Eutrophication alters Si cycling and litter decomposition in wetlands

Anthropogenic eutroph ication of wetlands may have a significant impact on the global biogeochem- ical silicon (Si) cycle, as Si fi ltering by wetland vegetation codetermines fluxes of Si towards the oceans. We experimentally investigat ed how macronutrient (NPK) enrichment alters total Si storage and Si stoichiometry in litter from six wetland species of Carex , which we related to other parameters of litte r quality and litter decompo- sition rates. Nutrient enrichment stimulated primary biomass production, which res ulted in an increased total Si storage in plants. However, this eutrophication- induced stimulatory effect on Si fixation in plant biomass was counterbalanced by consistently lower (up to 50 % reduction) litter Si con centrations in all species, suggest- ing a plant-physiological res ponse following the relief of nutrient stress. Moreover, com petitive species (typical for eutrophic conditions) tende d to accumulate less Si (per g DM) than slow-growing species (typical for nutrient-poor conditions). Finally, a negativ e correlation between litter Si concentrations and litter decomposition rates in nutrient-poor environments s uggested an inhibiting effect of Si on decomposition. However, negative correlations between litter Si concentrations and litter macronutrient concentrations as well as po sitive correlations between litter Si concentrations and C:N and lignin:N ratios indicated a strong interdep endence of Si with other litter quality parameters that de termine decomposition. We conclude that stimulatory effects of eutrophication on total Si storage in wetland vegetation (following an increase in biomass produc tion) need to be balanced with the plant-physiological re sponse of lower tissue Si concentrations. We argue that rates of Si cycling are likely to be altered through shifts in litter quality and decomposition rates.