Phosphorus Leaching in Soils: Effect of Soil Type, Soil Phosphorus Saturation, and Animal Manures Generated from Modified Diets.
Gurpal Toor, Univ of Arkansas, Biological & Agricultural Engineering, 203 Engineering Hall, Fayetteville, AR 72701, J. Thomas Sims, Univ of Delaware, Dept of Plant & Soil Sciences, Newark, DE 19717-1303, and Brian Haggard, USDA-ARS/Bio & Ag Eng. Dept, 203 Engineering Hall, Fayetteville, AR 72701.
Land application of animal manures is a cost-effective alternative to beneficially recycle manures. However, long-term addition of manures can result in application of large amounts of phosphorus (P) to soils thereby saturating the P sorption sites and increasing the risk of P leaching to waters. The greater P losses to fresh waters are of environmental and ecological concern as they can lead to decreased water quality, such as the incidence of harmful algal blooms. We now know that all soils do not possess the same risk of P leaching, as soil P saturation along with subsoil physical (macropore flow pathways i.e. root channels, soil cracks, worm holes) and chemical (Fe, Al) properties can affect the movement of P leached from topsoil horizons. The key to reduce P leaching from P saturated soils is by lowering soil P surpluses. One of the most promising options to reduce soil P build up is dietary modification, where use of dietary additives (e.g., phytase) for poultry and reductions in over-feeding of P for both poultry and dairy has been shown to decrease total P in manures by as much as 40%. Currently, we lack the understanding of the extent of P leaching from P saturated soils and soils that would become P saturated in few years (if future manure applications are continued) and how modifying diets affects the solubility and transport of P in these P saturated soils that often receive manure applications. This Information would help in identifying the “critical source areas” where P loss by leaching is of greatest concern and where P-based management practices should be implemented to protect and improve water quality. We investigated the risk of P leaching from three typical soils of Mid-Atlantic US that each had two P saturation levels (determined by Mehlich 3- P saturation ratio (PSR): optimum PSR: <0.10; environmental PSR: >0.15) by using intact soil columns (30 cm diameter, 50 cm deep) collected with an advanced tractor-mounted soil corer. Our objectives were to quantify the effect of soil type and soil P saturation and to evaluate the effect of dietary modifications of dairy and poultry on P leaching. Columns were irrigated at 2.5 cm/hour to apply equivalent of 50 mm of irrigation each week for a total of 16 weeks. In the first experiment, columns were leached for 8 weeks without application of treatments. Then, six treatments were established: control (no fertilizer or manure), superphosphate, two dairy manures generated from low (3.6 g/kg) and high (5.3 g/kg) P in diets, and two poultry manures generated from normal and reduced mineral P and phytase diets. Columns were amended with 85 kg total P/ha from superphosphate or manures and were irrigated for another 8 weeks. Leachate was collected for analyses of different forms of P. Concentrations of Dissolved Reactive P (DRP) in leachate were not significantly different between optimum and environmental PSR soil columns before superphosphate application but were significantly greater following superphosphate application for environmental PSR soil columns. Total P concentrations in leachate were higher from environmental PSR columns before superphosphate application, and increased by two- to five- folds after superphosphate application. Our results provide clear evidence of a greater risk of P leaching from high P saturated. For example, P leaching was greatest from the soils saturated with P (Matapeake silt loam>Pocomoke sandy loam) suggesting a need to reduce build up of soil P. While the risk of P leaching from least P saturated soil (Woodstown sandy loam) following application of P inputs is lower, which indicate the ability of this soil to retain applied P. The major forms of P (>60% of total) determined in the leachate included dissolved (both DRP and dissolved unreactive P) rather particulate (total particulate P) following superphosphate, dairy manures, and broiler litters application. Greatest concentrations of DRP were leached from superphosphate-amended soils than other treatments because of the greater solubility of P in superphosphate than other manures/litters. In the soils amended with low and high P dairy manures, P leaching losses were not significantly different. This highlights the importance of other manure chemical constituents (e.g., Ca, Mg) in modifying P leaching, which requires further investigation. Lower losses of all forms of P were occurred from phytase broiler litter-amended soils compared to normal broiler litter-amended soils. This shows that dietary modification for poultry involving use of phytase and reducing mineral P in diets was effective at reducing P leaching from soils. This may aid in successful adoption of these dietary management strategies to reduce total P contents in manures and decrease soil P build up thereby resulting in lower P losses through surface runoff and leaching to waters.