Greenhouse gas mitigation through dairy manure acidification

Liquid dairy manure storages are sources of methane (CH4), nitrous oxide (N2O), and ammonia (NH3) emissions. Both CH4 and N2O are greenhouse gases (GHGs), whereas NH3 is an indirect source of N2O emissions. Manure acidification is a strategy used to reduce NH3 emissions from swine manure; however, limited research has expanded this strategy to reducing CH4 and N2O emissions by acidifying dairy manure. This study compared control dairy manure (pH 7.4) with two treatments of acidified manure using 70% sulfuric acid (H2SO4). These included a medium pH treatment (pH 6.5, 1.4 mL acid L−1 manure) and a low pH treatment (pH 6, 2.4 mL acid L−1 manure). Emissions were measured using replicated mesoscale manure tanks (6.6 m2) enclosed by large steady state chambers. Both CH4 and N2O were continuously measured (June-December 2017) using tunable diode laser trace gas analyzers. Ammonia emissions were measured three times weekly for 24 h using acid traps. On a CO2 equivalent basis, the medium pH treatment reduced total GHG emissions by 85%, whereas the low pH treatment reduced emissions by 88%, relative to untreated (control) manure. Total CH4 emissions were reduced by 87 and 89% from medium and low pH tanks, respectively. Ammonia emissions were reduced by 41 and 53% from medium and low pH tanks, respectively. Additional research is necessary to make acidification an accessible option for farmers by optimizing acid dosage. More research is need to describe the manure buffering capacity and emission reductions and ultimately find the best approaches for treating farm-scale liquid dairy manure tanks.