Carbon dioxide sampling and analysis technologies for aquatic and terrestrial ecosystems

Abstract

Forest ecosystems and wetlands are considered important carbon sinks for alleviating climate changes, but the sequestration processes and regulations of climate factors on controlling the variability of carbon fluxes of these ecosystems may differ [1]. Increase of CO₂ in the atmosphere can be reduced through terrestrial ecosystems, by absorption of CO₂ in vegetation and soils, resulting in carbon sinks [2-3]. There are several direct and indirect methods for quantifying the amount of carbon stored in soils [4-5]. Direct methods for measuring soil carbon in the land use, land use change and forestry (LULUCF) sector are based on commonly accepted principles of soil sampling and in situ studies. It is known that the production of CO₂ in soil and their release into the atmosphere (gas diffusion and mass flow) is a component of soil respiration [6].

This paper intends to present two complementary methods for the quantification of greenhouse gases exchange between aquatic and terrestrial land use. For this purpose, we conducted a comprehensive CO₂ field activity along two types of ecosystems. The aim of using those methods is to improve the management of in-situ measurements by increasing the efficiency of resources.

One of the tested methods for assessing emissions and removals from reservoirs in the LULUCF sector is the respiration chamber method. Carbon budgets are a simplified way to measure the additional emissions that can enter the atmosphere, and the EGM-5 respiration chamber method is a means of determining the CO₂ exchange between the soil/water and the atmosphere. Measurements of soil respiration rate in terrestrial and aquatic ecosystems aim to define the annual flux of CO₂ and to identify uncertainties in estimating global carbon flow.

Tests were performed for the implementation of the method in experimental locations both to determine soil respiration with the standard equipment purchased (EGM-5 CO₂ analyzer with SRC respiration chamber) and with equipment designed to adapt to aquatic ecosystems. In order to perform CO₂ flux measurements on water, the chamber system was placed on a floating device.

An alternative is the injection kit sampling device which is a technique used to measure the concentrations of small samples of gas, usually collected in sampling chambers and transferred to the EGM-5 with a syringe. This technique is inexpensive, small, portable and have low human and material resource requirements, making them useful for field campaigns in remote and inaccessible locations. Also, the ability to measure simultaneously in different areas is a major advantage of the injection kit.

The principle of the method of measuring the CO₂ flux with closed static chamber EGM-5 is based on the difference between the atmospheric CO₂ concentration and the CO₂ emissions from the plot surface, calculated automatically by the device. On the other hand, the injection kit method measures the CO₂ dioxide accumulated inside the chamber placed on the plot over a period of time. The CO₂ concentration in a gas sample is calculated by injecting the sample into a fixed gas stream with a known CO₂ concentration (baseline CO₂ level) and calculating the difference in value between the initial or atmospheric concentration and the CO₂ concentration from the sample.

By testing the injection kit in relation to the values obtained with EGM-5 for the same gas, the aim was to perform the measurements under identical conditions and in the same plots. Measurements were made in two types of ecosystems: forest and wetland. The portable CO₂ gas analyser, EGM-5, has been adapted for monitoring the CO₂ flux in specific wetland conditions, with a floating device.

The results shown in Fig.1a and Fig.1b indicate that the instruments used responded similarly to changes in the CO₂ concentration of the source gas.

       

Fig.1a Correlation of CO₂ flux values obtained by the injection kit method and the respiration chamber method EGM-5 in the forest ecosystem

Fig.1b Graphical representation of CO₂ flux values obtained by the injection kit method and the EGM-5 respiration chamber method in the wetland

On average, the Injection kit reported a CO₂ concentration higher than the EGM-5, and a direct correlation between the values obtained by the two methods applied in the forest ecosystem and an average difference of 23,9 ppm in the wetland study area.

The future perspectives of the research activities presented in the paper aim to improve and apply the injection kit method especially in wetlands where accurate data are deficient and to identify a potential gap between the two techniques, based on which the dimensions of the sampling devices and the sampling time will be adjusted.