Long-term evolution of shrub prescribed burning effects on topsoil organic matter and biological activity in the Central Pyrenees (NE-Spain).

Since the last half of the 20th Century, scrubs have been invading subclimatic grasslands in the montane and subalpine stages of Spain due to the decrease of the grazing activity. This shrub encroachment reduces biodiversity and the ecopastoral value of the region and leads to the accumulation of woody fuel, which represents a high fire risk. To control the encroachment, prescribed burnings are performed but their effects on soils over the years are still undetermined. This study aims to research about the long-term effects of a prescribed burn of Echinospartum horridum (Vahl) Roth. on topsoil organic matter and biological activity. Soil sampling was carried out in Tella-Sin (Central Pyrenees, Aragón, Spain) and four treatments were selected: unburned (UB), immediately burned (B0), burned 6 years before (B6, mid-term) and burned 10 years before (B10, long-term). Among the obtained results, an immediately after burning decrease on ß-D-glucosidase activity (GLU) was found, which did not recover over time. Other properties did not have an immediate reduction but did so over time: total soil organic carbon (SOC), labile carbon (DOC), total nitrogen (TN), basal soil respiration (bSR). And others were not affected at all: microbial biomass carbon (MBC), and the microbial metabolic quotient (qCO2). Moreover, the normalized soil respiration (nSR) increased with the time, which implies an acceleration of the potential mineralization of soil organic carbon. In short, although the elimination of the dense shrubs by fire has not entailed major immediate soil modifications, which would be typical of a low severity prescribed burn, several mid- and long-term effects in the C cycle have been observed. Future studies will have to discern what is the main cause of these modifications (soil microbial composition, edaphoclimatic changes, lack of soil cover and soil loss, soil fertility, etc.).

Fire does not transform shrublands of Echinospartum horridum (Vahl) Rothm. into grasslands in the Pyrenees: Development of community structure and nutritive value after single prescribed burns.

Prescribed fire has been extensively used in recent years to control woody encroachment into mountain and other grassland-dominated landscapes. In the Aragon Pyrenees, prescribed burns have been mainly used to remove the native thorny shrub Echinospartum horridum (Vahl) Rothm., whose populations are spreading to the detriment of grasslands. To study the effectiveness of the burning of E. horridum to preserve grasslands for livestock grazing, the vegetation of six sites burned 0.5, 2, 3, 6, 15 and 35 years ago was sampled and compared with that of nearby unburned shrubland (control) and grassland (reference). In addition, the nutritional quality of E. horridum was examined and compared to that of the reference grassland to evaluate to what extent shrub growth can be controlled by herbivores after burning. Initial shrub cover recovered as early as 15 years after fire, with E. horridum being dominant. Plant diversity was greatest at intermediate number of years after fire. Initial floristic composition and life-form spectrum were restored 15-35 years after burning. Echinospartum horridum exhibited early lignification that restricts its availability as a palatable forage for the first two years after burning and makes it unlikely to be consumed thereafter, highlighting the difficulty in controlling the expansion of this species by livestock herbivory. The analysis of the nutrient levels suggested an increased shortage of limiting elements, such as phosphorus or sulfur, in the mid-term after burning due to substantial nutrient losses and exports during and after the burn. Our results question the suitability and sustainability of a single prescribed burn as management tool alone to control the expansion of E. horridum and call for caution in its application for fighting shrub encroachment in the Central Pyrenees.

Soil-geomorphology relationships determine the distribution of the main subalpine grasslands in the Central Pyrenees (NE-Spain).

The two most widely extended mountain grasslands in Europe (Nardus-mat grasslands and chalk grasslands) are distributed forming complex patterns. In the Ordesa and Monte Perdido National Park (Central Pyrenees, NE-Spain), they grow as secondary pastures within the treeline ecotone at the subalpine belt. This work aims to show the influence of soil properties on the spatial distribution of these pastures, under a dynamic geomorphology. Soils are sampled under both grasslands, which grow on different cumulative levels: Nardus-mat grasslands in the upper level (L1) and chalk grasslands in the lower level (L2). Soils in L1 have a significantly higher acidity, lower soluble ions and exchangeable calcium content than those in L2, reflecting a more intense leaching process, consistent with a longer period of slope stability. Qualitative differences are detected in the soil organic matter of the soil, using carbon and nitrogen isotopes, lighter in L2 soils than in L1 soils, due to a higher proportion of legumes growing in L2 (chalk grasslands). Soils in L1 and L2 shared many physical properties, such as a fine and homogeneous texture in the whole profile (silty clay or silty clay loam), and high aggregate stability and porosity in the topsoil. In contrast, the soils in L2 are shallower than in L1, which reduces their water-holding capacity. The soil is classified as Orthoeutric Cambisol (Clayic, Humic) in L1 but its rejuvenation, by gully erosion, transforms it into an Hypereutric Leptosol (Loamic, Ochric) in L2 (Typic Haplocryept and Lithic Haplocryept, respectively by Soil Taxonomy system). Definitely, the distribution of both grasslands for the studied area is linked to two cumulative levels of different ages, which in turn is strongy related to different soil properties.

Plant-topsoil relationships underlying subalpine grassland patchiness.

Approximately half of the area in the Spanish Central Pyrenees is dedicated to pastures. A decrease in stocking rate coupled with changes in livestock management in recent decades have favoured the expansion of Nardus grasslands, which are considered undesirable for grazing use and for diversity conservation. The objective of this study was to analyse how topsoil properties are related to grassland plant composition occurring in erosion-disturbed (chalk grasslands) and undisturbed (Nardus mat-grasslands) soils in a subalpine area of the Spanish Central Pyrenees. We selected six paired sampling points for a side-by-side comparison of both communities. At each point, we 1) estimated the plant cover of each species through inventories and 2) analysed a set of physical-chemical topsoil properties (0-5 and 5-10 cm depth). Data were analysed through multivariate analysis. We found typical species of Nardus mat-grasslands in the undisturbed sites growing on non-eroded and well-structured soils that were low in calcium and acidic, with high contents of organic matter. In turn, we found earlier-successional grassland communities growing on slopes recently affected by soil erosion processes. The species composition was mainly species from stony slope grasslands and, to a lesser extent, from the long-term snow-covered environments of the high mountains. These soils were shallower and stonier and had a less-stable structure, higher pH, and lower organic matter and calcium content than undisturbed soils. Our results suggest that the differences between both communities emerge and are maintained by soil-plant feedback mechanisms mediated in Nardus mat-grasslands through soil stabilization and acidification and in chalk grasslands through soil erosion and basification. These findings suggest that the subalpine grassland mosaic results from a model of non-equilibrium plant coexistence due to soil disturbance and inexorable succession. Management should be focused on maintaining a disturbance regime, through grazing, sufficient to prevent the spreading of Nardus mat-grasslands.

Changes in soil organic matter composition after Scots pine afforestation in a native European beech forest revealed by analytical pyrolysis (Py-GC/MS).

The introduction of coniferous species in former deciduous forests may exert changes in soil organic matter, particularly in its molecular composition. In this work, pyrolysis-gas chromatography-mass spectrometry was used to study changes in SOM quality related to the centennial afforestation of Scots pine in an area formerly covered by European beech forest in the NE-flank of the Moncayo Natural Park (NE-Spain). For each soil profile three organic layers (fresh litter, fragmented litter and humified litter) and mineral soil horizons (Ah, E, Bhs and C) were studied. A total of 128 compounds were identified in the pyrograms, and composition differences were detected among the organic and mineral soil layers as well as between soils under beech and pine, for the main compound classes: nitrogen compounds, aromatics, lignin methoxyphenols, polycyclic aromatic hydrocarbons, lipids and polysaccharide-derived moieties. Such chemical differences were found to be derived from the biomass composition of the predominant vegetation type that was incorporated into the soil and from its progression into the soil profile. The analysis of the distribution of alkanes indicated higher SOM stabilization in the native beech forest soil. The signal of beech biomarkers (long chain n-alkanes C31-C33) found in the pine E horizon indicates the permanence of SOM derived from the natural forest ca. 100 years after the afforestation.

Dynamics of topsoil carbon stocks after prescribed burning for pasture restoration in shrublands of the Central Pyrenees (NE-Spain).

Prescribed burning has been recently readopted as a management practice in the Central Pyrenees (NE-Spain) to stop shrub encroachment processes and recover pasturelands. The immediate effects of prescribed burning on soil C stocks and related biological properties and their evolution in the short-to mid-term after burning were assessed. The study was conducted during three autumnal prescribed burnings in the Central Pyrenees in the municipalities of Buisán, Asín de Broto and Yebra de Basa. At each site, the topsoil Ah horizon was sampled at soil depths of 0-1, 1-2 and 2-3 cm immediately before and immediately after burning. Additionally, seasonal samplings were conducted every 6 months up to one year in the case of the Asín and Yebra sites and up to 24 months at the Buisán site. The total soil organic C stock (SOCS) total N stock (NS), microbial biomass C (MBC), soil basal respiration (SR) and ß-D-glucosidase activity were analyzed. The maximum temperatures recorded at the soil surface were 438 °C (Buisán), 768 °C (Asín) and 595 °C (Yebra). At the Buisán site, burning significantly decreased the SOCS (-52%), NS (-44%), MBC (-57%), SR (-72%) and glucosidase activity (-66%) at 0-1 cm depth, whereas fire had no direct effects on soil at the Asín and Yebra sites. The contrasting effects of burning on soil that were observed among sites were found to be related to differences in fire residence time. The prescribed fire at the Buisán site was on a plain slope under slow winds (<8 km h-1) at a burning rate of 0.64 ha h-1, which produced greater impacts on the soil properties than the burnings at the Asín and Yebra sites, where fire spread rapidly (2.72 and 1.43 ha h-1, respectively). At the Buisán site, the SOCS and NS recovered to the unburned values 24 months after burning. One year after burning, the SOCS at Asín were 60% higher than those of the unburned soils at 0-1 cm depth. At all sites a decreasing trend in soil biological activity in the short- and mid-term was observed. From the results it can be concluded that: 1) the direct effects of burning on soil are highly dependent on the environmental conditions, 2) in the mid-term, the reduction in soil biological activity and the incorporation of ashes and charred plant remains led to an increase in the SOCS of the burned soils.

Pyrogenic organic matter from palaeo-fires during the Holocene: A case study in a sequence of buried soils at the Central Ebro Basin (NE Spain).

We studied the fire record and its environmental consequences during the Holocene in the Central Ebro Basin. This region is very sensitive to environmental changes due to its semiarid conditions, lithological features and a continuous human presence during the past 6000 years. The study area is a 6 m buried sequence of polycyclic soils developed approximately 9500 years ago that is exceptionally well preserved and encompasses four sedimentary units. The content and size distribution of macroscopic charcoal fragments were determined throughout the soil sequence and the analysis of the composition of charcoal, litter and sediments via analytical pyrolysis (Py-GC/MS). The high amount of charcoal fragments recovered in most horizons highlights the fire frequencies since the beginning of the Neolithic, most of which were probably of anthropogenic origin. In some soil horizons where charcoal was not found, we detected a distribution pattern of lipid compounds that could be related to biomass burning. On the other hand, the low number of pyrolysates in the charcoal could be attributed to high-intensity fires. No clear pattern was found in the composition of pyrolysates related to the age of sediments or vegetation type. The most ancient soil (Unit 1) was the richest in charcoal content and contains a higher proportion of larger fragments (>4 mm), which is consistent with the burning of a relatively dense vegetation cover. This buried soil has been preserved in situ, probably due to the accumulation of sedimentary materials because of a high-intensity fire. In addition, the pyrogenic C in this soil has some plant markers that could indicate a low degree of transformation. In Units 2-4, both the amount of charcoals and the proportions of macrofragments >4 mm are lower than those in Unit 1, which coincides with a more open forest and the presence of shrubs and herbs. The preservation of this site is key to continuing with studies that contribute to a better assessment of the consequences of future disturbances, such as landscape transformation and climate change.

Effects of prescribed burning for pasture reclamation on soil chemical properties in subalpine shrublands of the Central Pyrenees (NE-Spain).

The abandonment of the traditional pastoral activities in the subalpine grasslands of the Central Pyrenees (NE-Spain) has resulted in shrub encroachment processes that are dominated by species such as the Echinospartum horridum. Therefore, prescribed burning has been recently readopted in this region as a management tool to stop the spread of shrubs and recover grasslands. We aimed to assess the effect that this practice may have on soil chemical properties such as SOC, N, pH, EC, water-extractable and exchangeable cations (Ca2+, Mg2+ and K+), cation exchange capacity, inorganic N forms (N-NH4+ and N-NO3-) and available P. We studied two prescribed burnings conducted at the subalpine level of the Central Pyrenees in the municipalities of Tella-Sin (April 2015) and Buisán (November 2015). At each site, the topsoil was sampled in triplicate at soil depths of 0-1, 1-2 and 2-3 cm immediately before (U), immediately after (B0) and one year after (B12) burning, and litter and/or ashes were removed prior to sampling. The results indicate that in the B0 samples, burning significantly reduced the SOC and N contents as well as the exchangeable Ca2+ and Mg2+ at 0-1 cm, whereas the rest of the studied properties remained virtually unchanged. However, in the B12 samples we detected a decrease of nutrient content that was probably related to leaching and/or erosion processes.

Effects of prescribed fire for pasture management on soil organic matter and biological properties: A 1-year study case in the Central Pyrenees.

Prescribed burning has been readopted in the last decade in the Central Pyrenees to stop the regression of subalpine grasslands in favour of shrublands, dominated among others by Echinospartum horridum (Vahl) Rothm. Nevertheless, the effect of this practice on soil properties is uncertain. The aim of this work was to analyse the effects of these burnings on topsoil organic matter and biological properties. Soil sampling was carried out in an autumnal prescribed fire in Buisán (NE-Spain, November 2015). Topsoil was sampled at 0-1cm, 1-2cm and 2-3cm depth in triplicate just before (U), ~1h (B0), 6months (B6) and 12months (B12) after burning. We analysed soil total organic C (TOC), total nitrogen (TN), microbial biomass C (Cmic), soil respiration (SR) and ß-D-glucosidase activity. A maximum temperature of 438°C was recorded at soil surface while at 1cm depth only 31°C were reached. Burning significantly decreased TOC (-52%), TN (-44%), Cmic (-57%), SR (-72%) and ß-D-glucosidase (-66%) at 0-1cm depth while SR was also reduced (-45%) at 1-2cm depth. In B6 and B12, no significant changes in these properties were observed as compared to B0. It can be concluded that the impact of prescribed burning has been significant and sustained over time, although limited to the first two topsoil centimetres.