Phosphorus pools in tree and intercanopy microsites of a juniper-grass ecosystem

Gradients of soil-nutrient distribution between trees and intercanopy areas are common in many semiarid woodland ecosystems. To test if microsites under and between canopies influenced P pool distribution in a semiarid woodland dominated by one-seed juniper [Juniperus monosperma (Engelm.) Sarg.] and galleta grass [Hilaria jamesii (Torr.) Benth.], we compared inorganic, organic, and microbial P pools under trees and intercanopy areas of two Aridisols. Soils collected (5-15 cm depth) under eight tree canopies and in eight intercanopy areas from a Calciorthid and a Camborthid were subjected to a sequential P fractionation scheme. Soils and microsites were significant independent factors determining total soil P, which ranged from 814 μg P g^-1 soil (SE = 25) to 1123 μg P g^-1 soil (SE = 21). Resin P was significantly influenced by the interaction of soils with microsite. Organic hydroxide P was the largest organic P fraction and exceeded or equaled the amount of resin P. It differed significantly between the Calciorthid at 10.1 μg P g^-1 soil (SE = 1.0) and the Camborthid at 22.1 μg P g^-1 soil (SE = 1.6). Microsite and soil did not significantly affect microbial P, which ranged from 12.9 μg P g^-1 soil (SE = 2.1) to 17.0 μg P g^-1 soil (SE = 0.7). Nutrients and microbial activity are usually concentrated under canopies in semiarid and arid ecosystems. This research shows that P pools distribution in the studied ecosystem did not follow this general pattern, and that soils may be more important in determining P pool distribution than microsites.