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

Abstract

Abstract. Peatlands are significant global methane (CH4) sources, but processes governing CH4 dynamics have been predominantly studied in 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. Intensive root activity throughout a rhizosphere stretching over 2 m in depth accompanied by molecular oxygen release created aerobic microsites in water-saturated peat, leading to a thorough CH4 oxidation (< 0.003 mmol L−1 pore water CH4, enriched in δ13C-CH4 by up to 10 ‰) and negligible emissions (0.09±0.16 mmol CH4 m−2 d−1) from Astelia lawns. In sparsely or even non-rooted peat below adjacent pools pore water profile patterns similar to those obtained under Astelia lawns, which emitted very small amounts of CH4 (0.23±0.25 mmol m−2 d−1), were found. Below the A. pumila rhizosphere pore water concentrations increased sharply to 0.40±0.25 mmol CH4 L−1 and CH4 was predominantly produced by hydrogenotrophic methanogenesis. A few Sphagnum lawns and – surprisingly – one lawn dominated by cushion-forming D. fascicularis were found to be local CH4 emission hotspots 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 a pronounced CH4 emission pattern was evaluated on a conceptual level aiming to exemplify scenarios in densely rooted bogs. We conclude that presence of cushion vegetation as a proxy for negligible CH4 emissions from 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 to intensive root activity of A. pumila, for example.

Cushion bogs are stronger carbon dioxide net sinks than moss-dominated bogs as revealed by eddy covariance measurements on Tierra del Fuego, Argentina

“…Zealand is small (Goodrich et al, 2015a). Methane C loss at the latter site was with up to 21.75 g m −2 a −1 (Goodrich et al, 2015b) considerably larger than at the studied Fuegian cushion bog where soil-atmosphere CH 4 exchange is very low (Fritz et al, 2011;Münchberger et al, 2019). The intense transport of oxygen through the dense root system of A. pumila results in 35…”

“…Atlantic blanket bogs that develop in similar relief settings but receive more than three times more precipitation (Laine et al,5 2006) compared to Fuegian cushion bogs show generally lower annual NEE-C uptake rates as reported by Kiely (2005, 2010) what has been determined for northern hemisphere raised bogs and much higher than in Moat (Münchberger et al, 2019).…”

“…Biogeosciences Discuss. Münchberger et al (2019) at Moat yields an once again most likely overestimated annual CH 4 -C flux of 0.39 ± 0.70 g m −2 a −1 for cushions dominated by A. pumila and an even lower flux sum of 0.23 ± 0.25 g m −2 a −1 from the pools between cushions. On the other hand, CH 4 -C fluxes from the seldom S. magellanicum patches at Moat are with 6.66 ± 4.82 g m −2 a −1 larger than at Pipo.…”

“…With the EC method, we were able to measure CO 2 net ecosystem exchange (NEE) on ecosystem scale and at high temporal resolution. To date, 20 plant community-scale gas flux measurements in Fuegian peatlands have been conducted within three methane (CH 4 ) flux studies with manual chambers (Fritz et al, 2011;Lehmann et al, 2016;Münchberger et al, 2019). The measurements in these studies were taken at the same sites where data presented in this study were acquired.…”

Cushion bogs are stronger carbon dioxide net sinks than moss-dominated bogs as revealed by eddy covariance measurements on Tierra del Fuego, Argentina

“…Fritz et al (2011) estimated fine root biomass accumulation to be 4 times larger in cushion bogs than in Northern Hemisphere raised bogs. The aerenchymatic roots of vascular cushion plants lead to additional oxygen transport to the rhizosphere and thereby to highly decomposed peat also near the surface and to closeto-zero methane emissions (Fritz et al, 2011;Münchberger et al, 2019).…”