Agriculture Reference
In-Depth Information
are (at dressing), though at a lower rate. The amount of volatilized nitrogen increases with
temperature.
Wind speed is another major determinant, controlling volatilization through its effect on
mixing in the liquid phase, and the rate of transport of NH
3
away from the air-water or air-soil
interface (Denmead et al., 1982; Fillery et al., 1984).
Ammonia loss from surface applications of urea can be reduced by the use of a urease
inhibitor which allows urea to move into the soil before hydrolysis. The ammonia then
released is retained by the soil. One compound which has been widely tested for its capacity
to reduce ammonia loss from urea is the phosphoroamide N-(n-butyl) thiophosphoric triamide
(NBTPT) (Byrnes & Freney, 1995). In upland fields Bronson et al. (1989) found that addition
of NBTPT markedly reduced ammonia volatilization from loamy sands.
Ammonia loss from cropland tends to be important when anhydrous ammonia or urea
fertilizers are misapplied to dry soil, such that the NH
3
that is added as anhydrous ammonia or
formed from urea escapes to the atmosphere before it can be dissolved in the soil solution as
NH
4
+
. Fertilizer misapplication in this way is inefficient and is more likely to occur during
extended dry periods (Robertson et al., 2013).
E
FFECT OF
N
F
ERTILIZATION ON
N
ITROGEN
O
XIDES
(NO
X
)
E
MISSION
Nitrogen oxides (NO
x
) are produced by agricultural soils (Karl et al. 2009; Hudman et al.
2010), even if they are a minor but significant source (Robertson & Vitousek, 2009).
Production and emissions are typically enhanced by N fertilizers application, precipitation,
and elevated temperature (Butterbach-Bahl et al., 2009). Jaeglé et al. (2005) estimated that the
emissions from agricultural soils summed to about 14 % of global surface emissions. In most
cropped soils NO
x
, together with N
2
O, NH
3
, N
2
and NO
3
-
, represent a form of nitrogen
compounds emitted when they escape to plant uptake, so a net loss of these compounds
indicate a bad N management. Emission of NO
x
from agriculture include also residue burning
and land clearing (Robertson & Vitousek, 2009).
NO
x
(NO and NO
2
) carry out, directly and indirectly, several functions in some chemical
transformations in the atmosphere. They affect the production of O
3
in the low atmosphere
(troposphere), which acts as a greenhouse gas and as a harmful chemical to human health and
to plant productivity (Derwent et al., 2008). Ozone (O
3
), by reducing plant photosynthesis,
leads to a reduction of atmospheric CO
2
sequestration by the plant biomass and resulting in
more CO
2
-driven warming (Felzer et al., 2004; Sitch et al., 2007).
Another indirect effect of NO
x
is on atmospheric CH
4
. The largest removal process of
CH
4
is oxidation by the hydroxyl radical (OH), accounting for 88% of the total sink.
Emissions of NO
x
can increase atmospheric OH and accordingly, decrease CH
4
concentrations (Boucher et al. 2009).
NO
x
can lead to increasing O
3
on daily time scales while, on a decadal time, they can lead
to decreases in O
3
, the net result of these competing effects depends on where the NO
x
emissions occur (Collins et al., 2010; Fry et al., 2012). However, the net impact of NO
x
on
atmospheric radiative property is likely to be cooling, by (1) decreasing the CH
4
atmospheric
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