Climate and soil micro-organisms drive soil phosphorus fractions in coastal dune systems

The importance of soil phosphorus (P) is likely to increase in coming decades due to the growing atmospheric nitrogen (N) deposition originated by industrial and agricultural activities. We currently lack a proper understanding of the main drivers of soil P pools in coastal dunes, which rank among the most valued priority conservation areas worldwide. Here, we evaluated the joint effects of biotic (i.e. microbial abundance and richness, vegetation and cryptogams cover) and abiotic (i.e. pH and aridity) factors on labile, medium-lability and recalcitrant soil P pools across a wide aridity gradient in the Atlantic coast of the Iberian Peninsula. Climate determined the availability of medium-lability, recalcitrant and total P, but had a minor net effect on labile P, which was positively and significantly related to the presence of plants, mosses and lichens. Medium-lability P was significantly influenced by soil bacterial richness and abundance (positively and negatively, respectively). Our results suggest that micro-organisms transfer P from medium-lability pool to more labile one. At the same time, increases in bacterial richness associated to biofilms might be involved in the thickening of the medium-lability P pool in our climosequence. These bacterial-mediated transfers would confer resistance to the labile P pool under future climate change and uncover an important role of soil micro-organisms as modulators of the geochemical P cycle.