J. agric. Engng Res. (1998) 69, 85 — 93 Nitrogen Transformations and Losses following Pig Slurry Applications to a Natural Soil Filter System (Solepur Process) in Brittany, France D. R. Chadwick; T. van der Weerden; J. Martinez; B. F. Pain IGER, North Wyke, Okehampton, Devon EX20 2SB, UK; Cemagref, Waste and Environmental Engineering Division, 17 avenue de Cucille´, 35044 Rennes, Cedex, France (Received 10 February 1997; accepted in revised form 10 October 1997) The soil filter system, Solepur, has been shown to be highly successful at removing organic matter and nitro- gen (N) from pig slurry during its first five years of operation. The system involves three operations; ap- plication of large volumes of pig slurry to a managed field; collection and treatment of the nitrate-rich leachate; and irrigation of the treated water over other fields. The purpose of this study was to determine the environmental implications of applying excessive volumes of slurry to the managed field and to improve the understanding of the N cycle in the soil filter system. Nitrous oxide (N  O) and ammonia (NH  ) emissions were measured from the managed field following two slurry applications under different soil condi- tions. Denitrification losses, nitrate leaching, N uptake by plants and soil immobilization of N were deter- mined and methane (CH  ) emissions were measured. Emissions were monitored following slurry applica- tions 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), 23% (as nitrous oxide) and 12% (as dinitrogen gas). Losses as ammonia following the previous June application were greater (31%) but losses as nitrous oxide were less ( ( 1%) coinciding with lower emissions of methane. Measured losses and transformation of N were in agreement with the estimated N balance made in earlier work. Methods for reducing NH  and denitrification losses from the system are discussed, e.g. shallow in- jection of slurry to reduce ammonia losses and the addition of nitrification inhibitors to slurry to reduce the production of a suitable nitrogen source at times when conditions are favourable for denitrific- ation.  1998 Silsoe Research Institute 1. Introduction Intensification of livestock production systems has re- sulted in large quantities of slurries being produced in limited geographical areas 1 with insufficient land area to utilize the slurry efficiently. In France, 55% of pig pro- duction, 46% of the poultry production and 21% of the dairy production is located in Brittany. 2 Approximately 75% of the excreta collected from these animal produc- tion systems is as slurry. Annually this material is esti- mated to contain 210 t of nitrogen (N), with 57, 25 and 13% of the total N being derived from the cattle, pig and poultry systems, respectively. 3 The intensive live- stock systems in Brittany tend to be situated on freely draining soils, so the safe disposal of this waste with minimum risk of polluting the watercourses with high nitrate (NO  ) concentrations, is of greater concern than fully exploiting its value as a fertilizer. In order to reduce the risk of NO  pollution, the period of the year in which spreading of slurries onto land is permitted is controlled by legislation in Brittany. 4 The conventional treatment system available for remov- ing N from slurry prior to spreading is aerobic digestion 5 but the process requires sophisticated, expensive equip- ment and skillful operation. The Solepur process which was designed to treat slurry using biological processes only, offers a potentially simpler, less expensive alternative. The Solepur pilot experiment has been described else- where. 6,7 Briefly, it is a pilot-scale system for treating pig slurry using natural soil processes. There are three main stages (a) a managed field (grass or arable) to which slurry is applied and all drainage is recovered by means of an impermeable layer 80 cm below the surface, (b) a system of storage lagoons or ‘‘reactors’’ for promoting denitrification of drainage water and (c) a non-managed field (grass) for irrigating the denitrified drainage water. The Solepur process involves applying large volumes of slurry onto the managed site and collecting and treating 0021-8634/98/010085#09 $25.00/0/ag970227 85  1998 Silsoe Research Institute