Persistence of wildfire effects on soil colour: implications for sediment tracing in burned catchments

Abstract

[eng] Purpose: This study analyses the capacity of soil colour parameters to discriminate sediment origin in terms of channel banks, burned surface soil and unburned surface soil in a fire-affected catchment. The main objectives were to assess the persistence of fire effects on soil and sediment colour 11 years after a severe wildfire, testing their ability as long-term tracers in burned catchments. Methods: Soil colour parameters were analysed using a resampling approach, based on soil samples collected from the same locations in 2013 and 2024. The discriminative power of the two source sample sets was tested using artificial mixtures created with 2013 and 2024 soil samples. Finally, 25 suspended sediment samples collected throughout the study period 2013–2024 were unmixed by using MixSIAR, encompassing the 2013 and 2024 sources separately for comparing results. Results: Significant colour shifts were detected in burned surface soils (ΔE = 11.83), with moderate changes in channel banks (ΔE = 4.91) and minimal variation in unburned soils (ΔE = 1.74). These changes were correlated with total carbon concentration. Source discrimination tests showed greater accuracy using the 2013 dataset. MixSIAR results for 25 suspended sediment samples indicated that the 2024 source data failed accuracy tests in the upstream sub-catchment site but performed similarly to the 2013 data in the downstream site. Conclusion: The soil colour at the catchment showed significant variations due to ash exhaustion. These changes reduce the ability to distinguish between unburned and burned sources. It is therefore necessary to reappraise the categories of potential sediment sources.