Plant–microbe interactions underpin contrasting enzymatic responses to flooding intensity variation in the cascade reservoir riparian areas

Abstract

Periodic anoxia, induced by flooding and erosion, has been regarded as a protective mechanism for soil organic carbon (SOC) by limiting microbial decomposition. However, this effect was not observed uniformly across riparian zones. This study distinguishes S-type from Non-S-type riparian areas, where the former retains SOC through microbial limitation, while the latter experiences accelerated SOC loss due to high reducible metals and organic carbon. To explore these dynamics, field experiments were conducted across 19 plots in the riparian areas of cascade reservoirs along the Jinsha River, China. In older reservoirs with prolonged flooding, phenol oxidase activity was suppressed (12 years: 4.033 ± 0.240 μmol g⁻¹ h⁻¹; 13 years: 5.442 ± 0.199 μmol g⁻¹ h⁻¹) compared to a 5-year-old reservoir (8.430 ± 0.340 μmol g⁻¹ h⁻¹). Non-S-type riparian areas exposed to high flooding intensity (FI) showed significantly higher phenol oxidase activity (high FI: 6.336 ± 0.236 μmol g⁻¹ h⁻¹ vs. low FI: 3.801 ± 0.181 μmol g⁻¹ h⁻¹), along with reduced lignin-derived compounds. These Non-S-type riparian areas exhibited higher microbial diversity, dominated by r-strategist taxa (e.g., Proteobacteria), enhanced bacterial connectivity, and hosted three fungal guilds acting as network connectors—traits absent in S-type areas. Furthermore, microbial communities in Non-S-type riparian zones showed structural stability through iron-reducing and phenol oxidation-related carbon metabolism. Mantel's test analysis showed that the weak correlation between lignin-derived compounds and phenol oxidase-producing microbes dynamics in these areas suggested that lignin-degrading taxa, resilient to hydrological perturbations (S-type: P < 0.05, Non-S-type: P > 0.05), mitigate the impact of lignin on carbon metabolism. These findings disclosed the presence of distinct microbial resilience mechanisms in response to hydrological extremes, highlighting the capacity of Non-S-type zones to maintain ecological functions under fluctuating environmental conditions.