Elsevier

Geoderma

Volume 406, 15 January 2022, 115539
Geoderma

Soil and organic carbon redistribution in a recently burned Mediterranean hillslope affected by water erosion processes

https://doi.org/10.1016/j.geoderma.2021.115539Get rights and content

Highlights

  • SOC content was homogenized at hillslope positions in burned Mediterranean soils.

  • Burned soils have different distributions of aggregate mass (size and density) and SOC (density).

  • Four rains mobilised high sediment and SOC amounts (role of soil erosion as C sink).

  • OC was initially transported as non-protected (FLF > OLF > HF) and later as protected (HF > FLF > OLF).

Abstract

Forest fires cause many changes in the physical, chemical and biological soil properties such as aggregation and soil organic carbon contents (SOC) as well as on soil hydrology and erosion processes. Most studies on post-fire soil erosion in Mediterranean environments have been plot-based and research at hillslope or broader scale is scarce. Understanding SOC nature, distribution and modifications, as produced by forest fires and erosion, has become crucial to model and define the role of soil erosion as source or sink of C, and to sustainably manage ecosystem services related to the soil resource. This research provides data about the loss and redistribution of soil and SOC in a Mediterranean forest hillslope burned with high severity, at the Natural Park of Sierra de Espadán, Spain. Soil was sampled in coupled hillslopes (ca. 0.25 ha) (BU: burned, CO: control) at bottom (depositional), middle (transport) and top positions (eroding) at two depths (0–2 cm, 2–5 cm), and under two environments (UC: under canopy soil, BS: bare soil). Sediments were collected after each erosive event along one year, and yields were calculated. Samples were analysed to assess aggregate stability (AS), size and density fractionations, SOC contents and stocks. The main hypothesis is that fire affects soil characteristics related to aggregation and SOC stabilization and, together with erosion processes, may modify SOC distribution within aggregates and the burned hillslope.

Soils were in general very stable, but some differences in the results of the methods used were observed. Significant differences were found for the environment (under canopy vs bare) and soil depth but not for slope position. SOC content was high both at BU and CO with no significant differences. In the BU hillslope, a homogenization of SOC contents was observed along the hillslope, while in the CO, a higher SOC content was measured in the depositional and transport sites than in the eroding one. Similar trends were observed for SOC stocks. Only four erosive rain events were registered in this study, which generated no sediment yields in CO hillslope. In the BU one, sediment yields were measured (0.05–0.58 Mg ha−1, total 0.925 Mg ha y−1), which mobilised OC amounts ranging between 0.005 and 0.04 MgC ha−1. When samples were fractioned, changes were observed in the mass distribution of soil and sediment aggregates by size and density, and in the OC content between density fractions of BU soils with regard to sediment and CO soils.

According to the results, effective post-fire management should be oriented to control and reduce the erosion of aggregates < 2 mm, which present the highest SOC content and are very prone to be transported off-site. This fraction should include all the partially burned biomass (free light material), which acts as a first mulching and contains high amounts of OC that should be kept within the burned hillslope to increase soil fertility, promote vegetation recovery and act as a C sink. In the BU hillslope, eroded free light material might be buried at the depositional site and if the conditions are favourable for its conservation, SOC accumulation would be promoted, which may have implications for its stabilization, and the role of soil erosion as a C sink.

Introduction

Organic matter (OM) in soil is the largest and most dynamic reservoir of carbon (C) on Earth and, thus, a key factor in global carbon cycling (Cerli et al., 2012, Kirkels et al., 2014). Organic C is stabilized in soils through key mechanisms as physical isolation (occlusion), chemical interaction of OM with reactive soil minerals, and preservation of recalcitrant compounds (Berhe, 2012, von Lützow et al., 2006, Wang et al., 2014). These mechanisms relate to soil aggregation by providing physical and chemical protection of soil organic carbon (SOC) against decomposition, so that OM becomes non-accessible for soil microorganisms and fauna (Doetterl et al., 2016, Six et al., 2002), and by aggregate formation through SOC sorption to pedogenic metal oxides, clay minerals or by co-precipitation with polyvalent cations (Mikutta et al., 2006, Schmidt et al., 2011). It has been found that aggregate formation appears to be closely linked with SOC storage and stability (Hu and Kuhn, 2016, Wiesmeier et al., 2019). On the other hand, soil erosion can promote breakdown of aggregates at the eroding landform positions leading to exposure of previously protected SOC (Doetterl et al., 2012, Lal, 2003), which potentially increases its mineralization rate (Berhe, 2012, Zhang et al., 2006) and releases soluble compounds (Badía et al., 2014, Caon et al., 2014). However, transported and deposited SOC can be protected from decomposition if efficiently buried in slow turn-over environments, leading to large C sinks in colluvial and alluvial sediments depending on the rate of burial, the time since burial, the nature and amount of mobilized C, and the post-depositional conditions (Doetterl et al., 2016, Kirkels et al., 2014).

There are a number of disturbances affecting soil aggregation and SOC as changes in land use (Martínez-Mena et al., 2012, Wiesmeier et al., 2019) or forest fires (Shakesby, 2011, Shakesby et al., 2015), among others. Forest fires are considered one of the main causes of soil degradation in the European Mediterranean region (Caon et al., 2014, Garcia-Ruiz et al., 2013, Shakesby, 2011), affecting their physical, chemical and biological soil properties (Badía et al., 2014, Bento-Gonçalves et al., 2012, Campo et al., 2008), as well as an increase in water runoff and sediment loss (Campo et al., 2006, Cawson et al., 2012). Particularly important are the effects of forest fires in the first few centimetres of topsoil, in relation to changes in SOC quantity and quality (González-Pérez et al., 2004), aggregate stability and erosion processes (Mataix-Solera et al., 2011, Shakesby and Doerr, 2006).

Soil erosion disturbs topsoils and, preferentially, removes SOC from upslope places, resulting in the redistribution and burial of SOC in depositional environments (Martínez-Mena et al., 2012, Wang et al., 2014). There is a number of factors that can influence the net effect of transport and deposition on eroded SOC (and consequently on its fate) as the rate and nature of soil erosion, the amount and nature of the eroded C, soil texture, soil aggregation, the transport distance, and terrain attributes such as slope gradient and surface roughness (Doetterl et al., 2016). Slow but long-range transport may lead to a higher degree of decomposition of mobilized SOC, whereas fast but short-range transport might lead to the burial of mobilized SOC, with a lower degree of decomposition, at the depositional site (Berhe and Kleber, 2013, Kuhn et al., 2009, Quinton et al., 2010). At this zone, the rate of decomposition of eroded SOC can be reduced by a combination of processes. These are biochemical (recalcitrance of organic constituents mainly those associated to pyrogenic carbon), physical (protection with burial, aggregation, and changing water, air, and temperature conditions) or chemical (mineral-OM associations). As stated by Berhe et al. (2007), in this scenario, eroded SOC remaining near the surface of foothills could contribute to enhanced decomposition, higher mineralisation rates (Quinton et al., 2010, van Hemelryck et al., 2010, van Hemelryck et al., 2011), whereas the decomposition rate of buried C stocks is likely to be reduced. In general, initial sediment C-enriched will be buried by the sediments transported during subsequent rains (depending on their magnitude and on the frequency of episodic heavy rainfall events or floods) (Nadeu et al., 2012).

Redistribution of SOC can be affected by forest fires, which can increase or reduce SOC stocks depending on several factors (González-Pérez et al., 2004, Shakesby et al., 2015). The role of soil aggregation and stabilization in SOC dynamics during erosion and deposition has attracted scientific attention in recent decades (Nadeu et al., 2011, De Nijs and Cammeraat, 2020). However, the effects of such processes on SOC stabilization have not been studied in Mediterranean forest soils, where bare-soil areas act as sources of runoff water, sediment, seeds and nutrients (including C) that move downslope and are captured by, and concentrated in, vegetation patches (Urgeghe and Bautista, 2015). Even less is known about SOC stabilization when disturbances, such as forest fires, lead to a decrease in vegetation cover and litter, therefore increasing the connectivity of runoff-source areas and the transport capacity of the flow, reducing the hillslope storage potential for water and sediments (Cammeraat, 2004, Mayor et al., 2011), and exposing the already fire-affected SOC.

Understanding SOC nature and reactivity upon changes, as those produced by forest fires, has become crucial to model and define the role of soil as source or sink of C, and to sustainably manage ecosystem services related to the soil resource (Faria et al., 2014). This knowledge starts by the identification of organic fractions with distinct chemical and biological functions and turnover times, characteristics strongly related to the form and/or the type of interactions with minerals (e.g. Rasmussen et al., 2005). Therefore, isolating fractions of OM occurring either inside or outside of aggregates or being part of organic–mineral associations, all of them different in terms of biochemical properties and functional relevance, has become a major research topic during the last two decades (Berhe, 2012, Cerli et al., 2012, Doetterl et al., 2012, Grünewald et al., 2006, Wang et al., 2014).

Physical fractionation by density has been proven useful to separate SOM and to identify meaningful soil fractions, which can be related to different stability and stabilization processes (von Lutzow et al., 2007, Wagai et al., 2009, Wagai et al., 2015, Nadal-Romero et al., 2016, Yeasmin et al., 2017). In contrast to chemical extractions, density fractionation allows for isolation of unmodified C fractions, and is theoretically related to the spatial arrangement and interactions of organic compounds and minerals (Cerli et al., 2012). The method separates light and heavy fractions (Christensen, 1992), taking advantage of the difference in density between minerals and organic material, and often by additional physical dispersion (e.g. sonication), aiming an aggregate disruption and subsequent release of the OM occluded therein (Golchin et al., 1994).

The free light fraction (FLF), which floats in a solution of given density without additional dispersion, comprises undecomposed, easily accessible OM, i.e. large organic fragments that underwent little physical and/or chemical transformation. The occluded light fraction (OLF) comprises much finer organic material with similar composition as the FLF but slightly more altered, stabilized by aggregation, and protected within aggregates, i.e., OM floating in the solution after aggregate disruption. The remaining OM fraction in sediments represents the heavy fraction (HF), in which C is strongly bound to minerals and cannot be completely separated from them, i.e. organic–mineral associations (e.g. Cerli et al., 2012, Kaiser and Guggenberger, 2007). Both floating (light) fractions are supposed to comprise mainly plant-derived debris (leaves, branches, and roots) plus some animal residues, charcoal, seeds, pollen, and microorganisms (Golchin et al., 1994, Wagai et al., 2009). The main differences between the two light fractions should be their size and location within the soil matrix. Together with size fractionation, this technique would be helpful to study how erosion, transport and deposition can induce transitions in SOC from one fraction to another (i.e. active to passive or vice versa) for example by aggregate disruption or deep burial, which may change C mineralisation rates (Wang et al., 2014).

The present study intends to obtain a better understanding of SOC accumulation and stabilization in a post-fire Spanish Mediterranean hillslope under soil erosion and deposition processes. The main hypothesis is that fire affects soil characteristics related to SOC stabilization and, together with erosion processes, can modify the SOC distribution within aggregates and in the burned hillslope. Accordingly, the main objectives are: (a) to determine differences caused by fire in soil aggregation, SOC content and stock, at hillslope scale; (b) to evaluate the influence of different variables as hillslope position, environment (under canopy and bare soils) and depth on the changes of soil aggregation and SOC distribution; (c) to use density and size fractionations in burned and unburned soils, as well as in sediments, to assess the effects of fire and erosion on SOC distribution within aggregates; and, (d) to estimate the role of fire and erosion in the possible changes of SOC accumulation and stabilization (i.e. SOC stock) in a Mediterranean hillslope, in order to shed some light in the discussion about the role of soil erosion as source or sink of C.

Section snippets

Study site and sampling

This work was carried out in the municipality of Azuébar, Natural Park of Sierra de Espadán, in the Province of Castellón, Spain (Fig. 1). Coupled hillslopes (burned: BU, and control: CO, ca. 0.25 ha each one) belonging to the coastal foothills of the Iberian Mountain System were selected (BU: 39°50′45.11″N, 0°22′20.52″W; CO: 39°51′08.7″N, 0°22′17.6″W). Both slopes are located on forested concave hillsides, with ENE aspect, 25 − 28° of slope and an altitude around 370 m a.s.L. (more information

Soil characteristics

Table 1 summarizes the soil properties determined in this study. In relation to the WDT, significant differences were only found between BU and CO, and between UC and BS (M−W, p < 0.05). In CO soils, generally, >200 drops were needed to break aggregates, and no differences could be observed for position, environment or depth (Table 1, Table 2). In BU soils, only UC (200 drops) and BS (120 drops) were significantly different (M−W, p < 0.05, Fig. S5). Results from the wet-sieving test also showed

Soil characteristics

Soils were in general very stable, but opposite results were observed (Table 1), which can be attributed to the slower wetting rate of the WDT as compared to the wet sieving. Imeson and Vis (1984) stated that WDT test is very suitable for soils of low aggregate stability, and the method would not be appropriate for the stable aggregates of this study, and other Mediterranean soils burned at high and moderate severity fires (Campo et al., 2008).

The response to AS to forest fires is complex since

Conclusions

Wildfire impact on Azuebar’s hillslope caused changes in several of the soil properties, which confirms the research hypothesis. Burned and control hillslopes showed differences in the stability of soil aggregates but trends depended on the size analysed and the method used. Factors as hillslope position (related also to hillslope steepness, length, and curvature), soil depth and environment (under canopy soil vs bare soil) influenced significantly the movement, and stock of OC in the

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

This work has been supported by the VALi + d postdoctoral contract (APOSTD/2014/010) of the Generalitat Valenciana. J. Campo also wants to acknowledge C. Celi, J. Westerveld and J. Schoorl for the help with laboratory work; and A. Revynthi, B. Peñarroya, M.I. Montoya and P. Yousefi for their great support.

References (82)

  • G. Certini et al.

    Wildfire effects on soil organic matter quantity and quality in two fire-prone Mediterranean pine forests

    Geoderma

    (2011)
  • E.A. De Nijs et al.

    The stability and fate of Soil Organic Carbon during the transport phase of soil erosion

    Earth-Sci. Rev.

    (2020)
  • S. Doetterl et al.

    Erosion, deposition and soil carbon: A review of process-level controls, experimental tools and models to address C cycling in dynamic landscapes

    Earth-Sci. Rev.

    (2016)
  • J.M. Garcia-Ruiz et al.

    Erosion in Mediterranean landscapes: changes and future challenges

    Geomorphology

    (2013)
  • J.A. González-Pérez et al.

    The effect of fire on soil organic matter–a review

    Environ. Int.

    (2004)
  • G. Grünewald et al.

    Organic matter stabilization in young calcareous soils as revealed by density fractionation and analysis of lignin-derived constituents

    Org. Geochem.

    (2006)
  • Y. Hu et al.

    Erosion-induced exposure of SOC to mineralization in aggregated sediment

    Catena

    (2016)
  • A.C. Imeson et al.

    Assessing soil aggregate stability by water-drop impact and ultrasonic dispersion

    Geoderma

    (1984)
  • P. Jimenez-Pinilla et al.

    Advances in the knowledge of how heating can affect aggregate stability in Mediterranean soils: a XDR and SEM-EDX approach

    Catena

    (2016)
  • K. Kaiser et al.

    Distribution of hydrous aluminium and iron over density fractions depends on organic matter load and ultrasonic dispersion

    Geoderma

    (2007)
  • F.M.S.A. Kirkels et al.

    The fate of soil organic carbon upon erosion, transport and deposition in agricultural landscapes — a review of different concepts

    Geomorphology

    (2014)
  • R. Lal

    Soil erosion and the global carbon budget

    Environ. Int.

    (2003)
  • M. Martínez-Mena et al.

    Organic carbon enrichment in sediments: effects of rainfall characteristics under different land uses in a Mediterranean area

    Catena

    (2012)
  • M. Martinez-Mena et al.

    Effect of water erosion and cultivation on the soil carbon stock in a semiarid area of South-East Spain

    Soil Till. Res.

    (2008)
  • J. Mataix-Solera et al.

    Fire effects on soil aggregation: a review

    Earth-Sci. Rev.

    (2011)
  • Á.G. Mayor et al.

    Scale-dependent variation in runoff and sediment yield in a semiarid Mediterranean catchment

    J. Hydrol.

    (2011)
  • E. Nadal-Romero et al.

    How do soil organic carbon stocks change after cropland abandonment in Mediterranean humid mountain areas?

    Sci. Total Environ.

    (2016)
  • D. Peña-Angulo

    Spatial variability of the relationships of runoff and sediment yield with weather types throughout the Mediterranean basin

    J. Hydrol.

    (2019)
  • V.O. Polyakov et al.

    Soil organic matter and CO2 emission as affected by water erosion on field runoff plots

    Geoderma

    (2008)
  • S.A. Prats et al.

    Can straw-biochar mulching mitigate erosion of wildfire-degraded soils under extreme rainfall?

    Sci. Total Environ.

    (2021)
  • S.A. Prats et al.

    Mid-term and scaling effects of forest residue mulching on post-fire runoff and soil erosion

    Sci Total Environ.

    (2016)
  • B.M. Rau et al.

    Influence of prescribed fire on ecosystem biomass, carbon, and nitrogen in a pinyon juniper woodland

    Rangeland Ecol. Manag.

    (2010)
  • R.A. Shakesby

    Post-wildfire soil erosion in the mediterranean: review and future research directions

    Earth-Sci. Rev.

    (2011)
  • R.A. Shakesby et al.

    Impacts of prescribed fire on soil loss and soil quality: an assessment based on an experimentally-burned catchment in central Portugal

    Catena

    (2015)
  • R.A. Shakesby et al.

    Wildfire as a hydrological and geomorphological agent

    Earth-Sci. Rev.

    (2006)
  • T. Terefe et al.

    Influence of heating on various properties of six Mediterranean soils. A laboratory study

    Geoderma

    (2008)
  • M. von Lutzow et al.

    SOM fractionation methods: Relevance to functional pools and to stabilization mechanisms

    Soil Biol. Biochem.

    (2007)
  • R. Wagai et al.

    Nature of soil organo-mineral assemblage examined by sequential density fractionation with and without sonication: Is allophanic soil different?

    Geoderma

    (2015)
  • X. Wang et al.

    Soil aggregation and the stabilization of organic carbon as affected by erosion and deposition

    Soil Biol. Biochem.

    (2014)
  • M. Wiesmeier et al.

    Soil organic carbon storage as a key function of soils – a review of drivers and indicators at various scales

    Geoderma

    (2019)
  • S. Yeasmin et al.

    Organic carbon characteristics in density fractions of soils with contrasting mineralogies

    Geochim. Cosmochim. Ac.

    (2017)
  • Cited by (7)

    View all citing articles on Scopus
    View full text