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

DH% = [(C_HA + C_FA) × 100 / TEC] (1)

while humification rate (HR%) was calculated as:
[21]

HR% = [(C_HA + C_FA) × 100 / TOC] (2)

Soil experiments
The experiment was carried out in Motta San Giovanni (LAT:38°0′15″12 N; LONG: 15°41′45″24 E) in Reggio
Calabria, southern Italy, an agricultural area representative of Mediterranean horticultural systems. The
region has a typical Mediterranean climate, characterized by mild, wet winters and hot, dry summers, with a
mean annual temperature of approximately 17-18 °C and annual precipitation of 600-700 mm, concentrated
mainly between autumn and early spring.

A sandy-loam soil (11.85% clay, 23.21% silt, 64.94% sand) was used for the trial, which was carried out
[19]
from December to June. The experimental field was divided into two main plots of 0.5 hectares each, and
subsequently into smaller subplots. Four fertilizer treatments were tested, applied on a nitrogen-equivalent
basis of 30 kg N ha :
-1

• SBO (476 kg S ha ): sulfur bentonite with olive pomace amendment;
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• CTR: unfertilized control;

• CTR0: soil at initial state;

• Compost: 2,123 kg/ha;

• Vermicompost: 1,769 kg/ha;

• Digestate: 2,123 kg/ha.

The trial followed a randomized complete block design with three replicates per treatment. Each replicate
consisted of three subplots, and six soil samples were taken per subplot for analysis. The experiment lasted
two consecutive years, with results presented as the mean of three independent trials. Irrigation was managed
at 70% of field capacity, monitored using a direct-read pH/moisture probe (R181). The application rate of
SBO was determined based on published references, which recommend elemental sulfur inputs ranging from
2,200 kg S ha to 3,300 kg S ha depending on soil characteristics [15-20] . After 180 days, soil samples were
-1
-1
collected for chemical and biological analyses. Samples were air-dried, sieved to < 2 mm, and stored at 4 °C
for no longer than 24 h before microbial and enzymatic assays. Chemical and biochemical properties were
analyzed in triplicate to ensure reproducibility.

Soil chemical analysis
The following soil properties have been detected. Soil texture with the hydrometer method ; electric
[27]
conductibility (EC) in 1:5 soil/water suspension, after stirring at 15 rpm for 1 h, was measured with a Hanna
instrument conductivity meter; pH was determined in soil/solution ratio 1:2.5 with a glass electrode. Organic
carbon was detected with the Walkley and Black method . Total nitrogen (TN) was assessed with the
[28]
Kjeldahl method . C/N was quantified as a carbon:nitrogen ratio. Water-soluble phenols were extracted and
[29]
analyzed as described by Kaminsky and Muller and monomeric and polyphenols were determined with
[30]