Effects of reforestation, deforestation, and afforestation on carbon storage in soils.

This study reviews the effects of changes in land use and land management on SOC pools in forest soils. In the 1990s, deforestation remained the most important land-use change in tropical regions (-142 x 10(6) ha per year). In non-tropical regions the forested area increased in developed countries as a result of natural reforestation (+26 x 10(6) ha per year). Deforestation also continued in under-developed countries in temperate regions. Without intensive site preparation, harvest followed by natural regeneration or reforestation has little impact on SOC pools in the mineral topsoil (0-0.3 m). Intensive site preparation results in losses of 6-13% of the initial SOC from the topsoil in the first decades. On average, deforestation followed by conversion to cropland results in SOC losses of 42% (or 0.1-1500 g (C) m(-2)) from the mineral topsoil, whereas conversion to pasture results in gains of 8%. The largest changes in SOC storage occur within the first two decades. After reforestation, SOC accumulation depends on the kind of managed forest established. Under productive deciduous reforestation (excluding eucalypts), SOC in the mineral topsoil accumulates at a rate of 20-50 g (C) m(-2) per year, and SOC pools could recover from cultivation-induced losses within 40 years. Under coniferous reforestation, the rate of accumulation of carbon is highest (95 g (C) m(-2) per year) in the organic layer, which is very susceptible to site preparation practices. In the mineral topsoil, the rate of accumulation is much lower (4 g (C) m(-2) per year), and recovery of the initial SOC pools might take several hundred years. The resulting land-use 'memory effect' has introduced large variation of the SOC pools in contemporary carbon budget studies. Thus, there seems to be a large temporal asymmetry between the period of time over which depletion of SOC occurs and the time needed for recovery of the SOC pools in the mineral soil. This should be taken into account when considering land-use and land-management activities to decrease atmospheric CO2 concentrations over this century.