Alternative Scenarios for the Development of Oil Palm in the Tropics: Carbon and Nutrients Dynamics

Expansion of agriculture in forested areas poses significant threats and pressure to ecosystems and potentially the global climate. More specifically, expansion of oil palm (OP) plantations has been found to exert great impact on key ecosystem functions like C storage, water cycling and support biodiversity. Alternative pathways for a more sustainable development of agriculture have been proposed. However, lack of evidence through comprehensive studies constrains the potential of deforestation free options to be accounted as a fundamental part or as priority scenarios of future sustainable OP development. This thesis work looks to provide insights into the effects of two alternatives land use change options into OP: planted pastures and savanna conversion on soil biogeochemical aspects and ecosystem C storage. Pastures conversion into OP was C neutral at the ecosystem level (Manuscript 1), however soil C storage decreased by 39% during the first OP cycle, ca. 30 years, and then remained stable along the second OP cycle. This ecosystem C neutrality contrasts remarkably with the deforestation scenario for OP development, where about 173 Mg C ha-1 are lost due to the establishment of OP plantations. Another positive implication of establishing OP in pasture areas was the increased chemical fertility. However, this trade-off between soil C storage and chemical fertility demands close attention so that soil fertility could be enhanced mainly by means of soil natural processes rather than by adding chemical inputs. Such reliability on SOC to enhance soil fertility in OP plantations is possible in the long-term as indicated by the found recovery of labile C fractions and bulk soil C content in the topsoil (Manuscript 4). In the conversion of savannas into OP plantations, a rather positive ecosystem C balance was found (Manuscript 2). Such C gain at the ecosystem level was mainly explained by the no change in soil C stocks over an entire OP rotation cycle, which in turn was due to the slow decomposition of savanna-derived C and effective OP-derived C accumulation that offset the small savanna C losses. Importantly, soil biological activity and SOC stocks were found to be maintained or even enhanced, when organic amendments (i.e. crop residues from frond piles) were used. This contrasted, with the practice that relied heavily on mineral fertilizers application to increase chemical fertility. It was also demonstrated that changes in SOC stocks within plantations are mainly driven by C inputs that are controlled by OP fine roots (Manuscript 3). The fine OP roots were directly affected by nutrients addition and drive changes in soil microbial properties. This work demonstrates that alternatives LUC to OP can reduce significantly the C losses associated with deforestation. Future OP development should be directed toward degraded pastures areas so that the initial SOC losses are not as high as found in the well-managed pastures of this work. Notably, the management decisions will play a key role on the direction and strength of SOC stock changes and can further increase the positive ecosystem C outcomes and soil C storage capacity of OP plantations in former savanna or pasture areas. Therefore, OP cultivation should shift toward a more ecologically oriented intensification with a better balance between environmental and productivity goals.