J. Agric. Engng Res. (1999) 73, 235 d 243 Article No. jaer.1999.0412, available online at http://www.idealibrary.com on Factors a!ecting Nitrogen Transformations and Related Nitrous Oxide Emissions from Aerobically Treated Piggery Slurry F. Be H line; J. Martinez; D. Chadwick; F. Guiziou; C.-M. Coste Cemagref, Livestock and Municipal Waste Management Research Unit, 17 avenue de Cucille & , CS 64427, 35044 Rennes Cedex, France; e-mail: fabrice.beline@cemagref.fr Institute of Grassland and Environmental Research, North Wyke, Okehampton, Devon EX20 2SB, UK; e-mail: david.chadwick@bbsrc.ac.uk Perpignan University, Centre for Agrochemistry, 52 avenue de Villeneuve, 66860 Perpignan, France; e-mail: coste@univ-perp.fr (Received 20 April 1998; accepted in revised form 3 February 1999) A laboratory treatment system was designed to study the fate of nitrogen during aerobic treatment of pig slurry. Di!erent aeration strategies, and more particularly the in#uence of residence time and aeration level, were manipulated. A series of six experiments was carried out to determine the nitrogen mass balance, including measurement of the gaseous nitrogen forms particularly ammonia and nitrous oxide. Further nitrogen transformations were examined during the subsequent anaerobic storage of aerated pig slurry at 7, 21 and 60 days. Aeration level and carbon content of raw slurry were identi"ed as the main factors in#uencing nitrogen transformation during treatment. A high aeration level (2}4 mg O  /l) and/or low carbon content (biological oxygen demand of the raw slurry +2 g/kg) resulted in nitrite accumulation (up to 33% of the total nitrogen content of the raw slurry) while a low level of aeration (redox potential"0 mV  ) and high carbon content (biological oxygen demand of the raw slurry +16 g/kg) led to simultaneous nitri"ca- tion and denitri"cation which removed 66% of the total nitrogen in the raw slurry. Nitrous oxide emissions were observed in all treatments and represent up to 30% of the total nitrogen content of the raw slurry. Both nitri"cation and denitri"cation appear to be sources of nitrous oxide during the treatment. Further nitrous oxide emissions were recorded during subsequent storage, especially when the biological oxygen demand to NO  -N ratio was lower than 1)3. However, during closed storage experiments, the nitrous oxide emitted was dissolved into the slurry and "nally reduced to di-nitrogen after 60 days of storage.  1999 Silsoe Research Institute 1. Introduction In France, aerobic treatment of surplus slurry is seen as necessary in order to protect water courses from ni- trate pollution. 1 Aeration systems have been designed to remove nitrogen from the slurry as di-nitrogen gas, via nitri"cation and denitri"cation. 2 Several parameters are known to control the behav- iour of nitrogen (e.g. the level of aeration and the resi- dence time). Indeed, nitri"cation has been shown to occur with dissolved oxygen concentration of above 1}2% of saturation 3 and with a residence time of more than three days. 4 Denitri"cation can occur when the dissolved oxygen concentration is less than 10}15% of saturation. 5 As a consequence, simultaneous nitri"cation and denitri"cation are possible with dissolved oxygen concentration between 1 and 10% of saturation, especially with a redox potential between 0 and !200 mV (standard hydrogen probe). For residence times of less than three days, the removal of nitrogen is solely due to emissions of ammonia. This emission can represent up to 40% of total nitrogen of raw slurry 6 and is largely in#uenced by the aeration rate and the temperature. 7 During the aeration process, nitrous oxide (N  O) can also be emitted. Nitrous oxide is an important green- house gas which may lead to global warming and climate change, and is also implicated in stratospheric ozone depletion. 8}10 On a molecular basis, N  O has a global warming potential about 250 times that carbon dioxide (CO  ). Nitrous oxide in the atmosphere accounts for about 6% of the direct radiative forcing of the long-lived greenhouse gases. Agriculture is presently estimated to contribute from 65 to 80% of the total anthropogenic N  O, 10 which represent approximately 2 Tg N  O}N/yr. 0021-8634/99/070235#09 $30.00/0 235  1999 Silsoe Research Institute