Dissolved organic matter characteristics and composition of small saline lakes in the Badain Jaran Desert, China

Elucidating the characteristics and molecular composition of dissolved organic matter (DOM) is critical to understanding carbon cycling in increasingly saline lakes in arid and hyper-arid areas; however, the ages and molecular signatures of DOM in these widely distributed saline lakes remain poorly understood. Here, variations in DOM ages and other characteristics with salinity were investigated across 10 small saline lakes within the Badain Jaran Desert based on ¹⁴C-isotopic analysis, absorption and fluorescence spectroscopy, in addition to Fourier transform ion cyclotron resonance mass spectrometry. The results showed that brine lakes had dissolved organic carbon (DOC) levels 3-fold higher than brackish lakes, and the ¹⁴C age of DOC in brine lakes was twice as old. Compared to brackish lake DOM, brine lake DOM had more significant signatures of microbial contributions, as indicated by the higher fluorescence index values and greater relative abundances of microbial humic-like fluorescent components, in addition to sulfur-containing compounds. Moreover, brine lakes showed lower relative abundances of aromatic lignin-like components, as well as higher relative abundances of lipid- and condensed-aromatics-like components. Regarding halogen-containing DOM, the brine lakes showed higher abundances of halogen-containing lignin- and lipid-like components, supporting the preferential transformation of aromatic compounds into halogenated lignin- and lipid-like components. This study demonstrates the strong impacts of salinization on DOM aging, accumulation, and halogenation in inland saline lakes and highlights the critical role of brine lakes as a persistent carbon pool.