Understory ferns removal downregulates microbial carbon use efficiency and carbon accrual in previously degraded lands.

• Restoration of degraded lands increases microbial C use efficiency and C storage. • Removing understory ferns decreases the quality of litter inputs to soils and microbial C use efficiency. • Fern removal decreases total soil respiration in a young restored pine forest. • Litter quality exerts a greater role than litter quantity in SOC dynamics. • Understory ferns promotes SOC accumulation in restored pine forests. When reforesting degraded lands, understory ferns can take root and cover the forest floor, which can change the quality and quantity of litter added to soils. However, it is unclear how ferns and the ongoing succession of trees impact soil organic carbon (SOC) dynamics. Here, we established experimental sites in Masson pine (Pinus massoniana) stands for understanding the underlying mechanisms of how ferns affect SOC stock and soil respiration (R s) in subtropical China. The established three pine stands covered with ferns (Dicranopteris dichotoma (Thunb.) Berhn.) were used to represent a 37-year restoration sequence: two restored sites with vegetation cover (VC) established in 1984 and 2002 (referred to VC-1984 and VC-2002), and an unrestored site (VC-0). In August 2014, two treatments were set i.e., ferns were retained in half area of the sites ("control"), and continuously removed ferns from the other half ("fern removal") for seven years. Our results showed that fern removal significantly decreased R s by 25.8% in VC-2002 compared to the control over a three-year period from 2015 to 2018, but did not significantly affect R s in VC-0 and VC-1984. We found that fern removal decreased fine root biomass by 50.4% in VC-0, 31.4% in VC-2002, and 54.7% in VC-1984 in 2019, with seven-year averaged SOC stocks decreased by 37.2%, 13.9%, and 22.7% in VC-0, VC-2002, and VC-1984, respectively. Moreover, fern removal significantly increased carbon- and nitrogen-degrading enzyme activities and decreased microbial carbon use efficiency, suggesting that fern removal may increase SOC decomposition. Taken together, we concluded that fern removal may increase SOC-derived CO 2 efflux counteracting the reduced root respiration in VC-0 and VC-1984 but not in VC-2002, leading to a divergent response of R s to understory fern removal. Our study emphasizes the necessity of incorporating understory plants regulates SOC decomposition and accumulation in assessing forest recovery effects on soil carbon cycling. [ABSTRACT FROM AUTHOR]