Soil Carbon Dynamics and Nutrient Cycling - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 101

grass roots would have been larger than from
roots of the arable crops in treatment (i); or
(iii) 3 years of a pasture comprising a mix-
ture of grass and clover - this treatment
would lead to organic inputs at least as large
as in treatment (ii) plus additional N from
biological N fixation by the clover. Where no
N fertilizer was applied to the spring barley
crop (Fig. 7.1b) , grain yield doubled from
2 to 4 t ha − 1 when comparing treatments
(ii) and (iii) with the lower organic matter
treatment (i). That this was a result of add-
itional N supply in the two treatments that
included 3 years of pasture was shown be-
cause where N fertilizer was applied to spring
barley, yield increased substantially in treat-
ment (i), such that crop yields in all three
treatments became approximately equal. The
benefit to crop yield was proportionately
greater than the increase in soil C content,
indicating that the additional N was coming
in large part from relatively fresh fractions of
SOM - especially from legume residues in
the case of the pasture that included clover.
There were similar trends with winter wheat:
grain yield in the absence of added N fertil-
izer increased from about 3 t ha − 1 in the low
SOM all-arable treatment to 5.5 t ha − 1 where
wheat followed the grass/clover pasture.

in SOM content are highly significant for a
soil's ability to retain nutrients such as Ca,
Mg, potassium (K), Fe and Zn, plus a range of
micronutrients that occur in cationic forms.
An additional and related benefit of in-
creased SOM content is that it contributes
buffering capacity to soil, such that a soil
with higher SOM content can resist the nat-
ural tendency of soils to become more acid
under the influence of rain which is slightly
acidic or additions such as nitrogen fertilizer.
This is important in soils used for arable
crops. An extreme example was seen in a long-
term fertilizer experiment in China (Zhao et al .,
2010). In a subtropical soil in a high rainfall
region that started at pH 5.7, soil pH in treat-
ments receiving N fertilizer but no additional
organic matter fell to the range 4.2-4.7 after
12 years and led to virtual crop failures of
wheat and maize. By contrast, where manure
was applied in addition to N fertilizer, pH
increased slightly to 5.9, and where manure
alone was applied, it rose to 6.6.

Impacts of Organic Carbon on Soil
Physical Properties and Implications
for Nutrient Availability to Plants

Organic matter in soil is highly influential in
favouring the formation of aggregates at differ-
ent scales. A soil with a pronounced aggregate
structure, with stable pore spaces between the
aggregates, is favourable for root growth and
the uptake of water and dissolved nutrients.
The strong relationship between the stability
of aggregates and organic C content has been
demonstrated in studies covering many hun-
dreds of soils worldwide (Tisdall and Oades,
1982). Microaggregates (regarded as those
< 250 μm in diameter) are formed through dir-
ect interactions between clay particles and or-
ganic matter through the carboxyl or phenolic
groups in organic matter, usually via cations
such as Ca, Fe or aluminium (Al) associated
with clay surfaces. Organic matter in such mi-
croaggregates is either composed of relatively
stable forms and/or is further stabilized against
decomposition through association with the
clay particles. Microaggregates are thought to
be assembled into larger units (macroaggregates)
through the action of more transient organic

Role of Organic Carbon in
Retention of Nutrients

Plant nutrient species that occur as positively
charged ions (cations) are retained in soil
through the action of negatively charged sites
on soil constituents. Many such sites occur at
the surfaces of clay minerals and are associ-
ated with non-crystalline oxides in soil, and
are also due to carboxyl groups in organic
matter. On an equal mass basis, organic mat-
ter has a much greater ability to retain cations
(termed cation exchange capacity, CEC) than
soil inorganic constituents. For example, the
CEC of soil organic matter is typically around
300 cmol c g − 1 compared to values for clays
in the range < 10- 100 cmol c g − 1 . Even though
the organic matter content in soil is typically
only a few per cent by mass, because of its
large CEC, organic matter often contributes
50% or more of soil total CEC. Thus, changes