Chemical and biochemical properties of soils developed from different lithologies in Northwestern Spain (Galicia)

Article Chemical and Biochemical Properties of Soils Developed from Different Lithologies in Northwestern Spain (Galicia) Valeria Cardelli 1,*, Stefania Cocco 1, Alberto Agnelli 2, Serenella Nardi 3, Diego Pizzeghello 3, Maria J. Fernández-Sanjurjo 4and Giuseppe Corti 1 1Department of Agricultural, Food and Environmental Sciences, UniversitàPolitecnica delle Marche, Ancona 60131, Italy; [email protected] (S.C.); [email protected] (G.C.) 2Department of Agricultural, Food and Environmental Sciences, Universitàdegli Studi di Perugia, Perugia 06121, Italy; [email protected] 3Department of Agronomy, Food, Natural Resources, Animals and the Environment, Universitàdegli Studi di Padova, Legnaro 35020, Italy; ser[email protected] (S.N.); [email protected] (D.P.) 4 Department of Soil and Agronomic Chemistry, Universidade de Santiago de Compostela, Lugo 27001, Spain; [email protected] *Correspondence: v[email protected]; Tel.: +39-338-599-6867 Academic Editors: Adele Muscolo and Miroslava Mitrovic Received: 15 March 2017; Accepted: 19 April 2017; Published: 22 April 2017 Abstract: Physical and chemical soil properties are generally correlated with the parent material, as its composition may influence the pedogenetic processes, the content of nutrients, and the element biocycling. This research studied the chemical and biochemical properties of the A horizon from soils developed on different rocks like amphibolite, serpentinite, phyllite, and granite under a relatively similar climatic regime from Galicia (northwest Spain). In particular, the effect of the parent material on soil evolution, organic carbon sequestration, and the hormone-like activity of humic and fulvic acids were tested. Results indicated that all the soils were scarcely fertile because of low concentrations of available P, exchangeable Ca (except for the soils on serpentinite and phyllite), and exchangeable K, but sequestered relevant quantities of organic carbon. The scarce soil fertility was common to all soils independently of the parent material, and we attributed this similarity to the pedogenetic pressure induced by the climatic conditions. Also, the hormone-like activity of humic and fulvic acids, similar for all the soils, was probably due to pedogenesis. We hypothesized that the hormone-like activity of the humic substances helps growth and diffusion of vegetation in low fertile soils and, consequently, soil organic carbon sequestration too. Keywords: amphibolite; serpentinite; phyllite; granite; humic substances; humic acids; fulvic acids; auxin-like activity; indoleacetic acid 1. Introduction Lithology is one of the driving factors to determine soil chemical, physical, and mineralogical properties. Studies have confirmed correlations between parent material and soil texture, pH, porosity, and moisture content [ 1 , 2 ], but also nutrient cycling [ 3 ] and fertility [ 4 ]. Thus, these characteristics are able to select the aboveground vegetation [ 5 , 6 ] and the soil biological features [ 7 ] even in environments that are extreme for climatic or geochemical reasons. For example, Kooijman et al. [ 8 ] observed that the bedrock type influenced the ecosystem functioning in a Mediterranean forest of northeast Spain, with small tree height and scarce vegetation cover on granite derived soils. However, although granitic rocks are generally nutrient poor parent materials, some varieties have a chemical and mineralogical Forests 2017,8, 135; doi:10.3390/f8040135 www.mdpi.com/journal/forests Forests 2017,8, 135 2 of 16 composition that may induce good soil fertility [ 9 ]. The parent material composition may largely influence the rate of soil carbon sequestration, even though pedogenesis and element biocycling can affect this rate, with consequences on environment and human health [ 10 ]. For example, metamorphic rocks such as serpentinite and gabbro produce soils with a natural abundance of heavy metals like Ni, Zn, Mn, Cr, Cd, Co, and Cu [ 11 , 12 ]. Additionally, soils derived from these rocks are often poor of essential nutrients such as Ca, K, and P, have a high Mg/Ca ratio [ 13 ], and high levels of Mg and Fe [ 14 ]. Nonetheless, under certain environmental conditions these soils may accumulate an amount of organic matter that, through the formation of stable complexes between humic substances and heavy metals, reduces the bioavailability of these latter [ 15 , 16 ]. However, the vegetation growing on soils developed from these lithologies is adapted to the various disadvantageous conditions and, apart from the positive activity exerted by the humic substances, plants have developed biochemical and biological adaptations to limit heavy metals uptake and translocation (e.g., [ 17 ]) and overcome chemical and nutritional stresses (e.g., [ 14 , 18 ]). Among the strategies adopted, symbiosis with arbuscular mycorrhizal fungi allow vegetation to increment its edaphic tolerance (e.g., [ 19 ]). Among the soil-plant relationships that are functional in terms of improving plant resilience and, consequently, biomass production and soil carbon sequestration, the hormone-like activity of the humic substances seems to have an important role [ 20 – 22 ]. Humic substances exert a hormone-like activity due to the presence of indole-3-acetic acid (IAA), a plant hormone of the auxin class [ 23 , 24 ]. The influence of humic substances on plant development is strictly correlated to the quantity of IAA, which can have an inhibitory effect at concentrations between 0.1 and 1 mM, but may stimulate the growth of plants at concentrations ranging from 0.1 µ M to 0.1 mM through direct influence on physiological targets [ 20 ]. In soils under cold climatic conditions, the hormone-like acitivity of the humic substances is fundamental for allowing some species to survive [25]. In Galicia (northwest Spain), a wide lithological variability generated a considerable pedo-diversity [ 26 , 27 ]. Here, the humid climate provides precipitations much higher than the evapotranspiration for long periods. This fact, together with an accelerated chemical weathering due to the mild temperature and a soil acidification favored by the non-carbonatic nature of the lithologies, induces an intense cation leaching and the genesis of poor soils [ 28 ]. On granite and quartz schists, these environmental conditions cause the release of Al in different forms and, because of the acidic soil pH and the scarcity of clay minerals, this Al can enter the soil solution and threaten the vegetation [ 29 ]. However, a high organic matter content, and specifically that of humic substances, may promote the formation of stable organo-Al complexes, and thereby reduce the soil Al activity [ 30 ]. In soils developed from serpentinite, the abundance of heavy metals is high, but the formation of organo-metal complexes reduces the availability of these elements (e.g., [ 31 ]). Also, in these soils, we suspected that humic substances might have a hormone-like activity and favor soil carbon sequestration by stimulating resilience and biomass production of the vegetation. The aim of the present study was to evaluate the effect of the lithology on chemical and biological properties of the superficial mineral horizon of soils developed from four parent rocks. In particular, we assessed the differences in terms of soil evolution, soil carbon sequestration, and hormone-like activity of humic and fulvic acids. The selected soils developed from amphibolite, serpentinite, phyllite, and granite in a restricted area of Galicia (northwest Spain) where the soil temperature and moisture regimes were similar. Forests 2017,8, 135 3 of 16 2. Materials and Methods 2.1. Study Sites The study area was located in the provinces of La Coruña and Lugo, in Galicia (northwest Spain) (Figure 1). Taking advantage of the elevated lithological variability of the region, we selected five locations developed from four different parent rocks: amphibolite, serpentinite, phyllite, and granite. On granite, we took into consideration two areas of the same location with different vegetation: a former Pinus sylvestris L. wood that was cut four years before this study, and a Quercus robur L. wood 40–60 years old. The sampling areas receive a mean annual precipitation ranging from 845 to 1619 mm, and have a mean annual temperature from 10.5 to 12.2 ◦ C, with mean summer and mean winter temperatures that differ by more than 6 ◦ C. Because of this, the soil moisture regime is udic and the soil temperature regime is mesic for all the soils. Details on vegetation and environmental conditions of each area are provided in Table 1. Forests2017,8,135 3of16 2.MaterialsandMethods 2.1.StudySites ThestudyareawaslocatedintheprovincesofLaCoruñaandLugo,inGalicia(northwest Spain)(Figure1).Takingadvantageoftheelevatedlithologicalvariabilityoftheregion,weselected fivelocationsdevelopedfromfourdifferentparentrocks:amphibolite,serpentinite,phyllite,and granite.Ongranite,wetookintoconsiderationtwoareasofthesamelocationwithdifferent vegetation:aformerPinussylvestrisL.woodthatwascutfouryearsbeforethisstudy,andaQuercus roburL.wood40–60yearsold.Thesamplingareasreceiveameanannualprecipitationrangingfrom 845to1619mm,andhaveameanannualtemperaturefrom10.5to12.2°C,withmeansummerand meanwintertemperaturesthatdifferbymorethan6°C.Becauseofthis,thesoilmoistureregimeis udicandthesoiltemperatureregimeismesicforallthesoils.Detailsonvegetationand environmentalconditionsofeachareaareprovidedinTable1.    Figure1.MapofGalicia(northwestSpain)indicatingthestudylocations. Figure 1. Map of Galicia (northwest Spain) indicating the study locations. Forests 2017,8, 135 4 of 16 Table 1. General features and morphology of one of the two soil profiles opened on different lithologies in Galicia (northwest Spain). Horizon aDepth (cm) Color bStructure cRoots dBoundary eThickness (cm) Other Observations Parent rock: Amphibolite Location: Arzúa (La Coruña). Soil use: Pinus radiata D. Don. wood. Altitude: 386 m. Mean annual precipitation: 1619 mm; mean annual air temperature: 12.2 ◦ C; absolute minimum temperature: 2.9 ◦ C; absolute maximum temperature: 24.6 ◦ C. Vegetation: Pinus radiata, Eucalyptus globulus,Quercus robur,Betula pubescens. Understory: Pteridium aquilinum,Ulex europaeus,Erica arborea,Rubus sp., Gramineae. Soil: coarse-loamy, mixed, mesic, Typic Udorthent [32]. Oi1 9–5 - - 0 aw 3–7 Stems, leaves, and bark of P. radiata and E. globulus, stems and leaves of Q. robur,B. pubescens,U. europaeus, and P. aquilinum; few mesofauna, mostly midges. Oi2 5–1 - - v1, vf, f cw 3–6 Stems, leaves, and bark of P. radiata and E. globulus, stems and leaves of Q. robur,B. pubescens,U. europaeus, and P. aquilinum. Oi3 1–0 - - 0 aw 1–2 Fragmented stems, leaves, and bark of different species. A 0–26 7.5YR 2.5/2 3 f,m cr, fr 3 mi,vf,f,m,co cw 23–30 - Bw 26–38 7.5YR 4/2 3 m sbk, fr 3 mi,vf,f,m; 2 co cw 10–15 - BC 38–48 7.5YR 5/2 2 f abk, fr 2 mi,vf,f,m,co cw 7–10 - Cr 48–56+ 7.5YR 5/2 - 1 mi,vf,f; 2 m,co - - - Parent rock: Serpentinite. Location: Melide (La Coruña). Soil use: Natural environment. Altitude: 454 m. Mean annual precipitation: 1195 mm; mean annual air temperature: 11.9 ◦C; absolute minimum temperature: −4.4 ◦C; absolute maximum temperature: 33.2 ◦C. Vegetation: Erica umbellata,Erica scoparia,Calluna vulgaris,Ulex europaeus, Pinus radiata (scrawny), Rubus ideaus, gramineae, Cladonia lichen enveloping the E. umbellata stems. Soil: loamy-skeletal, mixed, mesic, Oxyaquic Udorthent [32]. Oi 2–0 - - 0 aw 1–2 Stems and leaves mostly of E. umbellata,E. scoparia, and U. europaeus; very few mesofauna, mostly ants. A1 0–2 10YR 4/4 3 f m cr, fr 2 mi,vf,f,m cw 1–5 In the micro-depressions there is a thin crust. A2 2–8 10YR 5/4 2 f,m cr, fr 1 mi,vf,f,m cw 4–6 - BCg 8–17 10YR 6/3 2 f,m abk-sbk, fr 0 cw 4–9 Abundant mottles; small pockets of translocated fines. R 17–22+ 10YR 4/2 - 0 - - Thick cutans of translocated fines at the surface of the rock. Parent rock: Phyllite. Location: Guntín (Lugo). Soil use: Natural environment. Altitude: 460 m. Mean annual precipitation: 845 mm; mean annual air temperature: 11.7 ◦C; absolute minimum temperature: −6.6 ◦C; absolute maximum temperature: 32.0 ◦C. Vegetation: Erica scoparia,Erica umbellata,Erica cinerea,Calluna vulgaris, Pterospartum tridentatum,Ulex europaeus,Quercus robur,Betula pubescens,Erica scoparia,Halimium lasianthum. Soil: loamy-skeletal, mixed, mesic, Typic Udorthent [32]. Oi1 3–2 - - 0 cw 1–3 Stems and leaves mostly of E. scoparia,E. umbellata,E. cinerea,C. vulgaris,P. tridentatum, and U. europaeus; very few mesofauna, mostly ants. Oi2 2–0 - - 3 mi,vf aw 2–3 Stems and leaves mostly of E. scoparia,E. umbellata,E. cinerea,C. vulgaris,P. tridentatum, and U. europaeus. A1 0–8 7.5YR 3/4 2-3 f,m,c cr, vfr + sg 3 f,m; 2 mi,vf,co cw 6–8 Skeleton: 30%. A2 8–14 7.5YR 4/4 3 f,m cr, vfr + sg 2 mi,vf,f,m cw 5–8 Skeleton: 60%. A/C 14–23 7.5YR 4/2 sg to 1 f,m cr, vfr v1mi,vf,f cw 5–8 - Cr 23–44 2.5Y 4/2 - v1mi,vf cw 15–22 - C 44–80+ 2.5Y 4/2 - 0 - - - Forests 2017,8, 135 5 of 16 Table 1. Cont. Horizon aDepth (cm) Color bStructure cRoots dBoundary eThickness (cm) Other Observations Parent rock: Granite. Location: Mount Segade (Lugo). Soil use: Former Pinus sylvestris L. plantation that was cut four years before this study. Altitude: 397 m. Mean annual precipitation: 845 mm; mean annual air temperature: 10.5 ◦C; absolute minimum temperature: −6.8 ◦C; absolute maximum temperature: 29.2 ◦C. Vegetation: seedlings of Quercus robur,Pteridium aquilinum,Calluna vulgaris, Ulex europaeus,Lotus corniculatus,Rubus sp., Lonicera periclymenum,Cytisus sp., Hedera elix. Soil: loamy, mixed, mesic, Typic Humudept [32]. Oi 1–0 - - 0 cw 1–2 Remainders of the P. sylvestris wood and cut, stems and leaves of Q. robur,P. aquilinum,C. vulgaris,U. europaeus. A1 0–9 10YR 2/1 2 fcr, fr 3 mi,vf,f,m; 2 co cw 8–11 - A2 9–27 10YR 2/2 2 f,m cr, fr 2 mi,vf,f; 1 m cw 16–19 - AB 27–40 10YR 3/4 2 f,m sbk-cr, fr 2 mi,vf,f,m cw 13–15 - Stone line 40–45 10YR 4/4 2 f,m sbk, fr 2 mi,vf,f,m cw 5–7 Mostly made of quartzite stones. Bw1 45–58 10YR 5/6 2 f,m abk, fr 2 mi,vf,f cw 12–15 - Bw2 58–77 10YR 6/6 2 f,m abk, fr 1 mi,vf,f cw 18–21 - Cr 77–84+ 10YR 7/310YR 6/3 - v1mi, vf - - - Location: Mount Segade (Lugo). Soil use: Quercus robur L. wood 40–60 years old. Altitude: 423 m. Mean annual precipitation: 845 mm; mean annual air temperature: 10.5 ◦C; absolute minimum temperature: −6.8 ◦C; absolute maximum temperature: 29.2 ◦C. Vegetation: Quercus robur. Understory: few Quercus robur seedling. Soil: loamy, mixed, mesic, Typic Humudept [32]. Oi 8–5 - - 0 cw 3–4 Stems and leaves of Q. robur. Oe 5–0 - - 1 vf,f cw 5–7 - A1 0–22 10YR 4/2 3 f,m,c cr, fr 3 mi,vf,f,m,co cw 20–22 - A2 22–37 10YR 4/3 2 f cr, vfr 3 mi,vf,f,m; 2 co cw 13–16 - AB 37–48 10YR 3/3 2 f,m cr-sbk, vfr 2 mi,vf,f,m,co cw 10–13 - Bw1 48–61 10YR 4/4 2 f,m sbk, vfr 2 mi,vf,f; 1 m,co cw 12–15 - Bw2 61–81 10YR 4/6 2 f,m sbk, vfr 1 mi,vf,f,m cw 17–22 - Cr 81–91+ 10YR 7/410YR 7/3 - v1mi,vf,f - - - a horizons’ designation according to Schoeneberger et al. [ 33 ]; b moist and crushed, according to the Munsell Soil Color Charts; c sg = single grain, 1 = weak, 2 = moderate, 3 = strong; f = fine, m = medium, c = coarse; cr = crumb, abk = angular blocky, sbk = subangular blocky; fr, friable; vfr, very friable; d 0 = absent, v 1 = very few, 1 = few, 2 = plentiful, 3 = abundant; mi = micro, vf = very fine, f = fine, m = medium, co = coarse; ea = abrupt, c = clear; w = wavy. Forests 2017,8, 135 6 of 16 2.2. Soil Sampling In each location, we conducted a soil survey with the opening of auger holes and mini-pits to select a study site representing the average condition of the place. In every study site, two soil profiles were dug until the parent rock; each profile was 8–12 m away one from the other. The soils were morphologically described according to Schoeneberger et al. [ 33 ]. Except for the soils on amphibolite, in all the other cases there were two A horizons, which were collected together to form a whole sample. Hence, a large amount of the A horizon (at least 2 kg) was collected from each profile. At each site, two additional samples of A horizon were taken in the surrounding of the profiles in order to have four samples per site. The samples were stored in a portable refrigerator. Once in the laboratory, the samples were allowed to air-dry and were subsequently sieved using a 2 mm mesh. 2.3. Physical and Chemical Analyses Mineralogy was assessed by X-ray diffraction with a Philips PW 1830 diffractometer (Fe-filtered Co K α 1 radiation, input voltage of 35 kV, and current of 25 mA). After identification of the minerals on the basis of their characteristic peaks, a semi-quantitative estimation of the mineralogical assemblage was obtained by estimating the area of the diagnostic peak by multiplying the peak height by the width at half-height. Soil pH in H 2 O and in KCl (solid:liquid ratio 1:2.5) was measured by a combined glass-calomel electrode. Total C and N were determined by a Carlo Erba EA1110 dry combustion analyzer, and the available P was estimated with the Olsen method [ 34 ]. According to Agnelli et al. [ 35 ], exchangeable cations were determined on 5 g sample placed into a plastic container, submerged with a 0.2 M BaCl 2 solution (solid:liquid ratio 1:10), and gently shaken on a horizontal shaker for 90 min at room temperature. Then, the suspension was left to stand for 30 min and decanted. The solution was filtered through Whatman 42 paper and analyzed for exchangeable Ca, Mg, K, and Na with a Shimadzu AA–6300 atomic absorption spectrophotometer, and for Al with a Varian SpecttrAA 220Z atomic absorption spectrophotometer equipped with graphite furnace. Exchangeable H was calculated as the difference between the pH of the 0.2 M BaCl 2 solution before and after the contact with the sample [ 36 ]. Effective cation exchange capacity (ECEC) was determined as the sum of exchanged Ca, Mg, K, Na, Al, and H; Al saturation as the percent of exchangeable Al over ECEC; and base saturation as the percentage of exchangeable Ca + Mg + K + Na over ECEC was also calculated. To estimate the amount of carbon sequestered in the A horizon of each location, the bulk density of the A horizon was determined on the base of the pedofunction proposed by Hollis [ 37 ]. The amount of soil organic carbon sequestered was expressed in T of carbon per hectare of A horizon (T ha−1). 2.4. Organic Matter Fractionation The organic matter was fractionated following a sequential fractionation procedure. Briefly, 40 g of sample was placed into a beaker, submerged with water (solid:liquid ratio 1:2.5), and shaken overnight. Afterward, the suspension was left to stand for a few hours and the supernatant was separated from the extraction residue by decantation. The decanted suspension was filtered through Whatman 42 paper. The solution that passed through the filter contained the water-extractable organic matter (WEOM), while the material that remained on the filter was the free particulate organic matter (POM), which is mainly made of partially decomposed vegetal and animal debris. The solution was immediately recovered in refrigerator, while the POM was dried at 40 ◦ C. To submit the extraction residue to alkaline separation [ 38 ], it was transferred into a centrifuge tube and added to 100 mL of a 0.1 M NaOH solution. The suspension was gently shaken overnight; then, it was centrifuged for 15 min at about 15,000 × gand filtered through Whatman 42 paper. The extraction with 100 mL of 0.1 M NaOH solution was then repeated. The extraction residue was washed twice, each time with 50 mL of distilled water; the obtained solutions were recovered after centrifugation and filtration, and added to the NaOH extract. The organics dissolved in this extract represented the total extractable Forests 2017,8, 135 7 of 16 organic matter (TEOM), which was treated with a 6 M H 2 SO 4 solution until pH 1.2 and left to stand one night. After standing overnight the samples were centrifuged for 10 min at about 15,000 × g to separate the precipitated humic acids (HA) from the fulvic acid (FA) that remained in solution. The extraction residue containing the non-extractable organic matter (NEOM) was dried at 40 ◦ C and weighed. All solutions and solid fractions were analyzed for their organic C content by dichromate digestion at 180 ◦ C for 30 min [ 39 ], and their contents were expressed in terms of C as WEOM (water extractable organic carbon, WEOC), POM (particulate organic carbon, POC), TEOM (total extractable organic carbon, TEOC), and NEOM (non-extractable organic carbon, NEOC). For the bioassay, HA and FA of only one profile per site were purified through permeable membranes (cutoff 1000) immersed in distilled water. 2.5. Bioassay of Auxin-Like Activity of the Humic Substances The auxin-like activity of HA and FA was assessed by the growth reduction of watercress ( Lepidium sativum L. ) roots [ 40 ]. Watercress seeds were surface-sterilized by immersion in 1% hydrogen peroxide for 15 min. After rinsing five times with sterile distilled water, 20 seeds were placed on sterile filter paper in a sterile Petri dish. The filter paper was wetted with 1.2 mL of 1 mM CaSO 4 (control), or 1.2 mL of 20, 10, 5, 2.5, 1, 0.5, 0.1, 0.01, and 0 mg L −1 indole-3-acetic acid (IAA) (Sigma) solution; another set of seeds placed in Petri dishes was added to 1.2 mL of 0.5, 0.005, 0.0005, 0.000005, and 0.00000001 mg L −1 of purified HA or FA. The seeds germinated in the dark at 25 ◦ C in a germination room. After 48 h, the seedlings were removed and the root lengths were measured with a TESA-CAL IP67 electronic caliper (TESA, Renens, Switzerland) and Data Direct software, version 1 (ArtWare, Asti, Italy). 2.6. Statistical Analysis Except mineralogy and bioassay, all the analyses were run in double on each sample. The values obtained for each sample were averaged, and these averages were used to calculate the mean for each A horizon. In all cases, the standard deviation was calculated for n= 4. These data were statistically treated with R software [ 41 ] and differences between samples were tested by the analysis of variance (ANOVA), after checking the parametricity of the dataset by the Shapiro-Wilk statistic test (“stats” package) for normality and by Levene’s test (“car” package) for homoscedasticity, both at 5% significance level. Data that did not confirm the assumptions were log, exponential, and reciprocal transformed. We considered the ANOVA test as significant when p< 0.05; differences between means were compared using a post-hoc Tukey’s Honest Significant Difference (HSD) test with p< 0.05. For each site, the mineralogical analysis was conducted only on the two samples collected from the soil profiles, and the results were averaged. However, because of the approach used for the mineralogical analysis, only a semi-quantitative estimation was obtained; hence, we provide an estimate of the mineralogical assemblage with no statistical treatment of the data. Bioassay tests were run using HA and FA of one of the two A horizons collected from the soil profiles. The data obtained were processed by linear regression analysis to calculate the HA and FA dose–response curves by using SPSS for Windows software, version 18.0 (SPSS, Chicago, IL, USA). Before regression, data were transformed on a natural logarithmic scale to obtain the best linear fitting. Three replicates per experiment were performed and standard errors were always less than 5%. 3. Results 3.1. Soil Morphology Soil development in terms of both horizon differentiation and thickness of the solum ( A + B horizons ) was the poorest for the soils developed from serpentinite and phyllite, followed by those on amphibolite and granite (Table 1). Generally, all the soils were moderately to well structured, usually with abundant roots, at least in the A horizons. Only the soil on serpentinite displayed Forests 2017,8, 135 8 of 16 a relatively low number of roots. The soil originated from amphibolite, serpentinite, and phyllite, because of their small development, were Entisols, while those on granite were Inceptisols. 3.2. General Properties The mineralogical composition of the soils mostly reflected that of the parent rock (Table 2). A greater variety of minerals was in the soil developed from serpentinite, with relatively high concentrations of serpentine, chlorite, pyroxenes, and amphiboles, and lower contents of quartz, plagioclases, and albite. In contrast, a simplified mineralogical composition was detected in the soils derived from granite, which were made of quartz, plagioclases, micas, and kaolinite. Small amounts of kaolinite were found in all the soils except that on serpentinite, while traces of 2:1 secondary clay minerals were detected in all the soils. The calculated bulk density of the A horizons was 0.84 (standard deviation 0.06) g dm −3 for the soil on amphibolite, 0.85 (0.07) g dm −3 for that on serpentinite, 0.79 (0.03) g dm −3 on phyllite, 1.03 (0.10) g dm−3 on granite under pine, and 0.84 (0.05) g dm −3 on granite under oak. These values well agreed with those reported by Rodríguez-Lado et al. [42] for topsoils of Galicia. The soils displayed acid to sub-acid pHs, with values of pH KCl 0.5–0.9 pH units lower than those in water (Table 3). The content of available P was always lower than 10 mg kg −1 (Table 3), much less than the deficiency limit considered for the method used, 23 mg kg −1 [ 34 ]. In the soils on amphibolite, phyllite, and granite under oak, this condition of low fertility was worsened by the low ECEC, in all cases dominated by Al, which amounted to about 69% of the ECEC (Table 3). The soil on serpentinite showed the highest ECEC and the lowest Al saturation, but a very low Ca/Mg ratio of about 0.20. The total nitrogen (TN) content was relatively higher in the soil on amphibolite, while the lowest amount was detected in the soil on granite under oak (Table 4). Conversely, the total organic carbon (TOC) content was relatively high in all the soils, except in that on granite under pine (Table 4). The WEOC represented the lowest component of TOC, never exceeding the proportion of 1.21% as in the soil on serpentinite (Table 4). The POC was relatively high in the soil on granite under oak, while the lowest value was found in that under pine (Table 4). The contents of HA and FA were similar in all the soils, with HA always higher than FA (Table 4). The total content of HA + FA assumed proportions to TOC ranging from ~20 to ~49%, with the lowest proportion in the soil on amphibolite and the highest in that on granite under oak. The NEOC, which represent the organic carbon fraction most recalcitrant to degradation, was higher in the soils on amphibolite and phyllite than in the others (Table 4). The amount of organic carbon sequestered by the A horizons was calculated considering the mass-based concentration of TOC, the bulk density, and the thickness of each A horizon. This quantity was 228 (standard deviation 17) T ha −1 for the soil on amphibolite, 76 (20) T ha −1 for that on serpentinite, 145 (39) T ha −1 on phyllite, 148 (15) T ha −1 on granite under pine, and 343 (81) T ha−1 on granite under oak. Table 2. Semi-quantitative mineralogical estimation for the A horizons of soils derived from different lithologies in Galicia (northwest Spain). Parent Rock Q Pl Mi C K S CM Ma T Pp Am Ab Py Amphibolite +(+) ++++ tr tr tr +++ + Serpentinite + + - +(+) ++ tr tr tr +(+) + +(+) Phyllite +++++(+) +++ + (+) tr tr Granite—P. sylvestris +++ +++(+) +++ (+) tr Granite—Q. robur +++ +++ +++ + tr Q = quartz, Pl = plagioclases, Mi = micas, C = chlorite, K = kaolinite, S = serpentine, CM = 2:1 clay minerals, Ma = margarite, T = alc, Pp = pyrophyllite, Am = amphiboles, Ab = albite, Py = pyroxenes. + = ≈ 10%, (+) = ≈ 5%, tr = traces. Forests 2017,8, 135 9 of 16 Table 3. Values of pH in H 2 O and KCl, available P, exchangeable cations, effective cation exchange capacity (ECEC), Al saturation, and base saturation (BS) for the A horizons of soils derived from different lithologies in Galicia (northwest Spain). Parent Rock pHH2O pHKCl Available P Exchangeable Cations ECEC Al Saturation BS Ca Mg Na K Al H mg kg−1cmol(+) kg−1% % Amphibolite 5.50 a4.59 b2.9 a0.22 cd 0.24 c0.62 a0.39 a4.58 a0.64 a6.69 b68.5 a22.0 b −0.35 −0.01 −1.8 −0.05 −0.03 −0.01 0 −3.15 −0.09 −3.07 −18.2 −17.9 Serpentinite 5.97 a5.06 a6.1 a3.26 a16.08 a0.38 bc 0.46 a0.18 b0.73 a21.09 a0.9 b95.7 a −0.16 −0.01 −2.4 −0.22 −2.29 −0.08 −0.29 −0.04 −0.11 −0.28 −0.2 −0.4 Phyllite 4.69 a3.91 c9.6 a1.14 ac 0.38 c0.33 bc 0.39 a6.59 a0.68 a9.51 b69.3 a23.6 b −1.1 −0.3 −7.9 −0.17 −0.12 −0.05 −0.01 −0.19 −0.07 −0.05 −0.7 −0.6 Granite-P. sylvestris 5.31 a4.80 ac 5.3 a0.98 bc 2.64 b0.29 c0.27 a3.94 a0.43 b8.55 ab 46.1 a48.9 ab −0.32 −0.23 −4.3 −0.68 −1.65 −0.05 −0.01 −0.64 −0.1 −3.94 −14.2 −9.9 Granite-Q. robur 5.13 a4.39 c2.7 a0.30 bc 0.07 d0.52 ab 0.35 a3.97 a0.52 ab 5.73 b69.3 a21.6 b −0.06 −0.01 −0.5 −0.12 −0.01 −0.13 −0.01 −0.41 −0.13 −1.37 −4.2 −4.7 Numbers in parentheses are the standard deviations (n= 4). BS = (Ca + Mg + Na + K)/(Ca + Mg + Na + K + Al + H) · 100. For each column, mean values with different letters significantly differ for p< 0.05 for the Tukey’s post-hoc test. Table 4. Total nitrogen (TN), total organic carbon (TOC), carbon/nitrogen ratio (C/N), water extractable organic carbon (WEOC), particulate organic carbon (POC), humic acid (HA), fulvic acid (FA), and non-extractable organic carbon (NEOC) for the A horizons of soils derived from different lithologies in Galicia (northwest Spain). Parent Rock TN TOC C/N WEOC POC HA FA NEOC g kg−1 Amphibolite 6.64 a104.63 a15.76 c0.55 a1.26 bc 14.44a6.49 a81.89 a −0.37 −7.8 −2.81 −0.14 −0.78 −7.18 −0.86 −0.9 Serpentinite 4.00 b110.68 a27.67 b1.34 a5.22 ad 36.06 a11.46 a56.60 b −0.35 −29.16 −1.93 −0.08 −1.31 −28.29 −8.77 −8.42 Phyllite 4.98 ab 131.89 a26.48 b1.15 a2.68 bd 37.23 a10.16 a80.67 a −1.23 −35.84 −5.85 −0.58 −2.46 −15.37 −60.9 −14.54 Granite—P. sylvestris 3.14 bc 53.34 b16.99 c0.45 a0.09 c13.97 a4.68 a34.15 bc −0.46 −5.43 −2.16 −0.32 −0.01 −3.39 −1.81 −1.46 Granite—Q. robur 2.74 c110.67 a40.39 a0.42 a15.22 a38.87 a15.37 a40.79 bc −0.18 −29.95 −1.72 −0.11 −7.18 −17.95 −11.6 −8.36 Numbers in parentheses are the standard deviations (n= 4). For each column, mean values with different letters significantly differ for p< 0.05 for the Tukey’s post-hoc test. Forests 2017,8, 135 16 of 16 51. Bonifacio, E.; Zanini, E.; Boero, V.; Franchini-Angela, M. Pedogenesis in a soil catena on serpentinite in north-western Italy. 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