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United States Department of Agriculture

Agricultural Research Service

Title: Evaluation of Phosphorus Source Coefficients as Predictors of Runoff Phosphorus Concentrations

item Smith, Matthew
item White, Kathryn - Kate
item Coale, Frank

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2008
Publication Date: 1/6/2008
Publication URL:
Citation: Smith, M.C., White, J.W., Coale, F.J. 2008. Evaluation of Phosphorus Source Coefficients as Predictors of Runoff Phosphorus Concentrations. Journal of Environmental Quality. 44(3):742-753.

Interpretive Summary: Phosphorus source coefficients (PSC) for manures, composts, and other organic phosphorus (P) sources are indicators of P availability for transport in runoff from agricultural soils. They are an important parameter of the P Site Index (PSI) which is used in some states as part of comprehensive nutrient management planning. Different methods for determining PSCs for various organic P sources have been proposed and adopted in different states. Most involve relating the concentration of water soluble P (WS-P ) in the P source to soil WS-P or to P dissolved in runoff from surface-applied manures. When an organic P source is incorporated, source P equilibrates with the soil and these methods are not necessarily applicable. To overcome this limitation a PSC determination method that accounts for the manure P - soil P reaction by incubating the P source in soil has been proposed. Source coefficients for a given organic material are determined from the extractability of soil P relative to inorganic fertilizer P following two or eight weeks incubation. We conducted two parallel studies to assess the effectiveness of PSCs calculated by this method as predictors of P concentrations in runoff from simulated rainfall. Six manures (liquid dairy manure (separated and unseparated), digested dairy manure, dairy manure solids, poultry litter, and compost) were examined. The manures were incorporated in a Mattapex silt loam soil at a rate to supply 150 lb acre-1 plant available nitrogen, packed into shallow boxes, and placed under rainfall simulators. Runoff was collected at 1 day, two weeks and eight weeks after manure incorporation and analyzed for dissolve P, bioavailable P (the P immediately available to promote eutrophication; a combination of dissolved P and readily available sediment-bound P), and total P. The same manures were incorporated in the Mattapex soil at a constant P rate, incubated, sampled, and analyzed for WS-P, readily desorbable P, and Mehlich 3 P (M3-P)(an agronomic soil P test) for PSC calculation. All of the manures increased soil M3-P to the upper end of the optimum agronomic P range. Immediately following manure incorporation P transport was dominated by sediment P transport as P enriched soil and manure particles were eroded from the soil surface. The measured runoff P concentrations at two and eight weeks were well related to PSCs calculated from the two week incubation soil extractions. Desorbable P and M3-P proved to be the best predictors of dissolved and bioavailable P in runoff. In runoff at two weeks desorbable P proved to be the best predictor of both runoff P fractions, however, after eight weeks soil equilibration M3-P PSCs was the better predictor of these same fractions. The results of this study indicate that PSCs determined from soil incubations can effectively predict P concentrations in runoff under controlled conditions. More research is necessary to determine PSCs in a variety of soils and evaluate the resulting values with runoff P collected under field conditions. Overall, PSCs calculated from relative P extractability in soil incubations have potential as predictors of P availability for use in a PSI.

Technical Abstract: The objectives of this study were to determine Phosphorus Source Coefficients (PSC) for organic phosphorus (P) sources, and to examine the relationship between PSCs and P concentrations measured in simulated rainfall runoff. The PSC is an important parameter in the P Site Index (PSI). An incubation study was utilized to calculate PSCs based on the extractability of P from organic P sources (liquid dairy manure (separated and unseparated), digested dairy manure, dairy manure solids, poultry litter, and compost) relative to P from triple superphosphate fertilizer. Runoff phosphorus concentrations were regressed on the calculated PSCs. Immediately following treatment runoff dissolved reactive P (DR-P) and iron oxide strip P (FeO-P) were significantly related to soil water soluble P (WS-P) and FeO-P (R2=0.74*** and 0.74***, respectively). However, PSCs from the 14 day incubations were the best predictors of runoff P after 14 days soil equilibration in the runoff boxes. The values for FeO-P PSC ranged from 78% for compost to 28% for poultry litter and were significantly related to runoff DR-P (R2=0.80***) and FeO-P (R2=0.76***) during the 14 day runoff event. Mehlich 3 PSCs ranged from 59% for compost to 30% for unseparated dairy manure and were better predictors of DR-P and FeO-P during the 56 day event (R2=0.73*** and 0.65***, respectively). Throughout the study runoff total Kjeldahl P was better related to total sediment concentrations (R2=0.87***) than to the PSCs. The mean PSC for all treatments and extraction methods was 49%, similar to the value currently used in the Maryland PSI (50%) for comparable manures.

Last Modified: 06/26/2017
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