Comparing photoactivities of dissolved organic matter released from rice straw return under varying alkali concentration: Insights from molecular structure and photoactive species

Alkali treatment can prevent soil acidification caused by rice straw returning to the field. However, the photochemical behavior of dissolved organic matter (DOM) released from rice straw after alkali addition and its underlying mechanism remain unclear. This study investigated the molecular composition and photochemically produced reactive intermediates (PPRIs) of DOM from rice straw extracted using water (CRDOM) and extracted from rice straw with varying alkali concentration (ARDOM). The experimental results indicate that alkali additive facilitates DOM release, ranging from 384.2 to 2054.4 mg C/L. Fourier transform ion cyclotron resonance mass spectrometry revealed the dominance of unsaturated and phenolic compounds in both CRDOM (63.6 %) and ARDOM (ranging from 55.8 % to 57.7 %). Under simulated solar irradiation, ARDOM-Low exhibited the highest apparent quantum yields (AQYs) for ³DOM* (54.13), ¹O₂ (1.52 × 10^-2) and ^•OH (4.46 × 10^-5). This phenomenon maybe attributed to the lower molecular weight and higher unsaturated compounds of ARDOM-Low. Additionally, as a representative pollutant in modern ecological agriculture park, sulfadimidine (SM2) was chosen to be investigated for its photodegradation by DOM. The results suggested that ARDOM-Low effectively facilitated the photodegradation of SM2, with ³DOM* as the main photoreactive species, providing evidence for higher AQYs of PPRIs facilitated by ARDOM-Low. This study provides new insights into understanding the molecular composition and photochemical processes of DOM released during rice straw return, thereby offering theoretical guidance for rice straw return practices in agriculture.