Eutrophication alters Si cycling and litter decomposition in wetlands

Anthropogenic eutrophication of wetlands may have a significant impact on the global biogeochemical silicon (Si) cycle, as Si filtering by wetland vegetation codetermines fluxes of Si towards the oceans. We experimentally investigated 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 litter quality and litter decomposition rates. Nutrient enrichment stimulated primary biomass production, which resulted 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 concentrations in all species, suggesting a plant-physiological response following the relief of nutrient stress. Moreover, competitive species (typical for eutrophic conditions) tended to accumulate less Si (per g DM) than slow-growing species (typical for nutrient-poor conditions). Finally, a negative correlation between litter Si concentrations and litter decomposition rates in nutrient-poor environments suggested an inhibiting effect of Si on decomposition. However, negative correlations between litter Si concentrations and litter macronutrient concentrations as well as positive correlations between litter Si concentrations and C:N and lignin:N ratios indicated a strong interdependence of Si with other litter quality parameters that determine decomposition. We conclude that stimulatory effects of eutrophication on total Si storage in wetland vegetation (following an increase in biomass production) need to be balanced with the plant-physiological response 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.