Differences in dissolved organic matter (DOM) composition of soils from native eucalypt forests and exotic pine plantations impacted by wildfire in Southwest Australia

In recent decades, plantations of exotic species have transformed large tracts of forested landscapes, where they often form a mosaic with the surrounding native forest. Wildfires have also increased in frequency and intensity globally, impacting both plantation and native forests and affecting soil organic matter (SOM), including dissolved organic matter (DOM). DOM of soils is considered highly labile and likely sensitive to effects of fire and changes in land use. However, it is unclear how wildfire influences key characteristics of DOM in exotic plantations compared to native forest. In this study, we investigated the impacts of wildfire on chemical properties of DOM in exotic Pinus radiata plantations and adjacent native jarrah (Eucalyptus marginata) forest in southwest Australia. We coupled fluorescence excitation-emission matrix parallel factor analysis (EEM-PARAFAC) and ¹H-NMR with soil nutrient analyses to characterise soil DOM (0–5 cm) from burned and unburned plots of each forest type, four months after the fire. Four fluorescent components (humic-, fulvic-, microbial and protein-like substances) were derived from soil DOM using the PARAFAC model. DOM from burnt forest soils was predominantly of lower molecular weight (E2:E3) and contained a greater microbial-derived component and a lower fulvic component than unburnt forest. As expected, soil pH and labile inorganic P increased in both forest types after fire. By contrast, an increase in the amount of DOC after fire was only recorded in native forest, while δ¹⁵N signatures were more enriched in pine plantation soils after fire but not in native forest soils. Differences in impacts of fire on DOC and δ¹⁵N between forest types suggest a different response to fire according forest type, even though these differences were not evident above ground. The ¹H NMR spectra of DOM revealed all samples were largely dominated by aliphatic structures. However, there were no significant differences in the overall spectra between forest types or between burnt and unburnt forests. Overall, our findings suggest that transformations in soil DOM after fire were affected by feedbacks between forest type and fire (where soils from pine plantations appeared to be more affected by fire).