EFFECTS OF ACIDIFICATION ON NITROGEN TRANSFORMATIONS IN STORED DAIRY EFFLUENT

Executive Summary A trial was carried out to determine the efficacy of using sulphuric acid (H2SO4) to acidify farm dairy effluent, with the aim of preserving the nitrogen (N) resource during storage. The acidified (pH 6.0, 5.5, 5.0 and 2.5) and control (without acid) effluents were placed into a series of plastic containers and buried with the effluent surface at ground level to simulate effluent storage conditions. The trial was outdoors under a covered clear plastic shelter. Samples of the effluent were regularly collected during an 81 day trial period and analysed for N fractions and pH. The total N content of the effluent decreased in all treatments over the storage period. The largest N loss was observed from the control treatment, where 62% of the initial N content was lost after 81 days’ storage. Ammonia (NH3) volatilisation was likely to be the main reason for N loss from the effluent. When the effluent pH was lowered with addition of acid, N loss was reduced and ranged between 14 and 58% over 81 days. The lowest pH of 2.5 was the most effective at N retention (86%) as urea hydrolysis was slowed, which delayed the availability of ammoniacal N (NH4 plus NH3) for volatilisation. The duration of the lowered pH was short-lived in the effluents adjusted to pH 6.0, 5.5 and 5.0. The data suggests that either the frequent application of acid to keep effluent at pH of 5-6, or an initial high dose rate that slows urea hydrolysis, would substantially reduce N losses from stored effluent. We propose that acidification should be further tested for reducing N loss under more dynamic effluent storage conditions, replicating regular additions of both effluent and acid in order to better simulate a working pond system.