• Energy Research

La destruction des prairies semées est souvent liée à une baisse de productivité et/ou un changement de flore avec le temps. Ces évolutions rarement objectivées ont fait l'objet de deux projets de recherche-action récents dans le Grand-Ouest, dans lesquels des observatoires de jeunes parcelles bien implantées ont été suivis pendant 4 (projet PERPeT) et 5 ans (projet PTD). Les suivis portaient sur la biomasse annuelle récoltée et la composition botanique, afin de déterminer comment vieillissent les prairies en fonction du semis initial, du pédo-climat et des pratiques d'utilisation des parcelles. La synthèse des principaux résultats permet de conclure que « bien faire vieillir des prairies, c'est possible », que leur évolution est multifactorielle avec parmi les principaux déterminants des facteurs climatiques (alimentation en eau), puis liés aux pratiques et aux sols. Plusieurs types de prairies et de trajectoires d'évolution ont été caractérisés, certaines espèces jouant un rôle important dans ces dynamiques. La flore se diversifie et le fond prairial s'équilibre au bout de 5-6 ans, avec un développement racinaire rendant les sols plus résistants au piétinement. Enfin, ces évolutions plus ou moins favorables en termes de quantité et qualité de l'herbe récoltable peuvent amener à reconsidérer la fonction de chaque parcelle dans le système fourrager. Often, the destruction of sown grasslands is linked to a decrease in productivity and/or a change in flora over time. These evolutions are rarely objectified and are the subject of two recent research-action projects in the Great West, in which observatories of young, well-established plots were monitored for 4 years (PERPeT project) and 5 years (PTD project). The monitoring focused on the annual biomass harvested and botanical composition, in order to determine how the grasslands age according to the initial sowing, the pedo-climate and the use practices of the plots. The synthesis of the main results allows us to conclude that "it is possible to age grasslands well", that their evolution is multifactorial with climatic factors (water supply), among the main determinants, then linked to practices and soils. Several types of grasslands and evolutionary trajectories were characterized, with certain species playing an important role in these dynamics. The flora diversifies and the prairie bottom becomes balanced after 5-6 years, with a root development making the soils more resistant to trampling. Finally, these evolutions, more or less favorable in terms of quantity and quality of harvestable grass, can lead to reconsider the function of each plot in the forage system.

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The rate of N uptake of crops is highly variable during crop development and between years and sites. However, under ample soil N availability, crop N accumulation is highly related to crop growth rate and to biomass accumulation. Critical N concentration has been defined as the minimum N concentration which allows maximum growth rate. Critical N concentration declines during crop growth. The relationship between critical N concentration and biomass accumulation over the growth period of a crop is broadly similar within major C(3) and C(4) cultivated species. Therefore, the critical N concentration concept is widely used in agronomy as the basis of the diagnosis of crop N status, and allows discrimination between situations of sub-optimal and supra-optimal N supply. The relationship between N and biomass accumulation in crops, relies on the interregulation of multiple crop physiological processes. Among these processes, N uptake, crop C assimilation and thus growth rate, and C and N allocation between organs and between plants, play a particular role. Under sub-optimal N supply, N uptake of the crop depends on soil mineral N availability and distribution, and on root distribution. Under ample N supply, N uptake largely depends on growth rate via internal plant regulation. Carbon assimilation of the crop is related to crop N through the distribution of N between mature leaves with consequences for leaf and canopy photosynthesis. However, although less commonly emphasized, carbon assimilation of the crop also depends on crop N through leaf area development. Therefore, crop growth rate fundamentally relies on the balance of N allocation between growing and mature leaves. Nitrogen uptake and distribution also depends on C allocation between organs and N composition of these organs. Within shoots, allocation of C to stems generally increases in relation to C allocation to the leaves over the crop growth period. Allocation of C and N between shoots and roots also changes to a large extent in relation to soil N and/or crop N. These alterations in C and N allocation between plant organs have implications, together with soil availability and carbon assimilation, on N uptake and distribution in crops. Therefore, N uptake and distribution in plants and crops involves many aspects of growth and development. Regulation of nitrogen assimilation needs to be considered in the context of these interregulatory processes.

This is the 2024-1 release of the ICOS final quality ecosystem data product that includes eddy covariance fluxes, meteorological observations, ancillary and biometric data and full metadata at 73 labelled ICOS stations in the ecosystem domain. The archives contain more detailed description of the different data files contained in the archives. Measurements have been collected using the following instructions: ICOS Ecosystem Instructions for Air Meteorological Measurements (TA, RH, PA, WS, WD), https://doi.org/10.18160/NHEG-4KWW ICOS Ecosystem Instructions for Turbulent Flux Measurements of CO2, Energy and Momentum, https://doi.org/10.18160/QWV4-639G

Quantifying crop responses to nitrogen and avenues to improve nitrogen-use efficiency

The aim of the study was to evaluate the impact of defoliation intensity, defoliation frequency, and interactions with N supply on N uptake, N mobilization from and N allocation to roots, adult leaves, and growing leaves. Plants of Lolium perenne were grown under two contrasted N regimes. Defoliation intensity treatments consisted of a range of percentage leaf area removal (0, 25, 50, 75, or 100%). These treatments were applied in parallel to a set of plants previously undefoliated, and to a second set of plants which had been defoliated several times at a constant height. A (15)N tracer technique was used to quantify N uptake, mobilization, and allocation over a 7 d period. A significant reduction in plant N uptake was observed with the removal of more than 75% of lamina area, but only with high N supply. As defoliation intensity increased, the amount of N taken up and subsequently allocated to growing leaves during the labelling period was maintained at the expense of N allocation to roots and adult leaves. Increasing defoliation intensity increased the relative contribution of roots supplying mobilized N to growing leaves and decreased the relative contribution of adult leaves. Defoliation frequency did not substantially alter N uptake, mobilization, and allocation between roots, adult and growing leaves on a plant basis. However, tiller number per plant was largely increased under repeated defoliation, hence indicating that allocation and mobilization of N to growing leaves, on the basis of individual tillers, was decreased by defoliation frequency.

Projected climate change and increased pressure for adopting more sustainable agricultural practices calls for new approaches in breeding forage crops for the future. In the cool temperate regions of Europe they may benefit from a warmer and prolonged growing season, even though new stresses may emerge during autumn and winter, whereas further south risk of drought will increase. In addition, forage crops have to be able to use both nutrients and water more efficiently in the future in order to maximize production per unit area. Examples are given how perennial forage crops can be adapted to the projected envi-ronmental conditions in Europe through breeding. In the Nordic region the focus is on identifying traits that are important for high yields under changed overwin-tering conditions, as well as management practices. In maritime, oceanic Europe the focus is on forage grass and legume root systems for ecosystem service, nutri-ent and water use, and the advantages and potential for Festulolium, including its role in ruminant nutrition. In temperate and southern Europe, the aim is to develop varieties able to survive long drought periods and to recover rapidly following autumn rains, as well as improving adapted legume species to reduce the use of synthetic fertilizers, the environmental impacts of ruminant production systems and their dependency on external protein-rich feeds. Forage production systems, commonly found in areas less suited to grain production, can contribute significantly to future food security if the adaptation of forage crops to the future environmental challenges is successful.