Rainfall re-engineered by representative tree species in the urbanized Chaohu Lake Basin: Differential regulation of DOM molecular size, nutrient, and metal dynamics

Abstract

The vegetation canopy is a critical interface that actively modulates the chemistry of atmospheric deposition, yet the species-specific mechanisms driving these transformations remain poorly resolved. This study reveals the canopies of three typical species in the urbanized Chaohu Lake Basin-Camphor, Osmanthus, and Heather-function as dynamic chemical reactors that fundamentally re-engineer the molecular architecture of rainfall. All canopies acidified rainfall and massively enriched it with nutrients; notably, dissolved total phosphorus (DTP) concentrations were amplified by over 20-fold under Osmanthus. The net nitrogen flux from the Camphor canopy (603.46 kg/km²) far exceeded that from Osmanthus (152.58 kg/km²), which actively retained atmospheric NH₃-N. Critically, we identified a novel decoupled mobilization mechanism for Iron (Fe): unlike base cations which are driven by physical washout, Fe transport was strictly governed by specific organic complexation. UV₂₅₄ absorbance increased by one to two orders of magnitude in throughfall. FT-IR analysis demonstrated that canopies catabolized atmospheric aliphatic compounds while leaching vast quantities of plant-derived metabolites. High-resolution SEC-OCD analysis further uncovered species-specific molecular footprints: while Osmanthus and Heather primarily released high-molecular-weight (MW) humic substances, Camphor uniquely exhibited a bimodal DOM leaching pattern, releasing massive pulses of low-MW labile neutrals (<0.5 kDa) alongside humics. Integrating these findings, we propose a unifying humic-microbial co-control model. This model posits that the ecosystem's hydrochemistry is governed by a synergistic interplay between a stable humic-like fraction and a dynamic microbial by-product-like fraction. This study demonstrates that tree species identity is a primary driver of the quality and ecological function of carbon and nutrient fluxes, offering crucial mechanistic insights for predicting the biogeochemical consequences of vegetation change.

Keywords: Cations; DOM; EEMs; Moleculare size distribution; Nutrient; Rainfall; Throughfall.