Source, composition and molecular diversity of dissolved and particulate organic matter varied with riparian land use in tropical coastal headstreams.

Source, composition and molecular diversity determine the reactivity and stabilization of organic matter (OM, dissolved [DOM]/particulate [POM]), affecting its behavior and fate. Here, multiple spectral and mass spectrometry techniques were applied to examine how riparian land-use shaped the source, composition and molecular diversity of POM and DOM (HDOM) in adjacent headstreams. Compared to HDOM with abundant lignins, microbially-transformed heteroatoms and carboxyl-rich alicyclic acids (CRAMs), POM exhibited higher allochthonous characteristics and more bioactive components, but lower molecular weight and diversity in different land-use-dominated streams. Compared to wetland-dominated headstreams, both POM and HDOM exhibited more terrestrial origin and condensed aromatics/tannins molecules for agriculture-impacted headstreams and bio-labile lipids, proteins and carbohydrates for forest-impacted headstreams. Structural equation mode (SEM) showed that soil-derived DOM (SDOM) showed the most prominent influence on the source, composition and molecular diversity of POM and the source of HDOM. The molecular composition and diversity of HDOM were mainly influenced by soil properties/SDOM and aquatic microorganisms, respectively. Redundancy analysis (RDA) revealed that autochthonous, bio-labile compositions of POM in forest and wetland streams were positively related to aquatic Bacteroidetes/Cyanobacteria, and carbohydrates/biogenic index of SDOM, while that of HDOM were positively linked with aquatic Bacteroidetes/Cyanobacteria, and SDOM molecular diversity. Terrestrial and aromatic POM in agricultural headstreams were associated with aquatic total nitrogen/Actinobacteria, and humification degree, aromatic/phenolic substances of SDOM, while that of HDOM were mainly regulated by aquatic nitrate/total nitrogen/Actinobacteria, and aromatic/carboxylic-containing moieties of SDOM. Noteworthily, the molecular diversity of agricultural OM increased along the soil-stream continuum due to the input of soil condensed aromatics and tannins. The opposite trend was observed in forest and wetland streams due to the input of bioactive carbohydrates and the microbial-degradation in-stream. These results are helpful to predict the behavior and fate of OM and determine effective management strategies in tropical coastal regions undergoing intense anthropogenic alterations.