Phosphate leaching responses from unperturbed, anaerobic, or cattle manured mesotrophic sandy loam soils

Control of P from sewage outlets has not led to expected improvement of Dan ish freshwater quality, calling the diffuse P losses especially from draine d agricultural land into question. We studied some mechanisms for P loss fr om the fertilized topsoil from selected catchments that have been monitored for P losses: (i) losses due to desorption of P from A horizons enriched t hroughout the last decades by fertilization, (ii) losses of P associated wi th temporarily reducing conditions imposed on A horizons, and (iii) losses associated with application of fresh cattle manure to the soil surface or b y mixing into the soil matrix. The effluent concentrations of P from 15 int act topsoil samples were 10 to 40 times lower than what could be expected B orn batch studies, presumably due to nonhomogeneous flow conditions. Additi on of solid cattle feces by incorporation or directly on the surface had ma rkedly differing effects on P leaching. While incorporation did result in a 10- to 20-fold increase in effluent P concentration, surface application r esulted in a further 10- to 20-fold higher effluent concentration of molybd ate-reactive P. These results indicate that while desorption from the soil matrix is likely to be much less than expected from batch chemical consider ations, special attention should be given to drained pastures, where the im portant P source may not be the soil matrix per se, but fecal remnants on t he surface, that may he transported directly to the drains through macropor es. Furthermore, reductive dissolution of Fe associated P may be of some im portance.