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Increasing soil organic matter (SOM) is of primary importance for maintaining soil fertility and mitigating climate change. Leaving crop residues on top of soil is not always an efficient means of increasing SOM because (i) of the high mineralization of the crop residues, (ii) crop residues may increase the mineralization of existing SOM (priming effect) and (iii) wetting-drying cycles may increase mineralization of SOM and crop residues. Little research has been carried out into these mechanisms under Sudano-Sahelian conditions where the rainfall is mostly irregular with wetting-drying cycles during the transition between the wet and the dry season. To evaluate the effect of wetting-drying cycles on the mineralization of SOM and crop residues and the priming effect, an agricultural soil from the North Region of Cameroon with or without (controls) 13 C-labeled rice straw amendment as crop residues was either subjected to five wetting-drying cycles or maintained at constant water potential after a single rewetting event. Soil samples were incubated for 70 days at 28 °C and the CO 2 and 13 CO 2 emissions and mineral N were monitored. Adding straw (+ 833 μg C g − 1 soil) increased the cumulative CO 2 emissions from the soil (+ 921 μg C-CO 2 g − 1 soil). A positive priming effect was observed (+ 92 μg C-CO 2 g − 1 soil). Only the first wetting cycle created a mineralization flush of the SOM and the straw. This flush did not recur probably because of a lack of labile SOM. However an extra addition of straw after 28 days increased CO 2 emissions but did not result in further mineralization flushes after re-wetting while SOM mineralization was not limited by N availability. We conclude that SOM depletion under Sudano-Sahelian conditions was not explained by SOM mineralization enhancement due to multiple rewetting events or to priming effect following crop residues addition. Indeed, SOM depletion could rather be explained by a high level of mineralization of both, SOM and crop residues, when the soil reached its water retention capacity.

High resolution and low uncertainty deforestation maps covering large spatial areas in tropical countries are needed to plan efficient forest conservation and management programs such as REDD + (Reducing Emissions from Deforestation and Forest Degradation). Using an open-source free software (R, GRASS and QGis) and an original statistical approach combining multi-date land cover observations based on Landsat satellite images and the random forests classifier, we obtained up-to-date deforestation maps for the periods 2000–2005 and 2005–2010 with a minimum mapping unit of 0.36 ha for 7.7 M hectares, i.e. 40.3% of the tropical humid forest and 20.6% of the tropical dry forest in Madagascar. Uncertainty in deforestation on the maps was calculated by comparing the results of the classification to more than 30,000 visual interpretation points on a regular grid. We assessed accuracy on a per-pixel basis (confusion matrix) and by measuring the relative surface difference between wall-to-wall approach and point sampling. At the pixel level, user accuracy was 84.7% for stable land cover and 60.7% for land cover change. On average for the whole study area, we obtained a relative difference of 2% for stable land cover categories and 21.1% land cover change categories respectively between the wall-to-wall and the point sampling approach. Depending on the study area, our conservative assessment of annual deforestation rates ranged from 0.93 to 2.33%·yr− 1 for the humid forest and from 0.46 to 1.17%·yr− 1 for the dry forest. Here we describe an approach to obtain deforestation maps with reliable uncertainty estimates that can be transposed to other regions in the tropical world.

This paper presents the results of the EX-Ante Carbon-balance Tool (EX-ACT) application on two rural development projects in Brazil. The analysis provides an estimate of project impact on GHG emissions and C sequestration indicating net mitigation potential: results show that the Santa Catarina Rural Competitiveness Project has the potential to mitigate 12.2 Mt CO 2 e and the Rio de Janeiro Sustainable Rural Development Project 0.85 Mt CO 2 e. Both projects are successful at promoting activities aimed at reducing rural poverty and also contribute to climate change mitigation, demonstrating the potential importance of sustainable agriculture (improved cropland and grassland management, expansion of agro-forestry systems and protection of forested areas) in delivering environmental services. EX-ACT has also been used as a tool to guide project developers in refining components and activities to increase project environmental benefits. Cost–benefit analysis shows that while both projects generate environmental benefits associated with climate change mitigation, the Santa Catarina Rural Competitiveness Project has significantly higher potential due to the size of the project area and the nature of activities, thus a higher likelihood of potential co-financing from climate finance sources.

To provide a better understanding of soil–atmosphere gas exchange processes, this study describes the atmospheric nitrous oxide (N2O) uptake by xylophagous termites and the biological process involved. The N2O consumption rates of three xylophagous termite species (Hodotermes mossambicus, Nasutitermes voeltzkowi and Hodotermopsis sjoestedti) were determined in incubation vials with ambient, artificially enhanced N2O concentrations in the headspace. Live individuals of the three termite species significantly decreased N2O concentrations (88%) in the headspace of the vials after 24 h incubation in the dark. The acetylene reduction assay method applied to N. voeltzkowi, a xylophagous termite species, showed a decrease in N2O uptake in acetylene-treated individuals, indicating the potential involvement of termite gut denitrifying microbes. The N2 formed is potentially subjected to assimilation via nitrogenase reductase into termite biomass through biological fixation as demonstrated by the reduction of acetylene to ethylene at an average rate of 18.21 ± 1.34 nmol C2H4 g−1 dw d−1. Further studies should focus on measurements of N2O-reductase (nosZ) gene activity in termite guts to gain a better understanding of the N2O reduction process in xylophagous termite species.

Increasing attention is being paid to near-infrared reflectance (NIR) spectroscopy for the rapid and cost-effective determination of soil carbon (C) and nitrogen (N) contents. The objective of the present paper was to compare the performances of two spectrometers: one covered the visible and NIR ranges (Vis–NIR, 350–2500 nm) and included a fibre-optic probe and the other covered the NIR range only (1100–2500 nm) and included a sample transport module. The comparison was carried out on two sets of clayey (n=97) and sandy (n=72) soil samples from tropical Africa and America. On the whole, both technologies provided good calibrations (R2>0.74) and predictions (R2>0.62) of soil C and N contents. The most accurate calibrations were achieved with the NIR spectrometer (R2>0.86), which also yielded the most accurate predictions for the sandy soils (R2=0.90 and standard error of prediction

National and sub-national scale estimates of soil organic carbon (SOC) stocks and changes can provide information land degradation risk, C sequestration possibilities and the potential sustainability of proposed land management plans. Under a GEF co-financed project, ‘The GEFSOC Modelling System’ was used to determine SOC stocks and projected stock change rates for four case study areas; The Brazilian Amazon, The Indo-Gangetic Plains of India, Kenya and Jordan. Each case study represented soil and vegetation types, climates and land management systems that are under represented globally, in terms of an understanding of land use and land management systems and the effects these systems have on SOC stocks. The stocks and stock change rates produced were based on detailed geo-referenced datasets of soils, climate, land use and management information. These datasets are unique as they bring together national and regional scale data on the main variables determining SOC, for four contrasting non-temperate eco-regions. They are also unique, as they include information on land management practices used in subsistence agriculture in tropical and arid areas. Implications of a greater understanding of SOC stocks and stock change rates in non-temperate areas are considered. Relevance to national land use plans are explored for each of the four case studies, in terms of sustainability, land degradation and greenhouse gas mitigation potential. Ways in which such information will aid the case study countries in fulfilling obligations under the United Nations Conventions on Climate Change, Biodiversity and Land Degradation are also considered. The need for more detailed land management data to improve SOC stock estimates in non-temperate areas is discussed.

The common GIS-based approach to regional analyses of soil organic carbon (SOC) stocks and changes is to define geographic layers for which unique sets of driving variables are derived, which include land use, climate, and soils. These GIS layers, with their associated attribute data, can then be fed into a range of empirical and dynamic models. Common methodologies for collating and formatting regional data sets on land use, climate, and soils were adopted for the project Assessment of Soil Organic Carbon Stocks and Changes at National Scale (GEFSOC). This permitted the development of a uniform protocol for handling the various input for the dynamic GEFSOC Modelling System. Consistent soil data sets for Amazon-Brazil, the Indo-Gangetic Plains (IGP) of India, Jordan and Kenya, the case study areas considered in the GEFSOC project, were prepared using methodologies developed for the World Soils and Terrain Database (SOTER). The approach involved three main stages: (1) compiling new soil geographic and attribute data in SOTER format; (2) using expert estimates and common sense to fill selected gaps in the measured or primary data; (3) using a scheme of taxonomy-based pedotransfer rules and expert-rules to derive soil parameter estimates for similar soil units with missing soil analytical data. The most appropriate approach varied from country to country, depending largely on the overall accessibility and quality of the primary soil data available in the case study areas. The secondary SOTER data sets discussed here are appropriate for a wide range of environmental applications at national scale. These include agro-ecological zoning, land evaluation, modelling of soil C stocks and changes, and studies of soil vulnerability to pollution. Estimates of national-scale stocks of SOC, calculated using SOTER methods, are presented as a first example of database application. Independent estimates of SOC stocks are needed to evaluate the outcome of the GEFSOC Modelling System for current conditions of land use and climate.

Conversion of the tropical forest to pasture results in changes in the quality and quantity of the soil organic matter (SOM) and other physical and chemical soil properties. Most studies concerning the carbon (C) cycle have focused on soil C stocks. However, little information is available on tropical soil respiration rates. The objectives of this paper are, firstly, to determine how conversion of a natural forest to a pasture and P fertilization affect (1) some soil properties and carbon and nutrient content, and (2) CO2 and CH4 exchanges at the soil–atmosphere interface. For such purposes, a forest-to-pasture chronosequence (pasture established in 1983, 1987, and 1994) was selected at the Fazenda Nova Vida in Rondonia. Soil sampling was done during the dry season (July 1996) and 6 months later during the rainy season (January 1997). Results show increased pH levels, exchangeable cations, and carbon stocks with the pasture installation. On the contrary, the level of available phosphorus decreased with the increasing age of the pastures, justifying the phosphorus fertilization practice. Regarding the CO2 and CH4 fluxes, we found that the forest and pasture soils had greater respiration (CO2 release) rates during the wet season than during the dry season but no specific and significant relationships between the emission and the pasture age was found. Forest soil CH4 consumption rates were three times lower during the wet season, whereas pasture soils showed a net emission of CH4 even during the dry season.

In natural ecosystems, soil organic carbon (C) is derived almost exclusively from the residues of plants growing in situ. In agroecosystems, it has at least two origins: one is the remains from the previous native vegetation, and the other is the remains of the crop and the decomposition of its residues. Where vegetation has changed from plants with the C3 photosynthetic pathway to C4 pathway or vice versa, changes in the natural abundance of 13 C in soil organic matter (SOM) over time can be used to identify sources of organic C in soil and to determine the turnover rate of SOM. For example, large areas of C3 tropical forest have been replaced with C4 pasture or cropland. Changes in the δ 13 C values of soil organic C in these areas reflect soil organic matter turnover rate, and provide insight regarding the functional role of tropical ecosystems in the global C cycle. This paper illustrates how the stable isotope 13 C can be used to estimate SOM turnover rates and the sensitivity of different models and different model parameters, using a chronosequence of forest and pastures of different ages from the Brazilian Amazon. A single-compartment exponential decay model and a two-compartment model in which SOM was divided into stable and labile components yielded similar estimates of soil C turnover time at the surface but divergent estimates at depth. The one-compartment model gave the least variable estimates of model parameters and turnover times and was also relatively insensitive to individual C stocks in single pastures of a particular age. Estimates of soil stable and labile C pools obtained using changes in forest soil δ 13 C with depth differed from estimates obtained using the chronosequence. This suggests that upon burning and pasture creation, a portion of the previously stable soil C pool is rendered less stable. Model r 2 was a poor criterion for selecting an appropriate soil C turnover model to apply to chronosequence data. In the absence of substantial justification for segregating SOM into different compartments based on lability, modeling should be done with the simplest models possible.

Conversion of natural forests to agriculture in the humid tropics leads to a reduction in ecosystem carbon storage due to the immediate removal of aboveground biomass and a gradual subsequent reduction in soil organic carbon. A considerable part of soil carbon is protected from microbial attack by a range of physical and chemical mechanisms and is not sensitive to landuse change. We analyzed the soils data base for Sumatra (Indonesia) developed by the Center for Soil and Agroclimate Research (CSAR) to estimate effects of landuse on soil C content. Sumatra has a considerable diversity of soils ranging from those of recent origin in the highlands, to older sedimentary and heavily leached soils in the pedimont peneplain and large areas of wetland soils along the coast. Peat soils (Histosols) and other wetland soils (Aquic and Fluvic suborders) contain the greatest soil C reserves, followed by young volcanic soils (Andisols). Agricultural use of these soils can have a disproportionately large effect on C release to the atmosphere. On the major part of the upland soils the difference in (top) soil C content between natural forest and agricultural land is in the range 0.5–1.0% C, equivalent to a change in total C stock of 10–20 Mg ha−1. These results agree with data collected in S. Sumatra in the 1930s. Corg of forest soils is related to soil pH, and is lowest in the pH range 5.0–6.0. Wetland conditions, lower pH, higher altititude (lower temperature) and higher clay and silt content all contributed to higher soil C contents in a multiple regression analysis of the whole data set. Existing models and data sets are insufficient to predict changes in soil C contents under various landuse practices. Carbon isotope studies, and especially the δ13C method may be used to study the effects of landuse change, especially when the vegetation was changed from one dominated by C3 plants (most forest species) to one dominated by grasses and crops with a C4 photosynthetic pathway. Results from Brazil documented a gradual decline of organic matter originating from the forest system and its partial replacement by organic matter derived from inputs of sugarcane during the first fifty years of cultivation. Forest conversion to well managed grasslands may lead to an increased soil C storage, after an initial decline. The consequences of erosion on losses of soil C depend on the scale at which these losses are considered, because of sedimentation processes. When net erosion losses are not expressed per unit area, but per length scale to the power l.6, erosion losses appear to be largely independent of scale. The `fractal dimension of erosion’ (on average around 1.6) probably is a landscape characteristic and estimates of its value are needed for extrapolation. Better understanding of soil C deposition sites is needed to evaluate overall erosion effects and test whether or not erosion can contribute to net C sequestration.

We examined the consequences of deforestation and pasture establishment for soil chemical and physical properties and for soil organic matter content, in Rondonia, in the southwestern part of the Brazilian Amazon basin. Two chronosequences were selected. One chronosequence consisted of a forest and pasture established in 1989, 1987, 1983, 1979 and 1972. The main soil type in this area is the red yellow podzolic latosol (Kandiudult). The second chronosequence consisted of a forest site and pasture established in 1987, 1983, 1972 and 1911, and the main soil type is a red yellow podzolic soil (Paleudult, Tropudult). The first soil type is the most base-depleted soil and has a higher clay content than the second one. Despite the initial differences in clay and cations contents between the forest sites the total soil carbon content at 0–30 cm in both forest were circa 3.7 kg C m−2 . After pasture installation soil bulk density were higher in the first 0–5 cm soil layer, mainly in one chronosequence but small changes were detected in deeper soil layers. Forest conversion to pasture caused appreciable increases in soil pH and exchangeable cation content, at least until nine years after pasture installation. pH levels were greater in the first chronosequence, with highest values (6.8 to 7.6) found in 3 and 5 years old pastures respectively. In the most base-depleted soil Ca content increased from 0.07 kg m−2 in the forest site to 0.25 kg m−2 in the 5 year old pasture. After normalization by clay content total soil carbon contents to 30 cm in the 20 year old pastures were 17 to 20% higher than in the original forest sites. Calculations of carbon derived from forest (Cdf) and from pasture (Cdp) using soil δ13C values showed that Cdf decrease sharply in the first 9 years after pasture establishment in both chronosequences and reached stable values of 2.12 kg C m−2 and 2.02 kg C m−2 in chronosequences 1 and 2, respectively. Soil carbon derived from pasture increased with time and represented 50% of total soil carbon in the top 30 cm after 20 years of pasture. In general we observed that forest conversion to pasture is associated to a pattern of increasing of soil cations and pH levels for at least 5 years under pasture establishment. The removal of the original forest for pasture establishment resulted in an accumulation of carbon derived from pasture in the soil.