## 3 Results and Discussions
### Release of Nutrients  
Some studies have hypothesized that LDHs could delay the discharge of nutrients because of the mechanism of the anion exchange reaction taking place within its structure, and LDHs could therefore be targeted as a slow-release fertilizer. Consequently, the feasibility of P-BC-LDHs for controlled nutrient release for agricultural uses was also investigated. In the present experiments, for the first time, the release characteristics of magnesium and aluminum in real substrate systems were examined, in addition to those of phosphorus because current studies regarding biochar-based LDH composites have primarily focused on phosphorus dissolution under water conditions. The actual cumulative release profiles in the substrate cultures of these elements (Figure 5) suggest that a progressive loss of nutrients is linked to increased sample dosages, except for phosphorus release tests, wherein the highest P-BC-LDHs applied did not result in extensive dissolution. However, a preferable sustained release behavior was obtained from all supplemented growing media. The sum of the phosphorus content on the first day (Figure 5a) for columns amended with 0.5%, 1.0%, 2.5%, and 5.0% (_w/w_) was 0.015, 0.019, 0.021, and 0.028 mg, respectively, indicating that all treatments accommodated approximately 0.8% of the total loaded phosphorus. This finding, which meets the criteria of no more than 15% of released nutrients within 24 h, indicates good compliance with the standard of slow-release fertilizers according to the Committee of European Normalization [48]. Phosphorus leaching gradually occurred over time and at the end of the study, the cumulative release was 0.087, 0.349, 0.389, and 0.204 mg in 0.5%, 1.0%, 2.5%, and 5.0% (_w/w_) treatments, respectively. The magnesium solubilizing patterns for every column exceeded that of phosphorus during the entire leaching experiment, particularly in the substrate amended with the maximum P-BC-LDH level (Figure 5b). The overall Mg losses from 0.5%, 1.0%, 2.5%, and 5.0% (_w/w_) were 0.196, 1.009, 0.896, and 3.875 mg, respectively, which is at least two times higher than that of phosphorus. The changes in the aluminum concentrations in the leachates collected are shown in Figure 5c. The changing tendency of released aluminum showed certain differences, for which P-BC-LDHs displayed significantly slower release behavior compared to both phosphorus and magnesium. The total amounts of aluminum from the leachates sample under applications of 0.5%, 1.0%, 2.5%, and 5.0% (_w/w_) on the last day reached 0.078, 0.090, 0.127, and 0.112 mg, respectively. Moreover, the ease of dissolution for these three nutrients was detected for zero-treatment columns, and their release profiles remained almost unchanged over the observed range of the experiments because of insufficient nutrients. The reason for this small phosphorus release was probably a phosphorus fixation reaction, whereby the surface functional groups, including carboxyl and hydroxyl enriched in partially carbonized biochar, could utilize bridged cations (Al3+ and Mg2+) to form  
Figure 5: Cumulative nutrient release patterns as a function of time for different incorporations of P-BC-LDHs. (**a**) Phosphorus, (**b**) magnesium, and (**c**) aluminum. stable six-membered ring complexes with phosphate [49]. It is also estimated that the pH of growing substrates is an important factor controlling phosphorus release from these materials because phosphorus stabilization will take place upon increased pH, resulting in reduced phosphorus leaching. Another possible explanation is that under such high-pH conditions, the substrate cultures are rich in basic salts, such as K\({}^{+}\), Na\({}^{+}\), and Ca\({}^{2+}\), and as such, phosphorus retention is dominated by the poor mobility of formed phosphate minerals induced by strong reactivity with those anions [50; 51; 52]. At the same time, the reduced acidity in the growing media after the application of P-BC-LDHs was capable of reducing the solubility of aluminum.