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Box 1 CO2 mitigation potential of managed grassland: An exampleFranzluebbers 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.
Nitrous oxide emissions and grassland managementSøren O. PetersenDanish Institute of Agricultural Sciences, Research Centre Foulum, Denmark
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’).
Grazing experimentA 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.
Nfix Nfix
1st year pasture
2nd year pasture
8th year pasture
10 m
Mark 3
Temporal dynamics of N2O emissionsTemporal 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).
-1
5 g-1 urine, 5 kg
N kg-1
5 g N kg urine, 2.5 kg
N kg .0
10 g urine, 2.5 kg
Urine composition and microbial stressIt 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.
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).
Tall fescue,grazed
Bermudagrass,hayed
C storage (g C m-2 yr-1) 65 22CO2 equivalents 238 81
N fertilizer input (g N m-2 yr-1) 9.3§ 15N2O from fertilizer (g N2O m-2 yr-1) 0.18§ 0.29CO2 equivalents -57§ -91