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.)
Redox complements, but does not replace other measures of
soil
health
The Eh correlated significantly with CO2burst
(p<0.001) and explained 36% of variability
(R2=0.36). In contrast, the correlations of pH
(p=0.06) and clay (p=0.80) with CO2burst were
nonsignificant. Organic matter (OM) by itself was not-significant (p
> 0.10; R2 = 0.08), but when included in
a linear regression with Eh, it was significant (p < 0.001)
and together the two variables explained 50% of the variability in
CO2burst. This supports earlier findings, that the
CO2burst 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
CO2 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 CO2burst. Even when including the pH correction to Eh (i.e. hydrogen
potential rH2), redox explained only 35% of the
variability (R2=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 (rH2< 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 CO2 burst was highly
variable under oxidized conditions, suggesting that redox cannot replace
CO2 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 CO2burst (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 CO2 burst
events.