Biological and geochemical controls on phosphorus fractions in semiarid soils

This study examined the concentration of organic and inorganic phosphorus in surface soils of a Bouteloua gracilis-Bouteloua eriopoda grassland, and a Larrea tridentata shrubland, in the northern Chihuahuan Desert, New Mexico, U.S.A. In this desert where the grassland vegetation has a uniform spatial distribution and individual shrubs have a patchy spatial distribution across the landscape, vegetation strongly influences the distribution of soil nutrients. Most studies of soil phosphorus fractions in desert soils have focused primarily on inorganic P fractions and have demonstrated the importance of geochemical controls on soil P cycling. The research presented here addressed the question of whether organic phosphorus, determined by the presence of different vegetation types, also contributes to soil P cycling. Within soils of similar age, topography, parent material, and climatic regime, samples were collected under and between vegetation and analyzed for P fractions following a modified sequential fractionation scheme. Most soil inorganic P was found in the HCl- and cHCl-extractable forms in both the grassland and shrubland soils, indicating CaCO3 control over phosphorus availability. In contrast, most soil organic P was bound to Al and Fe minerals. Labile, plant-available P fractions summed to 9.5% of total P in the grassland and 6.1% in the shrubland. Organic P comprised 13.3% of the total phosphorus pool in the grassland and 12.0% in the shrubland. Our results show that the organic P pool may represent an important, yet often overlooked, source of P in semiarid ecosystems.