Comparison of phosphorus distribution and cycling between adjacent native semidesert shrub and cultivated grass-dominated ecosystems

Summary P concentrations at several soil profile depths and for various plant parts were monitored over most of one year in adjacent native semidesert shrub and cultivated crested wheatgrass ecosystems. When multiplied by appropriate soil bulk densities and phytomass values, it was possible to calculate P mineralomasses in these two ecotessera. Knowledge of new to old growth ratios allowed estimation of P turnover. Although there was greater P in the vegetation of the native stand, the tissues lived longer and decomposed less rapidly than those in the grass stand that has been established in a portion of the original shrub dominated semidesert. The grass stand showed higher net primary productivity and had a much higher fraction of phytomass that is consumable by livestock. There was thus, at least a five-fold greater turnover of P in the tilled and seeded system. The introduced grasses are also withdrewing more P from the soil reserves. Whether nutrient export from the systems could lead to long-term declines in productivity will require further monitoring and experimental effort.