Increased flood frequency altered carbon fluxes via modifying plant and soil properties in a riparian grassland

Riparian ecosystems are hotspots for carbon dioxide and methane emissions, contributing significantly to global carbon budgets and ecosystem sustainability. Under global warming, increasing flood frequency is causing substantial changes in riparian plant-soil systems, with consequent shifts in their carbon dynamics. However, little is known about the effects of altered flood frequencies on such riparian plant-soil-carbon flux interactions. In this study we aimed to address this knowledge gap by using a combination of remotely sensed imagery and field-based monitoring approaches. Field campaigns for plant, soil and gaseous samplings were initially conducted in the minimally flooded period and again resumed in the annually flooded period. Results showed that the study area transitioned from an annually flooded period (1984-2006) to a minimally flooded period (2007-2018) and then back to an annual flooded period once again (2019-2023). Increased flood frequency altered plant composition and diversity, shifting the community towards annual species that are better adapted to flood disturbance. Plant total carbon increased from 38.2% to 44.0%, whilst soil total nitrogen decreased from 0.18% to 0.05%. These shifts increased ecosystem respiration by 85%, but not soil respiration, indicating autotrophic respiration drove the net rise. While methane production showed minimal response to flood frequency, a pronounced methane emission pulse (a 30-fold increase) was observed during active inundation. Our findings reveal flood frequency changes drive riparian carbon dynamics through synergistic vegetation-soil feedbacks, critically advancing the understanding of ecosystem responses to climate-amplified hydrological extremes. These findings provide valuable insights for flood management, ecosystem sustainability, and climate adaptation strategies. • Effects of altered flood frequency on riparian plant-soil and carbon dynamics were quantified. • A ‘regular-irregular-regular’ flooding pattern was identified between 1984 and 2023. • Flooding enhances plant diversity and shifts community composition towards annual species. • Flood-induced plant changes indirectly increased ecosystem respiration. • Methane emission altered only during inundation and was determined by methanogens.