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

Abstract

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.

Soil Carbon Stocks under Amazonian Forest: Distribution in the Soil Fractions and Vulnerability to Emission

“…Our results agree with other studies that demonstrate the importance of soil particle distribution in the control of soil carbon stocks (Lopez-Ulloa et al, 2005). Soil carbon stocks were greater in FLF and F-clay, stressing the importance of clay soils under primary forest in Amazonia in physical protection of soil carbon.…”

Geographic bias of field observations of soil carbon stocks with tropical land-use changes precludes spatial extrapolation

“…There is growing recognition that unless biophysical drivers are explicitly considered, we will not be able to estimate the consequences of land-use changes on soil C stocks (15) or predict the effects of management decisions, such as biochar amendment, to increase carbon sequestration (16). Precipitation strongly influences plant production, fluxes of soil C pools, and ultimately total soil C stocks and residence time (1).…”

Stabilization of recent soil carbon in the humid tropics following land use changes: evidence from aggregate fractionation and stable isotope analyses

“…We selected a subset of eight sites used by de Koning et al (2003) and López-Ulloa et al (2005). In addition to the sites which consisted of paired secondary forest (or tree plantation) and pasture we selected an additional natural forest site with similar overall environmental characteristics of pasture and secondary forest plots as close as possible.…”

“…Forest regrowth led to a yearly increase of soil C stocks in the upper 50 cm of about 1.4 Mg C ha -1 year -1 if the secondary forest had at least 20 years to grow (de Koning et al 2003). In a correlative study on a selection of these sites, long-term stabilization of soil C in volcanic ash soils was associated with the formation of metal-humus complexes and allophanes, whereas in soils with smectitic clay minerals soil C was stabilized primarily through sorption to clay (López-Ulloa et al 2005). In the present study, our objectives were: (i) to determine how important aggregation is for the stabilization of soil C in the topsoil of different soil types in natural forests, pastures and secondary forests of the humid tropics of Ecuador and (ii) to determine the stability of forestderived soil C in pastures and of pasture-derived soil C in secondary forests in different aggregate fractions.…”