Structure–Energy–Photochemical Activity Relationships in Fluorophoric Water-Extracted Organic Matter from (Un)charred Plant Materials

Abstract

Photobleaching experiments with water-extracted dissolved organic matter (wDOM) from uncharred or charred grass (SG25 and SG400), softwood (EC25, AJ25, EC400, and AJ400), and hardwood (HM25 and HM400) biomass revealed that photodegradation proceeds along an energy-dependent, component-specific trajectory dictated by the nature of the aliphatic attachments associated with single-ring aromatic domains. Trends in wDOM photobleaching for both uncharred [EC25 = AJ25 (94%) > SG25 (46%) ≥ HM25 (43%)] and charred [EC400 = AJ400 (76%) > HM400 (55%) > SG400 (44%)] biomass pointed to biomass type and chemistry and its response to processing (here, charring) as key factors controlling photodegradation. Specifically, photobleaching was lower in wDOM where aliphatic attachments to the aromatic backbone had shorter alkyl chain lengths, was involved in heterocyclic ring formation, or was more oxidized. Photobleaching behavior, across the wDOMs, was captured by a three-component, UVA photoenergy (E UVA-V )based model comprising a fast, wDOM f , component requiring a mean E UVA-V flux of 42−204 kJ m −2 for photobleaching; a slow, wDOM s , component requiring a 3−24-fold higher E UVA-V flux; and a photobleaching resistant, wDOM res , component. Combining long-term daily E UVA-V across the State of Texas, the statewide distribution of vegetation types used in this study, and our photobleaching model suggested that an increase in the number of vegetation fires is likely to decrease photodegradative contributions to wDOM cycling in the conifer-dominated east, have no effect in the grass-dominated central regions, and increase contributions in the hardwood shrub-dominated west.

The black carbon cycle and its role in the Earth system

“…The photolability of DBC is mainly controlled by the abundance of polycyclic aromatic structures, with larger condensed structures being more susceptible to photo-oxidation than less condensed structural features 46 . In laboratory experiments, the photo chemical half-life of polycyclic aromatic structures is one to two days and therefore is much shorter than the typical riverine transit time to the ocean 95 . However, the DBC content is relatively high in most river mouths (12 ± 5% of DOC) 2 .…”