The state of Paraná in Brazil underwent a period of great change in land use starting at the beginning of the twentieth century.  This period saw the replacement of native forests by agricultural lands (Gubert Filho, 1998). The removal of plant cover and adopting unsuitable soil management    practices     with     intensive     cultivation considerable accelerated the erosion and compaction process, resulting in the degradation of agricultural soils because of a rapid decline in organic matter content (Castro Filho and Logan, 1991). The change in ground cover also triggered changes to water balance, causing reduced water infiltration rates and increasing surface runoff, in addition to increasing evaporation and reducing transpiration.

 

Rates of water infiltration into soil, along with the hydraulic conductivity of saturated soils, are two of the most important physical properties used to understand accelerated erosion. Water infiltration into the soil profile is considered a property that controls leaching, runoff and crop water availability (Franzluebbers, 2002). Portable rainfall simulators are useful to measure infiltration rates in the field (Roth et al., 1985; Santos et al., 2016). Thus, determining the infiltration rate plays a crucial role, given the direct relationship between erosion and ability of water to infiltrate and move into deeper layers of the soil profile (Roth et al., 1985).

 

The direct drilling (DD) system has often been recommended as a soil management technique for tropical and subtropical climates. This is due to the accumulation of surface residues and tilling only the sowing line, which supports higher carbon content in surface soil (Calegari et al., 2006). Furthermore, permanent plant cover at the soil surface provided by this agricultural system favors the physical conditions necessary for seed germination and initial crop growth (Vanlauwe et al., 2014). The use of green manure has the potential to improve soil chemistry, as well as its biological and physical properties (Zaccheo et al., 2016). In addition to plant cover, seeders equipped with drilling shafts in direct drilling systems provide a physical environment beneficial for establishing crops by increasing total porosity, macroporosity, and by reducing soil bulk density and mechanical resistance to penetration in the sowing line (Nunes et al., 2014). In direct drilling systems, straw mulches applied to the soil surface improves soil water status, reduces hydraulic gradients and soil temperature during the growing season by providing an insulation layer, especially in the period of crop establishment (Siczek et al., 2015). In contrast, where conventional tilling is part of soil preparation, there is a reduction in organic matter content and soil macroporosity compared with forest and DD soils (Dalmago et al., 2009).

 

Soil bulk density at the surface layer may increase in the years following DD implementation, which is due to soil compaction caused by the machinery used to conduct growing operations (Secco et al., 2005). As organic matter levels and biological activities increase, however, soil structure improves while soil bulk density gradually decreases (Assis and Lanças, 2005). The water available for crop cultivation largely determines the level of climate risk. Soils with high organic matter content provide   better    conditions    for    profile    rooting    and exploitation, resulting in greater yields in non-irrigated conditions. The volume of water stored in soil tends to be greater at higher (more positive) potentials when planted without tilling in forest soils and becomes lesser as the potential declines (Dalmago et al., 2009). The aim of this work was to characterize the physical and hydraulic properties of a Dystroferric Red Latosol after conventional tillage and direct drilling, and compare it to soil in native forest.

This study was conducted at the Experimental Station of the Agronomic Institute of Paraná (IAPAR) in Londrina (PR), Brazil, (23°23′ S and 50°11′ W), where the altitude was 585 m. The region has a Cfa climate type, which according to Köppen classification, is described as humid and subtropical with hot summers. The average annual temperature is 21.1°C. The temperature during the hottest (January) and coldest (July) months are 23.9 and 16.9°C, respectively. The average annual precipitation is 1,610 mm. December, January, and February are the wettest months, while June, July, and August receive the least rainfall (Caviglione et al., 2000).

 

The soil at the study area is described as a clayey Dystroferric Red Latosol according to the Brazilian soil classification system (Santos et al., 2013). It is similar to a Ferralsol (Food and Agriculture Organization of the United Nations - FAO, 2006), and a very fine, ferruginous, isothermic rhodic happludox according to USDA Soil taxonomy (Soil Survey Staff, 1998). Additional details regarding the mineralogical and chemical characteristics were reported by Castro Filho and Logan (1991). In all three soil management types, particle size composition exhibited clay levels greater than 75 dag kg^-1 throughout the soil profile (Table 1). Conventional tillage (CT) uses one operation with disc plough at a depth between 20 and 25 cm and two disking with the light disc harrow for leveling the ground and preparing the seedbed. This area was maintained for 10 years by cultivating winter cover crops black oats (Avena strigosa Schieb), oil seed radish (Raphanus sativus L. var. Oleiferus Metzg), or white lupins (Lupinus albus L.). In the summer, it was seeded with black mucuna (Mucuna pruriens) and Crotalaria spectabilis.

During the flowering stage, cover crops were treated with herbicides while the soils were broken up and then smoothed using a harrow. The area was kept uncovered and weeded by hand to collect physical and hydraulic data. To determine soil-physical hydric properties, the area was cleaned with hand hoeing weeding.

The direct drilling system (DD) area was, for 8 years, maintained with soybean with an inter-row spacing of 0.5 m during the summer, and black oats (IAPAR Ibiporã 61 variety) with an inter-row spacing of 0.17 m during the winter. The seeder for direct drilling was equipped with narrow tires, which opened a narrow slot into the mulch-covered soil. This equipment was pulled using a medium tractor according to National Association of Motor Vehicle Manufacturers (ANFAVEA, 2011) classification, New Holland^®, model TL 75, 4 × 2 TDA with 78 horse power). This tractor was equipped with 12.4 to 24 front tires (0.31 m) with a diagonal structure and 18.4 to 30 rear tires (width: 0.47 m). The native forest (NF) comprised two hectares in a secondary mixed hardwood with a highly diverse flora that was maintained as a legal reserve. Soil samples were collected using a Dutch auger.

 

This was to determine the total organic carbon content in each of the three systems (DD, CT, and NF). Samples were taken from four different depths (0 to 0.10 m, 0.10 to 0.20 m, 0.20 to 0.30 m, and 0.30 to 0.40 m), with three replicates for each. The samples were carbon oxidized by wet combustion with potassium dichromate (Walkley and Black, 1947). Data analysis was conducted according to the methods reported by Pavan et al. (1992). To conduct physical and hydraulic analyses, undisturbed soil samples were collected in between the rows after the summer harvest and/or following the management of undergrowth plants and before sowing winter species. For this purpose, trenches were dug in March 2011 and soil samples were collected horizontally and then were inserted in the soil, using Köpecky cylinders of 0.05 m height and internal diameter with sharp edges and an internal volume of 98.17 cm³.

 

 

 

These results are related to lower structure stabilization and to the  soil  load-bearing   capacity   of   soils   subjected   to conventional tillage (Dias Junior and Pierce, 1996). At depths of 0 to 0.10 m and 0.30 to 0.40 m, soil bulk density, total porosity, and macroporosity were not considered as constraints to plant growth and water infiltration into the soil. This was despite soil bulk density values in the surface layer increasing by 34% under DD conditions, when compared to NF soil, and by 42% under CT conditions (Figure 2). In clayey Dystroferric Red Latosol with reduced tillage and CT, Argenton et al. (2005) reported there were increased soil bulk densities between 71 and 86% for depths ranging from 0.05 to 0.10 m, and between 10 and 16% for depths ranging from 0.30 to 0.40 m. Machine use and unsuitable soil moisture conditions lead to permanent damage to the soil structure (Dias Junior and Pierce, 1996). This is the result of the pressure applied to the ground surface (Argenton et al., 2005) by tires or active parts of the equipment that exceed the soil’s ability to withstand the weight (Silva et al., 2003). Such heavy weights are transmitted to various depths through stress distribution (Araujo-Junior et al., 2011). Soil bulk density increase of 27 and 37% were recorded after applying an equivalent amount of pressure (900 kPa) on samples of oxidic-gibbistic red yellow latosol and clayey kaolinitic yellow latosol (Silva et al., 2006).

 

In two clayey Oxisols (81 and 83 dag kg^-1 clay) of the West region of the State of Paraná, Brazil, Assouline et al. (1997) reported that soil compaction differed when subjected to compressions ranging from 50 kPa to 1,000 kPa. Those researchers also showed that beyond the similarity in particle size distribution and soil bulk density of both soil types, there were several differences in their physicochemical properties, particle thickness and crystallinity, all of which affect soil stability. In addition to changes in soil bulk density, Silva et al. (2006) reported there was a reduction in the average diameter of stable aggregates in water.

This was at a total porosity volume of 17 and 23%, reflecting reductions in macroporosity by 53 and 67%, and increases in microporosity by 35 and 23%, respectively, for yellow red latosol and yellow latosol in samples subjected to 900 kPa. In the present study, the reduction in total porosity at depths ranging from 0.0 to 0.10 m, when compared to NF soil (0.75 cm³ cm^-3), was 14% for soil under the DD system (0.65 cm³ cm^-3) and 18% (0.62 cm³ cm^-3) under the CT system (Figure 2). At a depth range of 0.0 to 0.10 m, the sowing of winter undergrowth plants and commercial plants in the summer in CT and DD systems may have positively changed their physical properties based on the seeder’s active parts (circular blade, sowing drill, fertilizer dispensers, seed sowing drill, and tamping wheel). In addition, there was an accumulation of plant residues and land preparation through plowing and using a harrow.

 

Sowing both summer and winter crops may contribute to favorable structural conditions at the surface of DD soils (Nunes et al., 2014). Another aspect worth mentioning is that the pressure exerted by these parts of the seeder may change the pressure applied to the sowing layer, as noted by (Reis et al., 2006), for compacting components. Furthermore, Reis et al. (2006) noted that the sowing shaft reduced the soil bulk density from 1.14 kg dm^-3 to 1.00 kg dm^-3 at a maximum depth of 8.0 cm. The low soil bulk densities observed in the NF (Figure 2a) may be due to the thick layer of undergrowth that has been deposited over the years, making the NF surface soil levels highly organic and porous (Assis and Lanças, 2005; Centurion et al., 2007). Greater total porosity values (macro and microporosity) at a depth range of 0.0 to 0.15 m in CT soils compared with DD in Dystroferric Red Latosol were observed by Silva and Rosolem (2001).

 

The effect of the undergrowth on the physical properties of the soil is cumulative and requires years of management before a significant difference become apparent (Laurani et al., 2004). In the present study, the DD system was used for eight consecutive years and already showed changes in the soil’s surface levels. In the CT system, with management incorporating winter cover crops, such as black oats cultivar Iapar 61 used as green manure, improvements and differences in the soil’s properties were not very pronounced in comparison to those in the DD system. Forest soil contained higher total organic carbon content than that found in agricultural soils at depths of 0.0 to 0.10 m and 0.10 to 0.20 m (Figure 3). This may be attributed to greater organic residue provided by the tree canopy, in addition to the interception of incident radiation, which minimizes oxidation of the soil’s organic matter by directly radiating the soil surface. Conventional tillage promoted a mean decrease of 43% in soil organic carbon content at soil surfaces of 0 to 0.10 m (Figure 3). Due to the short duration under DD (8 years), no differences were observed in the organic carbon content in the soil compared to CT.

 

At a depth range of 0.20 to 0.30 m, the total organic carbon content observed in the soil under the CT system was similar to that observed in the NF and greater than that of DD soil. This may have been the result of soil layer inversion caused by soil preparation in the CT system, which distributes carbon to greater depths, increasing organometallic bonds and reducing microbial decomposition. The CT system introduced more carbon than the DD system in the layers evaluated as a result of crop rotation that incorporates undergrowth plants. Similar results, with respect to the total organic carbon content of the soil, were reported by Argenton et al. (2005), who confirmed that the association of cover crops with corn, stabilized the carbon content at certain depths regardless of the management system (DD or CT). It was expected that over the years, the DD system will gradually increase the carbon content in the soil profile because of the cumulative effect of adding plant residues to the surface layers, thereby improving the soil’s physical properties (Calegari et al., 2006). The results of the adjustment parameters using the Mualem-Genuchten model for the water retention curves for the LVdf in native forest and different soil management systems are presented in Table 2.

 

The model was adjusted to the data, with a coefficient of determination (R²) between 0.87 and 0.99, all of which were significant at 1% (P < 0.01) probability according to the F-test. Soil water retention curves from the three environments at four depths are displayed in Figure 4. At a depth range of 0.0 to 0.10 m, it was observed that CT soil provided greater water retention values among the potentials, ranging from -0.32 kPa to -1,500 kPa. Soil porosity was modified by CT soil preparation (reducing total porosity and macroporosity, while increasing microporosity) (Figure 2). With regards to NF soil (v–10 kPa = 0.2973 cm³ cm^-3), the Dystroferric Red Latosol’s water retention rate, at a matrix potential equal to -10 kPa (field capacity) under the CT system (v–10 kPa = 0.4026 cm³ cm^-3) increased by 30%. The DD system (v–10 kPa = 0.3761 cm³ cm^-3), however, increased by 20% in relation to water retention at -10 kPa when compared with NF soil. On the other hand, the CT system increased the water retention rate by 6% relative to the DD system (Figure 4). This result    was   also   observed   by    Sidiras    and    Vieira (1984), who noted that a modification of the porous space in the traffic line of an 85 HP tractor showed a positive effect on the ability of the soil to retain water. A similar effect was observed when growing wheat, soybeans, and turnips, relative to the soil between the wheel tracks, because pore volumes were reduced if their diameters were greater than 10 µm.

 

In the 0.10 to 0.20 m layer, NF soil retained less water at all matrix potentials, when compared to soil from CT and DD systems (Figure 3). Due to the increased macropore volume and low soil bulk density (Figure 2), these physical and hydraulic properties do not contribute to water storage. In the 0.30 to 0.40 m layer, there was greater water retention at less negative potentials than in the other environments.

 Nevertheless, the total volume of water retained in the NF was less at all depth ranges, which bolsters the forest’s role in maintaining soil quality, providing water absorption, preventing run-off, replenishing the groundwater table, and recycling water via transpiration. Furthermore, the possibility that forest soil at the higher depths could hold more water should not be discarded. However, these measurements were not evaluated in the   present   study.  The   volume   of available water varied between 9 and 12% at all depths, which was similar to values (9 and 10%) reported by Faria and Caramori (1986) for the same soil under CT conditions. The volume of water stored in the soil profile at a maximum depth of 0.40 m was greater under DD conditions (42 mm) and lower in the forest (38 mm), whereas soil under CT provided 39.71 mm of storage.

 

1. Soil use in CT and DD systems lead to changes in the physical and hydraulic attributes of the soil, thus, increasing soil bulk density and microporosity, reducing total porosity volume and macroporosity beneficial for water movement by the clayey Dystroferric Red Latosol in relation to native forest soil.

 

2. The CT system maintained total organic carbon content at 0.20 to 0.30 m depth similar to that observed in NF and greater than that of DD.

 

3. Permanent soil cover in the DD system, proportioned by plant residues, protected the soil structure against damage caused by direct impact of raindrops, maintained soil bulk density lower than critical limits, total porosity, macroporosity and enhanced water infiltration compared to conventional tillage soil.

The authors have not declared any conflict of interests.