## 3 Results and Discussion
### Biological Characterisation of Biosolids
The qFDA expresses the ratio of hydrolys fluorescein diacetate (FDA) per C unit of microbial biomass [126; 132]. This index has been used mainly for the evaluation of soil disturbance and stress due to the presence of xenobiotics [126]. The interest in using qFDA in the analysed biosolids was based on establishing if this parameter can be a useful tool, on the global hydrolytic activity and as a function of the C stored in the BM of the biosolids, which is very similar to how they have been used in soils [132; 133]. The qFDA results obtained in the biosolids analysed for each season of the year were 356.27, 223.27, 381.57, and 636.39 mg Fluorescein g\({}^{-1}\)\(C_{\text{mic}}\) h\({}^{-1}\) for spring, summer, autumn and winter, respectively (Table 2). The winter, qFDA was significantly different (F = 23.371, \(p\) = 0.000) from the rest of the year. This behaviour was like that obtained for qCO\({}_{2}\), presenting the highest values. [PERSON] et al. [130] conducted studies on the effect on the physiological state of the microbial community in compost and vermicompost, revealing higher qCO\({}_{2}\) and qFDA, indicating that organic matter is less accessible to microorganisms and that there is a greater consumption of energy for oxidation of substrates. [PERSON] et al. [126] evaluated the effect on qFDA dynamics during the application of biosolids compost to agricultural soils in an incubation experiment at 60 days. These researchers observed that qFDA was characterised by stable values during incubation time, i.e., qFDA dynamics were not affected by incubation time and addition of biosolids. Thus, it can be deduced that in winter the biosolids under study presented a more complex chemical composition, probably due to the presence of recalcitrant compounds, with respect to the other seasons of the year, presumably making the organic matter of the biosolids less accessible in winter. So, it can be established that the qFDA parameter in biosolids could be a useful quality and functional indicator, giving valuable information on the accessibility of organic matter in biosolids before its application to soils. Biological parameters evaluated in biosolids, such as MBC and MBN content, as well as, the C\({}_{\text{mic}}\)/N\({}_{\text{mic}}\) ratio, demonstrated that biosolids have a significant content of active microorganisms, which could be produce a \"positive synergistic\" effect on the activity of soil microorganisms. In addition, in the biosolids evaluated over time, it has been possible to elucidate a high metabolic activity (DSH, FDA, and URS activity), presumably due to the content of substrates such as polysaccharides, proteins, fats and lipids that are easily assimilated by the microorganisms present. In addition, the DSH/FDA ratio established a predominance for secondary metabolism due to the presence of energy reserve compoundsalready mentioned. Due to the active enzymatic machinery, this will facilitate the mineralisation of the organic matter of the biosolids, reducing the energetic barrier for the decomposition of the foreign complex organic matter for the microorganisms of the soil. The qCO\({}_{2}\) and qFDA allowed for the evaluation of the physiological state of the microorganisms of the evaluated biosolids, giving information through time about their capacity to assimilate the organic matter present in the biosolids, which will depend on the labile or complex chemical composition it has. Thus, having information on the biological/biochemical/molecular status of biosolids before their application to soils will allow predicting possible positive as well as negative effects on them, and thus will allow making better decisions on the management and disposal of these biosolids as an amendment or soil improvers, especially agricultural ones.