Sustainability of traditional pastoral fires in highlands under global change: Effects on soil function and nutrient cycling

In Europe, rural depopulation and the abandonment of pastoral practices in mountain areas trigger deep changes in the landscape, which result in the accumulation of lignified fuels and the increased risk of fires, a sensitive issue in southern areas of the continent. From prehistory, a pyric herbivory has been practiced in European mountain regions. Pastoral fires created open communities, amenable to wild and domestic grazing, and herbivores perpetuated them by controlling fuel accumulation. In the last decades, the declining of extensive herbivory has given a prominent role to prescribed fires in order to preserve open communities. As a consequence, a new scenario of increased burning frequency and reduced herbivory emerges, which may affect the soil function in different ways. Our aim was to evaluate the effects of experimental burnings on soil function and nutrients cycling of a mountain gorse (Ulex gallii Planch.) shrubland, with the absence/presence of extensive grazing. We performed traditional “bush-to-bush” burnings in three experimental mountain plots and analysed seasonally along a 2-year period the soil function in relation to C-cycle (dissolved organic C, microbial biomass C and glucosidase activity), N-cycle (inorganicN forms, dissolved organic N, microbial biomass N and urease activity), P-cycle (phosphatase activity) and overall bacterial catabolic activity. Fire effects were time dependent and extensive grazing had a low influence on them. Fire originated a transient pulse of inorganic-N forms in the short term, which disappeared after 1year, and increased dissolved organic N forms, which attenuated with time. In burned areas, a decrease of total N and microbial biomass N, and a slow-down of N- and P-cycle enzymatic activities were observed in the mid-term coinciding with a decrease of soil moisture. Since a higher burning frequency is a feasible situation that may affect mountain, nutrient-poor soils, the enduring effects of prescribed fires need to be taken into account to establish the optimal date of burning and the adequate recurrence regime that avoid negative impacts on the soil function and minimize the loss of nutrients from the soil reservoir.