Temperature Sensitivity of Soil Organic Matter Decomposition During Forest Restoration: The Rhizosphere Effects

Forest restoration strongly contributes to the current terrestrial carbon (C) sink of the world under global warming. The magnitude and mechanisms of soil contribution to this C sink are largely unknown due to the uncertain temperature sensitivity (Q10) of soil organic matter (SOM) decomposition. To investigate the Q10 of SOM decomposition responding to warming, bulk and rhizosphere soil samples from a chronosequence of Chinese subtropical forests (i.e., restored from coniferous, mixed, to evergreen broadleaf forests) were incubated for 5 days under continuously increasing temperatures (5-33°C). Soil CO2 efflux rates and the corresponding soil parameters including SOM quality and bioavailability, microbial gene copy numbers, and microbial taxonomic composition were analyzed. Forest restoration increased the rhizosphere and bulk soil organic carbon content by 122.7% and 43.0%, respectively. CO2 efflux rates in the rhizosphere increased with forest restoration. In contrast, those in the bulk soil peaked in mixed forests, in line with dissolved organic carbon (DOC) and microbial gene copy numbers. The rhizosphere Q10 of SOM decomposition stayed around 2.2-2.5 during forest restoration, because of continuous available C input. Soil water content increased by rhizosphere SOC and fungal gene copy numbers increased by rhizosphere DOC both positively affected rhizosphere Q10, whereas rhizosphere C/N ratio negatively influenced it. In contrast, bulk soil Q10 decreased from 2.9 to 2.2 during forest restoration. Rhizosphere SOC input motivated an increase in bulk soil basal respiration and Acidobacteria (r-strategists), and a decrease in Proteobacteria (K-strategists), negatively affecting bulk soil Q10. Our results indicated that rhizosphere C inputs, microbial community composition, and soil properties, resulting from the restoration of pure plantations to evergreen broadleaf forests, can depress the Q10 of SOM decomposition, leading to negative feedback on climate change. This study provides a better understanding of how soil contributes to ecosystem carbon sequestration in subtropical forest sustainable management.