Biological Processes in the Soil and Floodwater - The Biogeochemistry of Submerged Soils - page 162

et al ., 2002). A solution is to measure the utilization of substrates added directly
to soil, and practical methods for doing this are being developed (Degens and
Harris, 1997).
Through such techniques evidence is emerging for the importance of biodiver-
sity for macro-scale soil processes (Usher et al ., 2004). Though there is evidently
a great deal of redundancy in most microbial populations, there are threshold
levels of biodiversity below which important soil functions are impaired. For
example, the decomposition of recalcitrant organic matter can only be achieved
by consortia of organisms operating together, such as in the anaerobic decom-
position of organic matter in submerged soils in sequential reduction reactions
mediated by microbes (Section 5.1.1). Also the growth and activity of individual
organisms is necessarily constrained by the nature of the prevailing community,
which is important, for example in the persistence of rare organisms in the soil
and management of soil-borne diseases.
Given that biodiversity is important, it is important to understand how soil
management affects it. But as yet there is not much information on this. It has
been assumed that intensification of rice production and more widespread use
of fertilizers and pesticides in the past few decades will have diminished the
diversity of microbes and invertebrates in ricefields compared with those under
traditional practices. Roger et al . (1991) compared the diversity of arthropods in
farmers' fields in the Philippines and at the International Rice Research Institute,
and found the greatest diversity in fields at the Institute, where there has been
heavy use of fertilizers and pesticides for many years, and the least in fields in
the Ifugao rice terraces at Banaue, where there has been little use of fertilizers
and pesticides. This goes against the often hypothesized trend of intensification
reducing biodiversity. Simpson et al . (1993b, 1994a, b) measured the effects of
fertilizer and pesticides on populations of algae and invertebrates in ricefields,
and found complicated interactions. Whereas N fertilizer inhibits N 2 fixation by
cyanobacteria, P fertilizer stimulates it, and the overall productivity of the flood-
water is generally increased by fertilization. Likewise pesticides have various
effects. Part of the community of organisms responsible for mineralizing organic
matter may be killed by pesticide, but the subsequent collapse of predators may
allow other mineralizing organisms to bloom. Several insecticides reduce the
numbers of ostracods that graze on N 2 -fixing cyanobacteria, and so N 2 fixation
is enhanced.