Soil Carbon Transition Curves: Reversal of Land Degradation through Management of Soil Organic Matter for Multiple Benefits - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 30

Increased
terrestrial
C storage

Oceanic C
storage or ?

Reduced CH 4
emissions

Formation
of water-
stable soil
aggregates

Less phys-
ical ero-
sion from
hill slopes

Less sedi-
mentation
in surface
waters

Less sedi-
ment load-
ing in coas-
tal zone

Less marine
eutrophi-
cation

Increased water
infiltration

Reduced
nutrient loss

Reduced coastal
fisheries?

Fig. 3.1. Example of an impact chain that relates changes at the small scale of soil aggregate dynamics
at the soil surface to long-term carbon storage at the global scale, with many further relationships
unspecified.

Box 3.1. Quantifying the buffering function of soil organic matter

Under the heading of regulating functions, Table 3.1 includes a number of references to 'buffering.
Soil organic matter increases the soil's water-holding capacity in the plant-available range, and hence
increases the buffering of vegetation against irregular rainfall or irrigation. It also binds nutrients and
buffers plants against irregular nutrient inputs or mineralization. Organic matter may also increase
root development and rooting depth, making vegetation more buffered. This buffer is no substitute for
long-term shortfalls in supply, however. In advanced horticultural systems with fully regulated water
and nutrient supply, there is no need for soil as such, as the buffer functions have been substituted.
In a generic way, buffering is defined on the basis of its effect on a response variable of interest.
Technically, the reduction of variance ( 1 - variance with/variance without) is a measure of buffering,
independently of how it is achieved (van Noordwijk and Cadisch, 2002). The conceptual definition of
buffer can be expressed quantitatively in mathematical models of coupled soil processes that use
measured soil properties and where state variables correspond to flows of material and energy at the
different scales of impact. This type of calculation quantifies the resilience of ecosystem services that
are provided by these flows (biomass, nutrients, clean water, etc.) against changes in environmental
conditions.

with each other. There is one notable excep-
tion, however. Many traditional agricultural
systems derive their productivity from the
nutrients mineralized during the decom-
position of soil organic matter. Many food
systems and regional economies have been
based on such resource use, without a con-
current replenishment of the soil organic
matter. In this first stage of human land use,
a negative trend in soil carbon stocks is
linked functionally to a positive result,
namely agricultural production or harvested
yield. However, the depletion of nutrients
for crop production through enhanced soil
carbon decomposition has limits. When soil
organic carbon stocks become very low, the

soil structure collapses, and soils become
very susceptible to erosion, compaction and
flooding due to poor drainage. The US Mid-
west dust bowl of the 1930s marked the end
stage of grain production living off the prairie
soil organic matter stocks and started a
search for more sustainable soil management
that increased soil carbon stocks (see discus-
sion below). This pattern of decline followed
by either a crash or a timely switch to more
sustainable land management practices ap-
pears to be a common story across continents
and ecological zones. The curve describing
this story of transitions in soil carbon can be
divided into three stages - Stage I: 'the fall',
Stage II: 'the dip' (or collapse) and Stage III: