Divergent seasonal enzyme dynamics reveal functional shifts in the Jingjiang River floodplain

Seasonal enzyme dynamics in floodplain soils reflect shifts in hydrological regime, yet drivers shaping the magnitude of these responses remain poorly quantified in regulated river systems. We characterized seasonal enzyme dynamics in the Jingjiang River floodplain (middle Yangtze River, China), comparing high-water (August 2024) and low-water (November 2024) periods across eight sites (n = 48). Results showed divergent responses: urease decreased 89.3%, invertase 39.0%, and alkaline phosphatase 17.2%, while catalase increased 104.7% (all p < 0.05). Soil moisture declined 39.1% and pH increased from 7.45 to 8.44. The pH-urease correlation (ρ = -0.877) exceeded the moisture-urease correlation (ρ = 0.576), a pattern consistent with pH being a prominent factor in the observed urease suppression. Initial soil organic carbon (SOC) was associated with phosphatase response magnitude at the site level (r = 0.74, p < 0.05, n = 8): high-SOC sites (>7 g kg^-1) exhibited a mean phosphatase increase (+34.7%), whereas low-SOC sites declined (-57.1%). Collectively, these findings provide a baseline for understanding seasonal biogeochemical dynamics in dam-regulated floodplains and generate the hypothesis that soil organic carbon content may serve as a practical indicator for identifying sites with greater functional stability across hydrological states.