This study aims to explore the effects of different biochar applications on soil physical and chemical properties in a Eucalyptus plantation in Northern Guangxi, find the best biochar application amount, and provide scientific guidance for the efficient utilization of forest residue and soil improvement. The soil of a four-year Eucalyptus plantation at the Huangmian forest farm in Northern Guangxi was selected as the study area, and six treatments including 0 (CK), 0.5% (T1), 1.0% (T2), 2% (T3), 4% (T4), and 6% (T5) were set through a field-positioning experiment to analyze the changes in soil physical and chemical properties under different application rates. Compared with the 0-30 cm soil layer of the control treatment, biochar application decreased the mean soil bulk by 3.82%-33.55%, while it increased the soil natural water content, capillary porosity, and total capillary porosity by 7.67%-31.75%, 8.95%-33.19%, and 9.28%-35.86%, respectively. The contents of exchangeable acid, exchangeable aluminum, exchangeable hydrogen, and exchangeable sodium in the soil decreased by 8.28%-70.03%, 5.55%-70.34%, 5.10%-21.78%, and 12.81%-49.27%. Biochar application increased the cation exchange capacity, electrical conductivity, exchangeable magnesium, and exchangeable calcium by 27.08%-160.39%, 117.00%-546.64%, 17.10%-66.14%, and 17.38%-71.38%, respectively. Soil pH increased by 0.17-1.29 after biochar addition. Similarly, the contents of soil organic carbon, total phosphorus, total potassium, available nitrogen, available phosphorus, and available potassium increased by 10.94%-51.37%, 14.29%-59.45%, 6.48%-59.57%, 6.28%-29.41%, 4.79%-19.81%, and 7.72%-75.87%. There was a positive correlation among the main physical and chemical factors. The physical and chemical properties reached their maximum values in the T4 or T5 treatment (4% or 6%). Biochar application provided considerable relief from soil acidification in the Eucalyptus plantation and had a positive effect on soil physicochemical properties. The addition 4%-6% of ripe Eucalyptus biochar produced the optimum results. The results show that biochar can improve the physical and chemical properties of soil, increase soil fertility, and enhance the soil's ability to retain water and fertilizer after twelve months. The findings of this study can be used as a reference in practical applications for soil improvement and sustainable management of Eucalyptus plantations.
Keywords: Eucalyptus plantations; biochar; soil chemistry; soil improvement; soil physical properties.