Acidification of manure reduces gaseous emissions and nutrient losses from subsequent composting process

Manure acidification is recommended to minimize ammonia (NH3) emission at storage. However, the potential for acidification to mitigate NH3 emission from storage and the impact of manure acidification (pH range 5–8) on composting have been poorly studied. The effects of manure acidification at storage on the subsequent composting process, nutrient balance, gaseous emissions and product quality were assessed through an analysis of literature data and an experiment under controlled conditions. Results of the data mining showed that mineral acids, acidic salts and organic acids significantly reduced NH3 emission, however, a weaker effect was observed for organic acids. A subsequent composting experiment showed that using manure acidified to pH5 or pH6 as feedstock delayed organic matter degradation for 7–10 days, although pH6 had no negative effect on compost maturity. Acidification significantly decreased NH3 emission from both storage and composting, however, excessive acidification (pH5) enhanced N2O emissions (18.6%) during composting. When manure was acidified to pH6, N2O (17.6%) and CH4 (20%) emissions, and total GHG emissions expressed as global warming potential (GWP) (9.6%) were reduced during composting. Acidification of manure before composting conserved more N as NH4+ and NOx− in compost product. Compared to the control, the labile, plant-available phosphorus (P) content in the compost product, predominately as water-soluble inorganic P, increased with manure acidification to pH5 and pH6. Acidification of manure to pH6 before composting decreases nutrient losses and gaseous emissions without decreasing the quality of the compost product. The techno-economic advantages of acidification should be further ascertained.