Seasonal changes in soil phosphorus (P) availability are not yet known for many ecosystems. We report seasonal changes in several pools of soil phosphorus, including plant available P from the Mojave Desert and Colorado Plateau. In addition we show that cheatgrass changes soil P fractions in unexpected and ecologically significant ways. Monthly soil samples (0-10 cm) from four sites in Canyonlands National Park in southeast Utah, are analyzed for P with a modified Hedley P fractionation method. Labile P (plant available) peaks in spring and autumn with significant monthly variation. Surprisingly, HCl extractable P changes as well, with a pattern inverse to that of labile P. Each of these sites has considerable Bromus tectorum L. (cheatgrass) invasion. In a separate greenhouse study with four contrasting soils (0-10 cm) from the Mojave desert CA and NV, we grew cheatgrass for 100 days to test for soil P changes. Regardless of the soil type, cheatgrass causes large (2x) increases in labile P and large (90%) decreases in both NaOH and HCl extractable P. In addition, root biomass, root nitrogen (N) and P as well as shoot biomass and shoot N and P vary widely according to soil type. We propose that cheatgrass roots solubilize large amounts of soil phosphorus seasonally, from P pools that are considered unavailable to most plant species, and that this solubilized P becomes available for Broumus tectorum L. uptake. These results are discussed in the contexts of root exudates, mycorrhizae and root rhizosphere effects on desert soil P. Published in 88th Annual Meeting of The Ecological Society of America, volume M.S., in 2003.
