Francisco de Paula Santander Ocaña University

Area of specialization: Critical Zone is the “heterogeneous, near-surface environment in which complex interactions involving rock, soil, water, air, and living organisms regulate the natural habitat and determine availability of life-sustaining resources”. Its limits range from the top of the canopy down to the bottom of the aquifer. The successful applicant to this position will join the Global Critical Zone Science Chair to develop and conduct a research program to better understand forest nutrition and nutritional stress in Eucalyptus forest stands in Brazil. Research context: Forests cover 1/3 of the continental surfaces and play a crucial environmental role both at local and global scales. Forests sequester 25% of the carbon injected each year to the atmosphere by human activities and thus regulate the Earth’s global climate at short time scales. In its 2022 report for policymakers, the Word Resource Institute (Seymour et al., 2022) draws attention to the fact that forests do not only play a role by absorbing carbon from the atmosphere but also influence global and local temperature, rainfall patterns through albedo, evapotranspiration (forest ecosystems provide 2/3 of the continent precipitation), surface roughness and aerosols emission. At longer timescales, forests, via the formation of soil organic matter, erosion and deposition in the ocean, play an essential role in the regulation of climate at the geological time scale. As a consequence, protection and better management of forests is crucial for climate warming mitigation through carbon sequestration, as well as for other ecosystem services such as wood production or recreation. However, forests are “not just carbon”. For forests to play their role on the planet, not only do they need carbon withdrawn from the atmosphere and water from the soil, but they also require a number of major and minor nutrients. Among the factors that limit the energetic yield of photosynthesis to 0.1%, and thus the productivity of forest ecosystems, the availability of soil nutrients is probably one of the most important. With the exception of nitrogen (N) that can be fixed from the atmosphere, major nutrients such as potassium (K), silicon (Si), phosphorus (P) or minor nutrients such as metals (zinc, Zn; magnesium, Mg; calcium, Ca; boron, B; molybdenum, Mo,…) are ultimately derived from the transformation of soil minerals into secondary phases such as cation-poor clays and oxides, a process known as chemical weathering and taking place in soils or affecting atmospheric mineral aerosols. There is now ample evidence that forests are under increasing nutritional stress (Penuelas et al., 2020). The limitation of forest productivity by nutrients like N and P has been extensively studied (Du et al., 2020; Hou et al, 2020), while the role of other mineral-derived nutrients has attracted less studies. This situation is all the more critical now that we know that the pure “liebigian” limitation (one factor limits the growth) is not true for most ecosystems (Wurzburger et al., 2012), implying that the effect of other nutrients and micronutrients must be studied in detail. Hence a prerequisite for our ability to assess forest ecosystem evolution and maintenance of biomass productivity of agroforests, in the face of environmental change, is a better understanding of how plant nutrient requirements are met beyond C, N, and P. K is one of those mineral nutrients that has been investigated. Fertilization experiments have confirmed that K deficiency limits tree growth and forest productivity and mechanistic models have been developed that confirm the strong response of GPP (gross primary production) to a nutritional stress for K (Cornut et al., 2022) The overall objective of this postdoc project is to improve our knowledge of nutritional dynamics in forest ecosystems based on the balance between organic (dead biomass recycling) and mineral (chemical weathering or added sources by fertilization) sources. To achieve this goal the project aims at make use of isotopes, in particular boron isotopes, and potentially K isotopes. Recently, the analytical and conceptual development of so-called non-traditional stable isotopes opened up a new avenue for the study of nutrient cycling in forest ecosystems, the main idea being that biogeochemical processes will generate measurable discriminations between metal isotopes that can be used to trace their routes through living individuals, ecosystems, or the critical zone (e.g. Cividini et al. 2010, Dessert et al., 2015). In this respect, the trace element boron (B) appears as a powerful tool as it is a micronutrient involved in a wide variety of physiological processes where it undergoes significant isotopic fractionation of the two stable isotopes: 10B and 11B (Gaillardet et Lemarchand, 2018, Roux et al., 2021, Chetelat et al., 2021). In the Eucalypus stands, Boron is frequently added to the soil as it has been observed that boron fertilization improves the resistance of the trees to drought. The aim of the project is to focus on forest plantations as a “model forest” to better understand the behavior of boron and other major nutrients (and in particular potassium) and how their cycles is linked to the ecosystem services. It is coupling experimentation, isotopic measurements and modeling aspects taking advantage of a network of international collaboration and collaborations with the private sector. Importantly, this project is associated to a broader project funded by the French National Research Agency (Nutribor project, PI Pr. Jérôme Gaillardet) which aims at applying boron isotopes to a range of critical zone observatories covering environmental and geological gradients. The successful postdoc will integrate the scientific community of the Nutribor project.  Briefly, the Nutribor project consists of different workpackages. 1/in-situ experimentation at the Ile de France Ecotron near Paris. 2/ boron isotope measurements in the critical zone of three natural catchments from the OZCAR network (French Critical Zone Observatory network) in Northern France, Southern France and the French Lesser Antilles. 2/ecophysiological and reactive transport modelling. The postdoctoral work will particularly be interesting for the comparison with the controlled experiments at the Ecotron facility near Paris. It will benefit for a pluridisciplinary research environment.  Eucalyptus plantations in Brazil and methodology: Brazil has one of the world’s largest surface areas of planted forest (9.9 million ha), of which more than 70 % is covered by eucalyptus (Pena-Vergara et al. 2022). These fast-growing forest plantations have high wood productivity, coming from the intensive management practices including short rotation, fertilization and genotype selection. These plantations are providing an increasing share of wood biomass for producing pulp and paper, charcoal, firewood, and panels. The rapid growth rates of eucalyptus with large wood exports at harvest make this ecosystem particularly interesting for studying and modeling biogeochemical cycles (Cornut et al. 2021), and poses important challenges in finding the right levels of fertilization to limit their environmental impact. Boron-poor soils are commonly found in the ‘Cerrados’ region of Brazil, where there is the greatest expansion of eucalyptus spp plantations on degraded pastures (José et al. 2009, da Silva Damasceno et al. 2023). Boron is one of the most limiting nutrients to eucalyptus seedling growth in these soils (Sgarbi et al. 1999, Sakya et al. 2002). Fertilization in boron is therefore necessary in the more depleted soils, but is also important in other areas where chemical weathering is no longer sufficient for sustaining the high exportations. The primary field site in Brazil, EucFlux, is a 200 Ha Eucalyptus instrumented plantation that has been highly monitored since 2008 with an eddy-covariance flux tower together with numerous ancillary data related to water, carbon and nutrient cycles (Christina et al. 2017). The soils at this site are deep Ferrasols (FAO classification) developed on Cretaceous sandstone, with approximately 80 % sand content down to the water table at 17 m. The mean annual rainfall is 1430 mm year-1. Harvest is planned for September 2025, following which a nutrient omission design for boron and potassium will be included for the next plantation cycle (of 6-7 years). The primary objectives of the postodoctoral work at this site will be to use existing infrastructure and additional experimental equipment to establish a partial boron mass balance at two developmental stages of Eucalyptus spp subject to different fertilization regimes: in a mature fertilized Eucalyptus spp stand after canopy closure in the final months leading to harvest, from January to September 2025, as well as post-harvest over the early developmental stages of the same clone of a Eucalyptus spp plantation subject to a fertilization design with boron and potassium omissions (Figure 1). Figure 1. Schematic design of nutrient omission in the Eucflux experiment made in collaboration with the private companies of forest exploitation. Various critical zone compartments will be analysed for B content and isotopic signature, as well as for the various macro- and micronutrient contents of these compartments (soil, soil water, different parts of the vegetation, rivers). More specifically, soils will be collected at four depths to 150 cm and multiple belowground and aboveground tree compartments at these different development stages (roots at three depths, branch wood & branch bark, stem wood & stem bark, leaves (high canopy and low canopy), and monthly litterfall samples. In addition, will be collected monthly composite samples of : 1/throughfall (using a funnel system connected to reservoirs), 2/stemflow (using PVC pipes spiraling down a section of the trunk to reservoirs), 3/soil solutions (using gravitational flow to lysimetric plates at the litterfall-soil surface interface and at 15 cm, with collections planned for further analyses at 40 cm and 100 cm post-harvest), 3/atmospheric deposition (with a receptor above the tree canopies at the top of the flux tower), 4/groundwater samples (collected via the piezometers using a weighted tube receptor). In addition, samples will be collected from water points and micro-watersheds in Itatinga close to the EucFlux site during the second phase of the project. In parallel, experiments will be run at the Ecotron (Ile de Paris) that will allow various levels boron application, as well as drought simulation to be applied to these eucalyptus (of the same clone, AEC144). Running these experiments in parallel will be of great value, between the precisely contrived conditions of the Ecotron to the in-situ field measurements at Itatinga. The Ecotron experiments are not part of the postdoctoral work and will be made by a French PhD student. Combining data from Eucalyptus stands and experimentations in Ecotron will allow boron fractionation between different soil, tree and water reservoir compartments to be determined, as well as the relationship between boron and other macro- and micronutrients will be evaluated. These findings have the potential to improve knowledge on the nature of nutritional dynamics in these Eucalyptus plantations in typically nutrient-deficient soils and subject to the increasing intensity and frequency of droughts. Boron isotopic measurements, major elements and complementary analyses will be conducted in Brazil as much as possible. These measurements will open perspectives of of collaboration with a modelling team at Cirad.  Job application: The Global Critical Zone Science Chair at the Mohammed VI Polytechnic University (UM6P) invites applications for a two years postdoc fellowship (candidates from Moroccan or African universities). The successful applicant will conduct research on the boron and potassium biogeochemistry in the critical zone of instrumented sites in Brazil. He.She will be mostly based in Brazil. The different compartments of the system will be investigated (soil, vegetation, soil pore water, river water) in the framework of the in-situ research experiment described above. The work will be done in collaboration with international teams and teams in Brazil. Boron isotopic measurements will be made in Brazil at least in a routine phase. Collaboration will also associate the private sector in charge of the forest exploitations. High precision isotopic measurements will be conducted. The candidate must have a background in either isotope geochemistry and if possible, an experience working with MCICPMS. The candidate is expected to adopt a system approach in its way to understand the behavior of boron and other nutrients in the studied agrosystems (/planted forestry systems). Modeling skills will be appreciated even if the main part of the postdoctoral work in field and lab-related. Writing skills are necessary as the results of the postdoctoral work will be published in high standard scientific journals. Criteria of the candidate: PhD in environmental science, soil science, surface geochemistry, or related fields from a recognized Moroccan or African university. At least one or two high publications record in international well-ranked journals Significant knowledge in environmental science studies, including experienced in soil field work and abilities in isotopic geochemistry techniques and/or modeling capabilities. Excellent verbal and written communication skills in English. Skilled in both field and lab work Proactive, ethic, and respectful person Tentative Schedule: Year 1 1 Collection of flux data B isotopic analysis, macro- and micronutrient analysis Monitoring and analysis of flux data 2 Collection of flux data B isotopic analysis, macro- and micronutrient analysis Monitoring and analysis of flux data 3 Monitoring and analysis of flux data Relationship between B and other macro- and micronutrients 4  Monitoring and analysis of flux data Relationship between B and other macro- and micronutrients Modelling boron and potassium fluxes Year 2 1 Monitoring and analysis of flux data Establishment of boron and potassium budgets Relationship between B and other macro- and micronutrients Paper 1: Boron budget in tropical soil-tree planted systems. Modelling boron and potassium fluxes  Paper 2: Boron and potassium dynamics in a planted tropical forestry system 2. 2 Monitoring and analysis of flux data Establishment of boron and potassium budgets Relationship between B and other macro- and micronutrients Paper 1: Boron budget in tropical soil-tree planted systems Modelling boron and potassium fluxes Paper 2: Boron and potassium dynamics in a planted tropical forestry system 2. 3 Relationship between B and other macro- and micronutrients Paper 1: Boron budget in tropical soil-tree planted systems. Modelling boron and potassium fluxes Paper 2: Boron and potassium dynamics in a planted tropical forestry system 2. Executive summary for policy makers 4  Paper 2: Boron and potassium dynamics in a planted tropical forestry system 2. Executive summary for policy makers Final report References: Chetelat, B., Gaillardet, J., Chen, J.Bin, 2021. Dynamic of boron in forest ecosystems traced by its isotopes: a modeling approach. Chem. 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