UNDERSTANDING PHOSPHORUS CHEMISTRY IN MANURE AND SOIL AND THEIR INTERACTIONS TO TREAT AND CONTROL PHOSPHORUS MOVEMENT IN THE ENVIRONMENT
Title: Determination of Phosphorus Retention and Flux in Soil
Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: November 7, 2006
Publication Date: July 10, 2008
Citation: Dao, T.H. 2008. Determination of Phosphorus Retention and Flux in Soil, pp. 65-70. In: Kovar, J. and Pierzinski, G. (eds). Methods of P Analysis for Soils, Sediments, Residuals, and Waters. Rev. Edition. Southern Region SERA-IEG 17. Souther Coop Series Bull.
Interpretive Summary: In soils and sediments, many physical, chemical, and biological processes jointly act to regulate and control the quantity of phosphorus that exists in the soil solution. Once liberated to the soil solution, soluble phosphorus fraction can be taken up and assimilated in plant tissues; it can also be re-immobilized in soil, or transported offsite to contaminate the surrounding environment. Although the primary mechanism for movement of phosphorus from agricultural soils is by erosion and in runoff from agricultural fields, specific instances of leaching and movement below ground have been reported. We described in this article an improved method for studying the retention and transport of reactive chemicals for other researchers and those interested in studying and developing management practices to mitigate phosphorus transport and inputs to surface water resources. Flow methods have been used to study water and movement of dissolved chemicals, the retention and release processes, for phosphorus in particular. Flow methods in soil columns are open systems where the chemical of interest and the reaction byproducts with soil and sediment particles are removed, minimizing re-adsorption, secondary side reactions, or inhibitory processes. Important aspects that can affect the retention and movement include the water flow rate, the exchange between solids and solution species, and chemical and biological transformations that can affect the amount of phosphorus in solution. Column studies can provide insights into the rate of phosphorus release and physical and chemical interactions that may influence mineralization of organic sources of nutrients such as animal manure and their transport in soil.
In soils and sediments, physicochemical and biological processes jointly act to control phosphorus (P) in solution. The soluble reactive P fraction is taken up by plants, sequestered in soil, or disperses in the surrounding environment. Although the primary mechanism for environmental transport of P from agricultural soils is by erosion and surface runoff, specific instances of subsurface movement have been reported. Agricultural P inputs to nearby surface waters have been associated with toxic algal blooms and the depletion of oxygen in aquatic systems. An improved understanding of P retention and transport mechanisms is needed to develop management practices to mitigate P transport and inputs to surface waters. Flow methods have also been used to study water and dissolved solute movement, the retention and desorption processes, for P in particular. Flow methods are open systems where solute and the reaction products with soil and sediment constituents are removed, minimizing re-adsorption, secondary precipitation reactions, or inhibition of desorption. Important parameters include water flux, chemical and hydrodynamic dispersion, sorption, exchange and desorption characteristics, and transformation rates coefficients. A flow displacement approach facilitates the simulation of the dynamic sorption- desorption, transformations, and transport of P in the soil and water system. Displacement studies provide insights in the kinetics of P release and physical and chemical non-equilibrium conditions that may influence nutrient mineralization and transport in soil.