Assessment of post-wildfire soil quality and its recovery in semi-arid upland rangelands in Central Iran through selecting the minimum data set and quantitative soil quality index
Introduction
Like global climate change, fire is an important disturbance in natural ecosystems (Snyman, 2015a, Fultz et al., 2016, Neary, 2019), often driving the processes of soil erosion, degradation and desertification worldwide (Hubbert et al., 2012, Pereira et al., 2018, Neary, 2019, Stavi, 2019). Historically, fire is considered a major ecological disturbance factor in fire-prone grasslands/rangelands, which can reshape the interactions/feedbacks between aboveground and belowground communities (Johnson and Matchett, 2001, Fynn et al., 2003, Harris et al., 2008), and affects critical soil functions for the maintenance of ecosystem services (Zhou et al., 2009, San Emeterio et al., 2016, Fultz et al., 2016, Alcañiz et al., 2018).
Soil properties and processes are often negatively affected by wildfires or prescribed fires both directly through heating and indirectly through burning and destruction of plant communities in grassland/rangeland ecosystems (Harris et al., 2007, Snyman, 2015b, Girona-García et al., 2019). The immediate and direct effects of fire are usually short-term and are restricted to the first few centimeters of the topsoil (Snyman, 2015b, Pereira et al., 2018, Girona-García et al., 2019). The indirect influence of fire in rangelands is the partial or total loss of plant and litter biomass, as well as changes in plant community composition and structure (Teague et al., 2008, Vermeire et al., 2014, Snyman, 2015a, Clark et al., 2018). In grassland and shrubland ecosystems, fire reduces soil organic matter (SOM) pools and substrate availability for the microbial community by decreasing carbon (C) inputs into the soil (Rice and Owensby, 2001, Fynn et al., 2003, Fultz et al., 2016, San Emeterio et al., 2016, Girona-García et al., 2019). Several studies have demonstrated a significant influence of fire on numerous soil properties, with burnt grassland and pasture soils frequently having less C, nitrogen (N), moisture, aggregate stability, and consequently the conditions are worse for the microbial life (Liu et al., 2000, Fynn et al., 2003, Snyman, 2003, Mills and Fey, 2004, Fultz et al., 2016, Alcañiz et al., 2018). Soil microbial communities (activity, biomass, composition) can be affected by fire through its short- and long-term effects on physical and chemical properties of the soil, such as pH, moisture, SOM and structure (Fynn et al., 2003, Muñoz-Rojas et al., 2016, Alcañiz et al., 2018, Girona-García et al., 2019, Pressler et al., 2019, Hinojosa et al., 2019). Fire can negatively affect soil microbial and biochemical functions by changing the composition of microbiota, substrate availability, microbial activity, microbial biomass and enzymatic activity (Zhou et al., 2009, Muñoz-Rojas et al., 2016, San Emeterio et al., 2016, Hinojosa et al., 2019, Pressler et al., 2019).
Although the effects of fire on individual soil properties were widely studied, the integration of soil variables into an overall soil quality index (SQI) has not been carried in burnt areas, particularly in fire-prone semi-arid rangelands. Soil quality was defined as the capacity of a specific kind of soil to function within ecosystem boundaries, to sustain plant and animal productivity, to maintain or enhance water and air quality, and to support human health and habitation in both natural and managed ecosystems (Doran and Parkin, 1994, Karlen et al., 2001). It is a theoretical concept that integrates a combination of different soil physical, chemical and biological properties into a single index value to assess soil resources, sustainable land management practices and ecosystem sustainability (Doran and Parkin, 1994, Karlen et al., 2001, Andrews et al., 2002, Carter, 2002). Soil quality assessment characterizes the ecological functions of a given soil by selecting dynamic soil properties, which are considered as indicators of soil quality (Doran and Parkin, 1994, Andrews et al., 2002, Burns et al., 2006). The selected indicators of soil quality should be sensitive enough to detect changes in soil functions, and should be rapid, simple and cheap to quantify (Karlen et al., 1997, Gregorich et al., 1994, Carter, 2002, Burns et al., 2006). Characteristically, the soil properties are often “interdependent” but might also respond differently or have different sensitivities to a disturbance factor or management practices (Li et al., 2013, Raiesi and Kabiri, 2016, Bünemann et al., 2018). Development of a SQI is, therefore, an important tool to select and integrate a set of different soil properties into an overall index value, and to determine the soil response to disturbances and management practices (Andrews et al., 2004, Paz-Kagan et al., 2014, Bünemann et al., 2018). This tool is also useful to identify a general pattern for the effect of natural and anthropogenic disturbances such as fire on soil functions (Adeyolanu et al., 2013, Wang and Fu, 2020). However, to the best of our knowledge, very few studies used integrative SQI to study and determine the effect of wildfire on soil quality of natural ecosystems (Adeyolanu et al., 2013, Wang and Fu, 2020). Specifically, the impact of fire on soil functions and quality using integrated soil quality indicators in fire-prone semi-arid rangeland ecosystems of Central Iran and other regions in the world has not been explored to date.
In Iran, native rangelands, covering about 55% of the country area, face critical soil degradation and subsequent low productivity due to the depletion of SOM and nutrients as well as deterioration of soil structure by land-use changes and heavy grazing pressure (Ghorbani et al., 2012, Raiesi, 2017). The incidence of accidental or unplanned fire events at a large scale might also contribute to a decline in soil fertility and quality of the semi-arid rangelands throughout the country. Increasing wildfire occurrence can escalate the vulnerability of landscape to extreme flooding and erosion events due to the reduced plant cover, aboveground biomass, loss of surface SOM and accelerated runoff after fire (Hubbert et al., 2012, Stavi, 2019), especially in arid regions with poor vegetation cover. Therefore, occasional wildfires would make rangeland ecosystems more susceptible to desertification. Furthermore, the negative effect of fire could be aggravated by climatic conditions, especially drought (Hinojosa et al., 2019, Sadeghifar et al., 2019), and thus the post-fire recovery of soil might take more time in arid and semi-arid environments. Future climate change is also expected to increase wildfire frequency and intensity in many ecosystems (Macias Fauria et al., 2011). Considering the importance of maintaining soil quality for a sustainable use of rangeland ecosystems, the objective of this study was to develop a SQI for assessing soil quality status based on chemical and microbial properties two years after a wildfire. Our key research question is how and to what extent fire influences soil quality in semi-arid rangeland ecosystems, in particular the soil microbial and biochemical functions. Here we tested the main hypothesis that soil quality would decline after a fire and that post-fire recovery soil quality would be slow in these rangelands. In the current study, an integrative SQI was used to determine the effects of wildfire on soil quality based on 22 soil properties. The quantitative assessment of soil quality would have important implications for resource management and may be of particular importance for both land managers and policy makers to assess the sustainability of management practices after disturbance by fire.
Section snippets
Description of the study area and sites
The six sites selected in this study were located in an area (35 km × 25 km) southwest of Shahrekord city, West-Central Iran (Fig. 1). The climate of the study area is semi-arid (BS, according to Köppen climate classification) with a range of mean annual rainfall of 380–560 mm and mean annual temperature of 10.5–11.6 °C, with large inter-annual variability. The area is characterized by a mountainous topography over an altitude range of 2174–2675 m above sea level. The topography of the study
Effects of fire on soil properties
Results showed that fire affected most soil properties and processes evaluated in this study, except silt and sand contents, BD, pH, CCE, qCO2, FR and BR (Table 3, Table 4). Soil OC, POC and TN contents were significantly (P < 0.001) lower in burnt than unburnt rangelands, while soil EC, AK and AP showed a reverse trend (Table 3). Several authors have reported that burnt soils had less OC, TN and labile C contents than unburnt soils in semi-arid rangelands/grasslands (Snyman, 2003, Fynn et al.,
Conclusions
A SQI could successfully be used to evaluate the impact of wildfire on soil quality status. Our study demonstrates that wildfire reduces soil quality of semi-arid rangelands in West-Central Iran. This finding is consistent with the hypothesis that soil quality is reduced and may recover following a fire perturbation, but the recovery (resilience) could be impaired by post-fire overgrazing in these semi-arid climates. Consequently, a longer recovery period is needed for the full recovery of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Financial support for this research was provided by Shahrekord University (under Grant/Award Number: 96GRN1M1932). We would like to thank the handling editor and the anonymous reviewers for their excellent comments and suggestions.
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