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Litter decomposition is a key ecosystem process that drives carbon (C) and nutrient cycling, which could be affected by shifts in plant community composition caused by plant invasion or expansion. However, how changes in leaf litter composition (e.g. litter‐mixing effect) and soil microbial community induced by shift in plant community composition affect decomposition remains elusive.Here, by deploying 432 litterbags (50‐μm mesh screen), we treated leaf litter of bamboo Phyllostachys edulis and tree species (in a mixture or alone) and decomposed them in forest sites expanded or not expanded by bamboo at three regions, to assess the effects of bamboo expansion on decomposition, microbial community structure and function.Bamboo expansion reduced microbial C to nitrogen (N) ratio and nutritional stress indicator (cy/pre), which was very likely ascribed to elevated N availability as a result of accelerated litter N release. The N loss of bamboo litter (28.1 ± 3.03%) was more than double of tree species litter (13.2 ± 3.1%), and decomposing litter of bamboo and tree species in a mixture increased litter N loss from 15.3 ± 3.1% to 25.9 ± 3.1% (by 69%). However, no consistent effect of litter type and litter treatment on C loss was observed. The non‐additive litter‐mixing effect is attributable to increased microbial nutrient use efficiency, which is illustrated by the interactive effect between litter treatment and cy/pre on decomposition. Moreover, the forest type interactively affected decomposition with cy/pre as ignoring litter treatment, where higher decay rate and litter N loss at bamboo expanded than non‐expanded sites would be expected when both sites having comparable cy/pre, but the reality is that bamboo expansion tends to have lower cy/pre. These results imply a trade‐off between introduced litter and nutrient use efficiency of local microbial community to drive decomposition.Our results demonstrate that litter decomposition affected by bamboo expansion was jointly modulated by litter‐mixing and microbial composition, which sheds new insights on predicting changes in soil C and nutrient cycling induced by shifts in plant community composition. A free plain language summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

• 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]

Increasing plant species richness generally enhances plant biomass production, which may enhance accumulation of carbon (C) in soil. However, the net change in soil C also depends on the effect of plant diversity on C loss through decomposition of organic matter. Plant diversity can affect organic matter decomposition via changes in litter species diversity and composition, and via alteration of abiotic and/or biotic attributes of the soil (soil legacy effect). Previous studies examined the two effects on decomposition rates separately, and do therefore not elucidate the relative importance of the two effects, and their potential interaction. Here we separated the effects of litter mixing and litter identity from the soil legacy effect by conducting a factorial laboratory experiment where two fresh single root litters and their mixture were mixed with soils previously cultivated with single plant species or mixtures of two or four species. We found no evidence for litter-mixing effects. In contrast, root litter-induced CO2 production was greater in soils from high diversity plots than in soils from monocultures, regardless of the type of root litter added. Soil microbial PLFA biomass and composition at the onset of the experiment was unaffected by plant species richness, whereas soil potential nitrogen (N) mineralization rate increased with plant species richness. Our results indicate that the soil legacy effect may be explained by changes in soil N availability. There was no effect of plant species richness on decomposition of a recalcitrant substrate (compost). This suggests that the soil legacy effect predominantly acted on the decomposition of labile organic matter. We thus demonstrated that plant species richness enhances root litter-induced soil respiration via a soil legacy effect but not via a litter-mixing effect. This implies that the positive impacts of species richness on soil C sequestration may be weakened by accelerated organic matter decomposition. [ABSTRACT FROM AUTHOR]