Intrinsic linkage mechanisms of DOM properties to organic phosphorus in lake sediments: Evidence from coupled molecular weight ultrafiltration and spectral analysis

Dissolved organic matter (DOM) is ubiquitous and significantly affects organic phosphorus (OP) biogeochemical cycles and subsequent environmental risks. However, to date, DOM molecular properties and their intrinsic linkage mechanisms with OP remain unknown. Here, combined characterization methods were developed to explore DOM molecular properties and their intrinsic linkage mechanisms with OP in NaOH-ethylenediaminetetraacetic acid (EDTA) extractants (the most ideal extractant for OP) in lake sediments. We found that NaOH-EDTA-DOM mainly consisted of humic acid rich in functional groups and substances derived from terrestrial material. As molecular weight (MW) increased, the aromaticity, hydrophobicity, functional groups and humification of NaOH-EDTA-DOM increased. One interesting finding is that DOM with higher aromaticity, hydrophobicity, humification degree, MW, and more substituents quantity may restrain extraction of P by NaOH-EDTA, implying OP components that combine DOM with a complex structure might be ignored by 31P NMR (the most popular OP characterization method). Additionally, 85 % of dissolved organic carbon (DOC) occurred mainly in low MW (LMW, < 1 kDa), while 82 % of OP was concentrated in high MW (HMW, > 1 kDa), and these showed a more significant statistical relationship in LMW than HMW, which is attributed to the combination complexity of HMW-DOM with OP . For OP components, DOM humification favored the transformation of some diester P species to RNA-mononucleotides and β-glycerophosphate (especially LMW), while inositol hexakisphosphate was easily associated with humus with HMW material to form more complex OP species, which testified DOM molecular properties can determine OP accumulation and composition by affecting OP transformation with different MWs. This work provides critical information about NaOH-EDTA-DOM molecular properties and forms novel insight into the linkage between DOM and OP at the molecular level.