English abstract
Biochar is a carbon-rich material used to increase the fertility of the soil and improve its
properties. It is produced through a process called pyrolysis which determines its stability.
Biochar produced at low temperatures is known to have a lower half-life compared to those
produced at higher temperatures. In this study, we investigated the effect of different clay
co-amendments on biochar stability as measured through microbial respiration (cumulative
CO2 production during the first month of incubation in soils). The effect on microbial
enumeration, activity, and diversity was also assayed. We hypothesized that a) the biochar
treated with inorganic clays increase biochar stability and reduce its degradation rate in
calcareous sandy soils of Oman, and b) based on the available literature, allophane clays
would be the most effective biochar additive for this purpose. Local clays were also tested
as a sustainable alternative to Allophane and Ferrihydrite synthetic clays and were
separated from the Al Khoud dam sediment after flocculation with Fe, Mg, Ca and HCl.
Biochar clay homogenous mixtures were achieved in stirred suspensions at different doses
(0, 0.032, 0.064, 0.128, 0.256, and 0.512) % by mass. Biochar that was obtained from date
palm leaves was pyrolyzed at 450 °C and applied to the soil at 2.5% by weight. The results
showed that biochar increased the soil microbial respiration rate, and the addition of
different clays to biochar showed a stabilization effect bringing respiration rates closer to
unamended soils. In general, the clays significantly reduced the average respiration rate
except for soil biochar with local clays coated with calcium (SB-cCa) and soil biochar with
local clays treated with hydrochloric acid (SB-cHCl) treatments. Moreover, ferrihydrite
(Fh) at the maximum tested dosage (0.5%) was the most effective treatment to achieve this
effect. The reduction in respiration rate was not reflected in a reduction in bacterial
enumeration. The 16S rRNA prokaryotic community taxonomic diversity characterization
showed more diversity in control soil compared with treated soils with 2.5% biochar and
0.512% clays and revealed a decreased abundance of archaea in samples amended with
biochar and clays, with an increase in the dominance of firmicutes, especially in SB-cHCl
treated samples. The differences in Shannon and Evenness diversity indexes for total
operational taxonomic units (OTUs) between control soil and soil amended with biochar
and clays were small, while for the class reduced compared with control soil. The addition
of both locally sourced natural clays and synthetic clays caused a strong increase in the
flavobacteriia class. The pH of biochar amendments appears to be driving the abundance
of actinomycetia and betaproteobacteria. These findings highlight the potential of biochar
and clays as effective soil management strategies for enhancing biochar stability and the long-term effect on carbon sequestration and other soil features offering a sustainable
solution for agriculture and ecosystem restoration in arid regions.
