## 3 Results and Discussion
### Biological Characterisation of Biosolids  
Biostability is a key feature in biosolids to evaluate their use as organic amendment in soils. In the last decade, studies in soils with various organic amendments have focused on using biological properties for their sensitivity to change, as indicators of the dynamics of organic matter transformation, nutrient cycling, stress and regeneration processes [66; 97; 98; 99]. Therefore, it was important to evaluate biological parameters for 1 year to establish indicators of quality and functionality of biosolids that have been added to soils, and thus be able to elucidate the positive or negative effects of adding these biosolids to soils before their application. The biological parameters evaluated in the biosolids obtained in the UWWTP can be found in Table 3. Microbial biomass (MB), defined as the living microbial component of an ecosystem [26], is the primary terrestrial ecosystem agent responsible for decomposition of organic matter, nutrient cycling and energy flow [100; 101]. In addition, the MB is considered an important ecological parameter in environmental ecosystems, because it represents the amount of energy stored by microorganisms [102; 103]. Through the establishment of MB in biosolids and by carefully interpreting their values, it is possible to evaluate and monitor the functionality and quality of biosolids as an organic amendment, and thus predict their effects on soil microbial activity and their relationship with productivity, fertility and resistance to degradation forces. In this case, the BM in biosolids was determined by 3 parameters: C for microbial biomass (MBC), N for microbial biomass (MBN) and the relationship between C and microbial N (Cmic/Nmic). These biological parameters were selected because they have been widely used in previous studies of agricultural soils with application of biosolids as organic amendment because they are considered as sensitive and rapid indicators of restoration or ecological stress of the soil or environmental system investigated [63; 104; 105]. The MBC and MBN are the most labile C and N stores in soils [106; 107], so their values are likely to be altered quickly in the event of any disturbance in the ecosystem. In addition, MBC and MBN are general  
Figure 3: Distribution of absorbance intensity for each set of peaks found in the FTIR analysis of biosolids samples for each season of the year. Each box in the bars represents the absorbance peaks ordered according to wavelength: (A) 3400–3500 cm\({}^{-1}\), (B) 2850–2930 cm\({}^{-1}\), (C) 1650–1750 cm\({}^{-1}\), (D) 1630–1650 cm\({}^{-1}\), (E) 1540–1550 cm\({}^{-1}\), (F) 1430–1450 cm\({}^{-1}\), (G) 1380–1400 cm\({}^{-1}\), (H) 1230–1250 cm\({}^{-1}\) and (I) 875–1050 cm\({}^{-1}\). indices of microbial activity in soils [102; 107]. Different soil management practices such as the addition of biosolids greatly affect the MBC and MBN in the soil [108]. For this reason, in order to understand the effect on soils of the application of biosolids, it is necessary to know the values of MBC and MBN in the biosolids before they are added to the soil. The results obtained from MBC in biosolids for different times of the year ranged between 587.22 and 1251.95 mg C\({}_{\text{mic}}\) kg\({}^{-1}\) dry biosolids. These results were significantly different (F = 26.318, \(p\) = 0.000) between seasons of the year (Table 3). In winter the lowest MBC content was presented (587.22 mg C\({}_{\text{mic}}\) kg\({}^{-1}\) dry biosolid) and the highest content was in summer (1251.95 mg C\({}_{\text{mic}}\) kg\({}^{-1}\) dry biosolid). The previous MBC values represented a range of 0.16 to 0.26% of the TOC present in biosolids for the different seasons of the year. However, these values are considerably lower than those established for soils where MBC corresponded to 1 to 3% of TOC [94; 96].