## 3 Results and Discussion
### Biological Characterisation of Biosolids
This behaviour may also be related to the quality of organic matter present in biosolids [92; 93], which according to the FTIR spectra observed (Figures 1 and 2), was stable and with little variability during the different seasons of the year. Therefore, the type and quantity of substrates available in biosolids has a significant influence on the development and predominance of the endogenous phase of the microorganisms present. In the same way, it can be established that the evaluated biosolids possess, besides labile organic matter, extracellular enzymes ready for the process of mineralisation of organic matter. In reference to the evaluation of extracellular enzymatic activities, the activity of urease (URS) in biosolids in different seasons of the year was evaluated. URS is an enzyme that catalyses the hydrolysis of urea into carbon dioxide and ammonia by acting on the C-N bonds of different peptides [129]. Therefore, the activity of the URS is of great importance in the biotransformation processes of nitrogenous organic compounds [99]. The values reached for the activity of URS in the biosolids analysed at different times of the year ranged from 109.82 to 194.55 mg N-NH4+ kg-1 dry biosolids h-1 (Table 3). The activity of the URS in the biosolids evaluated for the autumn presented significant differences (F = 17.771, \(p\) = 0.001) with respect to the rest of the year and was also the lowest value obtained (109.82 mg N-NH4+ kg-1 dry biosolids h-1). This could indicate that in autumn there could have been fewer nitrogenous organic compounds than in the rest of the year, so less urease enzyme activity was required. Previous research has focused on evaluating the effect of URS activity in agricultural soils with added biosolids but has not considered evaluating URS activity in biosolids per se for a better understanding of the mineralisation processes of nitrogen compounds after the addition of biosolids to soils. In investigations on the application of biosolids in agricultural soils, an increase in the activity of URS has been observed; however, the values obtained have oscillated from 20 to 100 mg of N-NH4+ kg-1 dry soil h-1 [129; 122; 127; 108]. However, the results of the URS activity obtained in these studies were lower than those obtained in this paper. This enzymatic activity then becomes an indicator of the presence and potential contribution of nitrogenous sources of biosolids when applied to soils. On the other hand, with the results obtained, the importance of evaluating the activity of the URS in agricultural soils when biosolids are applied as organic amendment in short (maximum 180 days of incubation in laboratory experiments) and long (>1 year in field experiments) treatments was also established. [PERSON] and [PERSON] [129] evaluated the effect of the addition of biosolids on URS activity at different doses of biosolids (0, 100, 200, and 300 ton ha-1) in agricultural soil in a short-term laboratory-scale incubation experiment (90 days incubation). In these experiments it was observed that the activity of the URS increased rapidly until reaching a maximum activity in 15 days after the application of the biosolids; then, the activity of the URS decreased significantly until reaching values below the initial values (day 0) at 90 days of incubation. However, other long-term studies (1 and 4 years, respectively) conducted by [PERSON] et al. [122] and [PERSON] et al. [127] agreed that URS activity was not affected by the addition of biosolids to agricultural soils. Thus, the increase in URS activity in soils with short-term application of biosolids could be a consequence of the stimulation of endemic soil microorganisms by the addition of N present in biosolids, increasing their metabolic capacity, and therefore, the production and activity of the urease enzyme.