So for the last couple of blogs looking at nitrous oxide
before we move onto something else, a big subject which hasn’t really been
tackled yet is manure management and the impact of manure storage and application on nitrous oxide.
Animal manure is a nutrient resource containing most of the
essential elements required for plant growth and can be a significant source of
N in both intensive and subsistence production systems.
Applying this manure to fields (as we all know) has numerous
benefits including building soil organic matter levels, microbial biomass and
mineralisation rates, and improves soil tilth, water holding capacity, oxygen
content and fertility, as well as reducing soil erosion, reduces nutrient
leaching and increases yields. There is
also the added benefits that come from using these manures as a source of
fertiliser and when this is synchronised with crop requirements and uptake is a
very cost effective resource of nutrients.
If these materials are not managed correctly however they
can present environmental risks, including emitting Nitrous oxide (alongside
methane and ammonia), which we want to avoid, not only because of its
deleterious effects in terms of our GHG credentials, but also (more importantly
from the farmer’s perspective) because it’s a source of nitrogen that we could
potentially use to improve yield but is being lost to the environment.
Nitrous oxide emissions from manure
Nitrous oxide emissions from soil application of manure are
a major contributor to total GHG emissions from agriculture.
Direct emissions of nitrous oxide from manure storage are
small when compared with methane emissions. For nitrous oxide emissions to
occur, manure must first be handled aerobically (with oxygen present) when
ammonium or organic Nitrogen is converted to nitrate and nitrite during
nitrification and then handled anaerobically (without oxygen) when the nitrate
and nitrite are reduced to nitrogen and nitric oxide through denitrification.
Most of the nitrous oxide resulting from manure is produced
in the soils where manures have been applied.
The bits of the nitrogen cycle that produce nitrous oxide (bad) are the
microbes present in the soil working on the manure under aerobic conditions
(part of the natural process that can’t really get away from) and partial
denitrification under anaerobic conditions (what we want to try and minimise),
as it is the anaerobic conditions which
produces more nitrous oxide.
Manure contains most of what it needs to kick-start the
processes in the soil of nitrification and denitrification. However these processes are pretty fickle beasts
and the rate of them occurring will depend on the amount and type of nitrogen
present, the available carbon sources, water content, temperature of the soil
and what bugs are there to work on the manure.
As you may be appreciating it’s a fairly complex system
which depends on lots of variables as to where the emissions occur and whether
its nitrous oxide, ammonia or methane that is the main issue. As nitrous oxide is emitted as a result of
different microbial processes the rate of emissions is highly variable. This causes a bit of a problem when looking
for those nice ‘one size fits all’ answers which we all want which will tell us
how to minimise the emissions of nitrous oxide when we are applying manures
(with all the great agronomic and economic benefits that occur).
What does the research say?
Scientists have been puzzling with this for a while, and
clever computer models alongside very sophisticated measuring and monitoring
equipments, means that it is now possible to look at how effective different
mitigation practices are at controlling nitrous oxide and look at the effect
this may have on the livestock production system.
However there is a further complication (nothing is ever
Due to the nature of the processes that result in methane
and nitrous oxide being produced, some practices that result in the reduction
of methane emissions (yay!), increase nitrous oxide emissions. For example, one
practice advocated by some is the aeration of slurry and manure during storage
which reduces methane emissions. However
if the aeration rate is sufficient to create an aerobic environment, the rate
of nitrous oxide emission will often be increased.
It’s an area where we are not completely there yet. There is still much to learn about the
benefits (and potential drawbacks) of particular mitigation practices, the
effect of combining mitigation practices, the response of environmental
indicators and the effect on the environmental and financial performance of the
farm as a whole.
What this report does highlight is some indications of where
efforts should be addressed.
To save the blog being massive, I’ll delve into some of
these next week.
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