The change of aggregate fraction by sediment hardening affects nutrient cycling in water-plant-sediment system

The growing impact of climate change, coupled with the widespread expansion of dam construction, has significantly disrupted the natural hydrological patterns of river-connected lakes. This disruption has led to the emergence of a novel phenomenon known as sediment hardening. This process alters the aggregate fraction of sediments, yet its potential effects on biogeochemical cycling within aquatic ecosystems remain largely unexplored and poorly understood. In this study, we investigated the effects of sediment aggregates of varying sizes on plant functional traits, water quality parameters, sediment properties, enzyme activity, and microbial characteristics through controlled simulations. Additionally, we explored how changes in aggregate fraction due to sediment hardening might influence nutrient cycling within water-plant-sediment systems. Our findings indicate that small macroaggregates (0.25–2 mm) play a pivotal role in determining the growth and stoichiometric properties of submerged macrophytes. Notably, hardened sediment was associated with a reduction in root anchorage force, suggesting that sediment hardening may contribute to root anchorage failure. Furthermore, we found that small macroaggregates fostered the highest microbial diversity, with distinct differences in microbial community structures across various aggregate size fractions. The partial least squares model revealed that shifts in aggregate fraction due to sediment hardening not only directly influenced the release of nitrogen and phosphorus from sediments but also indirectly affected nutrient cycling by altering the belowground traits of submerged macrophytes. Our results underscore the critical role of sediment hardening in regulating nutrient dynamics within aquatic ecosystems, providing new insights into the ways sediment alterations impact river-connected lakes.