Climate and soil microbial traits shape carbon use efficiency after large-scale afforestation

Afforestation of abandoned croplands opens a huge opportunity for carbon sequestration and climate change mitigation. Microbial carbon use efficiency (CUE) controls carbon sequestration. However, the drivers of microbial CUE after large-scale afforestation remains virtually unknown. Here, we conducted a large-scale paired-survey of 336 soil samples (168 top-soils and 168 deep-soil) in northern China to fill this knowledge gap. Using metagenomic sequencing and 18O-H2O tracer methods, we found that afforestation significantly enhances microbial CUE in top-soils, though its impact on deep soils is weaker, and such CUE enhancement is more substantial in arid (warm) regions than in humid (cold) regions. During the process of afforestation, soil microbiomes can boost CUE by optimizing their intrinsic microbial traits (i.e., genome size, body size, and maximum growth rate). Notably, in cropland soils-where the baseline traits of microbiomes are relatively low-the CUE-enhancing effect of afforestation is even more prominent. Collectively, this study offers robust empirical evidence and novel mechanistic insights into how climate interacts with soil microbiomes in shaping CUE after large-scale afforestation, highlighting the importance of incorporating climate and microbial traits in the predications and modelling of future soil carbon sink potential.