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

Abstract

Abstract: 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.