Chemical Structure of Weoc from Thermally-Altered Soils of Sierra Nevada, California.

Wildfires are expected to play a pivotal role in controlling the cycling and composition of organic matter (OM) in the soil system. While effects of fire on soil organic matter (SOM) have been extensively studied, the extent at which soil burning affects the soluble component of SOM remains unclear. In this study, we investigated the thermal transformations of water extractable OM (WEOM) by examining the changes in the distribution of C functional groups of water-soluble OC from soils heated at low and intermediate temperatures. WEOM (<0.7 µm particle size) was extracted from topsoils (0-5 cm depth) of five soil series sampled along a Sierra Nevada climosequence after the soils where heated in a muffle furnace at 150^oC, 250^oC, and 350^oC. The extracted solution was analyzed for water extractable carbon concentration and its major structural components by applying liquid-state ¹H NMR spectroscopy. Our findings revealed significant structural changes in water-soluble OC with increasing heating temperature across all soil types. At 250^oC, we observed an increase in OC concentrations relative to the thermally unaltered samples, followed by losses of O-alkyl functional groups and enrichment in O- and N-substituted aliphatic structures. Conversely, OC concentration decreased as heating increased from 250^oC to 350^oC, with an enrichment in both aromatic and amide structures in the leachate. Our results suggest that heat induced changes in surface soils during low and intermediate fire intensities can significantly alter the composition and structure of dissolved OM mobilized from soils and transported to rivers in the Sierra Nevada and beyond.