Agriculture Reference
In-Depth Information
floodwater-soil interface efficient, diffusion through the floodwater is a much less
important conduit for CH
4
emission from ricefields than escape by ebullition or
by passage through the plant (Section 8.1).
5.2 MACROBIOLOGICAL PROCESSES
Superimposed on the microbiological processes are processes driven by the
macroflora and -fauna in the soil and floodwater. These are responsible for the net
primary production of the system, which ultimately drives the biogeochemistry.
Organic matter produced by photosynthetic organisms is the source of energy and
nutrients for grazing organisms, populations of which can be very large. Many of
the invertebrate species found in wetlands create burrows through the soil which
provide conduits for the movement of oxygen, nutrients and carbon. Hence rates
of interchange between aerobic and anaerobic zones can be much greater than
expected from simple physical transport processes.
For discussion of the ecology of wetland soils and water see Mitsch and
Gosselink (2000) for natural wetlands, Roger (1996) for wetland ricefields, and
Catling (1992) for the additional niceties of deepwater ricefields.
5.2.1 NET PRIMARY PRODUCTION AND DECOMPOSITION
The net primary production is often far greater in wetlands than in drylands in
similar climate zones (Chapter 1). In a given climate, NPP depends on hydro-
logical conditions—the frequency and duration of submergence and the rate of
water flow—and on the concentrations of nutrients and toxins. Hydrological
conditions regulate primary producers and decomposers in the soil by limiting
the availability of oxygen for aerobic respiration and by affecting supplies of
nutrients and toxins. In general the frequency of inundation is more impor-
tant than the duration, and the more open the system the greater the NPP
because periodic inundation brings in oxygen and nutrient-rich sediment and
flushes out toxins. Likewise decomposition is faster under a fluctuating water
regime and the accumulation of organic matter is greatest in wetlands with
prolonged inundation and stagnant water (Moore and Bellamy, 1974; Mitsch
and Gosselink, 2000). Maximum productivity occurs under intermediate periods
of inundation.
In many wetlands NPP and decomposition are most limited by the availability
of nutrients, especially N and P. For example, in a review of published data on
nutrient limitations in North American bogs, fens, marshes and swamps, Bedford
et al
. (1999) found that a large proportion of the wetlands were either P limited
or limited by both N and P, especially those occurring on organic soils. Only
marshes had N:P ratios in both live tissues and soils that consistently indicated N
limitation, though the soil data suggested that the majority of swamps were also
