## 2 Materials and Methods
### Pot Experiment  
After the completion of the laboratory NH\({}_{3}\) loss incubation study, a pot experiment was conducted in a netted house located at the University Malaysia Kelantan Jeli Campus, Malaysia. Only 5 treatments were selected and carried forward to the pot experiment from the ammonia volatilization study. Treatments with 15 and 20 t ha\({}^{-1}\) RSRH biochar were excluded. Application of 15 and 20 t ha\({}^{-1}\) did not minimize NH\({}_{3}\) loss significantly compared to 5 and 10 t ha\({}^{-1}\) RSRH biochars. Hence, low rates of RSRH biochar application (5 and 10 t ha\({}^{-1}\)) were chosen since they were more economical. Treatments with soil only, soil + urea, and 50% and 100% commercial potting media were carried forward to the pot experiment to serve as a comparison to RSRH biochar effectiveness in minimizing NH\({}_{3}\) loss and nutrient retention in soil, and improving plant nutrient uptake. Rice plant (cultivar MR297) was used as a test crop in the pot experiment and the seedlings were planted in pots (23 height, 23 wide, and 23 cm diameter) which were filled with 5 kg of 5-mm sieved soil. Before planting, MR297 rice seeds were germinated in plastic trays filled with germination medium. The biochar rates of 5 and 10 t ha\({}^{-1}\) were mixed thoroughly with the soil 24 h before transplantation of rice seedlings into the pot on the seventh day. Three rice seedlings were planted in each pot, equivalent to three seedlings per hill [30]. The water level in each pot was maintained at 3 cm from the soil surface. After seven days of transplantation, N, P, and K fertilizers in the form of urea (46% N), Christmas Island Rock phosphate (32% P\({}_{2}\)O\({}_{5}\)), and Muriate of Potash (60% K\({}_{2}\)O) were applied at rates of 175, 97.8, and 130 kg ha\({}^{-1}\), respectively. These rates were scaled down based on the recommendation of Muda Agricultural Development Authority, Malaysia [31] with some modifications where the urea was increased to 175 from 151 kg ha\({}^{-1}\) for 5 kg soil per pot. The fertilizers were applied in a three-equal split at 7, 30, and 55 days after transplantation (DAT) by surface application. The lists of treatments evaluated in the pot experiment are listed in Table 2.  
The pot experiment was carried out in a completely randomized design with three replications in a net house. Plants were checked regularly and monitored up to the heading stage (70 days). The plants were harvested at 70 DAT. This is because the amount of soil used in the pot was not sufficient to support rice plants up to the flowering and ripening stage; thus, it was not economically practical to estimate the yield of rice based on pot experiments. This was in agreement with [PERSON] et al. [32]. \begin{table}
\begin{tabular}{c c} \hline \hline
**Treatment** & **Description** \\ \hline T0 & 100 g soil only (negative control) \\ T1 & 100 g soil + 0.97 g urea (positive control) \\ T2 & 100 g soil + 0.97 g urea + 0.14 g rice straw biochar + 0.14 g rice husk biochar \\ T3 & 100 g soil + 0.97 g urea + 0.28 g rice straw biochar + 0.28 g rice husk biochar \\ T4 & 100 g soil + 0.97 g urea + 0.42 g rice straw biochar + 0.42 g rice husk biochar \\ T5 & 100 g soil + 0.97 g urea + 0.56 g rice straw biochar + 0.56 g rice husk biochar \\ T6 & 50 g soil + 50 g commercial biochar potting media + 0.97 g urea \\ T7 & 100 g of commercial biochar potting media + 0.97 g urea \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Treatments evaluated in ammonia volatilization study. At the heading stage (70 DAT), the plant height was measured by using a measuring tape. Plant greenness was measured by using the SPAD Meter 502-nm. The number of fillers and number of panicles were counted and recorded. The aboveground parts of the plants were harvested and dried in an oven at 60 \({}^{\circ}\)C until a constant weight was attained [33]. The oven-dried plant samples were then grounded by using a grinding machine, after which they were analyzed for total N, P, and K. Total N was determined by using the Kjeldahl method.