Afforestation can lower microbial diversity and functionality in deep soil layers in a semiarid region

Afforestation is an effective approach to rehabilitate degraded
ecosystems, but often depletes deep soil moisture. Presently, it is not
known how an afforestation-induced decrease in moisture affects soil
microbial community and functionality, hindering our ability to understand
the sustainability of the rehabilitated ecosystems. To address this issue,
we examined the impacts of 20 years of afforestation on soil bacterial
community, co-occurrence pattern and functionalities along vertical
profile (0-500 cm depth) in a semiarid region of China’s Loess Plateau. We
showed that the effects of afforestation with a deep-rooted legume tree on
cropland were greater in deep than that of in top layers, resulting in
decreased bacterial beta diversity, more responsive bacterial taxa and
functional groups, increased homogeneous selection, and decreased network
robustness in deep soils (120-500 cm). Organic carbon and nitrogen
decomposition rates and multifunctionality also significantly decreased by
afforestation, and microbial carbon limitation significantly increased in
deep soils. Moreover, changes in microbial community and functionality in
deep layer was largely related to changes in soil moisture. Such negative
impacts on deep soils should be fully considered for assessing
afforestation’s eco-environment effects and for the sustainability of
ecosystems because deep soils have important influence on forest
ecosystems in semiarid and arid climates.