Age-hardening phenomena in an oxisol from the subtropical region of Brazil

Soil & Tillage Research 170 (2017) 27–37

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Age-hardening phenomena in an oxisol from the subtropical region of Brazil Moacir Tuzzin de Moraesa,* , Henrique Debiasib , Reimar Carlessoc , Julio Cezar Franchinib , Vanderlei Rodrigues da Silvad , Felipe Bonini da Luzd a

Federal University of Paraná, 80035-050, Curitiba, PR, Brazil Embrapa Soybean, PO Box 231, 86001-970, Londrina, PR, Brazil Federal University of Santa Maria, 97105-900, Santa Maria, RS, Brazil d Federal University of Santa Maria, 98400-000, Frederico Westphalen, RS, Brazil b c

A R T I C L E I N F O

Article history: Received 14 July 2016 Received in revised form 26 January 2017 Accepted 2 March 2017 Available online xxx Keywords: No-tillage system Rhodic eutrudox Soil penetration resistance

A B S T R A C T

Soil strength is not only affected by water content and bulk density, but also by the age-hardening phenomena, which plays a key role in increasing the soil strength as a function of time. It has been demonstrated that soil penetration resistance in no-tillage is higher when compared with other tillage systems at the same bulk density and water content. The objectives of this study was to investigate the effects of the age-hardening phenomena on soil penetration resistance in a long-term soil management system, running since 1988 in a very clayey Oxisol, in southern Brazil. Soil samples were collected from three soil layers (0.0–0.10 m; 0.10–0.20 m and 0.20–0.30 m) and five soil tillage systems: conventional tillage; minimum tillage with chiselling performed every year or every three years; and no-tillage for 11 or 24 years. Age-hardening was investigated using soil penetration resistance analysis and modelling. We used the area under the soil resistance to penetration curve to compare the age-hardening phenomena under the different tillage systems. For the same bulk density and water content, the soil resistance to penetration increased with time under no-tillage or without soil chiselling. For the same bulk density, no differences were found for macroporosity and microporosity among the tillage systems. Higher soil penetration resistance values in long-term no-tillage at the same soil bulk density and water content were attributed to the age-hardening phenomena, which increased the number and strength of bonds among soil particles, leading to higher soil cohesion. It is necessary to establish critical limits of soil penetration resistance as a function of the soil tillage system, and the time without soil chiselling or under no-tillage system. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Soil penetration resistance (SPR) is widely known to change as a function of water content and soil bulk density (Moraes et al., 2012). Therefore, the soil water content dependency may influence the interpretation of the soil’s compaction level when using SPR as a quantitative indicator. This problem may be circumvented by measuring the SPR in a drying soil at different bulk density values, either in the laboratory using undisturbed soil samples, or directly in the field (Busscher, 1990). To further the understanding of soil

* Corresponding author. E-mail addresses: [email protected], [email protected] (M.T. de Moraes), [email protected] (H. Debiasi), [email protected] (R. Carlesso), [email protected] (J.C. Franchini), [email protected] (V.R. da Silva), [email protected] (F.B. da Luz). http://dx.doi.org/10.1016/j.still.2017.03.002 0167-1987/© 2017 Elsevier B.V. All rights reserved.

strength variation, the soil penetration resistance curve (SPR curve) can be utilised. The SPR curve is the reading of SPR variation as a function of bulk density and soil water content. The SPR curve may be a useful parameter for evaluating soil physical quality in areas under annual crops (Gao et al., 2016), native forest or fruit trees (Fidalski et al., 2010), because it is closely related to the effects of soil strength on crop growth (Bengough et al., 2011). However, the SPR curve depends on more parameters than only the soil water content and bulk density (Busscher, 1990; Moraes et al., 2012). Accordingly, the soil organic carbon content and soil textural composition (Gao et al., 2011), or time without soil disturbance are assumed to contribute to change in soil particle arrangement, and particle cementation (Dexter et al., 1988). The agricultural area managed under no-tillage has increased continuously over the last decade mainly in tropical and subtropical regions, because long-term no-tillage can preserve

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the structural quality of soils over time, and provide suitable soil physical conditions for crop growth (Moraes et al., 2016). However, excessive soil compaction in untilled surface layers is regarded as one of the major reasons for crop yield reductions, especially during dry years (McKenzie et al., 2009), in weakly structured soils (López-Garrido et al., 2014) or in absence of diversified crop rotation systems (Abdollahi et al., 2015). The increase in soil compaction increases SPR (Moraes et al., 2012) and reduces soil porosity, macroporosity, aeration, water infiltration capacity (Valentine et al., 2012) and hydraulic conductivity (Silva et al., 2009). These soil physical alterations lead to poor root growth (Schmidt et al., 2013), and thus limit the soil depth and volume explored by the roots for the uptake of water and nutrients (Bengough et al., 2011) However, continuous pores or biopores may attenuate deleterious effects of the soil mechanical restrictions on plant growth (Moraes et al., 2016; Calonego and Rosolem, 2010). Thus, soil management is the most important factor in changing soil structural quality, for example through the creation of a continuous and stable network of biopores (Moraes et al., 2016). Traditionally, SPR values critical to plant growth have been indicated without taking into account either the soil management system or the adoption time. Only a few studies have considered the effects of soil management systems and their adoption time on the determination of SPR critical limits on crop growth and yield (Moraes et al., 2014a). For instance, several studies have assumed that a SPR value greater than 2 MPa at field capacity is limiting to root growth (Lipiec et al., 2012), slowing root elongation to less than half of its rate under unimpeded soil conditions (Gregory, 2006; Bengough et al., 2011). In many agricultural areas, no-tillage has led to SPR values above 2 MPa, however, in these areas no reduction in crop yield (Moraes et al., 2014a), or root growth (Martínez et al., 2008) was observed, revealing a cementation process that strengthened the soil structure in a way that meant the soil functioning was preserved. This strengthening may be ascribed to the formation of a pore network encompassing continuous and vertically oriented biopores that enable root elongation and adequate water and air fluxes in the soil, even under a high SPR (Moraes et al., 2016). Additionally, a wellcemented pore network is more resistant against collapse when the soil is exposed to heavy agricultural machinery traffic (Jin et al., 2013). The processes that result in soil strength increasing over time without soil disturbance have been reported in the literature, such as age-hardening phenomena (Utomo and Dexter, 1981). These phenomena are the result of two major processes, particle rearrangements, and particle cementation (Dexter et al., 1988). The first process has been called the type A mechanism, and is the true thixotropic effect, involving the rearrangement of soil particles (mainly clay) into new positions of minimum free energy (Dexter et al., 1988; Dexter, 1990). The second, is known as the type B mechanism, and involves the reformation or strengthening of cementing bonds at new points of contact or near-contact between pairs of mineral particles (Dexter, 1990). This higher number of contact points between soil particles, and the strengthening of the bonds among them, leads to greater soil cohesion and internal friction (Fuentes et al., 2013), thus inducing increases in soil penetration resistance without significant alterations to the volume, size and arrangement of pores (Moraes et al., 2014a; Ortigara et al., 2015). Soil strength and the strength of aggregates formed from disrupted soils increases with time (Utomo and Dexter, 1981). This process has a close association with the Mohr-Coulomb’s equation (cohesion and angle of internal friction), which determines the soil shear resistance (Conte et al., 2011). In addition, the soil cohesion is affected by time (Kemper and Rosenau, 1984), water content (Secco

et al., 2013), number of pores and cracks (Fuentes et al., 2013), organic matter, temperature, texture (Kemper et al., 1987), mineralogy, and iron and aluminium contents (Sánchez-Girón, 1996). Consequently, increases in soil strength are expected to occur over time after conservation tillage adoption, as a result of age-hardening processes (Horn, 2004). The definition of critical values of SPR for long-term no-tillage has been widely discussed in the literature, but still remains unclear (Moraes et al., 2014a; De Jong van Lier and Gubiani, 2015), possibly due to the influence of cracks and biopores on root growth (Dexter, 1991). However, there is little information regarding the age hardening phenomena in subtropical clayey soils managed under no-tillage. Thus a better understanding of this process is necessary to establish more accurate critical limits of SPR to allow a better understanding and monitoring of soil compaction and physical quality (Moraes et al., 2014a). We hypothesised that the absence of soil disturbance (notillage system) increases SPR values under the same bulk density and water content over time, as a result of the age hardening process. Thus, distinct SPR critical limits are needed as a function of tillage system, and the use of SPR curves as a soil physical quality indicator is a valuable option. We aimed to study the age hardening phenomena in no-tillage systems, and quantify its influence on the SPR curve of a very clayey soil for describing the evolution of soil physical quality. 2. Material and methods 2.1. Study site The study was carried out in a long-term experiment established in 1988 at the Experimental Station of Embrapa Soybean, in Londrina (latitude 23
110 S; longitude 51
110 W; and 620 m in altitude) State of Paraná, Southern Brazil. According to the Köppen classification, the climate of the region is humid subtropical (Cfa), with an annual average temperature of 21
C and with 1651 mm of rainfall (Moreno, 1961). The experiment was established on an Oxisol (Latossolo Vermelho Distroférrico, Brazilian classification; Rhodic Eutrudox, USA classification) with 755 g clay kg1 soil, 178 g silt kg1 soil and 67 g sand kg1 soil. The soil particle density at 0–0.3 m depth is 2.90 Mg m3, and the mean slope of the experimental area is 0.03 m m1. Before the establishment of the experiment, the area had been cropped with coffee (Coffea arabica L.) for approximately 40 years, with the entire area receiving similar management and inputs. 2.2. Experimental design, treatments, and field management The experiment was laid out in a 5  2factorial (soil tillage  cropping system), distributed in a randomised block design with four replications. The treatments consisted of the following tillage systems: conventional tillage with heavy disking to a depth of 0.15 m, then light disking (0.1 m depth), performed before each winter and summer growing season (CT); minimum tillage with annual chiselling (MTC1), performed before each winter crop planting, and no-tillage for the summer crop; minimum tillage with chiselling every three years (MTC3), performed before the winter crop planting, and no-tillage for the other winter/summer crops; continuous no-tillage for 11 years, established in 2001 (NT11); and continuous no-tillage for 24 years (NT24), established in 1988. Between 1988 and 2001, the soil under NT11 was tilled with a mouldboard plough (average working depth of 0.32 m), followed by light disking before planting the summer crop, and heavy disking (average working depth of 0.15 m) followed by light disking (0.07 m work depth) before the planting of the winter crop. The MTC1 and MTC3 plots were chiselled using a mounted chisel

M.T. de Moraes et al. / Soil & Tillage Research 170 (2017) 27–37

plough with rollers and four shanks spaced 0.40 m apart, working at an average depth of 0.30 m and an angle of 45
. Soil sampling was conducted at 10 and 22 months after final soil chiselling in the MTC1 and MTC3 plots, respectively. The soil tillage systems were established under two cropping systems: (i) wheat (Triticum aestivum L.) in the winter and soybean (Glycine max (L.) Merr.) in the summer every year (crop succession); and (ii) a four-year crop rotation system (crop rotation), with the following species (winter/summer): year 1 = white lupine (Lupinus albus L.) or radish (Raphanus sativus L.)/maize (Zea mays L.); year 2 = white oat (Avena strigosa Schreb.)/ soybean; year 3 = wheat/soybean; and year 4 = wheat/soybean. Each plot was 30 m long  10 m wide (area of 225 m2), with a space of 7 m in width left between each plot to allow tractor manouvering during operations. The average shoot dry biomass production of the species in succession and crop rotation systems was approximately 5.3 and 7 Mg ha1 yr1, respectively. The average soil organic carbon content in 2012 for the 0.0–0.10 m surface layers were 18.9 (CT), 19.9 (MTC1), 19.8 (MTC3), 20.6 (NT11) and 21.9 g kg1 (NT24). 2.3. Soil sampling and laboratory analysis In January 2012, when all the evaluated plots were grown with soybean, core soil samples were collected from the centre of the layers (0.0–0.10, 0.10–0.20, and 0.20–0.30 m) using stainless steel rings, with a volume of 100 cm3 (5 cm internal diameter and 5 cm height). Thus, 40 undisturbed cores were sampled per tillage system (5) for each soil layer (3) in the soybean interrows, totalling 600 soil samples of preserved structure. The 600 undisturbed samples were divided into five groups of 120, encompassing eight samples per tillage system and soil layer, regardless of the cropping system. Subsequently, each group of samples was subjected to the following soil matric potentials: 3 and 6 kPa using suction

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tables (EMBRAPA, 1997); 10; 33; 100; and 500 kPa in Richards’ chambers with porous plates. Once equilibrated at each matric potential, the samples were weighed, and the SPR was determined using a static bench penetrometer (Model: MA 933 Marconi) (Moraes et al., 2014b). The penetrometer consisted of a metal rod with a cone at the end, with a semi-angle of 30
, 4 mm diameter, and with a base area of 0.1256 cm2 connected to a load cell with a nominal capacity of 20 kgf. The penetration rate was 20 mm min1, so that for each sample, 120 readings were performed to a depth of 40 mm. The SPR was calculated as the average of the readings from 5 mm to 40 mm soil depth for each core sample. After SPR determination, the soil samples were oven-dried at 105
C for 48 h to quantify the soil bulk density – BD (Mg m3) and the volumetric soil water content – u (m3 m3). The soil microporosity (pores <50 mm) was equivalent to the volumetric water content at 6 kPa, known from measurements from the suction table. The macroporosity (pores >50 mm) was calculated as the difference between total porosity and microporosity. The SPR curve was adjusted in relation to the SPR values (independent variable) for the respective soil water content and bulk density (dependent variables), using the non-linear model described by Busscher (1990) (Eq. (1)) for each soil tillage system irrespective of the cropping systems. The coefficients of determination (R2) of the fitted Busscher’s non-linear models were calculated according Eq. (2). SPR ¼ aBDb u

c

ð1Þ

Where, a, b and c are the parameters of the model. R2 ¼ 1 

SSres SSreg

ð2Þ

Where, SSres = residual sum of squares; and SSreg = regression sum of squares.

Table 1 Non-linear regression parameters adjusted for the soil penetration resistance curve (SPR = a*BDb*uc), and the respective coefficient of determination (R2), at three layers in a Rhodic Eutrudox under different soil tillage systems. Parameter

Layers 0.0–0.10 m Estimated1

a b c

CT 0.0014  0.001 10.2964  1.03 5.3125  0.50

a b c

MTC1 0.00426  0.002 7.9621  0.71 4.6272  0.46

a b c

MTC3 0.0017  0.001 9.9153  0.69 5.3255  0.48

a b c

NT11 0.0193  0.014 5.3898  0.92 3.8757  0.60

a b c

NT24 0.00402  0.002 8.8037  0.73 4.8792  0.43

0.10–0.20 m

0.20–0.30 m

R2

Estimated

R2

Estimated

R2

0.94*

0.0740  0.06 3.4985  0.95 3.1692  0.75

0.78*

0.0998  0.05 0.7897  0.99 3.8852  0.63

0.91*

0.93*

0.0214  0.01 4.5706  0.75 3.9248  0.61

0.78*

0.0624  0.04 3.8942  0.81 3.0966  0.66

0.84*

0.96*

0.0276  0.02 4.9886  1.04 3.7445  0.63

0.84*

0.0217  0.01 4.3161  0.106 4.3430  0.61

0.91*

0.87*

0.0450  0.02 3.0957  0.94 3.7821  0.42

0.89*

0.0356  0.02 3.9592  1.19 3.7502  0.53

0.89*

0.96*

0.0009  0.0004 9.7291  0.87 6.22421  0.34

0.98*

0.0022  0.001 8.3026  0.92 6.0537  0.41

0.97*

CT: conventional tillage; MTC1: minimum tillage with chiselling every year; MTC3: minimum tillage with chiselling every 3 years; NT11: continuous no-tillage for 11 years; NT24: continuous no-tillage for 24 years. R2 = [1-(SSres/SSreg)]; u: soil volumetric water content (m3 m3); BD: soil bulk density (Mg m3); SPR: soil penetration resistance (MPa). 1parameters values the standard error. *significant by F-test at the 5% level.

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2.4. Data analysis The adjustment of SPR curves (Eq. (1)) to the measured data was performed using the routine “PROC NLIN” from the Statistical Analysis System 8.0 (SAS). Adjusted SPR equations were subjected to analysis of variance (ANOVA) with significant differences reported at P < 0.05. For all layers, the SPR curves were compared between the soil tillage systems using the area under the curve, calculated by the integral of the adjusted Eq. (1), considering water contents ranging from 0.35 to 0.42 m3 m3 and bulk densities ranging from 1.21 to 1.30 Mg m3. The area under the curve was calculated using the Matlab1 software. The area under the curve from tillage systems was submitted to ANOVA, and, when F-values were significant (p < 0.05), means were separated by t-test (p < 0.05). 3. Results and discussion The fitted soil penetration resistance (SPR) curve parameters for the soil tillage systems evaluated at the 0.0–0.10, 0.10–0.20, and 0.20–0.30 m layers are shown in Table 1. The models explained over 87% (0.0–0.10 m), 78% (0.10–0.20 m), and 84% (0.20–0.30 m) of the SPR variability. The relation between the measured and calculated SPR values showed that the models were adequate for all tillage systems, therefore, SPR can be accurately estimated from bulk density and water content values using Busscher’s model (Eq. (1)) (Fig. 1). The SPR equations were significant (p < 0.05) for all the tillage systems and layers evaluated. The SPR range for all the tillage systems and layers showed differences in the function of soil structure under field conditions (Fig. 2). Thus, the range of bulk density and u was higher in CT (Fig. 2-II) and MTC1 (Fig. 2-II, 2-III) compared with the other tillage systems. This higher range in the 0.10–0.20 m layer of CT reflects a transition region between the uppermost layer (0–0.10 m) and the layer containing a plough pan at 0.20–0.30 m. In addition, MTC1 showed large ranges of u and bulk density, probably resulting from differences in soil disturbance that occurred due to their positions near to and between shanks. Conversely, NT24 resulted in a smaller bulk density and u range at 0.10–0.20 m (from 1.19 to 1.36 Mg m3) and 0.20–0.30 m (from 1.13 to 1.33 Mg m3) layers. This bulk density range reflects an intermediary soil compaction level (maximum bulk density for this soil was 1.5 Mg m3); therefore, the best physical condition for root growth and yield is provided by the no-tillage system (Moraes et al., 2016).

Fig. 1. Calculated vs. measured values of soil penetration resistance in a Rhodic Eutrudox, very clayey soil, obtained at different layers and from different soil tillage systems. The dashed line represents the one-to-one relationship.

The SPR curve was plotted as a function of u (from 0.35 to 0.42 m3 m3) and bulk density (from 1.21 to 1.30 Mg m3) for all soil tillage systems at 0.0–0.10 m (Fig. 3a), 0.10–0.20 m (Fig. 3b), and 0.20–0.30 m (Fig. 3c) layers, enabling us to observe changes in the exponential increase of SPR as a function of u and bulk density, for each soil tillage system. The bulk density and u ranges were chosen because these values were observed in all soil layers and tillage systems according to the field observations described in Fig. 3. Moreover, the option of plotting the SPR curves using the same bulk density and u ranges allowed reliable comparisons among the treatments through the area under the curve (Table 2). The area under the SPR curves was greater for NT24 than for the other tillage systems, except the area under the curve for CT in the 0.20–0.30 m layer, which did not differ from NT24 (Table 2). The main increase in SPR on CT, at the layer 0.20–0.30 m, was due to a reduction of soil water content associated with lower soil organic content, which can increase the soil strength because there is a mechanical effect of soil particle cementation under lower as well as higher bulk density. Additionally, the area under the SPR curve was higher in NT24 than NT11, as well MTC3 had a higher SPR than MTC1, for all layers. This increase in SPR under the same bulk density and u emphasises that the soil strength increases over time, enabling more stable aggregates, and the formation of continuous pores within the soil profile. Therefore, the soil susceptibility to compaction increased with soil perturbation by chiselling. When soil disturbance through tillage is more frequent, the soil load-bearing capacity is strongly reduced, leading to a soil that is highly-susceptible to compaction (Ortigara et al., 2015). This may have a direct relation with the soil strength, and the agehardening phenomena can help in understanding the effects of notillage on soil load-bearing capacity. Considering the tillage system at the same bulk density and u, the SPR values were higher under NT24 than NT11, while MTC3 had higher SPR values than MTC1, for all the layers (Table 2). This increase in SPR under the same bulk density emphasises that the soil strength increases over time, enabling more stable aggregates, and the formation of continuous pores within the soil profile. Therefore, the soil’s susceptibility to compaction is increased with soil perturbation by chiselling. When soil disturbance through tillage is more frequent, the soil load-bearing capacity is strongly reduced, leading to a soil that is highly-susceptible to compaction (Ortigara et al., 2015). This may have a direct relation with the soil strength, and the age-hardening phenomena can help in the understanding of effects of no-tillage on soil load-bearing capacity. The bulk density and u effects on SPR in the NT24 treatment were greater compared with all the other tillage systems (Fig. 3). Hence, we observed that for the same bulk density and u, the SPR values in the NT24 treatment were higher than in the other tillage systems, indicating that the absence of tillage resulted in an increase in soil resistance over time (Fig. 3a and Table 2). This finding was even more apparent when comparing NT24 with NT11, and MTC3 with MTC1. Thus, considering the same bulk density (1.30 Mg m3), when the u decreased from 0.42 m3 m3 to 0.35 m3 m3 at the 0.0–0.10 m depth the SPR difference between NT24 and NT11 increased from 0.5 MPa (0.42 m3 m3) to 2.15 MPa (0.35 m3 m3). Similarly, SPR increased when longer intervals between soil chiselling were used. For the same bulk density (1.30 Mg m3), the differences between MTC3 to MTC1 increased from 0.45 MPa to 1.80 MPa when u varied from 0.42 m3 m3 to 0.35 m3 m3. Therefore, SPR increased as the time without soil disturbance increased for both tillage systems (no-tillage and minimum tillage) (Table 2). Thus, the general effect for the same bulk density and u was the increase in SPR values in response to longer periods under notillage or without soil chiselling, as a result from the age-hardening phenomenon. The effects of “cementation” among soil aggregates

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Fig. 2. Soil penetration resistance as affected by soil water content and bulk density, for the tillage systems (a) CT; (b) MTC1; (c) MTC3; (d) NT11; and (e) NT24, at (I) 0.0–0.10 m; (II) 0.10–0.20 m and (III) 0.20–0.30 m layers, in a Rhodic Eutrudox. CT: conventional tillage system; MTC1: minimum tillage system chiselled every year; MTC3: minimum tillage system chiselled every 3 years; NT11: continuous no-tillage system for 11 years; NT24: continuous no-tillage system for 24 years. Dotted lines show the common range, for all treatments, of bulk density and soil water content that were used for the statistical test of the models in Fig. 4.

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Fig. 3. Variation of soil penetration resistance estimated by the Busscher (1990) model for each soil tillage system as a function of soil volumetric water content and bulk density at (a) 0.0–0.10 m; (b) 0.10–0.20 m; and (c) 0.20–0.30 m depth, in a Rhodic Eutrudox in Londrina, PR, Brazil. NT24: continuous no-tillage system for 24 years; NT11: continuous no-tillage system for 11 years; MTC1: minimum tillage system chiselled every year; MTC3: minimum tillage system chiselled every 3 years; CT: conventional tillage system. BD: bulk density.

and particles are intensified as u decreases and departs from field capacity (0.34 kg kg1). Even at the deepest layer (0.20–0.30 m), SPR increased with time without chiselling, when considering the same bulk density and u (Fig. 3c). At this layer, when considering the same bulk density (1.30 Mg m3) and u (0.35 m3 m3), the MTC1 treatment had an average SPR value of 4.5 MPa, whereas in MTC3, the average SPR was 7.3 MPa. Likewise, the area under the curve was greater for MTC3 compared with MTC1 at the 0.20– 0.30 m depth, reinforcing that SPR increases under longer intervals between chiselling (Table 2). There are two types of age-hardening processes (Dexter et al., 1988), referred to as Type A and Type B (Fig. 4). Type A agehardening occurs when new particle–particle bonds are formed by the rearrangement of soil particles. However, in the Type B mechanism, new bonds are not formed, instead existing bonds

Table 2 Area under the curve of soil penetration resistance for a range of bulk density and water content from Fig. 1, for different tillage system and layers in a Rhodic Eutrudox. Tillage systems

Soil layers (m) 0.0–0.10

0.10–0.20

0.20–0.30

CT MTC1 MTC3 NT11 NT24

0.174  0.04 c1 0.162  0.03 d 0.203  0.04 b 0.195  0.05 b 0.238  0.02 a

0.243  0.02 b 0.187  0.02 d 0.226  0.03 c 0.244  0.06 b 0.260  0.02 a

0.356  0.01 a 0.209  0.02 c 0.268  0.03 b 0.228  0.02 c 0.365  0.07 a

1 Means the standard deviation, when followed by the same letter for the same soil layer did not differ by t-test at the 5% level.

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Fig. 4. Schematic representation of particle–particle bonds as affected by the age-hardening process in mechanism type A and B, according to the concepts described by Dexter et al. (1988).

become stronger (Dexter et al., 1988). Additionally, the critical u for the Type B mechanism increases as a function of the soil organic carbon content. This indicates that u must be higher to enable the Type B age-hardening process in soils with higher organic matter content. Conversely, Type-B age-hardening may occur at much lower water contents when the organic matter content is low (Dexter et al., 1988). At 0.20–0.30 m layer, except for the NT24 treatment, SPR values were higher in CT than other soil tillage systems considering the same bulk density and u. In CT the effects of the heavy harrow disks every year were limited to 0.15 m depth. Thus, regarding the time without soil disturbance at 0.20–0.30 m depth, both CT and the NT24 were identical, so that SPR was expected to be similar under the same bulk density and u. However, for bulk density higher than 1.23 Mg m3, and when the u is reduced from 0.42 m3 m3 to 0.35 m3 m3, the differences increased (Fig. 3c), but still without significant differences for the area under the curve (Table 2). These findings indicate that the high SPR values in NT24 are likely associated with other factors and processes besides the time without soil disturbance. Accordingly, the soil organic carbon may play a key role in both age-hardening mechanisms. For CT, u had more effect than bulk density on the variation of SPR values. Conversely, the effects of bulk density and u on SPR in NT24 were similar, indicating that root growth is likely less impaired when the compacted soil layer has higher organic carbon contents. Another hypothesis for these differences in SPR, for the same bulk density and u, between NT24 and CT is related to the formation of a “pan layer” with an absence of biopores, continuous pores and small root systems in CT, leading to changes in aggregation at the 0.20–0.30 m layer. Thus, in CT, the increase in soil strength was lower than NT24 because the bonds between soil aggregates in the CT were probably weaker, resulting in a lower aggregate structural stability. The absence of suitable physical conditions in CT at the 0.20–0.30 m layer may also have reduced the microbial activity, impairing the formation and binding of soil aggregates (Silva et al., 2014). Changes in the absolute value of each regression coefficient led to different SPR values for the same bulk density and u as a function of soil tillage systems and soil layers. For a better understanding of the effects of Busscher’s model coefficients on soil strength, the relationship between the estimated SPR values in NT24 was compared with all the other tillage systems (Fig. 5), for three soil layers. With the exception of CT in the 0.20–0.30 m layer (Fig. 5dIII), the SPR observed in NT24 was higher than in the other treatments (NT11, MTC3, MTC1 and CT), as indicated by the values located above the 1:1 line. In general, at higher SPR values, i.e., at

greater bulk density and lower u, the differences between NT24 and the other soil tillage systems increased, for all layers studied. Thus, these results prove the occurrence of the age-hardening phenomena in NT24, leading to higher cohesion and SPR for the same bulk density and u values comparatively for soil tillage systems disturbed more frequently and/or recently. Besides the absence of soil disturbance, the increase in SPR in NT24 compared with the other treatments may be linked to higher soil organic carbon contents, which can foster the age-hardening (type B mechanism) by strengthening the existing bonds between soil aggregates and particles (Dexter et al., 1988). It has to be highlighted that high SPR values may not be limiting to plant growth, since they do not necessarily mean a reduction in the pore space, as indicated by the SPR variation for the same bulk density and u. Furthermore, in systems without soil disturbance, the root growth may occur through biopores or low resistance zones in the soil profile (White and Kirkegaard, 2010). In this context, the definition of critical limits to root growth depends on other soil structural characteristics, such as the presence of cracks, biopores, inter and intra-aggregate regions with lower resistance that are perceived by the roots, but not by cone penetration (Bengough et al., 2011; Gubiani et al., 2013). Therefore, it is necessary to establish differentiated critical values of SPR as a function of soil management systems (Moraes et al., 2014a). Critical SPR values for a given management system may not be considered critical under another management system, with better soil structure, well developed aggregates, and more continuous pores such as biopores, providing easier root penetration even under high soil strength. The age-hardening phenomenon was defined by Dexter (1988) as the process in which the soil strength increases with time by two mechanisms, Type A and B, where the age-hardening is either related to the formation of new inter-particle bonds (Type A), or to the strengthening of existing inter-particle bonds (Type B). Regardless the mechanism involved in the age-hardening increases the soil strength as a function of the time without soil disturbance. This process can also be considered as the recovery of soil resistance with time by cohesion (Horn, 2004), or because the connections between soil particles that have been broken due to _ soil disturbance tend to be reconstructed with time (Błazejczak et al., 1995). Processes associated with physical degradation by soil mobilisation, may reduce the soil aggregation and strength (Veiga et al., 2007). The mechanisms involved in the age-hardening phenomena have been related to the rearrangement of soil particles via flocculation of clay particles accompanied by changes in pore size distribution and restoration of cementing bonds

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Fig. 5. Relationship between soil penetration resistance (SPR) estimated by Busscher’s model for the same bulk density and water content, in continuous no-tillage system for 24 years (NT24) compared with others systems: (a) NT11; (b) MTC3; (c) MTC1; (d) CT, for the (I) 0.0–0.10 m; (II) 0.10–0.20 m and (III) 0.20–0.30 m soil layers. NT24: continuous no-tillage system for 24 years; NT11: continuous no-tillage system for 11 years; MTC1: minimum tillage system chiselled every year; MTC3: minimum tillage system chiselled every 3 years; CT: conventional tillage system.

between soil particles, reflecting increase in tensile strength (Tormena et al., 2008). Debiasi et al. (2008) working on the effects of winter cover crops and wheeled tractor traffic on load bearing capacity and compressibility of an Alfisol also discussed processes that increase soil strength. These authors observed increases in the load-bearing capacity at the 0.03–0.06 m layer as a function of the soil sampling time. The increases in pre-consolidation stress were not attributed to bulk density, total porosity, and macroporosity because these properties were not affected by the sampling period. Therefore, the most likely reason for the increase in the preconsolidation stress over the soil sampling times is the longer period after soil mobilisation, enabling the age-hardening phenomenon. It is worth noting that the increase in SPR values observed in NT24 compared with the other treatments did not reduce the yield of both soybean and wheat during the 2011/12 growing season (Moraes, 2013). Moreover, Franchini et al. (2012), while analysing the data obtained in the same long-term experiment, showed higher soybean and wheat yields in NT than in CT from the 1994/95 to 2010/11 growing seasons. This result reinforces that higher SPR values may be not limiting to crop growth if the pore space enables adequate root elongation, air and water fluxes in the soil profile.

The SPR values are determined by several soil factors, such as bulk density, soil moisture content, particle density, particle size and pore size distribution. However, particle density and size distribution were constant among the experimental plots; and during the modelling process, bulk density and u were kept constant to enable comparisons among the tillage systems. In order to verify if the SPR values were influenced by the modification of the pore size distribution resulting from variations in the bulk density, we established the relationship between soil macro and microporosity (calculated from saturation) and bulk density for all the layers (Fig. 6). The macroporosity decreased exponentially with increasing bulk density, irrespective of the soil tillage system. Thus, macroporosity cannot be considered as a variable directly responsible for the SPR variation under the same bulk density. In addition, soil microporosity varied with bulk density according to a quadratic model for all the layers (Fig. 6d–f). Importantly, for the same bulk density, microporosity did not differ among tillage systems. Considering a bulk density of 1.30 Mg m3, the average macroporosity values were 0.43, 0.45 and 0.46 m3 m3 for the 0.0–0.10, 0.10–0.20 and 0.20–0.30 m layers respectively. These results show that changes in pore size distribution are not related to the increase in SPR values for the same bulk density and

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Fig. 6. Relationship between soil bulk density and macroporosity (a,b,c) or microposority (d,e,f) in a Rhodic Eutrudox as a function of soil tillage systems at 0.0–0.10 m (a,d), 0.10–0.20 m (b,e) and 0.20–0.30 m (c,f) layers.

u, observed in treatments with longer periods without soil disturbance. Therefore, the changes in SPR under the same bulk density and u as a function of time in NT system or without soil chiselling are directly linked to an increased bond number and strength of soil aggregates by age-hardening phenomena. Soil strength was increased as a function of time under notillage or without soil chiselling, and age-hardening phenomena was described for SPR by Busscher’s model. In general, the soil structure in no-tillage systems undergoes intense changes due to wetting and drying cycles (Gregory et al., 2009), soil management (Moraes et al., 2016), and crop rotation (Tivet et al., 2013). The pore network and soil structure are strengthened with time, positively influencing the hydraulic or gaseous fluxes (Horn, 2004). Soil strength is increased as a function of time without soil mobilisation due to higher soil cohesion and internal resistance of aggregates (Kemper and Rosenau, 1984). In addition, the rigidity of the pore system is affected by the internal rearrangement of particles (Horn, 2004). The main effects of age-hardening phenomena are observed in higher SPR values, which occur in lower u, because the cohesive forces are manifested more strongly in such conditions (Secco et al., 2013). No-tillage system changes the soil structure, soil aggregation and arrangement of particle–particle contact, leading to larger SPR values. Increased duration since the adoption no-tillage, associated with crop rotations encompassing plant species with abundant root systems, plays a key role in improving soil physical quality (Moraes et al., 2016), increasing the soil organic carbon content, the aggregate stability (Devine et al., 2014), the soil load-bearing capacity (Ortigara et al., 2015), the soil cohesion (Braida et al., 2007) and tensile resistance of aggregates (Ferreira et al., 2011). Soil chiselling affects the soil structure and aggregation, especially due to it breaking the particle–particle bonds, resulting in reduced SPR values in relation to no-tillage system, even when considering similar bulk density and u values. In addition, the continuous pores and network in the soil are disrupted, increasing

the soil resistance to root growth, and impairing water and gas fluxes in the soil. This occurs because the soil aggregation processes are related to the temporal persistence of the binding agents (e.g., transient, temporary, persistent), which foster the formation of micro-aggregates (persistent binding agents, primary particles and iron and aluminium oxides) or stable macroaggregates resulting from the enmeshing effect of fungal hyphae, and roots, which are temporary binding agents (Tivet et al., 2013). The physical protection of soil macroaggregates takes time (Cates et al., 2016), and the soil disturbance may disrupt such macroaggregates fostering the organic matter decomposition (Salvo et al., 2014), and reducing the soil tensile resistance (Ferreira et al., 2011)and cohesion (Silva et al., 2004), Thus, the soil chiselling directly affects the age-hardening phenomena, reducing the number of particle–particle bonds (type A mechanism) and the weakening of particle–particle bonds (type B mechanism). In general, the soil strength increases due to an increase in cohesive forces of capillary bound water and the increased effectiveness of cementing materials (e.g. drying and hardening of formerly dispersed clay) (Munkholm, 2011). The increase in soil organic carbon concentration under no-tillage compared with conventional tillage usually is attributed to a higher stabilisation of associated-C within macro and microaggregates (Tivet et al., 2013). This soil organic stabilisation confers a high tendency to increase soil aggregation through a higher number of particle–particle bonds (Moraes et al., 2014c). Soil strength is an important parameter for soil physical quality, directly related to the root growth (Bengough et al., 2011). However, the SPR is affected by many soil and penetrometer parameters (Moraes et al., 2014b), particularly by soil structure, water content, and cohesion (Mulqueen et al., 1977). Soil cohesion is influenced by management systems and is associated with bulk density, water content (Mulqueen et al., 1977), organic carbon (Braida et al., 2007), and soil content of Fe, Si and Al oxides (Silva and Cabeda, 2005). Soil structure affects water availability,

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nutrient uptake, and leaching, thus the soil aggregation can be improved by management practices (Bronick and Lal, 2005). Thus, our study showed that SPR is changed, for the same bulk density and u, by soil tillage as a function of the age-hardening process, revealing increases in soil cohesion due to the strengthening of existing inter-particle bonds as described in short term experiments (Dexter et al., 1988) and now described in a long term experiment encompassing different tillage systems. 4. Conclusions The age-hardening phenomena in a very clayey, Rhodic Eutrudox under different soil management practices can occur as a function of time in no-tillage system (NT11 and NT24), or with increasing time without soil chiselling (MTC1 and MTC3). Under the same soil bulk density and volumetric water content, the soil strength is increased with the time without soil tillage, leading to greater values of soil penetration resistance in no-tillage system. It is necessary to establish distinct critical limits of soil resistance to penetration, depending on soil tillage system and time of no-tillage adoption. Acknowledgments We thank the staff of the Crop and Soil Management Department of Embrapa Soybean, Donizete Aparecido Loni, Mariluci da Silva Pires, Everson Balbino, Eliseu Custodio, João Ribeiro de Macedo, Agostinho Aparecido da Silva, Ildefonso Acosta Carvalho, and Gustavo Garbelini, for their assistance over the experiment and data collection period. We would like to thank the Brazilian Federal Agency for the Support and Evaluation of Graduate Education (CAPES) for the scholarship awarded to the first author. References Abdollahi, L., Hansen, E.M., Rickson, R.J., Munkholm, L.J., 2015. Overall assessment of soil quality on humid sandy loams: effects of location, rotation and tillage. Soil Till. Res. 145, 29–36. _ Błazejczak, D., Horn, R., Pytka, J., 1995. Soil tensile strength as affected by time, water content and bulk density. Int. Agrophys. 9, 179–188. Bengough, A.G., McKenzie, B.M., Hallett, P.D., Valentine, T.A., 2011. Root elongation, water stress, and mechanical impedance: a review of limiting stresses and beneficial root tip traits. J. Exp. Bot. 62, 59–68. Braida, J.A., Reichert, J.M., Reinert, D.J., Soares, J.M.D., 2007. Cohesion and angle of internal friction associated with soil organic carbon and water content in Hapludult. Ci. Rural 37, 1646–1653. Bronick, C.J., Lal, R., 2005. Soil structure and management: a review. Geoderma 124, 3–22. Busscher, W.J., 1990. Adjustment of flat-tipped penetrometer resistance data to a common water content. Trans. ASAE 33, 519–524. Calonego, J.C., Rosolem, C.A., 2010. Soybean root growth and yield in rotation with cover crops under chiselling and no-till. Eur. J. Agron. 33, 242–249. Cates, A.M., Ruark, M.D., Hedtcke, J.L., Posner, J.L., 2016. Long-term tillage, rotation and perennialization effects on particulate and aggregate soil organic matter. Soil Till. Res. 155, 371–380. Conte, O., Levien, R., Debiasi, H., Strümer, S.L.K., Mazurana, M., Müller, J., 2011. Soil disturbance index as an indicator of seed drill efficiency in no-tillage agrosystems. Soil Till. Res. 114, 37–42. De Jong van Lier, Q., Gubiani, P.I., 2015. Beyond the Least Limiting Water Range: rethinking soil physics Research in Brazil. R. B. Ci. Solo 39, 925–939. Debiasi, H., Levien, R., Trein, C.R., Conte, O., Mazurana, M., 2008. Capacidade de suporte e compressibilidade de um Argissolo, influenciadas pelo tráfego e por plantas de cobertura de inverno. R. Bras. Ci. Solo 32, 2629–2637. Devine, S., Markewitz, D., Hendrix, P., Coleman, D., 2014. Soil aggregates and associated organic matter under conventional tillage, no-tillage, and forest succession after three decades. PLoS One 9, e84988. Dexter, A.R., Horn, R., Kemper, W.D., 1988. Two mechanisms for age-hardening of soil. J. Soil Sci. 39, 163–175. Dexter, A.R., 1988. Advances in characterization of soil structure. Soil Till. Res. 11, 199–238. Dexter, A.R., 1990. Changes in the matric potential of soil water with time after disturbance of soil by moulding. Soil Till. Res. 16, 35–50. Dexter, A.R., 1991. Amelioration of soil by natural processes. Soil Till. Res. 20, 87–100.

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