Internship: Modeling nitrogen fluxes in a mixed crop–dairy cattle system

You will work in the Herbivores joint research unit (UMRH). The mission of this unit is to produce, integrate and disseminate knowledge, and share expertise to design multi-performing herbivore farming systems that meet the challenges of global change (environmental, socio-economic and digital transition). The unit has 118 permanent staff and welcomes 70 non-permanent staff each year. It is organized into 4 research teams, a management team and a support team. You will work within the Livestock Systems, Agroecological Transition, Resilience and Product Quality (STARQ) team and the Digestion, Nutrition, Feed, Metabolism and Microbes (DINAMIC) team. You will also interact with researchers and engineers from INRAE’s unit such as Experimental unit for Forage, Ruminants, and the Environment (Ferlus), joint research unit Soil Agriculture and hydrosystem Spatialization (SAS) and joint research unit Physiology, Environment and Genetics for Animals and Livestock Systems (PEGASE). The project in which you will be involved aims to understand and model the interactions between nitrogen fluxes and natural nitrogen isotope signatures (δ¹⁵N) in an experimental mixed crop–dairy cattle system. Nitrogen is an essential element for agricultural productivity, but its increasing use over the past century has led to a substantial decline in nitrogen use efficiency. A significant fraction of this nitrogen is lost to the environment, contributing to water eutrophication through nitrate leaching, deterioration of air quality through ammonia volatilization, and climate change through nitrous oxide emissions. Improving nitrogen use efficiency and enhancing the recycling of nitrogen within agricultural systems are therefore key levers for reducing these losses and improving the sustainability of farming systems. Nitrogen naturally occurs as two stable isotopes: the light isotope, ¹⁴N (99.63%), and the heavy isotope, ¹⁵N (0.37%). Analysis of δ¹⁵N is a promising approach for assessing the degree of nitrogen cycling and recycling within agricultural systems. Indeed, microorganisms involved in nitrogen transformations, for example during organic matter decomposition or nitrification, generally prefer to use the lighter ¹⁴N isotope, thereby leaving a higher proportion of ¹⁵N in the remaining substrate. In particular, processes associated with nitrogen losses, such as nitrification, ammonia volatilization, and denitrification, preferentially take the lighter ¹⁴N isotope relative to ¹⁵N. These processes can therefore be associated with substantial isotopic fractionation, resulting in a relative depletion of ¹⁵N in the nitrogen forms that are lost (notably gaseous forms), and a relative enrichment in ¹⁵N in the remaining compartments, such as soils, plants, and animals. Among these compartments, soils and animal hair are of particular interest because their relatively slow dynamics allow them to integrate the history of nitrogen cycling and record isotopic signatures reflecting the degree to which the nitrogen cycle is open or closed in livestock systems (Kriszan et al., 2014). However, the complexity of the processes determining ¹⁵N signatures means that their interpretation remains poorly documented. The challenge is therefore to link the rapid dynamics of nitrogen cycling with the slower, integrative dynamics recorded in soils and animal hair, in order to better understand the determinants of δ¹⁵N and assess its potential as an indicator of nitrogen use efficiency and the degree of nitrogen cycle closure. This internship is part of the NATAN15 research project, which aims to analyse and model the interactions between fast and slow dynamics of the nitrogen cycle in livestock systems. The project seeks to assess the potential of δ¹⁵N signatures in ruminant hair and soils as integrated indicators of nitrogen use efficiency and the degree of nitrogen cycle closure. The project is based on the OasYs experimental mixed crop–dairy cattle system, comparing two contrasting areas over a 12-month period:  a grazed area, where direct deposition of animal excreta is expected to limit gaseous nitrogen losses; a mown area, where the application of previously stored manure during winter is expected to increase such losses.  Different compartments associated with the use of these areas—including soils, plants, animal hair, and manure—have been sampled and are currently being analysed for their δ¹⁵N signatures. The objective of the proposed internship is to adapt an existing multi-compartment mechanistic model (Poupin et al., 2014) to the context of the project, in order to describe the temporal dynamics of ¹⁵N isotope signatures in integrative compartments (animal hair and soils) based on nitrogen flux dynamics. This will ultimately allow the potential of these isotopic signatures to be assessed as indirect markers of the degree of nitrogen cycle closure. More specifically, you will be responsible for: conducting a literature review and critically analysin