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Changes in soil properties after prescribed burning for pasture restoration in shrublands of the Central Pyrenees (NE-Spain)
Abstract
ISSN: 2254-7606
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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.
Woody encroachment has been an ongoing process in the subalpine belt of Mediterranean mountains, after land abandonment, the disappearance of the transhumant system and the decrease of the livestock number. The main objectives of this study were: (i) to identify land use/land cover (LULC) changes from 1956 to 2015, and (ii) to investigate the effects of LULC changes in physical and chemical soil properties and soil organic carbon (SOC) and nitrogen (N) stocks. It is hypothesized that woody encroachment in the subalpine belt may lead to significant changes in soil properties, and will generate an increase in the SOC stocks. A land use gradient was identified in the subalpine belt of the Central Spanish Pyrenees: (i) subalpine grasslands, (ii) shrublands, (iii) young forests, and (iv) old forests. Mineral soil samples were collected every 10 cm, down to 40 cm, at three points per each LULC and a total of 48 samples were analyzed. The results showed that (i) woody encroachment has occurred from 1956 to 2015 due to the expansion of coniferous forests and shrublands (at the expense of grasslands), (ii) land cover and soil depth had significant effects on soil properties (except for pH), being larger in the uppermost 0–10 cm depth, (iii) SOC and N contents and stocks were higher in the grassland sites, and (iv) the woody encroachment process initially produced a decrease in the SOC stocks (shrublands), but no differences were observed considering the complete soil profile between grasslands and young and old forests. Further studies, describing SOC stabilization and quantifying above-ground carbon (shrub and tree biomass) are required.
Soil organic matter, aggregation and water repellency are relevant interrelated soil properties that can be af- fected 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 pre- scribed fire in the Central Pyrenees (NE-Spain) at 0–1, 1–2, 2–3 and 3–5 cm depths just before and ~1h, 6 months and 12 months after burning. Samples were separated as whole soil (<10mm) and 6 sieve frac- tions,<0.25, 0.25–0.5, 0.5–1, 1–2, 2–4 and 4–10mm. 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 2mm sieve fractions than the 2 to 10mm sieve fractions. Fire severely and significantly decreased the SOC content in the whole soil and the<0.25mmfraction at 0–1cm depth and in the 0.25–0.5mmfraction at 0–2cm depth. SWR was reduced by burning mainly at 0–1cm depth for the whole soil and the<0.25 to 2mm sieve fractions. Nevertheless, the AS of the 0.25–0.5mm 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
We determined the short and medium‐term effects of prescribed burns on soil organic matter content and biological activity in grazing areas invaded by the shrub Echinospartum horridum (Vahl) Rothm. in the Pyrenees of Huesca (Spain). Soil samples were collected at 0‐1, 1‐2 and 2‐3 cm depths in triplicate just before the burn, immediately after the burn (T0), and one and five years later (T1, T5). We analysed the contents of total soil organic C (SOC) and N, soil respiration, microbial biomass C, and β‐D‐glucosidase and acid phosphatase activities. Fire provoked an immediate high decrease in the contents of SOC (‐40.2 %) and N (‐26.3 %) in the first 3 cm, which were even lower at T5 (‐50.3 % and ‐46.5 %, respectively). This can be explained as follows: (i) low incorporation of burned organic matter due to removal by wind and runoff; (ii) changes in microclimate increasing soil temperature and enhancing mineralization; and (iii) a stimulating effect on decomposition due to the release of nutrients. Soil biological activity was affected at T0 in the first 3 cm (‐49.3 % glucosidase, ‐48.2 % phosphatase and ‐54.5 % respiration rate). Microbial biomass C content was also affected by fire at T0 (‐32.3 %) but was close to its initial value at T5. The results suggest that these soils are particularly sensitive to fire. Adjusting the frequency and intensity of the burns is necessary to minimize their impact on the soil and to ensure the suitability of this management practice.
Grassland vegetation is dominated by grasses, which covers one quarter of earth land, while legumes are the second most important species. Consumption of milk, meat, and eggs has substantially increased in past decades especially in developing countries. To meet the increasing demand of animal-based food, forage and feed production need to be increased simultaneously. Grasslands are the principal source of livestock feed, and management decisions play significant roles on the productivity and sustainability of grasslands. Animal grazing improves pasture productivity and long-term farm profitability. Hay and silage can provide nutrients to grazing animals when forage is limited. Grasslands provide habitat for wildlife and are considered an important source of carbon stored in the vegetation and soil. Grasslands significantly contribute to the climate and ecosystems and ultimately to the sustainability of the world.
Aims
Sub‐alpine grassland ecosystems have some of the highest biodiversity in Europe and constitute high‐value natural resources. These grasslands are under threat because of the abandonment of traditional agro‐pastoral activities and subsequent invasion by woody species. In the Central Pyrenees (Spain) several management techniques have been used to stop expansion of the highly encroaching shrub Echinospartum horridum . However, the ways in which these techniques affect recovery of sub‐alpine grasslands are poorly understood. The final goal of this study is to gain information about the effects of E. horridum management practices and provide recommendations for the local stakeholders.
Methods
This study evaluated the efficacy of controlled fires and mechanical removal of above‐ground vegetation in controlling expansion of E. horridum into sub‐alpine grasslands in the Central Pyrenees. E. horridum demography (germination and survival), soil seed bank and soil properties were recorded in two E. horridum stands where vegetation was previously removed by (1) fire (burning treatment) or (2) mechanical removal (cutting treatment) and (3) an undisturbed E. horridum stand (control).
Results
The burning treatment increased germination and survival of E. horridum seedlings more than the cutting treatment, relative to the control. Therefore, cutting appeared to be a better option for controlling E. horridum . Soil seed density was higher in the management treatments than in the control, but it did not harbour sub‐alpine grasslands species. E. horridum removal favoured recharge of the soil with seeds that arrived by dispersal. The soil seed bank in the burning treatment had higher seed abundance and seed diversity than in the cutting treatment; however, fire promoted a loss of soil nutrients.
Conclusions
The soil seed bank composition (low abundance and diversity of native species) coupled with rapid regeneration rate of E. horridum would prevent recovery of the sub‐alpine grassland based on the soil seed bank alone. Traditional shepherding has been reported to favour seed dispersal, and here we recommend E. horridum removal with cutting treatment as an additional practice for integrated management and recovery of the sub‐alpine grasslands.
This study examines the direct impact of a moderate/high-severity prescribed fire on phosphorous (P) stocks and partitioning in oligotrophic soils of a dry eucalypt forest within Sydney's water supply catchments, Australia. We also quantify and characterize the P present in the ash produced in this fire, and explore its relationships with the maximum temperatures recorded in the litter layer during the burn.
In these oligotrophic soils, P concentrations were already relatively low before the fire (< 130 mg kg− 1, mainly in organic forms). The fire consumed the entire litter layer and the thin Oa soil horizon, creating 6.3 ± 3.1 t ha− 1 of ash, and resulted into direct net P losses of ~ 7 kg ha− 1. The P lost was mostly organic and there was a moderate net gain of inorganic and non-reactive P forms. Importantly, only a small proportion of the post-fire P was bioavailable (equivalent to ~ 3% of the total P lost during fire). Higher total P concentrations in ash corresponded with higher maximum temperatures (> 650 °C) recorded in the burning litter layer, but effects of fire temperature on ash P partitioning were not significant.
Fire not only transformed P chemically, but also physically. Our results show that, immediately after fire, up to 2 kg ha− 1 of P was present in the ash layer and, therefore, highly erodible and susceptible to be transported off-site by wind- and water erosion. Even if most of this P was, initially, of low bioavailability, its transfer to depositional environments with different geochemical conditions (e.g. anoxic sediments in water reservoirs) can alter its geochemical forms and availability. Further investigation of potential P transformations off-site is therefore essential, particularly given that SE-Australian water supply catchments are subject to recurrent perturbation by prescribed fire and wildfires. The latter have already resulted in major algal blooms in water supply reservoirs.
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 β-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 β-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.
Soils constitute one of the most valuable resources on earth, especially because soil is renewable on human time scales. During the 20th century, a period marked by a widespread rural exodus and land abandonment, fire suppression policies were adopted facilitating the accumulation of fuel in forested areas, exacerbating the effects of wildfires, leading to severe degradation of soils. Prescribed fires emerged as an option for protecting forests and their soils from wildfires through the reduction of fuels levels. However such fires can serve other objectives, including stimulating the regeneration of a particular plant species, maintaining biological diversity or as a tool for recovering grasslands in encroached lands. This paper reviews studies examining the short- and long- term impacts of prescribed fires on the physical, chemical and biological soil properties; in so doing, it provides a summary of the benefits and drawbacks of this technique, to help determine if prescribed fires can be useful for managing the landscape. From the study conducted, we can affirm that prescribed fires affects soil properties but differ greatly depending on soil initial characteristics, vegetation or type of fire. Also, it is possible to see that soil's physical and biological properties are more strongly affected by prescribed fires than are its chemical properties. Finally, we conclude that prescribed fires clearly constitute a disturbance on the environment (positive, neutral or negative depending on the soil property studied), but most of the studies reviewed report a good recovery and their effects could be less pronounced than those of wildfires because of the limited soil heating and lower fire intensity and severity.