Geoscience Reference
In-Depth Information
anoxic medium. As nitrification occurs in aerated medium, this
presumes prior migration of nitrates to lower layers by diffusion.
In Figure 12.15 we have, therefore, drawn the nitrates strad-
dling the three compartments of the medium to avoid having to
represent these migration processes. The complexity of this path-
way (nitrification, then downward diffusion, followed by deni-
trification) certainly explains why the losses of gaseous nitrogen
are limited in rice paddies.
When all is said and done, rice benefits from substantial additions
of nitrogen coming from the atmosphere.
Emissions of CH
4
by rice paddies occur when the potential Eh falls
below -150 or -200 mV (Table 12.4). They contribute to the greenhouse
effect. Studies are under way to examine if it would be possible to
drain the paddies periodically to oxygenate the soils when excessively
reducing conditions are established (Minamikawa and Sakai 2005). But
the method is not yet perfected because it leads to great decline in
yield.
Methane
CONCLUSION
All over the world, generations have been involved in reducing wet lands
by drainage to extend cultivable areas. In some countries, continuing the
process is justifiable. In others, it is necessary to protect these rare wet
environments because their biological and physicochemical functions are
of great value. In Europe, the remaining wetlands serve as nesting sites
for sedentary birds and as rest areas for migrators. They are also
sanctuaries for numerous plants on which evolution has conferred
amazing capabilities: sub-aquatic respiration, ability to capture and
digest insects, etc. Such is the case of the Camargue in France and the
Everglades National Park in U.S.A.
On the other hand, it is nearly everywhere acceptable to continue
draining wetlands (Gleysols and Stagnosols) to ameliorate them. It has
also been shown that draining out of puddles and elimination of rivulets
that affect wetlands lead to reduction of malaria in the countries where
it is rife (Fanica 2006). The same is true for chikunguniya. We must
reflect before rashly criticizing this agricultural improvement.
