Agriculture Reference
In-Depth Information
5.3
IS BIODIVERSITY IMPORTANT?
Submerged and non-submerged soils have huge biological diversity and contain
orders of magnitude more biological species than above ground habitats and
aquatic systems (Liesack
et al
., 2000; Conrad and Frenzel, 2002; Usher
et al
.,
2004). The origins of this diversity are in the physical and chemical heterogeneity
of soils at micro- and macro-scales, and intense competition for substrate. In
submerged soils there are extreme gradients of redox potential from oxic to
anoxic zones due to the slow transport of O
2
through the soil, and gradients of
substrate from rich to poor zones due to the non-uniform distribution of plant
debris and root exudates. Heterogeneity also arises from soil physical structure
and the labyrinthine network of soil pores which constrain the movements of both
organisms and substrates. Although submerged soils have weak macro-structure,
especially rice soils that have been deliberately puddled, they retain considerable
micro-structure in water-stable micro-aggregates and a corresponding network
of pores. These factors in combination result in a near infinite combination of
opportunities and constraints for different organisms.
But does all this biodiversity have any consequences for soil processes at the
macro-scale? This is a seemingly straightforward question, but the answer has
been surprisingly elusive. Progress has been hampered by the absence of suitable
experimental methods for analysing biological diversity and its relation to soil
functions. Three types of method are used (Ritz, 2004):
(1)
genotypic
analysis, which assesses the basic genetic information about the
community of microbes present;
(2)
phenotypic
analysis, which assesses the expression of the genetic information,
i.e. the living form of the microbial community; and
(3)
functional
analysis, which assesses the processes that the microbial commu-
nity is actively or potentially engaged in.
The greatest diversity is revealed in genotypic analysis, but there is a corre-
sponding lack of discrimination. Analyses of soil DNA often do not show clear
differences between soils from widely differing environments, including in sub-
merged soils (Liesack
et al
., 2000; Reichardt
et al
., 2000). Phenotypic analysis,
such as by assaying membrane-bound phospholipids from living microbes, is
more discriminatory, and there is now good evidence for phenotypic 'signatures'
in soil microbial communities, modified by the environment the microbes are
operating in, including for submerged soils (Reichardt
et al
., 1996). In func-
tional analysis, the actual or potential activities of the microbial community are
measured. Techniques for this have been developed based on the ability of soil
communities to utilize different C-containing compounds using Biolog
plates.
The results often match phenotypic analysis and the expected effects of soil and
environmental differences. However the method is biased towards those microbes
that thrive under the particular conditions of the assay
in vitro
(Preston-Mafham
