Spatial Variation of Soil Phosphorus within a Drainage Ditch Network

Agricultural drainage ditches serve as P transport pathways from fields to surface waters. Little is known about the spatial variation of P at the soil‐water interface within ditch networks. We quantified the spatial variation of surficial (0–5 cm) soil P within vegetated agricultural ditches on a farm in Princess Anne, MD with an approximately 30‐yr history of poultry litter application. Ditch soils from 10 ditches were sampled at 10‐m intervals and analyzed for acid ammonium oxalate‐extractable P, Fe, Al (Pox, Feox, Alox), and pH. These variables were spatially autocorrelated. Oxalate‐P (min = 135 mg kg−1, max = 6919 mg kg−1, mean = 700 mg kg−1) exhibited a high standard deviation across the study area (overall 580 mg kg−1) and within individual ditches (maximum 1383 mg kg−1). Several ditches contained distinct areas of high Pox, which were associated with either point‐ or nonpoint‐P sources. Phosphorus was correlated with Alox or Feox within specific ditches. Across all ditches, Alox (r = 0.80; p < 0.001) was better correlated with Pox than was Feox (r = 0.44; p < 0.001). The high level of spatial variation of soil P observed in this ditch network suggests that spatially distributed sampling may be necessary to target best management practices and to model P transport and fate in ditch networks.