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The role of organic acid exudates in liberating phosphorus from seagrass‐vegetated carbonate sediments

Sediment‐bound phosphorus (P) is a potential nutrient source for P‐limited seagrasses inhabiting carbonate sediments. We explored the role of organic acid (OA) exudation by seagrasses in liberating mineral P from carbonate sediments. Organic acids can act to increase available P by dissolving carbonate sediment, competing with P for binding sites and complexing dissolution end products, and also by fueling microbial processes that change pore‐water pH. We used dialysis tubing placed around individual roots in situ to quantify dissolved species immediately adjacent to roots (root zone) and compared these to bulk pore‐water concentrations in vegetated and nonvegetated sediments. Total OA concentrations were highest in the root zone (29.8 ± 1.8 [mol L−1) compared to bulk measures of 15.5 ± 1.9 and 7.5 ± 0.6 [mol L−1 in vegetated and nonvegetated sediments, respectively. Phosphate concentrations were also highest in the root zone and were linearly related to OA concentrations (R2 = 0.63). Organic acid concentrations increased along a seagrass productivity gradient, and ratios of OA concentrations to productivity showed a significant response to a gradient in P‐limitation of seagrasses. Organic acid concentrations found in and around roots, compared to those found in bulk sediment measures, indicate that seagrasses are a significant source of OA. Sampling at small spatial scales (mm) immediately adjacent to the roots is critical, because bulk sediment pore‐water measures did not capture the observed fluctuations caused by the rapid reaction and consumption of OA in the sediment. Root‐zone processes can liberate considerable quantities of P, and OA exudates likely contribute significantly to the success of T. testudinum in P‐limited environments.