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Despite their small size, vernal pools are hotspots for biogeochemical processes and biodiversity. To better understand their functioning, we studied the influence of vegetation (Sphagnum palustre, Molinia caerulea (L.) Moench) on organic-matter (OM) decomposition in soils, during periods of drought. OM decomposition rates were quantified by using a cotton-strip assay and field mesocosms. Decomposition rates were slower in the water-logged soils because hypoxic conditions decreased microbial activity. The vegetation type is also shown here to be related to decomposition rates. Indeed, the combined effect of Molinia caerulea and Sphagnum palustre reduced them compared to the presence of Sphagnum palustre alone or bare soil. We hypothesize that M. caerulea may have negative impacts on the microorganisms through a negative priming effect or allelopathic interactions. [ABSTRACT FROM AUTHOR]