Deep roots versus pumps: comparison of deep nutrient removal in dry tropical eco- and agrosystems (ANR project Nutrilift)

Processes linking lower and upper parts of the Critical Zone (CZ) are crucial for sustaining life on continents and ecosystem services provided by eco- or agro-systems. Rock weathering at depth is expected to be an essential source of nutrients and deep-rooted trees are believed to induce water and nutrient ‘lift’, benefiting 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 dynamics. The Nutrilift project aims at quantifying the role of deep critical zone in the supply of nutrients to eco- and agrosystems, based on the hypothesis that while in natural forests, deep-rooted species can derive part of their nutrient resources from increased mineral weathering at depth, the relative importance of this process in shallow-rooted agrosystems is much less - and agroforestry systems represent an intermediate situation. Conducted within the framework of the Indo-French Cell for Water Sciences (IRD - CNRS - INRAE - UPS - Indian Institute of Science, Bangalore, India), the project is based on long-term monitoring in the Mule Hole (diversified forest) and Berambadi (irrigated agriculture and agroforestry) watersheds of the M-TROPICS Observatory in Peninsular India. For this purpose, we study the vertical evolution of soil properties and associated pedological processes as a function of plant cover/land-use. Weathering processes and/or plant uptake will be studied in the vicinity of the roots using micro-characterization techniques, which will allow to calibrate combined hydro-geochemical models. The deep contribution to the nutrient budgets of each site will be quantified by intra-plant isotopic balances as well as by the identification of specific geochemical signatures to the deep contribution of the critical zone. An originality of the project is the observation of the deep critical zone (up to 10m) via instrumented pits with continuous pCO₂ and moisture measurements, scanners (root dynamics) imaging and pore water collection. The effects of future changes -associated with climate and land uses- on the dynamics of the deep critical zone will be explored from scenarios co-constructed with local stakeholders.