## 3 Results and Discussion
### Biological Characterisation of Biosolids
The observed increase in FDA activity was also related to a high content of labile organic matter in the evaluated biosolids, i.e., easily mineralizable organic compounds available as substrates for microbial metabolism [105; 125]. However, the values obtained for biosolids evaluated independently of the season of year were higher than those reported in the literature in soils and soils with added biosolids [66; 126; 127]. [PERSON] et al. [124] evaluated the changes in FDA activity of 2 types of soil, 1 silty and the other sandy, without addition of organic amendments and with the same types of tillage, finding that FDA activity ranged from 50.50 to 126.70 mg fluorescein kg\({}^{-1}\) dry soil h\({}^{-1}\) and from 40.20 to 77.20 mg fluorescein kg\({}^{-1}\) dry soil h\({}^{-1}\), respectively. These results were lower than those obtained in this study for biosolids of all seasons of the year; even in summer the value obtained was 2.2 times higher than the highest value obtained in the silty soil evaluated in that study. On the other hand, [PERSON] et al. [127], evaluated the effect on FDA activity by adding organic amendments to an agricultural soil with 5 treatments: (1) control; (2) 137 mg urban vegetative compost ha\({}^{-1}\); (3) 202 mg biosolids ha\({}^{-1}\); (4) 404 mg biosolids ha\({}^{-1}\) and (5) designed mixture (202 mg biosolids ha\({}^{-1}\), 10.3 mg biochar ha\({}^{-1}\), and 5.7 \(\mu\)g Fe\({}_{2}\)O\({}_{3}\) oxide ha\({}^{-1}\)), obtaining values of 38, 90, 75, 50, and 70 mg fluorescein kg\({}^{-1}\) dry soil ha\({}^{-1}\), respectively. Thus, [PERSON] et al. [127], established that the organic amendments they used significantly increased FDA activity in soil, especially with biosolids, and suggested that organic amendments may contain extracellular hydrolytic enzymes or microorganisms that have these enzymes. With the results obtained in this study, the FDA's activity in biosolids could be demonstrated as suggested by [PERSON] et al. [127], and then it would be expected that by adding the biosolids evaluated in the present study to an agricultural soil, they would provide labile organic matter and enzymatic machinery already prepared for their degradation. In summary, an increase in FDA activity is expected by adding the evaluated biosolids to soils, as similarly reported in previous studies [66; 125; 127]. [PERSON] et al. [56], established a biosolids stability index expressed as DHS/FDA, providing information about the predominance between the primary metabolism (breathing processes, etc.) and the secondary metabolism (consumption of energy reserves) in the microorganisms present. If the DHS/FDA quotient is small, it can be mentioned that the secondary phase of metabolism referring to endogenous processes predominates in the system and vice versa. The results obtained in the biosolids analysed for this DHS/FDA index ranged from 5.55 to 7.99 mg INF mg\({}^{-1}\) Fluorescein, and there were no significant differences (F = 3.96, \(p\) = 0.069) between the seasons of the year (Table 3). The results obtained from DHS/FDA were lower than those obtained by [PERSON] et al. [56], who obtained values of 58 to 10 \(\mu\)mol O\({}_{2}\)\(\mu\)mol\({}^{-1}\) FDA hydrolys (equivalent to 164.49 to 28.37 mg INF mg\({}^{-1}\) Fluorescein) during the monitoring of an aerobic process of stabilisation of residual sludge. With this it could be established that the secondary metabolism was the dominant one in the evaluated biosolids. The biosolids evaluated were a stabilised solid residue that is in a more markedly endogenous phase. Thus, when adding biosolids to agricultural soils, total enzyme activity in the soil would be expected to increase because the biosolids are in the endogenous phase, i.e., the microorganisms present are adapted and consume energy reserves for the production of hydrolytic enzymes that transform current organic matter into energy for the maintenance process [43; 128].