Single biochar application enhances soil methane sink and underlying microbial mechanisms in a subtropical forest after twelve years of treatments.

Abstract

Biochar is an effective amendment for enhancing soil carbon sequestration and regulating soil greenhouse gas emissions, such as methane (CH4), yet its decadal-scale effects in forest ecosystems remain unclear. Here, based on a 12-year field experiment in a subtropical forest where biochar was applied once at rates of 0 (CK), 10 (Bio10), and 30 t ha-1 (Bio30), we monitored in-situ CH4 fluxes over one year and simulated daily soil CH4 fluxes using a Soil Methanotrophy Model (MeMo). Combined with laboratory incubations and metagenomic absolute quantification sequencing, we found that a single biochar application enhanced the soil CH4 sink even after twelve years, in a dose-dependent manner. Compared with CK, Bio10 increased the annual cumulative CH4 uptake by 24.9% (P < 0.05), although its mean annual CH4 uptake flux showed no statistically significant increase (10.3%). In contrast, Bio30 significantly increased both the mean annual flux and annual cumulative uptake by 56.0% and 76.4%, respectively. Biochar addition significantly increased soil extractable organic carbon (EOC) contents, but significantly decreased ammonium nitrogen (NH4+-N) content (P < 0.05). Furthermore, relative to CK, Bio30 increased methanotrophic abundance (e.g., pmoA gene abundance) by 56.5% and methane oxidation potential by 170.5%. Biochar did not significantly alter the overall methanotrophic community structure. Structural equation modeling revealed that biochar enhanced methane oxidation potential directly, and indirectly through increasing soil EOC contents, and methanotrophic abundance, thereby promoting CH4 uptake (total effect = 0.60). In summary, a single application of 30 t ha-1 biochar still exerts a significant enhancement of the soil CH4 sink in subtropical forest soils twelve years after application. These findings provide critical scientific evidence for evaluating the long-term climate mitigation benefits of biochar in forest ecosystems and offer theoretical support for nature-based strategies to enhance forest soil CH4 sinks.