Contrasting molecular structures and photooxidation behaviors between dissolved organic sulfur released from rice straw-biochar and aerobically decomposed rice straw

Photooxidation of dissolved organic sulfur (DOS) in soils and natural waters plays an important role in the sulfur biogeochemical cycle. However, the structural-dependent photoliabilities of DOS from different sources remain unclear. Here, the molecular structures and photooxidation behaviors of DOS in pyrogenic dissolved black matter (PyDOM) derived from rice straw-pyrolyzed biochar (referred to as PyDOM-S and considered to be representative of black carbon from prairie fires) were thoroughly characterized and compared with those of DOS in leached dissolved organic matter (LDOM) derived from aerobically decomposed rice straw (referred to as LDOM-S and considered to be generally representative of organic-rich horizons in soils and peats). The Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analysis revealed that both PyDOM-S and LDOM-S were, respectively, dominated by aliphatic (60.4 % and 41.1 %) and lignin-like compounds (35.1 % and 40.1 %), followed by minimal aromatic and polyaromatic compounds (2.8 % and 11.9 % in total). As demonstrated by the sulfur K-edge X-ray absorption near-edge structure (S-XANES) analysis, PyDOM-S consisted mainly of organosulfate (80.4 %) contrasting to the diversified and mingled reduced sulfurs (62.7 %) and oxidized sulfurs (37.3 %) of LDOM-S. Under simulated sunlight irradiation, 74 % of sulfur in PyDOM-S was photomineralized to sulfate within 24 h and totaling 89 % after 168 h, but only 9 % and 42 % for LDOM-S given the same periods of time, confirming the much faster photomineralization of PyDOM-S. After 168-h irradiation, almost all molecules in PyDOM-S disappeared, whereas a large proportion (44.2 %) of LDOM-S molecules (mainly aliphatic and lignin-like compounds) were photo-resistant. Furthermore, the photomineralization of PyDOM-S was mainly contributed by the final and complete oxidation of organosulfate to sulfate; however, the photooxidation of LDOM-S was dominated by the sequential oxidation of exocyclic sulfur and heterocyclic sulfur to organosulfate prior to releasing sulfate. These results highlight that pyrogenic-sourced PyDOM-S and diagenesis-derived LDOM-S exhibit contrasting photooxidation behaviors due to the associated distinct molecular structures.