11:00 〜 11:15
[AHW27-08] Integrated multi-isotope model to assess nutrient balances in watershed ecosystems: Application of phosphate oxygen isotope
キーワード：Ecosystem metabolism、Landscape stoichiometry、phosphate oxygen isotope、Nutrient balance、Reoligotrophication
Introduction
Since the Anthropocene, human activities have disrupted the natural biogeochemical cycles of phosphorus (P) and nitrogen (N), leading to cultural eutrophication. While wastewater treatment plants (WWTPs) have mitigated eutrophication in developed countries, emerging concerns about reoligotrophication have been linked to declines in fishery production. The N:P ratio plays a crucial role in regulating ecosystem processes, with ecological stoichiometry suggesting an optimal nutrient balance. However, the ideal ratio for entire ecosystems remains unknown, despite evidence that organismal metabolism peaks around the Redfield ratio (N/P = 16) in microcosm studies. Identifying the optimal nutrient balance could inform watershed management strategies by adjusting nutrient inputs accordingly. 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 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, we will introduce an application of PO4 oxygen isotope (δ18OPO4) for measurement of the P metabolism.
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, NO3 and PO4) and δ18OPO4 values in the downstream of these rivers during the irrigation period (May) and non-irrigation period (September) of 2024. Isotopic fractionation associated with biological P uptake and release (biological equilibrium) reportedly dominates the δ18OPO4 signature of the PO4 pool in most aquatic systems (Paytan and McLaughlin, 2012). We hypothesized that in systems with high P metabolism, the measured δ18OPO4 values would approach the biological equilibrium values, which can be estimated by water temperature and oxygen isotope ratio of water (δ18OH2O) (Chang and Blake, 2015). Therefore, we utilized the deviation between the measured δ18OPO4 values and estimated biological equilibrium values (PO4-δ18Oσ) as an indicator of P metabolism.
Results & Discussion
The δ18OPO4 values were higher in the irrigation period (14.4 ± 1.2‰) than in the non-irrigation period (11.9 ± 2.1‰). Most of these values were lower than the biological equilibrium values, suggesting the contribution of P supply from sources with low δ18OPO4 values and/or isotopic fractionation associated with the mineralization of organic P. The δ18OPO4 values showed a correlation with geological properties, suggesting that rocks were likely the primary P source to the rivers. Additionally, a positive correlation between δ18OPO4 values and the δ18OH2O values may be evidence of biological P metabolism, including both biological equilibrium and mineralization. PO4-δ18Oσ increased with TN/TP ratios up to 40–50, then declined during both the irrigation and non-irrigation periods. These findings indicate that δ18OPO4 have the potential to serve as indicators of P sources and P metabolism influenced by the N:P balance. However, challenges remain in distinguishing between the effects of P sources and P metabolism. To address this issue, predictive models of δ18OPO4 values from P source mixing based on land use and geological properties (Ishida et al., 2019), as well as the utilization of triple oxygen isotopic compositions (Sambuichi et al., 2023), could be effective approaches. By integrating these measures, δ18OPO4 has the potential to serve as a robust indicator of P metabolism in aquatic ecosystems.
Since the Anthropocene, human activities have disrupted the natural biogeochemical cycles of phosphorus (P) and nitrogen (N), leading to cultural eutrophication. While wastewater treatment plants (WWTPs) have mitigated eutrophication in developed countries, emerging concerns about reoligotrophication have been linked to declines in fishery production. The N:P ratio plays a crucial role in regulating ecosystem processes, with ecological stoichiometry suggesting an optimal nutrient balance. However, the ideal ratio for entire ecosystems remains unknown, despite evidence that organismal metabolism peaks around the Redfield ratio (N/P = 16) in microcosm studies. Identifying the optimal nutrient balance could inform watershed management strategies by adjusting nutrient inputs accordingly. 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 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, we will introduce an application of PO4 oxygen isotope (δ18OPO4) for measurement of the P metabolism.
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, NO3 and PO4) and δ18OPO4 values in the downstream of these rivers during the irrigation period (May) and non-irrigation period (September) of 2024. Isotopic fractionation associated with biological P uptake and release (biological equilibrium) reportedly dominates the δ18OPO4 signature of the PO4 pool in most aquatic systems (Paytan and McLaughlin, 2012). We hypothesized that in systems with high P metabolism, the measured δ18OPO4 values would approach the biological equilibrium values, which can be estimated by water temperature and oxygen isotope ratio of water (δ18OH2O) (Chang and Blake, 2015). Therefore, we utilized the deviation between the measured δ18OPO4 values and estimated biological equilibrium values (PO4-δ18Oσ) as an indicator of P metabolism.
Results & Discussion
The δ18OPO4 values were higher in the irrigation period (14.4 ± 1.2‰) than in the non-irrigation period (11.9 ± 2.1‰). Most of these values were lower than the biological equilibrium values, suggesting the contribution of P supply from sources with low δ18OPO4 values and/or isotopic fractionation associated with the mineralization of organic P. The δ18OPO4 values showed a correlation with geological properties, suggesting that rocks were likely the primary P source to the rivers. Additionally, a positive correlation between δ18OPO4 values and the δ18OH2O values may be evidence of biological P metabolism, including both biological equilibrium and mineralization. PO4-δ18Oσ increased with TN/TP ratios up to 40–50, then declined during both the irrigation and non-irrigation periods. These findings indicate that δ18OPO4 have the potential to serve as indicators of P sources and P metabolism influenced by the N:P balance. However, challenges remain in distinguishing between the effects of P sources and P metabolism. To address this issue, predictive models of δ18OPO4 values from P source mixing based on land use and geological properties (Ishida et al., 2019), as well as the utilization of triple oxygen isotopic compositions (Sambuichi et al., 2023), could be effective approaches. By integrating these measures, δ18OPO4 has the potential to serve as a robust indicator of P metabolism in aquatic ecosystems.