English abstract
Soil salinity is a critical issue that reduces global agricultural productivity, affecting 1,128
million hectares worldwide. In Oman, 44% of the land, especially in the north and south of
Al-Batinah, is salt-affected, threatening crop yields and food security. Biochar, a carbon-rich
solid derived from biomass pyrolysis, has emerged as a promising technique to alleviate soil
salinity and improve soil health in arid environments. However, the effectiveness of biochar
as a soil amendment is influenced by its physicochemical properties, which are determined by
feedstock source and pyrolysis temperature. Moreover, their alkaline nature can be a limitation
in arid regions with alkaline soils. The main objective of this study was to investigate the
synergistic effects of biochar and elemental sulfur on the reclamation of saline soils in Oman.
In this study, the physicochemical characteristics and impact on soil microbes of biochar
amendments from three feedstock sources (date palm leaves, mesquite plants, and sludge
compost) pyrolyzed at 450, 600, and 750°C were investigated. The biochar quality for arid
land applications was improved by co-applying elemental sulfur (0.013%) as an acidifying
agent and incubating it with compost or vermicompost (10%) as biological activators for 50
days. We then evaluated the effects of applying pretreated (incubated) biochar with direct soil
co-amendment on the soil physicochemical and microbial properties. Finally, the effects of
these two types of biochar application (pretreated biochar and direct co-amendment) were
compared in the saline soil of Oman on salt leaching and plant response when irrigated with
poor water quality (EC 3 (dS.m-1
)).
The results showed that increasing the pyrolysis temperature increased the pore size,
thermostability, and C-C bonds while decreasing the O-H and C-O bonds. Mesquite biochar
had the highest surface area (600 m2g-1) and carbon content (94.98%), but reduced soil
microbial enumeration by up to 50% and respiration by up to 80%, especially at higher
temperatures. Sludge compost biochar's high mineral content (45-66%) and potential
contaminants make it unsuitable as a soil amendment. Based on the physicochemical
properties and microbial impact, date palm biochar prepared at 600°C was selected for the
next steps. In the second stage, elemental sulfur significantly decreased biochar pH by up to
1.0 unit, with the highest reduction from 8.1 to 7.2 occurring when co-amended with
vermicompost. Sulfur increased water-soluble concentrations of calcium by 147% and 105%,
and magnesium by 929% and 447% in compost and vermicompost, respectively, indicating
reduced biochar basicity. The sulfate levels increased by up to 300% when compost was coapplied with sulfur, suggesting efficient oxidation. Sulfur amendments stimulated substrateinduced respiration by up to 200%, microbial diversity (Shannon H) increased from 3.08 to
4.52 in compost treatments and altered the bacterial community structure. The comparison
showed that sulfur reduced soil pH by up to 0.8 units when combined with vermicompost,
leading to a maximum pH decrease from 8.05 to 7.25 with pretreated biochar and 7.00 with
direct co-amendment. Direct co-amendment led to up to 50% higher soil NH4+ concentrations
and 25% higher respiration rates but 40% lower substrate-induced respiration than pretreated
biochar. A three-fold higher number of prokaryotic zOTUs (980 vs. 292) was observed with
direct co-amendment than with the pretreated biochar. Co-amendment of biochar with sulfur
lowered soil pH by up to 1.0 unit and decreased topsoil electrical conductivity by nearly 50%,
which was consistently lower than that with pretreated biochar. Sulfur addition increased the
Na and Ca contents in the leachates by over 20% and 60%, respectively. Although compost
and vermicompost did not affect salinity management, their integration with biochar and
sulfur nearly tripled wheat yield (200% increase) and increased grain size by over 20%.
Overall, this study suggests considering feedstock type and pyrolysis temperatures during
biochar production to improve soil health in arid-land agroecosystems. Integrating biochar
with sulfur and compost is a promising sustainable technology for managing biochar alkalinity
and improving soil fertility and plant growth in arid regions by up to 200%. Although
pretreated biochar showed positive effects, the direct co-amendment approach exhibited more
pronounced responses, up to 50% higher, and is considered a more effective option for bio-saline agricultural soil management.
