Agriculture Reference
In-Depth Information
Plants shade the floodwater and soil surface, and lower the temperature and
concentration of CO
2
above the floodwater during the day. These effects increase
as the plant canopy develops during a growing season, with corresponding
changes in the ecology of the soil and water. Plants also act as substrate for
epiphytic growths—for example for nitrogen-fixing cyanobacteria—and they
provide mechanical support for animals, for example allowing animals to escape
high temperatures in the floodwater during the day. Plant roots support a partially
oxic
rhizosphere
. The rhizosphere is non-photic, but root-derived carbon provides
substrate for microbial growth. The thickness of the oxic rhizosphere is only a
few tenths of a millimetre, but because the roots can occupy a large part of the
anaerobic soil layer, a significant proportion of it may be oxic (Chapter 6).
5.2.3 FLOODWATER PROPERTIES
Temperature and Radiation
Under submerged conditions, temperatures in the soil and water depend on the
depth of the water and on the density of the plant canopy, as well as on mete-
orological conditions. The water transmits incident short-wave radiation to the
soil but it also insulates the soil against emission of long wave radiation. The
full plant canopy transmits 90% of the short-wave infrared radiation (i.e. half the
total short-wave). Hence there is a 'greenhouse' effect and consequently the soil
and water temperatures tend to be higher than the air temperature. Evaporative
cooling reduces the surface water temperature and drives convection currents, so
the water tends to be well mixed.
Figure 5.8 shows temperatures in the air, water and soil in a tropical ricefield
over a year. For the conditions in Figure 5.8, the water and the top 2 cm of soil
were 2-5
◦
C warmer than the maximum air temperatures and they continued to
rise for some weeks after the annual peak air temperature. The 2-10 cm layer
of soil is 2-3
◦
C cooler than the top 0-2 cm. Because of the greenhouse effect,
diurnal changes in soil and floodwater temperatures tend to be smaller than the
changes in air temperature. However the effects depend on the depth of the
floodwater and its source. Under continuous flowing irrigation, water tempera-
tures tend to be lower than air temperatures in hot areas and vice versa in cool
areas. Sediment load and algal cover also have effects.
Incident radiation varies with season, cloud cover and latitude, and the fraction
reaching the floodwater varies with the density of the plant canopy and with the
presence of floating macrophytes and plankton and floodwater turbidity. Algae
use different wavelengths of light to green plants, and this may in part com-
pensate for shading by plants. Nonetheless the intensity of photosynthetically
active radiation at the floodwater surface becomes deficient at some point dur-
ing the season. Figure 5.9 shows the decline in light intensity at the floodwater
surface as the canopy of a rice crop develops. Within 6 weeks of transplanting,
