**1. Introduction**

Nutrients (nitrogen, phosphorus, and potassium) and soil organic carbon (SOC) are critical components of a healthy soil, which is the foundation of a strong food system [1]. Tropical ecosystems are particularly susceptible to the loss of nutrients through soil erosion or leaching processes [2]. Intense rainfall in tropical or subtropical areas results in the leaching of fertilizer containing N, P, and K from soil bodies. Nutrient leaching could diminish soil fertility, accelerate soil acidification, increase fertilizer costs for farmers, and reduce crop yields [3]. The deposition of leached nutrients into water bodies adversely affects aquatic environments because of potential risks such as eutrophication. Leaching of N and P and agricultural runoffs are among the leading contributors to non-point source (NPS) pollution, which has a detrimental effect on drinking and ground water, aquatic habitats, and other water resources. Agricultural runoffs often contain several contaminants, including nutrients, pesticides, pathogens, sediment, salts, trace metals, and other substances, which contribute to biological oxygen demand [4]. Moreover, SOC, comprising nutrient and soil biota, leaches out over time [5], which could aggravate nutrient losses and water pollution. An enormous quantity of fertilizers must be applied to counter the dwindling fertility of agricultural soil.

Biochar is a solid bioresource obtained through the pyrolysis of organic waste. Residues from agricultural and forestry production processes are suitable raw materials for the production of high-quality biochar [6,7]. Biochar is a porous substance containing high levels of carbon and various functional groups. Accordingly, the addition of biochar to agricultural soil has emerged as a feasible strategy to enhance soil water retention capacity [8–10], soil quality [11–14], soil organic matter stability and nutrient retention [15,16], organic carbon (OC) sequestration [17], and greenhouse gases emission reduction [18–21]. Furthermore, biochar can affect soil microbial properties, including microbial activity [22] and microbial diversity [23]. However, the interactions between biochar and microbial properties in soil are not fully understood [24]. The application of biochar to soil could increase soil fertility and crop productivity by reducing leaching or even supplying nutrients [25–27]. However, the effects of biochar on nutrient leaching and OC retention has been reported to vary with the applied biochar pyrolysis temperature, raw material, and soil type [28,29]. Biochar produced from secondary forest residues could reduce fertilizer leaching and increase plant growth and nutrition [26]. Furthermore, the addition of biochar produced from hardwood to a typical Midwestern agricultural soil in the United States considerably reduced the leaching of total N and P by 11% and 69%, respectively [3]. Yao et al. [29] reported that the effect of biochar on nutrient retention and release varied with the nutrient and biochar type.

In this study, we conducted a 42-day column leaching experiment by using loamy sand soil samples that were obtained from a tropical/subtropical area and treated with two types of wood dust biochar pyrolyzed at 300 and 600 ◦C. The objective of this study was to determine the effects of biochar application on water, nutrient, and OC retention and leaching from the observed soil. The results are expected to be valuable for assessing the potential of biochar for the retention and immobilization of nutrients in soils and inhibition of water body contamination.
