Effects of slurry acidification with sulphuric acid combined with aeration on the turnover and plant availability of nitrogen

Ammonia (NH3) losses from animal houses, slurry storage and slurry application can be significantly reduced by acidifying the slurry in livestock houses. The acidification may potentially influence organic matter turnover during slurry storage and therefore also the following release of inorganic nitrogen (N) after application to soil. We studied the effects of pig and cattle slurry acidification with sulphuric acid on (1) N release in a loamy sand soil and (2) the N fertilizer replacement values after slurry application to spring barley (Hordeum vulgare L.) by incorporation and to winter wheat (Triticum aestivum L.) by surface application. Pig and cattle slurries were acidified to pH 5.5 before storage but pH increased to above 6 at application in the field. The composition of slurry after storage indicated that the organic matter turnover during storage is inhibited by acidification, probably due to the presence of acetate in combination with low pH. The acidified slurry contained more butyric acid than the untreated slurry. However, there was no clear effect of acidification on the subsequent mineral N release in soil. Effects of aeration of the acidified slurry were also studied, but 4 days’ aeration had no detectable effects on slurry composition and N availability. Slurry acidification had no detectable effect on the mineral N fertilizer equivalence (MFE) when slurry was incorporated before sowing a barley crop, whereas after surface-banding the MFE of cattle slurry N increased from 39 to 63% and of pig slurry N from 74 to 101% due to the acidification. After acidification, the MFE of slurry was similar in the two crops, indicating that NH3 volatilisation from acidified cattle and pig slurry was low both after incorporation and after surface application.