Liquid manure storages are an important source of greenhouse gases (GHG) on dairy farms. Methane (CH 4) and nitrous oxide (N 2 O) are the predominant GHGs, while ammonia (NH 3) is an indirect source of N 2 O. Addition of acid to manure has shown promising emission reductions, however, cost of acidification may be unfeasible for farmers. Fully cleaning storages has also shown to reduce CH 4 , due to removal of inoculating effects of residual manure ("inoculum") on fresh manure (FM). However, complete removal of inoculum is practically impossible on large farms, thus acidifying only the inoculum may reduce GHGs without requiring acidification of all FM. This study aimed to quantify the effect of acidified inoculum on CH 4 , N 2 O, and NH 3 emissions from stored manure and quantify the changes in methanogen abundance and activity. Emissions were measured from six 10.6 m 3 storages filled with 20% inoculum (1-year-old manure) and 80% FM. Inoculum was treated in three ways: untreated (control); previously acidified (1-year prior); and newly acidified with 70% H 2 SO 4 (1.1 L m −3 manure). The CH 4 and N 2 O emissions were continuously measured from June-November using tunable diode trace gas analyzers coupled with venturi air flow systems. The NH 3 emissions were measured at 24-h intervals 3 × weekly using acid traps. The activity and abundance of methanogens were quantified by targeting the Methyl Coenzyme M Reductase A (mcrA) gene and transcript which encodes a subunit of the key enzyme that catalyzes the final step of methanogenesis. Bacterial abundance was quantified by targeting the bacterial 16S rRNA gene. Quantifications were performed using quantitative real-time PCR. CH 4 emissions were reduced by 77% using newly acidified inoculum and 38% using previously acidified inoculum, compared to the control with untreated inoculum (36.1 g CH 4 m −2). Significant treatment reductions in mcrA gene and transcript abundance suggest that CH 4 reductions were caused by disruption of methanogen activity. NH 3 and N 2 O emissions were reduced by 33 and 73% using acidified inoculum and 23 and 50% using previously acidified inoculum, respectively, compared to the control. Results suggest that lower acid rates and acidifying less frequently may still have good treatment effects while minimizing cost.