Presentation Information
[O12-P42]Evaluation of Soil Aggregate Formation and Red Soil Runoff Reduction Induced by Seaweed Compost
*Oto Ikeda1, *Yume Ikebata1, *Shana Kawakami1 (1. Kagoshima Prefectural Yoron Senior High School)
Keywords:
Red soil runoff,Seaweed composting,Soil aggregation,ICP-AES,XRF
Soil organic matter on Yoron Island, Kagoshima Prefecture, is rapidly decomposed in subtropical environments because biological activity by soil fauna and microorganisms is enhanced under the warm temperature through the year1. Therefore, soils on Yoson Island tend to develop a single-grain structure that is easily eroded by rainfall, resulting in enhanced runoff of agricultural soils into the ocean. This affects marine ecosystems, fishing industry, and tourism. For example, when red soil runoff increases, the marine environment becomes worse in terms of eutrophication and turbidity of the coastal ocean. A decrease in soil nutrient concentration can also hinder crop growth. A large amount of seaweed such as Sargassum washed ashore, affecting the coastal landscape, and their disposal has also become an additional challenge. In this study, we aimed to evaluate the effectiveness of seaweed compost in forming soil aggregate and reducing red soil runoff2. We tested whether compost made from seaweed can help prevent red soil runoff by improving soil structure.
To evaluate the current status of red soil runoff on the island, we collected seawater samples from 10 coastal sites from May 15 to June 9, 2025, and compared with previously reported values3. A soil runoff experiment was conducted using soils with composted seaweed and untreated soils4. Tap-water was poured from above onto both soils placed in containers inclined at 45°, and the passing time of runoff water was measured. Eroded soil samples were also collected and their dry weight was measured. Water and soil samples were analyzed by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and X-ray fluorescence (XRF), respectively in July 2025 at the Atmosphere and Ocean Research Institute, The University of Tokyo to determine Fe concentrations (an indicator of red soil runoff) and P and K concentrations (indicators of nutrients promoting plant growth).
The ICP-AES results showed that Fe concentrations in seawater increased at one site, while both Fe and P concentrations decreased at the other sites from 2022 to 2025. According to the Japan Meteorological Agency (2026)5, precipitation in June 2025 decreased by approximately 4.7% compared to June 2024, suggesting that changes in rainfall may have influenced the amount of red soil runoff. In the soil runoff experiment, the passage times of runoff water were 16.4 seconds and 24.3 seconds for the soils with composted seaweed and untreated soils, respectively. The amounts of eroded soil were 87.6 mg and 157.3 mg, respectively. These results suggest that the formation of soil aggregates improved water infiltration and reduced soil erosion. XRF analysis showed that Fe concentrations in the soils with composted seaweed increased by 0.966% compared to the untreated soils, whereas P and K concentrations decreased by 0.024% and 0.031%, respectively.
Our results suggest that changes in soil structure promoted aggregate formation, resulting in reduced susceptibility to soil erosion. Overall, the application of composted seaweed to soil may be effective in enhancing soil aggregation and reducing red soil runoff. Future studies will further investigate whether seaweed compost also promotes plant growth.
To evaluate the current status of red soil runoff on the island, we collected seawater samples from 10 coastal sites from May 15 to June 9, 2025, and compared with previously reported values3. A soil runoff experiment was conducted using soils with composted seaweed and untreated soils4. Tap-water was poured from above onto both soils placed in containers inclined at 45°, and the passing time of runoff water was measured. Eroded soil samples were also collected and their dry weight was measured. Water and soil samples were analyzed by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) and X-ray fluorescence (XRF), respectively in July 2025 at the Atmosphere and Ocean Research Institute, The University of Tokyo to determine Fe concentrations (an indicator of red soil runoff) and P and K concentrations (indicators of nutrients promoting plant growth).
The ICP-AES results showed that Fe concentrations in seawater increased at one site, while both Fe and P concentrations decreased at the other sites from 2022 to 2025. According to the Japan Meteorological Agency (2026)5, precipitation in June 2025 decreased by approximately 4.7% compared to June 2024, suggesting that changes in rainfall may have influenced the amount of red soil runoff. In the soil runoff experiment, the passage times of runoff water were 16.4 seconds and 24.3 seconds for the soils with composted seaweed and untreated soils, respectively. The amounts of eroded soil were 87.6 mg and 157.3 mg, respectively. These results suggest that the formation of soil aggregates improved water infiltration and reduced soil erosion. XRF analysis showed that Fe concentrations in the soils with composted seaweed increased by 0.966% compared to the untreated soils, whereas P and K concentrations decreased by 0.024% and 0.031%, respectively.
Our results suggest that changes in soil structure promoted aggregate formation, resulting in reduced susceptibility to soil erosion. Overall, the application of composted seaweed to soil may be effective in enhancing soil aggregation and reducing red soil runoff. Future studies will further investigate whether seaweed compost also promotes plant growth.