COS 60-2 - Water use and nutrient retention in fertilized Black and Red Mangroves of Southwest Florida

Background/Question/Methods

Eutrophication is a major threat to mangroves, exposing these ecosystems to an excess surge of nutrients from either natural or human-influenced events. Besides causing water quality issues, this phenomenon potentially alters ecosystem processes and tree physiology. In order to understand how increased nutrients affect mangrove water use on a whole forest scale, we fertilized red (Rhizophora mangle L.) and black (Avicennia germinans [L.] L.) mangrove plots with inorganic forms of nitrogen (N) and phosphorus (P). Sap flow data were collected over two growing seasons and one dry season, and used to model outer sapwood water use. Comparisons were made between water use in trees fertilized with N or P vs. control trees. We additionally tested the spatial and temporal efficacy of solid-form fertilization methodology in a tidally restricted site. Porewater data were collected from 0.5, 1.0, and 1.5 m from fertilization cores for 6.5 months and analyzed for inorganic nutrient concentrations.

Results/Conclusions

While the concentration of soil porewater N (NH₃⁺) and P (PO₄^-) decreased from 0.5 to 1.0 m from the fertilization microsite, there was a weak effect of time on nutrient concentration, indicating that fertilization methods described in the literature should be sufficient in fertilization studies over a 6-month interval. Average daily maximum sap flow rates showed that N fertilization caused an increase in average daily maximum rates of water transport through the outer black mangrove sapwood (5, 15mm), even during the dry season. Additionally, daily water use modeled in outer black mangrove sapwood (5, 15 mm) during the growing season was higher in both N and P fertilized trees compared to control plot trees. However, since red mangroves did not respond to either N or P fertilization with increased water use patterns by sapwood depth (5, 15 mm), this species may be better equipped than assumed to face the challenges of excess nutrients at the individual tree scale, at least among trees that have received fairly high P loading rates from agricultural drainage over the previous decade.