Bridging the Gap: Integrating Top-Down and Bottom-Up Measurement Approaches to close the Amazon CH4 emissions budget

This study is part of an international mission in the Amazon rainforest, involving researchers from the Federal University of Rio de Janeiro, Universities of Leeds, Linköping, British Columbia, and coordinated by Prof. Vincent Gauci and Dr. Sunitha Pangala. The primary objective is to reconcile top-down methane (CH₄) emission estimates, derived from remote sensing data over Amazon floodplains, with bottom-up measurements obtained from field studies. Previous satellite observations indicated a discrepancy of 20 million tons of CH₄ emitted annually, a significant gap that could not be fully explained by ground-based sources. This project aimed to resolve this difference by integrating both remote sensing satellites and field data using ABB’s Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) to understand and bring to light the different dynamics involved in methane emissions in the Amazon rainforest.

Reconciling top-down and bottom-up carbon budgets can be particularly challenging in specific ecosystems where topography complicates site access and sampling. Under such conditions, the availability of compact and rugged cavity-enhanced laser-based analyzers offering sub-ppb precision is invaluable for environmental scientists. ABB’s portable greenhouse gas ultraportable analyser, GLA132-GGA (47 cm × 35.56 cm × 17.78 cm) is capable of monitoring CH₄ with a 1 s precision of 1.4 ppb, which can be improved to 0.2 ppb with 100 s averaging time. The analyser is based on OA-ICOS technology that combines high precision capabilities through enhanced cavity path length and robustness to mechanical vibrations, which is crucial to field applications. Scientists used semi-rigid custom chambers wrapped around the trunk of floodplain trees and connected them to the GLA132-GGA to measure individual CH₄ emissions from 2357 trees in 13 floodplain sites.

The findings provide a crucial link to reconcile the 20-million-ton discrepancy in Amazon's methane budget. Scaled estimates of methane flux emitted from floodplain trees align closely with the missing methane observed in previous satellite data. During the rainy season, when Amazon tree roots become submerged, trees have evolved specialized adaptations to enhance oxygen supply to their roots by enlarging pores in their stems. This adaptation inadvertently facilitates the release of methane, produced by microorganisms in the waterlogged soil, through the same pore openings. Floodplain trees thus function as natural chimneys, venting substantial quantities of methane into the atmosphere. These large emissions from floodplain trees play a pivotal role in closing the Amazon methane budget.

Furthermore, a second campaign revealed that methane produced deep within the soil column can also escape to the atmosphere via tree roots, even when the water table is below the surface. Regression analysis demonstrated that, while methane emissions show negligible response to increased flood levels above the soil surface, there is a clear dependence of whole-tree methane emissions on the presence of submerged roots. This highlights the importance of floodplain trees in regulating methane fluxes across varying hydrological conditions, underscoring their significant role in the global methane cycle.