Initial litter chemistry and seasonality drive potentially toxic element cycling during decomposition in a subtropical forest: insights from monospecific and mixed litter.

Abstract

The cycling of potentially toxic elements (PTEs) in leaf litter is a crucial component of the geochemical cycle within forest ecosystems. However, the influences of initial litter chemistry and seasonality on PTE release remain poorly understood. A total of 150 litter samples (5 litter types × 6 initial month treatments × 5 replicates), each enclosed in a litterbag, were deployed over a one-year decomposition period in a subtropical evergreen broad-leaved forest in the National Nature Reserve of the Ailao Mountains, with setups initiated in January, March, May, July, September, and November. The concentrations and accumulation indices (AIs) of six PTEs (Co, Sb, Sr, Ti, Tl, and V) were quantified in the decomposed or decayed leaf litter of four tree species (Schima noronhae, Castanopsis wattii, Lithocarpus hancei, and Ternstroemia gymnanthera), and their mixture. The results indicated that after one year of decomposition, the average concentrations of Ti, V, Co, Sr, Sb, and Tl all increased compared to their initial concentrations, with an increase factor ranging from 1.46 to 8.19. Except for Sr, the AIs of Ti, V, Co, Sb, and Tl in the four species and their mixture exceeded 100%, indicating significant net accumulation. Initial litter chemistry and seasonality exhibited significant effects on the AIs of PTEs (p < 0.05), although their interactions did not. Specifically, Mn concentration positively correlated with the AIs, whereas Mg, water-soluble sugar, sulfur, hydrogen, and the C/N ratio exhibited negative associations (p < 0.05). Notably, dry-cool season initiation (Jan-May) enhanced PTE retention compared to wet-warm periods (Jul-Nov), aligning with decomposition kinetics observed in subtropical forests. Mixed litter showed a slight non-additive antagonistic effect on the accumulation of PTEs. These findings prominently demonstrated the influence of initial litter chemistry as well as the regulatory function of seasonality on the accumulation and release of PTEs, thereby providing valuable insights for ecological risk assessment and forest management.