Effects of microbial groups on soil organic carbon accrual and mineralization during high- and low-quality litter decomposition

Litter input regulates the mineralization and accrual of soil organic carbon (SOC), depending on microbial groups and litter quality. Stable isotope probing was used after the addition of 13C-labeled litter of different recalcitrance, including Populus davidiana (characterized by low quality, high C:N, and lignin content) and Quercus wutaishanica (characterized by high quality, low C:N, and lignin content). The flow of litter C to microbial biomass and necromass was followed by quantification of 13C incorporation into phospholipid fatty acids (13C-PLFAs) and amino sugars (13C-AS), and their respiration. The CO2 efflux and positive priming effects (PE) were higher, but the 13C-necromass was lower (8.5% vs. 10%) in the soil with P. davidiana than with Q. wutaishanica. The 13C-necromass continuously increased with P. davidiana litter decomposition, but reached a peak at day 14 after Q. wutaishanica litter addition. These results indicated that the decomposition of resistance compounds mainly resulted in necromass formation after low-quality litter addition. The P. davidiana-derived 13C in fungal PLFAs increased with decomposition, suggesting the importance of these microorganisms in decomposing resistance compounds in low-quality litter despite the bacterial-dominated decomposition process of both litters. Bacteria frequently exhibit versatility in utilizing C from different sources, whereas a larger proportion of fungi display specialization. SOC mineralization increased with the remaining litter but decreased with 13C-G- bacterial PLFAs and 13C-actinobacterial PLFAs after both types of litter addition because starving microbes need to invest more soil C in maintenance respiration vs. biomass formation. 13C-necromass increased with 13C-PLFA because microbial biomass is the primary precursor of necromass. Altogether, low-quality compounds are more readily respired, whereas C from high-quality biopolymers is relatively more assimilated into the necromass. This study further explains the effect of soil microbial groups on SOC under different qualities of litter decomposition and deepens our understanding of the mechanism of SOC sequestration.