[1] We studied the changes in soil carbon contents when pastures are converted to either secondary forest or plantation forest in north-western Ecuador. At 40 sites within the region, paired pasture and forest plots were compared. We related the observed soil carbon concentrations, stocks, and changes (in the 0-0.25 m and 0.25-0.5 m layers) to land use history, climate, and soil characteristics. Variation in carbon concentrations over sites in volcanic soils could be well predicted for both pastures (R 2 = 0.96) and forests (R 2 = 0.93) on the basis of soil mineralogy, while for sedimentary soils, clearly less variation could be explained (R 2 = 0.14 for pastures and 0.39 for forests). The dominant factor explaining changes in carbon stocks following pasture to forest conversion was pasture age. Forests, paired with pastures less than 10 years old, had on average 9.3 Mg ha À1 less soil carbon than the pastures, while forests paired with pastures between 20 and 30 years old had on average 18.8 Mg ha À1 more soil carbon and forest paired with pastures older than 30 years had on average 15.8 Mg ha À1 more carbon than the pastures. In this region, reforestation of old pastures will generally lead to an increase of soil carbon stocks. These results can be used for optimal site selection for carbon sequestration projects and for including soil carbon in the estimated benefits of these projects.

The influence of soil C stabilization mechanisms is normally not considered in studies on the effects of land use changes. Instead, observed changes are typically explained by differences in litter input. As a result, it is not well known if and how quickly newly incorporated C is stabilized in soils. Our goals were to find out how much soil C was stabilized in two different soil orders (Andisols and Inceptisols) and which are the responsible mechanisms of C stabilization. Furthermore, we looked for evidence that newly incorporated soil C was stabilized in these contrasting soil orders. We selected 25 sites in northwestern Ecuador with two paired plots per site: one plot where pasture was converted to secondary forest and one plot where forest was converted to pasture. In all the plots, soil C content, stocks, and stable isotope (δ13C) signal were measured in the surface soil. The δ13C values were used to estimate the stocks of soil C derived from forest (Cdf) and from pasture (Cdp) in all plots. We calculated correlations between these stocks and soil and environmental characteristics to identify mechanisms of soil C stabilization. Our results show that long‐term stabilization in Andisols was through formation of metal–humus complexes and allophane, while in Inceptisols long‐term stabilization was through sorption to clay minerals. We found evidence that recently incorporated C was not stabilized in Andisols, while in Inceptisols, poorly crystalline (hydr‐) oxides seemed to have stabilized part of this soil C. We conclude that unless soil C stabilizing mechanisms are explicitly considered, we will not be able to predict the direction and magnitude of changes in soil C stocks following land use changes in the tropics.

Quantitative knowledge of stabilizationand decomposition processes is necessary to understand, assess and predict effects of land use changes on storage and stability of soil organic carbon (soil C) in the tropics. Although it is well documented that different soil types have different soil C stocks, it is presently unknown how different soil types affect the stability of recently formed soil C. Here, we analyze the main controls of soil C storage in the top 0.1 m of soils developed on Tertiary sediments and soils developed on volcanic ashes. Using a combination of fractionation techniques with 13 C isotopes analyses we had the opportunity to trace origin and stability of soil carbon in different aggregate fractions under pasture and secondary forest. Soil C contents were higher in volcanic ash soils (47-130 g kg -1 ) than in sedimentary soils (19-50 g kg -1 ). Mean residence time (MRT) of forest-derived carbon in pastures increased from 37 to 57 years with increasing silt + clay content in sedimentary soils, but was independent from soil properties in volcanic ash soils. MRTs of pasture-derived carbon in secondary forests were considerably shorter, especially in volcanic ash soils, where no pasture-derived carbon could be detected in any of the four studied secondary forests. The implications of these results are that the MRT of recently incorporated organic carbon depends on clay mineralogy and is longer in soils dominated by smectite than non-crystalline minerals. Our results show that the presence of soil C stabilization processes, does not necessarily mean that recent incorporated soil C will also be effectively stabilized.

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Costs of reforestation projects determine their competitiveness with alternative measures to mitigate rising atmospheric CO2 concentrations. We quantify carbon sequestration in above-ground biomass and soils of plantation forests and secondary forests in two countries in South America-Ecuador and Argentina-and calculate costs of temporary carbon sequestration. Costs per temporary certified emission reduction unit vary between 0.1 and 2.7 USD Mg(-1) CO2 and mainly depend on opportunity costs, site suitability, discount rates, and certification costs. In Ecuador, secondary forests are a feasible and cost-efficient alternative, whereas in Argentina reforestation on highly suitable land is relatively cheap. Our results can be used to design cost-effective sink projects and to negotiate fair carbon prices for landowners.