Comparison of different methods of soil respiration measurement in field conditions

Abstract

The aim of the study was to provide a comparative assessment of the average daily intensity of soil respiration (soil CO₂ emission rate in field conditions) using three methods, including two modifications of the chamber method and an absorption method.

Location and time of the study. The study was carried out in 2024–2025 in the grass and forest biogeocenoses located in the suburbs of Novosibirsk (Akademgorodok area, 54°85′09′′ N, 83°18′34′′ E). The soil is a dark gray forest soil of medium loamy granulometric composition (Luvic Greyzemic Phaeozem (Siltic)).

Methods. Carbon dioxide emission from soil was measured by three methods. Two of them were modifications of the chamber method, and the third one was an improved version of the absorption method. In the dynamic version of the chamber method (hereinafter referred to as the EGM-5 method), the CO₂ concentration in the chamber air was determined using a high-precision (<1%) IR gas analyzer. In the static modification (the VENTpro method), the CO₂ concentration was determined using an inexpensive but less accurate (±5%) analog. In the absorption method, the soil was covered with an insulating vessel for 24 hours, and the emitted CO₂ was absorbed by 10 ml of a 1 M NaOH solution with a sorption surface equal to 38 % of the isolated soil area.

Results. In the model experiment in which spatial variability in soil biological activity was minimized, a decrease in the CO₂ flux was registered by chamber methods due to CO₂ accumulation in the above-ground space of the chambers. Ten minutes after chamber sealing, the CO₂ flux was 4–7% for the EGM-5 method and 9–10% for the VENTpro method. Moreover, the coefficient of variation for CO₂ emission from soil in this experiment was 8% for the EGM-5 method and 16% for the VENTpro method. It is concluded that with the respiration measurement procedure in which the chamber was sealed for only 5 minutes, a decrease in the natural rate of CO₂ diffusion from soil did not significantly affect the results. When measuring soil respiration in the field at small plots (≈10 m²), the coefficient of variation of the average daily soil CO₂ emission rate increased to 20% or more when registered by the chamber methods, whereas it did not exceed 10% by the absorption method. Due to the significant variability, the differences between the results of the three methods were not statistically significant. For this reason, it was also not possible to identify significant differences in the dynamics of soil CO₂ emission during the day in the forest and grassland biogeocoenoses. All three methods provided fairly close results for the total CO₂ emission for May–October (kg C-CO₂ /ha): 5674 by EGM-5, 6347 by VENTpro and absorption 6198 (LSD₀₅ = 607).

Conclusions. In most cases, when comparing soil respiration in the field, no statistically significant differences were found between the results of the three methods, although the differences often reached 30% or more. This is because of the high variability of soil CO₂ emission rate, especially observed when measured with the VENTpro device, which was largely due to the large spatial variability of soil biological activity in the forest and grassland. To record more subtle effects using chamber methods (for example, intra-daily respiration dynamics), a preliminary assessment of the variability of the CO₂ emission indicator at the experimental plot and the calculation of the required measurement replication rate are indispensable. It is also important to conduct detailed studies of possible errors in calculating the average daily soil CO₂ emission rate using chamber methods based on its current actual value (determined over 1–5 minutes of exposure). Apparently, under certain conditions, mainly with weak soil temperature dynamics during the day, these errors will be insignificant. The absorption method used in this study avoided this drawback as it measured soil respiration over a 24-hour exposure period. In our opinion, until a generally accepted method for determining the average daily soil CO₂ emission rate is established, the choice of method should be based on the purpose and feasibility of the study. The anticipated quantitative differences in soil CO₂ emissions, which are expected to be identified as a result of such a study, should be of key importance.