Simulation of Phosphorus Cycling in Semiarid Grasslands

Abstract

A simulation model of the phosphorus cycle in semiarid grasslands was developed and tested. When used with data sets for biotic and abiotic driving variables of the Pawnee (Colorado) and Matador (Saskatchewan) Sites, this model predicted plant and decomposer uptakes and turnover rates of the principal phosphorus compartments. Daily P requirements for plant and decomposer uptake are taken from pools of labile inorganic P in each soil layer. These pools are replenished mainly by mineralization of labile organic P as well as leaching of water—soluble forms from standing dead biomass and litter. Phosphorus solubility (recalculated daily from relationships to the labile inorganic pools), soil H 2 O content, and rates of diffusion of P through soil are the major controls on rates of uptake by the active fraction of the live root biomass. Model operation was found to be more sensitive to soil parameters than to plant parameters. Simulation results indicated rates of decomposer uptake 4—4x greater than plant uptake in semiarid grasslands. Simulated P concentrations in live plant tops were highly responsive to the pattern of seasonal rainfall which agreed well with published data. The most critical informational needs revealed by model development and operation were in the areas of activity and morphology of roots and the rates of mineralization of organic P as affected by soil depth.