QUANTIFYING NITROGEN FLUX AFTER APPLICATI ON OF N-15-LABELED PIG SLURRY ON TRITICALE IN THE LATE AUTUMN

Predicting nitrate leaching after spreading slurry in autumn is diffic ult because plant uptake, mineralization, immobilization, volatilizati on and denitrification occur and modify the nitrate pool available for leaching. To estimate these fluxes, pig slurry was labelled with ((NH 4)-N-15)(2)SO4 and spread in December (110 kg NH4-N.ha(-1)) on tritica le. Soil microbial immobilization, crop uptake and soil inorganic nitr ogen were measured at seven dates between day 2 and 63 after applicati on. NH4-N in the slurry follows three ways of transformation: volatili zation (38 kg N.ha(-1) in 16 days), immobilization (29 kg N.ha(-1) at day 30) and nitrification (42 kg N.ha(-1)). This last one was achieved 48 days after spreading, despite the cold mean temperatures measured during the experiment. Gross mineralization of soil and slurry organic nitrogen was large (35 kg N.ha(-1) for the 0-10 cm soil layer). The r eal utilization coefficient of N-15- labelled N was low, smaller than 4% at day 63. The leaching of nitrate was small because there was no r ainfall after day 48. Thus, from the balance of N-15-labelled N, it is suggested that 22 kg NO3 N.ha(-1) has been lost by denitrification.