Photochemical transformation of terrestrial dissolved organic matter derived from multiple sources in tropical plantations

Photochemical transformation is a critical geochemical fate of terrestrial dissolved organic matter (DOM) that drains from forests to downstream water bodies. However, the photo-degradability and photo-transformation of terrestrial DOM from various sources are yet to be explored. Here, optical spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry were used to analyze photochemical change in spectroscopic and molecular-level signatures of DOM derived from throughfall, stemflow, runoff, and soil pore water at depths of 20 and 40 cm in tropical typical Eucalyptus urophylla (EU) and Acacia auriculiformis (AA) plantations. The dissolved organic carbon concentrations were generally higher in the EU plantation than in the AA plantation and higher in stemflow than in other sample types. The greatest DOM degradation occurred in soil pore water and the least degradation occurred in stemflow. Similarly, the common photo-transformation of DOM, including decreased aromaticity and unsaturation degrees, was greatest for soil pore water and the least for stemflow DOM. Most of the photochemical products were high H/C and O/C molecules for throughfall, stemflow, and pore water, but high H/C and low O/C molecules for runoff because of strong decarboxylation. These findings reveal the highly varied molecular composition and photo-degradability of DOM from different terrestrial sources and highlight that stemflow is an important contributor to photo-resistant DOM among terrestrial DOM sources.