Trace Gases - The Biogeochemistry of Submerged Soils - page 254

with terms for N, C and base added or removed in urea hydrolysis, organic C and
N mineralization, and root uptake. The dynamics of CO 2 in the floodwater and
the coupled transfer of CO 2 and NH 3 across the air-water interface (Section 3.5)
are allowed for.
The model shows that cumulative volatilization of NH 3 is sensitive to the
initial distribution of urea in the soil, its rate of hydrolysis, and the rate of
absorption of N by rice roots. It is largely insensitive to other parameters. For
example, it might be thought that addition of organic matter to the soil to acidify
the floodwater should lessen NH 3 volatilization. However, the model shows that
although increased CO 2 production affects the diurnal change in floodwater pH, it
little affects the daily average pH and hence NH 3 volatilization. This is because
the relative rates of movement of carbonate species and acidity between the
soil and floodwater are such that the increased alkalinization of the floodwater
resulting from increased CO 2 loss is not matched by an equal inflow of acidity
from the soil.
The model shows that the spread of urea and NH 4 + into the soil is typi-
cally only a centimetre or two in a week (Figure 8.9). The recovery of broad-
cast fertilizer N in the crop must therefore depend entirely on the superficial
root system in the soil-floodwater interface. The good recovery of broadcast
fertilizer N obtained if the fertilizer is added when the crop demand is maxi-
mal (Peng and Cassman, 1998) therefore indicate rapid uptake by roots in the
soil-floodwater interface.
8.4 SULFUR COMPOUNDS
8.4.1 GLOBAL BUDGET
Submerged soils are important sinks for atmospheric sulfur (Howarth et al .,
1992). Sulfate washed into wetlands or deposited from the atmosphere is largely
reduced to sulfide by sulfate-reducing bacteria. Subsequent precipitation with
metals, especially as FeS, results in more or less permanent removal of the S
from the global S cycle.
Little sulfur is re-emitted from wetlands into the atmosphere. Table 8.7 gives
estimates of global emissions of volatile sulfur compounds from different sources.
Total emissions are in the range 98 to 120 Tg (S) year − 1 ; 75 % is anthropogenic,
mainly from fossil fuel combustion in the northern hemisphere. The main natural
sources are the oceans and volcanoes. Wetlands and soils contribute less than
3% of the total emission.
8.4.2 EMISSIONS FROM RICEFIELDS
The main source of S emissions from ricefields is the burning of crop residues,
during which most of the sulfur in the residues is converted to volatile oxides (Fox