Litter quality controls the contribution of microbial carbon to main microbial groups and soil organic carbon during its decomposition

A 163-day decomposition experiment with 13C-enriched leaf litter of Populus davidiana (low quality, with low N content, high C:N and high lignin content) and Quercus wutaishanica (high quality, with high N content, low C:N and low lignin content) was conducted to investigate the effects of litter quality on the microbial contribution to soil organic C (SOC). We used stable isotope probing (SIP) technology of phospholipid fatty acid (PLFA) and amino sugar, determined soil enzyme activities, and microbial C use efficiency (CUE) to study the microbial contribution to SOC formation as affected by litter quality. Gram-positive (G +) and Gram-negative (G -) bacteria rapidly assimilated the readily available C of high- and low-quality litter, whereas fungi selectively utilized more recalcitrant compounds. The ratio of 13C-fungal to 13C-bacterial necromass increased and then leveled off until the end of the incubation for both litters. Therefore, litter-derived C was first utilized by bacteria, then allocated presumably by the consumption of bacterial necromass to fungi, and, at the end, the litter C was mainly stabilized as fungal necromass. The addition of high-quality litter led to higher total necromass and SOC in comparison to the addition of low-quality litter. Likely this difference depended on the higher availability of easily available C compounds in the Q. wutaishanica than in P. davidiana litters. The efficiency of SOC formation, determined by the percentage of SOC content gain divided by the litter C content loss, correlated with the microbial incorporation of P. davidiana litter-derived 13C into PLFAs and amino sugars. However, it increased sharply in the late phases of Q. wutaishanica litter decomposition despite the decreased 13C incorporation in PLFAs and amino sugars, suggesting the dominance of physical litter C stabilization. Compared to the high-quality litter, the low-quality litter induced lower but steadier necromass accumulation, thus increasing the SOC content in the long term. Litter quality, litter-derived 13C in PLFAs, and microbial CUE are the main drivers of litter-derived C use pathways. Our findings underpin the microbial C pump-regulated SOC formation, whereby differences in litter quality shape the composition of main microbial groups, leading to differences in enzyme activities and CUE, which determine necromass turnover and thus SOC formation.