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Nitrous oxide emissions and grassland
management Søren O. Petersen Danish Institute of
Agricultural Sciences, Research Centre Foulum,
Denmark
Box 1 CO2 mitigation potential of managed
grassland An example Franzluebbers et al. (2000
Soil Biol. Biochem. 32 469-478) quantified C
sequestration in different long-term pasture
systems for a 30-yr period. The carbon stored can
be recalculated into CO2 equivalents as shown
below. Nitrogen inputs via inorganic fertilizer
were also reported and, using the emission factor
of 0.0125 recommended by IPCC (1997) and a Global
Warming Potential of 310 for N2O, this N input
can similarly be recalculated into CO2
equivalents.
Excretal returns are not included the IPCC
emission factor for N in excreta deposited
during grazing is 0.02.
As the table indicates, N2O derived
from fertilizers (and excreta) must be taken into
account when C sequestration strategies are
considered.
Overview This poster describes a field study
starting this year that will investigate sources,
distribution and mechanisms behind nitrous oxide
(N2O) emissions from grazed pastures. Carbon
storage in grasslands has been proposed as a CO2
mitigation option. However, the potential effect
depends significantly on the fate of N inputs,
since N2O derived from N turnover can partly or
completely off-set the removal of atmospheric CO2
(see Box 1). Nitrous oxide emitted from grazed
pastures may originate from labile soil organic
matter or from animal deposits. Any interaction
between these sources will be evaluated by
monitoring emissions and associated soil
characteristics from white clover-ryegrass
pastures of different age after a grazing period
(see Grazing experiment). The extreme
heterogeneity of N2O emissions hampers
quantification at the field scale. Focusing on
the individual urine spot as a major source of
N2O, the dynamics of N2O emissions and related
changes in soil solution chemistry will be
followed. The information may be used for
modelling of emissions based on soil analyses
(see Temporal dynamics of N2O emissions).
Nitrous oxide emissions may not be linearly
related to N input via urine. It is hypothesized
that high ammonia concentrations in urine spots
may influence N2O emissions from nitrification
and denitrification due to mechanisms such as C
release from scorched plant roots and lysed
cells, and microbial stress. This will be
investigated in urine spots using PLFA analyses
(see Urine composition and microbial stress).
Temporal dynamics of N2O emissions Temporal
dynamics of N2O emissions, soil solution
chemistry (electrical conductivity, soil moisture
TDR, urea, inorganic N and dissolved organic C)
will be followed on a daily basis in artificial
urine spots. The dynamics of plant N uptake will
be monitored by remote sensing. Urine will be
collected during milking and diluted or amended
as required (see Urine composition and microbial
stress). It will be attempted to develop a
simple model relating N2O emissions from urine
spots to soil parameters. Such a relationship
would then be integrated into the grazing module
of the whole-farm model FASSET (Jacobsen et al.,
1998 Dan. Inst. Agric. Fish. Econ. Rep. No.
102), which includes a dynamic creation of
spatial heterogeneity in soil nutrient status by
simulating urine and dung depositions (Hutchings
and Kristensen, 1995 Grass Forage Sci. 50
300-313).
Grazing experiment A selected experimental field
(gray areas in the small drawing), currently used
for a study of nitrate leaching in relation to
pasture management, will be fenced to keep out
cattle for 4 weeks, then opened for a 7-d period.
Soil characteristics (electrical conductivity,
soil moisture TDR, urea, inorganic N and
dissolved organic C) will be monitored at the end
of the grazing period in 33 sampling points as
indicated below. Nitrous oxide emissions will be
measured in 3 sampling points from each plot.
The grazing experiment will be repeated in
July-August in a different experimental field.
Urine composition and microbial stress It is
well-known that plant roots may be scorched by
urine deposition due to high levels of ammonia in
the soil following urea hydrolysis. It is
conceivable that ammonia is also a stress factor
for soil microorganisms, including nytrifying and
denitrifying bacteria. This may influence N2O
emissions in a way that is not easily predicted.
Nitrite oxidizers are typically more sensitive
than ammonia oxidizers, and accumulation of
nitrite is likely to increase N2O emissions.
Selective stress effects could indirectly
stimulate less sensitive organisms via a release
of C from lysed cells. Depending on the extent of
C release and the sensitivity of denitrifiers,
this could increase or mitigate N2O production
from this source. As a first attempt to
investigate these questions, general stress
indicators will be investigated by PLFA analyses,
while specific effects on nitrification and
denitrification may be reflected in the
dynamics of soluble N pools.
1st year pasture 2nd year pasture 8th year
pasture
10 m
This study contributes to the Danish project
Dinitrogen Fixation and Nitrous Oxide Losses in
Organically Farmed Grass-Clover Pastures An
Integrated Experimental and Modelling Approach,
and to the FP5 project Greenhouse Gas Mitigation
for Organic and Conventional Dairy Production
(MIDAIR).