Genesis of an atypical Podzol in the Iberian Range: micromorphological characterization

A podzol has been described in the Iberian Range (Moncayo Natural Park), which may represent the southernmost example in Europe. The fact that it occurs at the podzol distribution limit makes its morphology atypical. The aim of this work is to identify the components and formation processes of the soil, and to determine if the processes are active or paleo-processes by a micromorphological approach. The study podzol is located in the top of a northern hillside (30% of slope), at 1600 m altitude on quartzitic sandstones, under Scots pine (Pinus sylvestris) with an udic moisture regime and a frigid temperature regime. We found opaque, orthic iron oxide nodules at the base of the profile (Bhs1), which is related to a current oxidation-reduction process; silt caps are also observed on the coarse elements, pedofeatures evidencing the cryoturbation process, probably also current processes. In the Bhs2 horizon, coatings and micro-laminated clay infillings are identified that are interspersed with fine silts in the holes. Many of these coatings are fragmented and mixed with the basal mass of the soil, indicating that they are paleofeature, possibly prior to the current podzolization.


Introduction
Podzols represent 3.2% of the world soil cover (IUSS Working Group WRB 2015) and nearly 20% of European soils (Tóth et al. 2008) but they are rare in Spain, where they cover only about 0.1% of its surface (Gómez-Miguel and Badía-Villas 2016). Podzols, Spodosols in Soil Taxonomy System (Soil Survey Staff 2014), are present in NW Spain (Macías and Calvo de Anta 2001;Carballas et al. 2016) and in small and scattered areas in NE Spain such as the Catalan Pyrenees (Bech et al. 1981;Boixadera et al. 2008), the Basque Country (Camps and Aizpurúa 2007) and the Sierra de Urbasa, Navarra (Val Legaz and Íñiguez Herrero 1981a). In some cases, these soils have macro-and micro-morphologic podzolic characteristics but their B horizons do not always meet the spodic horizon diagnostic criteria (Val Legaz and Íñiguez Herrero 1981b). In contrast, other soils meet these criteria but their morphology is peculiar , probably because they represent the southernmost Podzols in Europe. In order to understand its formation, soil micromorphological investigation can be an excellent option (Stoops et al. 2018). The aim of this work is to characterize the micromorphology of one Podzol, located in the Iberian Range (NE Spain), to identify its components and to determine whether the processes are active or past (paleo-processes).

Study Area
The study area is located at the top of a hillside (slope inclination of 30%), in the northern slope of the Moncayo Massif (near San Gaudioso hermitage) at 1600 m above mean sea level (GPS coordinates: 1º 48' 53.59" W 41º 47' 13.72" N). The lithology of the study area consists of Early Triassic quartzitic and micaceous sandstones (IGME 1980). The studied soil develops on a colluvium of these sandstones, mainly quartzofeldspathic, with monocrystalline and polycrystalline quartz grains and K-feldspars (microcline and orthoclase) and mica (Arribas et al. 2007). The average annual rainfall is about 978 mm and the average annual temperature is 6.3 °C; the average annual maximum temperature is 11.9 °C and the minimum is 1.4 °C (Fick and Hijmans 2017). According to the available climatic data, we interpret the moisture regime as udic, and the temperature regime as frigid (Soil Survey Staff 2014). The oaks (Quercus petraea and Quercus pyrenaica), growing from 900 m to 1200 m, are succeed by beech (Fagus sylvatica), from 1200 m to 1600 m above sea level. The latter grove was replaced by Scots pine (Pinus sylvestris) due to a reforestation around 1920 (García-Manrique 1960). The understory consists mainly of heathers (Erica vagans, Erica arborea, Erica cinerea), hollies (Ilex aquifolium), blueberries (Vaccinum myrtillus), wavy hairgrass (Des-champsia flexuosa) and mosses. The soil described in the study site has an O-Ah-E-Bhs-C horizon sequence (Figure 1) that fulfills the requirements to be classified as Skeletic Umbric Albic Podzol (Loamic) (IUSS Working Group WRB 2015). The O horizon is very well developed, divided in Oe-Oi-Oa horizons, and classified as Humimor (Badía et al. 2018). More details on this Podzol and its factors of formation have been previously described ); a selection of its properties is provided ( Table 1).

Methods
Oriented topsoil clods were sampled from each soil horizon. Soil clods were placed in containers and transported undisturbed to the laboratory where they were air-dried to a constant weight. A thin section (5 x 13 cm) was obtained from each clod, according to the methods described by Benyarku and Stoops (2005). The thin sections were observed using a petrographic microscope under both plane-(PPL) and crosspolarized light (XPL) and described following the guidelines of Stoops et al. (2018).
Due to the irregular boundary and the thickness of the Bhs horizon, two samples of this horizon were obtained at the top, Bhs1 (24-35 cm) and in the middle, Bhs2 (50-60 cm).

Results
The results obtained through the micromorphology of the profile studied are shown in Table 2.  The b-fabric of the Ah horizon is undifferentiated due to the abundance of amorphous organic matter, including sections of phlobaphenized pine needles together with charcoal fragments (Figure 2a), evidence of past forest burning. In the E horizon (Figure 2b), coarse fragments (quartzitic sandstone) have silt cappings above and voids below (Figure 2d). The microstructure in Bhs1 horizon is crumb and the b-fabric is sericitic crystallitic (micaceous) (Figure 2c). The micromass is redder than in the upper horizons, due to the presence of Fe oxides. In the Bhs2 horizon micro-laminated clay coatings can be observed as well as clay infillings in pores, most of them fragmented (Figure 2e). Moreover, orthic Fe oxides nodules are frequent (Figure 2f).

Discussion
The Bhs horizon fulfill the morphological and analytical criteria of spodic horizon and, therefore, the soil can be classified as Podzol (IUSS Working Group WRB 2015) or Spodosol (Soil Survey Staff 2014). Especially striking is the fact that oxalate-extracted Al and Fe contents (Al ox +½Fe ox ) is 16 times higher in the Bhs horizon than in the E horizon (1.40% and 0.09% Al ox +½Fe ox , respectively). This pattern is also found in Podzols from the subalpine stage of the Pyrenees (Boixadera et al. 2008).
In spite of this, the typical micromorphological characteristics of Podzol (pedofeatures related to the accumulation of colloidal organic matter, Fe and Al in depth (Van Ranst et al. 2018)) are not clearly observed. Instead, micromorphology reveals clay illuviation pedofeatures in the base of Bhs (Bhs2) in spite of the fact that argiluviation and podzolization cannot occur simultaneously since podzolization needs a more acidic pH and the sesquioxides produced during this process do not allow clay movement (Lundström et al. 2000). The observed features related to clay illuviation are mainly fragments of clay coatings, not related to the present-day pore system, therefore the argiluviation has to be interpreted as a paleoprocess, probably occurring before podzolization. For this reason, the Bhs2 horizon is probably a former argic horizon (Bt), nowadays podzolized, which has been later transformed to Bhs (Van Ranst et al. 1980).
Cryoturbation can be responsible for the destruction of clay coatings. Frost action is also responsible for the formation of silt cappings on coarse fragments and voids below them, by ice formation and thawing (Van Vliet-Lanoë and Fox 2018), in both the E and Bhs horizons. This is probably an active process, since the average minimum temperature is -1.7 ºC in the winter months and the average of the minimum temperatures in the coldest month (January) is -4.3 ºC (Martínez del Castillo et al. 2012;Fick and Hijmans 2017).
In the Bhs2 horizon, Fe-nodules are common, opaque, orthic and typic, as results of redox processes. Due to the current udic moisture regime and the water infiltration decrease by the relative clay accumulation in this horizon, these processes are possibly active nowadays, but they should have occurred also during other cold periods by ice melting. Although it is not possible to determine the composition of the nodules with optical microscopy, analytical data show queluviation of OM, Fe and Al in the illuvial Bhs spodic horizon. Replacement of native beech by Scots pine might have caused the soil acidification and also an increase in the polyphenol content that slows down the OM decomposition causing the OM accumulation in the topsoil and forming Fe and Al sesquioxides (Labaz et al. 2014;Leuschner et al. 2013). Due to the relative recent podzolization, it is recognized more from analytical data than from micromorphological observation.
The acidic soil reaction, the horizonation (with a brownish gray horizon Ah, the presence of an E horizon, the increase of OM and Fe and Al oxides in Bhs), as well as the soil-forming factors (Scots pine forest with heathers, quartzitic sandstone, udic regime, etc.) are favourable to Podzol formation.

Conclusions
In spite of the fact that the studied profile is clearly classified as a Podzol in the WRB soil system based on its field description and chemical analyses, its micromorphology reveals several features not characteristic of these soils, which point to past processes of clay illuviation. Clear morphologies due to frost action (fragmentation of clay coatings, silt cappings) and redoximorphic features are also observed. These processes are probably active at present given the frigid and udic soil temperature and moisture regimes. In summary, we hypothesize that this profile has been formed by argiluviation processes and subsequent podsolization, which may have occurred by the replacement of the original beech forest by the pine forest and also most likely of climatic conditions.