Soil Carbon Stocks and Land Management

The annual fluxes of CO2 from atmosphere to land - global Net Primary Productivity (NPP) - and land to atmosphere (respiration and fire) are each of the order of 60 billion tonnes C/year (IPCC, 2000a). During the 1990s, fossil fuel combustion and cement production emitted 6.3 ± 1.3 Pg C/year to the atmosphere, whilst land-use change emitted 1.6 ± 0.8 Pg C/year (IPCC, 2001; Schimel et al., 2001). Atmospheric carbon increased at a rate of 3.2 ± 0.1 Pg C/year, the oceans absorbed 2.3 ± 0.8 Pg C/year with an estimated terrestrial sink of 2.3 ± 1.3 Pg C/year (IPCC, 2001; Schimel et al., 2001).

©CAB International 2007. Greenhouse Gas Sinks (eds D.S. Reay, C.N. Hewitt, K.A. Smith and J. Grace)

The size of the pool of carbon in the soil is therefore large compared to gross and net annual fluxes of carbon to and from the terrestrial biosphere. Figure 4.1 (IPCC, 2001) shows a schematic diagram of the carbon cycle.

Soil carbon pools are smaller now than they were before human intervention. Historically, soils have lost between 40 and 90 Pg C globally through cultivation and disturbance (Schimel, 1995; Houghton, 1999; Houghton et al., 1999; Lal, 1999). The goal of engineering carbon sequestration is to increase the carbon sink relative to the current stock.

Soil carbon sequestration can be achieved by increasing the net flux of carbon from the atmosphere to the terrestrial biosphere by increasing global NPP (thus increasing carbon inputs to the soil), by storing a larger proportion of the carbon from NPP in the longer-term carbon pools in the soil or by slowing decomposition. For soil carbon sinks, the best options are to increase carbon stocks in soils that have been depleted (i.e. agricultural soils and

Nitrogen Deposition Ipcc Soil Carbon

Fig. 4.1. The global carbon cycle for the 1990s (billion tonnes C). (a) The natural carbon cycle (DOC = dissolved organic carbon). (b) The human perturbation. (Redrawn from IPCC, 2001.)

Fig. 4.1. The global carbon cycle for the 1990s (billion tonnes C). (a) The natural carbon cycle (DOC = dissolved organic carbon). (b) The human perturbation. (Redrawn from IPCC, 2001.)

degraded soils). In many regions, agricultural soils are net emitters of CO2 as described in the following section.

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