The Role of Soil Organic Matter and Manures in Sustainable Nutrient Cycling - Sustainable Management of Soil Organic Matter - page 316

P. Smithet al. Carbon Mitigation Options in Agriculture

4.13

Carbon Mitigation Options
in Agriculture: Improving
our Estimates for Kyoto

P. S MITH , P.D. F ALLOON , D.S. P OWLSON AND
J.U. S MITH

Soil Science Department,IACR-Rothamsted,Harpenden,
HertfordshireAL5 2JQ, UK

Introduction

Soils are only one component of an ecosystem and often respond to global
change only indirectly via physiological responses in plants. In response to
raised concentrations of CO 2 in the atmosphere, for example, all ecosystem
responses are mediated by only three direct effects on plant processes:
photosynthesis, respiration and transpiration. Impacts on the soil are
mediated through secondary responses in the plant such as changes in
water use efficiency, specific leaf area, root : shoot ratio, C : N ratios and
many others, so that impacts of raised CO 2 upon the soil can be regarded as
tertiary in nature. Figure 4.13.1 demonstrates schematically the complexity
of the ecosystem response to raised CO 2 .
Given that soil responses are often so far removed from the direct
effects of global change drivers, it is not surprising that soil responses to
global change are so difficult to predict. However, it is vital that soils are
considered in global change studies since soils represent a major pool
of carbon in the biosphere, estimated at ~1500 10 15 g (Batjes, 1996)

globally, about twice that in atmospheric CO 2 . Given that the global stock
of soil carbon is so large, it is clear that only small changes in soil carbon
stocks can result in significant perturbations to the global carbon cycle.
Here we briefly review attempts to quantify European carbon mitigation
options in agriculture and examine opportunities and limitations for
improving these estimates in the future.

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