We included in this model aridity (Ar; composite variable formed from Ar and Ar2), percentage of plant cover (Plant), percentage of clay (Clay), spatial position (Spatial; composite variable formed from distance from Equator (De) and longitude (Lon)), activity of phosphatase, organic matter component (OMC; first component from a PCA conducted with organic C (OC) and total N (TN)) and total P. We built our structural equation model by taking into account all these relationship, as explained in Methods. There are some differences between the a-priori model and the final model structures owing to removal of paths with coefficients close to zero (Fig. 2). Hexagons are composite variables30. Squares are observable variables.