## 3 Results and Discussion
### Physicochemical Characterisation of Biosolids
[67] conducted a study of biosolids mineralisation in agricultural soils and established that the C/N ratio was inversely related to the mineralisation rate of C and N. These results coincide with [PERSON] and [PERSON] [80] \"organic amendments with low C/N ratio decompose more rapidly, once incorporated into the soil, than those with a high C/N ratio (>25). [PERSON] et al. [11], established that low rates of C mineralisation in soils are potentially related to high C/N ratios (>30) and organic matter stability. Therefore, biosolids with an average C/N of 6.94 can conserve the C/N of the soil-biosolids system, without affecting C mineralisation rates by avoiding organic matter losses in the soil. One of the advantages of using biosolids as organic amendments is that they contain nutrients other than N, such as P and K, as well as micronutrients [81]. The content of P and K in the analysed biosolids was from 1.48 to 2.65% and from 0.59 to 1.09%, respectively. The content of P and K was significantly different (F = 51.778, \(p\) = 0.000 and F = 28.732, \(p\) = 0.001, respectively) between the seasons of the year. In spring, the highest P and K contents were presented, at 2.65 and 1.09%, respectively. The lowest value for P was 1.48% and corresponds to summer, while for K it was 0.59% in autumn. The results obtained from P and K in the evaluated biosolids were higher than those obtained by [PERSON] et al. [13], who reported P and K values for aerobic biosolids of 0.14 and 0.46%, respectively. [PERSON] et al. [81] established that for different biosolids (aerobic, anaerobic and digested), the N, P and K content ranged from 2 to 8%, 1.5 to 3%, and 0.1 to 0.6%, respectively. The N and P content of the biosolids evaluated in this study were within the reported range; however, K was above that reported in other studies [81; 82; 83]. In addition, it is important to establish the amount of nutrients provided by biosolids as they are added to the soil, because although biosolids are a source of nutrients (N, P and K), they often do not replace the use of commercial fertilisers, since their content of N, P and K may be less than that required for crops. [PERSON] et al. (2020) conducted a long-term field experiment (4 years) with 3 doses of biosolids applied to the soil and found that it was necessary to add chemical fertilisers of P and K to cover the growth needs of the crops. However, the P released (organic P to inorganic P) from the biosolids increased in the soil depending on the application rate of the biosolids. Similarly, studies have reported that the content of K in biosolids (0.1 to 0.6%) is generally very small ([PERSON] et al., 2019; [PERSON] et al., 2020; [PERSON] et al., 2020; [PERSON] et al., 2020). The low K content in biosolids significantly increases the nutrient imbalance in terms of the N:P:K ratio, so the most common practice is to add K fertilisers independently of biosolids when necessary ([PERSON] et al., 2020). However, the biosolids analysed in this work presented a content of 0.59 to 1.09% K, being above the commonly reported value for biosolids; however, they cannot be considered high without analysing the K content in the soil and the K requirements depending on the type of crop. Therefore, the importance of knowing the content of these macronutrients in biosolids is reiterated, in order to avoid adverse environmental effects or low crop yields. The micronutrients present in the biosolids studied were Fe (16.10 to 39.28 g kg\({}^{-1}\) dry biosolids), B (122.41 to 172.62 mg kg\({}^{-1}\) dry biosolids), Mn (126.85 to 215.62 mg kg\({}^{-1}\) dry biosolids), Zn (602.76 to 813.51 mg kg\({}^{-1}\) dry biosolids), and Cu (95.22 to 132.42 mg kg\({}^{-1}\) dry biosolids). These results are similar to some previous studies of different biosolids that have reported the presence of micronutrients ([PERSON] et al., 2020; [PERSON] et al., 2020; [PERSON] et al., 2020).