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
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
Selecting for plants and animals that maximise N
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