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\begin{document}

\title{Redox potential as a soil health indicator -- how does it compare to
existing methods?}



\author[1]{Tuomas Mattila}%
\affil[1]{SYKE}%


\vspace{-1em}



  
  \date{June 18, 2024}


\begingroup
\let\center\flushleft
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\selectlanguage{english}
\begin{abstract}
Soil health is the capability of soil to provide ecosystem services.
These can be quantified through multiple separate indicators
(N-mineralization, water infiltration, aggregate stability, etc.) or by
a single proxy that integrates many soil processes. Two commonly used
integrative measurements are the soil 24h-respiration test (CO2burst)
and the visual evaluation of soil structure (VESS). Both are fast, but
capture only a part of whole phenomenon of soil health. Soil redox
potential is a promising soil and plant health indicator. The redox
potential is controlled by soil chemical oxidation-reduction reactions
and therefore integrates several processes. However, this method has
been tested only on a few soils. In this study, we evaluated redox by
comparing it with other established soil health indicators on 35 fields
in Finland. Based on the results, redox correlated well with soil
biological activity, structure, and texture. Soils with good structure
had an oxidized redox status. A low redox state was connected to high
biological activity. The carbon farming practices resulted in lower
oxidation. A combination of redox and pH could be used to classify
soils. The analysis supports the use of redox as a soil health
indicator, but further research is needed in identifying the processes
and properties the redox is an indicator for.%
\end{abstract}%



\sloppy


\section*{Introduction}

{\label{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, CO\textsubscript{2}-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 CO\textsubscript{2} 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
CO\textsubscript{2} (``CO\textsubscript{2}burst''). 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 CO\textsubscript{2} (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
((CH\textsubscript{2}O)\textsubscript{n}), which supplies electrons,
allowing the reduction of other compounds and producing
CO\textsubscript{2} and water, for example:\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

(eq 1.)

In fully aerobic soils, oxygen serves as the electron acceptor,
resulting in the production of CO\textsubscript{2} and
H\textsubscript{2}O. The redox potential is controlled by the rates of
reduction and oxidation processes in soil and is connected to soil
respiration, for example in rewetted agricultural soils, where redox is
a strongly correlated predictor of CO\textsubscript{2} flux (Bartolucci
et al., 2021). Based on this, redox could be a promising indicator for
soil carbon cycle (labile carbon pools) and structure (oxygen
availability), two key components of soil health (Kibblewhite et al.,
2008).

Redox can also serve as an indicator for nutrient availability and pest
regulation (Husson, 2013). Redox can be thought of as a parallel to pH,
which measures proton availability, as redox (Eh) measures electron
availability. And as pH, redox can influence nutrient availability
considerably. Redox is managed to avoid toxic As and Cd buildup in rice
paddies (Evans et al., 2021) and to improve Mn supply (Husson, 2013;
Zhang and Furman, 2021). A key challenge for using redox as an indicator
is however, that is changes rapidly with soil water and oxygen
conditions (Zhang and Furman, 2021). Husson et al. proposed to use redox
potential as a soil test from dried soil samples. (Husson et al., 2016).
In the test, dried soil would be rewetted and the redox would be
measured for 2 min. In theory, the test is similar to the
CO\textsubscript{2} burst test, but taking only 2 min instead of 24 h.
It therefore has great potential for a high-throughput indicator for
soil health, but it should be tested in different soils and under
different management to see how it correlates with established soil
tests and how it reacts to management.

To evaluate redox as a soil biological health indicator, we compared it
to existing measures of soil health (CO\textsubscript{2}burst, visual
evaluation of soil structure VESS and soil organic matter). We used 18
sites from an ongoing carbon sequestration experiment (Carbon Action,
2019-; (Mattila et al., 2022)), where each site had a carbon farming
trial plot and a control plot. We measured CO\textsubscript{2}burst and
redox from dried and rewetted samples and compared the results with each
other and other analyses of soil properties. The results were used to
classify soils and to evaluate the change in soil health from three
years of carbon farming. This allowed the evaluation of the redox
potential as an indicator of soil health.

\section*{Materials and methods}

{\label{materials-and-methods}}

The materials for the study were soils sampled from the Carbon Action
experiment (Mattila et al., 2022) intensive observation set (Mattila,
2020), where 20 farms test carbon sequestration (cover crops, compost,
grazing practices, intercrop leys and subsoiling). The experiment
started in 2019 and the samples were collected in July 2021. Each farm
had a split field, where one side had carbon farming practices
implemented and the other was held as a continued-normal-practice
control. The samples were collected from 3 GPS located points on each
field from a 10 m radius from the center of the point with a 16 mm soil
corer from a depth of 0-17 cm. Each field had 30 cores collected, which
were pooled, dried at room temperature (fan assisted) and gently sieved
through a 5 mm sieve. As the soil sieving and milling can influence the
results, all soil sample processing was done following established
guidelines (Franzluebbers and Haney, 2018). (Due to an unfortunate
laboratory accident, 5 samples were lost during processing, resulting in
an overall sample amount of n=35.) The sampled soils covered a large
range of soil texture and organic matter: the median clay content was
35\% (4-63\%) and the OM 6.8 \% (2.6-15.5\%). The farming systems
covered annual cropping, grass in rotation and perennial pastures.

For CO\textsubscript{2} burst analysis, the dry samples were rewetted to
approximately 50 \% pore space (i.e. 30 ml of soil and 9 ml of water;
Woodsend lab manual). The sample was placed in a 475 ml container and
sealed with a CO\textsubscript{2} measurement cap fitted with a
datalogger (Woodsend IRTH). The CO\textsubscript{2} concentration was
measured for 24 h and the increase in CO\textsubscript{2} level over
time was converted to mg CO\textsubscript{2}-C/kg\textsuperscript{3}soil
by multiplying with container air space, dividing by sample mass and
converting to mass units using the ideal gas law.

For the redox analysis, soils were also rewetted to 50\% pore space
(Husson et al., 2016). The Redox was measured with an Extech RE300
Exstik Platinum oxidation-reduction-potential sensor (platinum
electrode, silver/chloride reference electrode). The flat end ORP sensor
was pressed to the moist soil sample and allowed to stabilize 1-3
minutes, until the ORP reading changed only slowly. Three repeated
measurements were made of the same sample and the average value was
used. The sample pH was measured with a Horiba LAqua Twin pH meter,
using 1:1 ratio of distilled water:soil. The ORP reading was converted
to Eh (mV) by adding the reference electrode voltage (200 mV) and pH
corrected to a relative hydrogen score by the equation
rH\textsubscript{2} = Eh/30 + 2 pH. For interpretation, the readings
were compared to suggested norms for ''healthy'' soils (Husson, 2013).

For additional interpretation, the soil samples were classified
according to organic matter content, soil clay content, soil structure
(VESS (Ball and Munkholm, 2015)) and type of crop (perennial, annual).
These were collected and published as ongoing monitoring in the Carbon
Action experiment (Mattila and Girz, 2021).

The statistical analysis was done in R programming language (R Core
team, 2022). Correlations between measured variables were calculated
with Pearson's correlation. Significance between the differences in two
groups was tested with Mann-Whitney U-test (Wilcoxon). The effect size
of carbon farming practices was tested by fitting a linear model to the
data, using each farm as a blocking factor.

\section*{Results and discussion}

{\label{results-and-discussion}}

\subsection*{Redox potential can be used to classify soils and monitor
management
effects}

{\label{redox-potential-can-be-used-to-classify-soils-and-monitor-management-effects}}

The redox potential Eh of soils ranged from reduced to slightly oxidized
+340 to +560 mV and from acidic pH 5.8 to slightly alkaline pH 7.6
(Figure 1). Most of the sampled soils were in a region of the Eh/pH
space, which is considered favorable for plant growth (Husson, 2013).
Compared with the favorable region, one soil was too reduced and 10
samples were too oxidized. The reduced sample was a seasonally
waterlogged silty clay loam soil, the oxidized alkaline sample was a low
organic matter sandy loam soil. The acidic and oxidized soils were a
more complex set of soils with either a very high OM level
(\textgreater{}12\%) or a very high clay level (\textgreater{}60\%). The
two soils in the ''optimum'' range were silty clays with an OM 6\% and a
history of grass cultivation and horse manure application. Compared to
the four soils evaluated in Husson et al. (2016), the soils in this
study were more reduced (530 vs. 470 mV), which could be expected on the
basis of differences in climate (warm vs. cool temperate) and organic
matter level (2\% vs. 8\%).

The addition of carbon compounds to the soil through carbon farming
decreased Eh (Figure 1 and Figure 2). This supports the findings of
Husson et al. (2016) where conservation agriculture (residue retention
and minimum tillage) reduced Eh by 10-25 mV. In this study, the
reduction was on average -20 mV (p \textless{} 0.05; 95\% range -94-+26
mV). The largest reductions (-90 mV) were on sites that had compost soil
amendments applied. This would suggest that soil redox potential reacts
to the addition of readily decomposable organic matter. Overall, the
redox potential test on dried and rewetted soils could differentiate
soils based on their OM and clay status, but it also reacted on
short-term changes to soil management. Poor soil condition
(waterlogging, OM or tillage) was reflected by the placement in the
Eh/pH chart. This reactivity to soil conditions and management makes
redox a promising indicator of soil health.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Figure 1.} Tested soils in a pH/Eh Pourbaix plot. Triangles=
carbon farming plots, rounds = control plots. Color = OM content.
Favorable and optimum regions from (Husson et al., 2016).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Figure 2.} The effect of carbon farming practices on soil redox
potential. (Light shade= control plot; dark shade = carbon farming plot.
Samples taken in July 2021, on the third year of the experiment.)

\subsection*{Redox complements, but does not replace other measures of
soil
health}

{\label{redox-complements-but-does-not-replace-other-measures-of-soil-health}}

The Eh correlated significantly with CO\textsubscript{2}burst
(p\textless{}0.001) and explained 36\% of variability
(R\textsuperscript{2}=0.36). In contrast, the correlations of pH
(p=0.06) and clay (p=0.80) with CO\textsubscript{2}burst were
nonsignificant. Organic matter (OM) by itself was not-significant (p
\textgreater{} 0.10; R\textsuperscript{2} = 0.08), but when included in
a linear regression with Eh, it was significant (p \textless{} 0.001)
and together the two variables explained 50\% of the variability in
CO\textsubscript{2}burst. This supports earlier findings, that the
CO\textsubscript{2}burst is more related to the availability and quality
of organic matter than the quantity (Haney et al., 2012). Redox
potential explains the variation in biological activity better than
organic matter or texture. It may be, that this is due to the rewetting
experiment conditions, where labile carbon compounds are oxidized (eq.
1), and that the redox is not directly related to the biological
activity, but to the physicochemical factors controlling
CO\textsubscript{2} flush from rewetting (Barnard et al., 2020). In any
case, when combined with soil OM, the redox potential can explain the
majority of the biological activity related to the carbon cycle, making
it a promising soil health indicator or a rapid proxy for respiration
tests.

However, the redox should not be used to predict
CO\textsubscript{2}burst. Even when including the pH correction to Eh
(i.e. hydrogen potential rH\textsubscript{2}), redox explained only 35\%
of the variability (R\textsuperscript{2}=0.35) (Figure 3). Reduced
conditions co-existed with high biological activity: of the five soils
with high biological activity, four had reduced redox status
(rH\textsubscript{2}\textless{} 28). The one exception was a very high
OM level (14\%) pasture site. Thus, reduced conditions were a requisite
for high biological activity. However, as also three reduced soils had
only moderate biological activity, reduced conditions do not guarantee a
high biological activity. Likewise, the CO\textsubscript{2} burst was
highly variable under oxidized conditions, suggesting that redox cannot
replace CO\textsubscript{2} burst as a biological soil health test.

The trend of oxidized redox and low biological activity in the lab tests
is in direct contrast with field observations, where high oxidation
correlates with high respiration (Bartolucci et al., 2021). However, in
field conditions, the redox was changing over time (with soil drying)
and in our observations the redox changed between samples in an
artificial drying-rewetting experiment. In the field experiments, oxygen
limited respiration on a rewetted wetland. In the laboratory test,
respiration was more likely limited by carbon compound availability or
the amount of microbial biomass, similar to the CO\textsubscript{2}burst
(Barnard et al., 2020). In any case, field experiments on drained, low
organic matter soils would be needed to estimate how well the lab test
redox results correlate with in-field CO\textsubscript{2} burst events.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 3.} Soil biological activity vs. redox state (rH2). The
colors represent OM concentration and the shape is the treatment vs.
control group.

Redox potential tracked changes in soil management (Figure 2; Eh -20 mV
p = 0.048*), but the difference between management practices in
CO\textsubscript{2}burst was not significant (increase of 8 mg-C/kg, p =
0.06). CO\textsubscript{2} burst is considered as an intermediate
indicator, changing over a period of a few years (Weil and Brady, 2016).
In this test, three years was not enough to result in a detectable
difference in CO\textsubscript{2}burst. It may be, that the
CO\textsubscript{2} burst integrates more variables (Barnard et al.,
2020) than the redox and that some variables have developed in opposite
directions, confounding the effect of management. However,
CO\textsubscript{2}burst was found to correlate with texture but not
with structure, while redox correlated with both texture and structure
(Table 1), suggesting that redox can integrate soil properties that
change very rapidly, such as structure (Weil and Brady, 2016).

Soil structure had a marked effect on the redox potential (Figure 4)
(VESS vs. rH2, R\textsuperscript{2}=0.35, p=0.04). In soils with good
structure (VESS \textless{} 2.75) the redox status was more oxidized
(rH\textsubscript{2} 30.5) compared to soils with poor structure (VESS
\textgreater{}3.25; rH\textsubscript{2} 28.6) (Figure 4). Even in poor
structure soils, most soils were classified as oxidized, indicating that
redox cannot be used as a replacement for the soil structure evaluation,
but that good structure is often found together with oxidized soils. OM
correlated with VESS (R\textsuperscript{2}=0.42): soils with high OM
were often classified as having a good structure (Figure 4).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

Figure 4. The redox status (rH2) of soils with a good structure (VESS)
was more oxidized than for soils with a poor structure. The colors on
the points describe the soil OM concentration.

Overall, redox can be an important additional measure for soil health
quantification (Table 1). It correlates with soil structure (VESS) and
biological activity (CO\textsubscript{2}burst). As it does not correlate
with OM, it is a measure of the active organic matter pool, possibly
similar to water soluble organic carbon (Haney et al., 2012). However,
especially the pH-corrected rH\textsubscript{2} score correlates with
soil clay content, so the interpretation should be based on soil
texture. Although redox correlates with other soil health indicators, it
is not a replacement for those. For example, soils with a good structure
have a high redox (Figure 4), but also some poor structure soils may
also have similar values. Soils with high biological activity have low
redox potentials (Figure 3), but a low redox potential can also be found
in a soil with moderate biological activity.

As the redox potential changes more rapidly with management than
CO\textsubscript{2}burst (Figure 2), it is a promising indicator to
track management-induced changes in soil health. In this application, it
could also be used to classify soils based on their Eh and pH (Figure
1). These results supported the earlier findings on the use of soil
redox potential to classify soils and to follow their change due to
management (Husson, 2013; Husson et al., 2016). As the earlier studies
were conducted in a warm temperate climate with lower OM and clay
content, similar findings in high OM and clay environments support
continuing investigations into the role of redox in quantifying soil
health.

A major challenge in interpreting the redox results is the integrative
nature of the redox potential. It is a combined effect of all ongoing
soil oxidation and reduction reactions, which defines the redox
potential. It may be hypothesized that the organic matter decomposition
would drive the potential in a drying-rewetting test, but in this study,
soil texture was found to correlate with redox potential, suggesting
that other reactions than organic matter related were also driving the
potential. Further studies on the interpretation of redox in different
textures and farming systems would be needed to make it an applicable
tool for managing soils.

Table 1. Correlation between soil redox status (Eh and pH corrected rH2)
and commonly used soil health indicators (Structure VESS; CO2burst;
Organic matter; and Clay content) (Weil and Brady, 2016).\selectlanguage{english}
\begin{longtable}[]{@{}llllllll@{}}
\toprule
& & & & Rapid & -Change & - & Permanent\tabularnewline
\midrule
\endhead
& & \textbf{Eh} & \textbf{rH2} & \textbf{VESS} &
\textbf{CO\textsubscript{2}-burst} & \textbf{OM} &
\textbf{Clay}\tabularnewline
\textbf{Redox} & \textbf{Eh} & \textbf{1.00} & 0.88 & & &
&\tabularnewline
\textbf{Hydrogen score} & \textbf{rH2} & 0.88** & \textbf{1.00} & & &
&\tabularnewline
\textbf{Structure} & \textbf{VESS} & -0.35* & -0.41* & \textbf{1.00} & &
&\tabularnewline
\textbf{Biol.activity} & \textbf{CO\textsubscript{2}burst} & -0.60** &
-0.59** & 0.01 & \textbf{1.00} & &\tabularnewline
\textbf{Organic matter} & \textbf{OM} & 0.16 & 0.09 & -0.42* & 0.28 &
\textbf{1.00} &\tabularnewline
\textbf{Texture} & \textbf{Clay} & -0.31 & -0.34* & 0.48** & 0.47** &
-0.11 & \textbf{1.00}\tabularnewline
\bottomrule
\end{longtable}

\section*{Conclusions}

{\label{conclusions}}

Redox presents a rapid measurement of soil oxidative-reductive status.
When applied to a simplified soil drying-rewetting test, it could be
used to classify soils into oxidized and reduced. In addition, the redox
status correlated with CO\textsubscript{2}burst, indicating that it can
quantify the amount of readily available substrates for microbial
activity. When combined with organic matter concentration, it could
explain most of the variability in measured CO\textsubscript{2}burst
results. It also correlated with soil structure and texture, making it a
promising general indicator of soil conditions. Simultaneously, the
correlation with soil texture and structure and the fact that it is an
outcome of several reduction-oxidation reactions, makes interpretation
of redox challenging. Currently, it can be used as an index for soil
health, but more studies on the mechanisms of redox and it's change over
time could reveal, what are the soil processes it is an indicator of.

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{\label{literature-cited}}

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