Can a Red Wood-Ant Nest Be Associated with Fault-Related CH₄ Micro-Seepage? A Case Study from Continuous Short-Term In-Situ Sampling

Simple Summary

Methane (CH₄) is common on Earth but its natural sources are not well-characterized. We investigated concentrations of CH₄ and its stable carbon isotope (δ¹³C-CH₄) within a red wood-ant (RWA; Formica polyctena) nest in the Neuwied Basin, a part of the East Eifel Volcanic Field (EEVF), and tested for associations between methane concentration and RWA activity patterns, earthquakes, and earth tides. Methane degassing was not synchronized with earth tides, nor was it influenced by a micro-earthquake or RWA activity. Elevated CH₄ concentrations in nest gas appear to result from a combination of microbial activity and fault-related emissions. The latter could result from micro-seepage of methane derived from low-temperature gas-water-rock reactions that subsequently moves via fault networks through the RWA nest or from overlapping micro-seepage of magmatic CH₄ from the Eifel plume. Given the abundance of RWA nests on the landscape, their role as sources of microbial CH₄ and biological indicators for abiotically-derived CH₄ should be included in estimations of methane emissions that are contributing to climatic change.

Abstract

We measured methane (CH₄) and stable carbon isotope of methane (δ¹³C-CH₄) concentrations in ambient air and within a red wood-ant (RWA; Formica polyctena) nest in the Neuwied Basin (Germany) using high-resolution in-situ sampling to detect microbial, thermogenic, and abiotic fault-related micro-seepage of CH₄. Methane degassing from RWA nests was not synchronized with earth tides, nor was it influenced by micro-earthquake degassing or concomitantly measured RWA activity. Two δ¹³C-CH₄ signatures were identified in nest gas: −69‰ and −37‰. The lower peak was attributed to microbial decomposition of organic matter within the RWA nest, in line with previous observations that RWA nests are hot-spots of microbial CH₄. The higher peak has not been reported in previous studies. We attribute this peak to fault-related CH₄ emissions moving via fault networks into the RWA nest, which could originate either from thermogenic or abiotic CH₄ formation. Sources of these micro-seepages could be Devonian schists, iron-bearing “Klerf Schichten”, or overlapping micro-seepage of magmatic CH₄ from the Eifel plume. Given the abundance of RWA nests on the landscape, their role as sources of microbial CH₄ and biological indicators for abiotically-derived CH₄ should be included in estimation of methane emissions that are contributing to climatic change.

Characterization of N2O emissions and associated microbial communities from the ant mounds in soils of a humid tropical rainforest

Tropical rainforest soils harbor a considerable diversity of soil fauna that contributes to emissions of N₂O. Despite their ecological dominance, there is limited information available about the contribution of epigeal ant mounds to N₂O emissions in these tropical soils. This study aimed to determine whether ant mounds contribute to local soil N emissions in the tropical humid rainforest. N₂O emission was determined in vitro from individual live ants, ant-processed mound soils, and surrounding reference soils for two trophically distinct and abundant ant species: the leaf-cutting Atta mexicana and omnivorous Solenopsis geminata. The abundance of total bacteria, nitrifiers (AOA and AOB), and denitrifiers (nirK, nirS, and nosZ) was estimated in these soils using quantitative PCR, and their respective mineral N contents determined. There was negligible N₂O emission detected from live ant individuals. However, the mound soils of both species emitted significantly greater (3-fold) amount of N₂O than their respective surrounding reference soils. This emission increased significantly up to 6-fold in the presence of acetylene, indicating that, in addition to N₂O, dinitrogen (N₂) is also produced from these mound soils at an equivalent rate (N₂O/N₂ = 0.57). Functional gene abundance (nitrifiers and denitrifiers) and mineral N pools (ammonium and nitrate) were significantly greater in mound soils than in their respective reference soils. Furthermore, in the light of the measured parameters and their correlation trends, nitrification and denitrification appeared to represent the major N₂O-producing microbial processes in ant mound soils. The ant mounds were estimated to contribute from 0.1 to 3.7% of the total N₂O emissions of tropical rainforest soils.