Pyric herbivory decreases soil denitrification despite increased nitrate availability in a temperate grassland.

Pyric herbivory, the combination of controlled burning and targeted grazing, is an effective strategy for restoring abandoned, shrub-encroached rangelands to open ecosystems. This practice may impact soil nitrogen pools by altering soil nitrification and denitrification rates, and may lead to an increase of nitrogen losses through nitrate leaching and N-gas emissions. This research, located in the south-western Pyrenees, investigated the effects of pyric herbivory on soil nitrification and denitrification potentials and mineral nitrogen content in a gorse-encroached temperate rangeland six months after the burning was implemented. The study included three treatments: high-severity burning plus grazing, low-severity burning plus grazing, and unburned and ungrazed areas (control). We measured soil nitrification and denitrification potentials (net and gross), the limitation of denitrifiers by nitrogen or organic carbon, and the abundance of nitrite- and nitrous oxide-reducing bacteria. Additional soil and vegetation data complemented these measurements. Results showed that pyric herbivory did not significantly affect nitrification potential, which was low and highly variable. However, it decreased gross denitrification potential and nitrous oxide reduction to dinitrogen in high-severely burned areas compared to the control. Denitrification rates directly correlated with microbial biomass nitrogen, soil organic carbon, soil water content and abundance of nirS-harbouring bacteria. Contrary to the expected, soil nitrate availability did not directly influence denitrification despite being highest in burned areas. Overall, the study suggests that pyric herbivory does not significantly affect mid-term nitrification rates in temperate open ecosystems, but may decrease denitrification rates in intensely burned areas. These findings highlight the importance of assessing the potential impacts of land management practices, such as pyric herbivory, on soil nutrient cycling and ecosystem functioning.

Disruption of Traditional Grazing and Fire Regimes Shape the Fungal Endophyte Assemblages of the Tall-Grass Brachypodium rupestre.

The plant microbiome is likely to play a key role in the resilience of communities to the global climate change. This research analyses the culturable fungal mycobiota of Brachypodium rupestre across a sharp gradient of disturbance caused by an intense, anthropogenic fire regime. This factor has dramatic consequences for the community composition and diversity of high-altitude grasslands in the Pyrenees. Plants were sampled at six sites, and the fungal assemblages of shoots, rhizomes, and roots were characterized by culture-dependent techniques. Compared to other co-occurring grasses, B. rupestre hosted a poorer mycobiome which consisted of many rare species and a few core species that differed between aerial and belowground tissues. Recurrent burnings did not affect the diversity of the endophyte assemblages, but the percentages of infection of two core species -Omnidemptus graminis and Lachnum sp. -increased significantly. The patterns observed might be explained by (1) the capacity to survive in belowground tissues during winter and rapidly spread to the shoots when the grass starts its spring growth (O. graminis), and (2) the location in belowground tissues and its resistance to stress (Lachnum sp.). Future work should address whether the enhanced taxa have a role in the expansive success of B. rupestre in these anthropized environments.

Decoupling of traditional burnings and grazing regimes alters plant diversity and dominant species competition in high-mountain grasslands.

Over millennia, the combination of controlled burnings and extensive grazing has maintained mosaic landscapes and preserved mountain grasslands in southern Europe. In the last century, deep socio-economic changes have led to an abandonment of traditional uses, to a general decline of the domestic herbivory and to a misuse of burning practices. This study aims to quantify how the decoupling of burning and grazing regimes affects in the long-term the structure, diversity and dynamics of high-mountain, shrub-encroached grasslands. In spring 2012, four treatments (burned-grazed, burned-ungrazed, unburned-grazed and unburned-ungrazed) were set up at three sites in the Special Area of Conservation Roncesvalles-Selva de Irati, in southwest Pyrenees. During seven years, we monitored floristic composition and the height of the native tall-grass Brachypodium rupestre in four plots at each site. In the burned plots, we surveyed the resprout of the dominant shrub Ulex gallii and the dynamics of recovering of the herbaceous vegetation. Plant communities evolved differently in grazed and ungrazed plots. Extensive grazing, despite being lower than in previous decades, maintained plant diversity and limited shrub encroachment. The total absence of grazing fostered the encroachment of U. gallii at two sites and the expansion of B. rupestre at the other site. When B. rupestre cover was >60%, the encroachment of U. gallii was reduced. This research highlights the competition that occurs between shrubs and tall-grasses in the absence of grazing, and the modulating effect exerted by the burnings and the site-specific features. Understanding local plant dynamics is the first step to design the most appropriate practices that help to preserve diversity at the landscape and the community level in high-mountain grasslands of south Europe.

Changes in soil nitrogen dynamics caused by prescribed fires in dense gorse lands in SW Pyrenees.

Rural depopulation, abandonment of traditional land uses and decrease of extensive stockfarming is accelerating shrub encroachment in mountain areas. In NW Spain, gorse (Ulex gallii Planch.) is expanding, developing dense shrublands that accumulate high fuel-loads, ignite easily and persist during long periods as alternate stable states. Under this scenario, traditional bush-to-bush farming fires are being replaced by high fuel-load burnings performed by specialised teams to reduce fuels and promote mosaic landscapes. This research analyses the effects on soil function and nitrogen (N)-cycling of these new generation of prescribed fires practiced under similar conditions to traditional fires (winter time, moist soils), but differing in the biomass and the continuity of the surface burnt. The results showed significant changes in N-cycle parameters, such as increases in inorganic N and dissolved organic nitrogen (DON), but declines in N microbial biomass and urease activity. At the ecosystem level, potential N losses were high because the pulse of water-soluble forms, DON and nitrate, following fire overlaps with periods of low biological N retention by microorganisms and plants. Although most effects were similar to those observed in traditional burnings done in the same region, the primary concern is the high potential for DON losses following prescribed burning in highly gorse-encroached areas. In N-limited ecosystems, a crucial issue is to attain an equilibrium between frequent burnings, which may prevent an optimal recovery of the soil function, and uneven burnings, which burn high amounts of accumulated fuel and increase the risk of removing large quantities of dissolved N from the ecosystem in a unique fire event. Overall, the use of different techniques combined with fire are needed to promote and consolidate desired changes in dense gorse lands.