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Page 4 of 19 Maffia et al. Carbon Footprints 2026, 5, 7

composting process, temperature, moisture, and oxygen levels were monitored daily using a centrally placed
probe. Water was added when necessary to maintain optimal moisture content, and daily mixing ensured
proper aeration and oxygenation, thereby enhancing microbial activity and promoting the breakdown of
organic matter into stable humus. At the end of the composting cycle, the composts were air-dried, finely
ground to pass through a 2 mm sieve, and homogenized for uniformity. Both compost types reached full
maturity within six months .
[19]

Vermicompost
Vermicomposting was conducted in a 50 L capacity worm bin (Vevor, 5-Tray Worm Composter, model
WB25101). The feedstock mixture consisted of 45% olive residues, 45% organic food waste, and 10% straw, to
which 20% earthworm biomass was added. Red wigglers (Eisenia fetida) were introduced at a density of
approximately 1,000 individuals (≈1 lb) per square foot of surface area. The bedding was kept loose to
enhance aeration, and moisture was maintained at a level that was damp but not waterlogged. Over a period
of four months, the organic material was progressively decomposed by the worms, resulting in stable and
mature vermicompost .
[20]
Olive-based digestate
Olive-derived digestate was sourced from a biogas plant operated by the Fattoria della Piana cooperative
(Candidoni, Calabria, Italy). The facility has a total digester volume of 3,260 m and an installed capacity of
3
998 kWe. The feedstock used for anaerobic digestion consisted of 50% olive residues combined with 50%
animal manure and maize silage. The plant was operated under mesophilic conditions at 40 °C, with a daily
feed input of 120 m and a hydraulic retention time (HRT) of 60 days. The minimum guaranteed retention
3
time (MGRT) was 16 h at 40 °C. After production, the digestate was sampled and analyzed to determine its
chemical and biological characteristics, with the aim of evaluating its suitability as a humus-rich soil
amendment.

Humic substance detection
Humic substances (HSs) were extracted from air-dried samples using 0.1 mol L KOH (sample-to-solution
-1
ratio of 1:20, w/v). Extractions were performed at room temperature under a nitrogen atmosphere for 16 h.
The suspensions were centrifuged at 7,000 × g for 20 min to separate the supernatant containing soluble HS
from residual solids . The resulting extracts were analyzed to determine total organic carbon (TOC), total
[21]
extractable carbon (TEC), concentrations of humic acids (HA) and fulvic acids (FA). The corresponding
fraction of carbon (FC) were expressed as C_HA and C_FA, respectively. These measurements were used to
calculate several humification parameters, including humification rate (HR%), humification index (HI),
humification degree (HD, DH%), and the ratio of the absorbances at 465 and 665 nm (E4/E6) .
[22]
Characterization of HS functional groups was carried out by Diffuse Reflectance Infrared Fourier Transform
Spectroscopy (DRIFT). Measurements were conducted with a Nicolet Impact 400 Fourier transform infrared
(FTIR) spectrophotometer (Nicolet Instruments, Madison, WI, USA) equipped with a diffuse reflectance
accessory (Spectra-Tech, Stamford, CT, USA). For each spectrum, 200 scans were collected at a resolution of
4 cm and processed using Omnic software (Version 3.1, Nicolet Instruments, USA). Analytical samples
-1
were prepared by homogenizing 2 mg of dried extract with 148 mg of spectroscopic-grade potassium
bromide (KBr) (Aldrich Chemical Co., Milwaukee, WI, USA). Absorption bands were identified according
to standard assignments described in previous studies [23-25] . Carboxylic and total acidic functional groups
were quantified following the CaOAc and Ba(OH) titration methods , with filtrates passed through 0.45
[26]
2
μm membrane filters as recommended by Ritchie and Perdue . Phenolic acidity was calculated as the
[27]
difference between total acidity and carboxylic acidity. The degree of humification (DH%) was expressed
[21]
as: