Trace Gases - The Biogeochemistry of Submerged Soils - page 256

and Hue, 1986; Lefroy et al ., 1992). However these may be rapidly returned to
the soil in rainfall, particularly during tropical wet seasons, though not necessarily
at the same site. Sulfur may also be lost by volatilization as H 2 S, but in most
soils any S 2 − formed in reduction is promptly precipitated as FeS or other sulfides
(Chapter 4), and so net losses are small.
There are also modest emissions of organic-S compounds. Minami et al . (1993)
measured S emissions from field lysimeters treated with S-containing compounds
in amounts found in crop residues and organic manures. Emission of H 2 S,
OCS (carbonyl sulfide), CH 3 SH (methyl mercaptan), CH 3 SCH 3 (dimethylsul-
fide, DMS), CS 2 (carbon disulfide) and CH 3 SSCH 3 (dimethyl disulfide) were
detected, with DMS by far the largest. Emissions of DMS ranged from 4.1 to
7.3mg (S) m − 2 year − 1 , and varied diurnally and seasonally in ways indicating
mediation by the rice plants. The type of soil had little effect. The measured emis-
sions multiplied by the total global rice area indicate a potential global emission
from ricefields of 0.004 to 0.01 Tg (S) year − 1 . This compares with emissions
from all wetlands of 0.003-0.68 Tg (S) year − 1 , from other plants and soils of
0.05-0 . 16 Tg year − 1 , and from oceans of 15-25 Tg year − 1 . Likewise emissions
of CS 2 , OCS and other organic-S compounds from ricefields are small in com-
parison with other known sources.
8.5 CARBON SEQUESTRATION
Table 1.3 gives estimates of the global distribution of carbon in soils. Wetlands
are the single largest organic C pool, account for roughly 40% of the total soil
carbon. Destruction of wetlands therefore results in significant loss of the global
terrestrial carbon store. There have been large losses of wetlands in developed
countries in the past-almost half the original wetland area in the US in the last
200 years for example-but this is now largely under control. However preser-
vation of wetlands is less of a priority in developing countries. About half the
global area of natural wetlands is in the tropics.
Despite the burning of crop residues in the productive, irrigated rice areas of
tropical and subtropical Asia, and their removal for other purposes in the low-
producing rainfed rice areas, soil carbon levels are largely constant (Bronson
et al ., 1998). In any case, the amount of carbon in the shallow puddled layer of
ricefields amounts to only a few per cent of the amount in natural wetlands.