Deep roots versus deep pumps : Comparing deep nutrient uplift in dry tropical eco- and agro-systems – NutriLIFT

Organized around 4 workpackages, the first ambition of the project is to design a new methodological framework for characterizing soil properties (porosity, mineralogy, geochemistry), roots (morphology/anatomy) and water dynamics down to great depths (>10m) for each site, including the monitoring of several dynamic variables with depth: pCO2, humidity, temperature, pore water composition. WP1 compares the vertical evolution of these properties according to the different vegetation cover and calibrate the COMFORT hydrological model. WP2 focuses on the rhizosphere properties of each system using a set of micro-characterization techniques to detect subtle changes in mineralogy and nutrient bioavailability in the vicinity of the roots, a study of carbon dynamics from 13C tracer and 14C dating, and finally using specific geochemical tracers of weathering processes (d7Li, d30Si, d41K, d11B) and/or plant removal (d42/44Ca, d30Si). These complementary approaches will allow calibration of the WITCH geochemical model, specifically designed to reproduce weathering processes on a seasonal scale. WP3 aims at quantifying the deep contribution to the nutrient budgets of each site, in 2 steps: (1) quantification of the nutrient budget by identifying for the tree the share of recycling and re-sampling from an original approach, the intra-plant isotope balance applied to K, Ca, Si, B, (2) identification of deep flows from the geochemical signatures of the sap and transpiration flows calculated by COMFORT, which will provide for the first time an integrated view of nutrient uptake and redistribution within each system. Finally, WP4 will explore the effects of future changes -associated to climate and uses- on the dynamics of the deep critical zone, based on scenarios co-constructed with local stakeholders, in partnership with a Karnataka NGO. By quantifying the impact of the coupling between the visible and invisible biosphere on biogeochemical cycles and by modelling them, the results of this project will make it possible to optimize the sustainable management of agro-ecosystems.

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Processes linking lower and upper parts of the Critical Zone are crucial for sustaining life on continents. Deep roots fulfill essential functions, impacting water and biogeochemical cycles, plant ecophysiology and community ecology, not only in natural forests but also in agrosystems. Rock mineral weathering at depth is expected to be an essential source of nutrients and deep rooted trees are believed to lift water and nutrients that benefit to the whole community. However, quantifying this nutrient lift remains a challenge, linked on the one hand to the hidden nature of the roots and on the other hand to the complexity of the rhizosphere.
The Nutrilift project will take up the challenges of understanding and quantifying the role of nutrient lift in the functioning of the Critical Zone, guided by the hypothesis that while in natural forests, deep-rooted species can derive some of their nutrient resources from increased mineral weathering at depth. The relative importance of this process in agrosystems is much lesser, whereas agroforestry systems represent an intermediate situation. Conducted at the International Research Laboratory CEFIRSE (IRD-CNRS-INRA-UPS, Indian Institute of Science, Bangalore) in close collaboration with our Indian colleagues, the project will build on the long-term monitoring of the Kabini Critical Zone Observatory, Indian sites of the SNO M-TROPICS (par of OZCAR Research Infrastructure). It will benefit from the exceptional degree of characterization of these sites, where we revealed critical links between surface and depth, in particular the vertical sharing of water resources by the dominant tree species in the forest ecosystem and the importance of accounting for deep uptake for closing biogeochemical cycles.
The project will design and implement a new methodological framework for the combined characterization of soil and roots properties and the monitoring of solute and water dynamics from the surface down to great depths for 3 contrasted sites (forest, agroforestry and agriculture). We will use a unique combination of techniques to simultaneously characterize soil and root systems properties from the surface down to the deep regolith and use this information to constrain the hydrological model COMFORT, explicitly accounting for deep root uptake. We will encompass the different processes controlling the storage and mobilization of carbon and nutrients in the rhizosphere with a multiple tool approach, in order to connect the current weathering /nutrient dynamics and the dynamics of root-associated carbon. We will propose a new conceptual model based on intra-plant isotopic budgets to quantify the contributions of the deep regolith to the nutrients uptake. Finally, we will build and use a Reactive Transport Model (coupling COMFORT and WITCH) to simulate the impact of climate change on the Critical Zone functioning and to explore management strategies in a participatory approach with stakeholders.
By quantifying the impact of coupling/decoupling the “invisible and visible biosphere” on nutrient cycles, the project will allow a better assessment of ecosystem’s resilience and to design more sustainable management practices enhancing biologically mediated Critical Zone processes in agrosystems.