## 3 Results and Discussion
### Physicochemical Characterisation of Biosolids
The values obtained were lower than those obtained in this study ranging in a TOC range from 31.80 to 35.10%. In similar studies of different types of biosolids from aerobic digestion processes and activated sludge or anaerobic digestion systems, the average TOC values reported were below those obtained in this study [11; 68]. Thus, the biosolids analysed in this study, despite their variability in TOC content, probably due to the biochemical composition of wastewater, wastewater treatments and sludge stabilisation processes, presented a high C content. It is then argued that the biosolids in this study can be used as potential sources of labile or affordable sources of C, for application in agricultural soils or with fertility problems. However, due to their variability, the need to monitor the TOC content of biosolids produced in the WWTPs over time, especially before and after their application to agricultural soils, is highlighted. The above is needed for a better design or application plan (dosage) and does not only consider the total amount of biosolids (Ton Ha\({}^{-1}\)), as is currently practiced at the local or regional level. In addition, due to the variability of TOC presented in the evaluated biosolids, it is important to establish short- and long-term studies on its effects on C mineralisation rates (CO\({}_{2}\) emissions), incorporation into microbial biomass (MBC), enzymatic activities, and global C balances (inputs and outputs) when applied to soils. Biosolids have been characterised as an organic residue rich in nitrogen [19]. The TN content of the biosolids analysed at different times of the year ranged between 6.35% and 6.78%, with no significant differences (F = 0.200, \(p\) = 0.893) between them. From the values obtained, it was considered that the evaluated TN is a stable parameter regardless of the time of year in which it is analysed. In function to the non-variability in the N content observed in the evaluated biosolids, this parameter could then be used as an indicator or criterion of quality and functionality during the design of agronomic doses or application rates to agricultural soils or soils with N deficiencies. However, the values obtained were high (1.54 to 4.07%) compared to those reported in other studies where biosolids have been characterised for organic amendments to agricultural soils [11; 18; 75]. In the same context, the effect of applying biosolids to soils makes the N content increase and is also available to crops longer, especially when the organic N content is mineralised (nitrification) to inorganic forms, mainly N-NH\({}_{4}\)\({}^{+}\) and N-NO\({}_{3}\)\({}^{-}\)[68; 76; 77]. Thus, a physicochemical indicator of functionality or quality of biosolids to consider, which is linked to biological processes of N mineralisation, can be, not only the N content in soil and N requirements in crops, but also the N content in biosolids, especially during the design of soil application dosages [19; 78]. The content of C and N in biosolids are 2 parameters of interest during the application of organic amendment to soils. Although the C and N content in the analysed biosolids was high compared to the values reported in the literature, the C/N ratio was within the previously reported values [18; 68]. The C/N ratio of biosolids ranged from 5.64 to 8.64 for different times of the year and were statistically different (F = 10.243, \(p\) = 0.004). The highest C/N ratio was presented in summer (8.64) and the lowest in spring (5.64). These results are within the range of 4.4 to 22.0 obtained for the C/N ratio of biosolids analysed in other studies [13; 45; 66]. [PERSON] and [PERSON] [79], evaluated the mineralisation of N in 2 biosolids, 1 anaerobic and 1 aerobic, with a C/N ratio of 6.7 and 4.7, respectively, establishing that the C/N ratio is an important factor to evaluate in the mineralisation processes of biosolids in agricultural soils. [PERSON] et al.