Following on from the blog at the end of last week looking at mitigation of nitrous oxide emissions from livestock systems, this second part of the report looks at some of the science behind management that has been advocated to reduce GHG emissions (in particular nitrous oxide), and how these can integrate into management changes on the farm.
The diagram below comes from the paper (link here) that these blogs are based on, and highlights some of the areas to consider when looking to reduce nitrous oxide emissions from your system. If you click on the picture it will enlarge.
Animal management and housing
Structures used to house livestock do not directly affect the processes resulting in nitrous oxide and methane emissions, however the type of structure used determines the manure management methods used to handle, store, process and use the manure.
Housing systems with solid floors that use straw accumulate manure with a lighter DM which when stored in piles created conditions for nitrification and denitrification and thus greater nitrous oxide emissions. Farmyard manure and deep litter manure handling systems tend to produce greater nitrous oxide emission than slurry based systems.
Dietary effects on manure emissions
Manipulating rations to reduce nutrient excretion of N and P are well studied, but relating it to mitigation of methane and nitrous oxide is fairly new. Data in the effect of dietary protein on manure Nitrous oxide emissions is not consistent. Tannins as a dietary supplement has been studied, but more studies are needed in terms of relating tannin application through the diet to manure and GHG emissions.
Improving pasture quality in terms of forage digestibility is an efficient way of decreasing GHG emissions from the animal and the amount of manure produced. In pasture based systems, improving forage quality often means increasing Nitrogen fertiliser application rates which can have a negative impact on urinary N excretion and thus ammonia and nitrous oxide emissions.
Reduction of nitrous oxide emissions from intensive grazing systems can be achieved by several strategies:
Improving N use efficiency through reducing the amount of N excreted by grazing animal
Optimising soil management and Nitrogen inputs
Optimising pasture renovation
Manipulating soil N cycling processes through soil additives
Selecting for plants and animals that maximise N utilisation
Altering grazing and feeding management
Manure storage and treatment
Greenhouse gas emissions from stored manure are primarily in the form of methane (due to anaerobic conditions) although nitrous oxide emissions can occur and ammonia volatilisation losses are often large.
A direct way to avoid cumulative GHG emissions is to reduce the amount of time manure is stored.
Semipermeable covers tend to increase Nitrous oxide emissions because they provide optimal aerobic conditions for nitrification at the cover surface and at the same time create a low oxygen environment just below the cover favourable for denitrification and the production of nitrous oxide.
Due to the nature of the composting process N losses can be high and are influenced by a number of factors including temperature C to N ratio, pH, moisture and material consistency. Compost can be a source of nitrous oxide emissions with both nitrification and denitrification processes occurring during composting.
Application method and emissions
An important difference between mineral fertiliser and manure is that manure contains organic Carbon which, depending on soil condition may affect Nitrous oxide emissions.
Manure carbon may increase microbial respiration rates in soil, thus depleting oxygen providing the anaerobic conditions required for denitrification. Compared with mineral Nitrogen sources, manure applications increases soil Nitrous oxide flux in soils with low Carbon content. Soil nitrous oxide emissions can vary greatly and emissions factors of up to 12% of N input (for nitrate based fertiliser) and 5% for manure have been reported.
Incorporating manures can greatly reduce ammonia emissions, leaving more N susceptible to emissions as nitrous oxide. However reduction in ammonia losses with incorporation means that smaller quantities of manure are required and potential for nitrous oxide production is reduced.
Urease and nitrification inhibitors
Microbial processes that result in nitrous oxide production can be manipulated through the use of chemical additives (see earlier blogs).
Cover cropping can reduce soil erosion, improve soil quality and fertility, improved water, weed, disease and pest management and enhance plant and wildlife diversity on the farm,
Reduction of Nitrogen fertiliser use by growing leguminous cover crops has a direct mitigation effect on soil nitrous oxide emissions by reducing soil nitrate availability and potential leaching. Cover crops can increase plant N update and decrease nitrate accumulation and thus reduce nitrous oxide production through denitrification but the results on overall GHG emissions have not been consistent.
What does this show?
There are a number of animal and manure management practices that are feasible and can effectively reduce methane and nitrous oxide from manure storage and for land application. Therefore due to numerous interactions at the animal, storage and land applications phases of the manure management process, GHG mitigation practices should not be evaluated individually in isolation but as a component of the livestock production system as a whole.
Source: Montes et al (2014) Mitigation of methane and nitrous oxide emissions from animal operations: II. A review of manure management mitigation options, J. Anim.Sci. 2013.91:5070-5094