Agriculture Reference
In-Depth Information
50
Surface
40
5 cm
10 cm
30
20 cm
20
10
0
02468
0 214
16
18
20
22
24
Time from midnight (hours)
Figure 3.9
Diurnal variation in temperature at different depths in the soil (White 1997). Reproduced
with permission of Blackwell Science Ltd.
air movement due to larger scale convection processes induced by the topography
or by the presence of large water bodies. This is an important determinant of frost
susceptibility in vineyards, which is discussed further in chapter 8.
Soil Temperature and Respiration
Soil respiration is a combination of root respiration and respiration by soil or-
ganisms. The rate of soil respiration determines the demand for O
2
in the soil,
especially inside aggregates, and hence influences the rate of gas diffusion (box
3.7). The respiration rate (
R
) is normally measured as the rate of O
2
consump-
tion per unit volume of soil in a unit of time. For aerobic respiration, 1 mole of
CO
2
is produced per mole of O
2
consumed, so
R
can also be measured by the
rate of CO
2
production under these conditions. The effect of temperature on the
respiration rate is expressed by the equation
R
R
0
Q
T
/10
3.5.4
(3.9)
where
R
and
R
0
are the respiration rates, respectively, at ambient temperature
T
and at a temperature of minimum respiration rate (0°C). The term
Q
defines the
relative increase in
R
for a 10°C rise in temperature, called the
Q-10 factor
, which
lies between 2 and 3. This applies up to the optimum temperature, generally be-
tween 30 and 35
o
C, above which enzyme function deteriorates.
In regions of cool climate, temperature change is the cause of a large seasonal
variation in the soil respiration rate, which follows a sinusoidal trend as described
by equation 3.8. In such regions, the seasonal maximum in
R
lags 1-2 months
behind that of soil temperature. Superimposed on this seasonal pattern is the ef-
fect of weather, because within any given month the daily maximum in
R
may be
