Prescribed burning of scrubland only affects very sensitive topsoil properties: A decadal-scale study in encroached grasslands of the southern Pyrenees

The decrease in grazing activity during the last century is causing an encroachment of the subclimatic grasslands in most European mountains, accelerated by climate change. To control this expansion, prescribed burnings (PBs) are used. However, their use can affect sensitive soil properties, particularly soil microbiology. Although many studies have already reported the immediate effects of shrub PBs on soils, very few of them have studied the long-term post-fire recovery. Therefore, the purpose of this study is to evaluate the long-term effects of the PBs on the topsoil biochemical properties over time, at a cm-scale (0–1, 1–2, 2–3 cm). The soil sampling was carried out in Asín de Broto (Central Pyrenees, Spain) and four treatments, depending on the time-since-fire, were selected: unburned (UB), immediately post-fire (B0), 5 (B5) and 9 (B9) years after burning. The results showed an immediate increase (B0) in chemical properties because of the ash incorporation, highlighting values 3.35, 1.64 and 1.55 times higher than in the UB samples for the electrical conductivity, the dissolved organic C and the total N, respectively. However, these increases were no longer noticeable after the mid-term (B5, B9). Regarding the biological properties, β-D-glucosidase (GLU) activity suffered a significant decrease over time (67 and 72 % less in B5 and B9, respectively), without affecting either the microbial biomass C or its activity. This study has proven that GLU is a useful indicator that can be used to assess the effects of low-severity fires. • Prescribed burns effects on the soil have been studied after 0, 5 and 9 years • The prescribed burns immediately increased the pH, EC, SOC, DOC, and N of the soil • Microbial activity (bSR) was not altered by either burning or time • Glucosidase, unaffected at year 0, was reduced both at 5 and 9 years after the fire • Differences in fire effects among soil depths were only statistically found for DOC.