Bridging Food Webs, Ecosystem Metabolism, and Biogeochemistry Using Ecological Stoichiometry Theory

Although aquatic ecologists and biogeochemists are well aware of the crucial importance of ecosystem functions, i.e., how biota drive biogeochemical processes and vice-versa, linking these fields in conceptual models is still uncommon. Attempts to explain the variability in elemental cycling consequently miss an important biological component and thereby impede a comprehensive understanding of the underlying processes governing energy and matter flow and transformation. The fate of multiple chemical elements in ecosystems is strongly linked by biotic demand and uptake; thus, considering elemental stoichiometry is important for both biogeochemical and ecological research. Nonetheless, assessments of ecological stoichiometry (ES) often focus on the elemental content of biota rather than taking a more holistic view by examining both elemental pools and fluxes (e.g., organismal stoichiometry and ecosystem process rates). ES theory holds the promise to be a unifying concept to link across hierarchical scales of patterns and processes in ecology, but this has not been fully achieved. Therefore, we propose connecting the expertise of aquatic ecologists and biogeochemists with ES theory as a common currency to connect food webs, ecosystem metabolism, and biogeochemistry, as they are inherently concatenated by the transfer of carbon, nitrogen, and phosphorous through biotic and abiotic nutrient transformation and fluxes. Several new studies exist that demonstrate the connections between food web ecology, biogeochemistry, and ecosystem metabolism. In addition to a general introduction into the topic, this paper presents examples of how these fields can be combined with a focus on ES. In this review, a series of concepts have guided the discussion: (1) changing biogeochemistry affects trophic interactions and ecosystem processes by altering the elemental ratios of key species and assemblages; (2) changing trophic dynamics influences the transformation and fluxes of matter across environmental boundaries; (3) changing ecosystem metabolism will alter the chemical diversity of the non-living environment. Finally, we propose that using ES to link nutrient cycling, trophic dynamics, and ecosystem metabolism would allow for a more holistic understanding of ecosystem functions in a changing environment.

Ecohydrological interfaces as hotspots of ecosystem processes

Tom Ian Battin

The movement of water, matter, organisms, and energy can be altered substantially at ecohydrological interfaces, the dynamic transition zones that often develop within ecotones or boundaries between adjacent ecosystems. Interdisciplinary research over the last two decades has indicated that ecohydrological interfaces are often “hotspots” of ecological, biogeochemical, and hydrological processes and may provide refuge for biota during extreme events. Ecohydrological interfaces can have significant impact on global hydrological and biogeochemical cycles, biodiversity, pollutant removal, and ecosystem resilience to disturbance. The organisational principles (i.e., the drivers and controls) of spatially and temporally variable processes at ecohydrological interfaces are poorly understood and require the integrated analysis of hydrological, biogeochemical, and ecological processes. Our rudimentary understanding of the interactions between different drivers and controls critically limits our ability to predict complex system responses to change. In this paper we explore similarities and contrasts in the functioning of diverse freshwater ecohydrological interfaces across spatial and temporal scales. We use this comparison to develop an integrated, interdisciplinary framework, including a roadmap for analysing ecohydrological processes and their interactions in ecosystems. We argue that, in order to fully account for their non-linear process dynamics, ecohydrological interfaces need to be conceptualised as unique, spatially and temporally dynamic entities, which represents a step change from their current representation as boundary conditions at investigated ecosystems.

American Geophysical Union2017

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

Juan Carlos Quezada Rivera

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

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

Juan Carlos Quezada Rivera

EPFL2020