Least limiting water range as influenced by tillage and cover crop

Agricultural Water Management 225 (2019) 105777

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Least limiting water range as influenced by tillage and cover crop a,⁎

b

T

a

Ingrid Nehmi de Oliveira , Zigomar Menezes de Souza , Lenon Henrique Lovera , Camila Viana Vieira Farhatea, Elizeu De Souza Limaa, Diego Alexander Aguilera Estebana, Juliana Aparecida Fracarollic a

University of Campinas (UNICAMP)-School of Agricultural Engineering, Av. Candido Rondon, 500, Campinas, São Paulo, Brazil Department of Water and Soils, University of Campinas (UNICAMP) - School of Agricultural Engineering (FEAGRI), Av. Cândido Rondon, 508, Campinas, SP, 13083875, Brazil c Department of Post-Harvest, University of Campinas (UNICAMP) - School of Agricultural Engineering (FEAGRI), Av. Cândido Rondon, 508, Campinas, SP, 13083-875, Brazil b

A R T I C LE I N FO

A B S T R A C T

Keywords: Diviner 2000 No tillage Soil penetration resistance Conservationist agriculture

Brazil has been experiencing a trend of increased mechanization, although there are no studies addressing the relationship between tillage and cover crops, which affects soil physical attributes, cover crop dry biomass, roots dry biomass, yield, soil water content, and the influence on the least limiting water range (LLWR), defined as the range of volumetric soil water content in which limitations to plant growth occur. This study aimed to i) assess LLWR during two cycles in a sugarcane area using different cover crops and soil tillage systems; ii) correlate the LLWR with different soil physical attributes (soil bulk density, macroporosity and soil penetration resistance); and iii) evaluate the potential use of LLWR as an index of soil and crop quality. The study was conducted under field conditions in a sugarcane culture in the municipality of Ibitinga, São Paulo, Brazil. We used four cover crops (sunn hemp, millet, peanut and sorghum) and three soil tillage systems [no tillage (NT), minimum tillage (MT), and minimum tillage with deep subsoiling (MT/DS)] and compared them with a control treatment [conventional tillage with lack of plant cover (CT)] using an experimental design with split-plot scheme. The soil physical attributes were more affected during the cane cycle by the soil tillages and cover plants. Regarding soil water content, sunn hemp and sorghum obtained the highest soil water content over time with the use of MT/DS, also because the soil bulk density values using sunn hemp and sorghum MT/DS (1.64 and 1.59 kg dm−3, respectively) are 8% lower than the CT for the layer 0.15–0.30 m for the cane plant cycle. In what concerns LLWR, the treatments that maintained their soil water contents within the range for more than 3 months in a row were sunn hemp and millet MT/DS. LLWR was an important indicator, showing that the treatments that obtained LLWR equal to zero, even with high root growth and low penetration resistance, were not enough to express differences in productivity. This proved that the index aggregates all the information and produces satisfying results.

1. Introduction Worldwide, especially in Brazil, there is a gradual elimination of sugarcane fields by slash-and-burn before harvest. This has resulted in an increase of areas with mechanical harvesting, leading to soil compaction (Chagas et al., 2016). Due to this problem, management practices which ensure the production throughout the cultivation cycle are essential for the success of sugarcane production (White and Johnson, 2018). The alternatives found for the sector include more conservationist management systems, which principles involve the reduction of

tillage operations with permanent soil cover using cover crops (Bertioli Júnior et al., 2012), so that soil compaction is reduced throughout the growing cycle. The use of cover crops during sugarcane implantation or renewal has advantages, such as lower loss rate of soil moisture by evaporation (Carvalho et al., 2017); protection of soil against erosion, especially during the rainy season; increased soil organic matter (de Figueiredo et al., 2015); and lower ground temperature (Derpsch et al., 2014), all of these favoring the maintenance of adequate soil water content. Additionally, the tillage system employed also affects soil moisture,

⁎

Corresponding author. E-mail addresses: [email protected] (I.N. de Oliveira), [email protected] (Z.M. de Souza), [email protected] (L.H. Lovera), [email protected] (C.V. Vieira Farhate), elizeu.fl[email protected] (E. De Souza Lima), [email protected] (D.A. Aguilera Esteban), [email protected] (J.A. Fracarolli). https://doi.org/10.1016/j.agwat.2019.105777 Received 31 May 2019; Received in revised form 30 August 2019; Accepted 31 August 2019 Available online 10 September 2019 0378-3774/ © 2019 Elsevier B.V. All rights reserved.

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deep subsoiling at 0.70 m depth (MT/DS). For both MT and MT/DS – subsoiling operations were carried out using five-rod subsoiler, operations shown in Fig. 2. The treatments of soil tillage management were disposed in vertical rows, with three repetitions, established after cover crops in May 2015. Each plot was composed of six rows of sugarcane variety CTC4 with spacing of 1.5 m and 30 m in length, encompassing an area of 300 m2 per plot. For the control, conventional tillage (CT) was used, consisting of soil tillage for sugarcane using two light harrows applied in the plot without cover. The stalks were distributed through John Deere tractors, models 7205 j and a 205 HP, and a DMB PCP 6000 chopped cane planter. Assessments were carried out throughout the plant cycle (2015/ 2016) and measured every three months, as well as at harvesting time. The same procedure was followed during the first ratoon cycle (2016/ 2017). Root dry biomass samples were taken at the time of sugarcane harvest for both cycles, together with undisturbed soil samples, which were collected according to the methodology proposed by Otto et al. (2009). Results are shown in the Table 3 and yield in Table 4.

and conservationist systems are more beneficial to soil water retention than conventional tillage systems, by promoting higher water infiltration in the soil (Almeida et al., 2018). Therefore, the use of these systems in agricultural areas has been increasing, such as no-tillage, were the soil is not tilled, use of cover crops and soil covered maintenance (Panachuki et al., 2010). However, the mechanisms involving the maintenance of soil water contents suitable for systems that use cover crops and conservationist tillage systems are yet to be fully understood, especially in sugarcane areas. Thus, the least limiting water range (LLWR) – defined as the range of water content in the soil at which the limitations to plant growth associated with water potential, aeration, and soil resistance to root penetration are minimal (Tormena et al., 1999) – is an interesting tool for minimizing the compacting effects on soil structure (Gonçalves et al., 2014) induced by different management systems. In addition, the quantification of lower and higher LLWR limits can be useful to assess the periods in which the culture was subject to deficit or excess of water availability, and consequently subjected to stress in terms of available water, aeration, and soil penetration resistance (Blainski et al., 2012). Using this index along with the attributes that directly interfere with the LLWR, such as the soil physical attributes (soil bulk density and macroporosity) and soil water content, in addition to using as reference the values of dry biomass production of the cover crops, root dry biomass, and productivity, allows evaluating whether the LLWR produces effective results. For sugarcane in Brazil, there is no reference of using LLWR as broadly as in this study, comparing it with this many crop attributes, which may bring a new level of responses. Thus, there are no studies evaluating as many cover crops and conservationist soil tillage system in the same research. Hence, our objectives were to i) assess LLWR during two cycles in a sugarcane area using different cover crops and soil tillage systems; ii) correlate the LLWR with different soil physical attributes (soil bulk density, macroporosity, and soil penetration resistance); and iii) evaluate the potential use of LLWR as an index of soil and crop quality.

2.2. Soil water content Measures of soil water content started from the sugarcane planting (May 2016), always close to the fifteenth day of the each month – all the readings taken on the same day for a better standardization of results. The same procedure was followed for the second year of cultivation in the first ratoon cycle (May 2016 – May 2017). Soil water contents were measured by the method of Frequency Domain Reflectrometry (FDR) using the equipment Diviner 2000 (Sentek, 2001), which consists of a display with keyboard data collector (datalogger) coupled to a probe by cable. Monitoring the soil water content was held up to one meter deep, with ranges subdivided at every 0.10 m according to equipment configuration and shown in Fig. 3. Reading frequency in Diviner 2000 is stored by the datalogger at a fixed time (1 s), resulting in readings around 120,000 MHz (water-Fw) and 160,000 MHz (air-Fa) depending on the soil moisture content. The equipment was standardized before the measurements were taken, due to differences between probes; the values of Fa and Fw were 175.69 MHz and 123.33 MHz, respectively. The procedure was performed to record readings inside a PVC pipe exposed separately to air and water. The output of the data provided by the datalogger is called relative frequency (FR), defined by Eq. (1):

2. Material and methods 2.1. Experimental area The study was conducted under field conditions at Santa Fé mill, in an experimental area in the municipality of Ibitinga-SP (21°45′ S and 48°49′ W) at an altitude of 455 m above sea level. The region´s climate is classified as tropical with dry season (Aw) according to the Köppen climate classification (Alvares et al., 2013), with a cold and dry winter, as well as a hot and rainy summer. Mean annual temperature of 23 °C and annual precipitation of 1260 mm (CEPAGRI, 2015). Temperature and precipitation were measured using an Anova technology weather station configured to take measurements every five minutes which provided data for monitoring these attributes in the experimental area – shown in Fig. 1. Soil was classified as Ultisol Udult, according to the Soil Taxonomy System (Soil Survey Staff, 1998). Particle size distribution of the soil was obtained by pipette method, according to Teixeira et al. (2017), and values are shown in Table 1. The experiment occurred on rows, with cover crops planted in horizontal rows in February 2015: sunn hemp (Crotalaria juncea), millet (Pennisetum glaucum L), peanut (Arachis hypogaea L.), and sorghum (Sorghum bicolor L.). For biomass production of cover plants (Table 2), upon reaching the maximum flowering point, the cover plants were sampled for dry mass production (DM) analysis, in an area of two square meters per plot, in which the plants were cut close to the ground. Subsequently, the samples were dried at 65 °C for 72 h, weighed and the results expressed as Mg ha−1 and then the carbon and nitrogen contents (Table 2) determined by dry combustion in an elemental analyzer (Truspec model). The soil tillage managements were: no-tillage (NT), minimum tillage with subsoiling at 0.40 m depth (MT), and minimum tillage with

FR =

Fa − Fs Fa − Fw

(1)

where, Fa = frequency reading in the PVC pipe totally suspended in the air; Fs = frequency reading in the PVC pipe in the soil; Fw = frequency reading in the PVC pipe immersed in water. Even though the manufacturer supplies a global calibration curve, the manufacturer recommends an in situ equipment calibration for comparison of the obtained relative frequency with volumetric soil moisture. Therefore, equipment calibration was carried out in the experimental area using a single cover plant, sunn hemp, under different soil tillage systems. This allowed us to isolate the cover crop effect on soil structure, since the cover crops affect soil structure only at shallow depth. Eq. (2) was used to transform the relative frequency into volumetric moisture.

θ=

b

FR a

(2)

where, θw = soil volumetric moisture; a, b = constants of the soil calibration equation. Table 5 presents the constants associated with the regression curve resulting from calibrating the Diviner 2000 equipment for each tillage system. 2

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Fig. 1. Precipitation and temperature data from April 2015 to May 2017 for the plant cycle and first ratoon cycle in Ibitinga-SP, Brazil. Table 1 Soil particle size distribution for an Ultisol Udult in an experimental area in the municipality of Ibitinga, São Paulo, Brazil. Soil layer (m)

Sand %

0.00–0.05 0.05–0.10 0.10–0.20 0.20–0.30 0.30–0.70

73.9 73.0 69.4 63.1 57.1

± ± ± ± ±

1.6 2.0 2.8 2.8 5.9

Silt

Clay

9.7 ± 2.0 9.7 ± 3.0 11.1 ± 2.0 10.2 ± 6.0 10.7 ± 9.0

16.4 17.4 19.5 26.7 32.2

Textural class

± ± ± ± ±

1.5 1.5 1.8 1.7 5.8

Sandy Sandy Sandy Sandy Sandy

loam loam loam clay loam clay loam

Table 2 Production of dry biomass and C/N ratio in the cultural residues of cover crops, used at the time of expansion of the sugarcane field in an experimental area, in the municipality of Ibitinga, São Paulo, Brazil. Cover crop

Dry biomass Mg ha−1

C/N

Sunn hemp Peanut Sorghum Millet

11 b 5c 21 a 10 b

13 15 28 48

c bc b a

Values followed by the same letter do not differ between themselves by Tukey’s test (p < 0.05).

Fig. 2. Time lapse of the soil tillages that occurred in the area. CT = conventional tillage, MT/SP = minimum tillage with deep subsoiling, MT = minimum tillage, NT = no tillage.

2.3. Soil physical attributes Unaltered soil samples were collected to characterize the area before the implementation of the experiment, before the first harvest (cane plant) and after the second harvest of sugarcane (first ratoon cycle), aiming to evaluate the effect of cover crops and soil tillage systems on the soil physical attributes. Soil samples were collected in the layers at 0.00–0.15 and 0.15–0.30 m of depth, with three replicates using metallic cylinders of 0.05 × 0.05 m. Soil bulk density (BD) was obtained by the ratio between dry soil mass and soil sample volume. Macroporosity (MaP) was obtained by the difference between water content of the saturated sample (total porosity) and water content after submitted to tension table at 6.0 kPa (Teixeira et al., 2017). Soil penetration resistance (PR) was estimated in laboratory using undisturbed samples (Guedes Filho et al., 2014), using an MA 933 benchmark electronic penetrometer (MARCONI), with a 4.0 mm tip and constant penetration speed of 10 mms−1. To eliminate the effect of the variation of soil water content, the samples were saturated by capillarity and submitted to a matric potential of −6.0 kPa.

2.4. Least limiting water range (LLWR) LLWR (m3 m−3) was determined using the methodology described by Silva et al. (1994). Soil samples were collected in metallic cylinders of 0.05 × 0.05 m with three replicates, which were submitted to a matric potential of −2, −4, −6 kPa on a tension table, according to Teixeira et al. (2017); and −8, −10, −33, −100, −500, and −1500 kPa, using Richard’s extractor (Richards and Fireman, 1943). After each sample reached the equilibrium, they were weighed and then soil penetration resistance (PR) was determined, using the procedures described in Tormena et al. (1998). After PR assessments, the samples were kiln dried at 105 °C for 24 h, for determination of water content (θ) and soil bulk density (BD). To determine the LLWR, the soil penetration resistance curve was adjusted based on the model proposed by Busscher (1990), described by Eq. (3): PR = a*θb*BDc

(3)

where, PR = soil penetration resistance (MPa); θ = volumetric water 3

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Table 3 Dry root biomass (kg ha−1) of sugarcane in an Ultisol Udult with different cover crops and soil tillage for the depth.0.00–1.00 m. Treatment/Cover crop

Peanut Sunn hemp Millet Sorghum Mean CT

Plant cycle (2015/2016)

First ratoon cycle (2016/2017)

NT

MT

MT/DS

Mean

NT

MT

MT/DS

Mean

1042 Aa 914 Aa 1278 Aa 1349 Aa 1145 a

537 Aa 1273 Aa 1320 Aa 1351 Aa 1120 a

942 Aa 1219 Aa 973 Aa 1350 Aa 1112 a

840 B 1135 AB 1178 AB 1350 A

2508 2487 2163 2341 2375 a

2173 1976 2770 2413 2333 a

2401 2308 2824 2602 2534 a

2361 2257 2586 2452

822

A A A A

2186

NT = No-tillage; MT = minimum tillage; MT/DS = minimum tillage with deep subsoiling. Means followed by the same letter, uppercase in the column and lowercase in the row, do not differ between themselves by Tukey’s test (p < 0.05). *Significant by Dunnett’s test at 5% significance level. Table 4 Sugarcane yield (Mg ha−1) for an Ultisol Udult in expansion of sugarcane culture with different cover crops and soil tillage. Sugarcane yield (Mg ha−1) Plant cycle (2015/2016)

First ratoon cane (2016/2017)

Treatment/Cover Crop

NT

MT

MT/DS

Mean

NT

MT

MT/DS

Mean

Peanut Sunn hemp Millet Sorghum Mean CT

106 117 113 101 109 a

110 102 100 109 105 a

104 116 110 119 112 a

106 111 107 109

102 104 106 97 101 a

99 99 99 108 101 a

102 114 103 108 106 a

100 105 102 104

A A A A

105

A A A A

104

NT = No-tillage; MT = minimum tillage; MT/DS = minimum tillage with deep subsoiling. Means followed by the same letter, uppercase in the column and lowercase in the row, do not differ between themselves by Tukey’s test (p < 0.05). *Significant by Dunnett’s test at 5% significance level. Table 5 Constants of the regression curve obtained in the calibration of Diviner 2000 for different soil tillage systems in an Ultisol Udult. Treatment

a

b

R2

No-tillage (NT) Minimum tillage (MT) Minimum tillage with deep subsoiling (MT/DS) Conventional tillage (CT)

0.458 0.372 0.447 0.166

1.666 1.644 2.138 1.457

0.89 0.96 0.91 0.88

a, b = constants of the soil calibration equation.

The curve used to calculate the LLWR is expressed by the relation between θ and BD, being described mathematically by a nonlinear function, according to Eq. (5): θ= d* Ψe

(5) −3

where, θ = volumetric water content (m m ); Ψ = matric potential (kPa); d, e = equation adjustment coefficients. With a logarithmic transformation, we obtained Eq. (6). 3

In(θ) = In(d) +e * In(ψ)

(6)

The values of θ associated with Ψ, PR, and aeration porosity (AP) limiting to plant growth were: field capacity (θFC) or water content at Ψ −10 kPa (Reichardt, 1990); permanent wilting point (θPWP) or water content at Ψ −1,500 kPa (Savage et al., 1996), and soil water content in which the aeration porosity (θAP) is 0.10 m3 m−3 (Grable and Siemer, 1968). Fig. 3. FDR Diviner 2000 installation scheme and layers of measurement for the plant cycle and first ratoon cycle in Ibitinga-SP, Brazil.

2.5. Statistical analysis

content (m3 m−3); BD = soil bulk density (Mg m−3); and a, b, c = model adjustment parameters. Through a logarithmic transformation, we obtained Eq. (4): ln (PR)= ln(a)+ b*ln (θ) + c* ln (BD)

Significant differences between the treatments were assessed by analysis of variance (ANOVA) using t-test on subdivided plots with significance assessed through a Tukey’s test at 5% significance level. A block test was subsequently performed using Dunnett’s test (5% significance level) including the control treatment (CT). For the statistical

(4) 4

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Table 6 Soil bulk density for an Ultisol Udult in expansion of sugarcane culture with different cover crops and soil tillage. Bulk density (kg dm−3) Plant cycle (2015/2016)

First ratoon cycle (2016/2017)

Treatment/Cover crop

NT

MT

MT/DS

Mean

Peanut Sunn hemp Millet Sorghum Mean CT

1.72 1.67 1.67 1.70 1.69 a

1.69 1.61 1.69 1.73 1.69 a 1.66

1.65 1.54 1.67 1.55 1.61 b

Soil layer 0.00-0.15 m 1.69 A 1.78 1.62 A 1.67 1.67 A 1.71 1.67 A 1.76 1.74 a

1.56 1.64 1.70 1.59 1.62

Soil layer 0.15-0.30 m 1.60 B 1.72 1.67 A 1.69 1.66 AB 1.68 1.68 A 1.73 1.70 a

Peanut Sunn hemp Millet Sorghum Mean CT

1.65 1.66 1.70 1.72 1.68

aA aA aA aA a

1.61 1.72 1.61 1.72 1.66

NT

MT

MT/DS

Mean

1.82* 1.73 1.73 1.78 1.77 a

1.75 1.67 1.73 1.76 1.72 a

1.78 1.68 1.72 1.77

A B AB A

1.71 1.66 1.65 1.70 1.68 a

1.73 1.68 1.64 1.72

A A A A

1.71

aB * aA aB * aA ab

aB * aAB aA bB* b

1.72

1.79* 1.70 1.60 1.73 1.70 a 1.68

NT = No-tillage, MT = minimum tillage; MT/DS = minimum tillage with deep subsoiling. Means followed by the same letter, uppercase in the column and lowercase in the row, do not differ between themselves by Tukey’s test (p < 0.05). *Significant by Dunnett’s test at 5% significance level.

analysis of LLWR, we used Excel´s SOLVER for fitting linear models. Analysis of variance was performed by f-test for the regression, and the coefficients were analyzed using Student’s-t test, both at 5% significance level.

double interaction between soil tillage and cover crops was observed, as was the case for the superficial layer. The highest macroporosity value in the sugarcane plant was for millet (0.15 m3 m−3), followed by sunn hemp MT (0.14 m3 m−3), which were the only treatments that obtained higher values than CT (0.08 m3 m−3) by Dunnett’s test at 5% significance level (Table 7). In the first ratoon cycle, there were differences between soil tillage system for the subsurface and superficial layers (Table 7). MT had a higher MaP in the subsurface layer (0.07 m3 m−3) than in the surface layer (0.05 m3 m−3); NT was equal in both (0.06 m3 m−3); and MT/DS presented a lower value in the subsurface layer (0.05 m3 m−3) compared to the superficial (0.06 m3 m−3). All the values found are below the value considered as limiting for an adequate aeration of the soil (0.10 m3 m−3) and did not differ from CT. It can be observed that the treatment with MT which used sunn hemp and millet as cover crops differed from CT both in the superficial layer and in the 0.15-0.30 m layer during the sugarcane plant cycle. Results regarding cover crops were opposite to those for BD, but proved that MT was better for both physical attributes. Soil penetration resistance (PR) presented a similar behavior to MaP, with double interactions for both depths during the sugarcane plant culture, but not for ratoon sugarcane (Table 8). The superficial layer of the sugarcane cycle showed no difference between cover crops, but differences were observed between soil tillage systems, with the highest PR obtained by NT (1.54 MPa), followed by MT (1.07 MPa) and MT/DS (0.97 MPa). The highest PR value between the treatments was 1.88 MPa for peanut NT, whereas the lowest was 0.57 MPa for peanut MT/DS, and values lower than 0.97 MPa differed statistically from CT (1.56 MPa) by Dunnett’s test (p < 0.05) (Table 8). The first ratoon cycle presented difference between the soil tillage systems, with the highest PR obtained by MT (1.60 MPa), followed by MT/DS (1.31 MPa) and NT (1.22 MPa). For the 0.15-0.30 m layer, the PR had a double interaction between tillage and cover crop, with the highest value of 2.14 MPa for sunn hemp MT and the lowest of 0.69 MPa for sorghum MT/DS (Table 8). Differences in soil tillage were found, with MT and NT (1.49 MPa) higher than MT/DS (1.08 MPa), while for cover crops the highest PR occurred for sunn hemp (1.64 MPa), followed by millet (1.30 MPa), peanut (1.24 MPa), and sorghum (1.23 MPa). For the first ratoon cycle in the subsurface layer, only differences in soil tillage were found for PR, with MT (1.57 MPa) higher than NT (1.25 MPa) and MT/DS (1.18 MPa), following the same classification as

3. Results 3.1. Soil physical attributes In the cane plant cycle alone (2015/2016) there was a significant effect of soil tillage systems on BD for the two soil layers (Table 6), and, in the 0.00–0.15 m layer, the MT/DS treatment presented the lowest mean value (1.61 kg dm−3) when compared to MT and NT (1.69 kg dm−3). For the 0.15–0.30 m layer, a double interaction between the cover crops and soil tillage systems was observed, and the MT/DS treatment showed the lowest BD mean value (1.62 kg dm−3), while NT obtained the highest value (1.68 kg dm−3). Regarding cover crops, peanut presented the lowest BD (1.60 kg dm−3), whereas sorghum and sunn hemp presented the highest BD values (1.68 kg dm−3 and 1.67 kg dm−3, respectively). When compared to CT (1.72 kg dm−3), the treatments peanut MT (1.61 kg dm−3) and MT/DS (1.56 kg dm−3); millet MT (1.61 kg dm−3); and sorghum MT/DS (1.59 kg dm−3) obtained lower values of BD. During the production cycle of ratoon sugarcane (2016/2017), there was no double interaction between cover crops and soil tillage systems in any of the soil layers evaluated. However, in the 0.00–0.15 m layer, the mean BD values for sunn hemp (1.68 kg dm−3), sorghum (1.77 kg dm−3), and peanut (1.78 kg dm−3) were higher. We highlight that the BD for the treatment peanut MT (1.82 kg dm−3) differed from the CT (1.71 kg dm−3). Macroporosity (MaP) showed double interaction between treatments and cover crops in the sugarcane plant for the two soil layers (Table 7). For the 0.00–0.15 m layer, the combination of peanut-NT presented higher MaP (0.16 m3 m−3), and peanut-MT, millet-MT/DS, and CT showed MaP values lower than 0.10 m3 m−3. In the same layer, values higher than 0.12 m3 m−3 were considered significantly different from CT by Dunnett’s test (p < 0.05). Contrary to the behavior found in BD, millet (0.15 m3 m−3) and sunn hemp (0.12 m3 m−3) obtained the highest values of MaP, while, also unlike BD, NT (0.13 m3 m−3) obtained a higher MaP, followed by MT (0.11 m3 m −3), and finally MT/ DS (0.10 m3 m−3) obtained the lowest mean. For the 0.15-0.30 m layer, there was a reduction in MaP compared to the superficial layer (Table 7). For the sugarcane plant cycle, a 5

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Table 7 Macroporosity for an Ultisol Udult in expansion of sugarcane culture with different cover crops and soil tillage. Macroporosity (m−3 m−3) Plant cycle (2015/2016) Treatment/Cover crop

NT

Peanut Sunn hemp Millet Sorghum Mean CT

0.16 0.08 0.12 0.15 0.13

Peanut Sunn hemp Millet Sorghum Mean CT

MT

aA * aC bBC* aAB* a

First ratoon cycle (2016/2017)

MT/DS

0.08 0.12 0.15 0.09 0.11

bB aAB* aA* bB ab

0.10 0.12 0.05 0.13 0.10

Mean

NT

bA aA * cB abA* b

Soil layer 0.00-0.15 m 0.08 B 0.05 0.12 A 0.05 0.15 A 0.08 0.09 B 0.06 0.06 a

abA bA cA bA b

Soil layer 0.15-0.30 m 0.09 B 0.06 0.10 B 0.05 0.12 A 0.06 0.08 B 0.07 0.06 ab

MT

MT/DS

Mean

0.06 0.04 0.06 0.07 0.05 a

0.06 0.05 0.06 0.06 0.06 a

0.06 0.05 0.07 0.06

A A A A

0.05 0.06 0.05 0.04 0.05 b

0.07 0.05 0.07 0.06

A A A A

0.06 0.11 0.08 0.12 0.12 0.10

aAB bB bA aA ab

0.07 0.14 0.19 0.06 0.11

bC aB* aA* bC a

0.07 0.09 0.06 0.07 0.07 0.08

0.08 0.04 0.08 0.07 0.07 a

0.08

0.06

NT = No-tillage, MT = minimum tillage; MT/DS = minimum tillage with deep subsoiling. Means followed by the same letter, uppercase in the column and lowercase in the row, do not differ between themselves by Tukey’s test (p < 0.05). *Significant by Dunnett’s test at 5% significance level.

the sugarcane cycle and for the superficial layer of the first ratoon cycle (Table 8). During the sugarcane ratoon cycle, the PR values for the treatments were not statistically different from CT in any layer. The PR values were the only ones to show statistical differences between soil tillage systems, in all the cycles evaluated.

was a less pronounced decrease during the FC performance as higher limit, and a more pronounced decrease when aeration porosity became the higher limit. For the 0.15–0.30 m layer, the LLWR presented a continuous decrease because PR was the lower limiting factor for almost all the density values evaluated, and 50% were outside the range limit (LLWR = 0). For the ratoon cycle, peanut MT obtained a LLWR equal to zero in the superficial depth. For the 0.15-0.30 m depth, the highest LLWR values were found for millet MT (0.04 m3 m−3), followed by millet MT/ DS (0.02 m3 m−3) and CT (0.01 m3 m−3), with the other treatments presenting LLWR equal to zero (Fig. 5B). CT presented a LLWR equal to zero in the cane plant cycle, and 0.10 m3 m−3 in the ratoon cycle (Fig. 5A).

3.2. Least limiting water range (LLWR) For the 0.00-0.15 m depth, the LLWR did not reach a critical bulk density value (density in which it becomes null) (Fig. 4A). However, the bulk density would be close to 1.80 Mg m−3, while for the 0.15–0.30 m depth (B), the critical density would be 1.70 Mg m−3. For the superficial layer, the lower limit was soil penetration resistance (PR) and the higher limit was field capacity (FC) up to the density 1.57 Mg m−3 (Fig. 4); over this value, the higher limit became aeration porosity (AP). For the 0.15-0.30 m layer, the higher limit throughout the LLWR was field capacity (FC), but the lower limit was the permanent wilting point (PWP) up to the density 1.40 Mg m−3, being replaced by PR after this value. For the plant cycle, the LLWR was higher for the first layer, when compared to the second (Fig. 5A and B, respectively), in which there

3.3. Distribution of soil water content at critical LLWR limits Fig. 6 shows the soil water content obtained during the two sugarcane production cycles (plant and first ratoon sugarcane), with their higher and lower limits established according to the intervals obtained in Fig. 4. Fig. 6 also presents the phenological phases, namely: Phase I sprouting and establishment (0–30 days after planting (DAP)); Phase II -

Table 8 Soil penetration resistance for an Ultisol Udult in expansion of sugarcane culture with different cover crops and soil tillage. Soil penetration resistance (MPa) Plant cycle (2015/2016) Treatment/Cover crop

NT

Peanut Sunn hemp Millet Sorghum Mean CT

1.88 1.09 1.74 1.47 1.54

Peanut Sunn hemp Millet Sorghum Mean CT

MT

aA aB* aA abA a

0.97 1.07 0.79 1.43 1.07

bB* aAB* cB* aA b

First ratoon cycle (2016/2017)

MT/DS

Mean

NT

0.57 bB* 1.4 aA 1.25 aB 0.65 bB* 0.97 b

Soil layer 0.00-0.15 m 1.14 A 1.03 1.18 A 1.06 1.26 A 1.22 1.18 A 1.58 1.22 b

1.09 1.39 1.16 0.69 1.08

Soil layer 0.15-0.30 m 1.24 B 1.25 1.64 A 1.02 1.30 AB 1.22 1.23 B 1.5 1.25 b

MT

MT/DS

Mean

1.30 1.78 1.64 1.69 1.60 a

0.95 1.68 0.83 1.78 1.31 ab

1.09 1.51 1.23 1.68

A A A A

0.84 1.06 1.28 1.54 1.18 b

1.15 1.19 1.39 1.60

A A A A

1.56 1.45 1.40 1.52 1.58 1.49

aA bA aA aA a

1.17 2.14 1.23 1.42 1.49

aB aA* aB aB a

1.54 aAB bA aAB bB* b

1.57

1.36 1.48 1.66 1.76 1.57 a 1.85

NT = No-tillage, MT = minimum tillage; MT/DS = minimum tillage with deep subsoiling. Means followed by the same letter, uppercase in the column and lowercase in the row, do not differ between themselves by Tukey’s test (p < 0.05). *Significant by Dunnett’s test at 5% significance level. 6

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Fig. 4. Soil water content in relation to the soil bulk density in an Ultisol Udult under sugarcane at critical levels of field capacity (FC = −10 kPa), permanent wilting point (PWP = −1500 kPa), aeration porosity (AP = 0.10 m3 m−3), and soil penetration resistance (PR = −2000 kPa); the shaded area represents the least limiting water range. (A) Depth of 0.00–0.15 m and (B) Depth of 0.15–0.30 m.

peanut and sunn hemp with the same tillage. Regarding ratoon cane, between August 2016 and February 2017, all treatments were below the lower limits, and NT remained within the LLWR between May and August. For sorghum, during the plant cycle, the higher limit was reached with MT/DS and MT during January, when the soil water content for these two treatments was 70% higher (0.25 m3 m−3) than the maximum values found for peanut (0.15 m3 m−3), sunn hemp (0.17 m3 m−3), and millet (0.17 m3 m−3) (Fig. 6). The lower limits were reached for the most part of the evaluation period, especially for NT, which presented a low amplitude of LLWR due to of its high BD. However, even for the other treatments, such as MT/DS, which obtained the highest amplitude, the limits were achieved in some periods, such as from March to May 2016. For the 0.15–0.30 m layer, where the LLWR amplitude was lower, certain treatments, in addition to CT, did not present LLWR, and were within the critical limits of LLWR (Fig. 7). The soil tillage system that remained for a longer period of time within the critical limits of LLWR was MT/DS (Fig. 7), because it was the only treatment to obtain soil water content for peanut, sunn hemp, and sorghum during the plant cycle, while for millet it was MT. In the deeper layer, the restrictive conditions were more pronounced, since the soil water content was outside the critical limits of LLWR for a longer period, mainly due to the higher BD that caused shorter limits, making it more difficult for moisture to conform. Also, despite the higher BD and lower MaP, higher soil water contents were found, reaching 0.30 m3 m−3 for millet MT in November. It is notable that the treatments of millet NT and MT/DS, as well as sorghum MT and NT, obtained a LLWR of zero, therefore it is not possible to relate moisture to LLWR. For the 0.15–0.30 m layer in the ratoon cycle, the soil water content for all the cover crops were below the higher limit for all the treatments (Fig. 7), except for millet MT, which obtained water content in the intervals July-August 2016 and December 2016-March 2017. This treatment also obtained higher water content throughout the cycle, as in December, in which it obtained water content of 0.20 m3 m−3, while peanut, sunn hemp, and sorghum obtained 0.10, 0.15, and

establishment and tillering (31–109 DAP), Phase III - maximum growth (109–347 DAP) and Phase IV - maturation (348–389 DAP). Phases II and III need more water compared to the others, and for both cycles the phases obtained the highest soil water contents at both depths (Figs. 4 and 5). Phase I is the one that requires the least amount of water, although achieved high levels of soil water content in all treatments. In Phase IV the lowest water contents occured at both depths, but it is a phase which requires low water contents for the plants, because it is already established. For peanut in the ratoon cycle, the higher limit was not reached at any time during the evaluation period (Fig. 6). However, the lower limit was reached by all treatments. When compared to the same period in other treatments, MT/DS had a longer period within the LLWR and higher water content, reaching soil water content of 0.25 m3 m−3 in February 2016. NT obtained soil water content within the LLWR for the same period as MT/DS, but with lower soil water content. For the first ratoon cycle, only MT/DS obtained LLWR above zero, but at no time did it exibit soil water content within the intervals. Sunn hemp presented the highest values for soil water content compared to the other cover crops assessed (Fig. 6). For the sugarcane plant, the higher limit was not reached and the soil water content remained above the lower limit for a longer period of time during the evaluated crop cycles; for MT/DS and NT, as occurred with peanut, it reached 0.27 m3 m−3 in February 2016, which is higher than the maximum value found for peanut (0.25 m3 m−3) also for MT/DS. During the first ratoon cycle, MT/DS was also moister compared to the other treatments, remaining within the LLWR for over half of the cycle, a period longer than that of the plant cycle for the same treatment, but acquiring lower water content, with a maximum of 0.15 m3 m−3. For millet’s plant cycle, the higher limit was not reached by any treatment. The lower limit was reached in August 2015 and April 2016 by MT/DS and MT and in May 2016 by all treatments (Fig. 6), as was observed with sunn hemp and peanut for MT/DS. In this case, opposed to the other cover crops, MT completed a longer period within the LLWR, when compared to MT/DS, although the latter reached higher values in February (0.25 m3 m−3). This result was found for both

Fig. 5. Least limiting water range (LLWR) for an Ultisol Udult in area with expansion of sugarcane to the depths of 0.00–0.15 m (●) and 0.15–0.30 m (◼). (A) Plant cycle (2015/2016), (B) First ratoon cycle (2016/2017). 7

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Fig. 6. Distribution of soil water content on critical LLWR (horizontal lines) limits over time at the depth of 0.00–0.15 m obtained by different cover crops (peanut, sunn hemp, sorghum, and millet) and tillage systems [minimum tillage (MT), no-tillage (NT), minimum tillage with deep subsoiling (MT/DS), and conventional tillage (CT)]. The shaded area represents the times when moisture remained within LLWR. Sugarcane phenological phases: Phase I - sprouting and establishment (a); Phase II - establishment and tillering (b), Phase III - maximum growth (c) and Phase IV - maturation (d).

0.12 m3 m−3, respectively. During the ratoon cycle, peanut and sorghum were not within the limits of LLWR in any soil tillage system (Fig. 7). In turn, sunn hemp obtained soil water content only for MT, and millet for all the treatments. The highest amplitude of LLWR was observed for MT. This resulted in the water content being within the adequate moisture range for the full development of the plants during a longer evaluation period. Despite the low amplitude of LLWR presented in the 0.15-0.30 m layer, the soil water content was higher than that of the superficial layer, highlighting the treatment with MT/DS during the ratoon cycle (Fig. 7), both with peanuts, sunn hemp and millet, during which water content was above the remaining treatments.

tillage for the passage of the sugarcane harvester due to its high weight (Vischi Filho et al., 2015a), while cover crops have their effect maximized in the first ratoon cycle because of the decomposition of the root system. One of the hypothesis of this study was that sorghum would obtain lower BD values due to its high dry biomass production (Table 2), as well as higher root system biomass (Table 3), but this result was not obtained because the fibrous roots could not decompact the soil, leading to a more superficial root development, not reaching higher depths for nutrients. The lowest BD values (Table 6) were observed for millet and sunn hemp, in the latter due to the fact that it is a legume with higher nitrogen fixation, high C/N ratio (Table 2), and consequent delay in decomposition, protecting the soil for a longer time (Giacomini et al., 2003). The use of MT/DS provided lower BD values during the plant cycle in both depths. Certainly, the disruption of restrictive layers by the subsoiler in deeper layers (0.70 m) contributed to this result, since the use of deep subsoilers affects soil respiration, which, added to the break of the compacted layer, increases porosity and reduces BD (Shukla et al., 2017). In spite of this, Torres et al. (2008) when studying several cover crops, also found that sunn hemp and millet obtained lower BD and higher MaP compared to sorghum. In what concerns the soil tillage system, the high degree of soil disturbance caused by CT tends to generate lower density values in the

4. Discussion 4.1. Physical attributes of the soil and its effect on LLWR Soil bulk density (BD) is important for the least limiting water range (LLWR), because the intervals were evaluated from BD values, and it was observed that BD reduces the LLWR (Fig. 5). During the plant cycle BD was affected by soil tillage in both layers and by cover crops only in the subsurface layer (0.15–0.30 m), while in the first ratoon cycle there was only influence of cover crops on the superficial layer (0.00–0.15 m). This is a consequence of a reduction in the effect of soil 8

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Fig. 7. Distribution of soil water content on critical limits of LLWR (horizontal lines) over time, in the depth of 0.15–0.30 m, obtained by different cover crops (peanut, sunn hemp, sorghum, and millet) and tillage systems [minimum tillage (MT), no-tillage (NT), minimum tillage with deep subsoiling (MT/DS), and conventional tillage (CT)]. The shaded area represents the times when moisture remained within LLWR. Sugarcane phenological phases: Phase I - sprouting and establishment (a); Phase II - establishment and tillering (b), Phase III - maximum growth (c) and Phase IV - maturation (d).

recommended limiting value, which may cause restrictions on root development (Xu et al., 1992) and aerobic respiration. Higher MaP values were observed in the treatments with MT and NT by Everton et al. (2015), who justified the results by the greater contribution of organic matter that these systems offer to the soil, despite the short period of time. There is also greater accumulation of roots in the superficial layer, mainly up to 0.20 m (Blackburn, 1984), and of roots from the previous cycle, plant sugarcane (Faroni and Trivelin, 2006), favoring the formation of macropores from the decomposition of these roots by soil microorganisms. As occurred for BD, the root system of sorghum did not affect MaP in depth, but only in the superficial layer (Table 7), where it concentrates due to compaction. The reduction of macroporosity is a reflex of soil compaction, resulting from the increase in BD, which consequently impairs the LLWR because of the decrease in aeration (Chen et al., 2014). A reduced macroporosity leads to an increased microporosity, which in turn tends

superficial layer. However, in the 0.15–0.30 m layer, BD increases because the tillage is not effective in these depths (Cherubin et al., 2016). The increase in BD caused negative effects on LLWR, because of its influence on both the higher and lower limits, promoting lower water storage. Higher BD values may promote restrictive conditions to plant growth, mainly due to the proximity of the high values with critical density values, considered 1.60 Mg dm−3 by Silva and Rosolem (2001). In this study, most treatments presented BD values above this value, for both cycles, indicating soil compaction. Macroporosity (MaP) for the plant cycle, at both depths, was affected by the different tillage systems evaluated, but only the 0.15–0.30 m layer was affected by the cover crops. This effect was also found by Cherubin et al. (2016), who evaluated areas cultivated with sugarcane in several latitudes in Brazil and found values below the one considered restrictive for plant respiration of 0.10 m3 m−3. For the ratoon cycle, the MaP values at both depths were below the 9

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the superficial layer in the study by Ferreira et al. (2017), who analyzed potato cultures with different irrigation regimes. Throughout the sugarcane cycles, there is an increase in compaction caused by machines (Vischi Filho et al., 2015b). In our experiment, this hypothesis can be proved by the increased density and consequent reduction of LLWR, causing development problems for the plant, which develop in the root system (Table 3), for the ratoon cycle, root production became homogeneous in all the treatments. Gonçalves et al. (2014), evaluating the effects of traffic on LLWR, found that after 20 machines crossings, without differentiating them, in an Oxisol with sugarcane, its interval reaches zero. We highlight the performance of the treatments sunn hemp and sorghum MT/DS for the superficial layer in the plant cycle, which were the only ones to remain in the region where field capacity is the limiting feature and LLWR is higher. This fact translates into greater advantages compared to the other treatments that have AP as restriction, resulting in higher root growth (Table 3) in these treatments.

to affect penetration resistance (PR). The results of BD and MaP relate to those obtained by PR (Table 8). In this case, the most influential factor were soil tillage systems, because they are directly related to soil compaction and decompaction, due to the different levels of tillage that occurred in the experiment. Thus, the higher PR values for the 0.15–0.30 m layer also occur because of the presence of cultural residues on the soil surface generated by the cover crops. During the plant cycle, there is a consequent increase in organic matter content and stability of aggregates, reducing PR (Prado et al., 2014). MT/DS resulted in lower PRs, even in the first ratoon cycle, in which there was no influence of cover crops on any of the depths. This is an effect of a moderate tillage applied in the already compacted area for decompaction, producing positive effects. Excessive tillage, as in CT, cause soil disruption and reduce fauna and flora (Carvalho et al., 2017), leaving the soil more susceptible to compaction (Table 8). Despite this, it is expected that, with several cycles of sugarcane, NT will become more viable, because it reinforces long-term positive effects. Constant tillage, even if in a localized way as in MT/DS, disrupts the soil to the point of degradation (Silva et al., 2012a,b), mainly due to the fact that, during the transformation of the pasture into sugarcane, several soil disruptions take place, causing a long-term negative effect and affecting the tillage systems used in sugarcane (Oliveira et al., 2019). Cherubin et al. (2016) found that soil tillage brings benefits in the short term, but in the long run reduces the soil capacity to resist degradation due to erosion. Sugarcane is one of the main Brazilian crops, and the state of São Paulo, where the experiment was carried out, is its main national producer. The fact that the soils are already compacted raises concern regarding the ways in which the soil is used in the long term, and it is important to adopt more conservationist managements to maintain the productivity of São Paulo soils in the long run. The solutions include, along with the use of cover crops and soil tillage, field traffic control (Aguilera Esteban et al., 2019; Souza et al., 2015) and straw retention in the soil (Satiro et al., 2017), leading to a more sustainable culture in these regions.

4.3. Soil water content The soil water contents were strongly influenced by the precipitations occurring in the area (Fig. 1), taking into consideration that sugarcane in Brazil is planted from March to May, that is, the beginning of the dry season. Thus, during the development of the cane plant, the soil must remain moist for the longest possible period in case of water scarcity, especially in areas without irrigation, as is the reality of most sugarcane cultures in the state of São Paulo. For all soil tillage systems, there was a peak during rainy periods in areas using MT/DS due to a higher MaP (Table 7), which has a significant contribution to the total porosity; however, in times of drought, it presented lower soil water content, and MT began to stand out. Due to subsoiling at 0, 70 m, there is a higher formation of initial macropores caused by the physical deformation of the soil, leading to an accumulation of more water in times of saturation, affecting both layers evaluated in this study. After the machine traffic, this effect was maintained in the plant cycle, showing that, although the physical attributes are homogenized (Tables 6 and 7), caused by the lower PR (Table 8). This demonstrates that the soil is less compacted, with water accumulation over time due to the conversion of macropores into micropores. Silva et al. (2012), using the PENTA implement, which performs deep subsoiling in areas of sugarcane, also found that this treatment brings benefits in the short term, but that this effect decreases over time. MT generated lower water content than MT/DS, but was stable throughout the cycles, mainly in the subsurface layer for the plant cycle. For the first ratoon cycle in the surface layer, MT was higher for millet and sorghum, while in the subsurface layer, it was higher for sunn hemp, millet, and sorghum. Because this treatment has a moderate tillage of the soil, it tends to generated the best results in the medium term during the period without stabilization of NT. Sorghum was expected to have the best performance in soil water content, because of its higher dry biomass production (Table 2), which resulted in a higher root dry biomass (Table 3). However, this result occurred only for the plant cycle in the 0.00-0.15 m layer and in rainy periods, because its high densities (Table 6) and low MaP (Table 7) led to a less expressive effect on the soil water content compared to millet and sunn hemp. Among the coverage crops evaluated, sunn hemp obtained the best results, because of the high amount of straw produced (Table 2), a result also found by Ambrosano et al. (2014), in addition to its more aggressive root system, also found by Ambrosano et al. (2013). Rosolem et al. (2002), evaluating the roots of several cover crops, found that sunn hemp generated a larger root diameter at the depth of four compaction levels, which, when decomposing, generate a greater amount of large biopores, which then become macropores (Table 7). Peanut was the cover crop with the lowest soil water content,

4.2. Least limiting water range PR was the lower limit in both depths, indicating the degree of compaction in these areas. When PR and BD increase, there is an increase in cohesion caused by the action of soil moisture between the particles, which become closer as the soil is compacted or becomes denser (Tormena et al., 2007). The limiting density was about 1.80 Mg m−3 for the most superficial layer and 1.70 Mg m−3 in lower depth. These values were higher than those observed by other authors who have conducted studies on sugarcane (Gonçalves et al., 2014; Souza et al., 2015), however this is an expected result given that this is a soil with a history of compaction. For the cane´s plant cycle in both depths, the highest values for LLWR were found using MT/DS with sorghum, sunn hemp, and peanut, while the treatment that presented the lowest intervals was NT, because of the high BD values. CT generated results below the other treatments for its high BD, and in the subsurface layer its LLWR was equal to zero. The use of straw on the soil surface reduces soil density and penetration resistance in the superficial layer (Mishra et al., 2015). Thus, the use of this management practice, added to deep subsoiling, was beneficial for the treatments. BD is positively correlated with FC and PR, and negatively correlated with AP. This proves that the soil evaluated is compacted mainly on the superficial layer (0.00-0.15 m), since, from the moment AP becomes the higher limit, there is a physical limitation of aeration. Nevertheless, the LLWR interval for the superficial layer was still higher than that found for the subsurface layer (Fig. 4), proving that the organic matter originated from the straw decomposition (Table 2) had positive effects on LLWR. The organic matter also affected the LLWR in 10

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most of the evaluation period the soil water contents were below the lower critical limit of LLWR. Only for millet and MT/DS, the water content remained above the higher critical limit for a few months of the plant cycle, which can be harmful due to excess water, resulting in lack of oxygen for root respiration. Both water excess and deficit cause problems in the development of the plant. In the case of deficit, there is reduction of antioxidants (Boaretto et al., 2014), while the excess can impair root respiration (Iamaguti et al., 2015) and promote the percolation of nutrients (Santos et al., 2016). Most treatments resulted in LLWR equal to zero during the ratoon cycle, due to the structural degradation of the treatments, such as increased density and reduced macroporosity. Among the treatments that obtained positive LLWR, none can be considered as limit references because they are below 0.05 m3 m−3, a value below the limit considered critical in the literature of 0.10 m3 m−3, which means that, for ratoon sugarcane, LLWR cannot be used as a parameter for analysis (Xu et al., 1992). The results obtained from the physical attributes such as BD and MaP, in addition to an evaluation only considering soil water content, would lead us to the belief that their effects on crop productivity would be manifold. However, there was no statistical difference for productivity (Table 4), proving the importance of the analysis of LLWR as an index for evaluating crop quality. From the results found, it was possible to see that there was a depletion of the soil physical attributes, with increase of Bd, reduction of MaP and increase of PR. Although the LLWR is fixed for the evaluated soil for each layer, the result of these physical attributes over time by plotting the reduction of Bd reduced the LLWR found for the treatments. Due to their high BD, despite the balanced water content throughout the crop, the treatments did not differ in any of the productivity cycles, and there was a reduction in productivity with the evolution of the productive cycles. Also, the productivity of CT, which for all the other indices was more degraded, was above some treatments, such as peanut MT/DS in the plant cycle, and only below sorghum MT and MT/DS, millet NT, and sunn hemp MT/DS. This proves that the treatments, by not fitting within the LLWR, significantly affected crop productivity, which is the most important factor for the producer. Thus, LLWR intervals must be considered and measures should be taken to mitigate their effects on productivity. These results, in addition to the values of the LLWR limits, show that appropriate soil management (cover crops and tillage systems with less tilling) increase soil water content. The soil tillage system that generated water content within the LLWR limits was MT/DS along with the use of cover crops, such as sunn hemp and millet, which obtained better results in this study. The incorrect use of soil management can bring losses to crops which can be evaluated from the first harvest, resulting in soil degradation due to water stresses that the plant may suffer and directly affecting productivity. The importance of soil conservationist managements should be disclosed not only to the scientific community but also to producers. This has been the first study on sugarcane which denotes the importance of the interaction between physical attributes, such as density and macroporosity, dry biomass of the cover plant, root system, soil water content, productivity, and their effect on LLWR in sugarcane areas, proving by several variables the advantages of conservationist management systems, which are an innovation in the area. Long-term studies are recommended to evaluate these systems and their future inter-relationships, for studying the long-term effect of cover crops and tillage system in various crop cycles. The authors expect that this study will encourage other authors to research further physical, chemical, and biological characteristics to contributing to improve these new management systems.

because its characteristics are the opposite of sunn hemp, that is, less root biomass (Table 3) and amount of straw (Table 2). Moreover, its harvest causes greater disturbance of the soil, compared to the tillage systems, favoring its degradation (Miura et al., 2016). This degradation is also observed in CT, also characterized by excessive soil disturbance, in which it generated lower water content, higher BD (Table 6), and lower MaP (Table 7), compromising the root development of sugarcane (Table 3). Thus, this proves that soil tilling before or during the culture of sugarcane should be avoided, because it negatively affects several aspects. For the two sugarcane cycles, the alternation between excess and scarcity of water resulted in uneven distribution of soil water contents, which may affect the sugarcane phenological cycle (Silva et al., 2012a,b). The sugarcane plant in Phase 1 suffered an excess of water. According to Fritche and Iriarte (2015) the excess in Phase 1 can be beneficial because it allows a high growth of the plant and higher accumulation of sucrose for the dry period, as long as it is not accompanied by soil erosion, through which the seedlings or tails move in place, destroying the already developed root system. For phase 3 and 4 of the first cycle the amount of water supplied was enough. On the other hand, in the second cycle, the excess occurred only in Phase 1. This fact can be harmful to the plant, because it is in phases 2 and 3 that the most pronounced development of the root system and production of dry matter occur, hence, the water deficit may be considered the major cause of world productivity problems (Rampino et al., 2006). According to Machado et al. (2009), stress in these phases causes problems in the stem growth and leaf elongation, which are essential for plant photosynthesis. In addition, Medeiros et al. (2016) found out that sunn hemp is very resistant to water stress, being recommended for saline waters. Millet was also suggested as a drought-resistant plant by Yamane et al. (2018), when intercropped with rice, it causes its roots to search for deep water, and can also be used in semi-arid areas. Sorghum is also a droughtresistant plant according to Assefa et al. (2010), especially when using selected varieties for this resistance. Bandyopadhyay et al. (2005) found that peanuts need high amounts of water, and recommended their use in irrigated areas. With that, the peanut, is the least drought-resistant plant as a cover crop, obtaining less soil water contents. 4.4. Soil water content and its relationship with the Least limiting Water Range Based on the data obtained for the LLWR, it was possible to follow the periods in which the soil water contents were ideal and, with this, evaluate when there was excess or lack of water for the sugarcane culture. For the superficial layer, all cover crops obtained positive LLWR (above zero) during the plant cycle. Among the treatments evaluated, sorghum was the cover plant that provided the maintenance of the soil water content within the LLWR limits for a longer period (Fig. 7), because of its low densities during the plant cycle (Table 6). The other cover crops (sunn hemp, millet and peanut) maintained their soil water contents below the lower limit of LLWR, indicating that the content achieved in this period was not enough for the full development of the culture. The lack of water for the full development of the culture causes problems in its development and consequently in juice quality and productivity, which are important attributes for the industry (Ferreira et al., 2017). Cover crops affect the soil water content because of the amount of dry mass produced, which protects the soil from the direct incidence of solar rays and helps maintain the water content as a result of the lower soil temperature variations. Both in the extreme summer and extreme winter, straw positively affects the soil water content and temperature (Awe et al., 2015). However, as the crop residues of the cover crops remained on the soil surface only during the plant cycle, one can say that they directly affected the soil water content only for this cycle. For the other treatments (sorghum, millet, peanut and CT), during

5. Conclusions We aimed to evaluate the ability of the LLWR index to represent soil 11

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degradation by comparing conventional soil tillage systems and conservation managements. This index was important for interpreting soil physical attributes, root system and water content combined, leading to the belief that if the treatment does not obtain water contents within the intervals, there is a good chance of reducing productivity. The conservationist soil tillage systems, such as minimum tillage and cover crops, increase soil water ranges, when compared to the conventional system. Millet and sunn hemp used as cover crops improve soil structure and consequently, also the soil water content, when compared to peanut and sorghum. In comparison with conservationist tillage systems, the conventional system provides the soil with lower water content, higher soil bulk density, and lower least limiting water range for surface depths.

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