Prescribed fire application in a Brazilian mountain environment: Changes in soil organic matter quality in the short and medium term

Prescribed fire (PF) is a management practice used ecosystems where the occurrence of wildfires is high due to the accumulation of biomass on the surface. In general, studies dealing with fire-affected soils evidence the degradation of less thermo-labile structures, with the preservation of more thermo-stable phases. As studies in mountain environments in Brazil are scarce and considering that the practice of PF has been on the rise in recent years, the present study aimed to quantifying and characterizing the impact of PF on soil organic matter in the Itatiaia National Park, Rio de Janeiro-Brazil. In a 240 days period, we assessed the impact of PF on soil pH, base cations release, available P, C and N contents and stocks, soil organic matter (SOM) quality. Among other techniques, we applied ATR-FTIR and 13 C NMR spectroscopy. Soil chemical and physical attributes were also quantified. At 10 days, PF application caused the release of labile organic carbon (LOC) forms, inducing an increase in content of humic fractions in the form of C-FA and C-HA. The humification process, nevertheless, occurred at a lower speed than the incorporation of LOC. Humin contents, in fact, did not systematically increase with time. At 10 days following PF, a decrease in carbohydrates and peptides occurred, with greater HA aromaticity. After 240 days since PF, the structural characteristics of HA were restored to levels like the original (pre-PF) condition. At 120 days after PF, the pH value, base saturation, Mg, Na and P increased in soil. However, the amount of C and N accumulated in the soil did not increase with PF application. We observed that up to 120 days of PF, the mineralization process seems to occur in priority over humification. A period longer than 240 days seems anyway needed to restore soil C and N stocks.