Effects of Ditch Dredging on P and N Spiraling in the Delmarva Peninsula

Francirose Shigaki1, Peter Kleinman1, John Schmidt1, Andrew Sharpley2, Arthur Allen3

(1) USDA-ARS, University Park (2) Univ. of Arkansas, Fayetteville (3) Univ. of Maryland, Eastern Shore, Princess Anne

The Delmarva Peninsula abuts the Chesapeake Bay estuary and is considered a key source of nutrients to the Bay. Large concentrations of intensive poultry operations on the Peninsula coincide with extensively ditch drained fields, resulting in a high potential for nutrient loading by surface and sub-surface flow. Annual loads of total P from drainage ditches can exceed 40 kg ha–1, with much of the P in dissolved form. Stream nutrient dynamics are influenced by biotic and abiotic processes occurring in the watershed as well as within the stream. Knowledge of these processes, which contribute to in-stream nutrient uptake, is largely attributable to the concept of nutrient spiraling, a conceptual and empirical model of nutrient cycling in fluvial ecosystems. We sought to understand the effect of the widespread practice of dredging on P and N spiraling in ditches draining fields receiving poultry litter. Dredging is performed to remove sediments from ditches to ensure adequate for drainage and represents a severe ecological disturbance and may function to either increase or decrease nutrient loads. The sediment exposed by dredging may not have the same nutrient buffer capacity as the original sediment. Nutrient injections were performed to evaluate P and N spiraling variables as a key to identify the fate of agricultural P and N drainage ditches. With time after cleaning, P and N retention was diminished and ditch dredged sediments become a greatest nutrient contributor to the water bodies. Results of this research will target ditches for remedial management, and improve our ability to manage and model the fate of agricultural P.