Response of Size-Dependent Chemical Composition of Dissolved Organic and Inorganic Species to Land Use Types in Tropical Coastal Headstreams

Land-use types dramatically affect the source, composition, behavior and fate of headstream chemical species. However, the influence of land-use types on the size-dependent composition of dissolved organic matter (DOM) and dissolved inorganic matter (DIM) is still unclear. Here, we evaluated the size-dependent concentration, composition and relationship of chemical species from 9 coastal headstreams (i.e. forested, agricultural and wetland headstreams) using multiple spectroscopic techniques including absorbance, excitation-emission matrix, near-edge X-ray absorption fine structure spectroscopy (NEXAFS), X-ray diffraction (XRD), scanning electron microscopy combined with energy dispersive spectroscopy (SEM-EDS) and synchrotron-based micro-X-ray fluorescence (μ-XRF). Results showed that 68.57%-85.78% of dissolved organic carbon (DOC) with abundant humic/fulvic-like substances were distributed in the <1 kDa fraction in all samples, with a higher percentage in wetland samples. Agricultural land-use increased the export of aluminosilicate minerals in >1 kDa fractions and terrestrial fulvic/humic-like substances from land to stream. Forested streams showed high DOC in the 1 kDa-0.2 μm fraction, predominance of autochthonous DOM rich in amides, amines, carboxyl and O-alkyl groups, and abundance whitlockite and ferromanganese oxides. Wetland streams were characterized by abundant halite, sylvite, and microbially-transformed humic-like DOM rich in aromatic and phenolic groups. Morphology and correlation analysis implied DIM was closely combined with fluorescent DOM (FDOM) and colored DOM (CDOM), resulting in more irregular organo-inorganic complexes in forested and agricultural streams, and more halite and sylvite crystal blocks encapsulating by smooth and close organic–inorganic complexes in wetland streams. Redundancy analysis (RDA) analysis suggested that land-use types (17.20%) and molecular weight (14.29%) were the prominent factors impacting the composition and relationship of DOM and DIM. Land-use types altered the sources and transport patterns of chemical species in headstreams, influencing their size-dependent compositions and associated biogeochemical processes. Such information is imperative for monitoring and developing land-use policies in coastal regions undergoing intense anthropogenic alterations.