Reducing greenhouse gas emissions from pig slurry by acidification with organic and inorganic acids

Methane (CH 4 ) emission from pig slurry is a large contributor to the climate footprint of livestock production. Acidification of excreta from livestock animals with sulfuric acid, reduce CH 4 emission and is practiced at many Danish farms. Possible interaction effects with other acidic agents or management practices (e.g. frequent slurry removal and residual slurry acidification) have not been fully investigated. Here we assessed the effect of pig slurry acidification with a range of organic and inorganic acids with respect to their CH 4 inhibitor potential in several batch experiments (BS). After careful selection of promising CH 4 inhibitors, three continuous headspace experiments (CHS) were carried out to simulate management of manure in pig houses. In BS experiments, more than <99% CH 4 reduction was observed with HNO 3 treatment to pH 5.5. Treatments with HNO 3 , H 2 SO 4 , and H 3 PO 4 reduced CH 4 production more than acetic acid and other organic acids when acidified to the same initial pH of 5.5. Synergistic effects were not observed when mixing inorganic and organic acids as otherwise proposed in the literature, which was attributed to the high amount of acetic acid in the slurry to start with. In the CHS experiments, HNO 3 treatment reduced CH 4 more than H 2 SO 4 , but increased nitrous oxide (N 2 O) emission, particularly when the acidification target pH was above 6, suggesting considerable denitrification activity. Due to increased N 2 O emission from HNO 3 treatments, HNO 3 reduced total CO 2 -eq by 67%, whereas H 2 SO 4 reduced CO 2 -eq by 91.5% compared to untreated slurry. In experiments with daily slurry addition, weekly slurry removal, and residual acidification, HNO 3 and H 2 SO 4 treatments reduced CO 2 -eq by 27% and 48%, respectively (not significant). More cycles of residual acidification are recommended in future research. The study provides solid evidence that HNO 3 treatment is not suitable for reducing CO 2 -eq and H 2 SO 4 should be the preferred acidic agent for slurry acidification.