Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/12/2007
Publication Date: 9/15/2007
Citation: Green, V.S., Dao, T.H., Stone Jr, G.N., Cavigelli, M.A., Baumhardt, R.L., Devine, T.E. 2007. Bioactive Phosphorus Loss in Simulated Runoff From a P-Enriched Soil Under Two Forage Management Systems. Soil Science. 172:721-732. Interpretive Summary: While runoff from agricultural fields contain more than inorganic phosphate derived from applied fertilizer or animal manure, information on the biological significance of the remaining phosphorus forms is limited. Previous studies have looked at total phosphorus content in runoff water and one of its components, soluble and reactive phosphorus, but have not focused on the dozen or more phosphorus-containing compounds that make up what has been referred to “soluble, unreactive phosphorus”. The reactive phosphorus is primarily inorganic phosphate. The remaining forms of P were labeled “unreactive” because they don’t react chemically in the routine laboratory techniques used to measure phosphorus, such as the molybdate reaction that yields an intense blue color upon reaction with phosphate. The neglected forms include phytate, polyphosphates, and organic phosphorus sorbed onto mineral or organic compounds in the runoff water. These compounds have been thought to be biologically unimportant and some are difficult to measure. We conducted a study to characterize the distribution of inorganic phosphate and enzyme-labile P forms in simulated runoff from an orchardgrass and a forage-type soybean crop systems. Using an enzymatic assay, we developed the biological availability of the “unreactive” organic P forms was measured. Runoff P forms were directly associated with soil available P fractions that involved biological enzymatic processes. The results also suggested that knowledge of the pattern of release is as important as the total P loss because management factors and production potentials of these forage systems can change the characteristics of the release to runoff, therefore potentially impacting the delivery of phosphorus to freshwater systems.
Technical Abstract: While runoff from manure phosphorus (P)-enriched soils contain more than the functionally-defined molybdate-reactive P, information on the biological significance of the remaining fraction is limited. A study was conducted to characterize concentration and mass distributions of inorganic phosphate and enzyme-labile P forms in simulated runoff from a Christiana silt loam under an orchardgrass and a forage-type soybean system, and determine the effect of crop management on that composition and relationships of runoff WEP fluxes with soil bioactive P indices. Concentration and mass distribution of dissolved and suspended P in runoff over time were log-normally distributed and were described by four parameters that defined the amplitude and asymmetry of the distributions. Enzyme-labile and total bioactive P forms were found in greater concentration and mass than WEP in runoff. Peak concentrations or mass loads were higher in runoff from soil amended with manure P and under the orchardgrass cover as the pasture system utilized less P than the more intensively-managed, high-producing forage-type soybean system. Parameters “c” and “d” of the concentration-time distribution showed a tendency of the manure P-treated soil to show a greater tailing of more elevated P concentration and mass loss and for a longer period of time during simulation. Cumulative mass loss of WEP were related to soil P fractions, with the highest correlation of WEP lost with the soil total bioactive P pool, followed by EDTA-exchangeable inorganic P, and Mehlich-3 P pools, suggesting that runoff P were directly associated with soil available P fractions that involved enzyme-mediated processes. The results also suggested that knowledge of the pattern of the log-normally distributed release is as an important factor as the total mass loss in runoff because management intensity and production potentials of these forage systems can alter the characteristics of the loss in runoff process.