Characterization of soils beneath a Posidonia oceanica meadow

Abstract

The study of a 475cm core (spanning 4316 y BP) sampled in a Posidonia oceanica meadow (Portlligat Bay, NW Mediterranean) allowed us to make the first detailed description of the soil below this endemic seagrass. The sediments under P. oceanica (often referred to as mat or matte) are low density (average bulk density of 0.69gDWcm−3) marine soils mainly composed of siliciclastic (46%) and biogenic carbonated (46%) fine-grained sediments (particles <0.25mm constitute more than 60% of the total inorganic particles). They are also composed to a lesser extent by coarse organic matter (5% of COM; >1mm) and finer organic matter (3% of SOM; <1mm). The mat is heterogeneous, and has a high total organic matter (TOM) content in the upper layers (average of 32% in the top 50cm, after ca. 500years of burial), after which the layers become increasingly dominated by inorganic fractions (ranging from 20 to 1% of TOM in 50–475cm). The TOM content in the mat decreases exponentially at an overall rate of 0.0005y−1. P. oceanica sediments have been found to contain the highest areal stocks of TOM and organic C (194kgDWTOMm−2 and 79kgCorgm−2, respectively) out of all seagrasses. The average Corg refractory-burial rates were estimated to be 21gCorgm−2y−1. Carbon as carbonates accreted at a rate of 54gCcarbm−2y−1. For the mat thickness studied, the two C fractions yielded a total stock that is among the highest ever recorded in terrestrial and marine soils (282kgCTOTm−2). The mat was also found to trap large amounts of sediment (rate: 899gDWm−2y−1; stock: 3342kgDWm−2), and in particular muddy (silt and clay) fractions (rate: 120gDWm−2y−1; stock: 417kgDWm−2). The results obtained in this study provide sound additional proof of the valuable role P. oceanica plays in stabilizing coastal sediments and sequestering C, and also of its potential as a Holocene palaeorecord. The P. oceanica mats located at shallow depths (<2m) can be tentatively classified as a Limnic Subaquatic Histosols (Calcaric, Eutric) (World Reference Base for Soil Resources, 2007).