Emissions of ammonia, nitrous oxide, methane, and carbon dioxide during storage of dairy cow manure as affected by dietary forage-to-concentrate ratio and crust formation

Sixteen 200-L barrels were used to determine the effects of dietary forage-to-concentrate (F:C) ratio on the rate of NH3-N, N2O, CH4, and CO2 emissions from dairy manure during a 77-d storage period. Manure was obtained from a companion study where cows were assigned to total mixed rations that included the following F:C ratio: 47:53, 54:46, 61:39, and 68:32 (diet dry matter basis) and housed in air-flow-controlled chambers constructed in a modified tiestall barn. On d 0 of this study, deposited manure and bedding from each emission chamber was thoroughly mixed, diluted with water (1.9 to 1 manure-to-water ratio) and loaded in barrels. In addition, on d 0, 7, 14, 28, 35, 49, 56, 63, 70, and 77 of storage, the rate of NH3-N, N2O, CH4, and CO2 emissions from each barrel were measured with a dynamic chamber and gas concentration measured with a photo-acoustic multi-gas monitor. Data were analyzed as a randomized complete block with 4 replications. Dietary F:C ratio had no effect on manure dry matter, total N and total ammoniacal-N (NH3-N + NH4+-N), or pH at the time of storage (mean ± SD: 10.6 ± 0.6%, 3.0 ± 0.2%, 93.1 ± 18.1 mg/dL, and 7.8 ± 0.5, respectively). No treatment differences were observed in the overall rate of manure NH3-N, N2O, CH4, and CO2 emissions (mean ± SD over the 77-d storage period; 117 ± 25, 30 ± 7, 299 ± 62, and 15,396 ± 753 mg/hr per m2, respectively). The presence of straw bedding in manure promoted the formation of a surface crust that became air dried after about 1 mo of storage, and was associated with an altered pattern in NH3-N and N2O emissions in particular. Whereas NH3-N emission rate was highest on d 0 and gradually decreased until reaching negligible levels on d 35, N2O emission rate was almost zero the first 2 wk of storage, increased sharply to peak on d 35, and decreased subsequently. The emission rate of CH4 and CO2 peaked simultaneously on d 7, but decreased subsequently until the end of the storage period. In this study, C:N ratio of gaseous losses was 32:1, reflecting higher volatile C loss than volatile N loss during storage. On a CO2-equivalent basis, the most important source of non-CO2 greenhouse gas emitted was CH4 until formation of an air-dried crust, but N2O thereafter. Taken together, these results suggested that the formation of an air-dried crust resulting from the straw bedding present in the manure reduced drastically NH3-N, and CH4 emissions, but was conducive of N2O production and emission.