Nitrogen (N) is a vital nutrient necessary for the growth of tea trees (Camellia sinensis L.), since it has a direct role in crucial physiological processes including chlorophyll production, hence influencing plant growth and development (Li et al., 2016). Increased nitrogen supply has also been shown to increase amino acid concentrations and decrease polyphenol concentrations (Rebello et al., 2022). High rates of N fertilizer application have been shown to favour shoot growth over root growth and as a result increase the susceptibility of the crop to drought (Cheruiyot et al., 2009). Tea plants receive a substantial quantity of N fertilizer annually. The quality of green tea increases with increasing N rate, while the quality of black tea decreases with N additions greater than 200 kg N·ha-1 (Rebello et al., 2022). In China, for instance, the average quantity of N fertilizer that is applied to tea plantations is between 300 and 450 kg N·ha-1, and the average annual harvest of tea leaves is around 800 kg·ha-1 (Chen & Lin, 2016; Xiao et al., 2018). The N concentration of tea is around 4–6% (Chen & Li, 2016), which means that only 30–50 kg N is removed in the harvested leaves and approximately 90% of the applied N fertilizer is either lost or retained in the soil. Tea plantations are commonly associated with acidic soils that receive considerable rainfall, which result in high potential for loss of nutrients (Yan et al., 2018). The primary kind of fertilizer utilized in tea plantations is compound fertilizer containing multiple nutrients, accounting for 70% of overall application. This compound fertilizer is enriched with various nutrients, and ammonium (NH4+) serves as the principal N source (Venkatesan et al., 2004). In addition, soil N mineralization is a significant source of N supply for plants but is frequently not considered when calculating the amount of N fertilizer to apply.
Biochar is a carbon-rich product produced by the transformation of organic materials through the pyrolysis process, which produces a product that has a porous structure, large specific surface area and strong adsorption capacity (Chen et al., 2019). The application of biochar in agricultural production has attracted much attention. Research has demonstrated that the use of biochar has the potential to improve soil physical and chemical characteristics, soil microbial communities, soil fertility, and stimulate crop development (Singh et al., 2022). It plays an important role in regulating soil N transformation, e.g., N mineralization and nitrification, via its significant cation exchange capacity (CEC) and adsorption capabilities, which contribute to the improved retention of organic matter and NH4+ in soils. This enhanced retention subsequently leads to increased availability of NH4+ for plant uptake while simultaneously lowering N losses (Gai et al., 2014; Prayogo et al., 2014). It is important to note that tea plants have a greater ability to take up NH4+relative to nitrate (NO3-, Ruan et al., 2007). The impact of biochar on the composition of soil microbial communities and the processes of N transformation, especially N mineralization, can be attributed, in part, to its ability to elevate soil pH and boost soil carbon content (Prayogo et al., 2014; Palansooriya et al., 2019). In a study conducted by Cen et al. (2021), it was shown that the utilization of biochar-based fertilizers resulted in a higher retention rate of NH4+ in the soil, as compared to ammonium sulphate applied alone. Specifically, the retention rate increased from 41–66%.
Soil N exposure expresses the presence of various N forms in soil over time (Burton et al., 2008b; Burton & Zebarth, 2014). It quantifies the presence of soil mineral N (NH4+ and/or NO3-) over time compared to instantaneous measures of soil mineral N concentration (Burton & Zebarth, 2014). It reflects potential risks, particularly in the context of NO3- loss through N2O emissions. Previous studies reported that soil N exposure is correlated with cumulative N2O emissions, whereas instantaneous measures of soil mineral N seldom are (Burton et al., 2008a; Burton et al., 2008b; Maharjan & Venterea, 2013; Pelster et al., 2013). Nitrogen exposure can be calculated by integrating conventional soil sampling methods over time, but this approach is time-consuming, labour intensive, and discontinuous. The emergence of ion-exchange membrane technology addresses these limitations, offering a more efficient approach for the continuous measurement of soil N exposure (Harrison & Maynard, 2014; León Castro & Whalen, 2016). This innovative technique holds the promise of being more convenient and accurate, with potential widespread application in future research and practical applications.
Soil N supply to the plant is a result of the combination of carryover of inorganic N from the previous growing season and the mineralization organic N over the growing season (Zebarth et al., 2005; Nyiraneza et al., 2012). Measuring or predicting the N mineralization of soil has proven to be challenging because it is driven by soil biological processes. Traditional methods for determining soil N mineralization potential involve chemical extractions or long-term aerobic incubation, these approaches have practical drawbacks, such as time consuming and the exclusion of biological factors (Luce et al., 2011; Ros et al., 2011). These measures of the N mineralization potential are generally well correlated (Sharifi et al., 2007a; Sharifi et al., 2007b; Dessureault-Rompré et al., 2010) and been applied to predicting growing season N mineralization (Dessureault-Rompré et al., 2012; Laurence et al., 2024). Biological N Availability (BNA), the mineralization of N occurring over a 14-day aerobic incubation, corresponds to the soil labile organic N pool, or pool I as defined by Sharifi et al. (2007b) and is used to quantify N mineralization potential. Dessureault-Rompré et al. (2015) used BNA (Pool I) in combination with pedotransfer functions to reflect climate to predict growing season N mineralization.
Despite the increased interest in biochar in agriculture, most studies have concentrated on common crops like corn, wheat, and vegetables. The main response variables measured have been crop growth and soil microbial communities (Vaccari et al., 2011; Yu et al., 2019; Ogura et al., 2021; Wang et al., 2023). The majority of soil-related biochar research has focused on soil improvement and soil remediation (Sohi et al., 2011; Zhu et al., 2017). Few studies have examined the impact of biochar on N cycling processes in the acidic soils of tea plantations, particularly soil N supply in these areas and particular the period following early-summer supplementary fertilization where N loss potential is high. Therefore, in this study, we monitored soil N mineralization potential and soil N dynamics using a 2-week aerobic incubation method and ion exchange membrane (IEM) technology to examine the influence of various biochar application rates and fertilizer combinations on soil N dynamics during this summer period.