P limits PPN in both aquatic and terrestrial ecosystems. Indeed the major P pool in terrestrial ecosystems is the soil, which may not be readily accessible to plants. Extremely low P concentration has been reported in Cuatro Cienegas, a region of the Chihuahuan Desert. However this region shows very high plant species richness. We studied plant species richness and plant species identity as correlates of inorganic P pools in the soil. We used a sequential fractionation procedure (Hedley's procedure) to obtain separate soil inorganic fractions which vary in terms of their availability to growing plants. Thus, we used regression tree models to explore relationships between different fractions of soil P and attributes of the plant species community. We analyzed 10 sites of similar parental material of shrub land and grassland.

Results/Conclusions

We found a total of 235 µg P g^-1 in the soil, out of which 11 % was in a labile form, and almost 86 % was removed from apatite-type minerals and occluded P in the shrub land. In grassland however, we found a total of 36 µg P g^-1 in the soil, out of which 2 % was in a labile form and 97 % was removed from apatite-type minerals and occluded P. Plant abundance was the factor more closely related with labile P fractions in both shrub land and grassland. Also, presence of Jatropha dioica Cerv. Var. Gramineae Mc Vaugh (EUPHORBIACEAE) was partly related to a fraction of labile P in the shrub land. Plant species richness was related with the occluded P fraction in the shrub land, and the presence of one species, Sporobolus airoides (Torrey) Torrey (POACEAE), was related with the occluded P fraction in the grassland. These results suggest different dynamics of the P transformation in the soil related with the plant community. In this sense, a "niche differentiation" hypothesis could be a mechanism that explains the amounts of inorganic P in the soil of the shrub land, while in grassland a "functional type" hypothesis could be more important for that. It is possible that water in the ground controls the importance of plant species richness and plant species identity for the same process of ecosystem functioning, P transformation in the soil in this case.