Agriculture Reference
In-Depth Information
The penetration of the O
2
into the soil depends on its rate of consumption in
aerobic processes and its rate of transport by mass flow and diffusion, and in the
floodwater-soil interface, mixing by burrowing invertebrates. Various aerobic
processes take place in these oxygenated zones.
Nitrification
Depending on the population and growth rates of nitrifying bacteria, the meet-
ing of O
2
from the floodwater or roots and NH
4
+
from the anaerobic bulk soil
will lead to production of NO
3
−
. Subsequent movement of the NO
3
−
into the
anaerobic soil will lead to rapid loss by denitrification. The importance of this
process will be sensitive to the various factors affecting oxygenation of the inter-
face and transport through the soil. Measurements of denitrification in ricefields
have in fact failed to find very high rates of loss (De Datta and Buresh, 1989).
An important point is that conditions favouring high rates of oxygenation of
the floodwater-soil interface through algal activity during the day will also
favour volatilization of NH
3
because of the concomitant increase in floodwa-
ter pH (Section 3.2). Therefore NH
3
volatilization may out-compete nitrifiers for
NH
4
+
in the soil surface.
Figure 5.6 shows profiles of NO
3
−
concentration measured with microsensors
in soil cores taken from ricefields by Revsbech and co-workers (Liesack
et al
.,
2000). During illumination of the cores, O
2
generated in the floodwater penetrated
to a depth of 2-3mm and a clear peak of NO
3
−
was apparent, produced in
nitrification. However in the dark, entry of O
2
from the floodwater diminished,
nitrification ceased, and NO
3
−
moving into the soil from the floodwater was
rapidly consumed in denitrification. The rates of nitrification were slow and all
fields investigated had similar rates. Nitrification in the rice rhizosphere may be
more important (Chapter 6).
2
Dark
Light
1
0
−
1
−
2
3
−
0
1
2
3
4
Concentration of NO
3
−
(
µ
M)
Figure 5.6
Concentration profiles of NO
3
−
in soil cores from a ricefield, illuminated and
not illuminated (Liesack
et al
., 2000). Reproduced with permission from Elsevier Science
