THE FATE OF NITRATE IN CATTLE SLURRIES ACIDIFIED WITH NITRIC-ACID

The fate of NO3- in raw and anaerobically-digested cattle slurries aci dified with HNO3 was studied in a laboratory incubation experiment at the Agriculture and Food Science Centre, Belfast, in 1992. Four amount s (20, 30, 40 and 50 g) of each slurry type were acidified with five r ates of HNO3 supplying 0.7, 1.4, 2.1, 2.8 and 3.5 mg N/g enriched to 1 0 atom% N-15. Th, slurries were stored in 500 mi glass jars for 7 days at 20 degrees C. The jars were left open to allow O-2 supply by diffu sion through the slurry surface to create varying degrees of aeration depending on slurry depth. Fluxes of N-2 and N2O were measured from ea ch jar for 2h each day. The pH was measured after each gas flux measur ement and HNO3-N recovery as NO3--N measured at the end of the incubat ion. For raw cattle slurry, almost none of the HNO3-N was recovered as NO3--N at the 0.7 or 1.4 mg HNO3-N/g rates, but recovery was not sign ificantly different (P > 0.05) from 100% at the 2.8 and 3.5 mg HNO3-N/ g rates. At the 2.1 mg HNO3-N/g rate, recovery was higher when larger amounts of slurry were incubated, indicating that O-2 could promote NO 3- loss. Average pH during the incubation period had to be < 5.5 for r ecovery of HNO3-N as NO3--N to be > 90%. For digested cattle slurry, r ecovery was > 87% at the 0.7 mg HNO3-N/g rate and not significantly di fferent (P > 0.05) from 100% at all other acid rates. Fluxes of N-2 an d N2O were not detected from any digested slurry treatment but were me asured in all raw slurry treatments. Gaseous losses occurred after inc ubation for 1 day, and maximum flux rates occurred after incubation fo r 2 days. Fluxes were highest from incubations with the intermediate a mounts of slurry containing 1.4 mg HNO3-N/g. The largest observed flux from an individual jar was 322 mu g N-2-N/g/2 h with 40 g of slurry c ontaining 1.4 mg HNO3-N/g, which would have accounted for a loss of 23 % of the added HNO3-N during this period. Nitrous oxide and N, were al ways evolved simultaneously and when the pH was between 7 and 9. On av erage, 90% of the total loss of N-gases was as N-2. The raw slurry con tained much higher concentrations of volatile fatty acids than the dig ested slurry. If HNO3 is added to slurries containing such readily dec omposable substrates, the NO3- is susceptible to loss as N-2 and N2O b y denitrification. Oxygen supply, pH and substrate availability contro l the rate of denitrification.