drivers of soil respiration of root and microbial ... - Unitus DSpace

1.1. Global C balance: biosphere-atmosphere interactions and human influence.
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The earth contains approximately 10 8 Gt of C distributed in several pools which differ in
their size and turnover times (Fig.1).
Fig. 1 The global carbon cycle. Values are given in Gt C. Bold prints represent reservoirs and normal prints
represent fluxes. Mean residence time of the pools is given in parentheses. DOC = dissolved organic carbon,
DIC = dissolved inorganic carbon. Source: WBGU (Schubert et al. 2006). Adapted after Schlesinger (1997);
WGBU (2003). Numbers expanded and updated for ocean and fossil fuels: Sabine et al. (2003); Raven et al.
(2005); for atmosphere: NOAA-ESRL, (2006).
Natural systems and biogeochemical cycles have historically maintained these pools in dynamic
equilibrium. More recently, anthropogenic activities such as deforestation, agricultural practices, and the
burning of fossil fuels have resulted in large shifts among carbon pools, particularly since the beginning of
the industrial revolution (IPCC 1995, 2001) (Fig.2). Annual emissions of CO2 from fossil fuel
combustion are small relative to the natural flows of carbon; nevertheless, these anthropogenic
emissions are the major contributor to increasing concentrations of CO2 in the atmosphere and
above all they represent a transfer of carbon from the slow carbon cycle to the active carbon cycle.
From 1850 to 1998, 270 ± 30 Gt C were emitted from fossil fuel burning and cement production
(Marland et al., 1999). During the same period, emissions from land use change were estimated at
136 ± 55 Gt C (Houghton 2003). These last were related to deforestation, biomass burning,
conversion of natural to agricultural systems and the ploughing of soils. World soils historically
have been major source of atmospheric enrichment of CO2: until the 1950s more C was emitted into
the atmosphere from land use change and soil cultivation than from fossil fuel combustion (Lal,
2003). Presently, about 20% of the global emissions come from land use change (IPCC, 2001). In
fact, soils with a total global storage of approximately 1500 Gt C (Jacobson et al. 2000) hold three