Savanna

Summary

A savanna or savannah is a mixed woodland-grassland (i.e. grassy woodland) ecosystem characterised by the trees being sufficiently widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of grasses. According to Britannica, there exists four savanna forms; savanna woodland where trees and shrubs form a light canopy, tree savanna with scattered trees and shrubs, shrub savanna with distributed shrubs, and grass savanna where trees and shrubs are mostly nonexistent. Savannas maintain an open canopy despite a high tree density. It is often believed that savannas feature widely spaced, scattered trees. However, in many savannas, tree densities are higher and trees are more regularly spaced than in forests. The South American savanna types cerrado sensu stricto and cerrado dense typically have densities of trees similar to or higher than that found in South American tropical f

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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.

EPFL2020

Drivers of soil carbon stabilization in oil palm plantations

Alexandre Buttler, Thomas Guillaume, Juan Carlos Quezada Rivera, Johanna Katharina Rüegg, Mathieu Raymond Santonja

Increasing soil organic carbon (SOC) in agroecosystems is necessary to mitigate climate change and soil degradation. Management practices designed to reach this goal call for a deeper understanding of the processes and drivers of soil carbon input stabilization. We identified main drivers of SOC stabilization in oil palm plantations using the well-defined spatial patterns of nutrients and litter application resulting from the usual management scheme. The stabilization of oil palm-derived SOC (OP-SOC) was quantified by delta C-13 from a shift of C4 (savanna) to C3 (oil palm) vegetations. Soil organic carbon stocks under frond piles were 20% and 22% higher compared with harvest paths and interzones, respectively. Fertilization and frond stacking did not influence the decomposition of savanna-derived SOC. Depending on management zones, net OP-SOC stabilization equalled 16-27% of the fine root biomass accumulated for 9 years. This fraction was similar between frond piles and litter-free interzones, where mineral NPK fertilization is identical, indicating that carbon inputs from dead fronds did not stabilize in SOC. A path analysis confirmed that the OP-SOC distribution was largely explained by the distribution of oil palm fine roots, which itself depended on management practices. SOC mineralization was proportional to SOC content and was independent on phosphorus availability. We conclude that SOC stabilization was driven by C inputs from fine roots and was independent of alteration of SOC mineralization due to management. Practices favouring root growth of oil palms would increase carbon sequestration in soils without necessarily relying on the limited supply of organic residues.

WILEY2019

Land use and soil texture effects on organic carbon change in dryland soils, Senegal

Claire Guenat, Assize Touré

Soil organic carbon (SOC) losses due to poor soil management in dryland are now well documented. However, the in- fluence of soil properties on organic carbon change is not well known. The groundnut plant (Arachis hypogaea L.), and the dominant crop system in the Senegal’s Soudanian zone, have been compared with semi-natural savanna. Leaves, stems and roots biomass were measured, and soil characteristics were analysed. The total leaves and stems biomass was 1.7 and 2.7 Mg ha−1 dry matter in groundnut fields and savanna respectively. Total SOC stocks were low (8 to 20 Mg C·ha−1 within upper 0.2 m depth, 20 to 64 Mg C·ha−1 within upper 1 m depth) and were significantly lower (P < 0.05) in sandy soils than in sandy clayey soils, and lower (approximately 27% - 37%) in groundnut fields than in savanna soils. δ13C values show that SOC quality is transformed from the savanna plants (C4/C3 mixed-pools) to C3-pools in groundnut cultivated zone, with the organic matter signature more preserved in the clayey soils. This study confirms that converting woodland to groundnut fields provokes texture transformation and SOC loss. The results call for the extreme necessity to regenerate the wooded zone or encourage practices that favour SOC restitution.

2013