Mechanisms Underlying Aboveground and Belowground Litter Decomposition Converge over Time under Nutrient Deposition

Decomposition is vital for nutrient cycling and is sensitive to atmospheric nutrient depositions. However, the influences and underlying mechanisms of nutrient deposition on the long-term decomposition of leaves and absorptive roots remain unclear. Here, we explored the responses of leaves and absorptive roots to nutrient deposition (control, +N, +P, and +NP) in Pinus massoniana and Schima superba forests in subtropical China based on two stages (early-stage (1-year) and late-stage (3-year)) of a decomposition experiment. The chemical factions (acid-unhydrolysable residue (AUR), cellulose, and hemicellulose concentrations) and microbial enzymatic activities (hydrolase and oxidase) were also determined. The +N treatment had persistent negative effects on absorptive root decomposition, except for P. massoniana during the late stage. The +P treatment had a positive effect on leaf decomposition in all stages. The +NP treatment had a positive effect on leaf decomposition during the late stage. The increasing decomposition rates of foliar under +P treatment were more correlated with the increasing acid phosphatase activity than chemical factions, indicating a microbial mechanism. The decreasing decomposition rates of roots under +N treatment were weakly correlated with increasing AUR concentrations and strongly correlated with decreasing oxidase activity during the late stage, indicating both chemical and microbial mechanisms. Overall, our findings highlight that, despite contrasting responses to nutrient deposition, the mechanisms underlying aboveground and belowground decomposition tend to converge as decomposition progresses.