Zero to moderate methane emissions in a densely rooted, pristine Patagonian bog - biogeochemical controls as revealed from isotopic evidence

Abstract

Peatlands are significant global methane (CH4) sources, but processes governing CH4 dynamics have been predominantly studied on the northern hemisphere. Southern hemispheric and tropical bogs can be dominated by cushion forming vascular plants (e.g. Astelia pumila, Donatia fascicularis). These cushion bogs are found in many (mostly southern) parts of the world but could also serve as extreme examples for densely rooted northern hemispheric bogs dominated by rushes and sedges. We report highly variable summer CH4 emissions from different microforms in a Patagonian cushion bog as determined by chamber measurements. Driving biogeochemical processes were identified from pore water profiles and carbon isotopic signatures. An intensive root activity within a rhizosphere stretching over 2 m depth accompanied by molecular 25 oxygen release created aerobic microsites in water-saturated peat leading to a thorough CH4 oxidation (< 0.003 mmol L-1 pore water CH4, enriched δ13C-CH4 by up to 10?) and negligible emissions (0.09 ± 0.16 mmol CH4 m-2 d-1) from Astelia lawns. Root activity even suppressed CH4 emissions from non-rooted peat below adjacent pools (0.23 ± 0.25 mmol CH4 m-2 d-1), in which we found similar pore water profile patterns as obtained under Astelia lawns. Below the rhizosphere pore water concentrations increased sharply to 0.40 ± 0.25 mmol CH4 L-1 and CH4 was predominantly produced by hydrogenotrophic 30 methanogenesis. Few Sphagnum lawns and ? surprisingly ? one lawn dominated by cushion-forming D. fascicularis were found to be local CH4 emission hot spots with up to 1.52 ± 1.10 mmol CH4 m-2 d-1 presumably as root density and molecular oxygen release dropped below a certain threshold. The spatial distribution of root characteristics supposedly causing such pronounced CH4 emission pattern was evaluated on a conceptual level that aimed to reflect extreme examples of general scenarios in densely rooted bogs. We conclude that presence of cushion vegetation as a proxy for negligible CH4 emissionsfrom cushion bogs needs to be interpreted with caution. Nevertheless, overall ecosystem CH4 emissions at our study site were probably minute compared to bog ecosystems worldwide and widely decoupled from environmental controls due intensiveroot activity of e.g. A. pumila.