**5. Conclusions**

The results of this study demonstrate that the incorporation of Honduran mahogany (*Swietenia macrophylla*) wood sawdust biochar into sandy loam soil samples could improve soil health by increasing the capacity of the soil to retain nutrients and reduce nutrient leaching. The biochar materials applied in this study, particularly WB600, alleviated soil acidification. The incorporation of the biochar materials considerably increased SOC, inorganic N (NH4 <sup>+</sup>-N and NO3 −-N), and ava. P concentrations in the soil samples. The biochar application did not significantly affect *D*<sup>B</sup> or Ex. K in the soil. Furthermore, the total volume of leachates and cumulative quantity of DOC, inorganic N, P, and K leached from the soil samples decreased significantly in the biochar-treated samples. WB300 engendered the least quantities of NH4 <sup>+</sup>-N and K, both of which are cations. Thus, WB300 could be more negatively charged than WB600 because of its inhibitory effects on cation leaching. By contrast, WB600 was likely to have a higher density of P-bonding sites, which resulted in a stronger inhibitory effect on P leaching. Although the ability to retain various nutrients in soil differed with pyrolysis temperatures, both biochar materials effectively contributed to the conservation of groundwater and subsequent downstream water bodies. Increased retention of these nutrients in soils can increase the probability of absorption by plant roots, thereby decreasing the risk of leaching into rivers or groundwater reservoirs. In future studies, the on-site effects of biochar application on underground and water bodies should be determined in terms of eutrophication and potential pollution, especially for intensively fertilized cropped fields.

**Author Contributions:** Conceptualization and methodology, S.-H.J.; sample analysis, Y.-L.K.; writing—original draft preparation, S.-H.J. and Y.-L.K.; writing—review and editing, S.-H.J. and C.-H.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Ministry of Science and Technology of the R.O.C. (Grant number MOST 105-2628-B-020-001-MY2).

**Acknowledgments:** We would like to thank the students of the Soil Survey and Conservation Laboratory, Department of Soil and Water Conservation, National Pingtung University of Science and Technology, for their assistance during the column leaching experiment and analysis.

**Conflicts of Interest:** The authors declare no conflicts of interest.
