Effect of biochar amendment to the forest floor on net ecosystem production: An 8-year field experiment in a secondary oak forest

Applying biochar to forests as a carbon-negative technology holds great promise for sustainable forest management. However, given the inherently high carbon sequestration capacity of forest ecosystems, the potential impacts of biochar amendment on net ecosystem production (NEP) require careful evaluation. To address this, we conducted an 8-year field experiment in a secondary oak ( Quercus serrata ) forest to assess the long-term effectiveness of biochar on carbon sequestration. Biochar was manually applied to the forest floor at rates of 0 (Control), 5 (C5), and 10 Mg ha⁻¹ (C10). We estimated biometric-based NEP by quantifying net primary production (NPP) and heterotrophic respiration (HR). During the second and third years post-application, biochar accelerated HR. Consequently, NEP decreased by 53% in C5 and by 31% in C10 compared to the Control, although there was no statistical difference. During the later five-year period, NEP in C5 (1.23 ± 0.19 Mg C ha⁻¹ yr⁻¹) remained lower than the Control (2.26 ± 0.33 Mg C ha⁻¹ yr⁻¹). In contrast, the increase in HR in C10 completely subsided during this later phase. Driven by a sustained, significantly higher growth rate of canopy oak trees compared to the Control, NPP in C10 exhibited an increasing trend. Consequently, NEP in C10 (3.11 ± 0.74 Mg C ha⁻¹ yr⁻¹) trended higher than the Control, although the difference was not statistically significant. These findings highlight that a biochar amendment of 10 Mg ha⁻¹ yields a long-term net carbon gain by enhancing canopy tree growth and the NEP of the forest ecosystem.