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 β-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.publishe

Effects of prescribed burning on soil organic C, aggregate stability and water repellency in a subalpine shrubland: Variations among sieve fractions and depths

Soil organic matter, aggregation and water repellency are relevant interrelated soil properties that can be affected by fire. The aim of this work was to analyse the effects of shrub prescribed burning for pasture reclamation on the soil aggregate stability, organic carbon and water repellency of different soil depths and aggregate sizes in a subalpine environment. Soil samples were collected from an area treated by an autumnal low-intensity prescribed fire in the Central Pyrenees (NE-Spain) at 0-1, 1-2, 2-3 and 3-5 cm depths just before and ~1 hour, 6 months and 12 months after burning. Samples were separated as whole soil (<10 mm) and 6 sieve fractions, <0.25, 0.25-0.5, 0.5-1, 1-2, 2-4 and 4-10 mm. We analysed soil organic C (SOC), aggregate stability (AS) and soil water repellency (SWR). In the unburned samples, SOC and SWR were higher in the <0.25 to 2 mm sieve fractions than the 2 to 10 mm sieve fractions. Fire severely and significantly decreased the SOC content in the whole soil and the <0.25 mm fraction at 0-1 cm depth and in the 0.25-0.5 mm fraction at 0-2 cm depth. SWR was reduced by burning mainly at 0-1 cm depth for the whole soil and the <0.25 to 2 mm sieve fractions. Nevertheless, the AS of the 0.25-0.5 mm aggregates increased after fire, while the rest of the sieve fractions remained virtually unaffected. One year after the prescribed burning, SOC slightly increased and SWR recovered in the fire-affected fractions, while the AS for all aggregate sizes and depths showed a considerable decrease. The results suggest that the direct effects of burning are still present one year after burning, and the post-fire situation may pose an increased risk of soil loss. Furthermore, our results indicate that fine soil fractions are more likely to be affected by fire than coarser soil fractions and highly influence the whole soil behaviour

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 0–1 cm, 1–2 cm and 2–3 cm depth in triplicate just before (U), ~ 1 h (B0), 6 months (B6) and 12 months (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 1 cm depth only 31 °C were reached. Burning significantly decreased TOC (- 52%), TN (- 44%), Cmic (- 57%), SR (- 72%) and ß-D-glucosidase (- 66%) at 0–1 cm depth while SR was also reduced (- 45%) at 1–2 cm 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

Changes in pools of organic matter and major elements in the soil following prescribed pastoral burning in the central Pyrenees

High-mountain soils are rich in partially decomposed organic matter, which is highly sensitive to mineralization and fire. Prescribed burning is performed in the Pyrenees to keep subalpine grasslands open for grazing. The compositions of the ash, litter and duff layers, and the particulate organic matter (POM) of the topsoil in the 0–1, 1–2, 2–3, and 3–5 cm depths were analyzed in relation to the nutrient availability after the prescribed burning of a stand encroached by erizón (Echinospartum horridum). The concentrations of C, N, P, and S and organic components (nonstructural, hemicellulose, cellulose, and lignin-type) were determined before the prescribed burn and 0, 6, 12, 18, and 24 months after the prescribed burn. The fire consumed the aboveground biomass, the litter and part of the duff layer, and the most thermostable (i.e., lignin-type) components and the least volatile elements (P, S) were selectively preserved in the resulting ash. Prescribed burning caused significant losses of organic-C and N only in the 0–1 cm depth (–72% and –68%, respectively). The organic-C loss was mostly (82%) from the POM, whereas the N loss was from more similar proportions of the POM (57%) and the nonparticulate organic matter (NPOM) (43%). However, few changes were observed in the composition of the organic matter, which pointed to a largely uneven combustion that resulted in a substantial part of the organic matter remaining largely untouched by the fire. After 6 months, the duff layer was depleted in hemicellulose by 32% compared to immediately after the burn, and fragmentation of the POM into the NPOM was observed. During the second spring, N- and P-rich charred POM were incorporated into the top 1 cm, while C-rich charcoal particles underwent fragmentation and vertical transport into the deeper soil. The preburn ecosytem was limited by P, and likely also by S. The plant available N showed transient increases of 1.5–2.1 times the immediate postburn levels for nitrate-N at 12 months after burning, and of up to 10–20 times for ammonium-N at 18 months. In contrast, the concentrations of plant-available P and S gradually declined to 1.8–3.3 and 1.8–4.0 times, respectively, lower at 24 months after the burn. Results indicated that fire-induced increases in the nutrient availability can be short-lived in high-mountain habitats, but steadier and likely more persistent nutrient inputs can derive from the gradual breakdown of charred organic matter. © 2021 Elsevier B.V

Immediate effects of prescribed burning on C-related topsoil properties in Central Pyrenees

Prescribed burning, i.e. the deliberate use of fire under specific conditions, is a management tool for recovering pasturelands affected by shrub encroachment. The objective of this work is to determine the immediate effects of prescribed burning on topsoil properties related to C cycle in soils covered with dense scrubs (Echinospartum horridum) in Tella (Huesca, Central Pyrenees, Spain). Soils were sampled in triplicate immediately before and after burning at 0-1 cm, 1-2 cm and 2-3 cm depths. We analysed the content of total oxidizable C (TOC); the content and mineralization rates of labile and recalcitrant C pools C, as inferred from incubation assays (141 days); microbial biomass C (MBC); and the ß-D-glucosidase (GL) activity. All studied soil properties were significantly affected by fire, varying in terms of intensity and affected depth. Fire produced a significant decrease in TOC (-41% on average), similarly affecting the upper 3 cm of soil. The content of labile C decreased considerably (-87% on average) at depths up to 3 cm, but its mineralization rate increased (+ 150% on average). The MBC was particularly affected at 0-1 cm (-53%), while GL activity showed significant decreases throughout the upper 3 cm (-49% on average). These results show a strong impact on the studied soil properties just after burning. Monitoring the evolution of these soils is necessary to assess their resilience in the short and medium terms, and check the sustainability of controlled burning for pasture management in the Pyrenees. Keywords. Pastureland – Controlled fire – Soil organic matter – Soil biological activity – Carbon mineralization. Effets immédiats du brûlage dirigé sur des propriétés du sol superficiel liées au C dans les Pyrénées Centrales Résumé. Le brûlage dirigé est un outil de gestion permettant le rétablissement des pâturages touchés par l'embroussaillement. L'objectif de ce travail est de déterminer les effets immédiats du brûlage dirigé sur les propriétés du sol superficiel liées au cycle du C dans des sols couverts par le genêt hérissé (Echinospartum horridum) à Tella (Huesca, Espagne). Les sols ont été échantillonnés en triple avant et après un brûlage sur 0-1 cm, 1-2 cm et 2-3 cm de profondeur. Nous avons analysé la teneuren C oxydable total sur sol (COT); la teneur etle taux de minéralisation du C labile et récalcitrant, déduit à partir de tests d'incubation (141 jours); le C issu de la biomasse microbienne (CBM); l'activité des ß-D-glucosidase (GL). Toutes les propriétés du sol étudiées ont été significativement affectées par le feu, variant en termes d'intensité et de profondeur affectée. Le feu a produit une diminution significative du COT (-41% en moyenne) et identique dans les premiers 3 cm de sol.Le contenu de C labile a considérablement diminué (-87% en moyenne) jusqu'à 3 cm, mais son taux de minéralisation a augmenté (+ 150% en moyenne). Le CBM a été particulièrement touché sur 0-1 cm (-53%), tandis que l'activité des GL a montré des diminutions significatives sur les premiers 3 cm de sol (-49% en moyenne). Ces résultats montrent un fort impact du brûlage sur les propriétés des sols étudiés. Le suivi de l'évolution de ces sols est nécessaire afin d'évaluer leur capacité de résilience dans les court et moyen termes et de vérifier la viabilité du brûlage dirigé pour la gestion des pâturages dans les Pyrénée

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