The soils of the savannah region, Piaui State, Brazil, are favorable to agriculture; however, intensive use combined with inadequate management has caused soil degradation in the region. For this reason, studies focused on the use of alternative production systems as a no-till system (SPD), have become essential. Thus, the objective was to evaluate the changes in the physical-hydric attributes and organic matter in Oxisols under different times of no-tillage crop under straw in savannah areas of the Southwest region of the Piaui State, Brazil. Field data collection was carried out on commercial farms in the municipalities of Baixa Grande do Ribeiro (07° 48'10” S, 45° 00'60" W and altitude of 600 m), Uruçuí (08º14'07" S, 44º38'09" W and altitude of 550 m) and Bom Jesus (09º10'35" S, 44º50'36" W and altitude of 600 m), Piaui State, Brazil. The areas had different times of no-tillage crop under straw: SPD2 - 2 years, SPD3 - 3 years, SPD6 - 6 years, SPD10 - 10 years, SPD12 - 12 years, SPD15 - 15 years and SPD18 - 18 years old, as well as areas of native forest (control), in which the physical-water attributes were evaluated (distribution of granulometric fractions, soil density, total porosity, macro and microporosity, available water and field capacity), organic matter and the humid fraction. The database is organized as follows: identification of sampling sites, characterization of no-tillage systems under straw, soil physical-hydric attributes by layers and figures with the location of the farms and points sampling.

2.1. Data collection sites

The study was carried out on farms representing grain production systems, in three municipalities in the savannah region of the southwest of the Piauí State, Brazil: Baixa Grande do Ribeiro (07°48'10” S, 45°00'60" W and altitude of 600 m) , Uruçuí (08º14'07" S, 44º38'09" W and altitude of 550 m) and Bom Jesus (09º10'35" S, 44º50'36" W and altitude of 600 m) (Figure 1).

The region's climate is tropical savannah Aw, according to the Köppen classification, with a predominance of the rainy season between October and April and an average annual rainfall of 1,200 mm. Soils of the typical dystrophic Yellow Latosol class predominate, with a clay-sand-sandy texture (Embrapa, 2018), with the remaining vegetation of the savannah (Pragana et al., 2012).

2.2. Evaluated systems, history of use and soil sampling

Seven managed areas and one native forest were evaluated, used as a reference ecosystem to compare the original soil conditions, totaling eight areas (Table 1). The managed areas were arranged in a chrono sequence according to the time of no-tillage system crop (SPD) (Sá et al., 2004), in: SPD2 - 2 years (initial phase), SPD3 - 3 years (initial phase), SPD6 - 6 years ( transition phase) SPD10– 10 years (transition phase), SPD12 - 12 years (consolidation phase), SPD15 - 15 years (consolidation phase) and SPD18 - 18 years (consolidation phase). Details of the plots and location of the sampled areas on each farm used in the study are shown in Figure 2 and Table 1.

For the collection of soil data, in each farm, two sampling points were determined, one at the top of the slope and the other at the middle. At these points, mini trenches were opened in the soil, to allow the collection of samples with deformed structure and with undeformed structure, in the layers: 0.0-0.1; 0.1–0.2; 0.2–0.3 and 0.3-0.5 m.

At each sampling point and depth evaluated, an unformed sample with a volumetric cylindric of 100 cm³ (5 cm in height and diameter) and a deformed sample (300g) were collected. The granulometry, soil density, total porosity, soil water retention curve, granulometric fraction content (clay, sand and silt), volume of macro, micro and total porosity, moisture in the field capacity and permanent wilt, and total availability of water, organic matter and humine fraction were measured.

2.3. Analysis of physical-hydric attributes of soil and organic matter

To determine the granulometry, which aims to quantify the participation of each granulometric fraction in the soil sample, the deformed sample was used with the application of the total dispersion methodology described by Teixeira et al. (2017). The soil density, which corresponds to the relationship between the sample's soil mass and the volume occupied by the sample, samples collected with 100 cm³ volumetric rings at different depths were used. For these determinations, the methodology proposed by Teixeira et al. (2017).

The soil water retention curves, which demonstrate the relationship between the matrix potential and soil moisture, were constructed using undisturbed samples with the following stress points: saturation, 2, 4, 6, 10, 30, 300 and 1500 kPa, combining the methodologies of the tension table (for tensions of 2, 4 and 6 kPa) and the Richards chamber (for tensions of 10, 30, 300 and 1500 kPa) described by Teixeira et al. (2017) and Richards & Fireman (1943).

To determine the total porosity (TP) obtained by the difference between the saturated soil mass and the dry soil mass in an oven at 105ºC for 24 h, the method proposed by Teixeira et al. (2017). The micro, macro and total porosity of the soil was determined by the tension table method according to the methodology proposed by Teixeira et al. (2017).

The determination of moisture in the field capacity (θCC) was obtained at a tension of 10 kPa (Cassel & Nielsen, 1986; Reichardt, 1988), which corresponds to the soil moisture when it is retaining the largest possible amount of water in the soil. The point of permanent wilt (θPMP), which is equivalent to the lower limit of soil moisture capable of being absorbed by the plants, was obtained at the tension of 1500 kPa, in the Richards chamber (Klein, 1999).

The soil organic matter content was obtained by applying the correction factor of 1.724 to the soil carbon values, as recommended by Pribyl (2010), commonly used to estimate the organic matter (OM) content in soils, based on the total organic carbon (TOC) which was determined by the Walkly Black method, by hot oxidation with potassium dichromate (K2Cr2O7) and titration with ferrous sulfate (FeSO4) (Teixeira et al., 2017). For the determination of the humine fraction, chemical fractionation was carried out using the methodology that is based on solubility in alkaline or acidic medium and subsequent determination of the fraction carbon, which in the present study was used humine (C-HU) (Teixeira et al., 2017).

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