The average temperature for each windrow, as recorded twice a week, before each turning are reported at Fig. 2. Temperatures trends show that maximum temperatures are all falling in a comparable range and above the 55 °C target. The highest peak temperature was reached by CD (58 °C), while the lowest temperature level was observed for CB3 (55.6 °C). However, CB2 and CB3 reached peak temperatures at least 4 days earlier then control CD and CB1. The bio-oxidative phase of CB2 and CB3 lasted for ~ 15 days, while CD and CB1 needed ~ 19–20 days to reach the same temperature levels.
The range of 52–60 °C is considered as the most appropriate for adequate treatment of the OM [33, 38]. However, ECN-QAS for compost operation quality manual recommends 10 days over 55 °C or 3 days above 65 °C for full sanitization of the substrate from human pathogens in open systems .
Nevertheless, even if the peak temperature of 55 °C was reached in each windrow, the duration of this phase lasted for at least 10 days only in the case of CD and CB2. In order to verify the full sanitization of the products, they were all analyzed for biological contamination by Salmonella spp. The outcome was positive, with all samples resulting pathogen free. However, due to the nature of the main substrate utilized, digestate from manure, this study also investigated the presence of coliforms (E. coli): all products showed coliform concentrations below 10 UFC g−1, excluding the case of CB3, which presented a proliferation of 104 UFC g−1, probably due to a contamination during the co-composting process.
The characterization of the four products according to ECN-QAS quality standards is presented in Table 3.
Two further parameters (impurities, weed seed) were not analyzed (even if recommended by ECN-QAS), as the initial materials were substantially free from these thanks to the intrinsic nature of the feedstocks and the upstream processes (slow pyrolysis for biochar production, and anaerobic digestion for digestate). Furthermore, the experiment was conducted under greenhouse conditions, minimizing the risk of external seed contamination. Plant response, instead, is the subject of ongoing agronomic studies, whose results are not yet published. Maximum particle size was substituted by granulometry distribution analysis, presented in Table 4.
It should be noted that limits concerning OM content, contamination, and inorganic pollutants were met by all blends, which would thus qualify for commercial uses in the EU. Further analysis was however performed to better characterize the products in relation to the initial biochar content of the windrows, in particular as regards the OM transformation (Table 4). The water content of the COMBI blends was reduced by 9.8%, 11.5%, 9.5%, and 5.7% for CD, CB1, CB2, and CB3, respectively, if compared to the initial moisture. However, no water was added during the experiment and the ambient average relative humidity value of the location area, where the test was carried out, at the end of the experiment was 66.1% (see Fig. 5, online resource).
The stabilization of organic substrates through the composting process is the result of both degradation and humification of the organic matter (OM), leading to a final content of OM in composted material lower than in the initial windrow, which can be considered a measure of the intensity of the composting process [33, 39]. The OM content was analyzed in all samples, and a mass balance analysis was performed, considering the dry-weight reduction of the windrows, as recommended by M.P. Bernal et al. [33, 40]. Results are presented in Table 5.
As fixed carbon increases with the biochar content in the initial blend, due to the stable and recalcitrant form of C added with biochar, this fixed carbon amount remains substantially the same during the short-time co-composting process (60 days) [19, 29, 31]. Therefore, in order to adopt a measure representative for the OM yield, the amount of biochar-fixed carbon (BFC) was subtracted from the total OM content (Table 3), defining the new parameter net organic matter (NOM). NOM at the end of the process, expressed as a percentage of the initial NOM, increased almost linearly with the initial biochar rate added to the compost pile (CB3 > CB2 > CB1 > CD).
A higher percentage of OM in the composted material with higher percentages of biochar could lead to assume that co-composting negatively influences the intensity of the bio-oxidative phase, thus lowering the rate of degradation and stabilization of the OM in the initial material. However, three parameters listed below (PDRI, Humification NH4-N/NO3-N ratio) show a stabilized compost for all the blends.
The measure of the Potential Dynamic Respiration Index (PDRI) showed better stabilization for CB2 and CB3. Specific oxygen uptake rate is in fact a direct parameter to assess the compost stabilization at the end of the process. Following Bernal et al., which refer to the Californian Compost Quality Council (CCQC) maturity index [33, 41], PDRI values above 1000 mg O2 kgOM−1 h−1 are representative of unstable compost, while below 300 mg O2 kgOM−1 h−1, the compost is considered as very stable. CB1 showed the highest value, with 350 mg O2 kgOM−1 h−1, while the others resulted below the reference limit. However, it should be noted that only CB2 and CB3 showed PDRI 200 mg O2 kgOM−1 h−1.
Other parameters can also be investigated in order to evaluate the bio-stabilization level of the products, such as humic acid (HA) and fulvic acid (FA) content, which are representative of the humification degree. A higher degree of humic substances correspond to a more efficient stabilization of the OM during composting . HA and FA are heterogeneous complexes which can be classified by molecular weight, functional groups, and degree of polymerization and cyclization [33, 43, 44]. The four blends were analyzed for humic substance content and the results are shown in Table 6. CB1 content of HA and FA was the lowest observed, although comparable with CD. CB2 values for HA and FA were the highest. CB3 showed the lowest HA value and intermediate FA value. These results suggest that the amount of biochar in the initial windrow does not allow a linear prediction of the HA and FA content, and that the CB2 windrow apparently maximized the synergistic effects of co-composting on the microbial humification processes.
As reported in the table above, the two main indexes used in this study to evaluate the humification level of the four blends, following Roletto et al. , were the Humification Index (HI, representing the ratio between HA and organic carbon contents) and the Polymerization Index (PI, representing the ratio between HA and FA). HI, in this paper, was calculated considering NOM as the organic carbon content, thus excluding the fixed carbon content of the amount of biochar used in blends. All products showed HI index higher than the minimum reference threshold, but while the control CD and CB1 showed comparable HI values, CB2 and CB3 showed a humification index at least three times higher than the previous ones. PI resulted below the limit only for the case of CB3, whereas the other samples showed comparable values.
A high level of NH4-N forms is an indication of a low stabilization for the OM. The fate of nitrogen forms varies along the composting process: the NH4-N form is prevailing during the mineralization processes of the OM, typical of the bio-oxidation phase.
As shown in Table 7, the NH4-N contents for all the blends fall below the limit of 0.04% w/w d.b. proposed by Zucconi and de Bertoldi  for mature compost (though from the organic fraction of municipal solid wastes), even if the same parameter for CD was at least three times the other blends. On the other hand, since nitrification of ammonium mostly occurs after the thermophilic phase, NO3-N concentration can also be retained as a good indicator of compost stabilization. Bernal et al.  proposed a limit of 0.16 to the NH4-N/NO3-N index to define a compost sufficiently mature. All blends remained below the 0.16 limit for the NH4-N/NO3-N ratio, with CB2 and CB3 giving the lowest values.
A summary of COMBI characteristics compared to the composted digestate is given in Table 8; the main parameters chosen to evaluate the quality of the materials produced by composting and co-composting were the final biochar rate, the NOM yield, the humified organic matter, the compliance with the ECN-QAS limits for inorganic pollutants, and the product stabilization and sanitization indexes.