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: