Jeq50332 1151..1159

Manure water-extractable phosphorus (WEP) data are used in indices and models to assess P transport in runoff. Methods to measure WEP vary widely, often without understanding the effect on howmuch P is extracted.We conductedwater extractions on five dairy, swine, and poultry manures to assess single and sequential extractions, drying manures, solution to solid (cm g) extraction ratios, andPdetermination method. We found little difference in WEP of single or sequential extractions. Increasing extraction ratio from 10:1 to 250:1 resulted in more WEP recovered, but in a diminishing fashion so that ratios of 200:1 and 250:1 were not significantly different. Patterns of increased WEP with extraction ratio varied with manure type, presence of bedding material, and drying treatment. Fresh and air-dried manures had similar patterns, but differed substantially from oven-dried (90 C) manures. The differential effect of oven-drying on WEP was greatest for dairyandpoultrymanure,and less for swinemanure.Weanalyzedwater extracts colorimetrically before and after digestion, to examine the potential effect of P determination by inductively coupled plasma (ICP) spectroscopy. Digested extracts always contained more P. For manures withbedding, dryingdecreased thedifference inPmeasuredbefore and after digestion. The opposite was true for manures without bedding. Results highlight the influence of methodology on manure WEP measurement and caution needed when comparing data across studies using different WEP methods. Overall, our results point to a need for a standard manure water extraction method. N pollution of fresh surface waters by P is a water quality concern because it contributes to accelerated eutrophication and subsequently limited water use for drinking, recreation, and industry (Carpenter et al., 1998). Transfer of P from agricultural soils in surface runoff is a dominant transport mechanism in many situations (Sharpley et al., 2003). Soil and plant material contribute P to runoff, but their effect can be overwhelmed by P release from recently surfaceapplied manures (Eghball and Gilley, 1999; Kleinman and Sharpley, 2003; Moore et al., 2000). The quantity of water-extractable phosphorus (WEP) in surface-applied manures is an important factor controlling dissolved P concentrations in runoff (DeLaune et al., 2004a; Haggard et al., 2005; Kleinman et al., 2002b). Most studies have demonstrated this effect through regressions of WEP in surface-applied manure, typically expressed as a concentration (g kg) or mass (kg ha), and concentrations (mg L) of dissolved P in runoff (typically P in a runoff sample passing through a 0.45-mm filter) (Haggard et al., 2005; Vadas et al., 2004b). Therefore, WEP is seen as a key indicator of the potential for surface-applied manure to supply dissolved P to runoff (Kleinman et al., 2006), and manureWEP data are increasingly being used to guide management decisions. For example, the Arkansas P Index for pastures relies heavily on manure WEP surface-applied to fields to determine potential P transport in runoff (DeLaune et al., 2004b). Other P site assessment indices similarly propose to use manure WEP data, although to a lesser extent (Coale and Elliott, 2004).ManureWEP also plays a key role in improving the ability of fate-and-transport models to estimate P in runoff from surface-applied manures (Vadas et al., 2005). Manure WEP is generally estimated by shaking a small, representative quantity of manure with water and measuring P released into the solution. The concept is simple, but many variations have been used, often without acknowledgment or thorough understanding of how a specific method may affect the ultimate estimate of WEP or interpretation of results. Methods vary mostly in whether or not manures are dried or fresh (see references in Table 1), the use of single or sequential extractions (Dou et al., 2000, 2002), the solution to solids extraction ratio and times of shaking used during extraction (Kleinman et al., 2002b), and how P in the extracts is analyzed (Wolf et al., 2005). Because these variables may affect the quantity of manure P extracted and thus deemedWEP, methodology is critical to applications such as the Arkansas Pasture P Index, which is based on a specific method of estimating manure WEP (Self-Davis and Moore, 2000). Variations to this method that change WEP estimates will be a source of error in P Index results. Given the growing importance of WEP data in environmental interpretation of manure management, our objective in this study was to investigate the effects of variations in methodology on estimating WEP in animal manures. Key variables that we examined include the use of single or sequential extractions, method of drying manures before analysis, the solids to solution ratio used during extraction, and how P in extracts is analyzed. MATERIALS AND METHODS Manure Collection and Water-Extractable Phosphorus Analysis We collected five manures for P analysis that represented a range of livestock species, dry matter, and bedding contents P.A. Vadas, USDA-ARS, U.S. Dairy Forage Research Center, 1925 LindenDriveWest,Madison,WI 53706. P.J.A.Kleinman,USDA-ARS, Pasture Systems andWatershedManagement Research Unit, Building 3702, Curtin Road, University Park, PA 16802-3702. Received 1 Sept. 2005. *Corresponding author ([email protected]). Published in J. Environ. Qual. 35:1151–1159 (2006). Technical Reports: Waste Management doi:10.2134/jeq2005.0332 a ASA, CSSA, SSSA 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: ICP, phosphorus analysis by inductively coupled plasma spectroscopy; TP, total phosphorus in manures; WEP, manure waterextractablephosphorus;WEPI, inorganicmanurewater-extractablephosphorusmeasured colorimetricallywithout digestion;WEPT, totalmanure water-extractable phosphorus measured colorimetrically with digestion. R e p ro d u c e d fr o m J o u rn a l o f E n v ir o n m e n ta l Q u a lit y . P u b lis h e d b y A S A , C S S A , a n d S S S A . A ll c o p y ri g h ts re s e rv e d . 1151 Published online May 31, 2006