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 (P_ox, Fe_ox, Al_ox), 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 P_ox, which were associated with either point- or nonpoint-P sources. Phosphorus was correlated with Al_ox or Fe _ox within specific ditches. Across all ditches, Al_ox (r = 0.80; p < 0.001) was better correlated with P_ox than was Fe _ox (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.