Trace Gases - The Biogeochemistry of Submerged Soils - page 252

between rice yield and N fertilizer use (Figure 7.1), and the emission factors
used by Bouwman et al ., I estimate a total emission of NH 3 from wetland rice
of very roughly 3 . 6 Tg N year − 1 ,ofwhich1 . 2TgNyear − 1 is from China and
0.6 from India. This compares with a total global emission from N fertilizer of
9 . 0 Tg N year − 1 . Clearly wetland rice is an important source of NH 3 .
8.3.2 PROCESSES GOVERNING AMMONIA EMISSIONS FROM RICE
Urea is the main form of N fertilizer used in rice, together with, in China, ammo-
nium bicarbonate. At least two applications are generally made by broadcasting
the fertilizer onto the floodwater: the first 14-21 days after planting the crop
and a second at the maximum tillering stage 45-55 days after planting. The first
is subject to high rates of loss by volatilization (De Datta and Patrick, 1986;
De Datta, 1995). Losses are smaller once the crop canopy and root system are
established, because turbulence and hence gas exchange at the water surface
are less and absorption by the crop is greater. Rates of volatilization during the
early period measured by bulk aerodynamic and micrometeorological methods
often account for 30-40% and sometimes as much as 60% of the fertilizer
applied (Simpson et al ., 1984; Cai et al ., 1986; Fillery et al ., 1986 Freney et al .,
1990; De Datta et al ., 1989). Losses during the later stages are typically less than
half this, depending on how well matched the application is with crop demand.
Urea broadcast into the ricefield floodwater is hydrolysed to ammonium, bicar-
bonate and hydroxyl ions; the reaction is catalysed by the enzyme urease:
CO ( NH 2 ) 2 + 2H 2 O + H + −−−→ 2NH 4 + + HCO 3 −
One mol of H + is consumed in this reaction for every 2mol of NH 4 + formed.
In subsequent volatilization of NH 3 ,1molofH + is produced for every mol of
NH 4 + converted to NH 3 :
NH 4 + −−−→ NH 3 + H +
Because urease activities are much greater in the soil than in the floodwater, the
NH 4 + is largely formed in the soil as the urea moves downward by mass flow
and diffusion. The NH 4 + , H + and other reactants will also move between the
floodwater and soil-both upward and downward-with NH 3 being lost from the
floodwater by volatilization. The recovery of N in the crop therefore depends on
the rate of movement of urea and its reaction products through the soil and on
the rate at which the roots remove N from the downward moving pool.
Rachhpal-Singh and Kirk (1993a,b) developed a model of these processes
based on equations for the transport and reaction of urea, ammoniacal
species ( NH 4 + , NH 3 , NH 4 OH ) , carbonate species ( H 2 CO 3 ,HCO 3 − ,CO 3 2 − )
and mobile acid-base pairs ( H 2 CO 3 -HCO 3 − ,HCO 3 − -CO 3 2 − ,NH 4 + -NH 3 ,
NH 4 + -NH 4 OH, H 2 O-OH − ) . The equations are of the form of Equation (2.6)