Vegetation and microbial indicators of nutrient status: Testing their consistency and sufficiency in restored calcareous wetlands

Various indicators have been used to diagnose nutrient status of an ecosystem, but their consistency among different ecosystem components (e.g., soil microbes and plants) remains rarely examined. In this study, we selected three sites with a gradient of phosphorus (P) concentrations, i.e., two P-enriched restored sites and an oligotrophic, P-limited reference site in the Hole-in-the-Donut of Florida Everglades, USA. Three sets of indicators for plants, soils microbes, and periphyton were measured in February, 2010. For vegetation indicators, TN:TP ratio indicates P-limitation but N use and resorption efficiency suggest N limitation at the two restored sites. For soil microbial indicators, N- and P-acquiring enzyme activities (i.e., leucine aminopeptidase and alkaline phosphatase) indicate N and P limitation at the restored sites and the reference sites, respectively. For periphyton indicators, significantly higher nitrogenase activities suggest N limitation at the restored sites. Overall, soil microbial and periphyton indicators consistently showed N limitation at the two restored sites. Furthermore, the microbial enzyme activities were significantly correlated with soil/periphyton chemical properties (e.g., soil extractable inorganic and organic N and P, and TN:TP ratio of soil and periphyton) and were more powerful in diagnosing the nutrient status. The inconsistency between vegetation-based indicators and microbial indicators not only suggests the insufficiency of a single indicator, but also indicates the heterogeneity in nutrient limitation of different components of an ecosystem. It is important to evaluate those indicators in order to give implications for restoration, in particular in those ecosystems that are sensitive to even small changes of nutrient availability.