5:15 PM - 7:15 PM
[MIS22-P02] Are paddy fields a source of dissolved iron?—Seasonal and spatial variation in the Omono River Basin—
Keywords:Iron, River, Paddy Field
Dissolved iron (dFe) supplied from terrestrial areas to the ocean via rivers is an essential nutrient that supports primary production in coastal and open ocean environments. However, the average dFe concentration in Japanese rivers is low (0.024 mg/L, National Institute of Radiological Sciences, 2006), and iron deficiency has been considered a potential cause of rocky-shore denudation (decline and degradation of seaweed communities) in some coastal areas. In contrast, rivers on the Sea of Japan side (Tohoku and Hokuriku regions) have relatively high dFe concentrations, often exceeding 0.10 mg/L, suggesting the presence of a unique supply mechanism in this region. However, the underlying causes remain unclear. Given that iron tends to be mobilized from reducing environments, this study hypothesizes that the extensive rice paddy areas on the Sea of Japan side function as a source of dFe. The supply of dFe from rice paddies to rivers has received little attention thus far, and data on its concentration levels and seasonal variations are still insufficient. If it is revealed that rice paddies contribute to dFe transport to the ocean via rivers, this would provide new insights into the multifunctionality of rice paddies.
To clarify the actual supply of dFe from rice paddies to rivers, this study conducted biweekly water sampling for one year (April 2024–March 2025) in the Omono River watershed in Akita Prefecture. Sampling was conducted at 12 sites, including six locations along the main river, five tributaries, and one paddy surface water site. The Yokote Basin (Yokote Bonchi), located in the midstream region of the Omono River Basin, is a major production area of Akita Prefecture's branded rice, Akita Komachi. Since irrigation water from the Omono River is eventually discharged back into the river, this area is also of concern regarding the potential impact of agricultural activities on water quality. If our hypothesis is correct, dFe concentrations should increase as the Omono River flows through the Yokote Basin.
In many locations along the main river and tributaries, dFe concentrations increased from June to July, following the rice paddy puddling process. In particular, two major agricultural drainage channels in the Yokote Basin—Omiya River and Yokote River—exhibited the highest concentrations of the year, reaching approximately 0.75 mg/L in late June. dFe concentrations in the main river also increased significantly as it passed through the Yokote Basin, rising from 0.114 mg/L to 0.271 mg/L in late June, highlighting the influence of inflows from these drainage channels. Additionally, dFe concentrations in paddy surface water increased from 0.161 mg/L in June to 1.019 mg/L in July. These findings suggest that during the flooded period after puddling, reducing conditions in paddy soils intensified, leading to the release of large amounts of dFe from rice paddies into the drainage system and eventually into the Omono River. Subsequently, during the midseason drainage, intermittent irrigation, and final drainage periods from July to September, dFe concentrations decreased at most sites. From October to January, no distinct seasonal variations were observed.
Interestingly, however, dFe concentrations in Omiya River and Yokote River remained relatively high (0.20–0.50 mg/L) even after the irrigation season ended in September. The underlying mechanism is unclear, but given that most rice paddy soils in the Yokote Basin are gleysols, it is likely that sustained dFe supply occurs even during the non-irrigation period from autumn to winter. Furthermore, dFe concentrations at all sampling sites were strongly correlated with dissolved organic carbon (DOC) and dissolved manganese (dMn) concentrations throughout the study period (DOC: r = 0.56–0.98; dMn: r = 0.59–0.99). This suggests that the dFe present in the Omono River is predominantly organic-bound iron originating from reducing gleysols. Considering that dFe concentrations at the river mouth ranged from 0.072 to 0.153 mg/L, our findings indicate that despite seasonal fluctuations associated with irrigation, rice paddies serve as a significant and continuous source of dFe (primarily in organic-bound iron) to the Sea of Japan via the Omono River.
To clarify the actual supply of dFe from rice paddies to rivers, this study conducted biweekly water sampling for one year (April 2024–March 2025) in the Omono River watershed in Akita Prefecture. Sampling was conducted at 12 sites, including six locations along the main river, five tributaries, and one paddy surface water site. The Yokote Basin (Yokote Bonchi), located in the midstream region of the Omono River Basin, is a major production area of Akita Prefecture's branded rice, Akita Komachi. Since irrigation water from the Omono River is eventually discharged back into the river, this area is also of concern regarding the potential impact of agricultural activities on water quality. If our hypothesis is correct, dFe concentrations should increase as the Omono River flows through the Yokote Basin.
In many locations along the main river and tributaries, dFe concentrations increased from June to July, following the rice paddy puddling process. In particular, two major agricultural drainage channels in the Yokote Basin—Omiya River and Yokote River—exhibited the highest concentrations of the year, reaching approximately 0.75 mg/L in late June. dFe concentrations in the main river also increased significantly as it passed through the Yokote Basin, rising from 0.114 mg/L to 0.271 mg/L in late June, highlighting the influence of inflows from these drainage channels. Additionally, dFe concentrations in paddy surface water increased from 0.161 mg/L in June to 1.019 mg/L in July. These findings suggest that during the flooded period after puddling, reducing conditions in paddy soils intensified, leading to the release of large amounts of dFe from rice paddies into the drainage system and eventually into the Omono River. Subsequently, during the midseason drainage, intermittent irrigation, and final drainage periods from July to September, dFe concentrations decreased at most sites. From October to January, no distinct seasonal variations were observed.
Interestingly, however, dFe concentrations in Omiya River and Yokote River remained relatively high (0.20–0.50 mg/L) even after the irrigation season ended in September. The underlying mechanism is unclear, but given that most rice paddy soils in the Yokote Basin are gleysols, it is likely that sustained dFe supply occurs even during the non-irrigation period from autumn to winter. Furthermore, dFe concentrations at all sampling sites were strongly correlated with dissolved organic carbon (DOC) and dissolved manganese (dMn) concentrations throughout the study period (DOC: r = 0.56–0.98; dMn: r = 0.59–0.99). This suggests that the dFe present in the Omono River is predominantly organic-bound iron originating from reducing gleysols. Considering that dFe concentrations at the river mouth ranged from 0.072 to 0.153 mg/L, our findings indicate that despite seasonal fluctuations associated with irrigation, rice paddies serve as a significant and continuous source of dFe (primarily in organic-bound iron) to the Sea of Japan via the Omono River.