Short time effects of biological and inter-row subsoiling on yield of potatoes grown on a loamy sand, and on soil penetration resistance, root growth and nitrogen uptake

Europ. J. Agronomy 80 (2016) 55–65

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Short time effects of biological and inter-row subsoiling on yield of potatoes grown on a loamy sand, and on soil penetration resistance, root growth and nitrogen uptake Victor Guaman a , Birgitta Båth a,∗ , Jannie Hagman a , Anita Gunnarsson b , Paula Persson a a b

Department of Crop Production Ecology, Swedish University of Agricultural Sciences, P.O. Box 7043, SE-750 07 Uppsala, Sweden Rural Economy and Agricultural Society, Kristianstad, Box 9082, SE-291 09, Kristianstad, Sweden

a r t i c l e

i n f o

Article history: Received 25 June 2015 Received in revised form 23 June 2016 Accepted 23 June 2016 Available online 15 July 2016 Keywords: Soil penetration resistance Inter-row subsoiling Biological subsoiling Root length density N uptake Potato yield

a b s t r a c t Soil compaction, especially subsoil compaction, in agricultural fields has increased due to widespread use of heavy machines and intensification of vehicular traffic. Subsoil compaction changes the relative distribution of roots between soil layers and may restrict root development to the upper part of the soil profile, limiting water and mineral availability. This study investigated the direct effects of inter-row subsoiling, biological subsoiling and a combination of these two methods on soil penetration resistance, root length density, nitrogen uptake and yield. In field experiments with potatoes in 2013 and 2014, inter-row subsoiling (subsoiler) and biological subsoiling (preceding crops) were studied as two potential methods to reduce soil penetration resistance. Inter-row subsoiling was carried out post planting and the preceding crops were established one year, or in one case two years, prior to planting. Soil resistance was determined with a penetrometer three weeks after the potatoes were planted and root length density was measured after soil core sampling 2 months after emergence. Nitrogen uptake was determined in haulm (at haulm killing) and tubers (at harvest). Inter-row subsoiling had the greatest effect on soil penetration resistance, whereas biological subsoiling showed no effects. Root length density (RDL) in the combined treatment was higher than in the separate inter-row and biological subsoiling treatments and the control, whereas for the separate inter-row and biological subsoiling treatments, RLD was higher than in the control. Nitrogen uptake increased with inter-row subsoiling and was significantly higher than in the biological subsoiling and control treatments. However, in these experiments with a good supply of nutrients and water, no yield differences between any treatments were observed. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Soil compaction in agricultural fields has increased in recent decades due to intensive farming practices, including short crop rotations and use and intensification of heavy machinery. The compaction effect of heavy traffic may persist for more than a decade (Berisson et al., 2012) and is especially unfavourable when the pressure is applied while the soil water content is high (Heesmans, 2007; Hamza and Anderson, 2005). In cereal fields, yearly traffic wheel intensity of up to 150 Mg km ha−1 has been recorded, while in potato (Solanum tuberosum L.) fields the maximum value recorded is much higher, 300 Mg km ha−1 per year (Håkansson, 2000). Soil compaction occurs both in the topsoil and subsoil and

∗ Corresponding author. Department of Crop Production Ecology, Swedish University of Agricultural Sciences, P.O. Box 7043, SE-750 07 Uppsala, Sweden. E-mail address: [email protected] (B. Båth). http://dx.doi.org/10.1016/j.eja.2016.06.014 1161-0301/© 2016 Elsevier B.V. All rights reserved.

results in reduced porosity, especially of large pores, and increased bulk density. However, it is a more complex and costly problem to alleviate subsoil compaction than compaction in the topsoil (Zink et al., 2010; Håkansson, 1994). In most fields a compacted soil layer, known as the plough pan, can be detected in the upper subsoil. The shape, strength and thickness of the plough pan are often related to the pressure applied to the topsoil (Spoor et al., 2003; Håkansson and Reeder, 1994; Barraclough and Weir, 1988). High root penetration resistance in combination with reduced movement of water in the soil reduces water and nutrient availability to crops (Wolkowski and Lowery, 2008; Stalham et al., 2005; Arvidsson and Jokela, 1995; Håkansson, 1994) and may change the distribution of roots between soil layers and in some cases confines root development to the upper part of the soil profile wich restricts plant availability to water and minerals (Miransari et al., 2009; Lipiec et al., 2003; Unger and Kaspar, 1994; Douglas and Crawford, 1993).

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Potato is a crop with a sparse, shallow root system sensitive to drought and soil compaction at all stages of growth, from emergence to harvest (Stalham et al., 2005; Lynch et al., 1995). However, there is great variation in root biomass and overall structure of the root within the crop (Wishart et al., 2013; Iwama, 2008). In optimal soil conditions, it has been observed that potato roots can produce large amounts of root biomass, with a root depth of 1.40 m (Stalham and Allen, 2001). An explanation to the shallow roots of potato crops in practical field conditions may be the inability of the potato root system to penetrate compacted soil layers as for example the plough pan (Gregory and Simmonds, 1992). At soil penetration resistance greater than 1 MPa, potato root growth is greatly reduced, whereas roots of other crops can penetrate soil with resistance values of between 2 and 3 MPa (Stalham et al., 2007). The inability of the potato crop to penetrate compacted soil may be explained by the morphology of the potato root system, which is fibrous and branched with most of the root growth originating from lateral and basal roots (Weaver, 1926). When growing, roots rearrange the closest soil particles by pushing particles aside or in front of the root apex. However, at unfavourable levels of soil penetration resistance the root elongation rate decreases and the diameter of roots increases markedly, leading to clustered root growth and restricted root extension (Bengough and Mullins, 1990; Taylor and Ratliff, 1969). Agronomic practices such as deep cultivation or subsoiling designed to reverse soil compaction in deeper soil layers have been tested in many studies (Ekelöf et al., 2015; Canarache et al., 2000; Holmstrom and Carter, 2000). Subsoiling can improve plant growth and encourage deeper rooting (White et al., 2005). Bishop and Grimes (1978) observed a significant increase in rooting throughout a soil layer of 0.76 m depth when subsoiling was carried out on a sandy loam. Mechanical deep subsoiling has with a positive potato yield result recently been studied in Sweden (Ekelöf et al., 2015) and the growers are very interested to use this method. However, the drawback is that deep subsoiling has limited longevity and needs to be employed on an annual basis, as loosened soil is very susceptible to re-compaction (Raper et al., 2005; Busscher et al., 2002; Hamilton-Manns et al., 2002). For example, Busscher et al. (1986) found that on a loamy sand subsoiled to a depth of 0.5–0.6 m, soil strength increased to 1.5–2.5 MPa one year after subsoiling, although the effects of subsoiling were still visible. Subsoiling may also lead to acceleration of organic matter decomposition, resulting in weaker, less stable soil that is more susceptible to subsequent compaction (Brady and Weil, 2008; Canarache et al., 2000) and, in addition, has negative environmental effects as this method requires high traction force and fuel. The yield response to subsoiling is however inconsistent (Stalham et al., 2005). In some studies, tuber yield was found to increase by 6–14% (Henriksen et al., 2007; Bishop and Grimes, 1978), while other studies found no effect of subsoiling on yield (Holmstrom and Carter, 2000). According to Stalham et al. (2005) these diverging results may be because i) there was no real problem with compaction in the fields studied, ii) the subsoiling was carried out when soil was at or above its plastic limit, doing more harm than good to the soil structure, iii) the working depth was incorrect, or iv) good water supply masked the effects of subsoiling. Crops with deep root systems have been shown to have positive effects on soil structure by creating useful structural features such as root channels in deep soil layers that can be utilised by the following crop (Perkons et al., 2014; Löfkvist, 2005; Ishaq et al., 2001; Cresswell and Kirkegaard, 1995). In addition, plant root exudates stimulate the occurrence of soil organisms such as earthworms and microbes that contribute to the formation of bio-pores and a more stable soil structure through secretion of substances and humus compounds that bind soil particles together (Gregory, 2006; Dexter, 1990).

In the context of subsoil loosening, plants with root systems able to penetrate compacted soils is of special interest (Löfkvist, 2005). Materechera et al. (1991) proposed that crops with greater root diameter may be better at penetrating compacted soils than crops with small root diameter. This is in line with Chen and Weil (2010) who found that the taproot-forming species forage radish (cv. Daikon) and rapeseed (cv. Essex), both belonging to the Brassicaceae family, showed greater penetration capability than rye on fine loamy soils. Using a computer-assisted tomography technique, Hamza et al. (2001) observed that radish plants destabilised soil and loosened compaction by temporary decreases and increases in root diameter after the commencement of transpiration. Legumes may also be effective in improving soil structure, in some cases more effective than non-legumes, due to their strong root system and ability to produce substantial amounts of high quality residues (Snapp et al., 2005; Jones et al., 1998; Cochrane and Aylmore, 1994). Soil compaction modifies the soil nitrogen (N) balance. This is an effect of alteration of soil aeration status contributing to N losses and decreased N mineralisation, and of soil water properties leading to a change in the pattern of N transport and root growth (Lipiec and Stepniewski, 1995; Lipiec and Simota, 1994). By reducing soil compaction the crop root system may expand and grow larger which, in return, facilitates crop N uptake. This is of special importance considering N uptake efficiency from deeper soil levels (White et al., 2005; Westermann and Sojka, 1996; Pierce and Burpee, 1995; Miller and Martin, 1986). By monitoring the pattern of nitrate depletion, Asfary et al. (1983) found that potato roots were substantially more active below 0.30 m than at shallower depth. High root density in the subsoil is therefore of great importance at later stages of growth, when nitrate in the topsoil is depleted (Strebel et al., 1983). The use of different preceding crops for potato has not been extensively studied (Griffin et al., 2009). In addition, there is little information about the effects of combining a preceding crop with mechanical subsoiling in potato production. The objectives of this study were thus to determine the effects of mechanical, inter-row subsoiling after planting, biological subsoiling with preceding crops and a combination of these two methods on: i) soil penetration resistance, ii) root length density (RLD) and root distribution, iii) N uptake and iv) total yield. Our hypothesis was that combining inter-row and biological subsoiling would have a more pronounced effect in reducing soil penetration resistance, which in turn could encourage deeper rooting and thus lead to better N acquisition by potato plants and thus to increased yield.

2. Materials and methods 2.1. Experimental site and design Field experiments with potatoes were conducted in 2013 and 2014 on a soil classified as an Arenosol according to WRB (IUSS Working group, 2015), at the experimental farm Helgegården (56◦ 1 20 N, 14◦ 3 45 S), Kristianstad, Sweden (Table 1). According to soil penetration resistance data a clear plough pan was indicated (Fig. 1). The crop rotation during the previous 12 years is presented in Table 2. A conventional subsurface drainage system was installed at the experimental fields in 2000. The experiments were arranged in a split-plot block design with four replicates. The factor in the main plot was preceding crop and in the subplots inter-row subsoiling/no inter-row subsoiling. The experimental plots consisted of units measuring 12 m by 20 m, which were laid out in the year when preceding crops were established and divided into two (6 m by 20 m) in the year the potato crop was planted (8 rows per subplot) and the inter-row subsoiling treatment was applied. The outer rows were used as guard rows.

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Table 1 Physical and chemical properties of the soil used in field experiments at Helgegården, Kristianstad, in 2013 and 2014. Soil texture is classified as a loamy sand according to the Soil Survey Division Staff (1993). Soil properties

Clay ( < 0.002 mm ø) (%) Sand (0.06–2 mm ø) (%) Organic matter (%) P-AL (mg 100 g−1 )a K-AL (mg 100 g−1 )a Mg-Al (mg 100 g−1 )a K:Mg ratio Ca-AL (mg 100 g−1 )a pH in water—1:5b a b

0–0.20 m soil layer

0.40–0.60 m soil layer

2013

2014

2013

2014

7.5 76 2.8 24.3 7.6 12 0.6 1500 7.7

6 83.3 1.7 34 11.4 8.5 1.4 665 7.8

3 81 0.4 2.4 29 54 <0.1 >2000 8.3

3.3 89.3 0.3 5.8 4.5 38 0.1 >2000 8.4

P, K, Mg and Ca: AL-method according to Egnér et al. (1960). 1:5 soil:water ratio.

Fig. 1. Soil resistance during vertical penetration measured three weeks after planting of potatoes in the treatments: control, inter-row subsoiling, biological subsoiling and combination of inter-row subsoiling and biological subsoiling. Values presented are mean values from two field experiments (2013 and 2014) at Helgegården, Kristianstad, Sweden. Capital letters indicate significant differences between treatments with inter-row subsoiling on the one hand, and those without on the other hand, in four soil layers measured from the levelled soil surface: 0.15-0.20, 0.30-0.35, 0.40-0.45 and 0.50-0.55 m. At the different depths the standard error is <0.11 MPa.

Table 2 Cropping history of the studied fields previous the experimental years. Year 2014 2013 2012 2011 2010 2009 2008 2007 2006 2005 2004 2003

Experimental site 2011–2013

Experimental site 2012–2014

Potatoc Preceding cropsb Spring barley + red clovera Carrots Spring barley Sugar beet Winter rye Potato Spring barley Sugar beet Spring barley

Potatoc Preceding cropsb Spring barley + red clovera Spring barley Potato Spring barley Carrots Spring barley Sugar beet Spring barley Spring barley Potato

a The experiment started with the establishment of an under-sown red clover crop in spring barley. b The preceding crops red clover, oilseed radish, Chinese radish and spring barley were sown. Treatments presented in Table 4. c Potatoes (Solanum tuberosum L. cv. King Edward VII) were planted.

2.2. Weather conditions Except for a cold start (April) and end (September) in 2013 the difference in mean monthly temperature between the year 2013 and 2014 was below 1 ◦ C (Table 3). The total precipitation during the period was 11.5% higher in 2014 than in 2013. 2.3. Preceding crop management The preceding crops consisted of a red clover crop (Trifolium pratense cv. Ares) undersown in barley (Hordeum vulgare L. cv. Mercada) in 2011–2012 and 2012–2013 and annual crop species sown in 2012 and 2013. Chinese radish (Raphanus sativus L. ssp. longipinnatus cv. Structurator) and oilseed radish (Raphanus sativus L. ssp. oleiformis cv. Terranova) were sown in late August shortly after harvest of barley, while oilseed radish as green manure was sown in late May (Table 4). The red clover was followed by Chinese radish sown in July after harvest of red clover biomass and ploughing of the soil. Spring barley was used as control treatment. All

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Table 3 Mean monthly temperature, total precipitation and irrigation applied during the preceding crop growing season 2011–2013 and the potato crop growing season in 2013 and 2014 in field experiments at Helgegården, Kristianstad, Sweden. Temperature (◦ C)

2011 2012 2013 2014 30-year meana a

Precipitation (mm)

Total irrigation (mm)

April

May

Jun

Jul

Aug

Sep

April

May

Jun

Jul

Aug

Sep

Preceding crop

Potato

9.1 5.6 5.7 8.0 5.7

11.3 12.1 13.1 12.8 10.9

16.0 13.5 16.1 15.3 15.0

17.1 16.7 18.6 19.2 16.4

16.0 16.2 16.6 16.0 16.1

13.5 12.7 11 13.5 12.4

11 31 21 26 37

60 12 29 63 41

80 67 71 81 46

128 62 65 69 64

111 76 50 48 49

44 66 50 32 55

– 25 – –

– – 94 101

Data taken from the weather station in Karpalund, located 2 km from the experiment sites (Alexandersson et al., 2001).

Table 4 Sowing date, seed rate, fertilisation and growth time for preceding crops and potatoes included in two, two-year field experiments (2012–2013 & 2013–2014) at Helgegården, Kristianstad, Sweden. Preceding crop

Barley (Control) Barley and Chinese radish Barley and Oilseed radish Oilseed radisha Red cloverb and Chinese radish a b

Sowing date

Seed rate

N fertilisation

Growth time

2011

2012

2013

kg ha−1

kg ha−1

months

– – – – – – 5/5 –

29/3 29/3 9/8 29/3 8/8 28/5 4/5 6/7

6/4 6/4 2/8 6/4 2/8 24/5 – 4/7

150 150 15 150 20 15 4 10

100 100 30 100 30 54 0 0

4.5 4.5 3 4.5 3 5 18 4

Biomass left on the field when cut 31 July 2012 and 19 July 2013. The biomass was cut to a stubble height of 0.10–0.15 m. Biomass harvested 19 June 2012 and 17 June 2013.

Fig. 2. Subsoiler cultiplow with four shanks (Agrisem International SAS, France). A) Cultiplow shanks with 350 mm wide winged tips, B) subsoiler operating, the shanks were attached to a frame with a spacing of 0.75 m between outer shanks and 1.5 m between shanks in the central part of the frame, and C) soil surface after subsoiling to 0.45 m depth. Helgegården, Kristianstad, Sweden.

preceding crops except the mixture of red clover and Chinese radish were fertilised (Table 4) and they were all irrigated when required (Table 3). To determine the dry matter (DM) content of material left on the soil surface, the above-ground biomass was sampled on two occasions: after harvest and prior to autumn tillage, on two randomly selected 0.25 m2 squares in each plot (Table 5). In the three treatments that contained barley, the stubble was cut to 0.10-0.15 m height. The production of DM differed between preceding crops and between years. Chinese radish produced less above-ground biomass than oilseed radish. 2.4. Potato crop − agronomic practices On 24 April 2013 and 29–30 April 2014, potatoes (cv. King Edward VII) were planted (Underhaug UN3000, Norway) at 0.23 m within-row spacing and with 0.75 m between rows at a seed rate adjusted to seed piece weight, 2.5 t ha−1 in 2013 and 1.9 t ha−1 in 2014. Fertiliser was applied on four occasions. The first occasion was at 7 (2013) and 15 (2014) days before the potatoes were planted, when a starter dose of 99 kg N ha−1 , 41 kg phosphorus (P) ha−1 and 158 kg potassium (K) ha−1 was broadcast across all plots. On the same occasion, additional K was spread but the dose was adjusted to results from analyses of K in the 0-0.20 m soil layer and to compensate for the effects of the preceding crops. Hence the dose

varied between treatments (Table 6). The second N fertiliser dose, which was applied 17 (2013) and 14 (2014) days after planting, was based on mineral N in the 0-0.20 and 0.40-0.60 m soil layers seven (2012) and 15 (2013) days before the potatoes were planted (Table 6) and on estimated potential N mineralisation from the preceding crop biomass incorporated into the soil (Table 5). After another three weeks, the third fertilisation was carried out with a dose equivalent to 23 kg N ha−1 on all plots. The dose was based on the N concentration in petiole plant sap which is a good indicator of the N status of the plant (Gardner and Jones, 1975). The fourth N fertilisation was based on potential yield estimated by counting tubers and determining their specific gravity. As there were no differences between treatments, the fertilisation comprised urea equivalent to 10 kg N ha−1 (foliar fertilisation) equally on all plots. The preparation of the fields consisted of ploughing to 0.200.25 m depth in November and harrowing twice to 0.05 m depth in spring prior to planting. The same procedure was carried out in 2013 and 2014. The experiment field was irrigated on six occasions during the season, with approximately 20 mm on each occasion (Table 3). The experiments were treated with pesticides in accordance with local recommendations. Haulms were killed with a mechanical-chemical treatment on 22 August 2013 and 29 August 2014. Inter-row subsoiling to 0.45 m depth measured from the levelled soil surface was carried out one week after the potatoes were

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Table 5 Above ground yield of dry matter (DM) and nitrogen (N) and, the C/N ratio of preceding crops incorporated into the soil during two growing seasons 2012 and 2013 in field experiments at Helgegården, Kristianstad, Sweden. Autumn tilling was carried out at the beginning of November. Preceding crop

2012

2013

DM (kg ha−1 )

Barley (Control)a Barley and Chinese radish Barley and Oilseed radish Oilseed radishb Red cloverc and Chinese radish

N (kg ha−1 )

DM (kg ha−1 )

C/N

N (kg ha−1 )

C/N

Straw/stubble

Shoot

Straw/stubble

Shoot

Straw/stubble

Shoot

Straw/stubble

Shoot

Straw/stubble

Shoot

Straw/ stubble

Shoot

4018 4018

–

12 12

–

157 157

– 16

3480 3480

–

12 12

–

107 107

– 14

157

17

3480

107

23

– –

24 40

976

23

4018 – –

12 1019 8460d

– –

e

2505

21 120

2240

– –

30 24

– –

61 12

2640 8582d

28 156

– –

e

44

2340

24

Total barley harvest was 6150 kg/ha−1 in 2012 and 4015 kg ha−1 in 2013. Straw and stubble was measured from a cluster sample from all three treatments that contained barley. b Not harvested, biomass was left on the field until autumn tillage. c Harvested biomass was 5510 kg ha−1 (2012) and 5570 kg ha−1 (2013). d Sum of biomass cut and left in the field and regrowth. In 31 July 2012 biomass was 4100 kg ha−1 and regrowth until 15 October 2012 was 4360 kg ha−1 . In July 2013 biomass was 4080 kg ha −1 and regrowth until 19 October was 4500 kg ha −1 . e Biomass from Chinese radish incorporated into the soil. a

Table 6 Additional doses of fertiliser with potassium (K) between one and two weeks prior to planting and with nitrogen (N) between one and two weeks post emergence and total fertilisation in potato field experiments at Helgegården, Kristianstad in 2013 and 2014. Preceding crop

Fertilisation 2013

Fertilisation 2014

Additional a

Barley (control) Barley and Chinese radish Barley and oilseed radish Oilseed radish, no harvest Red clover and Chinese radish

Total b

Additional a

Total b

N

K

N

K

N

K

N

K

41 26 26 11 11

156 156 156 156 205

173 158 158 143 143

314 314 314 314 364

41 31 31 26 31

72 72 72 72 97

173 163 163 158 163

230 230 230 230 255

a Second N fertiliser dose applied 17 (2013) and 14 (2014) days after planting. The dose was based on mineral N in the 0-0.20 and 0.40-0.60 m soil layers and on estimated potential N mineralisation from the preceding crop biomass incorporated into the soil. b Additional K spread before the potatoes were planted. The dose was adjusted according to results from analysis of K-AL in the 0-0.20 m soil layer and to compensate for the effects of the preceding crops.

planted, using a subsoiler consisting of four shanks with 350 mm wide winged tips (Agrisem Cultiplow, Agrisem International SAS, France). The shanks were attached to a frame with a spacing of 0.75 m between outer shanks and 1.5 m between shanks in the central part of the frame (Fig. 2). The experiments were harvested on 12–13 September 2013 and 16 September 2014. The potato yield was determined after mechanical harvesting of two 15-m rows from each subplot using a potato harvester (ASA-Lift KT 100, Denmark).

2.5. Measurements and analyses Soil penetration resistance was measured in all treatments with a recording cone penetrometer (Penetrologger, Eijkelkamp, The Netherlands) three weeks after the potatoes were planted. The measurements were made at 10 locations randomly spread around the ridges and the furrows, to a depth of 0.60 m after levelling the soil surface. The penetrometer settings consisted of a cone with a base area of 2 cm2 penetrating the soil at an angle of 60◦ and a speed of 2 cm s−1 . In treatments with inter-row subsoiling the measurements were made at the bottom of the furrows where the subsoiler shanks operated and in the other treatments at the bottom of a random furrow where no tractor wheel had passed. Sampling of roots was carried out 15 July 2013 and 17 July 2014 two months after potato plant emergence, development stage 71 (Hack et al., 1993; 2-digit decimal code), with soil cores (0.073 m inside diameter, height 0.05 m) at three spatial positions (horizontal): beneath the centre of the bed, beneath the centre of the bed

and the bottom of the furrow, and beneath the bottom of the furrow. At each position and after removal of the ridge, core samples were taken from four different depth layers: 0.15–0.20, 0.30–0.35, 0.40–0.45 and 0.50-0.55 m (Fig. 3). The depths were measured from a levelled soil surface and were thus comparable to the depths presented for the penetrometer measurements and the subsoiling depth. The soil cores were placed in plastic bags and stored at −20 ◦ C until washing. Roots were recovered by washing the soil cores under running tap water on a sieve with a mesh size of 1 mm. The clean roots were scanned and root length was determined by image analysis using the WIN-RHIZO software system (version 2007a, Regent Instruments) and then converted to RLD (cm cm−3 ) based on the soil volume of the soil core (209 cm3 ). Nitrogen uptake and DM content were determined in haulm sampled in conjunction with haulm killing, performed prior to leaf senescence, and in tubers sampled at harvest. The N concentration in tubers and haulm was measured using a CHN-600 analyser (Leco Co.) after drying at 65 ◦ C for 12 h. The DM in tubers and haulm was calculated after drying at 110 ◦ C for another 12 h. Nitrogen in soil was determined according to the Dumas method (Bremner and Hauk, 1982) and potassium was analysed according to the ammonium acetate lactate (AL) method (Egnér et al., 1960). 2.6. Statistical analysis The RLD data were analysed using the following linear mixed model procedure: Yijklmn = u + ˛i + ˇj + k + ıl + (˛ˇ)ij + (˛)ik + (˛ı)il + (ˇ)jk

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+(ˇı)jl + ı

kl

+ (˛ˇ)ijk + (˛ˇı)ijl + (ˇı)jkl + (˛ı)ikl

+(˛ˇı)ijkl + (a)m + (b)imn + (c)ilm + (d)iklm + (e)ijklmn In this model, Yijklmn is the RLD obtained in the ith year at the jth depth and kth position with the lth treatment in the mth block with the nth preceding crop. Moreover, u is an intercept and ˛i , ˇj , k and ıl are fixed effects at years, depths, positions and treatments, respectively. Terms within brackets are fixed effects of interactions, and (a)m , (b)imn , (c)ilm , (d)iklm and (e)ijklmn are random effects of blocks, main plots, subplots, positions and residual errors, respectively. All random effects were assumed to be independent and normally distributed with expected value 0. The RLD data were square root-transformed as necessary to meet the requirements of analysis of variance (ANOVA) in terms of normal distribution and homogeneity of variance. ANOVA results for RLD, soil resistance, yield and N uptake are shown in Table 7. The N uptake in haulm and tubers, soil resistance and tuber yield were analysed using a linear mixed model according to: Yijkl = u + ˛i + ˇj + k + (˛ˇ)ij + (˛)ik + (ˇ)jk + (ˇı)jl



+ ˛ˇ



ijk

+ (a)il + (b)ijl + (e)ijkl

where Yijkl is the N uptake/soil resistance/tuber yield measured in the ith year at the jth preceding crop and the kth subsoiling in the ith block with the jth preceding crop. Term u is an intercept and ˛i , ˇj and k are fixed effects of year, preceding crop and subsoiling, respectively. The terms within brackets are fixed effects of interactions and (a)il , (b)ijl and (e)ijkl are random effects of blocks, main plots and residual errors, respectively. All random effects were assumed to be independent and normally distributed with expected value 0. The contrast statement of the mixed procedure was used to compare the control treatment with the average of all other treatments regarding RLD, N uptake, soil resistance and tuber yield. Values were considered significant at a probability value of p < 0.05. The models were adjusted in the SAS program (SAS Systems for Windows, release 9.1.3, SAS Institute). 3. Results Initial statistical analyses showed no significant differences between the different preceding crop treatments. The following division of treatments into four groups was therefore made: 1) control (i.e. barley as preceding crop without subsoiling), 2) inter-row subsoiling (i.e. barley and mechanical subsoiling post planting), 3) biological subsoiling (merging of preceding crops; barley + Chinese radish; barley + oilseed radish; summer-sown oilseed radish and red clover + Chinese radish), and 4) combination of

inter-row subsoiling and the preceding crops in group 3. Main effects are presented averaged, over years. 3.1. Effects on soil penetration resistance Treatments with inter-row subsoiling, on the one hand, and those without inter-row subsoiling, on the other, differed significantly regarding soil penetration resistance throughout the soil profile (Fig. 1). In the barley control and biological subsoiling treatments, a distinct increase in soil compaction was evident starting at 0.20 m depth, reaching around 3 MPa at 0.30 m depth and remaining at this value to 0.60 m depth. In the two treatments including inter-row subsoiling no such compaction boundary was evident and instead compaction began to increase at 0.35 m depth in the profile and reached the same compaction level as in the two other treatments at 0.6 m depth. Analysis of soil resistance for the 0.15-0.20, 0.30-0.35, 0.40-0.45 and 0.50-0.55 m soil layers showed differences within layers between the two treatment pairs. 3.2. Effects on RLD and root distribution The highest RLD value was recorded in the treatment in which inter-row and biological subsoiling were combined (Fig. 4), where RLD was more than twice as high as in the barley control. For the inter-row and biological subsoiling treatments, RLD was higher than in the control. In the 0.15-0.20 and 0.30-0.35 m layers, RLD was lower in the control than in the other treatments. The RLD values in the 0.15-0.20 and 0.40-0.45 m layers were higher in the combined treatment than in the inter-row and biological subsoiling treatments. In the 0.50-0.55 m layer, no differences in RLD were observed between treatments. There were no differences at any depth between the inter-row and biological subsoiling treatments. Differences regarding RLD between years were found (Table 7). In 2013, the mean RLD was 25% higher than in 2014 and also higher at all depths except the 0.15-0.20 m soil layer (not shown). The RLD at the three different horizontal positions was lower in the barley control than in the other treatments (Fig. 5). The highest values at positions 1 and 2 were recorded in the combined treatment, while at position 3 (bottom of the furrow) the RLD value in the inter-row subsoiling treatment was as high as in the combined treatment. No differences in RLD between positions were observed in the other treatments. 3.3. N uptake in haulm and tubers Total N uptake in haulm and tubers was higher in the inter-row subsoiling treatment than in the biological subsoiling and barley control treatments (Fig. 6). No differences were observed between

Fig. 3. Sampling of roots at four depths measured from the levelled soil surface: 0.15–0.20, 0.30–0.35, 0.40–0.45, & 0.50–0.55 m, and three horizontal spatial positions: (1) beneath the centre of the bed, (2) beneath the centre of the bed and the bottom of the furrow, and (3) beneath the bottom of the furrow. Helgegården, Kristianstad, Sweden.

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Fig. 4. Root length density (RLD cm cm−3 ) in four soil layers measured two months after potato plant emergence, development stage 71 (Hack et al., 1993; 2-digit decimal code). The values presented are mean values from two field experiments (2013 and 2014) at Helgegården, Kristianstad, Sweden. Different letter(s) indicate significant differences. Capital letters indicate differences between treatments, lower case letters differences between layers.

Fig. 5. Root length density (RLD cm cm−3 ) measured 2 months after emergence of potatoes at three different horizontal positions: 1 = beneath the centre of the bed, 2 = beneath the centre of the bed and at the bottom of the furrow, and 3 = beneath the bottom of the furrow. The values presented are mean values from two field experiments (2013 and 2014) at Helgegården, Kristianstad, Sweden. Different letter(s) for treatments and positions indicate significant differences.

Fig. 6. Nitrogen (N) uptake measured prior to leaf senescence in a potato crop. The values presented are mean values from two field experiments (2013 and 2014) at Helgegården, Kristianstad, Sweden. Different letter(s) for treatments indicate significant differences. Capital letters indicate differences between treatments regarding total sum of N uptake in haulm + tuber, lower case letters differences between treatments regarding N uptake in the different plant parts (haulm and tuber).

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Table 7 Summary of mixed model analysis used to calculate statistical differences in root length density (RLD), soil resistance, yield and nitrogen (N) uptake between treatments at different years, depths, spatial position and plant part. Bold letters indicate significance differences (p < 0.05). DF Den = degrees of freedom denominator. Source

Year Treatments (Treat.) Year × Treat. Depth Year × Depth Depth × Treat. Year × Depth × Treat. Spatial position (Spat.pos.) Year × Spat.pos. Depth × Spat.pos. Year × Depth × Spat. pos. Spat.pos. × Treat. Year × Spat.pos. × Treat. Depth × Spat. pos. × Treat. Year × Depth × Spat.pos. × Treat. Plant part (haulm/tuber) Treat. × Plant part Year × Plant part Year × Treat. × Plant part

RLD

Soil resistance

Yield

N uptake

DF Den

P-value

DF Den

P-value

DF Den

P-value

DF Den

P-value

57 57 57 660 660 660 660 660 660 660 660 660 660 660 660 – – – –

0.0026 <0.0001 0.1154 <0.0001 <0.0001 <0.0001 0.0524 0.2621 0.1453 0.6370 0.0776 0.0101 0.9091 0.9971 0.9777 – – – –

57 57 57 180 180 180 180 – – – – – – – – – – – –

0.0016 <0.0001 0.8919 <0.0001 0.0001 <0.0001 0.4970 – – – – – – – – – – – –

6 42 42 – – – – – – – – – – – – – – – –

0,127 0,4187 0,5899 – – – – – – – – – – – – – – – –

6 144 144 – – – – – – – – – – – – 114 114 114 114

0.9641 0.0405 0.0228 – – – – – – – – – – – – <0.0001 0.0246 <0.0001 0.1466

the biological subsoiling, the combined and the control treatments. However, N uptake in haulm was higher in the combined and interrow subsoiling treatments than in the control and, in the case of the combined treatment, also higher than in the treatment with biological subsoiling. There were no differences in N uptake in tubers between treatments. The statistical analysis revealed an interaction between treatment and year (data not shown). In 2014, total N in haulm and tubers in the inter-row subsoiling treatment was higher than in the other treatments while there were no differences between treatments in 2013. 3.4. Tuber yield The tuber yield was normal, around 50 t ha−1 , for the variety and for the area. When data from both experimental years were combined, no significant differences were observed in total yield (Table 7). The only notable effect on yield parameters was higher tuber yield in the combined treatment compared with the biological subsoiling treatment (data not shown). 4. Discussion General challenges when studying the benefits of management practices aimed at enhancing deeper rooting are the ability to isolate additional confounding effects which may arise with incorporation of preceding crop biomass, such as increased water use efficiency and soil nutrient uptake, lower incidence of insect pests and diseases and weed suppression (Snapp et al., 2005; Dabney et al., 2001; Cresswell and Kirkegaard, 1995). In our study, measures were taken to avoid differences in nutrient availability between treatments and thus examine only the effects of the subsoiling treatments. Interestingly enough we did not find significant differences between the preceding crops used with regard to any of the observed variables. We can therefore discuss them together as the ‘biological subsoiling’ treatment. 4.1. Effects on soil penetration resistance Our study could not confirm the hypothesis that combining inter-row and biological subsoiling has a more pronounced effect in reducing soil penetration resistance in the subsoil than applying these two treatments separately. There was no difference in

this respect between the inter-row subsoiling and the combined treatments. The benefit of using different crops to reduce soil penetration resistance may thus be a long-term process that is difficult to study in short-term field experiments. However, due to economically reasons the only option for potato growers in Sweden may be to use a biological subsoling crop once in a crop rotation, e.g. as a preceding crop before potatoes. This makes the short time effects of subsoling of special interest in this project. Even if most biological subsoling studies have been conducted over several years it was shown by Bodner et al. (2014) that short time preceding crop subsoiling may affect soil physical qualities. When mechanical and biological subsoiling was conducted separately, mechanical inter-row deep subsoiling to 0.5 m depth, in agreement with earlier studies (Ekelöf et al., 2015; Canarache et al., 2000; Holmstrom and Carter, 2000), showed significant reductions in soil penetration resistance compared to the barley control. However, we could not show a reduction in soil penetration resistance using a one-year biological subsoiling crop although earlier studies has shown that the use of crops with deep root systems has positive effects on soil structure by creating useful structural features such as root channels in deep soil layers that can later be utilised by the following crop (Löfkvist, 2005; Ishaq et al., 2001; Cresswell and Kirkegaard, 1995). In many of the studies the subsoiling crops have, however, been grown for several years. Abdollahi and Munkholm (2014) showed that autumn-established forage radish grown consecutively for five years on a sandy loam decreased soil penetration resistance substantially in the plough pan region. In another study performed by Perkons et al. (2014), growing two-year chicory, a taprooted crop, on a Haplic Luvisol (loess parent material), increased the number of biopores per unit surface area in deeper soil, allowing subsequent crop roots to grow deeper. On the other hand, Chan and Heenan (1996) observed no differences in either soil macroporosity or wheat yield in a 10-yearold experiment on a red earth (27% clay) where wheat was grown in rotation with either clover or lupin. The effective rooting depth of wheat was, however, found to be improved with lupin as preceding crop. Choice of plant species, soil type and environment are other factors that decides how the root development of a preceding crop influence soil structure (Monroe and Kladivko, 1987). In the biological subsoiling treatments of the present study, all preceding crops were dicotyledonous, while the barley used in the control treatment is monocotyledonous. According to Materechera et al.

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(1991), the roots of dicotyledonous plants are better than those of monocotyledonous plants at growing through compacted soil layers due to their larger root diameter, which allows higher pressure to be exerted on the soil. In the present study there was no difference in soil penetration resistance between the control and the biological subsoiling treatments. Similarly, Kautz et al. (2010), who studied effects of perennial alfalfa and grass/clover on soil resistance, found no general decrease in soil resistance in the subsoil and only slightly lower resistance in the topsoil. However, as there are other features of importance our study can not conclude that biological subsoiling did not affect soil structure.

4.2. Effects on RLD and root distribution As in previous studies (Iwama, 2008, 1998), the results obtained for potato root distribution in this study demonstrated that most of the roots were located in the topsoil. Total RLD and RLD in soil layers 0.15-0.20 and 0.40-0.45 were higher in the combined treatment than in the other treatments (Fig. 4) which confirms the hypothesis that combined treatment involving both inter-row and biological subsoiling can encourage deeper rooting than applying these two treatments separately. The higher soil penetration resistance in the treatment with biological subsoiling than in that with inter-row subsoiling (Fig. 1) did not result in lower mean RLD. Similarly, Seyed et al. (2011) found the highest values of RLD in soils with high penetration resistance. According to Unger and Kaspar (1994), only root distribution and not the total root length may be affected by soil compaction due to growth compensation by unimpeded roots. However, in the present study it was clearly observed that RLD in the control treatment was affected by soil compaction. Importantly, Stalham and Allen (2001) concluded that the inconsistent results show that soil resistance is not the sole limiting factor determining rooting depth, which may also depend on soil texture, water availability, cultivar and changes in soil biological environment. Despite differences in soil resistance between the biological and inter-row subsoiling treatments there was no difference in RLD between these treatments, which is most likely explained by pores and channels created by the preceding crops and utilised by the potato plants. In future experiments, it is important to determine whether the soil structure can become too loose for normal crop development, as water transport may be influenced negatively and become a problem, especially in dry conditions (Håkansson, 2000). In addition, as concluded by Spoor et al. (2003), excessive loosening in the subsoil makes soils more prone to re-compaction. However, in the present study the soil resistance in the inter-row treatment was not lower than in the combined treatment, where the highest RLD was observed. Although the differences in water supply (precipitation and irrigation) were not remarkable (380 mm in 2013 and 420 mm in 2014; Table 3), rooting depth was higher in 2013 than in 2014, when the root system was mostly confined to the upper part of the soil profile. This observation is in line with findings by Stalham and Allen (2001), who stressed the need to identify appropriate irrigation regimes that least affect deep rooting. The root system of potatoes has been reported to spread horizontally (Stalham and Allen, 2001), but the results in the present study showed that for all treatments except the inter-row subsoiling treatment, root distribution was uniform beneath the ridge and the bottom of the furrow (Fig. 5). In the treatment with inter-row subsoiling, RLD was higher beneath the bottom of the furrow than at the other horizontal positions, which according to Zhang and Davies (1989) may be due to improved aeration conditions in the furrow.

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4.3. Effects on nitrogen uptake Several studies in potatoes and other crops have shown that higher soil penetration resistance constrains root growth and hence nutrient uptake can be expected to decrease (Van Oijen et al., 1995; Haunz et al., 1992). This is in line with the results of the present study where high N uptake in the treatments with inter-row subsoiling, due to high N uptake in the haulm, may be explained by the reduced soil penetration resistance (Fig. 1). In addition differences in root distribution between positions (Fig. 5), and a possible increase in root surface area in the roots growing in parts of the soil with less resistance (Cai et al., 2014) may have contributed to the result. However, the results partly contradicts the hypothesis that deeper rooting leads to better N acquisition by potato plants as higher RLD in the 0.4–0.45 m soil layer in the combined treatment did not coincide with a higher N uptake than in the other treatments (Figs. 4 and 6). 4.4. Effects on tuber yield Our hypothesis that better N acquisition by potato plants leads to increased yield was not confirmed. Despite higher N uptake in the inter-row subsoiling treatments than in treatments without inter-row subsoiling (Fig. 6), there was a lack of differences in tuber yield (Table 7). Neither was the yield dependent on measured differences in soil penetration resistance and RLD. Although previous studies have shown a positive correlation between root length and yield (Iwama, 2008) it is likely that the results in the present study reflects good conditions regarding supply of water and nutrients to the crop. 5. Conclusion Our hypothesis that combining inter-row and biological subsoiling has a more pronounced effect in reducing soil penetration resistance in the subsoil encouraging deeper rooting leading to better N acquisition by potato plants and ultimately increased yield, was not confirmed. However, RLD values were higher in the combined subsoiling treatment than in the other treatments, confirming our hypothesis that the combined treatment generated added value to root development compared with using these two treatments separately. The increased root length had no effects on average tuber yield, however. Thus, further studies on the effects of subsoiling methods in potato crops growing under conditions of restricted water and nutrient supply are needed in order to identify the true potential of decreased soil resistance. This is especially important when supporting efforts aimed at improving water and nutrient use efficiency in e.g. organic farming, agriculture in dry areas of the world and in general future scenarios of more restrictions concerning water and nutrient supply. Acknowledgements The authors express sincere thanks to The Swedish Farmers’ Foundation for Agricultural Research (SLF) and the Swedish University of Agricultural Sciences (SLU) for providing the financial support for the project. Helpful advice and comments on the statistics by Johannes Forkman are much appreciated. References Abdollahi, L., Munkholm, L.J., 2014. Tillage system and cover crop effects on soil quality: i. Chemical mechanical, and biological properties. Soil Sci. Soc. Am. J. 78, 262–270. Alexandersson, H., Eggertsson, Karlström, C., 2001. Temperaturen och nederbörden i Sverige 1961-90. Referensnormaler. (2nd ed.) SMHI Meteorologi 99. SMHI (Swedish Meteorol. and Hydrological Inst.), Norrköping, Sweden.

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