What does manure have to do with climate change?
Storing liquid manure in tanks results in the emission of greenhouse gases. The emission of greenhouse gases through human processes are the main factor causing climate change. When we store liquid manure before applying it to the fields as fertilizer, several different greenhouse gases can be emitted. One of those greenhouse gases is methane. Methane is a very potent greenhouse gas, meaning that its potential to increase global temperatures is much higher than for example carbon dioxide. Therefore understanding how we can reduce the emission of methane during manure storage is an important climate change mitigation strategy.
After the manure is pumped out of the tanks for applying on the field, the tanks are often not emptied entirely before re-filling because of the practical difficulties. This practice of not complete emptying and cleaning tanks in-between storage has been shown to increase the emissions of methane. So why would completely emptying and cleaning liquid manure tanks before refilling with fresh manure reduce the emission of methane? Methane gets emitted from manure because of bacterial activity. Bacteria are “eating” the manure and releasing methane in the process. When manure is stored long enough, a large bacterial community consisting of many different species can establish. The larger the bacterial community - also called inoculum in technical terminology - the higher the emission of methane. Therefore, when the tanks get cleaned entirely, we remove the established bacterial community, and hence, reducing the emission of methane. If fresh liquid manure gets added to only partially emptied tanks, the bacterial community is already established and will produce more methane.
At dairy farms, manure storage tanks are usually filled gradually, meaning that daily or weekly, additional manure is added to the tanks. However, most research done on this topic has been using a batch-filling approach where all manure is added at once to a tank. One of our former graduate students Jemaneh Habtewold supervised by our Prof. Kari Dunfield was interested in understanding how different ways of filling manure storage tanks at a farm-scale level affect the emissions of methane. The resulting research paper written in collaboration with Wilfried Laurier University and Agriculture and Agri-Food Canada showed a significant decrease in methane emissions when tanks were emptied completely. This effect was even more pronounced in tanks that were gradually filled, the practice that resembles actual farm conditions.
The study also analyzed the bacteria community that established with different techniques. They found that the species of bacteria were very similar under all conditions, but they found a much larger number of bacteria in the tanks that had some manure left before being re-filled.
The authors concluded that greenhouse gas emissions can be reduced by almost 85% when tanks are emptied and cleaned entirely before being refilled, especially in tanks that are gradually filled with manure. However, if the manure is stored longer than 120 days, the bacterial community is able to catch up and the beneficial effect of the emptying is starting to level out. For this climate change mitigation method to be successful, the tanks need to be cleaned thoroughly, something that poses a practical challenge at the farm-level.
Citation: Habtewold, J., Gordon, R., Sokolov, V., VanderZaag, A., Wagner-Riddle, C., & Dunfield, K. (2018). Targeting bacteria and methanogens to understand the role of residual slurry as an inoculant in stored liquid dairy manure. Applied and Environmental Microbiology, 84(7), e02830-17.
You can find the original research article following this link:https://aem.asm.org/content/84/7/e02830-17.short