TECHNICAL ARTICLESPHOSPHORUS FRACTIONS AND DYNAMICS AMONG SOIL AGGREGATE SIZE CLASSES OF ORGANIC AND CONVENTIONAL CROPPING SYSTEMSSteven Green, V.1,2; Dao, Thanh H.3; Cavigelli, Michel A.1; Flanagan, Dennis C.4Author Information 1USDA-ARS, Sustainable Agricultural Systems Laboratory, Beltsville, MD. Dr. Green is corresponding author. E-mail: email@example.com 2College of Agriculture, Arkansas State University, State University, AR. 3USDA-ARS, Environmental Management and By-product Utilization Laboratory, Beltsville, MD. 4USDA-ARS, National Soil Erosion Research Laboratory, West Lafayette, IN. Received Dec. 30, 2005; accepted June 1, 2006. Soil Science: November 2006 - Volume 171 - Issue 11 - p 874-885 doi: 10.1097/01.ss.0000228055.92839.53 Buy Metrics Abstract Particulate nutrient transport from agricultural fields contributes to water quality degradation; however, insufficient data exist about potential bioactive P transport from erosion. Traditional measures of soil P such as total P or soil test P do not adequately assess the risk of water quality degradation due to particulate P because these measures do not necessarily reflect soil P bioactivity. To better understand the risk of water quality degradation due to sediment-associated P, we quantified the bioactive P fractions in five aggregate size classes from conventional no-till and chisel-till cropping systems as well as a tilled organic cropping system. Cropping system was not related to the concentration of bioactive P fractions in whole soils. However, aggregate size was related to bioactive P fraction concentrations. In general, macroaggregates had greater concentrations of bioactive P than did microaggregates and silt- and clay-sized particles across all cropping systems; the less than 0.053 mm aggregate size class had the lowest concentrations of bioactive P. To better understand the dynamics of these bioactive P fractions over time, we conducted an incubation study on the aggregates of the no-till system and quantified the bioactive P changes over time. Water-extractable P and complexed inorganic P concentrations did not change significantly during the incubation. However, complexed organic P concentrations increased up to 2-fold during the 56-day incubation. Using soil bioactive P fraction measurements from easily erodible aggregate sizes should aid in water quality degradation risk assessments. However, our research demonstrates the need to quantify the parameters that affect the dynamics of bioactive P fractions. © 2006 Lippincott Williams & Wilkins, Inc.