Abstract

Soils hold one of the largest near-surface pools in the global carbon cycle, containing at least 1,500 Pg C in organic forms, with a large proportion of this amount lying near the surface. Largely as a result of the human disturbance of soils, especially in cultivation, 36 Pg C was lost from soils between 1860 and 1960, with a current rate of loss of approximately 0.8 Pg C/yr. Thus, the loss of carbon from soils is a significant component of the biotic flux of CO2 to the atmosphere The soil carbon pool does not appear likely to house the missing sink. In fact, as a result of global warming, substantial amounts of CO2 are likely to be lost from soils.

Introduction

Soils hold one of the largest near-surface pools in the global carbon cycle, containing at least 1,500 Pg C in organic forms. Although some fractions of soil organic matter are very old, the global mean residence time for organic carbon in soils is approximately 30 years. The soil carbon pool is large and dynamic; increases or decreases in the amount of carbon in soils could have significant effects on the concentration of CO2 in the atmosphere (Trumbore et al., 1996). A large literature shows that human activities – especially cultivation – reduce the pool of carbon in soils and that most of this carbon is transferred to the atmosphere.