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 Danish freshwater quality, calling the diffuse P losses especially from drained agricultural land into question. We studied some mechanisms for P loss from the fertilized topsoil from selected catchments that have been monitored for P losses: (i) losses due to desorption of P from A horizons enriched throughout the last decades by fertilization, (ii) losses of P associated with temporarily reducing conditions imposed on A horizons, and (iii) losses associated with application of fresh cattle manure to the soil surface or by mixing into the soil matrix. The effluent concentrations of P from 15 intact topsoil samples were 10 to 40 times lower than what could be expected from batch studies, presumably due to nonhomogeneous flow conditions. Addition of solid cattle feces by incorporation or directly on the surface had markedly differing effects on P leaching. While incorporation did result in a 10‐ to 20‐fold increase in effluent P concentration, surface application resulted in a further 10‐ to 20‐fold higher effluent concentration of molybdate‐reactive P. These results indicate that while desorption from the soil matrix is likely to be much less than expected from batch chemical considerations, special attention should be given to drained pastures, where the important P source may not be the soil matrix per se, but fecal remnants on the surface, that may be transported directly to the drains through macropores. Furthermore, reductive dissolution of Fe associated P may be of some importance.