IntroductionTop

Soil structure is one of the main factors affecting crop growth over the long term (Passioura, 1991). Good soil structure consists of soil aggregation associated with high porosity, rapid infiltration rates, water retention capacity and increased air circulation which all facilitate root penetration and proliferation (Primavesi, 1982). A major problem of modern agriculture is the loss of soil aggregation, with compaction being one of the most important causes (Hamza & Anderson, 2005; Batey, 2009). Compaction alters the overall structure of soil pores, reduces pore number and size, and increases soil bulk density and resistance to soil penetration (Iler & Stevenson, 1991; Abu-Hamdeh, 2003).The effect of soil compaction on crop growth depends on soil texture. In general, soil bulk density values that limit root growth are 1.40–1.45 kg L-1 in silty to clay textured soil, and 1.65–1.75 kg L-1 in sandy soils (Daddow & Warrington, 1983).

The most frequent causes of soil compaction are use of heavy machinery, intensive cultivation, no crop rotation, and inadequate soil management (Hamza & Anderson, 2005; Batey, 2009). Soil compaction is accentuated in soils with low organic matter content and in soils with high moisture content (Hamza & Anderson, 2005). Organic matter loss can cause soil disaggregation, increasing susceptibility to compaction (Cochrane & Aylmore, 1994).

Soil tillage is a management practice that can alleviate soil compaction in diverse agricultural systems (Abu-Hamdeh, 2003; Quincke et al., 2007; Wortmann et al., 2008). Erbach et al. (1992) reported that different forms of tillage reduced differentially soil bulk density and resistance to penetration within the depth of tilled soil. The most effective tillage practices increase availability of soil water to crops due to increased infiltration (Lampurlanés et al., 2001). In crop rotations of wheat and maize, deep tillage increased yield due to a reduction of soil compaction and increased soil water holding capacity (Mu et al., 2016). Soil management practices that facilitate deep rooting are likely to improve the efficient use of N and water, thereby decreasing the likelihood of nitrate (NO3−) leaching (Thorup-Kristensen, 2011).

The compaction processes in soil-grown crops in greenhouse have become widespread worldwide. Indeed, higher soil bulk density values have been reported for greenhouse soils than in bare soils (Liang et al., 2013). In recent years, mechanization in greenhouse crops has increased notably, which resulted in an increase in soil compaction (Erdem et al., 2006). This has led to increased soil bulk density, with values above 1.60 kg L-1, whereas root growth is thought to be restricted above 1.75 kg L-1 (Primavesi, 1982). Serious compaction and soil degradation have been detected in greenhouse crops due to continuous cultivation, such as in cucumber crops (Liang et al., 2013).

In the area of Almería, in south eastern Spain, the system of cultivation in “enarenado” soil is used, which consists of covering the soil with a layer of siliceous sand that maintains humidity (Valera et al., 2014). A drip irrigation system is used with fertigation by tapes, in order to apply the dissolved fertilizers in the irrigation water, and the soil is kept constantly humid (Thompson et al., 2007a). Within greenhouses, most cultural practices such as transplanting, pruning, harvesting, and the application of plant protection products are carried out manually by farm workers (Valera et al., 2014; Padilla et al., 2017). Due to the presence of a sand mulch, tillage is hardly carried out, since the sand mulch layer must be removed prior to tillage and be replaced (Valera et al., 2014). The low frequency of tillage accentuates soil compaction processes in SE vegetable crops (Castilla, 1986; Martinez, 1987; Padilla et al., 2017).

The objective of this work was to assess the effects of tillage on soil properties, crop responses and root length growth and density of sweet pepper (Capsicum annuum L.) grown in soil in a greenhouse in SE Spain. We measured soil properties (bulk density, resistance to penetration, soil matric potential), crop responses (dry matter production, crop yield, nitrogen N uptake) and rooting pattern (root length density, relative root length distribution, root observation through minirhizotron tubes).

Material and methodsTop

Experimental details

The work was carried out in a plastic greenhouse located at the Experimental Farm of the University of Almería (Almería, Spain, 36° 51’ N, 2° 16’ W; 92 m elevation). The area of the greenhouse dedicated to the crop was approximately 1,300 m2. The greenhouse was similar to commercial greenhouses of the Almería region.

Two sweet pepper (cv. Melchor) cropping cycles were grown in soil with a summer-winter cycle, from 18 July 2016 to 24 March 2017 (248 days from transplant to end; hereafter the 2016-17 crop), and from 21 July 2017 to 20 February 2018 (214 days from transplant to end; hereafter the 2017-18 crop).

The soil was an artificial layered “enarenado” soil, which is typical of the region (Valera-Martínez et al., 2016). The “enarenado” soil consists of a 30 cm layer of silty clay loam soil, imported from a quarry, placed over the original sandy loam soil and a 10 cm layer of course river sand placed on the imported silty clay loam soil as a mulch (Padilla et al., 2017).

There were eight plots of 6 × 6 m, with four plots (i.e., replications) per treatment in a fully randomized block design. Polyethylene film sheets buried to 30 cm depth in the borders of the plots prevented water movement between plots. Five-week-old seedlings were planted 6-8 cm from each dripper; the plant density was 2 plants/m2.

Drip irrigation and fertigation were used. Drippers with a flow rate of 3 L h-1 were installed every 50 cm in drip lines, arranged in paired lines with an 80 cm separation. There was a 120 cm spacing between the paired driplines. There were 2 emitters/m2. Fertigation with complete nutrient solutions, applying all macro and micronutrients commenced at 9 and 10 days after transplant for the 2016-17 and 2017-18 crops, respectively. The N concentrations applied was the same for the two cycles, 9.7 mmol L-1. All cultural practices were consistent with local crop management.

Climatic conditions were recorded inside the greenhouse throughout both crops. Data were stored in a datalogger.

Tillage treatments

There was a tillage treatment and a control with no tillage (i.e., conventional soil management). For the tillage treatment, the gravel mulch was removed, and the soil was ploughed with a single pass with ripper to 15 cm depth, followed by a single pass with rotavator to 10 cm depth. A small tractor pulled the tillage implements. Following cultivation, the gravel was replaced on the surface of the cultivated soil and was evenly spread to form a 10 cm thick mulch layer. Tillage was conducted at the end of June 2016, before the commencement of the 2016-17 crop. No tillage was conducted before the 2017-18 crop. There were four replicated plots of the tillage and no tillage treatments.

In both treatments, irrigation volumes and frequency were modified to maintain soil matric potential between -15 and -25 kPa. Tensiometers (Irrometer Co., Riverside, CA, USA) were installed at 25 cm depth (relative to the surface of the gravel mulch layer) in each plot to measure soil matric potential. The range of -15 to -25 kPa range avoided crop water stress (Thompson et al., 2007b). The intended applied N concentration of fertigation of both treatments was of 10 mmol L-1; 92% of applied mineral N was in the form of NO3-, the rest as ammonium (NH4+).

Irrigation volumes were measured with water meters. Nutrient solution samples were collected in both treatments, two times per week, to determine the NO3- and NH4+ concentrations (SAN++, Skalar Analytical B.V., Breda, The Netherlands). Drainage was collected from free-draining lysimeters (Gallardo et al., 2014). Drainage was collected from free-draining lysimeters, the soil of the lysimeter reproduced the “enarenado” soil, their detailed description is in Gallardo et al. (2020).

Soil parameters measurements

Soil bulk density was determined one month after soil cultivation, on 27 July 2016. Soil coring rings (5.3 cm internal diameter, 4 cm wall height, Eijkelkamp Soil and Water, Giesbeek, The Netherlands) were used for determination at 15.5-19.5 and 30.5-34.5 cm soil depths. All soil depth values are relative to the surface of gravel mulch. One determination was conducted in each replicated plot. Each sample was taken 8 cm from a plant perpendicular to the line of plants.

Soil penetration resistance (kPa) was measured with a compaction meter (FieldScout SC 900, Spectrum Technologies, Inc., Aurora, IL, USA). Measurements were recorded automatically in a data logger for every 2.5 cm of soil depth, from 10 to 25 cm. The sampling point was at 8 cm from the plant perpendicular to the line of plants. In each of the two pepper crops, measurements were conducted at the beginning and at the end of the crop.

Mineral N (NO3-−N and NH4+–N) content of the soil was determined immediately before and at the end of each of the two crops. Sampling in each plot was located at 5 and 60 cm from the plant perpendicular to the line of plants. Soil mineral N was calculated as: 0.65 × position 5 cm + 0.35 × position 60 cm (Soto et al., 2015). The soil was sampled in each position to a depth of 70 cm in three depth intervals (10–30, 30–50, 50–70 cm). Soil mineral N content was determined following extraction with potassium chloride (40 g moist soil: 200 mL 2 mol L−1 KCl). NO3- and NH4+ concentrations in the extracts were determined with an automatic continuous segmented flow analyser (Model SAN++, Skalar Analytical B.V., Breda, The Netherlands).

Crop dry matter production and yield

Crop dry matter production and yield was determined from eight plants, in an area of 4 m2, in each plot. Plants were removed at ground level and the fresh weight material of all leaf, stem and fruit was determined. Dry weight was obtained after oven-drying subsamples at 65 °C. Mass of pruned material was determined as described above. Subsamples of leaf, stem and fruit were individually ground and sequentially in a knife mill and ball mill. Total N content (%) was determined (Rapid N, Elementar Analysensysteme GmbH, Hanau, Germany). The mass of N in leaf, stem and fruit was calculated from %N and mass of dry matter of that component. Crop N uptake (kg ha-1) was the sum of N in leaf, stem and fruit.

Root analyses

Towards the end of both crops, on 31 January 2017 and on 15 February 2018, soil cores were taken at 10 cm from the plant (P1) and at 30 cm from the plant (P2), perpendicular to the line of plants. In each position, the gravel mulch layer and 10-20, 20-30 and 30-40 cm depth layers were sampled; depth is expressed relative to the surface of the gravel mulch. A soil auger of 4.5 cm internal diameter was used for the gravel mulch layer and an auger of 3 cm internal diameter for the 10-20, 20-30 and 30-40 cm soil layers. Two replicate cores per sampling position were collected in each of four replicated plots of each of the two tillage treatments.

Roots of each gravel and soil sample were washed and were dyed with neutral red. Washed roots were scanned (Epson Perfection V800, Seiko Epson Corporation, Nagano, Japan) at 400 dpi in grey scale. The WinRHIZO Reg 2016 software (Regents Instruments Inc., Quebec, Canada) was used to measure root length in each sample. Relative root length distribution in each soil layer, relative to the whole soil profile sampled, was calculated as the percentage of root length in each soil layer divided by the total root length in the whole soil profile.

Transparent minirhizotron tubes were installed to non-destructively monitor root dynamics of sweet pepper throughout the 2017-18 crop. Installation of tubes occurred one year before, in July 2016, to allow for soil stabilization. The minirhizotron tubes were installed vertically to a depth of 48 cm from the surface of the gravel mulch, and at a distance of 10 cm from the plant in the direction of the line of plants. The tubes were of polymethyl methacrylate and were 60 cm long with 6.4 cm internal diameter. PVC caps were glued in the bottom of the tubes. The part of the tube that protruded above the gravel mulch layer was wrapped in aluminium tape to prevent light penetration. Two images (22 × 19 cm, 300 dpi) were taken per tube, the first one from the surface of the gravel mulch to a depth of 22 cm (0-22 cm, hereafter), and the second one from 22 to 44 cm depth (22-44 cm, hereafter). The 0-22 cm image comprised the 10 cm of the gravel mulch and the first 12 cm of the imported soil; the 22-44 cm image comprised the rest of imported soil and some of the original soil. Tube images were scanned (CI-600 Root Scanner, CID Inc., Camas, WA, USA) at 300 dpi and roots were digitized and analysed for root length per square meter of soil using the WinRHIZO Tron 2019 software (Regents Instruments Inc.). During the 207-2018 crop, tube images were taken every 43 days, on average; however, during the first 90 days of the crop, it was every 26 days on average.

Data analysis

In each of the two crops, differences in measured parameters between the tillage and no tillage treatments were tested by factorial analysis of variance (ANOVA) and pairwise LSD tests. Significant differences were established at p<0.05. Factors of ANOVA included block, tillage treatment and soil layer. Root length dynamics of minirhizotron tubes were evaluated by repeated-measure analysis of variance (RM-ANOVA). The ANOVA analysis were performed with the STATISTICA 13 software (TIBCO Software Inc., Palo Alto, CA, USA).