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Soil & Water Management & Conservation

Soil Carbon Stabilization in Converted Tropical Pastures and Forests Depends on Soil Type

^a Georg-August Univ., Institute of Soil Science and Forest Nutrition, Buesgenweg 2, 37077 Goettingen, Germany
^b German Technical Cooperation (GTZ), Ecuador Casilla 17-07-8721, Quito, Ecuador

^* Corresponding author (bioteam1{at}uio.telconet.net)

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 (¹³C) signal were measured in the surface soil. The ¹³C 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.

Abbreviations: ¹³C, stable isotope • Al_o, aluminium extracted by acid oxalate • Al_p, aluminium extracted by pyrophosphate • Cdf, carbon derived from forest • Cdp, carbon derived from pasture • C_p, carbon extracted by pyrophosphate • Fe_o, iron extracted by acid oxalate • Fe_p, iron extracted by pyrophosphate • LF, light fraction • masl, meters above sea level • Si_o, silica extracted by acid oxalate

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