PHOSPHORUS MOBILITY IN A KARST LANDSCAPE UNDER PASTURE GRAZING SYSTEM

Authors: ALLOUSH, G - VIRGINIA TECH; Boyer, Douglas; Belesky, David; Halvorson, Jonathan

Submitted to: Agronomie Agriculture Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/3/2003
Publication Date: 10/9/2003
Citation: ALLOUSH, G.A., BOYER, D.G., BELESKY, D.P., HALVORSON, J.J. PHOSPHORUS MOBILITY IN A KARST LANDSCAPE UNDER PASTURE GRAZING SYSTEM. AGRONOMIE AGRICULTURE ENVIRONMENT. 2003. v.23. pp. 593-600.

Interpretive Summary: About 86 trillion gallons of water annually is transported from the Appalachian Region to the eastern metropolitan US and the Gulf of Mexico. About 18% of the Region is underlain by karst, while 30% of the receipts from agricultural operations are generated in the karst region. In karst landscape, pastures occur on shallow limestone-derived soil where sinkholes sserve as direct connections to ground water and subterranean streams. Management practices including grazing and application of poultry manure could create a build-up of nutrients such as phosphorus and degrade water quality. This makes identifying the source and forms of P contaminating ground water important. We fractionated soils from sinkholes under different intensities of management. Phosphorus concentrations were greatest in the surface soil layer, decreased with soil depth and increased toward the lowest part of the sinkhole. Phosphorus bound to plant decomposition products arising from the plant community or in manure from the grazers was about one-half of the total in the surface layer and decreased gradually with depth. At least one-half of the phosphorus reaching ground water was organically-bound. Our findings will help nutrient management specialists and pasture managers develop grazing practices that preserve soil and water quality. High quality water and soil resources benefit the producer through sustained production capability of the farm and the consumer by providing a high quality agricultural product while preserving environmental integrity.

Technical Abstract: Long-term grazing of permanent pasture on karst landscape could increase soil organic matter especially in the surface soil layers. Manure deposited by grazing cattle can influence P dynamics leading to buildup of soil P to levels that can enrich surface and sub-surface runoff water. Relatively thin soils and porous limestone bedrock in karst allows rapid transport of water and sediments, which facilitates rapid P movement. We investigate spatial distribution of P and P transport in pasture on sinkhole topography. Surface water runoff collectors and lysimeters were used to quantify amounts and forms of P leaching toward ground water. Water extractable P of soil samples, was determined to describe environmental P distribution with soil depth and slope. Special focus was placed on organically bound-P as the most important fraction influencing water quality. Grazing increased concentrations of soi lN, P and organic matter, especially in the uppermost layer compared with an ungrazed, woode sink. All forms of P increased in the pasture. Molybdate reactive P was greatest in the 0-2.5cm soil layer and significantly decreased with soil depth until it was virtually nonexistent. Percent organic P, increasing with soil depth and averaged about 90% of the total P detected in water- extracts of the deep soil horizon (10-20cm). The distribution of inorganic and organic P in total P of surface runoff water, lysimeters, and underground water courses seemed to confirm observations made on soil. We show that organic P is the major form of P moving into ground water from pasture. Characterizing dissolved organic P fractions in soil will improve understanding of P leaching mechanisms from pasture.