Climate Change and Soil Carbon Impacts - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 235

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Climate Change and Soil
Carbon Impacts

Pete Smith*, Pia Gottschalk and Jo Smith

Abstract
Soils contain vast reserves (~1500 Pg) of carbon (C), about twice that found as carbon dioxide in
the atmosphere. Historically, soils in managed ecosystems have lost a portion of this C ( 40-90 Pg)
through land-use change, some of which has remained in the atmosphere. In terms of climate change,
most projections suggest soil C changes driven by future climate change will range from small losses
to moderate gains, but these global trends show considerable regional variation. The response of soil
C in future will be determined by a delicate balance between the impacts of increased temperature and
decreased soil moisture on decomposition rates, and the balance between changes in C losses from
decomposition and C gains through increased productivity. In terms of using soils to mitigate climate
change, soil C sequestration globally has a large, cost-competitive mitigation potential. Nevertheless,
limitations of soil C sequestration include time limitation, non-permanence, displacement and diffi-
culties in verification. Despite these limitations, soil C sequestration can be useful to meet short- to
medium-term targets, and confers a number of co-benefits on soils, making it a viable option for reducing
the short-term atmospheric CO 2 concentration, thus buying time to develop longer-term emission reduc-
tion solutions across all sectors of the economy.

Introduction

the predicted biospheric feedback to the
atmosphere. Central questions that still remain
when attempting to reduce this uncertainty
in the response of soils to global warming
are: (i) the temperature sensitivity of soil or-
ganic matter, especially of the more recalci-
trant pools (Davidson and Janssens, 2006;
Smith et al ., 2008a); (ii) the balance between
increased C inputs to the soil from increased
production (from CO 2 fertilization and crop
breeding advances) and increased losses
due to increased rates of decomposition
(Smith and Fang, 2010); and (iii) interactions

Soils contain a stock of carbon (C) to a depth
of 1 m that is about twice as large as that in
the atmosphere and about three times that in
vegetation. Small losses from this large pool
could have significant impacts upon future
atmospheric carbon dioxide concentrations,
so the response of soils to global warming is
of critical importance when assessing climate
C cycle feedbacks (Smith et al ., 2008a).
Models that have coupled climate and C
cycles show a large divergence in the size of