Soil Physical Attributes Under Eucalyptus stands With Non-living and Living Plants

The conservation of ecosystems has benefited from planted forests which provide reforested wood reducing the pressure on deforestation of natural forests. Soil physical attributes determine soil water storage capacity; therefore, they play an important role on plant roots’ development which may compromise plant’s survival. The study tested the influence of soil physical and water attributes on the survival of Eucalyptus spp. clones under dry tropical climate. Two areas were selected, including one with living plants and a second with non-living plants of Eucalyptus spp. clones. Moreover, five soil profiles were studied in each area and the parameters estimated were soil bulk density, total porosity, saturated hydraulic conductivity, soil water retention curve, pores size distribution, available water capacity, and S index. Soil physical and hydric attributes did not differ between the area with living plants and the one with non-living plants. The saturated hydraulic conductivity in the area surface layer was high for both the living plants and non-living palnts; 331 mm h and 294 mm h, respectively. The S index (to give the value) indicated that the structure was suitable for the development of Eucalyptus trees. Furthermore, it was possible to affirm that soil physical and water attributes of the studied areas were promising for the cultivation of Eucalyptus spp. in the dry tropical climate.


Introduction
The forest industry is recognized for its sustainable, social, and economic importance.Planted trees help conserving ecosystems because they reduce the deforestation pressure on natural forests leading to reforestation of degraded areas (Souza et al., 2014).
The total area of planted trees for industrial purposes in Brazil reached 7.8 million of hectares in 2016 (Brazilian Tree Industry, 2017).The Eucalyptus trees occupy 5.56 million of hectare of that total (71.3%),mainly spatialized in the states of Minas Gerais (25.2%),Sao Paulo (17.6%), and Mato Grosso do Sul (14.5%).Brazil has a great potential for cultivation of forest crops, mainly Eucalyptus species.
According to the Brazilian Tree Industry (2017), the Brazilian Cerrado biome, in the state of Piauí, is placed at the 15 th position among planted forest areas in Brazil with 37,176 hectares of Eucalypus spp.Some factors have contributed to the Eucalyptus cultivation in this region, such as well-drained and deeper soils adapted to mechanization.Moreover, another positive factor is the seasonal climate with half-year dry season and half-year wet season (Lopes, 2013).Note.pH = hydrogen potential; P = phosphorus; K = potassium; Ca = calcium; Mg = magnesium; Al = aluminium; H+AL = hydrogen+aluminium; SB = sum of bases; V = saturation/base; OM = organic matter.
On February 2015, three years after planting, two areas were randomly chosen to evaluate their soil physical and hydric attributes.In the first, Eucalyptus plants were alive and in the second area, they were dead.The selection process was based on the following criteria: (i) area with living plants, all plants around were alive; (ii) area with dead plants, all plants around were dead.Disturbed and undisturbed samples were collected from five soil profiles of both the areas with and without living plants.The distance between these profiles was approximately 5 m.Soil sampling was carried out at 0-0.10, 0.10-0.20,0.20-0.40,0.40-0.60,0.60-1.0 and 1.0 to 2.0 m depths and both the undisturbed and disturbed samples.
Physical analysis of granulometric distribution and particle density (Dp) were performed on disturbed samples.The granulometric analysis was performed using the pipette method (Gee & Bauder, 1986) and the particle density using the volumetric balloon method (Blake & Hartge, 1986b).
Table 2. Soil physical properties of the area with non-living plants and of the area with living plants Soil bulk density (Ds), total porosity (TP), soil water retention curve (WRC), soil hydraulic conductivity (Ksat), available soil water capacity (WRC) and S index (obtained at the inflection point of water retention curve) were determined from undisturbed samples.Soil bulk density was determined by volumetric ring method (Blake & Hartge, 1986a).Total porosity was function of soil bulk density (Ds) and particle density (Dp), through Equation ( 2).
WRC was determined on undisturbed and saturated samples submitted to the follwoing matrix potentials: -0, -10, -33, -70, -100, -500, and -1500 kPa, using Richards pressure chamber with porous plate (Kluteet al., 1986).At stability, the samples were weighted to quantify their moisture content at each applied potential.At the end of the last potential applied (-1500 kPa), the samples were dried in an oven at 105 ºC for 24 hours to determine their dry soil mass and gravimetric moisture.The soil WRC was obtained by adjusting the dataset to van Genuchten's model using the Retention Curve software (RETC) (van Genuchten, 1980) (Equation 3): where,  corresponds to soil volumetric moisture,  sat and  res are soil water contents representing saturation and residual moisture, respectively; h = soil water matrix potential (kPa); , n and m are empiracal parameters of the model in which m and n are coefficients and  in kPa.
Pore size distribution was calculated from WRC data according to (Boumaet al., 1977) classification proposal (Table 3).Pore size distribution was obtained based on the retention curve calculated by the equivalent radius on each potential (Libardi, 2012), as shown in Equation ( 4).
where, r is the pore radius equal to 50, 15 and 1 µm; σ is the surface water tension at 0.072 N m -1 ; φ is the contact angle between water and pore wall equal to 0; sol is the solution density, that is 1000 kg m -3 ; g is the gravity acceleration, 9.8 m s -2 ; and h is the water column height (applied potential).
Pore percentage with upper diameter was calculated for each potential using Equation ( 5): where, P is the soil pore percentage with upper diameter calculated to each potential; TP is the total soil porosity (%);  is the Ksat which was determined by descending loading method (Reynolds et al., 2002) decribed in Equation ( 6): where, ɸ is the cylindrical ring diameter without soil; ɸs is the ring diameter with soil; L, h 0 and h 1 are the height; t is time.
S index or WRC inclination value at the inflection point was determined based on the parameters obtained from retention curve, according to Dexter (2004)'s Equation ( 7): where, θs is the soil water content at saturation point on gravimetric basis (kg kg -1 ); θr is the residue water content (kg kg -1 ); n and m are the retention curve empirical parameters.
Later on, available soil water capacity (WRC) was calculated as a difference between soil water content at -10 and -1500 kPa potential multiplied by the soil bulk density and the thickness of each layer studied.Data was submitted to Shapiro-Wilk normality test at 5% probability.Then, soil hydric and physical attributes from the soil profiles were statistically compared by the paired t-test at 5% of probability.

Results and Discussion
Soil bulk density (Ds) had no significant difference between the area with living plants and that of dead plants (p > 0.05) (Figure 2A).The lowest Ds of the surface layer observed were 1.03 Mg m -3 and 0.96 Mg m -3 in the area with living and that with dead plants, respectively.These values might have been related to soil management operation during forest implementation period.In addition, it could also be attributed to root density in the first layers and the higher biological activity up to 0.30 depth (Lipiecet al., 1991;Trevisan et al., 2012).As consequen texture, Br root growt m -3 at 0-0. and areas w In forest s initial gro during pre     was mainly s ble to affirm th h.In fact, an S cates restrictiv Dexter, 2004).ins et to the water h the efore, yptus Thus, works aimed at validating S index, Tormena et al. (2008) suggested that more studies must be conducted under different soils, land-use in the tropical conditions.

Conclusion
Soil bulk density (Ds), total porosity (TP), soil water retention curve (WRC), soil hydraulic conductivity (Ksat), available soil water capacity (WRC) and S index did not differentiate the area with dead Eucalyptus from the one bearing living species' trees.Both areas presented favorable physical and hydric soil conditions for developement of Eucalyptus spp.Therefore, there is a need of conducting more research on edafoclimatic conditions that can contribute to clarifying the reasons of the premature death of Eucalyptus trees.

Table 1 .
Soil chemical properties of the two study areas

Table 3 .
Soil pore classification based on size distribution