DENITRIFICATION IN NITRIC-ACID-TREATED CATTLE SLURRY DURING STORAGE

Treatment of cattle slurry with HNO3 to lower the pH is a measure to r educe NH3 volatilization from this slurry during storage and after sur face application. Moreover, the addition of HNO3-N increases the ferti lizer value of the slurry. Incubation studies were carried out to exam ine possible NO3- losses and N2O emission from HNO3 treated slurry dur ing storage. Batches of cattle slurry were treated with various amount s of HNO3 to obtain a pH range of 6.0 to 3.0. The slurries were stirre d once or twice a week and stored for 6 months at 15-degrees-C. Change s in pH, Eh, concentrations of NO3- and NH4+, and emissions of N2O, CO 2 and CH4 were monitored. The loss of NO3- and the emission of N2O wer e related to slurry pH, being lowest at low pH. Cumulative loss of NO3 - ranged from about 40 mmol kg 1 for slurries of target pH less-than-o r-equal-to 5.0 to about 400 mmol kg 1 for slurries of target pH 6.0. H omogenization of the slurries via stirring and addition of H2O, decrea sed the NO3- loss and H+ consumption. The strong pH-dependance of the NO3- loss, the production of N2O and the stoichiometry of H+ denitrifi cation. Similar N2O production rates in the presence and absence of C2 H2 indicated that nitrification was not an important source of N2O. It is concluded that lowering the pH to values less-than-or-equal-to 4.5 as well as regular and complete homogenization of the slurry via stir ring are important for the success of the on-farm treatment of slurry with HNO3.