(PDF) Interplay between soil formation and geomorphic processes along a soil catena in a Mediterranean mountain landscape: an in

Environmental Earth Sciences (2020) 79:59 https://doi.org/10.1007/s12665-019-8802-2 ORIGINAL ARTICLE Interplay between soil formation and geomorphic processes along a soil catena in a Mediterranean mountain landscape: an integrated pedological and geophysical approach Massimo Conforti1   · Teresa Longobucco1 · Fabio Scarciglia2 · Giancarlo Niceforo2 · Giorgio Matteucci1 · Gabriele Buttafuoco1 Received: 12 April 2019 / Accepted: 24 December 2019 © Springer-Verlag GmbH Germany, part of Springer Nature 2020 Abstract Soil spatial distribution often shows high variability because of geo-environmental settings affecting soil formation and geomorphic processes. The paper describes a combined pedological–geophysical approach for investigating the interplay among soil-forming processes, geomorphic dynamics and physico-chemical soil properties in a soil catena in a Mediter- ranean mountain landscape (southern Italy). Stocks of soil organic carbon (SOC) and total nitrogen (STN) were determined and their interactions with slope position analyzed. Five slope units were defined along the soil catena (summit, shoulder, back, foot, and toe slopes), and in each of them, a soil profile was dug and analyzed. Thickness of soil horizons and contact between soil and parent material were determined by a 2D-electrical resistivity tomography survey. Results showed granitic bedrock is characterized by vary weathering degrees strongly affecting soil type development. Moreover, soil properties variation is related to topographic features and water erosion processes. Soil chemical composition showed high values of ­SiO2 and ­Al2O3, probably due to inheritance from underlying granitic parent material. Particularly, ­Fe2O3 content increased with weathering conditions as shown by chemical index of alteration values. Along foot and toe slopes, reworking and accumulation of colluvial soil lead to the formation of thicker accretionary A horizon. Consequently, summit areas, together with foot and toe slopes had greater SOC and STN stocks than back and shoulder slopes. That because soils are thicker and richer in SOC on summit and valley bottom than in shoulder and back-slope positions. The proposed approach has showed the benefit of an integrated methodology for investigating spatial variations along soil catenas. Keywords  Soil catena · Slope position · Parent material · Soil features · Southern Italy Introduction of environmental resources and it has a great influence for many ecological processes such as biogeochemical cycling, Soil as a component of natural and man-made ecosystems distribution of plant communities and agricultural productiv- (agricultural, forest, and urban), is a dynamic regulatory sys- ity (Lal and Moldenhaver 1987). In addition, large amounts tem generating a multitude of soil functions and supporting of carbon can be stored in soil profiles leading to a sequestra- the delivery of major ecosystem services for human well- tion of atmospheric ­CO2, which may mitigate global warm- being (Blum 2005; Costanza et al. 2017; Adhikari and Har- ing and improve soil fertility status (Batjes 1996; Scholes temink 2017). Soil is one of the most important components and Andreae 2000; Lal 2016). Soil development is a dynamic process depending on sev- * Massimo Conforti eral factors summarized following Jenny (1941) in climate,

organisms including humans, relief, parent material, and 1 time. Such a list of factors is important for understanding National Research Council of Italy, Institute for Agriculture which of them may be important for producing soil pattern and Forest Systems in the Mediterranean, 87036 Rende, CS, Italy within a region. In a more recent reformulation of Jenny’s 2 soil-forming factors, it has been added the spatial position Department of Biology, Ecology and Earth Sciences (DiBEST), University of Calabria, (McBratney et al. 2003). 87036 Arcavacata di Rende, CS, Italy 13 Vol.:(0123456789) 59   Page 2 of 16 Environmental Earth Sciences (2020) 79:59 In addition, many studies have long recognized that Studying soil along a slope represents a powerful tool weathering of parent rock and erosion are two major pro- for investigating the spatial variations in soil profiles cesses controlling development and spatial variability of (depth, assemblage, thickness and boundaries of different soil (Jenny 1941; Bockheim et al. 2005, Scarciglia et al. genetic horizons) on landscape at the hillslope scale (soil 2005; Yoo et al. 2009). Physical and chemical weather- toposequence). In this context, the objective of the paper is ing concern slow process of rocks disintegration produc- describing a combined pedological–geophysical approach ing materials, which form new soil. Conversely, erosion for investigating the interplay among soil-forming processes, causes the detachment, transport and redistribution of soil geomorphic dynamics and physico-chemical soil properties particles reducing soil thickness. Soil redistribution due across a representative soil catena in a Mediterranean moun- to erosion–deposition processes, is very important for tain landscape (southern Italy). In addition, the stocks in the biogeochemical cycles within soils and substantially soil of organic carbon (SOC) and total nitrogen (STN) were affects carbon and nutrient cycling (Quinton et al. 2010; determined, and the interaction among stocks of SOC, STN Doetterl et al. 2016). Generally, soil erosion processes and slope position was pointed out. take place at different landscape positions: water erosion detaches soil particles from hilltops and transport them to the valleys bottom. Consequently, topography coupled Materials and methods with geomorphic processes can significantly influence the spatial distribution of soil type and the their physico- Site description chemical properties as texture, bulk density, organic car- bon (OC), cation exchange capacity (CEC), trace elements, The study was conducted along a soil catena located on a etc. Many authors (e.g., Hook and Burke 2000; Lal 2004; slope of the Serre Massif (southern Calabria, Italy), within Sariyildiz et al. 2005; Quinton et al. 2010; Conforti et al. of the Biogenetic Nature Reserve ‘Marchesale’ (Fig. 1), cov- 2016; Ma et al. 2016) showed that soil properties vary lat- ered predominantly by beech trees (Fagus sylvatica L.) occa- erally and with depth and, often, these variations are due sionally associated with silver fir (Abies alba Mill.) trees. to small changes in topographic attributes, as slope gradi- Previous investigations showed the site is representative ent, concavities and convexities. Consequently, quantify- of highland landscape of the Serre Massif (Conforti et al. ing the spatial pattern of soil properties is the necessary 2016). The toposequence has a distance from upper to bot- premise for characterizing the status of development and/ tom part of the hillslope of about 60 m and a lateral width of or degradation of soils. Among soil properties, estimating about 20 m (about 0.12 hectares). The study area has eleva- soil organic carbon (SOC) storage within soil profile has tion ranging between 1162 and 1148 m a.s.l., slope gradient a particular importance. In addition, accurate and detailed varying from 0.5° to 32° (average 11°), and south-western knowledge about soil variation is essential to support slope exposition (Fig. 1). quantitative environmental studies that require accurate The climate of the study area is classified as Mediter- regionalized soil information, such as global and regional ranean upland (Csb-type, sensu Köppen 1936). Precipita- studies on sustainable land use management, soil modeling tion and temperature data are recorded at the meteorological and monitoring, biodiversity, climate change (Hartemink station of Mongiana (921 m a.s.l.), located at about 7 km to and McBratney 2008; Rodrigo-Comino et al. 2018). northwest of the study area (Fig. 1). The mean annual pre- The concept of ‘soil catena’ was first developed by Milne cipitation calculated using the last 30 years of observations, (1936, 1947) and Bushnell (1942) who have defined ‘catena’ amounts to 1800 mm with precipitation mainly concentrated a sequence of soils (toposequence) formed on a single par- between October and March with a maximum value of about ent material, although allowing the lower lying member to 330 mm in December and a minimum one of about 26 mm be formed from organic material entirely, whereas the more in July. The mean annual temperature is 11.3 °C with a mean elevated members were mineral soil (Buol et al. 2011). monthly maximum of 35.9 °C occurring between June and The concept of soil catena has been used in different August and a mean monthly minimum of − 9.9 °C in the parts of the world: in Uganda (Brunner et al. 2004), Mex- period December–March. The pedoclimate is characterized ico (González-Arqueros et al. 2017), Russia (Kosheleva by an udic soil moisture regime coupled with mesic soil et al. 2018; Ya Kudryashova et al. 2019), India (Singh and temperature regime (ARSSA 2003). Benbi 2018), Germany (Mayer et al. 2018), United States From a geological point of view, the area belongs to the of America (Evans and Hartemink 2014; Iggy Litaor et al. Late Hercynian batholith of the Serre Massif, composed 2018; Bergstrom et al. 2019), Malta (McLaughlin et al. mainly by crystalline rocks (Borsi et al. 1976; Angì et al. 2018), Poland (Wiśniewski and Märker 2019), South Africa 2010). Most of the upland landscape is covered by a thick (Khomo et al. 2011), France (Badía et al. 2015), only to regolith (up to 40 m, including saprolite and soil) (e.g., Cal- quote the most recent literature. caterra and Parise 2010). Exposed bedrock is common on 13 Environmental Earth Sciences (2020) 79:59 Page 3 of 16  59 Fig. 1  Location of the experimental site and schematic soil toposequence with location of studied soil profiles and electrical resistivity tomogra- phy (ERT) cross section 13 59   Page 4 of 16 Environmental Earth Sciences (2020) 79:59 the escarpments, where the regolith is typically shallow. In representative soil profile was dug down to the parent mate- particular, in the study area granitic rocks, emplaced around rial or C horizon. The profile locations were recorded using 295–393 Ma (Caggianelli et al. 2000), outcrop, which are a GPS receiver (Garmin GPS Etrex10) (Fig. 1). The five soil affected by intense fracturing and weathering processes profiles (P1–P5) were described in detail in field according (Ietto et al. 2016). However, these rocks, on topographic to the standard international guidelines (FAO 2006; Schoe- depressions as well as along valleys floor, are often covered neberger et al. 2012). Both disturbed (bulk) and undisturbed by colluvial deposits (Conforti et al. 2016). The landscape (core) soil samples were collected in every identified genetic of the study area is characterized by a gently rolling pla- horizon and analyzed in laboratory for some physical, chem- teau, probably having a partially inherited relief, uplifted ical and geochemical properties. by regional tectonic activity since the Middle Pleistocene In addition, seven hand auger drillings along the catena (Roda-Boluda and Whittaker 2017), controlled by NE–SW were carried out to improve the assessment of soil spatial normal and NW–SE strike-slip faults (Tripodi et al. 2018). variability (Fig. 1). They allowed to better identify the total This landscape is dissected by deep incisions and bordered soil depth and thickness of the different genetic horizons. by steep slopes, often structurally controlled (Conforti et al. 2016, 2018). The hydrographic network is highly controlled Geophysical investigation by tectonic lineaments (Conforti and Ietto 2019). In particu- lar, streams have an angular pattern, with narrow and well- Along the same transect, where the soil profiles were dug, defined streambeds and deep V-shaped valleys cut into the a geophysical prospection using 2D-electrical resistivity slopes. Although no detailed data are available on the age tomography (ERT) was performed (Fig. 1). The 2D-ERT of landscape formation, the main relief features and geotec- is an indirect method commonly used to detect geological tonic history are similar to those of the Sila Massif to the and pedological features as regolith or soil depth, particle north and the Aspromonte Massif to the south. Therefore, it size distribution, mineralogy, porosity, water content, solute can be supposed that the plateau was shaped during middle concentration (Samouelian et al. 2005; Rossi et al. 2011; Van Miocene–early Pliocene or Pliocene–early Pleistocene times Dam 2012; Perrone et al. 2014; Cavallo et al. 2016). In this (Molin et al. 2012; Robustelli 2019), and largely uplifted study, PASI-16SG24-N—Earth Resistivity Meter (Pasi S.r.l., during the Pleistocene (Olivetti et al. 2012; Robustelli 2019). Torino, Italy) was used and the investigation was aimed to Long-term average erosion rates estimated in these areas detect the thickness of soil horizons and contact between soil with a missing volume approach (Scarciglia 2015; Robustelli and bedrock. The length of ERT section was of 60 m with 2019) or using cosmogenic 10Be (Raab et al. 2018, 2019) 60 electrodes inserted with 1 m spacing. To obtain good range between ca. 0.10 to 0.40 mm/year, whereas short-term vertical resolution at sufficiently high depth of penetration, (last ~ 50 to 60 years) erosion rates obtained from fallout the Wenner–Schlumberger electrode array was selected. The 239+240 Pu are up to about 1–2 mm/year (Raab et al. 2018). apparent resistivity (Ω m) was subject to standard geophysi- According to Conforti et al. (2016), in the study area the cal inversion using RES2DINV software (Loke and Barker main soil types, classified using the USDA system (USDA 1996). The default smoothness constrained inversion formu- 2014), are Entisols and Inceptisols orders, representing rel- lation (last squares inversion) was applied, whereas to visu- atively shallow, poorly to moderately differentiated soils, alize inverse results, logarithmic contour interval was used. with high skeleton content and strongly dependent on the nature of the parent rock and the topographic features. In Laboratory analyses particular, gentle sloping and flat areas are characterized by Humic Dystrudepts, while steep slopes are dominated by All soil samples from the genetic horizons were air-dried, Lithic Udipsamments. gently crushed, using pestle and agate mortar, and then passed through a 2 mm sieve to remove coarse roots and Experimental design, pedological survey and soil gravel prior to laboratory analysis. Furthermore, for each sampling sample, dry soil color was estimated using the Munsell soil color chart. Physical and chemical soil properties such To evaluate the effects of slope position and geomorphic as bulk density (BD), particle density (PD), total porosity processes on soil development and SOC and STN stocks, (TP), particle size distribution, pH, cation exchange capacity five slope units were identified along the soil catena: sum- (CEC), organic carbon (SOC) and total nitrogen (STN) were mit, shoulder slope, back slope, foot slope and toe slope measured, according to the Italian Official Methods for Soil (Fig. 1). The slope units were identified combining a detailed Analysis (Pagliai 1997; Violante 2000). geomorphologic field survey and a morphometric analy- Soil bulk density was determined as the ratio of the mass sis, based on a digital elevation model (DEM) with a spa- of oven-dried solids to the bulk volume of the metal ring tial resolution of 5 m. Subsequently, in each slope unit a core (Blake and Hartge 1986). 13 Environmental Earth Sciences (2020) 79:59 Page 5 of 16  59 Soil particle density was measured by pycnometer method Al2 O3 (Blake and Hartge 1986), while soil total porosity (TP) was CIA = 100. (3) Al2 O3 + CaO + Na2 O + K2 O estimated as The CIA index has been widely used to estimate the 1 − BD intensity of chemical weathering in soils by comparing TP = × 100. (1) PD changes in major elements concentrations as ratios of labile Soil particle size distribution was determined using the (more mobile) to relatively stable (immobile) elements in hydrometer method (Bouyoucos 1962) after a pre-treat- soil and rock or other parent material (Düzgören-Aydin et al. ment with sodium hexametaphosphate as soil-dispersing 2002; Price and Velbel 2003). agent. The size classes used in this study are those defined Finally, total loss on ignition (LOI) was determined after by the United States Department of Agriculture (USDA) heating the samples for 2 h at 1000 °C. system: sand (2–0.05 mm), silt (0.05–0.002 mm), and clay (< 0.002 mm). Soil pH was determined on a soil to solution ratio 1:2.5 Results (soil: distilled water) using a potentiometer HI 9­ 2240® (Hanna Instruments Inc., Woonsocket, RI, USA). Exchange- Characteristics of soil profiles able cations capacity (CEC) was determined through the extraction in ammonium acetate at pH 7.0. Field observations and laboratory analysis provided clear Before determining SOC and STN concentrations, a insights about the influence of relief features, alteration subsample of fine earth (< 2 mm) was taken from each soil of granitic rocks and geomorphic processes on morpho- sample, ground in agata mortar and sieved at 0.25 mm. SOC logical, physical and chemical characteristics of the five concentration was determined using a Shimadzu TOC-L soil profiles along the catena. Therefore, the soils have analyzer with an SSM-5000A solid sample module (Shi- a profile weakly expressed and show a close similarity madzu Corporation, Kyoto, Japan), while STN concentra- to parent rock. Based on Soil Taxonomy (USDA 2014), tion was determined using an Elemental Analyzer NA 1500 the soil profiles from summit slope to foot slope (P1–P4) (Carlo Erba Instruments, Milan, Italy). can be classified as Humic Dystrudepts and to identify Finally, the stocks of SOC and STN for each soil profile A–Bw–C mineral horizons (Fig. 2a), while the soil profile were calculated according to Batjes (1996) as (P5) located in the toe slope can be classified as Lithic SOCStock = SOC × BD × D × (1 − Sske∕100) (2) STNStock = STN × BD × D × (1 − Sske∕100), where ­SOCStock and ­STNStock are the stock of SOC and STN for each soil horizon measured in Mg ha−1, SOC is measured in % (g C × 100 g−1 soil) and STN in % (g N × 100 g−1 soil), BD in g cm−3, D is horizon thickness (cm), Sske is soil skel- eton content, constituted by coarse fragments greater than 2 mm, expressed as percentage in volume. Sske contents were determined in the field. Stock of SOC and STN for whole-soil profile was determined as the sum of the stocks of every soil horizon. In addition, to evaluate the chemical interaction between parent material and soil, for each soil sample the major geo- chemical elements (Na, Mg, Al, Si, P, K, Ca, Ti, Mn, and Fe) were determined by X-ray fluorescence spectroscopy (XRF) (Beckhoff et al. 2006) using a Bruker S8 Tiger equipment at the Department of Biology, Ecology and Earth Sciences of the University of Calabria (Italy). The XRF analysis was Fig. 2  Two representative soil profiles studied along soil catena; the performed on pressed powder discs of whole-soil samples soil profiles were classified according to the USDA classification (USDA 2014); red letters denote the particular soil horizons, while (prepared by milling to a fine-grained powder in an agate white dotted lines represent horizon boundaries. a Soil profile P2 is mill). The geochemical elements were expressed in the form an Inceptisols (Humic Dystrudepts), located in shoulder slope, b soil of oxides and used to calculate the chemical index of altera- profile P5 is an Entisols (Lithic Udipsamments), located in toe slope tion (CIA) according to Nesbitt and Young (1982) as as Humic Dystrudepts and to identify A–Bw–C mineral horizons (a), while the soil profile (P5) located in the toe slope 13 59   Page 6 of 16 Environmental Earth Sciences (2020) 79:59 Udipsamments (Fig. 2b) with minimal horizon develop- The results of the particle size distribution showed that ment (A–C). The thickness of the pedons along the topose- the sand content increases with depth, while the percentage quence, varying from 81 to 176 cm (Table 1), showed a of silt and clay decrease moving towards the parent material high spatial variability. The variable thickness of surface A (Table 2). In particular, C horizons show a loamy sand tex- horizons (thinner in the upslope soil profiles of the catena ture class, with a content of sand greater than 70%, which is and thicker in the lowermost of the slope), clearly testify typical of poorly developed pedogenic horizons (Scarciglia for the influence of soil erosion and colluviation processes. et al. 2005). In addition, some original structures and fabric On the whole, reworked soil accumulation prevails at the of the parent rock are still preserved (Fig. 2). foot slope and toe slope, whereas erosion is active along The values of PD ranged from 1.61 to 2.42 g cm−3 with the shoulder- and back-slope position (Fig. 1). the lowest values in the A horizons, while it increased pro- The soil profiles are covered by a very dark brown lit- gressively in Bw and C horizons (Table 2). Soil BD varied ter layer or organic (O) horizon, which is generally about between 0.51 and 1.76 g cm−3 and gradually increased with 2–4 cm thick (Table 1), and consists of decomposed plant depth in all soil profiles analyzed (Table 2). Accordingly to remains, such as roots, branches, bark and leaves. others studies, this increase of soil BD might be related to Generally, A horizons, characterized by accumula- lower SOC content and higher compaction at greater profile tion of humified organic matter, exhibit a prominent dark depth, besides to the increase in coarse fragments (e.g., De brown color and a granular structure with a weak consist- Vos et al. 2005; Schrümpf et al. 2011; Grüneberg et al. 2014; ence, Bw or cambic horizons (USDA 2014), are yellowish Conforti et al. 2016) (Table 2). The C horizon shows some brown and have a subangular structure with weak to mod- differences in the bulk density that are most likely due to erate consistence, whereas soft mineral C horizons, poorly variations of weathering grade of the bedrock. affected by pedogenic processes and lacking properties of The values of TP were higher than the 30%, reaching a other genetic horizons (including some harder Cr layers, very high porosity (around 70%) for surface horizons and i.e., slightly weathered bedrock), show a brownish yellow slightly lower in the Bw horizons, due to the decrease of the color and exhibit a massive structure (Table 1). quantity of SOC and clay fraction (Tables 2, 3). In the hori- The field surveys have shown that the thickness of the zons examined, the TP varies from a minimum of 25.74% A horizon is linked to the local topography as well as to to a maximum of 68.32% with a light decrease with depth geomorphic processes that affect the site (Conforti et al. (Table 2). The main causes of the relatively high values of 2016). Therefore, the low depths (< 20 cm) of the A hori- TP for the A horizons can be related to granular structure zon was observed for the soils located along back-slope, and high bioturbation of these forest soils that created large characterized from high slope gradients (Fig. 1) and water planar voids which have been partially filled with granular erosion processes. On the contrary, along foot slope, as aggregates. well as in the toe slope, severe reworking and accumula- The results of the soil chemical properties are reported in tion of colluvial soil eroded from upslope areas lead to Table 3. Soil reaction varies from strong acidic to acidic with the formation of thicker A horizons (Table 1 and Fig. 2b). pH values included between 4.07 and 4.78, with no trend Selected physical properties of the soil profiles are with depth and significant differences between soil horizons. reported in Table 2. The skeleton was constituted by gra- Similar results have been observed by Vingiani et al. (2014) nitic rock fragments and within the soil profiles, shapes and Conforti et al. (2016) in others areas close to the study from sub-rounded to subangular with diameters ranging site. The high acidity of soils is due to the presence of acid- approximately from 0.5 to 10 cm were observed. Soil skel- parental granitic rocks, high precipitation in the study area, eton was abundant in the deepest horizons then gradually and then by the production of organic acids and high content decrease towards to top in all soil profiles (Table 2). In of organic substance (Conforti et al. 2016). particular, skeleton content ranged between 5.8% (P5: A Higher CEC values were obtained for the topsoil horizons horizon) and 32.1% (P3: C horizon). and decreased with soil depth for all the slope units; in par- Particle size distribution analysis showed the domi- ticular, CEC decreased abruptly in C horizon (Table 3). The nance of the sand and silt fractions with more than 80% CEC ranged between 3.45 and 26.38 cmol(+) ­kg−1 and due in all horizons and, consequently, low amounts of clay to the low amount of clay, the variability of CEC appears with the exception for profile P2 (Table 2). Following the mainly related to SOC (Scarciglia et al. 2008); therefore, a USDA soil texture classification, all the soil samples were significant positive correlation was found between CEC and classified as loam and silt loam (Table 1). In particular, the SOC (r = 0.92, p <0.001). lowest content of silt in an A horizon was measured in the SOC varied from 0.55 and 11.06% (Table 3) with the P3 profile (Table 2), where surface water erosion processes highest values in the surface horizons. As expected, a probably caused the loss of fine material. marked decrease of SOC for all profiles from A toward C horizons was observed (Table 3). The high values of SOC in 13 Table 1  Location and main morphological characteristics of soil profiles Environmental Earth Sciences Soil profile Coordinates (WGS84 Slope unit Altitude Slope gra- Slope curvature Surface Horizon Dry munsell colour Depth (cm) Structure Textural class UTM33N) (m a.s.l.) dient (°) stoniness (%) E (m) N (m) P1 608,179 4,261,925 Summit 1161 2.2 Flat 5 Oi/Oe 7.5YR 2/2 4–0 – – (2020) 79:59 A 7.5YR/3/2 0–26 gr, m 1 SLo Bw 10YR/5/4 26–73 sb m 2 L C 10YR/6/6 73 – 126 ma LS P2 608,167 4,261,913 Shoulder slope 1159 14.8 Concave 12 Oi/Oe 7.5YR 2/2 3–0 – – A 10YR/3/3 0–30 gr, sb m 1 L Bw 10YR/5/4 30–71 sb c 1 L C 10YR/6/6 71–109 ma LS P3 608,161 4,261,911 Back slope 1155 23.7 Concave 21 Oi 7.5YR 2/2 1–0 – – A 10YR/3/3 0–19 gr m 1 L Bw 10YR/5/4 19–45 sb c 2 SL C 10YR/6/7 45–76 ma LS P4 608,159 4,261,904 Foot slope 1151 9.4 Concave 8 Oi/Oe 7.5YR 2/2 2–0 – – A 10YR/3/3 0–44 gr, sb m 1 L Bw 10YR/5/6 44–105 sb c 2 SL C 10YR/7/6 105–176 ma LS P5 608,125 4,261,886 Toe slope 1149 4.3 Concave 4 Oi/Oe 7.5YR 2/2 4–0 – – A ­(A1 + A2) 7.5YR/3/2 0–63 gr, sb m 1 SLo C 10YR/6/6 63–81 ma LS Structure. Type: gr, granular; sb, subangular blocky; ma, massive; Size: f, fine; m, medium; c, coarse; Grade: 1, weak; 2, moderate. Soil texture class: SLo = silt loam; L = loam; SL = sandy loam; LS = loamy sand Page 7 of 16  59 13 59   Page 8 of 16 Environmental Earth Sciences (2020) 79:59 Table 2  Selected physical Soil profile Horizon Skeleton (%) Sand (%) Silt (%) Clay (%) PD (g cm−3) BD (g cm−3) TP (%) properties of the soil profiles P1 Oi/Oe – – – – – – – A 6.5 31.5 51.9 16.6 1.61 0.51 68.32 Bw 7.3 35.9 45.8 18.3 1.98 0.96 51.52 C 21.7 70.5 20.6 8.9 2.42 1.67 30.99 P2 Oi/Oe – – – – – – – A 9.1 33.0 46.5 20.5 1.96 0.73 62.76 Bw 10.6 51.4 34.8 13.8 2.20 1.02 53.64 C 18.3 75.2 19.4 5.3 2.37 1.76 25.74 P3 Oi – – – – – – – A 12.1 50.2 33.9 15.9 1.87 0.72 61.50 Bw 21.1 57.1 25.0 17.9 2.20 1.12 49.09 C 32.1 78.5 11.5 10.0 2.38 1.61 32.35 P4 Oi/Oe – – – – – – – A 17.8 48.1 42.4 9.5 2.19 0.84 61.70 Bw 22.1 68.1 22.8 9.1 2.31 1.15 50.22 C 29.2 74.7 17.2 8.1 2.41 1.57 34.85 P5 Oi/Oe – – – – – – – A 5.8 34.1 51.3 14.6 1.75 0.59 66.29 C 31.3 75.8 17.3 6.9 2.42 1.74 28.10 PD particle density, BD bulk density, TP total porosity Table 3  Selected chemical Soil profile Horizon pH (−) CEC SOC (%) STN (%) C/N (−) SOC stock STN stock properties, SOC stock and STN (cmol(+) (Mg ha−1) (Mg ha−1) stock of the soil profiles ­kg−1) P1 Oi/Oe – – – – – – – A 4.42 25.58 11.06 0.56 20.11 137.12 6.94 Bw 4.62 18.98 2.76 0.14 15.33 115.44 5.86 C 4.78 6.38 0.64 0.07 5.82 44.35 4.85 P2 Oi/Oe – – – – – – – A 4.33 16.78 5.52 0.33 17.25 109.89 6.37 Bw 4.62 10.38 2.04 0.15 13.60 76.27 5.61 C 4.68 3.45 0.55 0.07 7.86 30.05 3.82 P3 Oi – – – – – – – A 4.17 12.18 3.26 0.29 12.54 39.20 3.49 Bw 4.38 11.78 1.97 0.16 12.31 45.26 3.68 C 4.33 6.8 0.65 0.11 5.42 22.03 3.73 P4 Oi/Oe – – – – – – – A 4.26 18.78 4.54 0.31 16.21 137.73 9.40 Bw 4.21 9.38 1.91 0.14 13.64 104.38 7.65 C 4.18 5.86 0.84 0.09 7.64 66.29 7.10 P5 Oi/Oe – – – – – – – A 4.07 26.38 8.88 0.43 20.65 310.93 15.06 C 4.75 3.91 0.58 0.10 4.83 12.48 2.15 CEC cation exchange capacity the A horizons are related to a large amounts of organic mat- the results showed that, generally, different concentrations ter due to litter accumulation, which is promoted by humid of SOC are related to different slope position of A horizon and cool climatic conditions of the study area. Moreover, (Table 3) pointing out the importance of landform position 13 Environmental Earth Sciences (2020) 79:59 Page 9 of 16  59 in controlling soil water content and consequently the SOC concentration. Moore et al. (1993) pointed out that slope and wetness index can account for about one-half of the variabil- ity in SOC concentration. Lower positions are expected to receive additional runoff water from upper positions, which complicate the relationship between landform shape and SOC concentration. Higher SOC concentrations are often associated with lower bulk densities and vice versa (Con- forti et al. 2016). Large amount of carbon in the top soil horizon seems to be associated with the accumulation of plant remains and roots in this layer. The decay of these roots leaves carbon, which is then accumulated in soil. The concentration of STN is directly proportional to the SOC and covered a range from 0.07 to 0.55% (Table 3). Gen- erally, STN shows the same pattern of the SOC along down the profile confirming the strong relation between the two soil properties (r = 0.98, p <0.001). The carbon to nitrogen (C/N) ratio decreased from the surface to the deep horizons ranging from 20.65 to 4.83 (Table 3) and both values were observed in the profile P5. High C/N values in the A horizons indicate high biological activity (Callesen et al. 2003). The amount of stocks for SOC and STN, estimated for each soil profiles, are reported in Table 3. The SOC stocks Fig. 3  Vertical distribution of SOC stock (a) and STN stock (b) along ranged from 12.48 to 310.93 Mg h−1, whereas the STN stock the soil profiles opened within of each slope unit ranged from 2.15 to 15.06 Mg ha−1 (Table 3). The topsoil horizons stored generally more SOC and STN compared chemical composition of major elements in soil samples to towards the deepest horizons, as it was predictable and show the dominance of ­SiO2, ­Al2O3 and F ­ e2O3 (Table 4). observed by Conforti et al. (2016). The maximum amounts In particular, A horizons have mean values of 44.21% and in both SOC stock and STN stock were found in the A hori- 16.59% for silica ­(SiO2) and alumina ­(Al2O3), respectively, zon of the soil profile P5 (Table 3) located at the toe slope. and subordinate amounts of ­Fe2O3, with a mean value of Overall, the variations in the SOC and STN stocks were 9.82%. In all soil profiles, the content of S ­ iO2 decreased affect by the slope position. Figure 3 shows the SOC and regularly from C to A horizons, varying from 68.64% to STN stocks across the slope units: as predictable, highest 38.65, whereas the ­Fe2O3 content increased (Table 4). The SOC and STN stocks were found into upper and lower part overall decrease in S ­ iO2 in the A and Bw horizons is consist- of the soil catena, while the lowest contents were observed ent with a progressive weathering of primary minerals lead- along back-slope position, characterized by convex shape ing to the formation of secondary phases with an enrichment and where the water erosion processes have determined the in Fe and Al or to solubilized products (Egli et al. 2003). transport of soil fine particles and SOC. Summarizing, SOC That is in turn associated to an increase of ­Al2O3, probably and STN storage increased with a diminution in slope gradi- fixed in the neoformed phyllosilicate clays (Scarciglia et al. ent, and vice versa. 2016). This interpretation is also supported by the CIA index In addition, SOC and STN stock distribution results indi- (Fig. 4). This index is based on the progressive removal of cate that the storage of carbon and nitrogen in the individual soluble cations (e.g., Ca, Na, and K) from minerals dur- horizons is related to soil type and degree of pedogenetic ing chemical weathering, and reflects the proportion of pri- processes development (Rumpel et al. 2002). mary and secondary minerals in the bulk sample (Nesbitt and Young 1982). The CIA values for all soil samples were Geochemical data always within the range of 60–80, which indicates a moder- ate degree of weathering (Fedo et al. 1995). Furthermore, Table 4 reports the major soil elements determined by the the LOI concentrations (Table 4) show a continuous increase XRF analysis. There were no significant differences in major with increasing values of the CIA index from 4.19% (P2, elements content among the five soil profiles of the topose- C horizon) to 24.70% (P1, A horizon), which suggests an quence. The composition of the investigated soils clearly increase of hydrated neogenic phases such as clay minerals reflects its granitic parent material; therefore, the total and Fe(-Mn)-hydroxides (Perri et al. 2016), along with a 13 59   Page 10 of 16 Environmental Earth Sciences (2020) 79:59 LOI = loss on ignition; OM = organic matter, whereby the total organic C was determined via Shimadzu TOC-L analyzer and multiplied by the factor 1.72; IVC = inorganic volatile compounds, IVC (%) 5.68 9.60 6.55 8.44 7.56 3.25 8.61 7.85 4.98 4.96 6.26 6.25 8.51 4.28 OM (%) 19.02 6.14 1.27 9.49 3.51 0.95 5.61 3.49 1.12 7.81 3.29 1.62 15.27 1.00 LOI (%) 24.70 15.74 7.82 17.93 11.07 4.19 14.22 11.34 6.10 12.77 9.55 7.87 23.78 5.28 Fe2O3 (%) 11.55 11.12 5.45 11.33 8.86 2.06 8.40 8.40 5.56 8.43 7.10 5.82 9.80 2.88 MnO (%) 0.09 0.06 0.06 0.06 0.06 0.07 0.05 0.06 0.06 0.06 0.12 0.05 0.09 0.05 TiO2 (%) 1.29 1.33 0.66 1.29 1.01 0.18 0.96 1.07 0.70 1.04 0.86 0.67 1.08 0.38 CaO (%) Fig. 4  Comparison of chemical index alteration (CIA) values among the soil horizons of the five profiles analyzed along soil catena. The 0.97 0.62 1.00 0.81 0.86 2.73 0.75 1.09 1.85 1.05 0.97 0.98 0.82 1.41 Table 4  Total chemical elemental content (given as oxides) of all soil samples, obtained by XRF spectroscopy CIA index is widely used to evaluate the chemical alteration intensity in soils. High values indicate an intense alteration, while low values reflect limited chemical transformations K2O (%) 2.06 2.30 3.84 2.30 2.84 2.56 3.05 3.14 3.42 3.12 3.22 3.60 2.34 3.38 contribution of organic matter, especially in the A horizons. In line with this behavior, the vertical variations in the CIA P2O5 (%) were similar in all five soil profiles (Fig. 4): values gen- 2.65 2.48 2.67 1.39 2.42 0.09 2.97 0.64 2.37 1.02 0.98 2.95 2.67 0.11 erally increased from C horizons towards upper horizons. Moreover, the very high positive correlation between LOI SiO2 (%) and SOC clearly points to a dominance of organic matter in 38.65 43.54 53.63 44.51 48.34 59.78 47.24 52.46 54.01 51.43 53.99 55.56 39.22 68.64 the volatile components. The contents of other elements, for all hori- zons, progressively decrease in the following pattern: Al2O3 (%) ­Na2O > K2O > P2O5 > MgO > TiO2 > CaO > MnO (Table 4). 14.40 18.55 19.92 17.50 20.35 23.91 17.73 18.89 20.76 18.04 19.83 16.82 16.09 13.99 These results show a decrease in ­Na2O and K ­ 2O contents from the C to A horizons that is due to their high mobility controlled by the very low ionic potential (ionic charge/ionic MgO (%) radius) of ­Na+ and K­ + and their leaching, whereas ­TiO2 1.15 1.21 1.05 1.30 1.19 0.30 1.28 1.13 1.09 1.18 1.12 0.92 1.08 0.72 content slightly increases from C towards the A horizons, where it is likely concentrated in the fine fraction (Tyler Na2O (%) 2004; Scarciglia et al. 2016) (Table 4). 2.92 2.60 3.87 1.58 3.00 4.13 3.18 2.71 4.24 1.86 2.27 4.75 3.03 3.14 Geophysical data estimated by LOI minus OM Horizon In the 2D-ERT section (Fig. 5), the depth of prospection in the Bw Bw Bw Bw middle parts of the section reached about 4 m. Spatial distri- A A A A A C C C C C bution of resistivity shows a significant variation at different Soil profile depths along the section line (Fig. 5a). The resistivity values range between 20 and 4309 Ω m with the largest values mainly located at 3 m of depth-down (Fig. 5a). High variability in P1 P2 P3 P4 P5 13 Environmental Earth Sciences (2020) 79:59 Page 11 of 16  59 electrical resistivity values might be due to the combined effect a strong influence of coarse-grained parent material. In all of differences in soil properties, weathering condition of the soil profiles, a decrease in the clay fraction with depth was granitic bedrock, and soil water content. However, the water observed (Table 2) that also determined a decrease in TP and content should have little contribution to the electrical resis- an increase in BD, with a consequent increase of PD in the tivity variability, because soils are well drained and resistivity C horizons. Stolt et al. (1991) suggested that high values of measurements have been carried out in dry periods. BD and PD in the deep horizons are related to high content The 2D-ERT results were calibrated using the five soil of sand including quartz (particle density: 2.65 ­gcm−3) and profiles and the seven hand augers (Fig. 1). Therefore, the feldspar (2.55–2.76 ­gcm−3). 2D-ERT data coupled with field data have allowed con- Parent rock has shown to have a strong influence on the structing and mapping soil depth, spatial distribution of chemical composition of the studied soils. All soil profiles different soil horizons and showing the contact between had high values of ­SiO2 and ­Al2O3, mostly due to inherit- solum and parent material along the soil catena (Fig. 5a). ance from the underlying granitic parent material and sub- The electrical resistivity values show that transition from ordinately to the neoformed aluminosilicate minerals. Some soil to parent rock was progressive (Fig. 5a). Generally, soil of the highest correlation coefficient are clearly controlled resistivity varied from 200 to 930 Ω m; in particular, low by the parent rock and the intrinsic geochemical affinities values of resistivity were recorded in the A horizons which among chemical elements (e.g., all the negative correlations are characterized by low bulk density, higher content of between ­SiO2 and ­Fe2O3 (− 0.99), ­SiO2 and ­TiO2 (− 0.86) finer materials as silt and clay fractions, high porosity, and etc., because of silica strongly prevailing over other species high SOC content (Table 2). In addition, in the upper soil in quartz-dominated samples; or the positive correlations horizons, values of electrical resistivity less than 200 Ω m between ­TiO2 and F ­ e2O3 (r = 0.99, p  < 0.001), between ­TiO2 were found and they should be due to tree root systems, and MgO (r = 0.88, p  < 0.001), between ­Fe2O3 and MgO where high water contents frequently are located (Rossi et al. (0.83, p  < 0.001) related to ferromagnesian minerals), and 2011). Though values greater than 930 Ω m and up to about several intermediate values explained by secondary cations 1500 Ω m were found in the C horizon (Fig. 5a), which is included in crystal lattices of feldspars/plagioclases, micas, characterized by high values of bulk density, coarse texture, etc. (Table 5). In turn, the geochemical data indicate that the and low porosity (Table 2). ­Fe2O3 content increased with weathering intensity, as shown In agreement with field observations, the 2D-ERT data by the values of CIA, clearly indicating an increase of iron showed that electrical resistivity of bedrock varies with frac- release from the weathered host minerals (mainly biotite and turing and weathering of granitic rock conditions (Fig. 5b). other micas) and concentration as pedogenic iron oxides. The parent rock was mainly characterized by electrical resis- The SOC contents showed a significant negative correla- tivity values above 1500 Ω m (Fig. 5a). Nevertheless, in the tion with ­Al2O3, ­SiO2, ­K2O, and CaO, whereas highlighted upper part of the catena, low values of resistivity were also a significant positive correlation with MgO, ­TiO2, MnO, found and likely, due to the presence of highly weathered and ­Fe2O3 (Table 5). For the STN contents, a significant rocks, characterized by higher clay content and consequent positive correlation with MgO, T ­ iO2 and F­ e2O3, was also higher moisture content (Ritz et al. 1999). observed. Contrasting significant negative correlations were obtained with ­Al2O3, ­SiO2, and ­K2O (Table 5). These obser- vations, in agreements with White et al. (1996), suggest that Discussion the geochemical elements can control soil nutrient content. Besides, the CIA index showed a significant positive correla- As previously reported by many studies, soil heterogeneity tion (r = 0.61, p <0.01) with C/N ratio, highlight that weath- is elevated even at small spatial scales (e.g., Cambardella ering processes fulfill an important function in the biological et al. 1994; Pätzold et al. 2008). Therefore, the adoption of activity of the soil. (Riebe et al. 2004). accurate soil-landscape analysis is crucial for evaluating the The studied soil profiles are similar to those described in interplay between pedogenetic processes and geomorphic the Serre Massif by Vingiani et al. (2014), where the upper dynamics, as well as for monitoring SOC stock changes. organic-mineral (A) horizons, representing diagnostic umbric The granitic bedrock characterized by varying degrees of epipedons, are characterized by dark brown colors, high SOC weathering has a key role on the development of different content, low BD, relatively high TP, and a weak granular soil types, which vary from Inceptisol (Humic Dystrudepts) structure. The very low BD values measured in the topsoils to Entisols (Lithic Udipsamments). Moreover, the variabil- are also consistent with a possible allochthonous volcanic ity in soil characteristics is related to topographic features ash input contributing to soil formation in addition to the and water erosion processes. The five soil profiles denoted a local granite bedrock (Scarciglia et al. 2008; Vingiani et al. relatively young pedogenesis with weak horizon differentia- 2014). In addition, the results showed that along foot slope tion and, mainly, loamy to loamy sand textures, which show and toe slope, reworking and accumulation of colluvial soil 13 59   Page 12 of 16 Environmental Earth Sciences (2020) 79:59 Fig. 5  a 2D-ERT profile with location of soil profiles and auger drilling; b pedo-geological cross section interpreted by means 2D-ERT profile and soil characterization material, eroded from upslope areas, lead to the formation of coherently high in A horizons. Moreover, the results show thicker accretionary A horizons. In line with this behavior, that surface horizons have a much higher SOC content than these topsoils often consist of two overlying A horizons, as the deepest horizons, related to the abundance of beech litter in soil profile P5 (at the base of the soil catena), where the and continuous formation of carbon resulting from decom- upper half part of the organic-mineral horizon is darker than position of plant and root remains. In addition, colluvial its lower part (Fig. 2b). Consequently, SOC stock appears processes are clearly recorded by abrupt boundaries which 13 Environmental Earth Sciences (2020) 79:59 Page 13 of 16  59 Table 5  Person’s correlation between total chemical elemental content, SOC and STN Na2O MgO Al2O3 SiO2 P2O5 K2O CaO TiO2 MnO Fe2O3 LOI SOC STN Na2O 1.00 MgO − 0.52* 1.00 Al2O3 0.15 − 0.32 1.00 SiO2 0.28 − 0.66** 0.22 1.00 P2O5 0.42* 0.49* − 0.22 − 0.62** 1.00 K 2O 0.36 − 0.14 0.16 0.69** − 0.05 1.00 CaO 0.41* − 0.84*** 0.56** 0.57** − 0.51** 0.12 1.00 TiO2 − 0.60** 0.88*** − 0.37 − 0.86*** 0.42* − 0.56** − 0.78*** 1.00 MnO − 0.28 0.09 0.06 − 0.30 − 0.08 − 0.29 − 0.07 0.17 1.00 Fe2O3 − 0.56** 0.83*** − 0.35 − 0.90*** 0.46* − 0.63** − 0.77*** 0.99*** 0.18 1.00 LOI − 0.39 0.57** − 0.54** − 0.91*** 0.43* − 0.75*** − 0.59** 0.82*** 0.34 0.85 1.00 SOC − 0.35 0.40* − 0.55** − 0.80*** 0.30 − 0.73*** − 0.40* 0.66** 0.42* 0.70*** 0.95*** 1.00 STN − 0.34 0.43* − 0.58** − 0.76*** 0.29 − 0.68** − 0.39 0.66** 0.33 0.68** 0.92*** 0.98*** 1.00 Significant level: *p < 0.05; **p < 0.01; ***p < 0.001 separate the thick A horizons from the underlying, buried representative soil catena located in a mountain landscape C horizon, as clearly observed in the field (see profile P5). of the southern Italy. Sharp boundaries between different soil horizons were also Based on the results obtained, the granitic bedrock char- described in the other soil profiles upslope (e.g., soil profile acterized by varying degrees of weathering has a key role P2) and indicate coexistence or alternation of geomorphic on the development of different soil types, which vary from dynamics (erosion and/or deposition) with soil-formation Inceptisols (Humic Dystrudepts) to Entisols (Lithic Udip- processes. Thus, diffusive transport of soil particles along samments). Moreover, variability in soil characteristics is slope may explain the variations of the physical and chemical related to topographic features and water erosion processes. soil properties that were observed in the different slope units, Summarizing, the five soil profiles denote a relatively young as well as the SOC and STN stocks evaluated. pedogenesis with weak horizons differentiation and mainly The summit areas, foot and toe slopes have much greater loamy and loamy sand textures classes, which show a strong values of SOC and STN stocks than those at the back slope influence from the coarse-grained parent material. and shoulder slope. Indeed, foot and toe slopes have soils The proposed integrated approach confirms that the land- thicker and richer in organic matter than shoulder and back- scape position play a significant role on the development of slope positions, where steep gradients are interested by water erosion–deposition processes and consequently for the soil erosion processes (Gregorich et al. 1998). During field work, formation. In addition, the slope morphology affects greatly was observed that the back-slope unit is affected by surface the storage of SOC and STN. water runoff as revealed by the exposure of tree roots on the Finally, the electrical resistivity profile allowed to do surface and by a more high surface stoniness, compared to a quick screening on spatial variability of the soils and to others slope units (Table 1). predict the soil thickness variations along the toposequence Therefore, it can be hypothesized that soils located in the with limited disturbance and number of soil samples. There- steeper part of the catena (back-slope) have been truncated fore, the results show how 2D-ERT technique could applied by erosion processes, which have thinned the A horizon and to define sampling designs to analyze local soil variability. consequently reduced the content of SOC, STN, and fine Moreover, the results prove the 2D-ERT to be a valu- soil fractions. In turn, the soils in the lower slope position able tool for investigating variability of some soil features have received colluvial material and humus from the eroded and parent material. 2D-ERT profile allowed to define the upper part of the slope. horizontal and vertical variability of soil features from the interpretation of resistivity spatial pattern within soil catena. 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