NITROGEN TRANSFORMATIONS AND LOSSES FOLLOWING PIG SLURRY APPLICATIONSTO A NATURAL SOIL FILTER SYSTEM (SOLEPUR PROCESS) IN BRITTANY, FRANCE

The soil filter system, Solepur, has been shown to be highly successfu l at removing organic matter and nitrogen (N) from pig slurry during i ts first five years of operation. The system involves three operations ; application of large volumes of pig slurry to a managed field; colle ction and treatment of the nitrate-rich leachate; and irrigation of th e treated water over other fields. The purpose of this study was to de termine the environmental implications of applying excessive volumes o f slurry to the managed field and to improve the understanding of the N cycle in the soil filter system. Nitrous oxide (N2O) and ammonia (NH ,) emissions were measured from the managed field following two slurry applications under different soil conditions. Denitrification losses, nitrate leaching, N uptake by plants and soil immobilization of N wer e determined and methane (CH4) emissions were measured. Emissions were monitored following slurry applications in June and in the following October. Losses of nitrogen (expressed as a percentage of that applied ), following the October application, were measured as 6% (ammonia), 2 3% (as nitrous oxide) and 12% (as dinitrogen gas). Losses as ammonia f ollowing the previous June application were greater (31%) but losses a s nitrous oxide were less (<1%) coinciding with lower emissions of met hane. Measured losses and transformation of N were in agreement with t he estimated N balance made in earlier work. Methods for reducing NH3 and denitrification losses from the system are discussed, e.g. shallow injection of slurry to reduce ammonia losses and the addition of nitr ification inhibitors to slurry to reduce the production of a suitable nitrogen source at times when conditions are favourable for denitrific ation. (C) 1998 Silsoe Research Institute.