Characterisation of the effect of redox potential on the emission of greenhouse gases using wireless sensing techniques

Abstract

Soils act as both a source and sink of greenhouse gases (GHGs) and are widely considered to contribute to global warming. Soil N₂O emissions originate from microbial nitrification and denitrification processes. Reducing conditions in soils alter the biogeochemical processes and result in large emissions of N₂O and CH₄. Soil redox potential (Eh) measurements are a promising way to differentiate the major source mechanism in soil N₂O production and evaluate their functions within the N cycle and may contribute to the development of N₂O emission mitigation strategies.<br /> While soil GHG emissions have been studied in the recent past, the relationship between GHG production and Eh has not been systematically studied in detail. Eh monitoring can improve the assessment of soil chemical potential variations and GHG emissions, especially for CH₄ emissions, which mainly occur when soil is in highly reduced conditions because of the soil submerged below the water table (WT) continually, and for N₂O emissions, that have two distinct source processes at different Eh, i.e., nitrification at high Eh, and denitrification at intermediate Eh values. The change between oxidizing and reducing conditions in soil can be monitored and quantified by soil platinum (Pt) electrodes in combination with a reference electrode and a datalogger system with high temporal resolution (less than 1 min).<br /> The objectives of this thesis were to systematically investigate soil surface GHG emissions and their relationship with the spatial distribution and temporal variation of Eh. Because it is challenging to establish controlled conditions in natural soils, this study is based on a series of step-by-step laboratory experiments, exploring the effects of soil water content, N fertilization, and Eh on GHG emissions, followed by long-term measurements of Eh and GHG emissions in the field.<br /> In laboratory experiments, soil was exposed to varying WT levels to evaluate the utility of Eh monitoring for interpreting soil GHG emissions. To quantify soil GHG emissions, the static chamber method was used, in which gas samples were collected manually and analyzed by gas chromatography (GC). These measurements opened the possibility to interpret the long-term monitoring Eh data and to evaluate their influence on soil GHG emission under controlled soil moisture conditions.<br /> The Eh decreased steadily after the soil was submerged under water. It was found that CO₂ emissions had no clear relationship with Eh variations but were closely related to soil water potential. In addition, soil Eh variations showed different ranges of values at different depths. N₂O emission peaks occurred at different Eh ranges and were influenced by WT level changes or fertilization events. In order to obtain more accurate information on N₂O emission sources in cropland, we used an irrigation system in combination with the stable isotope labeling technique using a ¹⁵N-labeled fertilizer. This isotope tracer method provided better insight into N₂O source partitioning and provided an independent validation of the Eh-based N₂O source partitioning. It was found that the changes in soil Eh and N₂O emissions were induced by irrigation and fertilization events and were also related to the vertical distribution of dissolved NO_3^- and NH_4⁺ in the soil profile. Soil Eh values proved to be a suitable basis for identifying the two dominant N₂O sources, i.e., hydroxylamine oxidation (during nitrification) and nitrite reduction (during denitrification). It can be concluded from the laboratory experiments that measurements of Eh with high spatial and temporal resolution can make an important contribution to the study and interpretation of the temporally and spatially diverse N turnover processes in soils.<br /> As an application of the approach developed and tested in the laboratory to field conditions, we conducted continuous automated monitoring of the soil redox status along a transect in the riparian zone of a deforested Norway spruce forest for over a year. We found that the variability of soil Eh values increased during the transition from dry to wet conditions, while it decreased with soil depth. Most of the changes in soil Eh dynamics could be attributed to fluctuating WT depths. The GHG emissions from the study area were dominated by CO₂ and were mainly controlled by soil temperature and soil humidity. Only a few N₂O emission events were observed, mainly at the mid-slope position, and originated from both nitrification and denitrification. CH₄ emission was only observed at the position closest to the stream after the soil reached extremely reducing conditions (Eh < -150 mV). It could also be concluded from the field measurements that simultaneous monitoring of Eh and GHG emissions improves the understanding of soil biogeochemical processes and captures their dynamic variations in riparian zones. <br /> Overall, soil Eh monitoring with redox electrodes improved our understanding of the temporal and spatial distribution of oxidizing and reducing conditions within the soil profile and at different locations in the riparian ecosystem. In addition, continuous measurement of Eh variations have increased our understanding of soil biogeochemical process control of soil GHG production in soils and proved to be a valuable method for N₂O source partitioning. This work also suggests that Eh measurements could help improve the understanding of inorganic N turnover in soil. Therefore, we recommend the installation of redox sensors as standard components in long-term monitoring programs in critical zone observatories to assess the effects of climate or environmental changes on soil biogeochemical processes and GHG fluxes.