Dissolved Phosphorus from Undisturbed Soil Cores: Related to Adsorption Strength, Flow Rate, or Soil Structure? M. Saleem Akhtar, Brian K. Richards, Pedro A. Medrano, Maarten deGroot, and Tammo S. Steenhuis* ABSTRACT favor eutrophication (Chardon et al., 1997; Delgado and Torrent, 1999; Reddy et al., 1999). This study focused on While the rapid transport of nonadsorbed chemicals is relatively the subsurface transport of P for soils in the Northeast, well understood, preferential transport of adsorbable chemicals in- cluding P requires further study. Our objective was to characterize which are usually glaciated, with acid pH, and often subsurface P transport in glaciated soils of the northeastern USA. have a restrictive layer within 1-m depth. Large intact columns from five soils with differing structures were Phosphorus has a high affinity to soil and, therefore, subjected in duplicate to both synthetic acid rainfall at a low rate generally moves slowly downward through the soil ma- simulating natural rainfall, and ponded water for observing saturated trix (Eghball et al., 1990; Sims et al., 1998) or laterally flow. The rain was enriched with inorganic and organic P after baseline with interflow. Small but significant quantities of P may conditions were established. Drainage water P concentrations were move via preferential flow paths (Jensen et al., 1998; measured and adsorption isotherms were determined. Type of flow Ule ´ n et al., 1998; Simard et al., 2000) with little adsorp- was characterized using Cl breakthrough curves and visualizations of tion to the soil matrix (Mansell et al., 1985; Jensen et blue dye patterns. Baseline P concentrations in drainage water were al., 1998). High rainfall rates will cause water to bypass 0.02 to 0.04 mg L -1 . At low flow rates, P appeared in the drainage water soon after application of either inorganic or organic P for the the soil matrix and flow preferentially through a small silt loam soil (firm, moderate coarse prismatic parting to moderate portion of the soil (Scott et al., 1998a; Armstrong et medium subangular blocky structure) in which preferential flow paths al., 1999). Little is known about when the shift from carried most of the water flow. In contrast, the soils in which matrix predominantly matrix flow to preferential flow occurs type flow dominated (weak fine granular or weak medium subangular for different soils (Jensen et al., 2000). blocky structures) had little or no increase in drainage water P. How- Under preferential flow conditions, the greatest P ever, under ponded conditions all soils exhibited preferential flow concentrations appear in leachate shortly after P appli- and rapid P breakthrough. Elevated P concentration in the drainage cation (Scott et al., 1998a; Stamm et al., 1998; Laubel water could not be explained by the P adsorption strength with the et al., 1999; Jensen et al., 2000; Simard et al., 2000) possible exception of the sandy loam soil, where the outflow P concen- and concentrations vary spatially (Stagnitti et al., 1998). tration was consistently low. Variation in flow rate in conjunction with soil structure satisfactorily explained elevated concentrations of Because under udic moisture interpedal spaces, worm- dissolved inorganic and organic P in drainage water from surface holes, and decayed root channels are more prevalent in applied P. fine-textured soils regimes, macropore preferential flow is generally considered more significant in these soils, in contrast to coarse-textured soils (Boll et al., 1997; P hosphorus in runoff and subsurface flow from ag- Jensen et al., 1998; Ule ´ n et al., 1998) where fingered ricultural land contributes to eutrophication of sur- and funnel flow may occur. Although the chemical and face water bodies (Scott et al., 1998b; Sharpley et al., mineralogical composition of the soil determines P ad- 1999). Public awareness of eutrophication and its link to sorption isotherms and kinetics (Mansell et al., 1977; agriculture has lead to legislative measures encouraging Parfitt, 1978; Grant and Heaney, 1997), high pore-water agriculture to reduce its P contributions to streams velocities can override the kinetics (Armstrong et al., (Sharpley et al., 2000). The traditional approach to de- 1999; Simard et al., 2000) because the residence time creasing P loadings has been by reducing P in runoff by can be too short for the solute to diffuse to reaction controlling erosion and associated particulate P (Sharpley sites on the macropore wall (Jensen et al., 1998). et al., 1994). Recent studies suggest that particulate P The various forms and phase transformation reactions has a much smaller effect on lake or stream eutrophica- of P complicate the prediction of transport (Grant and tion than dissolved P because particulate P settles to Heaney, 1997). Evidence of dissolved organic P loss in the bottom while dissolved P is directly available for leachate is mixed. Transport studies on dissolved organic algal growth (Fozzard et al., 1999). Dissolved P can P in soil in pure glycerophosphate form (Castro and Rol- enter waterways via surface and subsurface flow paths, ston, 1977) indicated that the relative effluent glycero- especially through agricultural tile lines (Sims et al., phosphate concentration (C/C 0 ) reached 0.5 after 1.8 pore 1998; Stamm et al., 1998; Laubel et al., 1999), and can volumes, as opposed to Cl - reaching the same relative make up a substantial part of overall P loss during low concentration after only 0.8 pore volumes. Similarly, flow conditions in the summer, when high temperatures limited movement of phytate P was reported by Bow- man et al. (1967) who concluded that organic P tended to accumulate in the surface with little downward move- M.S. Akhtar, B.K. Richards, P.A. Medrano, M. deGroot, and T.S. ment through the root zone. However, long-term field Steenhuis, Dep. Bio. and Env. Eng., Cornell Univ., Ithaca, NY 14853. M.S. Akhtar was on leave from the Nat. Agric. Res. Center, Pakistan trials using manure application concluded that organic Agric. Res. Council, Islamabad, Pakistan. Received 2 Nov. 2001. *Cor- P actually traveled deeper in the profile than ortho P responding author (tss1@cornell.edu). Abbreviations: DOC, dissolved organic C. Published in Soil Sci. Soc. Am. J. 67:458–470 (2003). 458