Promoting soil carbon sequestration by diversifying tree species

Abstract

Forests cover about 30 % of the global land surface and store 861 ± 66 Pg C with 44 % being stored as soil organic carbon (SOC). Considering the huge amount, the variation of forest SOC would pose a considerable impact on C cycle-climate feedbacks. SOC pool is determined by the balance of C inputs mainly as plant detritus and C outputs via SOC decomposition, so that factors which affect C inputs or outputs may subsequently alter SOC pool. Cumulative evidence shows that increasing tree species diversity benefits SOC accumulation, but the underlying mechanisms remain poorly understood. We utilized a natural gradient of tree species diversity, and explored the impacts of tree species diversity on SOC dynamics in a subtropical forest. Tree species diversity increased microbial C growth and C use efficiency (CUE). Multiple analyses revealed that changes in microbial traits especially microbial life history strategies and substrate quality were mainly responsible for the increase of CUE in responsible for increasing tree species diversity. The increased CUE resulted in stimulated microbial biomass C, which is the precursor of microbial residue C. Meanwhile, increasing tree species diversity improved the mineral protection of microbial residue C and benefited its accumulation. The level of lignin as indicated by lignin phenols increased under higher tree species diversity due to stimulated plant detritus input, mineral protection and suppressed lignin decomposition by microbes. The increase of both microbial and plant residue C ultimately led to enhanced SOC pool under higher tree species diversity. Additionally, temperature sensitivity of SOC decomposition remains unchanged, priming effect of SOC decomposition decreases, and soil multifunctionality increases along with increasing tree species diversity. These findings suggest that diversifying tree species during afforestation would gain more than SOC accrue.