Soil Carbon Dynamics and Nutrient Cycling - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 104

fresh plant inputs, were much lower in the
bare fallow than in either of the others. In the
case of the bacterial communities, their abun-
dance , as assessed by culturing on a low-nutrient
agar, was much lower in the bare fallow soil
than in soil with fresh plant inputs - consist-
ent with the greatly decreased microbial bio-
mass C content. By contrast, bacterial diversity
was similar in the bare fallow soil to that in the
soils receiving fresh plant inputs from grass or
arable crops, whether assessed by phospho-
lipid fatty acid (PLFA) analysis, Biolog or ex-
tracted DNA. Because DNA examined in this
way is considered to represent the species pre-
sent, though the definition of species in bac-
teria is a matter of debate (Prosser et al ., 2007),
the distinct 16S rRNA gene PCR products re-
vealed by denaturing gradient gel electrophor-
esis (DGGE) were referred to as operational
taxonomic units (OTUs). Hence, the finding of
a high bacterial diversity in the 50- year, bare-
fallowed soil is a discovery that favours evidence
for the stability of the soil bacterial commu-
nity. The possible implications of these find-
ings for the organisms involved in nutrient
transformations are not known.
Besides their implication in the food
web chain, Six et al . (2006) stressed the con-
tribution of soil microbial communities to the
development of well-structured soils through
the production of a range of metabolites. Of
particular importance is the role of fungal ex-
ocellular polysaccharides and fungal hyphae
in the formation of macroaggregates. Soil mi-
crobial communities can derive energy and C
substrate for growth from both native SOM
and added organic materials such as plants
and manures, but community sizes are invari-
ably larger where there are added inputs. The
improved structural attributes arising from a
large and active microbial community is of
great significance for the growth of plant
roots and the effectiveness with which crop
plants can access water and nutrients.

strongly with inorganic ligands in complex-
ing metal cations. Generally, metal-organic
complexes have less positive charge than
the original metal ion, so complexation en-
hances desorption from soil minerals. Thus,
the process is highly significant in altering
the solubility, mobilization and availability
of metal cations: the overall result is de-
pendent on the properties of the cation-
organic complex, the type and quantities of
organic matter and cations and the influ-
ence of plants (Ondrasek and Rengel, 2012).
In the case of organic acids, the greater the
number of carboxylic groups present, the
stronger their metal-complexing ability: or-
ganic acids with only one carboxyl group
(lactate, formate and acetate) have very little
influence. For metals, there is generally a
decline in their propensity to form metal-
organic complexes in the order: trivalent
> bivalent > monovalent (Jones, 1998).
In addition to the role of organic matter
present in soil, organic acids released from
plant roots are also able to form complexes
with metal cations. Forming organic com-
plexes is an important mechanism by which
plants respond to nutrient deficiencies in
soils (Jones, 1998). It has been well known
that roots of graminaceous plants secrete
so-called phytosiderophores consisting of
organic acids such as citric acids to form Fe 3+ -
citrate complexes in soil solution, which can
be taken up readily by plants (Von Wirén
et al ., 2000). Microbial siderophores may
also contribute to iron nutrition in both di-
cotyledonous plants (Vansuyt et al ., 2007)
and monocotyledonous graminaceous spe-
cies (Shirley et al ., 2011). Organic complexes
may also be an uptake mechanism for plants
responding to manganese (Mn), and is also
observed in Mn-deficient soils. Soil organic
matter components do not directly form
complexes with anionic nutrients, such as P,
but their availability can also be enhanced by
forming Fe 3+ - and Al 3+ - organic complexes,
which release phosphorus from Fe-P and
Al-P minerals.
For some metals, forming organic com-
plexes is a process of detoxification to plants.
Typically, toxicity of Al can be eliminated
substantially by forming Al-organic acid
complexes in apoplast and soil solution.

Modification of Availability of Plant
Nutrients through Complexation

Soil organic matter carries carboxyl and
phenolic groups that are able to compete