Agriculture Reference
In-Depth Information
Gases Inventory (IPCC, 2006) the value of emission factor (EF) has been changed from
1.25% to 1%, as a result of new analyses of the available experimental data. For example,
Fierro and Forte (2012) reported an emission factor of 0.8% in a conventional tilled, N
fertilized and irrigated maize crop under Mediterranean climate conditions. As well as a GHG
inventory in Canada reported low N
2
O emissions in arid regions, on average of 0.16 and 0.8%
of N fertilizer applied was emitted as N
2
O, in the Brown-Dark Brown and Grey-Black soil
zones, respectively, compared to 1.19% in Eastern Canada (Grant & Wu, 2008). Some studies
concluded that emission factors values are appropriate only when N fertilization rates are less
than or equal to those required for maximum crop yields, because the percentage of N that is
emitted as N
2
O becomes more variable at higher N rates, so EFs may increase when the N
rate exceeds the crop and soil uptake capacity (McSwiney & Robertson, 2005; Grant et al.,
2006; Halvorson et al., 2008). Estimates of N
2
O emissions from croplands in the world in
1995 (IFA/FAO, 2001), estimated using IPCC emission factor of 1%, on the total amount of
N added (73.4 million t mineral form plus 20.66 million t animal manure) produced 3.50
million t of N lost as N
2
O, corresponding to a fertilizer induced of 0.735 million t. In terms of
GWP, this is equivalent to 4.65 kg CO
2
per kg of N applied, with an uncertainty range of 1.4
to 14.0.
Representative EFs values are also linked with the experimental design and monitoring
techniques used for N
2
O fluxes measurements. The length of measurement period and the
frequency (and intensity) of measurements were considered as key factors in any local or
large-scale estimations of N
2
O emissions (Parkin, 2008), since soil N
2
O fluxes result from
complex interaction among biological, physical and chemical factors, within a large spatial
and temporal variability (Clemens et al., 1999; McSwiney & Robertson, 2005; Wagner-
Riddle & Thurtell, 1998). Much of the challenge arises from the fact that small areas
(hotspots) and brief periods (hot moments) often account for high fluxes. N
2
O hotspots in
soils involve the interaction among patches of organic matter and physical factors controlling
oxygen diffusion in soil, and transport and residence time of N
2
O in soil pores. Thus, a series
of plant and soil factors, e.g., rooting patterns and soil structure at small (0.1 to 10 m) scales,
topography, hydrologic flow paths and geology at larger (
>
1 km) scales, need to be
considered to understand the spatial distribution of hotspots. Currently, soil N
2
O emissions
predicting models are calibrated on the basis of spatial variability. However, their reliability
to predict temporal variations is seriously undermined due to the very few data available in
literature to calibrate these models over time. The hot moments concept has been known since
long time but scarcely investigated by continuous monitoring, particularly in the small time
scale, since few experiments are based on high-time-resolution measurements systems
[dynamic chambers, Tunable Diode Laser (TDL) associated with eddy covariance technique].
The large part of data produced up-to-now, are referred to manual chamber measurements
limited in temporal resolution.
The increase of soil N
2
O emissions is not only the direct effect of N fertilizers on soil but
it is also due to the so-called indirect emissions of N
2
O from Nr sprinkled in the environment
(NH
3
, NO
x
, NO
3
-
). Volatilized N can affect N
2
O emissions because a portion of this N will be
deposited on agricultural and non-agricultural soils and in water and be subjected to
transformations that may result in N
2
O emissions; a portion of the NO
3
-
leached or discharged
in drainage can also be denitrified and result in N
2
O emissions (Del Grosso et al., 2006).
Indirect emissions are difficult to estimate because there is an uncertainty in both the amount
of Nr that escapes and the portion of N that is then converted to N
2
O. IPCC Tier 1
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