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Rain Forest GreenHouse Gases

Periodic Reporting for period 1 - RainForest-GHG (Rain Forest GreenHouse Gases)

Reporting period: 2018-09-01 to 2020-08-31

Carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) are three major greenhouse gases (GHG), whose increasing concentrations are responsible for global warming. Since 1750, the start of the industrial era, atmospheric concentrations of CO2, CH4 and N2O have dramatically risen, causing unprecedented changes in Earth’s climate. Tropical forests play a pivotal role in the global carbon balance and in natural climate change mitigation since they account for 68% of global forest carbon stocks and represent up to 30% of global soil carbon stocks. However, large uncertainties remain regarding the long term sustainability of tropical forests carbon sink capacity and, more especially, their role in GHG exchanges.
The overall goal of the research project “RainForest-GHG” was to advance knowledge in the understanding of GHG sink or source potentials of a tropical rainforest and determine the contributions of the main ecosystem compartments, i.e. soil, tree, atmosphere, to GHG fluxes. Specifically, I aimed at (i) developing a new automated system for simultaneous continuous measurements of GHG, i.e. CO2, CH4 and N2O fluxes to examine the temporal variations in GHG fluxes in soil and tree stems, (ii) estimating the relative contribution of each ecosystem compartment to the measured GHG, and (iii) determining environmental drivers responsible for the spatio-temporal variations in GHG fluxes.
To better describe the magnitudes and patterns of GHG in tropical forests, an automated system continuously measuring in situ soil fluxes of CO2, CH4 and N2O simultaneously, was elaborated and tested in the experimental study site of Paracou, French Guiana. The system was a combination of soil GHG chambers connected to two gas analysers running in parallel to enable the continuous long-term measurements of the three gases. Subsequently, optimal chamber closure time for each GHG were examined. Our automated soil GHG flux system run successfully during two years and this experimental setup was, for the first time, fully described and tested under tropical field conditions (Courtois et al. 2019). In this study we demonstrated that short closure time of 2 min was sufficient for reliable estimations of soil CO2 and CH4 fluxes whereas a closure time of 25 min was more appropriate to estimate soil N2O fluxes. Then, an original extension was designed to better quantify and understand the role of tropical tree stems in GHG exchanges in tropical forests, consisting of a flexible custom-made stem chamber system connected and controlled by the existing soil GHG flux system. Based on this system and an eleven-months period, we hypothesized that, as for the soil, the accuracy of CO2, CH4 and N2O flux estimates in tree stems is gas-specific, where the rate of gas diffusion and build-up within the chamber determines the minimum duration of the measurement cycle needed to compute fluxes most accurately. In our study, we presented the first automated flexible chamber system for continuously measuring in situ stem GHG fluxes at high temporal resolution (every second) under the warm and extremely humid environment of a tropical forest. We also demonstrated that, contrary to CO2, lengthening the stem chamber closure time not only improved the flux measurements but also largely affected final tree stem flux estimates of CH4 and N2O.

In addition, we proved that our new automated system for continuous stem and soil GHG flux measurements was not only able to capture the seasonal variations, but also the diel variations of stem and soil CO2, and to a lesser extent, CH4 and N2O fluxes. Our results showed clearly circadian rhythm in the stem CO2 efflux, as expected from temperature-driven Arrhenius kinetics. Opposite diel patterns between CO2 efflux from the stem and those emitted at an adjacent soil location were also noticeable, suggesting that in tropical forests stem and soil respiratory physiology is not necessarily governed by the same environmental drivers. No circadian rhythms were observed for CH4 and N2O on either the soil and the tree stem, likewise denoting decoupled regulation from that of diel CO2 fluxes.

In parallel to the abovementioned measurements and to disentangle environmental drivers, e.g. precipitations and nutrient availability, that may explain the spatial heterogeneity in soil GHG fluxes in tropical forests, an experiment combining drought and fertilisation treatments was set up in the Paracou study site, French Guiana (Bréchet et al. 2019). Our study revealed contrasted responses in soil fluxes of GHG, CO2 and CH4 in particular, to the treatments, where (i) nitrogen and phosphorus additions, mitigated by soil water content via imposed drought conditions, had a positive effect on CO2 efflux, and (ii) soil water content only strongly and positively affected CH4 fluxes. Surprisingly, fertilization only affected soil CO2 efflux, and drought caused soil to become sources of CH4 instead of sinks. These results suggested that changes in nutrients and water contents in soils most likely influence the complex processes of CO2 and CH4 exchanges, which are controlled by multiple biophysical and biogeochemical conditions, e.g. methanotroph activities.
RainForest-GHG is the first long term, continuous study of both ecosystem, tree stems and soil CO2, CH4 and N2O fluxes in a remote tropical rainforest, providing valuable insights and data to enhance our mechanistic understanding of the spatio-temporal variations in these fluxes.

Associated with the eddy covariance flux tower, the full system allowing to accurately examine the temporal variations of the soil and tree stems GHG fluxes and determine the contribution of these compartments to ecosystem GHG exchanges, ranked the Paracou study site first among GHG flux experimental sites. The next step in this line is to define a novel approach that allows flux upscaling for a highly complex forest, linking GHG flux data obtained in the soil, tree and canopy level, and improve carbon and GHG flux budget estimates.

Moreover, spatial variation in GHG fluxes have also been explored through individual measurements of the gases (i.e. CO2, CH4 and N2O) in soils and tree stems distributed along a topographical gradient in the Paracou study site. Surprisingly, preliminary results showed that, although none of the three GHG flux rates differed with topographical position, similar patterns were found in the soil and tree stems, excepted for N2O fluxes in the middle slope position. Compared with bottom-slope areas, top-hill areas, where the soil water content was lower, soils and trees were both sources of CO2 but sinks of CH4 and N2O. A possible explanation is that dissolved CO2, CH4 and N2O in soil water is taken up by roots, transported upwards with xylem stream and diffused across the root cortex or throughout above-ground plant organs, explaining part of the emissions of GHG observed in both soils and tree stems in bottom slope areas. In addition, the consumption of CH4 and N2O by the soils and tree stems in the top hill areas, rarely reported in tropical forest, can be explained by mechanisms responsible for their local production, either by microorganisms living within the trees or by physiological and photochemical processes. Indeed, only CO2 is directly synthetized by the trees and biophysical mechanisms related to CH4 and N2O exchanges between the soil, trees and atmosphere are not well known and warrant further investigations.