Spatial distribution of phosphorus speciation in marsh sediments along a hydrologic gradient in a subtropical estuarine wetland, China

In May and August 2013, a 360-m long transect (from a high to a middle tidal flat) was laid out in the Min River estuarine marsh to study the distribution of total phosphorus (TP) and its fractions (i.e., organic P (Org P), inorganic P (IP), aluminum-bound P (Al–P), iron-bound P (Fe–P), occluded P (O–P), and calcium-bound P (Ca–P)). The results showed that TP concentrations of the sediments ranged from 338 to 846 mg kg−1 (average 664 mg kg−1) in May and from 353 to 932 mg kg−1 (average 657 mg kg−1) in August. IP dominated the P fractions (accounting for 57–81% of TP) and was mainly composed of Fe–P (38%), O–P (30%), and Ca–P (25%). The TP, IP and Fe–P concentrations fluctuated along the hydrologic gradient during both measurement periods (except for the upper 10-cm sediments in August). Meanwhile, their concentrations decreased with depth (0–50 cm), but vertical variation declined in the middle tidal flat. A higher concentration of Org P was observed in the upper 20-cm sediments of the high tidal flat. The concentrations of Ca–P and Al–P increased from the high tidal flat to the middle tidal flat, but there was no significant difference between sediment layers (P > 0.05). The O–P concentration was significantly higher in Phragmites australis sediments compared with Cyperus malaccensis sediments (P < 0.05). Based on the space-for-time substitution rule, we predict that sea-level change will likely alter the composition and vertical distribution of TP in the Min River estuarine sediments.

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