Dynamics of phosphorus transfers from heavily manured Coastal Plain soils to drainage ditches

Understanding the dynamics of phosphorus (P) transport in agricultural drainage ditches is essential to their improved management for water quality protection. Seven ditches draining soils with a 20+ year history of receiving poultry litter were monitored: two for five years and five for one year. Ditches receiving runoff from point sources (e.g., barns) exported 4.3 to 25.3 kg total P ha^-1 (3.8 to 22.6 lb total P ac^-1) from 2005 to 2006, while ditches draining areas with only nonpoint source contributions exported 2.6 to 4.8 kg total P ha^-1 (2.3 to 4.3 lb total P ac^-1) during that period. High concentrations of P in field soils (Mehlich-3 P averaged 441 mg kg^-1, or parts per million) and ditch soils (Mehlich-3 P averaged 171 mg kg^-1) suggest that desorption is the key nonpoint source process controlling P in ditch flow. Over five years, annual total P losses from two ditches with only nonpoint source P contributions were 1.4 to 26.2 kg ha^-1 (1.3 to 23.4 lb ac^-1). Overland flow from the fields to these two ditches accounted for ≤ 8% of annual ditch P export, pointing to groundwater as a key pathway for P transport to ditches. Because P export from ditches was primarily in storm flow and groundwater sampling was primarily during base flow, this study does not provide compelling insight into the role of groundwater in ditch P transport. Only occasionally did dissolved P concentrations in groundwater and ditch flow correspond, and P export from the ditches occurred primarily in storm flow. Sampling of algal mats formed on the bottom of ditches suggests that floating algae may exacerbate sediment-related P transport. Results point to the need for new ditch management practices that can sequester dissolved forms of P and trap floating sources of P, in combination with traditional methods that primarily address sediment-bound P.