**4. Discussion**

#### *4.1. Soil Inorganic N*

The soil NO3 −–N and NH4 +–N content were both improved with biochar application. Differences in both soil NO3 −–N and NH4 +–N content were seen between experimental years. In 2018, significantly higher soil NO3 −–N and NH4 +–N content were observed than in 2019. This could be due to high amount of rainfall received in 2019 that could have caused substantial leaching of inorganic N compared to 2018. Overall, NO3 −–N across sites and years increased by 8.8% while NH4 +–N increased by 4.8% with biochar application. Similar observations were made by Yao et al. [31] who reported significant increase in NO3 −–N (34%) and NH4 +–N (35%) following biochar application. In addition, Singh et al. [7] observed up to 94% more soil NH4 +–N under biochar amendment than in the untreated plot. It is important to note that most of the studies that report high proportion of retained inorganic N were soil column leaching experiment compared to the current study that was conducted under field conditions as demonstrated by Libutti et al. [32]. In the current study, positive effects of biochar application in increasing the availability of NO3 −–N and NH4 +–N were observed at 10 and 15 t ha−1. With 20 t ha−<sup>1</sup> of wood biochar, Gao et al. [33] observed NO3 −–N and NH4 +–N recovery of 33 and 53%, respectively, under field conditions. In an attempt to offer explanations, Zheng et al. [34] indicated the increase in soil water holding capacity, NH4 +–N adsorption, and enhanced N immobilization as the main reasons for the increase in recovery of fertilizer-N following biochar soil application. Indeed, increasing the capacity of the soil to hold water increases chances of retaining both NO3 −–N and NH4 +–N within soil solution [35]. The enhanced adsorption of NH4 +–N has been attributed to increase in CEC as a result of the oxidation of aromatic C and formation of carboxyl groups [36]. Lawrinenko and Laird [25] reported an increase in the anion exchange capacity (AEC) of biochar, which reduces leaching of anionic nutrients. They explained that the increased AEC is due to the formation of oxonium functional group (–O+) and non-specific proton adsorption by condensed aromatic rings. Therefore, the rate at which biochar increases the availability of NO3 −–N and NH4 +–N is largely dependent on specific biochar production conditions.
