Increase in soil aggregate stability along a Mediterranean ...

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Plant Soil DOI 10.1007/s11104-015-2647-6

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Increase in soil aggregate stability along a Mediterranean successional gradient in severely eroded gully bed ecosystems: combined effects of soil, root traits and plant community characteristics Amandine Erktan & Lauric Cécillon & Frank Graf & Catherine Roumet & Cédric Legout & Freddy Rey Received: 26 November 2014 / Accepted: 18 August 2015 # Springer International Publishing Switzerland 2015

Abstract Background and aims Our objectives were to evaluate changes in soil aggregate stability along a successional gradient, located in severely eroded Mediterranean gully bed ecosystems and to identify predictors of soil aggregate stability variations among several soil, root traits and plant community characteristics. Methods We selected 75 plots in gully beds, representing five successional stages that differ in plant community composition, dominated by herbs, shrubs or trees according to successional stage. In each plot, we measured soil aggregate stability, basic soil characteristics, root traits and plant diversity indices. Results Soil aggregate stability increased along the successional gradient, being thrice higher in tree-dominated

communities as compared to grass-dominated communities. This increase was mainly driven by soil organic carbon (SOC) accumulation. In early successional stages showing low SOC (below 24 g.kg −1 or 12 g.kg−1 in some cases), fine sand content and the percentage of fine roots acted as co-drivers enhancing soil aggregate stability while silt content decreased it. Conclusion Plant succession in severely eroded Mediterranean gully bed ecosystems is accompanied by a strong stabilization of soil aggregates, mainly driven by SOC accumulation and for early successional stages, by soil granulometry and root traits as co-drivers. Stimulating succession thus appears as a promising restoration strategy for severely eroded ecosystems.

Responsible Editor: Kees Jan van Groenigen. Electronic supplementary material The online version of this article (doi:10.1007/s11104-015-2647-6) contains supplementary material, which is available to authorized users. A. Erktan (*) : L. Cécillon : F. Rey Irstea, UR EMGR Ecosystèmes Montagnards, 2 rue de la Papeterie, BP 76, Saint-Martin-d’Hères F-38402, France e-mail: [email protected]

F. Graf WSL Institute for Snow and Avalanche Research SLF, Flüelastrasse 11, CH-7260 Davos Dorf, Switzerland

A. Erktan : L. Cécillon : C. Legout : F. Rey Université Grenoble Alpes, F-38402 Grenoble, France

C. Roumet Centre d’Ecologie Fonctionnelle et Evolutive, UMR 5175 (CNRS – Université de Montpellier – Université Paul-Valéry Montpellier – EPHE), 1919 Route de Mende, 34293 Montpellier Cedex 5, France

A. Erktan INRA, UMR AMAP (botAnique et Modélisation de l’Architecture des Plantes et des végétations), TA A-51/PS1 Boulevard de la Lironde, 34398 Montpellier Cedex 5, France

C. Legout LTHE, Université Joseph Fourier, CNRS, G-INP, IRD, 70 Rue de la physique, 38400 Saint Martin d’Hères, France

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Keywords Plant communities . Succession . Soil aggregate stability . Roottrait . Plant diversity . Ecological restoration Abbreviations SOC CaCO3 RMD RLD SRL Herbs, Shrub, STree, TTree

Forest

Soil organic carbon Soil calcium carbonate Root mass density Root length density Specific root length Plant communities respectively dominated by herbs, shrubs, small trees (height2 m) Stabilized forested slopes used as a control

Introduction The stability of soil aggregates, describing their resistance to breakdown under disruptive forces, is a key soil property influencing several ecosystem processes, such as carbon storage (Jastrow et al. 1998), nutrient availability (Wang et al. 2001) and the resistance of soils to erosion (Barthès and Roose 2002; Frei et al. 2003). In severely eroded ecosystems, such as badlands, characterized by numerous active gullies and high level of disturbance, soil aggregate stability is a promising indicator of their ecological restoration status (Burri et al. 2009). The change in plant community composition through successional dynamics, occurring on these eroded ecosystems, is a key driver of their restoration (Walker and del Moral 2009), and thus potentially of soil aggregate stability variations. Recent studies showed that soil aggregate stability generally increased as succession proceeds (Duchicela et al. 2013; Cheng et al. 2015; Qui et al. 2015), the factors driving these modifications along successional gradients are however poorly known. Soil aggregates dynamics and stabilization are complex processes, driven by several abiotic and biotic factors related to soil characteristics (e.g. microorganisms activity, organic carbon content, inorganic binding agents), vegetation characteristics (e.g. vegetation cover and type, plant roots), land management and climate (Six et al. 2004; Bronick and Lal 2005; Abiven et al. 2009; Cécillon et al. 2010).

The influence of soil characteristics on soil aggregate stability has been fairly broadly documented, with negative effects of sodium, especially in marly terrains, and silt content (Tisdall and Oades 1982; Faulkner 2013) and positive effects of soil biota (mycorrhizae, earthworms, termites – Rillig et al. 2002; Graf and Frei 2013), calcium carbonate (CaCO3), clay (Tisdall and Oades 1982; Muneer and Oades 1989) and soil organic carbon concentrations (SOC – Six et al. 2004; Abiven et al. 2009). Despite an important literature on the topic, the conditions required for these drivers to play an effective role in soil aggregate stabilization under field conditions is not clear. For example, under many field conditions, SOC was found to be a crucial driver of soil aggregate stability (e.g. Tisdall and Oades 1982; Le Bissonnais and Arrouays 1997; Chenu et al. 2000), and in some other cases, studies failed to observe such an effect (e.g. Igwe et al. 1999; Lado et al. 2004). Le Bissonnais et al. (2007) suggested that the effect of SOC on aggregate stability may be SOC concentration-dependant. Further studies are thus needed to specify the range of SOC concentrations where it positively impacts soil aggregate stability. The effect of vegetation characteristics on soil aggregate stability has received relatively little attention, despite the multitude of mechanisms by which vegetation can modify soil aggregation. Aboveground, stems and leaves can protect the soil from erosion by acting as a wind breaker (Gray and Sotir 1996) and by modifying the raindrop energy of rainfall (Zuazo and Pleguezuelo 2008). Litter weakens surface erosion by reducing the Bsplash effect^ (Geedes and Dunkerley 1999) and by providing organic matter, through decomposition (Abiven et al. 2009). Belowground, roots contribute to the formation and the stabilization of soil aggregates in various ways (Six et al. 2004), by changing soil density via root penetration (Denef et al. 2002), influencing the hydrologic balance through evapotranspiration (Reid and Goss 1982; Rasse et al. 2000), producing exudates acting directly as binding agent (Morel et al. 1991; Bronick and Lal 2005) or indirectly as stimulator of microorganisms activity (Rillig et al. 2006), by providing organic compound through root decomposition (Gale et al. 2000; Puget and Drinkwater 2001) and by mechanical entanglement (Jastrow et al. 1998). Only few studies tried to identify, at the species level, which root characteristics – or traits – are responsible for these effects (but see Miller and Jastrow 1990; Carter et al. 1994; Graf and Frei 2013; Bardgett et al. 2014). For

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example, fine roots were highlighted as an important factor of soil aggregate stabilization, through entanglement mechanisms, alone and in association with mycorrhizae (Jastrow et al. 1998; Rillig et al. 2002; Gyssels et al. 2005; Hallett et al. 2009; Leifheit et al. 2014). At the plant community level, the links between root traits and soil aggregate stability remain largely unknown. Developing a trait-based approach was recently pointed as a promising way to improve our mechanistic understanding of soil aggregation at both species and community levels (Rillig et al. 2015). At the community level, some recent studies underpinned plant diversity as another important determinant of soil aggregate stability (Pohl et al. 2009, 2011; Martin et al. 2010; Pérès et al. 2013). Plant diversity is hypothesized to promote the formation and the stabilization of soil aggregates via several mechanisms. First, for a given plant trait, wide range of values can favor aggregate stability through complementary effects. As easily decomposable and recalcitrant organic matter respectively influence soil aggregation in the short (days) and long (months) terms (Abiven et al. 2009), plant communities with contrasting tissue chemical characteristics are likely to enhance aggregate stability at various time scales. Second, as several ecosystem processes positively related to soil aggregate stabilization are diversity-driven through overyielding mechanisms related to niche complementarity, it is very likely that plant diversity indirectly stimulate soil aggregate stability. For example, SOC concentration (e.g. Steinbeiss et al. 2008) and soil microbial biomass (e.g. Eisenhauer et al. 2010), both known to positively influence aggregate stability, increase with plant diversity. The relative importance of soil vs. vegetation characteristics in driving soil aggregate stability most probably depends on ecosystem type. The effect of vegetation characteristics on soil aggregate stability is suggested to be of particular importance in disturbed ecosystems (Pohl et al. 2009, 2011), such as severely eroded ecosystems. Because, disturbance intensity generally decreases along a successional gradient, the influence of vegetation characteristics on soil aggregate stability is thus potentially higher in early successional stages. As succession proceeds, changes in plant community composition are associated with strong variations in leaf an root traits (even though less studied as compared to aboveground traits), influencing the quantity and the quality if the litter and organic carbon compounds released into the soils (Garnier et al. 2004; Zangaro et al.

2008; Kazakou et al. 2009; Holdaway et al. 2011). In particular, plant traits point to the replacement of fastgrowing species which dominate the early stages, and whose litter tend to decompose rapidly, by slower growing species which tend to conserve internal resources more efficiently in recalcitrant structural compounds and decomposed slowly as succession proceeds (Garnier et al. 2004; Kazakou et al. 2009). These modifications in plant traits along plant succession are likely to influence soil aggregate stability through various direct and indirect mechanisms related to soil carbon dynamics but, to our knowledge, no study addressed this question yet. We focused on a successional gradient, located in severely eroded Mediterranean gully bed ecosystems from the French Southern Alps. More precisely, earlysuccessional plant communities were dominated by herbs, intermediate by shrubs and small trees (2 m) dominated in latesuccessional plant communities. The aims of our study were (1) to assess the variations of soil aggregate stability along this successional gradient and (2) to identify the predictors of soil aggregate stability variations among several soil, root and plant community characteristics. We expected soil aggregate stability to increase along the successional gradient. We hypothesized that more stable aggregates are associated with higher SOC, calcium carbonates, clay concentrations as well as with higher species richness and diversity. Fine root characteristics are expected to be positively associated with soil aggregate stability, through entanglement mechanisms, alone or in association with mycorrhizae. Finally, we hypothesized a stronger effect of vegetation characteristics in early successional stages.

Materials and methods Study site The study was carried out in summer 2012 on eroded terrains showing numerous active gullies in the Southern French Alps near Digne-les-Bains, at the Draix-Bléone Environmental Research Observatory (http://oredraixbleone.irstea.fr/; 44°08′N, 6°20′E). The study site was located in the Bouinenc catchment which has a total area of 40 km2. This catchment has an average altitude of 862 m and a mountainous

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and Mediterranean climate (Vallauri 1997). Annual mean precipitation is 900 mm and monthly average precipitations vary from no rain to more than 120 mm (Mathys 2006). Mean air temperature is 10.3 °C with average daily amplitude of 14 °C. The terrain is composed by Jurassic black marls (superior Bathonian and Callovo Oxfordian levels), formed by a mixture of carbonates and clay. These terrains are strongly sensitive to freeze and thaw cycles (Descroix and Mathys 2003), responsible for the weathering of bedrock. This leads to the formation of a regolith layer, composed by coarse particles embedded in a matrix of finer material (Maquaire et al. 2002). These particles are flat and thin and are called platelets. At our study site, vegetation cover was strongly reduced during the 19th century due to overgrazing and deforestation (Daily 1995; Vallauri 1997). Ecological restoration was conducted in the area between 1860 and 1914 (Vallauri et al. 2002). Currently, the Bouinenc catchment roughly shows two types of vegetation cover: areas under spontaneous dynamic showing scarce and patchy vegetation (herbs, shrub or tree dominated patches), and areas with fairly continuous vegetation cover, dominated by trees on gully slopes, indicating successful ecological restoration (Burylo et al. 2007). Selection and characterization of plant communities along the successional gradient We sampled 75 plots (1 × 2 m) across the catchment within spontaneous, sparse patches of vegetation in gully beds and on stabilized forested slopes. All plots were located on a homogeneous marly substrate, highly prone to soil erosion. We selected five types of plant communities corresponding to five successional stages. Early successional stages are characterized by recurrent erosive disturbances, limiting plant community development to an herbaceous and shrub layer only. Along the gradient, erosive dynamics decrease while plant dynamics increase (Cohen and Rey 2005). The five successional stages were defined according to the dominant species’ growth form, from the early to the late stages of the plant succession: (i) herbs (Herbs), (ii) shrubs (Shrub), (iii) small size trees (height2 m; TTree), and (v) stabilized forested slopes used as a control (Forest) (Fig. 1). For each successional stage, 15 plots were selected and studied. The ground projected cover of

the various growth form types characterizing each successional stage are reported in Appendix S1. The total ground cover ranged from 77 % in Herbs to 212 % in Tree and Forest communities, showing several vegetation layers. Herbs communities showed only one dominant growth form which covered 68±4.8 % of ground on average. Shrub communities showed both herbs and shrub growth forms, which covered 37±8.3 and 58± 6.9 % respectively. STree, TTree and Forest communities showed three growth forms: herbs (37±3.5 % on average), shrubs (47±4.6 % on average) and trees (85± 3.4 % on average) (Appendix S1). In these various plant communities, the dominant herbaceous species were Achnatherum calamagrostis L. (Herbs), Aphyllantes monspeliensis L. and Laserpitium gallicum L. (Forbs). The most common shrub species were Buxus sempervirens L., Ononis fruticosa L. and Genista cinerea Vill. Finally, Pinus nigra Arn. Ssp nigra and Pinus sylvestris L. dominated the tree cover. Plant species representing 80 % of total abundance are given in Appendix S2.

Soil characterization and soil aggregate stability measurements In each plot, soil sampling was performed within plant covered areas to increase the chances to collect rhizosphere soil. Soils were sampled at three points per plot in order to tackle spatial heterogeneity. For soil aggregate stability and root characterization (see below for the description of root sampling), topsoils (0–5 cm; the layer most affected by erosion at the study site; Mathys 2006) were sampled with a cylindrical core (diameter: 8 cm, length: 5 cm) to preserve soil structure. Intact soil samples (without removing roots) were air dried separately for 10 days; 100 g of air dried soil were

Fig. 1 Types of plant communities along the successional gradient in gully beds ecosystems. Pictures of representative plant communities of each successional stage (Grass, Shrub, STree, TTree) plus control treatment (Forest)

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then taken from each of the three samples to form one composite soil sample per plot. The composite soil sample was gently sieved to isolate the 3 to 5 mm soil fraction. Seven grams of soil macroaggregates from the 3 to 5 mm fraction were then selected manually using tweezers. This manual procedure was necessary to separate macroaggregates from marly macroplatelets, found in high proportion in this marly catchment (Mathys 2006). Soil aggregate stability was measured according to the standardized method NF X 31–515 (2005) derived from Le Bissonnais (1996) and Le Bissonnais and Arrouays (1997) and composed of three tests. We focused on the most disruptive test consisting of quick immersion in water, which best mimics the behavior of soil aggregate structural stability in case of Mediterranean heavy rainfall events that often occur on dry soils in summer or early fall at the study site. After oven drying at 40 °C during 24 h, the ca. 7 g of manually selected macroaggregates (3–5 mm size fraction) were weighted and, thereafter, immersed into 50 ml of deionized water for 10 min. Water was then gently discarded and the remaining soil aggregates were transferred onto a 50 μm sieve previously immersed in absolute ethanol to preserve the structure of the soil aggregates. Five helicoidal movements into two directions were done manually with constant amplitude (15 cm) and frequency (1 cycle s−1). Stable soil aggregates were collected, oven dried at 40 °C for 48 h and then gently sieved

% stable aggregate on each sieve ¼

through a six sieves column (2.00, 1.00, 0.5, 0.2, 0.1 and 0.05 mm) by running 20 identical helicoidal movements. This resulted in seven diameter classes for the study of stable soil aggregates. Marly platelets, released from macroaggregates during water immersion and retained in the 2 and 1 mm sieves, were removed manually using tweezers to limit the bias on the measurement. After drying (105 °C) to constant weight each aggregate size fraction, the mean weight diameter, MWD (mm) of soil macroaggregates was calculated as follows: X i¼7 MWD ¼

Mi  Di i¼1 X i¼7 Mi i¼1

Within each of the seven classes of diameter (i=1–7), Di [mm] is the central diameter of each size class. The extreme classes (D1 and D7) are respectively 3.5 and 0.025 mm (according to the norm NF X 31–515 2005). Mi [g] is the mass of stable soil aggregates isolated within a diameter class. The fractions above 2 mm (M7) and between 2 and 1 mm (M6) were corrected by removing the mass of marly platelets. To avoid overestimation, the MWD was also corrected for the primary particle of the same size, i.e. fine sand particles (50 μm< fine sand