Agriculture Reference
In-Depth Information
Box 3.3. Local indicators for soil organic carbon
Since soil carbon contents within a region with similar climate, and in soils derived from the same par-
ent material, depend mainly on the proportion of silt and clay size particles and human interventions, a
good local indicator is the ratio of soil organic carbon content to silt-plus-clay contents multiplied by 100,
using compatible units for carbon and silt-plus-clay contents (e.g. g kg -1 ). Indicator values of 4.5 could
be considered a threshold value below which soil functions would be severely affected, as, for example,
shown by barley yield decreases below the average (Quiroga et al ., 2006).
derived from global carbon storage issues
will at most be a co-benefit of actions that
primarily are motivated and financed on the
basis of local and watershed benefits.
There is a range of threshold values
where the buffer function of soil organic mat-
ter is affected negatively. With an increasing
climatic variability, and constant or reduced
tolerance to the variability of agricultural
productivity, it is easy to see that both farm-
ers and other stakeholders will need more
buffering capacity to sustain soil benefits,
while the actual trend in agricultural systems
is towards less buffering. The decline of soil
organic matter as the provider of buffering
is part of such a broader trend, and the recov-
ery of buffering needs to be part of the answer.
The answer is not only of interest from a food
production and climate change mitigation
point of view, but also from the point of view
of water quality, regulating stream flow and
aquifer recharge and supporting below- and
aboveground biodiversity.
Ecological-technical aspects of buffer-
ing soil water and nutrient availability,
which influence plant growth and harvest-
able yield, interact with social and economic
aspects of buffering at the level of livelihood
strategies of farmers and other land man-
agers. In a rapidly changing world, it is es-
sential to retain the buffering function of soil
carbon, to sustain benefits and to cushion
these from global change or shock events.
carbon stocks exist in the soil in the
form of soil organic matter.
The 'threshold' or 'transition points'
vary with different types of soils and cli-
matic conditions around the world.
To maintain soil functions, soils should
be managed using improved agricul-
tural or agroecological practices that re-
plenish soil carbon.
In mineral soils used for crop produc-
tion, recovery generally requires a
change in tillage and residue manage-
ment and/or a return of trees to agricul-
tural landscapes, plus maintaining ad-
equate nutrient levels.
In grasslands, a control of grazing pres-
sure is generally needed in order to re-
cover the vegetation, potentially fol-
lowed by the addition of organic matter
stocks derived elsewhere.
Where low potential/marginal land has
been converted to agriculture, on-site
photosynthesis might not generate suf-
ficient organic inputs to the soil to trig-
ger a recovery, and active amendments,
for example with agroindustrial organic
waste, might be necessary.
Societal changes to collect urban or-
ganic waste for land application are ne-
cessary to recycle sufficient organic
matter to close the nutrient loop be-
tween urban and rural areas.
In order to integrate efforts and thinking to
achieve the required management and pol-
icy changes, it may be desirable to work to-
wards a common indicator for measuring
minimum carbon threshold levels in the
soil in relation to various functions and
uses. Soil texture will probably need to be
included to allow functional interpretation;
alternatively, as in Plate 4, the carbon level
Conclusions
The analysis presented in this chapter sug-
gests the following:
Soil ecosystem functions and services
can be maintained only if sufficient
 
 
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