A geogenic CO2 emitting site (mofette U1) at the banks of the Laacher See, Eifel Mountains, was chosen to study the relationship between heavy postvolcanic soil degassing and vegetation during spring season. To test any interrelation between soil CO2 degassing and vegetation, soil chemism (pH, water content, conductivity, and humus content) and vegetation studies (number of species, plant-soil coverage) were performed. Geogenic soil degassing patterns of carbon dioxide and oxygen were clearly inhomogeneous, resembling soil porosity and distinct permeation channels within the soil. CO2 concentrations ranged from zero to 100%. Soil CO2 increased, while soil oxygen decreased with increasing soil depth. There was a reasonable correlation between CO2 degassing and soil pH as well as soil conductivity. Soil organic matter (SOM) resembled soil water distribution. The number of plant species (from a total of 69 species) as well as plant coverage strongly followed geogenic CO2 degassing. The total number of growing species was highest in low CO2 soils (max. 17 species per m2) and lowest at high CO2-emitting sites (one species per m2). Plant coverage followed the same pattern. Total plant coverage reached values of up to 84% in slightly degassing soils and only 5-6% on heavy CO2-venting sites. One plant species proved to be highly mofettophilic (marsh sedge,
Mofettes are sites with dry CO2 gas exhalations at ambient temperatures. The gaseous CO2 originates from magma chambers or from Earth mantle degassing. It moves upward through fissures and cracks within the rocks [
The present paper studies a natural, terrestrial mofette site at the Laacher See volcano, where CO2 from the Laacher See volcano still reaches the surface, influencing ecosystems. The quiet volcano is part of the east Eifel Volcanic field belonging to the West European Rift System. The last eruptive phase of the Laacher See volcano is about 13,000 years ago [
The study area “U1 mofette” (Figure
Photo from the U1 mofette as seen from north to south in June 2015. The dominating ground vegetation in the mofette is marsh sedge (
The area was chosen to study the relationship between geogenic soil degassing of carbon dioxide and the prevailing vegetation and between soil gas and selected geochemical and soil biological parameters. Seven transects with 2 m spacing and a total length of 54 m were laid out parallel to the lakeshore line. The hiking trail marked the western (lower) end of the study area (Figures
Gas flux was performed at each grid intersection using a portable diffuse carbon dioxide flux meter system (West Systems Portable diffuse flux meter carbon dioxide high-flux, Pisa, Italy). The device consists of an accumulation chamber (type B), a CO2 IR detector (Polytron, Dräger), and a PDA palmtop (Brand, Acer n300) for data communication, evaluation, and storage.
The CO2 flux measurement method is based on the measurement of rising CO2 concentration versus time in terms of ppm s-1. If the gas concentration inside the chamber is constant, linear regression is used to calculate the gas flux
After determination of the soil gas, soil cores were taken very close to the points of the gas measurements. A standard soil borer (Pürkhauer; Thomas Müller, Germany) was used, and samples were taken between soil depths of 7 to 13 cm. This depth range was thought to reflect the main rooting horizon of herbaceous plants [
Soil water content was determined by drying the soil samples of known fresh weight at 75°C for three days in an oven and reweighing after cooling to room temperature in a desiccator. Dry soil samples were ground, and soil pH was measured with a pH electrode (WTW, Germany) in bi-distilled water (750 mg dry soil in 15 ml water). After pH determination, soil conductivity was determined in the same solution using a conductivity meter (inoLab Multi 9420 IDS, WTW, Germany).
To determine soil organic matter (SOM), 1 g of ground, dry soil was heated (450°C for 18 hours) in an oven (muffle furnace; Heraeus, Germany), cooled to room temperature in a desiccator, and reweighed (for details see [
Vegetation cover was estimated according to [
All measurements were carried out at stabile weather conditions during a one-week period in February 2013. Vegetation analysis was done in May 2013.
Statistical analysis was carried out using MS Excel (Microsoft, USA), SigmaPlot 11.0 (Systat Software, USA), and SPSS (SPSS, USA). Multivariable statistical analysis was done using multiattribute analysis based on canonical correspondence analysis (CCA; CANOCO 4.5 program). Probable correlations between plant habitat parameters, plant species composition, and plant distribution can thus be detected [
Geogenic CO2 degassing was clearly heterogeneous within the selected area. At 10 cm soil depth, high [CO2] were found in a line running nearly diagonal from the left upper corner to the area close to the lower right part (Figure
(a–d) Soil CO2 concentrations within the U1 mofette (
Soil gas concentrations clearly changed with soil depth. Carbon dioxide increased with increasing soil depth whereas oxygen values decreased at deeper soil horizons (Figures
(a–d) Soil O2 concentrations within the U1 mofette. Measurements were performed at each intersection of the
There are two selected transects of the U1 mofette (Figure
Vertical sections of CO2 (a) and O2 (b) concentrations showing the prevailing gas channels. Two different transects for the two different gases were chosen. (a) CO2 concentrations are shown in linear transect 4. Up to five CO2 channels can be seen from which four are reaching the soil surface. (b) Transect 6 is used to illustrate oxygen channels through the soil. CO2 concentrations are given in %.
The same is true for oxygen, although in this case a narrowing of oxygen channels with increasing soil depths is visible (Figure
Also for oxygen, a more or less diagonal concentration pattern from the left upper corner to the right lower corner of the area is seen (Figures
Oxygen
Additional to the CO2 concentrations, soil CO2 fluxes within the U1 mofette were determined directly on the soil surface (Figure
CO2 soil fluxes in moles m-2 d-1 as measured at each intersection of the
Interestingly, soil penetration data underline the gas permeation pathways established by gas concentration measurements. Soil rigidity data obtained with a penetrologger clearly mark the microsites of low and high gas permeability (Figure
(a–f) Soil skeleton content (a) and soil rigidity (b) of the U1 mofette as measured by stone extraction and weight and by soil penetration using a penetrologger. Water content (c), soil pH (d), soil conductivity (e), and soil organic matter (SOM) (f) are also given. Orientation of the site as in Figure
Following the data from the soil surface down to a soil depth of 80 cm, it becomes clear that the higher the soil resistance, the lower the actual CO2 penetration or concentration. Soil hardness is extremely heterogeneous within the area but increases with soil depth. Due to the inhomogeneity of this parameter, a three-dimensional network of vertical and horizontal soil channels is formed allowing gas penetration only in distinct directions. A three-dimensional pattern of easy penetration and prohibited sites is thus formed. The picture of soil gas concentrations nicely mirrors this diffusional barrier pattern (Figures
Even the extractable skeleton content in the soil somehow reflects the gas emission pattern (Figure
Figure
Ascending geogenic carbon dioxide permeates through different soil phases, and due to its high solubility in water, it dissolves in soil water according to temperature, atmospheric pressure, and actual gas concentration and/or flux. Dissolved CO2 rapidly dissociates in water producing protons thus acidifying the aqueous soil phases as long as buffering capacities are below a certain limit [
Stagnant soil water and hypoxic soil conditions should also affect the formation of soil organic matter [SOM; [
In the present study, 69 different plant species were counted within the mofette area during a quantitative species survey in May 2013 (Table
List of species of the U1 mofette found during a quantitative survey in May 2013.
Herbaceous plants/ferns | ||
Shrubs and subshrubs | ||
Trees | ||
Mosses | ||
Comparing the pattern of the number of species per area within the U1 mofette with the prevailing CO2 degassing pattern, it becomes evident that high species numbers (up to 17 per 2m2) only occur at sites of diminished CO2 presence (Figures
Species number per area (a) and total plant coverage (b) within the U1 mofette. The number of different plant species per 2 m2 area is given. Plant/soil coverage is given in percent plant surface area per given soil surface. Orientation of the site as in Figure
The same observation is made when plant coverage is correlated with the CO2 degassing pattern (Figure
Correlating the growth patterns of selected plants and the degassing pattern within the designated mofette area, it turns out that just one single plant species clearly follows the degassing pattern of CO2. Marsh sedge (
Exact growth sites of
Quite in contrast to the growth pattern of
Growth sites (in % coverage) of selected mofettophobic plants within the study site. From top left to bottom right:
These species clearly avoid growing on degassing sites and occur only on control plots. The growth pattern of some species marks a linear or half-moon-like structure in the upper left corner of the site (running from 7/10-16 to 2/2). This is true for
Two other species like common snowberry (
Growth sites (in % coverage) of selected mofettophobic to mofettovague plants within the study site. From top left to bottom right:
Canonical correspondence analyses (CCA) were carried out on all environmental and individual species abundance data [
The canonical correspondence analyses CCA (Figures
(a–b) CCA (CANOCO 4.5) of selected soil parameters, CO2 concentrations in (a) 20 cm and (b) 40 cm soil depths (arrows), and plant species (triangles) in the U1 mofette. AnemNemo:
The strongest environmental vectors separated the plant species into two to three species groups (Figure
CCA thus corroborate the findings of the distribution of species according to the geogenic soil gas emission. Marsh sedge proved to be the only real “
Postvolcanic CO2 gas leakage still occurs around the Laacher See resulting in distinct, dry CO2 emanations sites (mofettes). Our results indicate that this CO2 degassing significantly affects the terrestrial soil ecosystem. The living organisms within this ecosystem appear to have adapted to the locally high CO2 concentrations through species substitution or adaption, with a shift towards hypoxic and acidophilic adaptations. In the case of plants, an azonally growing helophyte, namely, marsh sedge (
The competent, botanical characterization of mofette sites and the identification of
Whether knowledge on CO2 gas indicator plants will also help to identify gas leaks in artificial CO2 storage fields (CCS) is still a matter of debate.
We cannot share the data with other colleagues at present on a public scale as we are producing more scientific papers with these data. In this case, we have a time series over several years where these data are included. After publishing the follow-up papers, we may allow public access to the data.
The authors declare that there is no conflict of interest regarding the publication of this paper.
We want to thank Christa Kosch for her extremely valuable help in the field as well as in the laboratory. The help of Nina Hennigfeld, Annika Pelz, and Christian Baakes is gratefully acknowledged. Sabine Kühr helped in various soil analytics. Cordial thanks are also due to the Deutsche Vulkanologische Gesellschaft (Heinz Lempertz, Wolfgang Riedel, Rainer Hippchen, Wolfgang Kostka, and Walter Müller) as well as forester Karl Hermann Gräf for their help in getting necessary permissions as well as practical and financial support. We are indebted to Mrs. Claudia Uhl and Mr. Stefan Backes, Struktur- und Genehmigungsdirektion Koblenz Nord, Referat Naturschutz, for the permission to study the area and to the Department of Geosciences of the Friedrich Schiller University of Jena (Profs. Heide, Büchel, and Viereck) for many scientific discussions.