Effects of rainforest transformation systems on carbon cycle and soil fertility

Agriculture faces the challenge of producing, in the first half of the 21st century, a similar amount of calories to those produced during the last 400 years, while also reducing its negative environmental impacts, such as greenhouse gas emissions and soil fertility losses. Agricultural intensification in the tropics has been achieved at the expense of rainforest. Thus, tropical regions such as Sumatra, that have experienced extensive deforestation in the past, are close to a complete exhaustion of easily accessible forest. Jambi Province, located in Central Sumatra, has experienced a four-fold increase of the area under oil palm plantation in the last 20 years. Land-use changes in tropical ecosystems leads to major modifications of biogeochemical cycles. The decrease of soil organic carbon (SOC) following the conversion of natural forest to plantations raises major concerns about soil functions that regulate global climate and support plant growth. This thesis aims at assessing the impacts of land-use changes in a tropical region suffering from land scarcity on two soil functions provided by SOC; C sequestration and soil fertility. In a survey of randomly-selected plantations over the Jambi province, the soil degradation under rubber plantations of various land-use intensities and oil palm plantations was assessed by measuring the C and N content, and bulk density in the topsoil. The C content under rubber plantations was half of the C content under forests in Jambi province. On average, soils under oil palm were more degraded than under rubber plantations. Because the oil palm boom started when the lowland of Jambi province had already experienced extensive deforestation, oil palm plantations were frequently established either on soils degraded by previous use or on peat soils and in riparian areas. Nevertheless, the structure and management of oil palm plantations led to a further degradation between oil palms row and trees. In order to separate the land-use effect from the history of the plantations, SOC losses were quantified in a space-for-time substitution approach with forest sites as references. SOC losses under plantations were strong in the topsoil. Dissolved organic carbon concentrations under oil palm were higher than under other land-uses, but no differences in SOC were observed in the subsoil of any of the plantation types. The SOC losses in the topsoil were highest under rubber and oil palm monocultures. Due to low ground protection from canopy and litter layers, soil erosion, estimated by a shift of δ13C values in the subsoil of plantations, was especially high in these two intensive plantations. According to the 13C enrichment of SOC, the SOC turnover was not affected by forest conversion to rubber plantations. Therefore, the SOC stocks decreased because of reduced C input from the vegetation in rubber plantations, and of additional erosional losses in rubber monocultures. The 13C enrichment of SOC suggested that the turnover was slower under oil palm plantations due to a depletion of the labile SOM pool resulting from the absence of leaf litter input. The decrease of SOM availability under plantations, and especially oil palm, was confirmed by various techniques. Not only was the quantity of labile pools, such as dissolved organic carbon or the light fraction, lower, but also the quality of SOM. The latter was assessed by the aliphatic/aromatic ratio, the H/C ratio or the SOM thermal stability. The decrease of SOM availability was indicated by a decrease of microbial activity under plantations. Nonetheless, the microbial activity was resistant to the decrease of SOM and labile pools. The resistance of microbial activity showed a threshold at a C content similar to the average C content under intensive rubber plantations where microbial activity decreased faster than the SOC losses. Despite that the mechanisms leading to this resistance have not been assessed yet, the nutrient enrichment of SOM under plantations could support microbial activity up to a threshold where C becomes the limiting factor. In conclusion, forest conversion to plantation led to a strong decrease of SOC in the topsoil. However, the SOC losses were relatively small in comparison to the C losses from the plant biomass. Nevertheless, the decrease of SOC had strong impacts on all indicators of soil fertility (microbial biomass, basal respiration, phosphatase activity, DOC, light fraction, SOM chemical and thermal stability, N content and available P). The level of soil degradation in Jambi province and the establishment of new plantations on soils with high SOC stocks are raising major concerns about the sustainability of agricultural intensification in the tropics. Therefore, management practices should be improved to mitigate soil erosion and provide higher C input from the biomass in order to maintain soil functions.