Greenhouse gas and ammonia emissions from production of compost bedding on a dairy farm

Composting is a process being used on farms to help stabilize, recycle, and reuse the waste produced through livestock farming. Recent developments in composting technology enable dairy farms to produce their own bedding from composted manure. The production of bedding from composted manure changes the cycle of carbon and nitrogen as they are being reused in a new way, however, the effect of the process on gas emissions is not well studied. This study measured methane (CH4), carbon dioxide (CO2), nitrous oxide (N2O) and ammonia (NH3) from an on farm solid-liquid separation system followed by a composter reactor, which uses a continuously-turned plug-flow format. The gas emissions were studied over three seasons. Emissions were measured separately for two of the phases of composting. The first phase measured was the continuously-turned active compost phase. The second measured phase was during compost storage prior to the compost being used for bedding. During the active compost phase, there were low emissions of N20 and CH4 with most carbon being emitted as CO2-C, and most N emitted as NH3-N. During compost storage, there were higher emissions of both CH4 and N20 compared to the active phase, while NH3 and CO2 were emitted at a lower rate. Overall, combining the active composting and storage phases, the mean total emissions were 3.9 × 10−2 g CH4 kg−1 raw manure (RM), 11.3 g CO2 kg−1 RM, 2.5 × 10−4 g N2O kg−1 RM, and 0.13 g NH3 kg−1 RM. Emissions with solid-separation with composting were compared to calculated emissions for a traditional liquid manure storage tank. The total greenhouse gas emissions (CH4 + N2O) from solid separation, composting, compost storage, and separated liquid storage were reduced substantially compare to traditional liquid storage. Solid-liquid separation and well-managed composting could mitigate overall greenhouse gas emissions; however, an environmental trade off was that NH3 was emitted at higher rates from the continuously turned composter than reported values for traditional storage.