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a Inst. of Plant Sci., Swiss Federal Inst. of Technol., Universitätstrasse 2, CH-8092 Zurich, Switzerland
b Swiss Federal Res. Stn. for Agroecol. and Agric. (FAL), Reckenholzstrasse 191, CH-8046 Zurich, Switzerland
* Corresponding author (peter.stamp{at}ipw.agrl.ethz.ch)
Received for publication June 7, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: BD, bulk density • CT, conventional tillage • LDD, length per diameter-class distribution • MD, mean root diameter • NT, no-tillage • RLD, root length density
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Tillage-induced differences in the soil nutrient status may also have a significant impact on root growth. No-tillage often results in the stratification of soil nutrients, especially of immobile elements such as P (Logan et al., 1991; Holanda et al., 1998; Crozier et al., 1999), thus inducing a higher RLD in the topmost layer under NT (Gregory, 1994; Cannell and Hawes, 1994).
Information about the impact of tillage on the root growth of small-grain cereals is scarce. The root growth of winter wheat was similar at later growth stages under NT and CT in temperate regions (Ellis and Barnes, 1980; Dzienia and Wereszczaka, 1999), but the impact of tillage on root growth may depend on the length of time since the implementation of the NT system (Pearson et al., 1991).
The spatial distribution of the roots reflects the crop's potential to take up nutrients and water. The RLD is often used to describe root distribution. In general, the RLD is higher in the row than in the midrow (Rubino and Franchi, 1990). Roots are more abundant in the upper soil layer (Wilhelm et al., 1982; Barraclough et al., 1991; Dzienia and Wereszczaka, 1999). The impact of tillage on root distribution was evident in the layer affected by plowing (Gerik et al., 1987; Rasmussen, 1991). The roots in the NT system accumulated to a greater extent from 0 to 5 cm compared with the roots in the CT system (Chan and Mead, 1992; Rasmussen, 1991; Wulfsohn et al., 1996); the opposite was true in lower layers (Chan and Mead, 1992; Rasmussen, 1991). However, different effects of tillage intensity on the patterns of root distribution have also been reported (Ehlers et al., 1983; Cornish, 1987).
The root diameter may be indicative of the effects of soil strength on root growth and affects the utilization of nutrients in the soil. Sidiras et al. (2001) reported thicker barley (Hordeum vulgare L.) roots under CT than under NT, in contrast to Braim et al. (1992), while Pearson et al. (1991) found no effect of tillage on the diameter of wheat roots.
Impacts of tillage on the morphology and spatial distribution of wheat roots must be investigated further. At two sites in the Swiss midlands, we investigated the morphology and distribution of wheat root systems at harvest in NT and CT systems. With the analyses of fully developed root systems, the aims of the present study were to determine the impact of tillage intensity on the exploration of the soils by the roots.
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MATERIALS AND METHODS |
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Winter wheat after maize was studied. ‘Runal’ [breeder: Swiss Federal Research Station for Agroecology and Agriculture (FAL), Zurich, Switzerland] was used, a high-quality variety of winter wheat with an average yield potential. It was sown at Zollikofen on 4 Nov. 1996, 28 Oct. 1997, and 5 Dec. 1998 and at Schafisheim on 4 Nov. 1996, 24 Oct. 1997, and 9 Nov. 1998.
Two tillage systems (CT and NT) were compared. The CT treatment was plowed to a depth of 25 cm, and the wheat was sown with a rototiller rotary harrow (Rau, Weiheim, Germany) and drill (BS V6 drill with disc openers, Nodet, Montereau, France). In the NT plot, wheat was sown into the dead maize mulch using a "NT 750 A" no-till drill (Deere and Co., Moline, IL, USA) with a single-disc opener. In all the tillage systems, 400 grains m–2 was sown. The sowing depth was from 3 to 4 cm. The distance between rows was 14.3 cm at Zollikofen and 12.5 cm at Schafisheim in the CT plots and 16.6 cm in the NT plots.
According to the guidelines for fertilization of the Swiss Federal Research Stations (Walther et al., 2001), 22 kg P ha–1 and 8 kg Mg ha–1 were broadcast when the wheat was sown, with the exception of 1998–1999 when a soil test (Walther et al., 2001) had shown that there were enough reserves in the soil.
Nitrogen (ammonium nitrate) was broadcast three times. Based on the soil mineral N analyses (from 0 to 90 cm), N was applied for the first time at the beginning of tillering [BBCH Stage 25 (Lancashire et al., 1991)] at a rate of 120 kg mineral N ha–1. At the one-node stage (BBCH Stage 31) and at the beginning of the last leaf stage (BBCH Stage 37), N was applied on the basis of a quick test, i.e., analyzing the nitrate concentration in the sap of the basal stem (Lonza, Basel, Switzerland). Nitrogen fertilizer was applied at rates of 30 to 50 kg N ha–1 at both stages depending on the test results. The total amount of fertilizer N was 150 to 160 kg N ha–1.
Because wheat was sown right after the maize harvest, the plots were almost weed-free, and selective herbicides were used. At the BBCH Growth Stage 29 (Lancashire et al., 1991), 2.5 L ha–1 of the combined herbicides isoproturon {N,N-dimethyl-N'-[4-(1-methylethyl)phenyl]urea}, fluoroglycophen carboxymethyl {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoate}, and triasulfuron {2-(2-chloroethoxy)-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]benzenesulfonamide} (Lumeton, Novartis) and 1 L ha–1 of fluroxypyr {[(4-amino-3,5-dichloro-6-fluoro-2-pyridinyl)oxy]acetic acid} (Starane 180, Novartis) were sprayed. At the BBCH Growth Stage 31, 0.5 L ha–1 of the shortener trimexapac [4-(cyclopropylhydroxymethylene)-3,5-dioxocyclohexanecarboxylic acid] (Moddus, Novartis) was applied. At the BBCH Growth Stage 45, 1 L ha–1 of fenpropimorph {rel-(2R,6S)-4-[3-[4-(1,1-dimethylethyl)phenyl]-2-methylpropyl]-2,6-dimethylmorpholine} and difenoconazol {1-[2-[2-chloro-4-(4-chlorophenoxy)phenyl]-4-methyl-1,3-dioxolan-2-ylmethyl]-1H-1,2,4-triazole} (Avenir, Novartis) was sprayed against fungal diseases. At anthesis, 1 L ha–1 of the fungicide tebuconazol {
-[2-(4-chlorophenyl)-ethyl]--(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol} (Horizont, Bayer) and 0.0025 L ha–1 of the insecticide diflubenzuron {N-[[(4-chlorophenyl)amino]carbonyl]-2,6-difluorobenzamide} (Dimilin, Maag) were sprayed.
Root Sampling and Analysis
Roots were sampled at the maturity of wheat (BBCH Stage 92) using the soil-core method (Böhm, 1979). Four locations were chosen in each plot, and at each location, there were two sampling positions perpendicular to the crop row. At each position, the root samples in the row and between two rows were taken, using a vehicle-mounted soil coring system (Humax-Bohrsonden, Luzern, Switzerland) with a plastic tube (5 cm in diameter, 25 cm long) for sampling roots. The soil cores were 25 cm long and were taken from 0 to 100 cm in the row and from 0 to 50 cm in the midrow. The sampling locations were in areas free of wheel tracks.
For the root analysis, the soil cores were divided as follows: at 5-cm intervals from 0 to 30 cm, at 10-cm intervals from 30 to 60 cm, at 15-cm intervals from 60 to 90 cm, and from 90 to 100 cm. Using a semiautomatic hydropneumatic elutriation system (Gillison's Variety Fabrication, Benzonia, MI, USA), equipped with a 290-µm sieve (Smucker et al., 1982), the roots were washed and then stored at –20°C.
Before scanning, the roots were rinsed with tap water; organic debris and other materials were removed by decanting the samples. The cleaned roots were stained with fuchsin dye (Pararosaniline P-1528, Sigma Chemical Co., St. Louis, MO, USA) for at least 12 h at 4°C. The roots were distributed in a rectangular tray with a glass bottom. To maintain a uniform distribution, the roots were left in a thin layer of water containing a few drops of a dilute solution of Brij 35 (Polyoxyethylenlaurylether, Merck Schuchardt OHG, Hohenbrunn, Germany) (20% density). Then the roots were scanned, and the gray-scale images (resolution 600 by 600 dpi) were stored. Using the computer program ROOT DETECTOR (Walter and Bürgi, 1996), the root images were processed to determine the length and diameter of the roots. The program is based on an algorithm for the segmentation and local description of elongated, symmetric line-like structures developed by Koller et al. (1995). The length and mean diameter were calculated separately for each root segment. Thus, the total root length can be divided into user-defined diameter classes, which yield the length per diameter-class distribution (LDD). The theoretical lower limit of optical resolution was 127 µm, which is three times the pixel size of the scanner (42.33 µm at a resolution of 600 dpi); a root must be at least three pixels (dots) wide to be detected. The RLD was calculated by dividing the root length by the volume of the corresponding soil-core section. The MD was calculated from the projected surface area and length of the root.
Soil Sampling and Analysis
Soil samples were taken from the sampling locations to determine the soil BD at the time of sampling. A steel ring (467 cm3 and 5 cm high) was used to take samples every 5 cm from 0 to 30 cm. The fresh soil samples were weighed and dried at 105°C for 48 h. After drying, the soil samples were weighed, and the soil moisture content and soil BD were calculated.
Experimental Design and Statistical Analysis
The experiment was designed as a randomized complete block with three replicates. All the crops in the rotation were grown in each year. The plots were 12 by 35 m2. The soil BD was analyzed according to a split-plot design with tillage as the main-plot factor and soil depth as the subplot factor. The RLD and MD data were analyzed according to a split-split-plot design with year and site as the main plot factors, tillage as the subplot factor, and horizontal distance from the plant and depth as sub-subplot factors.
When subsamples were taken within the plots, their values were pooled for statistical analysis. An analysis of variance (ANOVA) of the plot averages was conducted, and the significant differences were separated by orthogonal contrasts or the Fisher's protected LSD test when appropriate. Some data sets were transformed to meet the assumption of ANOVA. However, all the results are presented in their original scale of measurement. Significant differences were accepted at P 
0.05, unless stated otherwise.
The data were analyzed by means of ANOVA using the procedures GLM (general linear models) and CORR (correlation analysis) of the SAS system (SAS Inst., Cary, NC, USA).
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RESULTS |
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The tillage system did not have a significant effect on the RLD across all the other factors; the average RLD under CT was slightly higher than under NT (Table 3). The effect of tillage on the overall RLD was consistent in each environment (year x site) and at all sampling positions because there were no significant interactions between the tillage system and year, site, and position.
However, a significant interaction of tillage and soil depth showed that the effect of tillage on the RLD depended on the soil layer. The RLD was significantly higher in the topmost soil layer (0 to 5 cm) (P < 0.001), similar from 5 to 10 cm, and lower from 10 to 30 cm under NT than under CT. Tillage had no effect on RLD below 30 cm. The trend was the same in the row as in the midrow (Fig. 1).
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Mean Root Diameter of Winter Wheat at Harvest
The MD of winter wheat was not affected by year, whereas site had a strong effect (P < 0.001); the MD of roots in loamy silt (Zollikofen) was 5% higher than in sandy loam (Schafisheim) (Table 3).
The MD was significantly higher in the row than in the midrow (P < 0.001) (Table 3) due to higher values from 0 to 25 cm; below 25 cm, the MD was the same at all depths (Fig. 2). No consistent trend was found in the row at all soil depths, whereas the MD in the midrow was similar from 0 to 25 cm and increased below 25 cm. The largest MD was found at depths of 0 to 5 cm in the row.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The higher RLD and larger MD at Zollikofen were probably due to the soil physical parameters. The soil at Zollikofen has more silt but less sand than the soil at Schafisheim, but the soil BD was higher at Zollikofen than at Schafisheim. A coarse aggregate system may result in shorter roots (Keita and Steffens, 1989; Donald et al., 1987) due to fewer macropores (de Freitas et al., 1999) and may also lead to thinner roots (Keita and Steffens, 1989). A higher soil BD reduces root length (Gerard et al., 1982; de Freitas et al., 1999) but increases the diameter of the root (Chassot and Richner, 2002). These results support the findings reported here to some extent; however, those studies were conducted in growth chambers, whereas ours were conducted in the field.
In general, the RLD of wheat decreases with soil depth and with the distance from the row (Rubino and Franchi, 1990). Our results show that the RLD decreased more sharply in the row than in the midrow from 0 to 10 cm. This pattern of vertical distribution of wheat roots has also been reported by others (Wilhelm et al., 1982; Barraclough et al., 1991; Dzienia and Wereszczaka, 1999).
The MD was larger in the row because of more thicker roots (i.e., seminal roots, crown roots, and the higher-order laterals roots) compared with in the midrow. The roots near the base of the plants are the oldest and, thus, the thickest (Pearson et al., 1991). As a consequence, the largest MD was found at depths from 0 to 5 cm in the row. There was no obvious trend in the MD below 5 cm, possibly due to the heterogeneous soil and the specific characteristics of the root (Hoad et al., 2001). In the midrow, the MD was smaller at depths from 0 to 25 cm and larger below 25 cm, as also reported by Holanda et al. (1998).
As in other studies of winter wheat under similar climatic conditions (Ellis and Barnes, 1980), NT and CT did not have a significant effect on the RLD. This may be due to the fact that the roots of winter wheat are insensitive to variations in the soil, as caused by the tillage system, or that the soil parameters did not differ significantly or the effects were balanced. The soil BD may have the strongest negative effect on root growth compared with the other factors (Ellis and Barnes, 1980). In our study, the soil BD was significantly higher under NT than under CT, which may affect the RLD. This may be counteracted by the macropores in the soil, which are usually more abundant and evenly distributed under NT (Ehlers et al., 1983; Ellis and Barnes, 1980; Martino and Shaykewich 1994). Both the soil moisture content and the nutrient contents were sufficient in the plots of our experiment, and thus, they could not result in different RLD values between NT and CT.
Similar to Italian ryegrass (Lolium multiflorum Lam.) (Rasmussen, 1991), the vertical distribution of the RLD was influenced by the tillage system in the uppermost soil layer (0 to 10 cm), with higher values for NT than for CT. From 10 cm to 30 cm (down to the plowpan), the values were higher under CT than under NT. Similar trends have been reported elsewhere for wheat (Chan and Mead, 1992; Wulfsohn et al., 1996). However, some studies showed an opposite trend (Cornish; 1987) or found no difference between NT and CT (Ehlers et al., 1983). This may be due to the different physical and chemical characteristics of the soil profile.
The great soil strength under NT may have contributed to our finding (Cannell, 1985; Cannell and Hawes, 1994). Due to the stratification of the soil P in the NT system, it was more abundant in the upper layers of the soil in the NT system and much less abundant in the lower layers, in contrast to in the CT system (Crozier et al., 1999; Franzluebbers and Hons, 1996; Holanda et al., 1998). This may also account for the differences in the RLD in the soil profile in both tillage systems.
The time at which the tillage system was implemented can have a significant impact on the root growth of winter wheat. Soil properties differ, depending on the time since the implementation of the tillage system (Rhoton, 2000). In 1997, there was no difference between the tillage systems, perhaps because the time since the implementation of NT was too short to markedly affect the soil environment. With each year, the difference between the NT and CT systems increased. From 1998 to 1999, the soil layer with a higher RLD increased under NT. Some long-term studies showed a similar pattern of root distribution as found in 1998 and 1999 (Lal et al., 1989; Blevins and Frye, 1993; Ruggiero et al., 1990).
Compared with RLD, MD was affected to a much greater extent by soil factors. However, tillage did not have a direct effect on the average MD, despite changes in the soil environment under NT. Pearson et al. (1991) also reported that tillage had no effect on the diameter of winter wheat roots at anthesis in contrast to barley roots (Sidiras et al., 2001; Braim et al., 1992).
However, the intensity of tillage altered the development of the MD in the soil profile. In the row, where a large number of the roots may be seminal roots, the larger MD in the top layer (0 to 15 cm) and the smaller MD from 25 to 50 cm in the NT soil may be related to a higher soil BD compared with CT soil. The relatively higher proportion of seminal roots with larger diameters was restricted to the topsoil. Chassot and Richner (2002) and Veen and Boone (1981) also reported thicker (maize) roots as a result of greater mechanical interference. A possible stratification of P in the soil under NT may play a role in the larger MD in the topsoil, as reported by Chassot and Richner (2002) and Veen and Boone (1981).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Effect of tillage on the RLD and MD were found mainly from 0 to 30 cm and from 0 to 50 cm down the soil profile, respectively. In general, NT resulted in a higher RLD in the topsoil and a lower RLD at greater depths as compared with CT. A similar trend, induced by tillage intensity, was also found for the MD in the row. However, the MD in the midrow was higher only from 0 to 10 cm under NT than under CT.
Differences in RLD down the soil profile between NT and CT increased with the years as the soil layer that contained more roots under NT was getting thicker. This finding indicated that the soil condition under NT is possibly being improved with the time after NT implementation. Therefore, adapted field management techniques such as fertilization, etc., should be devised in consideration of these changes.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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