Pulse labelling of deep soil layers in forest with ¹³CH₄: testing a new method for tracing methane in the upper horizons, understorey vegetation and tree stems using laser-based spectrometry

Abstract

Methane emissions from plants in wetlands are mainly due to internal transport, from the anoxic soil layers where methane is produced, to the atmosphere. This pathway has not yet been clearly demonstrated for upland forest vegetation, where methane can be produced in deep soil layers. We developed a new method to trace methane transfer from the deep soil. We conducted a ¹³CH₄ pulse labelling at 40-cm soil depth and then monitored ¹³CH₄ in the upper horizons, at the soil surface (with or without understorey vegetation) and emitted by tree stems until the total disappearance of the labelled gas. Most of the injected ¹³CH₄ was oxidized in the soil despite high soil water content. The understorey vegetation did not contribute to ¹³CH₄ emission by the soil. We prove that tree stems can emit methane produced in an upland forest soil, even when the said soil is a net methane sink. We conclude that pulse labelling with ¹³CH₄ and tracing by laser-based spectrometry is a promising tool approach to study the transport of methane from production to emission.