Phosphorus transformations in poultry litter-amended soils of the Atlantic coastal plain

Eutrophication is threatening water quality in Delaware's Inland Bays watershed, one of the largest aquatic ecosystems in the eastern USA and also the site of a highly concentrated poultry (Gallus gallus) industry. Since many (>85%) soils in this watershed are now high or excessive in P, a better understanding of P transformations in poultry litter-amended soils is needed. Our objectives were to determine the influence of poultry litter on P release from three soils from this watershed, on the amount and chemical forms of soil P, and on soil P sorption capacity. Phosphorus release from litter-amended soils was determined in a 110-d leached incubation study using three soils and two litter rates (18 and 36 Mg/ha). Phosphorus was separated into nonoccluded (NOC-P), occluded P (OC-P), and calcium bound P (Ca-P) by sequential fractionation. Changes in P sorption capacity were quantified by a P sorption index. Net soluble P (NSP) released from the litter-amended soils ranged from 1.1 to 15.0 mg/kg and was <4% of the total poultry litter P added. Most NSP was in the initial leachate. Soil test P (STP) was increased by an average of 167 and 279 mg/kg at the 18 and 36 Mg/ha rates (6.2 +/- 0.2 mg STP/Mg litter). Most of the P in the litter-amended soils (63-90%) was in the NOC-P and OC-P fractions. Phosphorus sorption index values decreased by 3 to 19% and 12 to 24% at the 18 and 36 Mg/ha rates. Future research should focus on the long-term fate and potential leachability of the P in these chemical fractions to determine whether animal waste management in this watershed should be based on P rather than N.