Vergara - 1976 - Physiological and morphological adaptability of ri

ciuziaon DIOXIDE AND RICE ‘new 215
Under field conditions, the soil supplies some CO, to crop photosynthesis.
The contribution of soil CO2 to net or gross photosynthesis has been estimated
for upland crops and under varied weather conditions (Moss et 211.. 1961; Monteith
et al.. 1964). Upland soils supply‘ CO, from 1.5 to 52.8 g - m"! - days‘. depending
on soils. temperature, soil moisture, and presence or absence of crops
(Lundegardh, 1964; Monteith et a1., 1964). Flooded paddy soils may supply less
CO, due to the insulation effect of standing ivater on CO, diffusion into the air.
Tanaka et a1., (1966) reported that where all straw from the previous crop had
been removed. any CO, released from a submerged soil was undetectable.
Murata et a1., (1957) also reported that only small amounts of CO, came out of
flooded paddy soils, ranging from —0.2 ' g - m4 - day" when 7.5 tfha of compost
ivas applied. to 2.2 g - m4 - day-M when 56.3 t/ha tvas applied.
Recently, Yoshida et al. (1974) found that much larger amounts of soil CO1
came out of paddy soils even when no compost was applied. It ranged from 3.9
to 5.’? g - m = - day l under flooded conditions and from 6.0 to 8.6 g - 111': - day‘
under drained conditions. This apparent discrepancy‘ among three reports is
perhaps due to measurement techniques rather than to environmental conditions.
Since severe CO, deficits occur around crop surfaces of rice it would be
reasonable to assume that soil CO; moving to crop surfaces could be efficiently’
used for photosyrnlhesis.
As shown in Table 2. the estimated contribution of soil CO, to gross photosynthesis
was 6% for the flooded soil and 7% for the drained soil‘. its contribution
to net dry matter production tvas 9% for the flooded soil and 12% for the
drained soil. The estimated contributions of soil CO, flux to rice photosynthesis
are about the same as those reported for upland crops (Moss et a1., 1961; Monteith
et a1. 1964;). Less than 10% contribution of soil CO, flux to gross photosynthesis
is expected for a good rice crop under sunny day's.
In wann areas of Japan some farmers drain paddy fields for 5 to 10 days about
the maximum tiller number stage. sometimes earlier or later depending on soil
conditions. This practice is known as “nakabosh? (midsummer drainage) and
is believed to increase rice yields. The reasons for the beneficial effect of “midsummer
drainage" are given elsewhere (Nojima, 1963). Considering that soil
CO1 flux is increased by drainage, the beneficial effect of midsummer drainage
Table 2. Carbon balance between rice photosynthesis and soil
CO, flux.
Flooded
Drained
Net dry matter production 22 26
(g CHsO- rn= day)
Gross photosynthesis 35 44
(g CH9 m1 day‘)
$0" CO; flux during daytime 2 0
(g CH;Oi m-l day-‘J
3.2
Estimated contribution of soil CO; to.
Net dry matter production ($6) 9 12
Gross photosynthesis ($6) 6 7