Gaseous Emissions from Grazing Lands

Abstract

Livestock production is responsible for most of the agricultural emissions, with about 22% from enteric
fermentation, 10% from managed waste, and 18% from grazed lands. Grazing lands can be both sources and
sinks of greenhouse gases (GHGs) as a consequence of vegetation and soil biological processes resulting in
the transformation of carbon (C) and nitrogen (N) compounds into GHGs. This entry addresses in detail the
factors influencing gaseous emissions, the types of gases produced, and the different management practices
to decrease these emissions from grazing lands. Of the GHG emissions from grazed lands, 98% are in the
form of nitrous oxide (N2O) and the remaining 2% is methane (CH4). Although grazing lands are considered
to be carbon dioxide (CO2) neutral, they have the potential to store over 100 Tg CO2/year. Gaseous production
from grazing land is sporadic both in time and space, and so it is a challenge to scale up the
measurements of gaseous emissions from a given location and time to regional and national levels. The exact
approaches also depend on local conditions and, therefore, may vary from region to region.

Full text

Gaseous Emissions from Grazing Lands
Lipismita Samal
Department of Animal Production, College of Agriculture, Odisha University of
Agriculture and Technology, Bhawanipatna, India
Veerasamy Sejian
Animal Physiology Division, ICAR-National Institute of Animal Nutrition and
Physiology, Bengaluru, India
M. Bagath
Animal Nutrition Division, ICAR-National Institute of Animal Nutrition and
Physiology, Bengaluru, India
R.U. Suganthi
Bioenergetics and Environmental Sciences Division, ICAR-National Institute of Animal
Nutrition and Physiology, Bengaluru, India
Raghavendra Bhatta
ICAR-National Institute of Animal Nutrition and Physiology, Bengaluru, India
Rattan Lal
Carbon Management and Sequestration Center, School of Environment and Natural
Resources, Ohio State University, Columbus, Ohio, U.S.A.
Abstract
Livestock production is responsible for most of the agricultural emissions, with about 22% from enteric
fermentation, 10% from managed waste, and 18% from grazed lands. Grazing lands can be both sources and
sinks of greenhouse gases (GHGs) as a consequence of vegetation and soil biological processes resulting in
the transformation of carbon (C) and nitrogen (N) compounds into GHGs. This entry addresses in detail the
factors influencing gaseous emissions, the types of gases produced, and the different management practices
to decrease these emissions from grazing lands. Of the GHG emissions from grazed lands, 98% are in the
form of nitrous oxide (N
2
O) and the remaining 2% is methane (CH
4
). Although grazing lands are considered
to be carbon dioxide (CO
2
) neutral, they have the potential to store over 100 Tg CO
2
/year. Gaseous pro-
duction from grazing land is sporadic both in time and space, and so it is a challenge to scale up the
measurements of gaseous emissions from a given location and time to regional and national levels. The exact
approaches also depend on local conditions and, therefore, may vary from region to region.
INTRODUCTION
Grazing lands can be both sources and sinks of greenhouse
gases (GHGs) as a consequence of vegetation and soil bio-
logical processes resulting in the transformation of carbon
(C) and nitrogen (N) compounds into GHGs. In grazing
systems, C and N can be exchanged in many forms among
atmosphere, plant, soil, and animal components. In the last
200 years, carbon dioxide (CO
2
), nitrous oxide (N
2
O), and
methane (CH
4
) emissions have increased by 36, 18, and
148%, respectively.
[1]
Although varied among years, atmo-
spheric concentrations of N
2
O and CH
4
have increased by
4.9 and 14.9% between 1990 and 2009.
[1]
Globally, grazing
lands and pasture cover about 3.6 billion hectares or
8.9 billion acres, representing 27% of the world land area
(13 billion hectares) and 70% of the world agricultural area
(5.2 billion hectares).
[2]
Broadly, grazing lands can be of
different types, for example, pasture lands, meadow, and
rangelands (grasslands, shrublands, savannas, etc.). Grass-
lands are particularly important sources of N
2
O emissions,
which are greater in grazed grasslands (pastures) than in
mowed (ungrazed) grasslands. Livestock grazing can
increase C and N cycling by consuming crop and weed
residues and returning C and N inputs into the soil through
manure (faeces and urine).
Livestock production is responsible for most of the
agricultural emissions, with about 22% from enteric fer-
mentation, 10% from managed waste, and 18% from grazed
lands.
[3]
Livestock manure is unmanaged when it is deposited
directly on grazed lands. Alternatively, livestock manure can
be managed in storage and treatment systems. Of the emis-
sions from grazed lands, 98% are in the form of N
2
O.
Encyclopedia of Soil Science, Second Edition DOI: 10.1081/E-ESS2-120053664
Copyright © 2015 by Taylor & Francis. All rights reserved. 1

The remaining 2% of GHG emissions from grazed lands is
CH
4
.
[3]
N
2
O, whose emissions into the atmosphere are influ-
enced by management of croplands and grazing lands, has a
lifetime of approximately 120 years and contributes 6.24% to
the overall global radiative forcing.
[4]
There exists a positive
correlation between CO
2
and N
2
O, that is, similarity in the
rates of gas accumulation and diffusive flux due to soil
moisture.
[5]
By employing proper management techni-
ques, grazing lands can both sequester C and reduce emis-
sions of CO
2
,CH
4
, and N
2
O, thereby reducing their GHG
footprint. This entry addresses the types of gases produced
from grazing lands, factors influencing these gaseous emis-
sions, and different mitigation strategies to reduce these losses.
DIFFERENT GASES PRODUCED FROM
GRAZING LANDS
Nitrous Oxide
Grazed lands emit N
2
O due to enhanced N cycling. Globally
livestock grazing is estimated to contribute about 1.55 Tg
of N
2
O-N flux/year.
[5]
N loss through volatilization during
cattle grazing is ~20 Gg N. Unlike the case of CO
2
and
CH
4
, there are no significant biological sinks of atmo-
spheric N
2
O. N
2
O emission is predominantly from grazing
lands amended with N-rich inputs such as synthetic and
organic fertilizers, sewage sludge, compost, unharvested
crop residues (both above and below ground), and
manure.
[4]
Grazing in grasslands reduce N uptake by plants,
leading to the temporary accumulation of mineral forms of
N in the soil. Increase in mineralization of soil organic
matter (SOM) may release inorganic N in the soil, thereby
stimulating microbial activity that produces N
2
O.
[5]
A por-
tion of the N cycled within the plant–animal–soil system
volatilizes to the atmosphere in various gaseous forms and
is eventually re-deposited onto the soils where it can con-
tribute to indirect N
2
O emissions. N
2
O gas is produced near
the soil surface and is accompanied by a net increase in soil
nitrate (NO
3
–
). Some N in the form of NO
3
–
can leach into
groundwater and surface runoff, undergo denitrification,
and also contribute to indirect emissions.
[3]
Mechanism of Nitrous Oxide Production
N
2
O is produced in soil by the microbial action during
autotrophic aerobic nitrification and subsequent heterotro-
phic anaerobic denitrification processes, which are affected
by soil and climatic factors such as composition and avail-
ability of organic matter (OM) as a source of energy,
decomposable organic C, N substrate supply or soil NO
3
–
concentration, partial pressure of O
2
,O
2
supply, tempera-
ture, soil pH, soil enzyme activities, rainfall/irrigation,
water-filled pore space (WFPS), and salinity. N
2
Oisa
Fig. 1 Nitrogen cycle for grazing land.
2 Gaseous Emissions from Grazing Lands

by-product of the first step of nitrification, whereas it is an
intermediate or end product in denitrification. Although
N
2
O production by nitrification is possible, the N
2
O emis-
sion peaks in grazed pastures are generally attributed to the
denitrification process. Fig. 1 describes the N cycle for
grazing lands.
Nitrification. It is a two-step process in which NH
4
+
is
oxidized to NO
2
–
by two groups of nitrifiers, that is,
ammonia-oxidizing bacteria (Nitrosomonas spp., Nitrosos-
pira spp., and Nitrosolobus spp.) and ammonia-oxidizing
archaea (Nitrosopumilus maritimus and Nitrososphaera
viennensis). During the second step, NO
2
–
is oxidized by
nitrite-oxidizing bacteria (Nitrobacter spp. and Nitrospira
spp.) to NO
3
–
.
Denitrification. It represents sequential dissimilative
microbial reduction of NO
3
–
via NO
2
–
to gaseous nitric
oxide (NO), N
2
O, and finally molecular dinitrogen (N
2
) via
four enzymatic complexes. It is the last step in the global
N cycle by which fixed N is returned back to the atmo-
sphere. Although denitrifying organisms normally respire
and metabolize aerobically when O
2
is present, a lower
partial pressure of O
2
causes them to use oxidized forms
of N as alternative terminal electron acceptor in their
respiration chain. Denitrification is the only biological
process of N
2
O consumption; however, this process
usually produces more N
2
O than it consumes because the
denitrification sequence is usually incomplete, that is, not
all of the NO
3
–
is reduced all the way to N
2
,resultingin
two main denitrification products: N
2
OandN
2
. This process
is usually carried out by heterotrophic denitrifiers, for
example, Thermoproteaceae, Cytophagaceae, Corynebac-
teriaceae, Streptomycineae, Bacillaceae, Rhodospirilla-
ceae, Rhodobacteraceae, Rhizobiaceae, Burkholderiaceae,
Nitrosomonadaceae, Neisseriaceae, and Pseudomonaceae,
which can reduce NO
2
–
into N
2
OorN
2
. Denitrification can
also be contributed by several autotrophic (Nitrosomonas
europaea) and heterotrophic nitrifiers (Psedomonas
denitrificans).
Other Nitrogenous Emissions
After the addition of N to farms as animal excreta, fertilizer,
crop residues, or biological fixation, it can be lost by
gaseous emissions to the atmosphere as ammonia (NH
3
),
NO, N
2
, and other NO
x
besides N
2
O, by runoff or leaching
of NO
3
–
andbysoilerosion.Inwell-aeratedsoils,NO
emissions are several times larger than N
2
O emissions.
Emission of NO is the principal gaseous N loss mechanism
during nitrification in soil.
Methane
Manure deposited on grazed lands (i.e., unmanaged
manure) produces and emits CH
4
. Yet, CH
4
emissions from
this source are relatively small, less than 3% of total grazed
land GHG emissions, because grazed lands do not often
experience the anaerobic conditions required for CH
4
production to exceed CH
4
uptake.
[3]
Carbon Dioxide
Although grazing lands are considered to be CO
2
neutral,
with a small uptake of 0.2 Tg CO
2
eq. through sequestra-
tion of CO
2
in soil organic carbon (SOC),
[3]
these lands can
be emission sources or net sinks for atmospheric CO
2
depending on whether C inputs to the soil from plant resi-
dues and manure exceed C losses from decomposition of
SOM. Grazed lands have the potential to store over 100 Tg
CO
2
/year.
[6]
Permanent grasslands used as pastures, range-
lands, and hayfields can maintain large soil C stocks due to
several characteristics. Perennial grasses allocate a high
proportion of photosynthetically fixed C below ground,
which is sequestered in the soil as OM after the residue is
returned to soil, maintain plant cover year-round, and pro-
mote formation of stable soil aggregates. CO
2
is emitted
from soils to the atmosphere through decomposition of
dead OM and respiration by roots and mycorrhizal fungi.
Soil C level is determined by the balance between the
amount of plant residue C added to the soil and the rate
of C mineralized as CO
2
emissions in unmanured soil. Plant
residues also vary in their inherent decomposability due to
differences in their chemical characteristics. Alterations in
plant residues also alter the soil microbial community, for
example, an increase in plant lignin content favors soil
fungi and thus leads to enhanced soil C storage. Fig. 2
describes the carbon cycle for grazing land.
FACTORS AFFECTING GHG EMISSIONS
FROM GRAZING LAND
Abiotic Factors
These include soil temperature, water content, pH, texture,
weather patterns, and various farm management practices.
Soil fluxes of GHGs are also affected by other factors
including air temperature, precipitation, net primary pro-
ductivity, soil respiration rates, substrate quality, soil
physical and chemical properties, and rates of decompo-
sition of OM.
Soil Temperature
It is the main environmental variable controlling CO
2
emis-
sions, while CH
4
is controlled primarily by soil wetness and
anaerobiosis. N
2
O emissions respond to both changes in
soil temperature and wetness. The temperature effect is
normally expressed as Q
10
value, which corresponds to the
increase of reaction rate due to the increase in temperature
by 10°C. For N
2
O emission, values vary between two
Gaseous Emissions from Grazing Lands 3

and ten. Soil temperatures in rangelands vary considerably
not only between seasons but also diurnally. The N
2
O
emissions show a diurnal cycle, varying with the tempera-
ture of surface soil. Peak N
2
O emission rates occur in the
afternoon and are minimum near sunrise. Temperature
influences enzyme kinetics and metabolic turnover rates
of nitrifiers and denitrifiers with optimum temperature at
30–35°C. Microbial activity, OM mineralization, urea
hydrolysis, and nitrification are higher at higher tempera-
ture and higher soil water content, thereby resulting in
higher CO
2
and N
2
O emissions.
Soil Moisture
Soil moisture or WFPS effect is mostly indirect, as it influ-
ences the rate of O
2
diffusion into the soil and controls the
intensity of aeration, thus determining the degree of anaero-
biosis. The interaction between moisture and aeration in the
soil matrix influences NO
3
–
formation and it's stability. The
nitrification process occurs with WFPS at 35–60% and
NH
4
+
should be available in the soil. At levels higher than
60% of WFPS, denitrification is the predominant source of
N
2
O emissions, due to N mineralization and hindered diffu-
sion of O
2
into the soil, favoring the formation of anaerobic
environments. N
2
O emissions often increase with increasing
aeration during drainage of anaerobic soils. With the soil
moist to less than field capacity (60% WFPS), average daily
emission can range from 6 to 25 gN/ha/day.
[7]
The magni-
tude of N
2
O emissions may increase about 12-fold from
pasture soils when WFPS increases from 60 to 80%.
[8]
Soil pH
Soil pH influences the rate of denitrification and the ratio of
denitrification products, N
2
O and N
2
, and subsequent N gas
emissions not only directly through the kinetics of denitri-
fication reductases but also indirectly through its impact on
composition and abundance of the denitrifier community.
[9]
Under acidic pH, the activity of N
2
O reductase is lowered
and the synthesis of new N
2
O reductase is inhibited, result-
ing in an increased accumulation of N
2
O.
[9]
In soils with
higher pH values, the activity and synthesis of N
2
O reduc-
tase is supported and the N
2
O/(N
2
O+N
2
) ratio is decreased
and, therefore, more N
2
is produced.
[9]
Soil Texture
CO
2
emission may increase with decreasing clay content
due to increased SOM mineralization. Soil texture also
impacts CH
4
and N
2
O fluxes. Soil structure and texture
affect hydrological properties and thus the leaching behav-
ior of soils and the mineralization rate.
Weather Patterns
Peak losses of N may occur in early spring and autumn in
the Mediterranean environment and lower N losses in sum-
mer due to low soil water contents. However, in subtropical
and tropical environments, large N losses may also occur in
summer. Substantial N
2
O emissions may occur in soils after
significant rainfall events, snow melts, and also during
Fig. 2 Carbon cycle for grazing land.
4 Gaseous Emissions from Grazing Lands

freeze-thaw or spring-thaw events. Even though the soil
temperature may be near 0°C, the N
2
O emission in cold
soils is caused by microbial activity and the production of
N
2
O exceeds its reduction to gaseous N
2
at low tempera-
tures.
[10]
Strategic application of dairy effluent during
dry summer and autumn can significantly reduce N
2
O
emissions from grazed pastures, and delaying effluent-
irrigation after a grazing event can reduce emissions by
decreasing the levels of surplus mineral N.
Farm Management Practices
Management practices also indirectly affect GHG emis-
sions by altering soil temperature and moisture regimes.
In the short term, drainage and fertilization accelerate GHG
emissions significantly, although their long-term effects are
likely moderated by C accumulation in the above- and
below-ground biomass.
Drainage. Machinery used for drainage may also affect
GHG fluxes by compacting the soil and thereby altering
properties such as bulk density, air-filled pore space, and
hydraulic conductivity. Drainage increases soil tempera-
ture, improves aeration by lowering water table depth, and
increases the production of highly decomposable fine roots
and exudates, thereby stimulating soil microbial activity
and enhancing OM decomposition, which increases CO
2
and
N
2
O and decreases CH
4
fluxes.
[11]
In nutrient-rich peat
soils, large fluxes of N
2
O occur after drainage when
increased O
2
availability stimulates nitrification.
[11]
The
low fluxes of CH
4
from the drained peatlands may be
attributed to decreased methanogenesis and/or increased
oxidation of CH
4
by oxidizing microbes in the aerobic part
of the peat, with part of the CH
4
oxidized in the aerobic peat
layers and converted to atmospheric CO
2
. The effect of
drainage may persist and may not decelerate the rate of
SOM decomposition for a long time.
Irrigation. It moderates soil water content and reduces
soil temperature fluctuations compared to no irrigation, but
N fertilization can reduce these parameters compared to no
N fertilization by increasing canopy cover and water uptake
through increased biomass production. Higher N losses are
expected from flood-irrigated pastures.
[7]
Addition of, as
little as, 5 mm of water to a grass sward can increase
N
2
O emission rates markedly.
[7]
Mounding. Mounding or raised beds may modify the soil
microbial processes and increase both CO
2
and CH
4
fluxes.
[11]
Mounding creates three sub-sites (mounds,
hollows, and undisturbed ground) with a different micro-
climate and OM distribution. Mounds increase soil temper-
ature, decrease soil moisture content, and accentuate OM
decomposition. N
2
O fluxes may increase in mounded hol-
lows and the top of mounds than in undisturbed grounds,
probably because soil moisture conditions, temperature,
and N availability favor nitrification and denitrification.
The two processes may occur simultaneously at shallow
depths and result in the gaseous loss of N
2
O and N
2
from
the soil mineral N pool.
[11]
N
2
O consumption may take
place in waterlogged conditions when denitrifiers reduce
N
2
OtoN
2
.N
2
O fluxes may be reduced in both the mounds
and the hollows.
[11]
Fertilization. N fertilization typically has a stimulatory
effect on N
2
O and CH
4
emissions but a variable effect on
CO
2
emissions. Extensive application of N fertilizer also
increases leaching of N. Both fertilized and unfertilized
soils emit N
2
O. While fertilizer-N is a source of N
2
Oin
case of fertilized soils, mineralization of SOM contributes
to the production of N
2
O from unfertilized soils. Perennial
grassland generally maintains higher levels of SOM com-
pared to those rotated cropland.
[12]
N
2
O escapes from soils
especially after applications of inorganic N fertilizer (e.g.,
urea, mono-ammonium phosphate) for crops and pastures
of mixed farming operations. Unlike crops, most pastures
have an active extensive root system throughout the grow-
ing season. N
2
O emission after application of anhydrous
NH
3
can be 2–4 times higher than surface applying urea,
ammonium nitrate, or broadcasting urea. Surface appli-
cation of fertilizer and manure is subject to greater vola-
tilization losses than injected fertilizer, predominantly as
NH
3
, which is eventually deposited downwind in envir-
onments where it may result in N
2
O emissions. More than
50% of the fertilizer N can be lost as N
2
O through the
process of denitrification or nitrification (depending on
soil conditions), to produce soluble NO
3
–
,whichmaybe
removed through leaching and runoff.
[13]
Fertilizing with
ammonium fertilizers, such as urea, increases the potential
for NH
3
emission, but under anaerobic flooded soil, it
could minimize gaseous N emissions through denitrifica-
tion.
[14]
Fertilization may stimulate the N-limited decom-
posing microorganisms. In addition, it also increases the
above-ground standing plant biomass, root biomass,
which increases rhizospheric deposition and autotrophic
respiration. It is the dominant practice affecting GHG
emissions in the short term, and its effects appear short
lived and of small magnitude. The effect of N fertilizer on
increase in CH
4
emissions has been attributed to NH
4
+
ions inhibiting CH
4
oxidation by competitive inhibition
of the mono-oxygenase enzyme and also inhibitory effects
of N on soil methanotrophs.
[11]
Biotic Factors
Species
In the United States, beef cattle are responsible for the
highest proportion of direct N
2
O emissions from grazed
lands because the vast majority of grazed lands are used
for beef production.
[3]
In general, emissions from managed
grazed land are about twice those of managed manure.
[3]
Gaseous Emissions from Grazing Lands 5

This is due to large numbers of beef cattle on grazing land
(more than 80% of all cattle) compared to feedlots, which
are a source of managed waste.
[3]
Distribution of manure
by animals during grazing at the soil surface is uneven;
however, distribution can be more uniform with sheep
than with cattle grazing.
[15]
Grazing sheep and cattle
together increases soil bulk density and OM and grass
yields compared with grazing sheep or cattle alone.
Manure
Deposition of animal excreta on pastures, including manure
and effluents in grazing lands, results in a patchy distribu-
tion of N and C and consequently enhanced biogenic trans-
formations of the nutrients. Uneven deposition of excretal
N by grazing animals can result in hotspots equivalent to an
application of 400–2000 kg N/ha/year in the small affected
area leading to wide spatial and magnitude variations in
N
2
O emissions. These hotspots correspond to 14.5%/ha/
year, considering a patch area of 0.4 m
2
per urine deposi-
tion.
[16]
Emission factor may be different for urine and
faecal depositions. Majority of CH
4
comes from feces and
on average 0.96 and 0.03 g CH
4
/day/cow is emitted from
faeces and urine, respectively, in grazing cattle
[17]
; whereas
N
2
O emissions are more from the urine, which contains
higher concentrations of N and also a more readily avail-
able mineral-N form than the feces. The amount of N lost
from urine patches depends on the N load deposited, the
time of year when urine is applied, and the soil type. N
2
O
emissions after urine application are higher in soils with
high moisture and high temperature. Addition of urine
causes the release of SOC by hydrolyzing OM, leading to
increased N
2
O production when oxygen diffusion is
restricted. Cattle urine N concentration is typically 6–7g
N/L when the cattle are grazing pasture while urine-
affected, compacted winter forage soils can result in rela-
tively large N
2
O fluxes. Winter grazing of Brassica crops
such as swedes and kale during high soil moisture conditions
leads to urine being excreted onto wet, compacted soils,
which is likely to result in significant N
2
O emissions.
N
2
O emission factor for cattle urine is 0.20% of the applied
urine N in Brazil and 0.66% in the United Kingdom.
[16]
IPCC (2007) lists a default N
2
O emission factor from cattle
urine of 2%, that is, 2% of the urine N is assumed to be emit-
ted as N
2
O.
[1]
In general, 0.1–3.8% of urine-N and 0.1–
0.7% of fecal-N is emitted to the atmosphere as N
2
O.
[16]
Grazing Type and Intensity
The quantity and form of N lost from grazing lands is
mainly a result of grazing intensity, which affects the spatial
distribution of urine patches. The intensity and timing of
grazing can also influence the growth, C allocation, and
plant species of grasslands, thereby affecting the amount
of soil C. Increased soil C on optimally grazed lands is
often greater than on un-grazed or over-grazed lands.
Overgrazing and continuous trampling decreases the
efficiency and viability of the pasture systems. There is a
significant difference in CH
4
emission rates depending on
whether pastures are rotationally or permanently grazed,
and a 9% lower CH
4
production/ha/day was measured on
rotationally grazed pastures compared to continuous graz-
ing.
[18]
Furthermore the use of pasture diminishes emis-
sions from manure storage (because there is less manure
in storage systems) but increases N
2
O emissions from
deposition by grazing animals.
Rangelands
Although the rates of N
2
O emissions from most rangelands
may be low, considering the extent of total rangeland area,
it is important to measure N
2
O emissions in the field over
an extended period, as small amount of rainfall may trigger
significant nitrification activity and hence N
2
O production
following a prolonged dry period. The N
2
O emissions from
the urine patches may be significant in rangelands. Usually
animal feces are voided in large areas, thus avoiding anaer-
obic N mineralization from animal waste.
[7]
In extensive
grazing system, large areas of rangelands are used for graz-
ing purpose. In view of increasing the pasture availability,
tree clearing is commonly practiced which may result in
increased soluble C even in deeper layers of the range-
lands.
[7]
These are associated with CO
2
concentration due
to microbial respiration and hence intense oxygen demand,
resulting in partial anaerobiosis.
Leguminous Pastures
In intensive grazing system, most of the N
2
supply for dairy
pastures is derived from pasture legumes, which are typi-
cally seeded in heavily grazed pastures. Forage legumes on
grazed lands contribute to N
2
O emissions because legumes
fix N
2
from the atmosphere into forms that can be used by
plants and by soil microbes, N
2
can become mineralized in
the soil and contribute to nitrification and denitrification.
Thus, more N
2
addedtosoilyieldsmoreN
2
Oreleased
to the atmosphere. N
2
O emissions from legume-based pas-
tures may be in the order of 1–2kgN/ha.
[7]
There is a
greater N
2
O emission with legumes than with non-
legumes due to the presence of rhizobium bacteria related
to root nodules. Because of lower C:N ratio and higher
N concentration, legumes decompose rapidly compared
to non-legumes, thereby increasing N
2
O emissions.
[15]
Burning of Savannas
In tropical savannas, where fire is frequently used as a
pasture management tool, the N
2
O and NO production rates
are much higher. In such a system, where N is mineralized
rapidly after fire, N
2
O emission rate could be higher, espe-
cially if fire is followed by moderate rainfall events.
[7]
GHG emissions can be significantly reduced following a
6 Gaseous Emissions from Grazing Lands

savanna fire early in the dry season, because the moisture
content of plant material is still relatively high resulting
in low-intensity fires that can reduce fuel loads without
killing trees.
[19]
MITIGATION STRATEGIES
A mitigation practice may affect more than one gas, by
more than one mechanism, for example, strategies intended
to sequester C in soils may impact the fluxes of N
2
O and
CH
4
.
[3]
Further, the temporal pattern of influence may vary
among practices or among gases for a specified practice.
One strategy that may be feasible for more intensely
managed pastures is the use of nitrification inhibitors (NI).
Nitrification Inhibitors
Application of NI to the soil can minimize N
2
O emission by
30–80%.
[20]
NI can reduce NO
3
–
leaching and also promote
better N utilization in the soil. It temporarily delays
the bacterial oxidation of NH
4
+
to NO
2
–
by inhibiting
Nitrosomonas spp. Bacteria.
[16]
An ideal NI should specif-
ically block NH
4
+
oxidation to NO
2
–
, but not NO
2
–
oxida-
tion to NO
3
−
. However, a NI does not inhibit nitrification
indefinitely and its effect depends on its rate of degradation
and persistence in soils. Examples of NI are dicyandia-
mide (DCD), piadin, acetylene, nitrapyrin (2-chloro-6-
(trichloromethyl)-pyridene), and 3, 4-dimethyl pyrazole
phosphate (DMPP). However, it is difficult to apply NI
on commercial pastoral farms because there is no technol-
ogy available to precisely apply only on urine patches and
the application over the whole farm is not economically
efficient.
[16]
Biochar/Agrichar
It is a promising technology to boost the SOC besides
producing bio-energy. The process involves pyrolysis of
forest or agricultural biomass to form a charcoal product
known as biochar. Under proper conditions, 30–50% of the
biomass C is retained in the porous biochar structure.
The biochar is highly recalcitrant in soils and can sequester
C for thousands of years. Application of biochar also
reduces N
2
O emissions from urine patches.
[12]
Grazing Management Practices
Grazing Intensity
Any practice that increases the photosynthetic input of C or
slows the return of stored C through respiration will
increase stored C, thereby enhancing SOC or building C
sinks.
[14]
So, light grazing should be practised instead of
heavy grazing. Optimal grazing management can enhance
accrual of soil C.
Controlled Grazing
Grazing management may affect N
2
O emissions from
pasture, for example, in inner-Mongolia, increasing stock-
ing rates of sheep reduced N
2
O emissions compared with
those from ungrazed pasture. Reducing the sheep stock
number and spring lamb production appears to be an option
for reducing N
2
O emissions from Australian grazing
lands.
[7]
Increasing the number of grazing animals and
grazing in the late fall/winter and in the early spring will
ensure that the manure will be spread over the field.
Another method of grazing called swath grazing may also
be followed. This practice spreads manure around the field
and will also decrease the feeding costs.
Rotational Grazing
Grassland practices such as rotational grazing can reduce
C losses and promote C sequestration in grazing lands.
[3]
By subdividing pastures and rotating cattle, producers can
manage stocking densities and grazing duration to allow the
vegetation time for rest and recovery.
Soil Conservation Practices
These practices may sequester soil C and enhance CH
4
con-
sumption. The largest potential is decreasing soil erosion and
restoring eroded and degraded soils so that they become net
C sinks. Grassland systems that have been degraded in the
past or maintained under suboptimal management conditions
are most conducive to sequestering additional C. Soils that
have been historically cropped using conventional tillage are
often depleted of C because tillage disturbs soil aggregates
and warms soil, both of which increase decomposition rates.
C-depleted soils can act as CO
2
sinks upon conversion to
grazing because grazed soils are typically not ploughed.
Although grazed mineral soils are a net sink of CO
2
,grazed
organic soils are a net source. If half of the grazed organic
soils were converted back to wetlands, CO
2
emissions from
this source could be reduced from approximately 4.6 to
2.3TgCO
2
eq. per year.
[3]
CONCLUSIONS
Production of GHGs from grazing land is sporadic in both
time and space, and so it is a challenge to scale up the
measurements of GHG emissions from a given location and
time to regional and national levels. The exact approaches
also depend on local conditions and, therefore, vary from
region to region. A national management strategy is needed
to reflect region-specific soil types and climatic conditions.
Regionally appropriate conservation agricultural practices
should be identified to build-up SOM, promote SOC, and
mitigate GHG emissions. However, there is large uncertainty
in the emission factor and different countries and regions
Gaseous Emissions from Grazing Lands 7

must have different specific gaseous emission factors due to
different soil, temperature, rainfall, and grazing systems. It is
also difficult to obtain definitive gas flux values because of
the analytical techniques, large spatial variability, and sea-
sonal variability. Simulation modeling should complement
these measurements over seasons and locations.
FUTURE PERSPECTIVES
Although GHG emissions derived from soil have been
researched for 50-60 years, there are still geographic regions
and grazing land systems that have not been well character-
ized. Forage quality should be improved for grazing animals
on smaller livestock operations through better pasture man-
agement. The effectiveness of NI should be assessed under
different conditions and for different soil types. Additional
incentives and educational programmes are needed for land
managers to adopt improved grazing management. More
research is needed on the potential use of biochar to
reduce GHG emissions from grazing systems. Improved
methodologies should be developed to improve the accu-
racy of determining changes in SOC and GHG emissions
from grazing lands. Comprehensive research programme
alongwithawholesystemanalysisisneededtoaccount
for the interactions to arrive at cost-effective and efficient
GHG mitigation management, policy, and legislation
options for sustainable management of grazing lands.
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8 Gaseous Emissions from Grazing Lands