Edaphic variables conditioning the habitat of oribatid mites in Luvic Phaeozems under forest plantations (Buenos Aires, Argentina)

The soil is a complex three-dimensional habitat and any changes in its structure and porosity are likely to affect the type and abundance of soil biota. Oribatid mites play an important role in the decomposition and mineralization of soil organic matter and their abundance depends on diverse soil parameters, including soil texture, organic matter content, pH, moisture, and the pore system. The aim of the present work is to analyze some of the edaphic variables that condition the habitat of oribatid mites in Luvic Phaeozems under Pinus radiata (site P) and Eucalyptus globulus (site E) plantations, in the Southeast of Buenos Aires, Argentina. Bulk density, penetration resistance, pH, moisture, pore system parameters, and oribatid abundance and species composition were analyzed. Site E had a greater total porosity than site P. The high tortuosity of the pores in both sites generates a complex habitat architecture for the development of oribatid mites. In both sites, oribatids of 70-400 μm in size predominated and were more abundant in site E. A positive correlation between the abundance of oribatids and the pore size in both sites was observed. In site E this correlation was lower for 70600 μm (R = 0.13) or negative for 70-400 μm (R = -0.78). Therefore, the oribatid abundance could be explained by a greater complexity of the structure of the organic horizon, lower bulk density and lower penetration resistance. These conditions favor the mineralization of organic matter, and therefore food availability. On the other hand, in site P, oribatid abundance is mainly influenced by the porous system, which conditions the access to food, competition between organisms and refuge from predators. Two new species were registered for Argentina: Mesotritia elegantula and Acrogalumna longipluma.


Introduction
Soil structure is defined as the arrangement of particles and pores of the soil (Oades 1993). The shape, size and spatial distribution of pores control the behavior of water and air, and provide information on transfer processes and life in the soil (Porta et al. 1999).
Soil management has negative effects on total porosity, quantity, distribution, and continuity of pores. This, in turn, causes changes in the flow of air, gases and water inside the pores, affecting the development of soil organisms (Kay and VandenBygaart 2002;Iglesias et al. 2007;Alvarez et al. 2008).
Soil is a complex system and provides space for the development of organisms of different sizes (Estrade et al. 2010). The concept of `habitable pore space' suggests that there is a relation between the size of organisms and the zones of soils they are physically able to inhabit (Young and Ritz 2000). Furthermore, there is a strong relationship between habitat complexity and the diversity and abundance of organisms (Downes et al. 1998;Bardgett 2002;Mac Nally and Horrocks 2007). As the complexity increases, available habitat space for organisms increases (Morse et al. 1985).
The relation between fauna and soil structure has been extensively studied in natural, cultivated and forested soils. Morphological burrow characteristics have been analyzed in relation to porosity, hydraulic conductivity and aggregate stability, amongst others (Jégou et al. 2001;Kladivko 2001;Langmaack et al. 2002). However, studies relating soil organisms with porosity as available habitat space are rare (Durcarme et al. 2004), although there is a clear, positive relation between the number and size of pores and the type of animals inhabiting them (Kamplicher and Hauser 1993;Hassink et al. 1993;Kamplicher 1999;Duhour et al. 2009;Alvarez et al. 2008Alvarez et al. , 2013. Mites are the most diverse and abundant organisms in the soil because of their size range (0.1-2 mm) they belong to the mesofauna group. The oribatid mites play an important role in the decomposition and mineralization of soil organic matter, and its transport through the profile (Lal 1991). Due to this ecological function and to the fact that they are sensitive to environmental changes, these organisms are used as soil quality indicators (Behan-Pelletier 1999;Bedano et al. 2008). The oribatids live within the pore spaces and their abundance depends on soil type, texture, amount and distribution of organic matter, pH and trophic relations, among other properties (Anderson 1988). Some studies (Anderson 1975;Noti et al. 1996;Hansen and Coleman 1998;Hansen 2000) have specifically investigated the relationships between the diversity of oribatid mites and microhabitats (presence of leaf fragments, roots, faeces, micro-organism clumps, etc.) (Martínez et al. 1999;Fredes 2013).
The soil uses in the southeast of Buenos Aires Province are mainly devoted to agriculture and pastures, besides some recreational-touristic areas. Some sectors in the basin de Los Padres Lagoon are ecologically suitable for the cultivation of Eucalyptus globulus and Pinus radiata and these plantations were installed more than 60 years ago with recreational purpose. The introduction of exotic forest species in these soils produces modifications in the structure of the soils, particularly in the moisture and organic carbon content, in the bulk density, penetration resistance and pH (Delgado et al. 2006;Gaitán et al. 2007;Jobbágy et al. 2006;O'Brien et al. 2003;Poore and Fries 1987).
There are precedents in the study of oribatid communities in cultivated soils (Osterrieth et al. 1998;Martínez et al. 1999;Scampini et al. 2000;Bernava et al. 2013) and forested soils (Levy et al. 2017); however, there are no studies linking the porosity and the habitat of the organisms in this area of Buenos Aires Province.
The aim of the present work is to analyze some of the edaphic variables that condition the habitat of oribatid mites in Luvic Phaeozems under tree cover of pines and eucalyptus, in the southeast of Buenos Aires. We propose the following hypothesis: "the abundance of oribatid mites in these soils is mainly influenced by the porosity characteristics".

Area description and soil characteristics
The study area is located in General Pueyrredón District, around de Los Padres Lagoon, Buenos Aires Province (37º56´ S and 57º44´ W) (Figure 1). The climate is oceanic temperate, with an annual precipitation of 800 mm and annual average temperatures of 14 °C (National Weather Service, according to the 1981-2010 records). According to Martínez (2001), the de Los Padres Lagoon belongs to the geomorphological unit "Perinange aeolian hills", which comprises a relief of morphologically complex hills, with heights of up to 30 m and slopes between 6 and 8% (Osterrieth et al. 1998 Working Group WRB 2015), formed on aeolian loess sediments linked to the latest arid cycle of the late Pleistocene-Holocene (Osterrieth and Cionchi 1985). These soils have an A horizon of 30 cm and a B horizon 50 cm thick approximately, with a silt loam texture. The organic matter content is high, approximately 9% in the A horizon decreasing with depth (INTA- EEA Rafaela 1989). The pH is slightly acid (6-6.3) in the A horizon, and close to neutrality (6.8-7) in the parent material. The soil temperature regime is mesic, and the soil moisture regime is udic (IUSS Working Group WRB 2015). In forested soils these characteristics are modified, the organic matter content and the pH are decreased (Delgado et al. 2006).

Soil measurements
The study was carried out in sites in the same topographic position, with the same soil type (Luvic Phaeozems): E): 60-year-old Eucalyptus globulus forest plantation, with recreationaltouristic use; P): 40-year-old Pinus radiata forest plantation, with ant insecticides use only, periodic turnover and mulch removal until 2002.
Previous to the plantation of pines the site was used for traditional agriculture.
Sampling was carried out in winter 2014. In each site, random readings were carried out at ten points corresponding to penetration resistance; extraction of samples was performed with cylinders to determine bulk density and moisture content. Bulk density (BD) was measured by the cylinder method (100 cm 3 ) (Blake and Hartge 1986) and penetration resistance (PR) was measured in the field with a cone penetrometer (Bradford 1986). The cone used follows the ASAE guidelines, with an angle of 30º and 14 mm basal diameter. The moisture was measured by the gravimetric method (Cassel and Nielsen 1986). To calculate PR, a correction with the moisture content was calculated according to the exponential regression model IC = a.W b , where IC is the cone index, W is the moisture content, a and b are empirical parameters (Busscher et al. 1997). The pH was measured in a soil paste (1:1) using an electrode. These measurements were performed at 5 cm depth.
For the micro-morphological analysis of porosity, three undisturbed samples were taken from the YEAR 2020 VOLUME 10 ISSUE 1 upper levels of the mollic epipedon and placed in 5 x 7 x 5 cm cardboard boxes. The samples were sprayed with polyester resin diluted in styrene monomer (Murphy 1986) and a fluorescent pigment (Uvitex OB). From each impregnated block one vertical thin section was obtained. These were scanned in transmitted light. For the quantification of porosity, areas of 3.5 cm x 4.5 cm were analyzed, according to previous studies (Poch 2005;Yagüe et al. 2016;Bosch-Serra et al. 2017).
The image was converted into a binary image using ImageJ® (Rasband 2014). The analysis was carried on pores larger than 70 μm to avoid interference with noise and also because this is the body size of the smallest oribatid mite, Microppia minus, found in these soils. The pore parameters were analyzed using the ImageJ® software. The solidity parameter (area divided by convex area for each pore) and the pore area fraction were obtained from pore images excluding pore edges. High values of solidity correspond to smooth pores, and low values to rough contours. Furthermore, the pore shape was assessed using a shape factor I=[Area/Perimeter 2 ] proposed by Bouma et al. (1977), where a factor I < 0.015 corresponds to elongated pores, a factor I between 0.015-0.04 is attributed to irregular pores and a factor I > 0.04 to rounded pores. The pore size distribution was calculated using an opening algorithm inserting circles of increasing diameters in the pores, provided by the Quantim4 library (Vogel 2008).

Analysis of oribatid mites
On each site, ten undisturbed soil cores (5 cm in depth and 10 cm in diameter) were extracted along with the samples for physical determinations in winter 2014. Extraction of soil mites was carried out using modified Berlese-Tullgren funnels (Southwood 1980). Mites were extracted and fixed simultaneously in a 70% ethanol collector tube placed beneath the funnels during a three week period after which, they were saved for posterior determination.
The determination of oribatid mites species was carried out under stereoscopic and optical microscopes after clarifying the specimens with warm lactic acid, and following the keys of Balogh and Balogh (1988, 1990, 1992. For the analysis of oribatid mites, abundance (number of individuals) and body size (microns) was measured using a graduated eyepiece in the optical microscope, and corroborated with bibliographic data.

Statistical analysis
Soils and biological data were analyzed by means of a T-test (p < 0.05) for mean comparison (Statistica®). In order to study interdependence relationships between the community of oribatid mites and the variables studied, a Canonical Correspondence Analysis was performed (Past®). This tool allows us to relate the species of a community with the environment where it lives (Ter Braak and Prentice 1988).

Soil properties
The forested Pinus radiata site (P) presented significantly higher bulk density values (0.93 g/cm 3 ) (p < 0.05) than the Eucalyptus globulus (E) site (0.74 g/cm 3 ). Site P also had higher pH values and penetration resistance, although these were not significant ( Table 1).
The moisture content was significantly lower in site P (Table 1).
YEAR 2020 VOLUME 10 ISSUE 1 The structure and porosity parameters in the thin section images did not show any significant difference between the studied sites. Both soils presented a granular microstructure with rough, interconnected compound packing pores. Frequent plant residues were also observed (Figure 2a).
The total porosity was slightly higher in site E (47.85%) in comparison with site P (41.46%) (Figure 2b). Similarly, the pores of both sites showed high values of solidity, indicating a high pore roughness (Figure 2b).

Analysis of oribatid mites
Oribatid mite abundance was higher in site E with 141 individuals, while in the P site 73 individuals were found ( Table 2). A predominance of organisms with size ranges of 70-400 μm and 500-600 μm was observed in both sites. Within these ranges, the most abundant organisms in site E showed sizes of 200-300 μm and 300-400 μm, while in site P the organisms of 70-100 μm predominated (Figure 4). No organisms larger than 600 μm were observed (Figure 4).
Twenty and sixteen species of oribatid mites were observed in E and P respectively. Five species were common to both sites: Carinogallumna clericata, Microppia minus, Oppiella nova, Suctobelbella variabilis, Totobates discifer ( Table  2). Species exclusive to site E have a greater size range (200-400 μm) than the exclusive species of site P (< 100 μm).
It should be noted that three of the species found represented new cites for Argentina: Mesotritia elegantula, Acrogalumna longipluma and Oxyoppia (Dzarogneta) taurus (Levy 2017, not published).

Relation between variables
The relation between the pore size distribution and the abundance of oribatids in both sites was analyzed. For the 70-600 µm pore size, a positive correlation was observed between both variables in site P (R = 0.77) but it was very low in site E (R = 0.13). Moreover, for the 70-400 µm pore size, a positive correlation was observed in P (R = 0.83), but it was negative in E (R = -0.78) indicating that the abundance of oribatids in site E could be influenced by other variables besides porosity.
In a third CCA considering all variables, the first 2 axes explained 42% of the variation and some associations were corroborated ( Figure  5c): some species were directly related to the  In site P, Cultroribula lata, Trypochthonius tectorum, Machuella ventrisetosa, Suctobelbella variabilis, Quadroppia circumita were principally associated with bulk density and solidity, although penetration resistance and rounded pores also influenced these species abundance (Figure 5c).

Discussion
The physical soil variables showed a behavior similar to that observed by other authors in these sites (Alvarez et al. 2012;Levy et al. 2015;. Bulk density values are related to the higher content of organic carbon. As mentioned by some authors (Borrelli 2001;Montti 2002;Alvarez et al. 2011), the organic carbon content under these plantations was high, E = 7-11% and P = 5-9%. This favors aggregation of particles, increasing the porosity (Lamas and Moreno 2000) and the oribatid mites abundance, which is higher in site E.
Although pH was slightly more acidic in P, differences were not significant between sites. However, in E some species were associated with pH ( Figure 5). The slightly acidic pH generated by the composition of the leaves from the mulch in both sites regulates the activity of microorganisms that act in the decomposition of organic matter (Oades 1993).
Moisture is also an important factor for the presence of organisms in the soil. The organic horizon present in E and P reduces evaporation and increases the organic matter in the soil, which in turn allows it to retain more water. This condition is more noticeable in E since a higher moisture content and a more developed organic horizon were observed when compared to P. At the same time, plant residues on the soil surface provide abundant food and regulate soil temperature. Also the structure and composition of the litter layer in E could explain the higher abundance of oribatids. Eucalyptus leaves 200-300 µm pore. D: 300-400 µm pore. E: 400-500 µm pore. F: 500-600 µm pore. G: 600-700 µm pore. H: 700-800 µm pore. I: 800-900 µm pore. J: 900-1000 µm pore. K: > 1000 µm pore. In blue: forested with Eucalyptus globulus site (E). In green: forested with Pinus radiata site (P). In red: oribatid species. In black: physical-chemical and porous system variables. generate a more complex three-dimensional structure that offers a more heterogeneous habitat and, as a result, a greater density of oribatids; this could be reflected below ground (Hansen 2000).
The higher total porosity of site E could be indeed related to higher values of bulk density and organic carbon (Kay and VandenBygaart 2002). In both sites, elongated pores (channels, fissures) predominate, followed by irregular (compound packing pores) and rounded pores (channels, chambers): part of this total porosity is probably caused by the effects of roots and macrofauna. The solidity parameter gives information about the irregularity, tortuosity and roughness of pores. This roughness would reflect a complex habitat architecture that offers a higher number of habitable pore spaces for soil organisms (Alvarez et al. 2018). Similar solidity values were observed for both sites, so other properties could influence in oribatid mites abundance.
Some studies indicate that the abundance and density of microarthropods are related to soil porosity (Vreeken-Buijs et al. 1998;Ducarme et al. 2004;Nielsen et al. 2008;Alvarez et al. 2018), bulk density, moisture and pH (Bedano et al. 2006;Socarrás 2013). In cultivated Luvic Phaeozems some authors (Osterrieth, et al. 1998;Martínez et al. 1999;Scampini et al. 2000) indicated changes in organic matter content, structural stability, pH and fauna abundance in the soil. Repeated anthropogenic disturbances produce modifications in the micro-and mesohabitats of organisms. This reduces the period of stability for reproduction and growth of the edaphic fauna, directly affecting its abundance. Therefore, it is possible that the oribatid abundance in these forested sites is related with the studied soil properties, bulk density and porosity in particular. Furthermore, site E is located in a recreational area, so it is subject to periodic anthropic impact such as surface compaction. Oribatids can respond in various ways to disturbances, in some cases they can disappear if the disturbance is strong and constant. They can also develop strategies to deal with unfavorable environmental conditions. Some species, such as Opiella nova, abundant in site E, are parthenogenetic, and they develop reproductive strategies against changes in the environment (Norton and Palmer 1991).
In both sites mites of 70-400 μm predominated, with a positive correlation between the abundance and the pore size in site P and negative in site E. In site P, some characteristics of the porous system conditioned the abundance of some oribatids, as it was also found by Vreeken-Buijs et al. (1998) who related biomass of mites and pore size. Conversely in site E, the pH and moisture variables were more important.
In this sense, Vreeken-Buijs et al. (1998), found a positive relationship between biomass of mites and pore size: oribatid mites were correlated to pores of 1.2-90 μm. In temperate forests, Ducarme et al. (2004) observed that the pores > 200 µm are a measure of the space available for soil microarthropods and this was correlated to their density and richness. Also, Nielsen et al. (2008) found a positive effect of increasing pore volume (60-300 µm) on mite abundance within two different habitats. Alvarez et al. (2018) conducted studies comparing cultivated and natural soils and observed the same relationship between mite abundance and pore size.
In these sites the relationship between abundance and pore size could be explained by the habitat selection of the organisms according to the availability of food. The oribatids are mainly saprophagous, but can feed on live plants, fungi and algae, some are nematode predators, and these occupy the small pores. Furthermore, larger species cannot invade small pores where fungi are more abundant (Elliott et al. 1980;Vreeken-Buijs et al. 1998).
On the other hand, with the physical limitation of available space, competition decreases between functional groups. Also, the pores of this size would constitute refuges from predation. The organisms residing in pores of appropriate size could be protected from predation by larger organisms since the latter are denied physical access to their prey (Young and Ritz 2000). This in turn, could explain the absence of mites in pores larger than 600 μm.
All these conditions directly influence the humification and mineralization processes of organic matter, promoting differences among the communities of soil organisms. Therefore, because the soil is a complex system, one or more variables could explain the abundance of oribatids in these soils.
It should be noted that the results obtained for these edaphic properties differ from those from other authors who indicate high soil degradation due to the implantation of forest species (Delgado et al. 2006;Gaitán et al. 2007;O'Brien et al. 2003;.
In the sites we have studied, despite the impact produced by the plantation of pine and eucalyptus, there is a tendency to recover the soil properties that were lost by its management, since these sites were agricultural prior to being forested.

Conclusions
The forested sites in the South of Buenos Aires (Argentina) studied here, have well-structured soils with similar edaphic properties. However, the site of Eucalyptus globulus had a greater total porosity than that of Pinus radiata. The high tortuosity (solidity) of the pores in both sites reflects a complex architecture habitat for the development of oribatid mites.
Although the number of mites was low, the oribatid abundance in the forested Eucalyptus globulus site could be explained by a greater complexity of the organic horizon, higher pH and moisture, and small pores (70-200 μm). These conditions favor the mineralization of organic matter, and therefore the availability of food. Conversely, the oribatid abundance in the forested Pinus radiata site is mainly influenced by the higher bulk density and penetration resistance which conditions porosity and access to food, competition between organisms and refuge from predators.