Environmental Research Web
Nitrous oxide is a powerful greenhouse gas and it plays a
key role in atmospheric chemistry and ozone destruction.
Atmospheric levels of the gas have been rising as a result
of man's activities, including growing crops and grazing
Intergovernmental Panel on Climate Change (IPCC) recommends
calculating nitrous oxide emissions from grasslands by
assuming that output of the gas increases linearly with
grazing density because more animals supply more nitrogen in
their waste and in decomposing plant residues. But now a
team from Germany, China and the UK has found that, at least
in grasslands in semi-arid, cool temperate regions, this
approach could overestimate nitrous oxide emissions by up to
contrast to general assumptions, grazing was found not to
increase nitrous oxide emissions in grasslands of cool
temperate climates," Klaus Butterbach-Bahl of Karlsruhe
Institute of Technology, Germany told
environmentalresearchweb. "There is a need to revise
and supplement current methodology to calculate effects of
livestock on N2O emissions in steppe/prairie
Butterbach-Bahl and colleagues measured nitrous oxide
emissions over a period of one year at 10 grassland sites
with various levels of grazing in Inner Mongolia, China, at
time intervals of three hours and one week. Other studies,
in contrast, have tended to focus on the growing season and
have taken relatively infrequent measurements over short
time periods with.
"Steppe/prairie systems cover huge parts of the terrestrial
surface, but are hardly studied outside of North America,"
said Butterbach-Bahl. "The investigated steppe region is
typical for huge parts of the Eurasian steppe belt."
team found that ungrazed steppe emitted a large pulse –
about 72% of its annual emissions – of nitrous oxide during
the spring thaw. Steppe grazed more heavily by livestock
emitted a smaller pulse of the gas in spring. The most
heavily grazed sites emitted just 8% of their annual
emissions during the thaw.
As a result of this spring-thaw pulse, grasslands supporting
more livestock emitted less nitrous oxide over the course of
a year, even though they had a higher output of the gas
during the growing season, in line with the theory behind
current calculation techniques. The researchers believe that
existing approaches may overestimate nitrous oxide emissions
for semi-arid cool, temperate grasslands by up to 72%. Such
grasslands probably account for one-third of temperate
grasslands worldwide, an area of around 10 million sq km,
Stephen Del Grosso
of the US Department of Agriculture.
Butterbach-Bahl and colleagues from Karlsruhe Institute of
Technology, the Chinese Academy of Sciences, and the UK's
Center for Ecology and Hydrology reckon that a higher stock
density crops vegetation lower to the ground, which in turn
allows snow to erode more quickly in spring, cuts soil
temperature and reduces the amount of soil moisture. This
provides less favourable conditions for the microbes that
respire anaerobically and cause denitrification in
water-logged surface soil above the frozen layer beneath
during the spring-thaw.
need to focus research on transition periods such as
freeze-thaw since major trace-gas fluxes may occur then, in
our case approximately 80% of the annual fluxes over 3–4
weeks," said Butterbach-Bahl. "We need to better constrain
sources to be able to develop sound mitigation strategies."
Grasslands cover about one-fifth of temperate land surface
and are widely used as pasture. Writing in
the team suggests that livestock grazing has the potential
to reduce natural background nitrous oxide fluxes, as might
cutting and hay-making.
"That possibility must be qualified because it is unclear
what portions of temperate arid grasslands are currently
grazed at levels that minimize N2O emissions,"
"In addition to N2O, the effects of grazing
intensity on other factors (such as plant productivity,
vegetation community structure, soil erosion, levels of soil
organic matter, and methane emissions) must be taken into
Butterbach-Bahl and colleagues are establishing a new
measurement network, analysing the effect of grassland
systems on greenhouse gases, such as methane and carbon
dioxide as well as nitrous oxide, and looking to model
possible options for reducing emissions.