient balances and ecosystem metabolism within the watershed: the study rivers greatly vary in their catchment size and land use pattern. We collected river waters for measurements of nutrient concentrations (TN, TP, nitrate and phosphate) and oxygen isotope ratios (δ18O and Δ17O) of NO3 in the downstream of these rivers during the irrigation period of 2024. Based on GIS data, we first examined how the land use pattern affects N and P loadings and consequently nutrient balances, indicated as TN/TP. Based on the isotope data, we next calculated upward deviation (NO3-δ18Oσ) of river water δ18ONO3 from an isotope mixing line of δ18ONO3 against Δ17ONO3 for atmospheric and remineralised NO3 sources in the study watershed, according to Tsunogai et al. (2016). Assuming that the NO3-δ18Oσ would increase through in-stream NO3 assimilation and denitrification, we used it as an indicator of ecosystem N metabolism.
3. Results & Discussion
Comparative approach revealed that TP increases with the increasing proportional area of rice paddy in catchment, while TN increases with the increasing residential area. As a consequence of NP loadings, TN/TP varied from 6.2 to 63.3 among the tributaries, excepting an outlier (265.4) for a densely populated river. From observed values of Δ17ONO3 on the isotope mixing line, we estimated that almost all NO3 is derived from remineralisation (98.1% on average). As an indicator of N metabolism, NO3-δ18Oσ values varied from 0.9 to 7.3, except for the densely populated river with a value of 13.9 comparable to those in wastewater from WWTPs. The highest NO3-δ18Oσ values were observed around 20 of TN/TP. A quadratic curve fitting against the TN/TP showed that the NO3-δ18Oσ values can reach the highest at 10.7 of TN/TP. The Redfield ratio was within a range between the observed and theoretical maxima, suggesting that there exists an optimal nutrient balance at which ecosystem processes can be maximized through the watershed management.