Agriculture Reference
In-Depth Information
Little CH
4
was emitted during the fallows. This study demonstrates that rice soils
in the fallow periods can be significant sources of N
2
O.
A common cropping sequence in the rainfed lowlands is wet season rice fol-
lowed by a dry season upland crop on residual soil moisture or supplemental
irrigation, followed by a 60- to 70-day fallow during the dry-to-wet transition.
Alternate soil wetting and drying in this system create particular difficulties for
the conservation of nitrogen in the soil (Buresh
et al
., 1993a; George
et al
., 1993,
1994, 1995). Soil N mineralized and nitrified at the onset of rains in the fallow
may be lost by leaching and by denitrification when the soil becomes submerged.
Commonly high-value vegetable crops are grown in the dry season with heavy
applications of fertilizers, leaving substantial amounts of residual nitrate in the
soil. This situation leads to large losses of N before the wet season rice is estab-
lished. Studies of N balances in an intensified rainfed lowland system of this
sort in the Philippines have shown N losses of up to 550 kg ha
−
1
year
−
1
through
nitrate leaching and denitrification (Tripathi
et al
., 1997).
8.3 AMMONIA
8.3.1 GLOBAL BUDGET
Ammonia has a lifetime of only a few hours to a few days in the atmosphere. It
and its reaction products are transported through the atmosphere and deposited
on terrestrial surfaces elsewhere. It is the main gaseous alkaline species in the
atmosphere and neutralizes a large part of the acid produced in oxidation of sulfur
and nitrogen oxides, probably up to a half though its dry-deposition is much faster
than that of NO
x
and SO
2
(Dentener and Crutzen, 1994). Dry- and wet-deposition
of ammonia contribute to soil acidification because 2mol of H
+
are produced in
the nitrification of 1mol of NH
4
+
. Also a large part of the ammonia deposited
on moist forest soils may be re-emitted as N
2
O (Section 8.2).
Table 8.5 shows a global inventory of ammonia emissions compiled by Bouw-
man
et al
. (1997). The main sources are the excreta of domestic animals (40%),
use of nitrogen fertilizers (17%), the oceans (15%) and biomass burning (11%).
About half of the global emission comes from Asia, and 70% is from food
production. Europe, the Indian subcontinent and eastern China have the largest
emission rates, reflecting the densities of domestic animals and the types and
intensities of fertilizer use. Anthropogenic emissions have probably increased
three-fold since 1950 in line with the increase in global population and food
production.
Ammonia volatilization from fertilizers is a function of the type of fertilizer,
soil conditions, meteorological conditions-temperature, wind speed, precipita-
tion-and fertilizer management. Table 8.6 shows the global use of nitrogenous
fertilizers and the corresponding NH
3
emissions based on empirical emission fac-
tors for different fertilizer types in temperate and tropical conditions (Bouwman
