## 3 Results and Discussion
### Biological Characterisation of Biosolids
Thus, fungal growth is promoted in a temperature range of 20 to 40 \({}^{\circ}\)C [113], while for bacteria the range is 4 to 60 \({}^{\circ}\)C [114]. With this information, it is possible to explain the predominance of fungi in spring and summer and bacteria in autumn and winter. With respect to the enzymatic activity of dehydrogenase (DSH), this enzyme is an indicator of microbial activity in biological systems such as biosolids, reflecting primary metabolism and cellular respiration because it only occurs in viable cells [115; 116]. In the biosolids evaluated at different times of the year, values of 2209.51 to 2426.10 mg INF kg\({}^{-1}\) dry biosolids h\({}^{-1}\) were found. The DSH of the biosolids did not present significant differences (F = 3.009, \(p\) = 0.095) at the different times of the year. With this it can be considered that, independently of the time of the year in which the biosolids were produced, their microbiota presented a similar cellular respiratory activity and primary metabolism. On the other hand, the DSH activity of the biosolids analysed was higher than that reported in the literature in soils with different management histories [117; 118; 119]. [PERSON] et al. [120] studied the activity of DSH as a biomarker of degradation and remediation processes in abandoned natural and agricultural soils. They observed that natural soils had higher DSH activity (61.2 mg INF kg\({}^{-1}\) dry soil h\({}^{-1}\)) compared to soils without agricultural activity for different periods. The decrease in DSH activity, which they reported over time, was attributed to the progressive erosion of soils without agricultural activity because of low levels of organic matter. Other studies applying biosolids to agricultural soils as an organic amendment have shown that DSH activity in soils increases with the addition of biosolids, which has been attributed to a high content of labile organic matter present in biosolids [121; 122; 123]. However, biosolids have not been considered to present a high DSH activity as evaluated in this study (DSH activity \(>\) 2000 mg INF kg\({}^{-1}\) biosolids h\({}^{-1}\)). Therefore, the addition of these biosolids to soils would be expected to increase DSH activity, not only because of their high organic matter content, but also because of the contribution of an active microbiota with available enzymatic machinery adapted to the organic matter present in the biosolids. Finally, to obtain information as observed from the evaluated enzymatic activity (DSH), before the addition of biosolids to agricultural soils, this information could be used to predict, with more ecological and biological arguments, the positive or negative effects of microbial interactions in agricultural soils with added biosolids. On the other hand, with respect to the secondary metabolism of microorganisms, which is related to reserve energy consumption [56], microbial activity was determined by hydrolysis of fluorescein diacetate (FDA) in biosolids. Fluorescein diacetate is hydrolys by proteases, lipases, and esterases, which are enzymes involved in the microbial breakdown of organic matter [32; 124]. At different times of the year, biosolids presented an FDA activity between 279.71 and 437.96 mg Fluorescein kg\({}^{-1}\) dry biosolids h\({}^{-1}\) (Table 3). Significant differences (F = 10.270, \(p\) = 0.004) from FDA between different seasons of the year were observed. In summer, it presented the lowest FDA activity (279.71 mg Fluorescein kg\({}^{-1}\) dry biosolids h\({}^{-1}\)), and in autumn, the highest activity (437.96 mg Fluorescein kg\({}^{-1}\) dry biosolids h\({}^{-1}\)). These values obtained at different times of the year could be due to the availability of energy reserve compounds in the biosolids, mainly lipids, esters and proteins as observed in the spectra of the FTIR analyses (Figures 1 and 2, Table 3).