Abstract

In this paper, we attempt to analyse the respective influences of land-use and climate changes on the global and regional balances of soil organic carbon (SOC) stocks. Two time periods are analysed: the historical period 1901–2000 and the period 2000–2100. The historical period is analysed using a synthesis of published data as well as new global and regional model simulations, and the future is analysed using models only. Historical land cover changes have resulted globally in SOC release into the atmosphere. This human induced SOC decrease was nearly balanced by the net SOC increase due to higher CO2 and rainfall. Mechanization of agriculture after the 1950s has accelerated SOC losses in croplands, whereas development of carbon-sequestering practices over the past decades may have limited SOC loss from arable soils. In some regions (Europe, China and USA), croplands are currently estimated to be either a small C sink or a small source, but not a large source of CO2 to the atmosphere.In the future, according to terrestrial biosphere and climate models projections, both climate and land cover changes might cause a net SOC loss, particularly in tropical regions. The timing, magnitude, and regional distribution of future SOC changes are all highly uncertain. Reducing this uncertainty requires improving future anthropogenic CO2 emissions and land-use scenarios and better understanding of biogeochemical processes that control SOC turnover, for both managed and un-managed ecosystems.

Highlights

  • During the past decades, research efforts have been made to improve our knowledge about the effects of climate, atmospheric CO2 and land-use change on terrestrial C cycle

  • It is interesting to note that most global carbon cycle models (Friedlingstein et al, 2006) show a net gain in high-latitude terrestrial carbon due to warming, while initial results using a permafrost-enabled version of ORCHIDEE (Koven C., Ringeval B., Ciais P., Friedlingstein P., personal communication) show carbon losses due to warming

  • For the most extreme emissions scenario (A1F1), three of the five dynamic global vegetation model (DGVM) (Hyland, LPJ, TRIFFID) simulate strong forest dieback and soil carbon loss in both the Amazon and African tropical forests, whereas there is a small increase of soil organic carbon (SOC) stocks for ORCHIDEE and Sheffield (Sitch et al, 2008)

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Summary

Introduction

Research efforts have been made to improve our knowledge about the effects of climate, atmospheric CO2 and land-use change on terrestrial C cycle. The mechanisms and factors that govern global uptake and release of C from the terrestrial reservoir, and their regional importance, are still poorly quantified This could only be achieved through C cycle modelling studies as long-term in situ measurements are very sparse and lack global coverage, and remote sensing techniques have limited capability for estimating below canopy processes such as soil respiration and are subject to significant absolute errors in carbon fluxes There exist several C cycle modelling studies for understanding and predicting the global terrestrial carbon cycle (McGuire et al, 2001; Cao et al, 2002; Peylin et al, 2005; Zeng et al, 2005; Friedlingstein et al, 2006; Sitch et al, 2008; Piao et al, 2009a,b).

Global distribution of SOC
Global vulnerability of SOC to climate
Global vulnerability of SOC to land-use changes
Historical global land cover change and SOC balance
Agricultural intensity and SOC balance
Forest management and SOC balance
Effect of climate and CO2 change
Effect of future agricultural change
Potential to increase SOC in agriculture
Uncertainties and future directions
Findings
Conclusions
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