Sediment hardening and deposition regimes interact to shape nutrient dynamics and aquatic plant growth in shallow freshwater ecosystems

Sediment physical properties and deposition regimes significantly influence nutrient dynamics and aquatic plant performance in shallow freshwater ecosystems. However, it remains unclear how interactions between sediment hardening, deposition depth, and depositional material influence biogeochemical processes and plant responses, particularly under conditions simulating natural or anthropogenic sedimentation events. This study examined the interactive effects of sediment hardening (hardened vs. unhardened), deposition depth (0 cm, 2 cm, 5 cm), and deposition type (mud vs. sand) on phosphorus (P) and nitrogen (N) exchange, microbial activity, and growth of the submerged macrophyte <i>Vallisneria natans</i>. Deposition depth was identified as the primary factor, explaining approximately 63.6% of variation in plant traits and 36.3% of nutrient exchange. Hardened sediments significantly restricted nutrient diffusion, resulting in localized high phosphorus zones near the sediment–water interface and promoting ammonium accumulation in porewater. Conversely, fine, organic-rich mud deposition increased phosphorus diffusion and stimulated microbial activity. Sand deposition acted predominantly as a physical barrier, limiting phosphorus release. However, when sand was applied over hardened sediment, considerable phosphorus efflux (~0.15 µg cm⁻² s⁻¹) occurred, likely due to accumulated phosphorus in underlying oxidized sediments and limited phosphorus sorption capacity of the sand layer. Similarly, a 5 cm mud layer atop hardened sediment caused a shift from net NH₄⁺ uptake to net release (~0.18 µg cm⁻² s⁻¹), reflecting rapid reactivation of mineralization processes under anoxic conditions. Microbial extracellular enzyme activities, specifically L-leucine aminopeptidase (LAP), β-1,4-N-acetyl-glucosaminidase (NAG), and acid phosphatase (ACP), varied distinctly with sediment conditions, indicating shifts in microbial nutrient acquisition strategies associated with oxygen availability and nutrient stoichiometry. <i>Vallisneria natans</i> exhibited reduced growth and altered nutrient allocation in hardened, nutrient-poor sediments. Deeper deposition further limited nutrient uptake and biomass accumulation, suggesting compounded stress from physical isolation and reduced nutrient access. Notably, interactions among sediment hardening, deposition depth, and deposition type often amplified negative effects on plant performance and sediment biogeochemistry. By linking physical sediment structure with biogeochemical fluxes and biological responses, our findings provide new mechanistic insights into how both natural deposition processes and sediment management strategies shape nutrient cycling and aquatic plant health in shallow freshwater ecosystems.