Sediment Aggregate Structure Regulates Nutrient Cycling and Plant–Microbe Interactions in the Water–Plant–Sediment Continuum

The growing impact of climate change, coupled with the widespread expansion of dam construction, has seriously 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. Here, we investigated how varying sediment aggregate sizes—large macroaggregates (LMA, >2000 μm), small macroaggregates (SMA, 250–2000 μm), and microaggregates (MA, <250 μm)—along with sediment hardening, affect nutrient bioavailability, microbial community structure, and growth of the submerged macrophyte Vallisneria natans. Our findings demonstrate significant aggregate-dependent differences: microaggregate-rich sediments promoted high nutrient release but created anoxic stress that limited plant growth, whereas sediments dominated by stable large macroaggregates restricted nutrient bioavailability, causing nutrient limitation. Intermediate-sized aggregates (SMA) provided optimal conditions, balancing nutrient release, oxygen availability, and microbial diversity, thus enhancing macrophyte growth and nutrient uptake. Aggregate fractions also shaped microbial communities, with SMA supporting the highest bacterial richness and most balanced nutrient cycling. Partial least squares path modeling indicated aggregate structure influenced porewater chemistry, microbial metabolism, and plant performance through interconnected direct and indirect pathways. This study highlights that sediment physical properties are crucial regulators of ecological functions in lake ecosystems. Strategic management of sediment structure could effectively enhance nutrient retention, microbial diversity, and macrophyte health, offering valuable insights for lake restoration and eutrophication control.