Introduction

The capability of soil to provide ecosystem services through four soil functions: carbon cycle, nutrient cycle, pest regulation and soil structure maintenance (Kibblewhite et al., 2008) is defined as “soil health.” It is closely related to soil quality and productivity, but is more integrative and focused on soil biology (Lal, 2016). Soil biology is a challenging phenomenon to map through indicators (Wade et al., 2022), especially compared to well-established methods for chemical (Gibbons et al., 2014) and physical (Hartge and Horn, 2009) soil quantification.
Biological soil health can be quantified through different approaches. Functional indicators focus on ecosystem processes (e.g. carbon metabolism: basal respiration, CO2-burst test, aerobic and anaerobic incubations). Compositional indicators measure key species groups (e.g. microbial diversity and biomass, nematode and earthworm counts) (Weil and Brady, 2016). One of the most commonly used functional indicators is the 24 h CO2 respiration rewetting soil test for biological activity (Franzluebbers et al., 1996). In this method, a soil sample is dried and rewetted, which results in a flush of CO2 (“CO2burst”). It responds to management and correlates with important soil processes such as nitrogen mineralization, making it a popular tool for managing soil health (Haney et al., 2018). Initially described in 1950’s (Birch, 1958), but research is still ongoing on the mechanisms of the burst (Barnard et al., 2020, p. 2; Hicks et al., 2022). Physical, chemical and biological soil processes control the flush of CO2 (Barnard et al., 2020), therefore it is an integrative indicator for many soil properties.
Soil redox potential status is another integrative indicator of soil properties and processes. The redox potential of soil is the product of oxidation-reduction reactions. It measures the general availability of electrons, or the relative difference between oxidation (loss of electrons) and reduction (gain of electrons) (Zhang and Furman, 2021). In soils, one of the main reactions is the oxidation of organic matter ((CH2O)n), which supplies electrons, allowing the reduction of other compounds and producing CO2 and water, for example: