New research on the long-term fertilisation experiment at Les Nouragues and Paracou in French Guiana

The main objective of the fertilisation treatments carried out in the forests of Les Nouragues and Paracou since 2017 is to gain a better understanding of the response of tropical forest ecosystems to changes in nutrient levels, particularly nitrogen and phosphorus.

At each forest site, three blocks of four 50 × 50 m (0.25 ha) plots were selected along a topographic gradient characterised by three forest zones with naturally contrasting soil conditions and with levels classified as hill-top (terra firme), slope and hill-bottom (Figure 1). The soils of the hill-top plots are well drained but poor in nutrients, in contrast to the soils of the hill-bottom plots, which are regularly saturated with water and rich in nutrients. The soils of the slope plots are intermediate. In each block, one plot served as a control plot (Ctrl; no nutrient addition) and the remaining three plots received one of three nutrient addition treatments: nitrogen (N), phosphorus (P) and a mixture of N and P.

The European "IMBALANCE-P" project (2014-2020) initiated this experiment by focusing on the role of plant-soil interactions in carbon sequestration and nutrient dynamics in tropical forests. At the end of the IMBALANCE-P project, funding from AnaEE-France (PIA) was mobilised through the FertiGuyane project to maintain the fertilisation treatment on the plots until 2023. The goal was to establish the fertilisation experiment and associated research in French Guiana on a long-term basis.

New research projects are currently underway, with measurement campaigns scheduled to run until at least the end of 2024. They focus in particular on the exchange of the main greenhouse gases (GHGs), i.e. carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O), not only at the soil-atmosphere interface, but also at the tree-atmosphere interface.

Soils are the main contributors to GHG fluxes in forests. Emissions and uptake of these gases are the result of plant- and soil-specific microbial processes at the soil-atmosphere interface. The results of several studies show that biotic and abiotic factors such as vegetation type, soil water content, soil temperature, soil type, the structure and composition of associated microbial communities, and the availability of nutrients (especially N and P) control the spatio-temporal variations of these GHG fluxes under certain conditions. However, although important, these observations of GHG fluxes in tropical forests are scarce and have not yet been reported in the context of nutrient imbalances in the environment simulated by fertilisation. In this context, the FertiSoil-GHG project (INRAE-EcoFoG and University of Antwerp) aims to assess for the first time the effect of seven years of N and P fertilisation on CO₂, CH₄ and N₂O exchanges at soil level, based on the results of flux measurements and soil properties for the first three years after the first fertilisation.

In addition to soils, it has been shown that trees can also make a significant contribution to the exchange of greenhouse gases in forests. CO₂, CH₄ and N₂O produced in the soil can be taken up in dissolved form by the roots and transported to the above-ground parts of the trees. Fluxes of these gases measured at the stem surface may also result from microbial processes inherent in the woody tissues, or from physiological and photochemical processes. There are currently no observations of these GHG fluxes at the tree level in the context of fertilisation in tropical forests. The main objective of the FertiSoilTree-GHG project (Labex CEBA; INRAE-EcoFoG-CzechGlobe, Czech Republic) is therefore to measure for the first time the effect of seven years of N and P fertilisation on the exchange of CO₂ and especially CH₄ and N₂O along the soil-tree-atmosphere continuum in two tropical forests.

Associated references:

Bréchet L., Daniel W., Stahl C., Burban B., et al. 2021. "Simultaneous tree stem and soil greenhouse gas (CO₂, CH₄, N₂O) flux measurements: a novel design for continuous monitoring towards improving flux estimates and temporal resolution" New Phytologist (IF = 8.51); doi: 10.1111/nph.17352.

Bréchet L., Courtois E.A., Saint-Germain T., Janssens I.A., et al. 2019. "Disentangling drought and nutrient effects on soil carbon dioxide and methane fluxes in a tropical forest" Frontiers in Environmental Science (IF = 3.88); doi: 10.3389/fenvs.2019.00180.

Courtois E., Stahl C., Van den Berge J., Bréchet L., et al. 2018. "Spatial variation of soil CO₂, CH₄ and N₂O fluxes across topographical positions in tropical forests of the Guiana Shield" Ecosystems (IF = 4.55); doi: 10.1007/S10021-018-0232-6.

Verryckt L.T., Vicca S., Van Langenhove L., Stahl C., et al. 2022. "Vertical profiles of leaf photosynthesis and leaf traits, and soil nutrients in two tropical rainforests in French Guiana before and after a three-year nitrogen and phosphorus addition experiment" Earth System Science Data (IF = 11.90); doi: 10.5194/essd-2021-142.

Vallicrosa H., Lugli L.F., Fuchslueger L., Sardans J., et al. 2023. "Phosphorus scarcity contributes to nitrogen limitation in lowland tropical rainforests" Ecology (IF = 6.43); doi.org/10.1002/ecy.4049.