Rice Husk Biochar Effects on Soil Carbon Storage and Greenhouse Gas Emissions in a Subtropical Poplar Plantation

Biochar amendment is widely used to enhance soil carbon storage and regulate greenhouse gas emissions, but field evidence from forest plantations remains limited. This 1.5-year field study investigated the responses of soil greenhouse gas (GHG) emissions and carbon storage to rice husk biochar applied at 15 t ha−1 (LB) and 30 t ha−1 (HB) in a subtropical poplar plantation. At 3.0 m from the tree stem, biochar did not significantly affect cumulative N2O emissions but significantly increased cumulative CO2 emissions and reduced CH4 emissions by 49.36% (LB) and 21.80% (HB), resulting in no significant change in non-CO2 GHG-based global warming potential. In contrast, biochar significantly enhanced measured soil organic carbon (SOC) storage in several soil layers, especially in the 0–20 cm layer, and increased mineral-associated organic carbon (MAOC) while decreasing the POC: MAOC ratio, indicating a greater allocation of SOC towards more recalcitrant fractions. However, when carbon storage was integrated over the 0–100 cm profile, a significant increase was observed only under the high biochar rate at D1. These findings suggest that biochar can enhance carbon storage in poplar plantation soils, although its GHG mitigation effect is limited and gas-specific. Additional measurements in biochar-amended plots showed higher N2O emissions closer to the tree stem (1.5 m), suggesting that future studies should use full-factorial treatment × distance designs to evaluate tree-proximity effects on biochar responses in plantation soils.