Trace Gases - The Biogeochemistry of Submerged Soils - page 234

Differences between Rice Production Systems
Early measurements were made mostly in temperate countries (Ciceron et al .,
1983; Seiler etal., 1984; Holzapfel-Pschorn and Seiler, 1986; Schutz et al .,
1989a,b), but a large programme of measurements was conducted in Asia in
the 1990s by the IRRI and partners using a common measurement system
(summarized in the topic by Wassmann et al ., 2000b). This revealed large
differences in emissions per season-more than an order of magnitude-between
different climatic zones across Asia, between types of rice culture, and between
management practices, particularly management of crop residues and use of
organic manures. For example, mean emissions in China are large because of the
widespread use of organic manures, in spite of the moderating effect of lower
temperature compared with tropical Asia; whereas in India, crop residues are
often largely removed from the fields and emissions are correspondingly smaller.
Seasonal emissions from irrigated rice are generally two- to four-fold greater than
from rainfed lowland rice under similar climates. Mean fluxes from deepwater rice
are smaller than from irrigated rice, but because of the far longer growing season,
the total seasonal emission may be similar. Extrapolating from measured seasonal
emissions for the different rice ecosystems and the area of each planted annually,
irrigated rice accounts for 70-80% of global CH 4 emissions from rice, rainfed
lowland rice for 15% and deepwater rice for 10% (Wassmann et al ., 2000d).
Differences within Seasons
Emissions from irrigated ricefields show distinct diurnal and seasonal variations
which illustrate the interactions between the governing processes. The diurnal
variation includes a maximum during the day and a minimum at night, and is
mainly linked to changes in the temperature of the soil solution which drive
changes in rates of CH 4 production and solubility and therefore changes in emis-
sion, whether through the plant or by ebullition (Schutz et al ., 1990; Yagi et al .,
1994; Wang et al ., 1999). The seasonal variation often has two peak periods:
one early in the season corresponding primarily to the decomposition of added
organic matter and ending abruptly when the organic matter has been used up;
and a second later in the season corresponding to emissions via the plant fuelled
by root exudation and turnover (Holzapfel-Pschorn et al ., 1986; Schutz et al .,
1989a,b; Yagi and Minami, 1990; Neue, 1997). Figure 8.1 shows the typical
pattern for a field to which organic matter has been added.
Various factors alter this basic pattern:
(1) The amount of organic matter added. This is highly variable across rice
production systems, depending on such factors as the time available between
crops, mechanization allowing residue incorporation, alternative requirements
for organic matter, and so forth. Often straw is entirely removed from the