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 38

during the mineralization of organic carbon,
and specific practices exist that can subse-
quently drive a turnaround that leads to in-
creased carbon content. In this final section,
we will discuss to what degree the many
local soil carbon transition scenarios add up
to a global pattern and what it implies for
the local, national and global policy making
that may be wanted or needed in order to
manage the trade-offs stage of the curves
and the choices to be made in order to en-
able recovery in Stage III, balanced with
avoided damage in Stage I.
Qualitatively, the soil carbon transition
curve paradigm aligns with agroecological
experience from around the world, but the
quantities involved in soil under the natural
vegetation at the start of the curve ( Fig. 3.2 ),
the relative and absolute decline in Stage I
( 10- 80%, depending on cropping practice
and length of time), the thresholds of land
abandonment in Stage II and the rates of re-
covery in Stage III can vary considerably.
New questions arise: over what period can
Stage III recover half of the Stage I losses?
Can Stage III exceed the starting point at the
farming system level? It certainly can lo-
cally, as shown by anthrosols (i.e. a soil that
has been formed or heavily modified due to
long-term human activity) and home garden
sites (Chapter 10, this volume) with sub-
stantial organic inputs derived elsewhere in
the landscape. Organic waste from the local
processing of agricultural products can also
be a source of local enrichment beyond that
achieved by the native vegetation's inter-
action with climate and inherent soil prop-
erties.
The decline of soil carbon in Stage I is a
consequence of land management decisions
that are driven mainly by the opportunity to
use productive soils, newly converted from
natural vegetation, where the buffering
functions provided by high soil organic
matter content appear to be in excess of
what is needed or appreciated by the farmer,
where there has not yet been a build-up of
soilborne crop diseases and where declin-
ing soil organic matter content provides a
nutrient source that makes the use of expen-
sive fertilizers unnecessary. While other
types of mining of natural resources are

taxed and regulated, this soil resource util-
ization (mining of soil carbon) tends to be
free, or even stimulated and subsidized by
governments that see an expansion of agri-
cultural production as key to their develop-
ment strategies. The subsequent decline of
productivity, loss of soil structure, reduced
infiltration, initiation of erosion problems
and emergence of pest and disease prob-
lems tends to be left to the farmer to deal
with - with land abandonment and migra-
tion to new land as an historical option for
which the Earth is now too densely popu-
lated.
The examples given in the previous
section speak of the potential for a rapid de-
cline in soil organic carbon, whereas recov-
ery may be slow. Preventive action on the
decline is strategic and ultimately econom-
ical from a macro perspective, even if it in-
volves short-term costs. There is a rationale
for the stakeholders at the watershed or
landscape scale to influence individual
farmer decisions to increase infiltration ef-
fectively, if that can reduce the risks of ero-
sion and spillover sedimentation effects,
leading to loss of watershed functions. These
influences normally come as a combination
of three primary policy instruments: regula-
tion, persuasion and incentives (Bemel-
mans-Videc et al ., 2003; van Noordwijk
et al ., 2012). Community control over indi-
vidual land access has been the historical
base of such feedbacks; land-use planning
and zoning decisions are the current ones -
contested by a sense that farmers should be
allowed to do what they want on land that
they own.
Multiple positive processes occur once
trajectories change towards enhancing soil
organic carbon levels during Stage III. As
the case studies illustrate, there are various
ways to increase soil carbon levels and deal
with trade-offs (Klapwijk et al ., 2014).
A wide range of practices/technologies and
approaches (such as participatory processes
in a watershed approach) are available to
realize the potential (Chapter 4, this vol-
ume). An enabling environment is critical
to allow their adaptation and adoption in
Stage III. How can policy encourage the actions
needed to move from Stage II to Stage III?