Understanding the influence of postfire oak afforestation on soil properties

The Mediterranean is a fire-prone region where fires have occurred for millennia. Since the late nineteenth century, restoration techniques like tree planting have been widely implemented following fire events. However, at soil level, postfire measures have been mostly focused on soil erosion processes. Depending on several factors, fire can cause more or less significant impacts on soils, such as changes in soil structure and in the availability of element concentrations, or loss of organic matter. Understanding fire impacts on soil is not only crucial for developing effective postfire rehabilitation strategies, but also to mitigate the long-term consequences on soil health and ecosystem functioning.

In our study, we evaluated the effect of postfire afforestation projects on soil characteristics in the long term. We studied 15 afforestation projects implemented in North-Centre Portugal in the period 1994-2006, in deciduous oak stands dominated by Quercus pyrenaica (including seven projects with pure oak stands and eight projects with mixed oak stands). For each project, we established a sampling plot and selected a nearby control area, affected by the same fire event but without oak afforestation or evident management. Fieldwork was conducted in 2021, when most projects (10) were between 12 and 17 years old, while the remaining projects (5) had been implemented between 21 and 25 years ago. One composite sample of superficial soil (0-5 cm of depth) was collected per plot, performing 15 soil samples in project areas and 15 samples in control areas. Each soil sample was characterized physicochemically for: proportion of fine/coarse fraction; pH(H₂O); organic Carbon (Corg); total N content; and available nutrients concentration. Posteriorly, we calculated two proxy variables: Soil Quality Index (SQI), which allows to evaluate the overall soil health condition; and the C/N ratio, as an indicator of organic matter mineralization process.

For both afforested and control areas, soils (mostly classified as Cambisols) presented pH values of approximately 5 (slight acidity), fine particles (< 2 mm) averaged between 67% and 69%, and no trace elements of enrichment. Soils from afforestation plots displayed higher K concentrations, while soils from control plots exhibited higher fertility levels based on Corg, N, and available P. No significant differences were observed in C/N ratios between afforested and non-afforested areas (14.9 vs 16.6), which indicates a relatively fast decomposition and N mineralization. Similarly, no significant differences were observed in the SQI. The lower Corg contents of afforested soils can be attributed to soil management (soil mobilization and management of the understory), which can contribute to the degradation of organic matter when the ecosystem is sensible due to the fire perturbation. Soil mobilization with disc harrowing was the predominant technique (40%). Further research should focus on understanding the effects of different postfire management options on soil properties over time, including in areas without evident enrichment/contamination problems, to improve postfire soil rehabilitation and sustainable forest management.

Acknowledgment: This research was supported by UID/AGR/04129/2020, UID/BIA/50027/2019, PD/BD/142963/2018 and PD/00157/2012.