Environmental Engineering Reference
In-Depth Information
related to the length of FIE, which varied from a minimum of 8-56 days. Overall
data suggest that the internationally applied emission factors for temperate crops,
which are empirically derived from sites with cooler and wetter climates than the
Mediterranean, would overestimate N
2
O emissions for Italian crops, in particular
those developing between spring and summer.
9.1 Role of Agricultural Ecosystem in the Total N
2
O
Atmospheric Budget
Nitrous oxide (N
2
O) is the third most important greenhouse gas and plays a key
role in the destruction of the ozone layer through the formation of NO
x
in the
stratosphere (Ravishankara et al.
2009
). Soils are the main source of N
2
O emis-
sions contributing to about 53 % of the total source strength (Denman et al.
2007
).
About 60 % of these emissions occur in the Northern Hemisphere. In temperate
areas, agricultural ecosystems are the primary source of N
2
O due to the elevated
input or reactive nitrogen (Nr) into the environment, mostly associated with ferti-
lization with mineral N and animal manure, N derived from N
2
-ixation (legumes
and N-fixing microbes) and N from enhanced soil mineralization. In fact, N
2
O
atmospheric concentration has increased constantly in the last centuries (270 ppb
before the Industrial Revolution to 320 ppb today) (Denman et al.
2007
) concomi-
tantly with the increase of Nr (Galloway et al.
2008
). Indirect emissions of N
2
O
due to the release of Nr into the environment in the form of leaching, runoff, nitro-
gen oxides (NO
x
) and NH
3
volatilization also play an important role in regional
and continental budgets and are directly related to agricultural management (IPCC
2007
).
9.2 Driving Factors of N
2
O Emissions
A complex interplay of factors concurs to the magnitude of direct N
2
O emissions
from agricultural soils. Controlling factors can be divided into proximal and distal
ones. The former are those having a direct influence on microbial processes which
control N
2
O emissions, i.e. nitrification and denitrification. N
2
O is a by-product of
nitrification, which although an aerobic process, leads to consistent emissions of
N
2
O only when oxygen tension decreases. N
2
O is also an intermediate product of
denitrification, carried out by anaerobic heterotrophic bacteria. Both processes are
directly limited by substrate availability (NH
4
+
−
, organic C) and are con-
trolled by temperature (Castaldi
2000
), soil pH, and presence of inhibiting mol-
ecules (Castaldi et al.
2009
). Soil water content exerts a direct effect on microbial
survival, as in prolonged and extremely dry conditions part of the microbial com-
munity will die or change its biological form (ex. formation of resistant spores).
However, water content also controls the solubility and hence the availability of
, NO
3
