Phosphorus and cover crop management practices affect phosphorus speciation in soils and eroded sediments

Abstract

Agricultural runoff often contains P in dissolved and sediment-bound forms, decreasing surface water quality. No-till and cover cropping conservation practices have been recommended for reducing erosion and nutrient loss from cropping systems. The overall aims of this study were to characterize and evaluate the effects of fertilizer (placement and source) and cover crop management on P speciation in surface runoff sediments and source soil. In 2014, a field-scale experiment was established in a no-till, corn (Zea mays L.)–soybean (Glycine max L.) cropping system with two cover crop treatments (with and without a winter crop; winter wheat [Triticum aestivum L.], rapeseed [Brassica napus L.], hairy vetch [Vicia villosa Roth], winter triticale [×Triticosecale Wittm.], and cereal rye [Secale cereale L.]) and three P fertilizer management treatments (no P, fall broadcast diammonium phosphate, and spring subsurface injected ammonium polyphosphate). Phosphorus fractionation in the source soil collected in the fall of 2019 and sediment samples collected throughout 2020 were analyzed using a modified sequential P extraction method to evaluate the cumulative effects of imposing the treatment factors over 5 years. The direct P speciation was done using X-ray absorption near edge structure spectroscopy. The indirect P speciation (fractionation) results showed that the management practices affected the exchangeable, organic matter-associated, and Fe-bound P fractions in sediments and the exchangeable and residual fractions in source soil. Direct P speciation results showed a depletion of Fe-associated P in soil and sediment from cover crop treatment, suggesting that Fe-associated P species were affected by cover crops. Changes in soil and runoff sediment P speciation would change the proportions and forms of soluble and particulate P in runoff sediments and may influence P bioavailability in aquatic ecosystems. Developing P fertilizer and cropping system management options with an understanding of soil P transformations helps maintain environmental sustainability.