Patterns and drivers of organic carbon in hydromorphic soils across inland aquatic-terrestrial ecotones at the global scale

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The aquatic-terrestrial ecotones of inland waters, including rivers, lakes, and reservoirs, are subject to occasional, recurrent, or even permanent flooding and play a vital role in the carbon cycles of aquatic ecosystems. However, the patterns and drivers of soil organic carbon in aquatic-terrestrial ecosystems have not been clearly documented on a global scale. To address this knowledge gap, we constructed a database of organic carbon content of 1970 globally distributed aquatic-terrestrial ecotone soils and utilized a random forest model to measure the primary drivers at global scale. We found that the average soil organic carbon content was 26, 33, and 18 g kg−1, and the mean soil organic carbon stock was 24.95, 22.38, and 30.66 t ha−1 for the aquatic-terrestrial ecotones of rivers, lakes, and reservoirs, respectively. Notably, the soil organic carbon content in lake ecotones was significantly higher than in river and reservoir ecotones. However, reservoir ecotones stored the highest soil organic carbon compared to lake and river ecotones. The random forest model explained 52 %, 49 %, 58 %, and 28 % variations of soil organic carbon content across all sites, river, lake, and reservoir ecotones, respectively. Soil physiochemical properties exert more pronounced effects across all ecotone types than climate factors when predicting soil organic carbon in global aquatic-terrestrial ecotones. However, it is noteworthy that the primary drivers of soil organic carbon content vary according to ecotone type. For river and lake ecotones, nitrogen content emerges as the dominant factor regulating organic carbon content. Conversely, in reservoir ecotones, pH stands out as the primary driver for soil organic carbon content. Beyond nitrogen content and pH, the composition of silt and clay also proves pivotal in predicting soil organic carbon in aquatic-terrestrial ecotones. These findings underscore the critical determinants of soil organic carbon accumulation in global inland aquatic-terrestrial ecotones.