Page 7 - Read online

Maffia et al. Carbon Footprints 2026, 5, 7 Page 3 of 19

organic matter decomposition and soil aggregation . Fungi allocate a greater proportion of assimilated
[12]
carbon to their biomass compared to bacteria, largely due to the synthesis of structurally complex and
decay-resistant compounds, such as chitin, melanins, and glucans . This investment in recalcitrant biomass
[13]
is considered an important pathway for the formation of stable SOC. However, recent studies highlight that
long-term carbon stabilization is also strongly influenced by the formation and persistence of microbial
necromass, particularly of bacterial origin. These two perspectives are not contradictory: fungal biomass
contributes to the formation of aromatic and structurally complex intermediates during early decomposition,
whereas bacterial necromass - often bound to mineral surfaces - constitutes a major component of
mineral-associated organic matter. Thus, the relative importance of fungi and bacteria in soil carbon
dynamics depends on the stage of decomposition and on environmental conditions. Soil management
practices that promote a balanced or fungi-enhanced microbial community - such as reduced tillage, organic
amendments, and diversified crop rotations - can therefore improve carbon sequestration by stimulating
both fungal transformation pathways and bacterial necromass accumulation . This functional divergence
[14]
between fungi and bacteria underscores the importance of microbial community composition in regulating
soil carbon dynamics and highlights the potential for microbiome-targeted management strategies to
mitigate atmospheric CO accumulation. Waste-derived fertilizers - particularly compost, vermicompost,
2
digestate, and sulfur - bentonite formulations enriched with olive pomace - are increasingly relevant in this
context, as they are produced from abundant regional waste streams in southern Italy and reflect current
circular economy objectives. Their contrasting stabilization levels, nutrient availability, and microbial activity
provide an ideal framework for comparing multiple carbon sequestration pathways, from soil aggregation
enhancement to microbial necromass formation. Understanding their effects is especially important in
Mediterranean horticultural systems, where high temperatures and intensive management accelerate organic
matter mineralization.

Integrating CFP assessment with a mechanistic understanding of soil carbon stabilization, this study
evaluates the dual contribution of these regionally relevant amendments to climate mitigation and soil
health. Specifically, we investigate how compost, vermicompost, digestate, and sulfur bentonite with olive
pomace (SBO) influence soil carbon accumulation, microbial activity, and the balance between fungal- and
bacterial-mediated pathways of organic matter transformation.

METHODS
sulfur bentonite with olive pomace
SBO was produced in tablet form (3-4 mm) by Steel Belt System srl (Varese, Italy), following the method
described by Muscolo et al. [15,16] . Elemental sulfur (S, 80%), a residue of hydrocarbon refining processes, was
combined with bentonite clay (B, 10%) as a support and carrier, and olive pomace (OP, 10%), a by-product
of the olive oil industry. Elemental sulfur represented the main component of the fertilizer . Prior to use,
[17]
the fertilizer was tested for potential contaminants, including pathogenic microorganisms (total coliforms,
faecal coliforms, Salmonella spp., and Escherichia coli) and heavy metals, to ensure its environmental safety
and suitability for soil application . Analyses confirmed the absence of both pathogens and heavy metals .
[16]
[16]
Compost
Compost was produced using specialized electric composters designed to promote efficient organic matter
decomposition. These composters contained separate chambers, which prevented the mixing of fresh and
decomposing material and allowed independent temperature regulation to optimize microbial activity. Each
composting process was carried out in triplicate and followed three controlled phases: (i) an initial
mesophilic phase of 8 days at 29 °C, (ii) a thermophilic phase of 20 days at 50 °C, and (iii) a prolonged
mesophilic phase of 92 days at 27 °C . Following these phases, all composts underwent a 30-day
[18]
stabilization stage at a constant temperature of 20 °C to ensure adequate maturation. Throughout the