The effect of land-use change on the net exchange rates of greenhouse gases: a meta-analytical approach.

One of the environmental impacts of land-use change (LUC) is a change in the net exchange of the greenhouse gases (GHGs) carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). Here we summarize findings based on a new global database containing data sets of changes in soil organic carbon stocks and soil CH₄ and N₂O fluxes. We combine that with estimates of biomass carbon stock changes and enteric CH₄ emissions following LUC. Data were expressed in common units by converting net CH₄ and N₂O fluxes to CO₂ equivalents (CO₂ eq) using established global warming potentials, and carbon-stock changes were converted to annual net fluxes by averaging stock changes over 100 yr. Conversion from natural forest to cropland resulted in the greatest increase in net GHG fluxes, while conversion of cropland to secondary forest resulted in the greatest reduction in net GHG emissions. Specifically, LUC from natural forest to crop and grasslands led to net fluxes of 6.2 ± 1.6 (Mean ± 95 % confidence intervals) and 4.8 ± 1.6 t CO₂ eq ha^-1 yr^-1 to the atmosphere, respectively. Conversely, conversion from crop and grasslands to secondary forest reduced net emissions by 6.1 ± 4.1 and 3.9 ± 1.2 t CO₂ eq ha^-1 yr^-1, respectively. Land-use change impacts were generally dominated by changes in biomass carbon. A retrospective analysis indicated that LUC from natural forests to agricultural lands contributed a cumulative 1326 ± 449 Gt CO₂ eq between 1765 and 2005, which is equivalent to average emissions of 5.5 ± 1.6 Gt CO₂ eq yr^-1. This study demonstrates how specific LUCs can positively or negatively affect net GHG fluxes to the atmosphere. [ABSTRACT FROM AUTHOR]