Effect of tree canopy on the spatial distribution of soil nutrients in a Mediterranean Dehesa

In arid and semi-arid ecosystems, isolated trees have an important effect on soil properties, which can determine the structure and function of the herbaceous and soil fauna communities in these types of ecosystems. The effect of the presence of a tree canopy on the spatial properties of soil nutrients were studied in this work. The hypothesis stated that the effect of the tree on the soil will be different for nutrients with varied biogeochemical characteristics. Nutrients such as N, which is retained in the ecosystem by biological mechanisms, must have a spatial distribution that is different from P and K, involved in geochemical processes (they are part of soil minerals), and in turn have a different distribution from non essential elements for plants such as Na and Li. To test this hypothesis, soil samples were taken under and around a tree in a Mediterranean Dehesa ecosystem, characterized by the presence of isolated trees within a herbaceous matrix. The samples were taken every 2 m and then at 50 cm intervals in a regular grid of 20 x 20 m around a holm oak (Quercus ilex), in order to analyse the spatial properties, using geostatistical techniques. All nutrients showed different patterns of spatial distribution. Mineral-N (NH4+NO3), like soil organic matter, presented higher concentrations which coincided with the tree canopy. The highest P concentrations occupied a space which was beyond the canopy projection. Potassium showed a lower spatial structure, with high concentrations which coincided with the position of the holm oak trunk, while Na and Li showed a sparse spatial structure and their distribution seems to be independent of the tree position. These results support the hypothesis that the presence of a tree differentially affects the spatial distribution of the various nutrients, depending on their biogeochemical characteristics.