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 N$_{2}$O emissions originate from microbial nitrification and denitrification processes. Reducing conditions in soils alter the biogeochemical processes and result in large emissions of N$_{2}$O and CH$_{4}$. Soil redox potential (Eh) measurements are a promising way to differentiate the major source mechanism in soil N$_{2}$O production and evaluate their functions within the N cycle and may contribute to the development of N$_{2}$O emission mitigation strategies. While soil GHG emissions have been studied in the recent past, the relationship between GHG productionand Eh has not been systematically studied in detail. Eh monitoring can improve the assessment of soil chemical potential variations and GHG emissions, especially for CH$_{4}$ emissions, which mainly occur when soil is in highly reduced conditions as a result of the soil submerged below the water table (WT) continually, and for N$_{2}$O 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 insoil 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). 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 longterm measurements of Eh and GHG emissions in the field. In laboratory experiments, soil was exposed to varying WT levels to evaluate the utility of Eh monitoring for interpretating 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. The Eh decreased steadily after the soil was submerged under water. It was found that CO$_{2}$ 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. N$_{2}$O 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 N$_{2}$O emission sources in cropland, we used an irrigation system in combination with the stable isotope labeling technique using a $^{15}$N-labeled fertilizer. This isotope tracer method provided better insight into N$_{2}$O source partitioning and provided an independent validation of the Eh-based N$_{2}$O source partitioning. It was found that the changes in soil Eh and N$_{2}$O 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 N$_{2}$O 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. [...