Characterization of soils beneath a Posidonia oceanica meadow

The study of a 475 cm 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 0.69 g DW cm− 3) marine soils mainly composed of siliciclastic (46%) and biogenic carbonated (46%) fine-grained sediments (particles < 0.25 mm constitute more than 60% of the total inorganic particles). They are also composed to a lesser extent by coarse organic matter (5% of COM; > 1 mm) and finer organic matter (3% of SOM; < 1 mm). The mat is heterogeneous, and has a high total organic matter (TOM) content in the upper layers (average of 32% in the top 50 cm, after ca. 500 years of burial), after which the layers become increasingly dominated by inorganic fractions (ranging from 20 to 1% of TOM in 50–475 cm). The TOM content in the mat decreases exponentially at an overall rate of 0.0005 y− 1. P. oceanica sediments have been found to contain the highest areal stocks of TOM and organic C (194 kg DW TOM m− 2 and 79 kg Corg m− 2, respectively) out of all seagrasses. The average Corg refractory-burial rates were estimated to be 21 g Corg m− 2 y− 1. Carbon as carbonates accreted at a rate of 54 g Ccarb m− 2 y− 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 (282 kg CTOT m− 2). The mat was also found to trap large amounts of sediment (rate: 899 g DW m− 2 y− 1; stock: 3342 kg DW m− 2), and in particular muddy (silt and clay) fractions (rate: 120 g DW m− 2 y− 1; stock: 417 kg DW m− 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 (< 2 m) can be tentatively classified as a Limnic Subaquatic Histosols (Calcaric, Eutric) (World Reference Base for Soil Resources, 2007).

► The first thorough description of a long continuous P. oceanica mat profile.
► The P. oceanica mat can be classified as a Limnic Subaquatic Histosols.
► P. oceanica soils can play a significant role in the global carbon cycle.
► The P. oceanica meadows can play a significant role in sediment retention.
► The P. oceanica mat is a privileged Holocene palaeorecord.