Nitrogen availability modulates the impacts of plant invasion on the chemical composition of soil organic matter.
Plant invasion can dramatically impact soil carbon (C) cycling and sequestration while, other global change factors, such as nitrogen (N) deposition, are predicted to promote plant invasion. However, questions remain as to whether the chemical composition of soil organic C (SOC) may alter with plant invasion and how N availability modulates the invasion effects on SOC. In this study, we conducted a 10-year mesocosm experiment simulating the invasion of Japanese knotweed (Polygonum cuspidatum) into a fallow soil, coupled with a simultaneous mineral fertilizer application scheme for the invasive plants. We investigated the invasion effects on the chemical composition of various SOC components at the molecular level, and examined how these effects responded to changes in soil N availability. Compared with the noninvaded soils, the knotweed-invaded soils exhibited a 17% increase in the microbial-derived C, mainly through the accumulation of fungal residue in the form of amino sugars. Despite receiving leaf litter which was abundant in polyphenolic compounds (40% and 3-times higher in lignin and tannins per unit biomass, respectively), the knotweed-invaded soils did not differ in the concentration of plant lipids and lignin monomers compared to the noninvaded soils inhabited by grasses. However, the concentrations of phytosterol in the knotweed-invaded soils were 1.5-fold as that in the noninvaded soils. Fertilizer application significantly increased the retention of plant-derived compounds in the knotweed-invaded soils, but also induced 45% greater degradation of lignin. Moreover, under fertilizer application, the knotweed-invaded soils accumulated 46% more microbial-derived C, primarily due to the altered microbial biomass and community composition. Collectively, our findings suggest that plant invasion has the potential to influence SOC chemical composition through changes in plant-derived and microbial-derived C. Furthermore, N deposition could reinforce the invasion effects on the molecular composition and accrual of SOC. Our results also highlight the need to understand the impacts of biological invasion in the context of other global change drivers that both affect invasion and modulate their effects.