Potential role of dissolved organic carbon in phosphorus transport in forested soils

Soils of the northern boreal forest have undergone significant pedogenic losses of P. This study was undertaken to examine the hypothesis that the leaching of dissolved organic carbon (DOC) is a mechanism for P removal from forested soils. The sorption of DOC and DOC fractions by soil horizons from the shoulder, backslope, and footslope soils of a catenary sequence was examined. Variation in DOC sorption among the soil horizons, and between positions in the catena, was related to differences in the clay content (r = 0.71) and citrate-dithionate-extractable Fe, Al, and Mn (r = 0.66). The hydrophobic acid and the hydrophilic acid fractions were the most abundant in the soil solution (72% of the total DOC) and accounted for most of the sorption of DOC in the Bt and C horizons (isotherm distribution coefficient Kd = 27.1 x 10(-4) to 337 x 10(-4) m(3) kg(-1)). The hydrophobic neutral fraction (14% of the DOC) was not sorbed by any of the soil horizons (average Kd = 20.0 x 10(-4) m(3) kg(-1)). The hydrophobic neutral fraction had a molar C/P ratio of 2800, compared with 21000 and 35600 for the hydrophobic acid and hydrophilic acid fractions, respectively. Phosphorus in the hydrophobic neutral fraction constituted 64% of the P measured in these three fractions. The high P content and low sorption of the hydrophobic neutral fraction suggests that the leaching of the hydrophobic neutral fraction may be a possible mechanism for the redistribution and loss of P in these soils.