An Investigation into the Effect of Ochre Amendments on Soil Carbon Storage, Aggregate Formation and Plant Growth

Ochre is an iron oxide waste that is currently stockpiled pending re-use or disposal
via landfill, providing motivation to research an environmentally beneficial application for
this accumulating waste product. It is known that iron oxide amendments to soils can
stabilise and increase soil organic carbon content, as well as having other potential benefits
to soil structure. This thesis researches whether the ochre amendments to soils could be
utilised as a land-based CO2 removal method, aiding in the mitigation of climate change.
It is hypothesised that additions of ochre to soils could reduce carbon availability, leading
to carbon sequestration. This hypothesis is tested through an adsorption experiment
which assesses whether ochre amendments to an Agricultural and Woodland Soil increase
organic carbon sorption and affect phosphorus availability, metal release and soil pH. Soil
incubation/plant growth experiments were also conducted which assess whether ochre
amendments reduce carbon availability and affect plant growth and aggregate formation.
The adsorption study concluded that the addition of the ochre to soils enhanced organic
carbon sorption, reducing soil carbon availability and lability, based on the assessment
of the change in quantity of carbon released into soil solution (5.89 ± 0.86 to 10.62 ±
1.29 mg/L and 7.15 ± 0.42 to 10.75 ± 3.01 mg/L for ochre amended Agricultural and
Woodland Soils, respectively), relative to control soils (10.95 ± 0.16 mg/L and 13.66 ±
0.21 mg/L for Agricultural and Woodland Soil controls, respectively) (p≤0.05: ANOVA
and Dunnett’s test). Ochres with a high goethite content (67.27 - 97.50 %) and a relatively
low - neutral pH (4.45 ± 0.05 to 8.34 ± 0.04) were favourable in increasing soil organic
carbon sorption. However, ochres with a high Na content (344 ± 159 to 846 ± 41.7 mg/kg)
were found to perform less favourably. The incubation/plant growth study found a lack of
difference in the extractable carbon content of ochre amended soils, compared to control
soils, but found a reduction in the cold water extractable carbon content in ochre treated
Woodland Incubation Soils (6.30 ± 0.96 to 14.88 ± 3.47 mg/L), indicating decreased
carbon availability and increased carbon storage, relative to the Woodland Incubation
control (20.88 ± 3.40 mg/L) (p≤0.05: ANOVA and Dunnett’s test). This provides
motivation for further experiments to take place, requiring a longer incubation period,
with thorough mixing, to ensure there is an increased opportunity of contact between the
iron oxide and dissolved organic carbon. In practical applications, a solution for increasing
contact chances is to plough the ochre into the soil as a slurry. Despite the addition of
ochre causing there to be more fine-grained material in the soils, generally no effects
on aggregate fraction size were found, implying that the fine ochres have supported the
aggregation of material to form coarse, stable macroaggregates, improving soil structure.
Future work should look into the organic carbon content of each aggregate size, as well
as measuring changes in carbon pools, as the results of this thesis suggest there is a shift
in carbon from a very labile to a less labile pool. Finally, additional experiments should
analyse for potential toxic metal uptake in the plant shoot biomass, given that some of the
ochre treated soils released more metals into solution than the control in the adsorption
study, despite the ochre amendments having no effect on plant growth.