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Background and aims: In the Jomoro district in Ghana, tree plantations were the first cause of deforestation in the past, drastically reducing the area occupied by primary forests. The aim of this study was to quantify soil organic carbon (SOC) losses due to a change in land use from primary forest to tree plantations (cocoa, coconut, rubber, oil palm) on the different substrates of the district. Secondary forests and mixed plantations were also included in the study. Methods: Soils were sampled at different depths up to 100 cm along a series of chronosequences in each of the three substrates (Granite, Lower Birrimian and Tertiary Sands) present in the area. Results: The highest SOC losses in the 0-30 cm layer were caused by the conversion of primary forests to tree plantations: cocoa −61 % of the original SOC stock, coconut −55 %, rubber −35 % and oil palm 28 %, while mixed plantations and secondary forests showed a loss of 23 % and 21 % of the original SOC stock, respectively. C losses were less apparent from the entire profile (to a depth of 100 cm). Conclusions: All conversions to tree plantations caused substantial SOC losses, comparable to the conversion of forests to agricultural systems. Secondary forests and mixed plantations were the only sustainable land uses that restricted SOC losses considerably. [ABSTRACT FROM AUTHOR]

The contribution of decomposing soil organic carbon (SOC) to total annual soil respiration (SR) was evaluated by radiocarbon measurements at a Scots pine stand growing on a plaggen soil in the Belgian Campine region. Two approaches were used to estimate the contribution of different C pools to SR. In the first approach, the variations in C content of soil CO efflux were monitored during one year (2003) and compared to the atmospheric and SOC C signatures to determine the contribution of 'fast' (root respiration and fast decomposing SOC) and 'slow' cycling C pools to total SR. In the second approach an estimate of the total heterotrophic soil respiration (Rh), comprising the slow cycling C and the heterotrophic part of the fast-cycling C pools, was derived applying a box model based on the amount of the bulk SOC pool and its C-derived mean residence time (MRT). The quantification of the Rh and the decomposition rate of the slow-cycling SOC allows to indirectly determining the contribution of the heterotrophic C that decompose within a year. Measurements of total SR performed in the field allowed assessing the contribution of the different C pools to total soil C efflux. On an annual basis, the fast-cycling C was the main contributor to SR, about 85%, while the contribution of the slow-cycling C (with MRT >1 yr) to total SR was 15%. Total annual Rh was 36% of total SR, which is in the lower range reported for temperate coniferous forests. The comparison of Rh with other estimates for the same site (47-50% of total SR) suggest a possible underestimation of the C flux from the mineral soil. In fact, the 'very old' C contained in the plaggen horizon strongly affects the signature of the mostly young C leaving the soil. In conclusion, our results indicate that the contribution of SOC decomposition to total soil CO flux in this forest is less than 40%, and at least half of it comes from organic compounds less than 1 year old. [ABSTRACT FROM AUTHOR]

The purpose of the present work is to asses the possibility of detecting changes in soil organic carbon (SOC) at the end of the 5-years of the first Commitment Period (CP) of the Kyoto Protocol of the United Nation's Framework Convention on Climate Change (1 January, 2008-31 December, 2012), by both direct measurement and the use of an opportunely evaluated SOC model, CENTURY. The investigated soil is young, developed since 28 years on virtually C-free spoil banks and under the influence of two managed forest stands, one a mix of English oak ( Quercus robur L.) and Italian alder ( Alnus cordata Loisel.) and the other pure English oak. The SOC stock of either stand was monitored since the time the stands were planted in 1981, and it was used together with other parameters for the model evaluation, while the future projections for the end of the first (2012) and second (2017) CP were made according to two extreme IPCC climatic scenarios: A1F1, the most dramatic, and B2, among the less impacting. Direct SOC measurements performed at the beginning and at the end of a time frame equivalent to a commitment period (2004-2008) had not shown significant variations in either stands. Compared to the 2008 SOC stock, in both stands the model shows variations at the end of the first CP from 0.7 to 1.8 Mg C ha for the A1F1 scenario and from 0.3 to 1.7 Mg C ha for the B2. These variations are within the standard deviations of the C stocks measured in 2008. On the contrary, at the end of the second CP, the modelled SOC increments range from 2.5 to 3.6 Mg C ha (A1F1) or from 1.9 to 3.4 Mg C ha (B2), indicating the possibility to detect the SOC changes by direct measurement, since the values well agree with the minimum detectable variation estimated for both sites in 3.3-4.5 Mg C ha. This work shows that SOC stock changes measured directly in the field can be minimal at the end of both CPs, and that CENTURY well simulates the SOC dynamics of the stands. The use of such a model, validated at long-term experimental sites, hence represents an effective tool for estimating future changes in SOC amounts in support of direct measurements when a short period of time, such as the CP, is considered. [ABSTRACT FROM AUTHOR]

Tropical primary rainforests of Africa are an enormous reservoir of carbon (C), most of which, in the common perception, is stored in the biomass. We studied one of these forests, Ankasa, in the south-western part of Ghana, in terms of quantity and 14C activity of soil organic carbon (SOC) to elucidate the little known important role of soil in storing carbon in such biomass-rich environments. The stock of carbon in the mineral soil to a depth of 1 m was measured to be 151 ± 20 Mg C ha−1, a similar value in magnitude to the one of the aboveground biomass being 138–170 Mg C ha−1, including live and dead wood. Surface litter C is roughly 10% (15 ± 9 Mg C ha−1) of the C in the biomass and soil. The radiocarbon measurements indicate that SOC was significantly affected by “bomb C” enrichment, so that “Modern C”, namely with a mean radiocarbon age lower than 200 years, is present also deeper than 45 cm in the Bo2 horizon. The mean residence time (MRT) estimated from radiocarbon content are of the order of a few decades in the topsoil and a few centuries in the deeper horizons. Altogether, the MRT values indicate a fast recycle of C compared to temperate or boreal forests, but not as fast as usually believed for tropical forest soils. Making a pondered mean, in the Ankasa forest the time an atom of C resides in soil is not much different from one atom of C in the woody aboveground biomass. Hence, the contribution of soil in storing C is substantial, implying that in primary rainforests it is mandatory to determine the SOC stock and its dynamics, too often neglected or underestimated. [ABSTRACT FROM AUTHOR]