Composition and lability of riverine dissolved organic matter: Insights from thermal slicing ramped pyrolysis GC–MS, amino acid, and stable isotope analyses

Abstract Dissolved organic matter (DOM) is one of the key carbon substrates transported by rivers from land to sea, yet only a small fraction of it has been characterized at the molecular level. Here we characterize high molecular weight DOM (HMW-DOM, >1 kDa) isolated by ultrafiltration from nine North American rivers using thermal slicing ramped pyrolysis gas chromatography mass spectrometry (TSRP-GC–MS), carbon and nitrogen stable isotopes, elemental composition and total hydrolyzable amino acids (THAA). Samples from the more anthropogenically altered watersheds carried more enriched δ15N-HMW-DOM signatures than observed in samples from the more pristine watersheds. The TSRP-GC–MS results, coupled with THAA data, constrained DOM degradation processe. Specifically, the diversity of pyrolyzates of HMW-DOM samples correlated well with the degradation index derived from the THAA data, with more degraded samples being more thermally stable and less molecularly diverse. The relationship between degradation index and pyrolyzate diversity observed in the HMW-DOM samples from across such a broad spatial scale suggests that degradative processes reduce riverine DOM complexity through the loss of more reactive moieties. Overall, TSRP-GC–MS, in combination of specific compound or isotope analyses, illuminates processes controlling DOM composition in different North American river systems with varying degrees of bioavailability and anthropogenic influence.