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Page 14 of 19 Maffia et al. Carbon Footprints 2026, 5, 7

This result suggests that CO emissions are higher in the presence of greater mineralization of organic matter
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and a dominant fungal activity, which favors the rapid degradation of organic compounds. In contrast, CFP
was positioned opposite to variables such as TOC, soil organic matter, humified carbon, total nitrogen, pH,
electrical conductivity, MBC, and CEC. This indicates a negative correlation, meaning that soils richer in
stable organic matter, nutrients, and microbial biomass exhibited a lower CFP, due to their greater capacity
to sequester carbon. From the perspective of treatments, SBO was located in proximity to CFP, confirming
its contribution to higher CO emissions. Compost and Vermicompost, on the other hand, were strongly
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associated with soil quality and stability parameters, confirming their ability to reduce CFP and enhance
carbon sequestration. Digestate showed an intermediate position, exerting some positive effects on fertility
but without the same efficacy in mitigating emissions. Overall, the first principal component (PC1), which
explained 68.7% of the variance, clearly represented the gradient between soils with higher emissions and
those with greater carbon accumulation potential. These results emphasize that the application of
high-quality organic amendments can be an effective strategy to improve soil fertility while simultaneously
mitigating the CFP. The correlation matrix heatmap further supports these findings by providing a global
overview of the pairwise relationships among soil parameters and CFP. CFP showed strong negative
correlations with TOC, SOM, HC, MBC, CEC, DHA and pH, confirming that higher soil fertility and
organic matter stability are linked to lower emissions [Figure 5]. Conversely, positive correlations were
observed between CFP and variables such as Fungi, FBC, and FC, highlighting the role of fungal-driven
mineralization and labile carbon pools in promoting CO release. This multivariate visualization reinforces
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the conclusion that a soil system enriched with stable organic carbon and balanced microbial communities is
more efficient in sequestering carbon and mitigating the CFP, while treatments dominated by labile pools
and fungal activity contribute to higher emissions [Figure 5]. When translating these results into future
policy actions, scalability and waste availability must also be considered. While vermicompost showed the
most favorable agronomic and environmental performance on a per-ton basis, its global production capacity
remains far smaller than that of agricultural waste compost and food-waste compost. Composting is
currently the dominant technology capable of processing millions of tons of organic residues annually,
including agricultural by-products, pruning residues, and municipal food waste. Therefore, although
vermicompost represents a high-quality amendment, compost is the option with the greatest scalability for
climate policy frameworks. Integrating estimates of regional waste availability into policy design would
support realistic deployment scenarios for waste-derived fertilizers. A methodological limitation of this study
is that the compost was produced using an electrically assisted system based on a traditional, slower
three-phase process. In recent years, however, composting technology has advanced substantially. Novel
microbial-driven systems are able to shorten the initial mesophilic phase to less than one day, rapidly reach
higher thermophilic temperatures (up to 70 °C), and complete thermophilic stabilization within 7-10 days
without the need for external energy inputs. These new process configurations - driven primarily by
optimized aeration, moisture control, and microbial community engineering - enable faster decomposition,
lower energy consumption, and reduced GHG emissions during composting [53,54] .

Beyond their experimental performance, these results have important implications for future GHG
mitigation and soil management policies. The strong humification capacity and low CFP of compost and
vermicompost indicate that these waste-derived fertilizers can simultaneously support agronomic
productivity and climate-smart nutrient management. By recycling regional organic residues and promoting
the formation of stable carbon pools, these amendments reduce dependence on synthetic fertilizers, enhance
soil resilience under Mediterranean climatic stressors, and contribute to long-term carbon neutrality goals.
Integrating such circular bio-based fertilizers into agricultural policy frameworks could therefore provide a
dual benefit, improving soil quality while mitigating GHG emissions. These outcomes are also consistent
with the goals of the “4 per 1000” Initiative launched at the 2015 Paris Climate Conference (COP21) , which
[55]
promotes increasing global SOC stocks by 0.4% per year to offset anthropogenic GHG emissions. The