Periodic Reporting for period 1 - RainForest-GHG (Rain Forest GreenHouse Gases)
Reporting period: 2018-09-01 to 2020-08-31
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.
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.
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.