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a Inst. of Plant Sci., Swiss Fed. Inst. of Technol., Universitätstrasse 2, CH-8092 Zurich, Switzerland
b Swiss Fed. Res. Stn. for Agroecol. and Agric. (FAL), Reckenholzstrasse 191, CH-8046 Zurich, Switzerland
c Current address: Swiss Fed. Inst. for Forest, Snow, and Landscape Research (WSL), Zürcherstrasse III, CH-8903 Birmensdorf, Switzerland
* Corresponding author (peter.stamp{at}ipw.agrl.ethz.ch)
Received for publication March 9, 2004.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: BD, bulk density • BSF, banded starter fertilizer • CT, conventional tillage • MD, mean root diameter • NT, no-tillage • RLD, root length density • SF, starter fertilizer
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Maize is sensitive to chilling, and the root growth of maize seedlings is often impeded by low temperatures (Richner et al., 1996; Schröder et al., 1996). Stamp et al. (1997) pointed out that a mild chilling stress often occurs during the early stages of maize growth in temperate humid climates and that it might affect the metabolism, architecture, and morphology of the roots. Chassot et al. (2001) reported that the temperature of the topsoil under NT may be the main reason for the poor growth of the roots and shoots of maize seedlings compared with CT under temperate humid conditions. A higher soil BD can decrease root length (Hill, 1990; Logsdon et al., 1987) and trigger the formation of lateral roots (Ball Coelho et al., 1998) because greater soil strength impedes root growth (Gerard et al., 1982; Logsdon et al., 1987). On the other hand, a more extensive system of macropores under NT can effectively counteract the impact of greater soil strength and, thus, have a beneficial effect on root growth (Rasse and Smucker, 1998; Wang et al., 1986).
Morphology and distribution of the roots are important traits for studying root growth and the uptake of water and nutrients by a crop. Few studies have dealt with the effects of tillage systems. According to the limited number of publications, maize roots tend to be thicker under NT (Barber, 1971; Holanda et al., 1998; Pereira de Mello Ivo and Mielniczuk, 1999). The reported differences in RLD throughout the soil profile between CT and NT systems are inconsistent, i.e., RLD in NT is either greater (Hilfiker and Lowery, 1988; Holanda et al., 1998) or less (e.g., Barber, 1971; Karunatilake et al., 2000) than in CT or similar in the two tillage systems (Hughes et al., 1992; Raczkowski, 1989). Root length density was often greater from 0- to 10-cm soil depth under NT due to the greater availability of water and nutrients (Ball Coelho et al., 1998; Pereira de Mello Ivo and Mielniczuk, 1999). However, at lower soil depths, the effect of tillage on the RLD was inconsistent.
No-tillage in combination with crop residues left on the soil surface may lower the temperature of the topsoil (Kovar et al., 1992). Cool soils may limit root growth and the uptake of nutrients by roots (Mackay and Barber, 1984). Therefore, application of SF (usually N and P) in a band near the seeds should be beneficial for the growth of seedlings due to the greater availability of nutrients in the soil area near their roots; an enhanced nutrient uptake sometimes results in a higher crop yield, especially in NT systems (Bullock et al., 1993; Vetsch and Randall, 2000, 2002).
The general effects of N and P on the growth of maize roots have been studied extensively. Nitrogen fertilization can increase root length and root surface area and decrease root mass per unit length (Anderson, 1987; Costa et al., 2002). Phosphorus can enhance number, length and diameter of the roots (Veen and Boone, 1981; Chassot and Richner, 2002); however, P did not always affect the total root length (e.g., Kuchenbuch and Barber, 1987). In general, the effects of N or P on the growth of maize roots are still unclear, especially with regard to their combined effects.
It is well known that roots tend to proliferate in nutrient-enriched soil zones (Drew et al., 1973); Russell (1977) refers to this as a "compensatory response." The results of pot experiments showed that maize roots are longer and thinner in zones that are rich in P or in NH4+ (Zhang and Barber, 1992, 1993). Under field conditions, the roots were longer in the area of a fertilizer band, but the length of the whole root system was not affected by fertilizer banding at early growth stage (Kaspar et al., 1991; Marsh and Pierzynski, 1998) or later growth stages (Durieux et al., 1994). Most of the previous studies compared the effects of different amounts of SF on root growth. Few studies have focused on the spatial distribution of maize roots as influenced by the asymmetrical application of SF to one side of the crop row.
In field experiments located in the Swiss Midlands comparing CT and NT systems, MD was less and RLD was greater in CT compared with NT at the V6 growth stage of maize (Chassot et al., 2001). Furthermore, RLD was greater in the SF band compared with the other side of the seed row where no SF was applied.
It is unclear whether these early-season effects of tillage and BSF on root growth persist until later growth stages of maize. To address this question, we designed this study to specifically investigate the morphology and distribution of maize root systems at anthesis as affected by tillage systems and BSF.
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MATERIALS AND METHODS |
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The crop rotation was winter wheat (Triticum aestivum L.), oilseed rape (Brassica napus L. var. napus), winter wheat, and maize. A white mustard (Brassica alba L. cv. Martigena) cover crop was grown between the winter wheat and maize. All crops of the rotation were grown in each year. Throughout the crop rotation, all the crop residues were left on the soil surface.
The early maturing maize hybrid ‘Granat’ (KWS, Einbeck, Germany; Menzi et al., 1996) was planted at a density of 100000 plants ha–1 in CT and 105000 plants ha–1 in NT on 3 May 1997 and 7 May 1999 in Zollikofen and on 2 May 1997 and 7 May 1999 in Schafisheim. Two weeks before sowing, the NT plots were sprayed with 3 L ha–1 glyphosate [N-(phosphonomethyl)glycine; Roundup, Monsanto, St. Louis, MO) and 10 kg ha–1 ammonium sulfate to eliminate weeds. Newly germinating weeds were controlled by spraying 1.5 kg ha–1 a.i. of atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] and 1.92 kg ha–1 a.i. of metolachlor[(S)-2-chloro-N-(2-ethyl-6-methyl-phenyl)-N-(2-methoxy-1-methyl-ethyl) acetamide)] before the emergence of maize on all the plots.
The maize seeds were dressed (90 mL per 50000 grains) with the systemic insecticide imidacloprid {1-[(6-chloro-3-pyridinyl)methyl]-N-nitro-2-imidazolidinimine; Gaucho, Bayer, Leverkusen, Germany} before sowing. The sowing depth was 3 to 4 cm.
Basal fertilization consisted of 35 kg ha–1 P, 133 kg ha–1 K, and 18 kg ha–1 Mg, broadcast before tillage in CT and superficially applied in NT at the same time. Starter fertilizer, a mixture of diammonium phosphate and NH4NO3, was banded at rates of 30 and 17 kg ha–1 of N and P at planting, 5 cm from the seeds on one side of each row and 5 cm below the seeds. The amounts of nutrients applied with the basal and starter fertilization, according to local recommendations, were the same for both tillage systems. Nitrogen was sidedressed as NH4NO3 at the V6 stage based on a soil Nmin test (Wehrmann and Scharpf, 1979) at the V4 stage; if the sidedressing N application exceeded 80 kg ha–1, it was split into two applications about 2 wk apart. The rate of N was calculated as follows: 200 kg N ha–1 minus Nmin. On average, the N rates were the same in both tillage systems; across all years, sites, and tillage systems, 107 kg N ha–1 was applied as sidedressings.
Root Sampling and Analysis
In each plot, three sampling locations were selected at anthesis. At each sampling location, the first three consecutive plants in a row were cut at ground level to determine the characteristics of the shoot. Afterward, by means of soil cores (Böhm, 1979), the roots next to the middle plant were sampled at two sampling positions in a line perpendicular to the row (9.5 cm from each side of the row, i.e., with and without BSF). The sampling periods were from 29 July to 2 August in 1997 and from 21 to 24 July in 1999 in Schafisheim and from 4 to 6 August in 1997 and from 26 to 28 July in 1999 in Zollikofen.
The intact soil cores (25 cm in length and 5 cm in diameter) were taken with an auger attached to a vehicle-mounted soil-coring system (Humax-Bohrsonden, Lucerne, Switzerland), with minimum destruction of roots; the sampling depth was 25 cm. All of the sampling positions were in areas that were free of wheel tracks. Soil cores were conserved at –20°C. For the analysis, the soil cores were separated every 5 cm after thawing. The roots were washed out of the soil section using a semiautomatic hydropneumatic elutriation system (Gillison's Variety Fabrication Inc., Benzonia, MI, USA), equipped with a 290-µm sieve (Smucker et al., 1982). After washing, the root samples were stored at –20°C.
The thawed roots were stained with fuchsin dye (Pararosaniline P-1528, Sigma Chemical Co., St. Louis, MO, USA) for at least 12 h at 4°C, and organic debris and other extraneous materials were removed from the samples by decanting. Using tweezers, the stained roots were placed on a rectangular tray with a glass bottom and of a size appropriate for the scanning area. To distribute roots uniformly, they were barely covered with tap water and several drops of surfactant Brij 35 (Polyoxyethylenlaurylether; Merck-Schuchardt, Hohenbrunn, Germany; 20% concentration). Using a flatbed scanner with a top-light adaptor (ScanJet 4C/T, Hewlett-Packard, Palo Alto, CA, USA), roots were scanned, and 8-bit grayscale images (resolution 600 by 600 dpi) were produced. The subsamples of the roots, taken from the same sampling position at the three sampling locations in each plot, were pooled for scanning. Using the computer program ROOT DETECTOR (Walter and Bürgi, 1996), root images were analyzed to determine the length and diameter of roots. The system's theoretical lower limit of resolution was 127 µm, which is three times the pixel size of the scanner (42.33 µm at a resolution of 600 dpi) because a root must be at least three pixels (dots) wide to be detected by the program.
Root length and root mean diameter were calculated separately for each measured root segment. Thus, the total measured root length was divided into user-defined diameter classes to give the length per diameter-class distribution of the roots. In this study, we used the following diameter classes: 0 to 100, 100 to 200, 200 to 300, 300 to 400, 400 to 500, 500 to 600, 600 to 700, 700 to 800, 800 to 1000, 1000 to 1200, 1200 to 1400, 1400 to 1600, 1600 to 3200, and >3200 µm. Root length density was calculated by dividing the total root length by the volume of the corresponding soil core section. Mean root diameter was calculated from the root surface area and the root length.
Experimental Design and Statistics
Root data were subjected to ANOVA analyzed according to a split-split plot design, with tillage as main-plot factor, BSF as subplot factor, and soil depth as sub-subplot factor, combined over years and sites (McIntosh, 1983).
If subsamples were taken from the same plots, they were pooled for statistical analysis. Means were separated by orthogonal contrasts or Fisher's protected LSD test. Significant differences were accepted at P 
0.05 unless otherwise stated.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Banded SF at sowing significantly affected the RLD at anthesis (Table 4). The effect of BSF interacted with soil depth in all combinations of years and sites; interactions of depth x BSF x tillage system were found only in 1999 (Table 4).
In 1997, the effect of BSF on RLD was very strong under both tillage systems, and the strongest effect was found to a maximum depth of 15 cm (P < 0.01) under NT (Fig. 2). As a consequence, no depth x BSF x tillage system interaction occurred in this year. In 1999, however, the effect of BSF was relatively weak. It significantly enhanced RLD in the uppermost 5 cm at both sites but decreased it in the layer from 5 to 10 cm of the NT soil in Schafisheim while it tended to decrease RLD in the CT soils in both sites, especially in Schafisheim (Fig. 2). The reason for the only significant interaction between tillage and BSF was that BSF decreased RLD in CT soil in the uppermost 10 cm, whereas it increased RLD in NT in the top 5 cm of soil. Across tillage systems and soil profile, BSF at sowing significantly enhanced the RLD in the SF band area at anthesis in 1997 whereas no impact (in Zollikofen 1999) or even a decrease in RLD (in Schafisheim 1999) was observed (Tables 4 and 5).
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Maize Root Diameter of Maize at Anthesis
Both year and site had significant effect on MD (Table 3). In 1997, the MD was larger than in 1999. The roots in Zollikofen were thinner than in Schafisheim. However, there was no interaction between year and site on MD (data not shown). In accordance to RLD, an individual ANOVA for each year-site combination was performed to analyze the effect of tillage, BSF, and soil depth on MD (Table 6). In Schafisheim, soil depth affected MD significantly, and the MD increased gradually from 0 to 25 cm while in Zollikofen, it did not affect MD distinctly (Tables 6 and 7).
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DISCUSSION |
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Throughout the topsoil, the RLD generally decreased and the MD increased with increasing soil depth. The main reasons might be the general decrease in the number of maize roots with depth (Liedgens and Richner, 2001) and possibly a reduced branching of the roots at lower depths as a result of unfavorable growth conditions.
In the present study, the RLD at anthesis was smaller under NT than under CT. This finding is consistent with the result of another study on maize root growth at an early growth stage (V6) in the same experiment (Chassot et al., 2001). Optimum amounts of nutrients were applied so that soil physical parameters are assumed to have been the main reason for the different root growth in NT and CT.
In general, the temperature of the topsoil is lower in NT than in CT, especially during the early stage of maize growth, whereas the soil moisture content and soil BD near the soil surface are greater (Chassot et al., 2001). Such changes in soil conditions are assumed to affect root growth and, indirectly, nutrient uptake, shoot growth, and crop yield.
A number of studies showed that a greater soil BD, which results in greater soil strength, generally impedes root growth (Gerard et al., 1982; Logsdon et al., 1987). In this experiment, the significantly greater soil BD under NT than under CT in the 0- to 25-cm layer (data not shown) may help to explain our observations on maize root response to tillage systems. On the other hand, the potentially positive effect of increased soil moisture on root growth under NT (Karunatilake et al., 2000; Lampurlanés et al., 2001) may have been limited by abundant precipitation in the Swiss Midlands. Meanwhile, the greater number of continuous macropores under NT might help to counteract the adverse effect of soil BD (Chassot et al., 2001) and have a beneficial effect on root development (Wang et al., 1986). However, the favorable effect of soil macropores may have been limited by the fact that NT was applied only a few years ago.
Low soil temperature often has a negative effect on the root growth of maize seedlings in cool temperate regions (Richner et al., 1996; Stamp et al., 1997). The lower temperature of the topsoil under NT was considered to be a main cause of poorer maize root growth until the V6 stage in a previous study in our experiment (Chassot et al., 2001). However, Chassot et al. (2001) showed that differences in the topsoil temperature (5-cm depth) between CT and NT in this experiment steadily decreased from mid-May (VE stage) to mid-June (V6 stage); at mid-June, the difference in daily mean topsoil temperatures between CT and NT at both sites was smaller than 0.4°C. It is hypothesized that during later growth stages of maize, the soil covered by the maize canopy overrides the effects of different soil conditions on topsoil temperature in these tillage systems. Thus, it is assumed that differences in maize root growth between CT and NT at anthesis are rather caused by an aftereffect of growing conditions during early growth than by soil temperature effects at later growth stages. This suggests that the root growth of the seedling can be decisive for the shape of the mature root system. Correspondingly, it was also found that the root traits of seedlings are important for the yield of maize silage (Richner et al., 1997). Attention should, therefore, be paid to improving the soil environment for seedlings under NT conditions.
The much greater decrease in the RLD under NT in Zollikofen compared with Schafisheim was probably related to the soil type (Chassot et al., 2001). The soil in Zollikofen is a loamy silt and poorly drained, whereas the sandy-loam soil in Schafisheim is coarser and well drained. Poorly drained silty soils tend to hinder root growth in NT systems (Hughes et al., 1992; Karunatilake et al., 2000), whereas well-drained coarse soils are more advantageous for root growth in NT systems (Hilfiger and Lowery, 1988). In contrast, Raczkowski (1989) reported an opposite effect of soil types on root response to tillage in field experiments in a continental climate.
Between 1997 and 1999, the rooting pattern in NT showed a temporal development. On the loamy silt of Zollikofen, the RLD was greater under CT than under NT in the whole sampled soil layer in 1997; in 1999, however, the RLD was greater in the top 5 cm under NT than under CT; below 5 cm, it was much greater under CT than under NT. On the sandy loam in Schafisheim, the RLD was greater under NT than under CT in the top 5 cm; and in the top 10 cm of soil in 1997 and 1999, respectively. Comparable results were reported for the same soil types (Ball Coelho et al., 1998). It is assumed that such temporal changes in root growth patterns in NT are a consequence of the gradual shift in soil conditions with time after the introduction of NT. In general, the development of a higher soil BD in NT is assumed to be the main reason for the accumulation of roots in the uppermost soil layer (Ball Coelho et al., 1998; Cannell and Hawes, 1994). The stratification of soil nutrients under NT may also contribute to the concentration of roots there.
The MD was larger under NT than under CT, probably for the same reasons as for the smaller RLD: (i) greater soil BD and (ii) lower soil temperature during the early growth stage. A few studies have similarly found thicker roots under NT than under CT (Barber, 1971; Holanda et al., 1998; Pereira de Mello Ivo and Mielniczuk, 1999).
In accord with the response of early growth of maize roots to BSF (Chassot et al., 2001), our results also showed that at both sites in 1997, the RLD at anthesis in the top 0 to 25 cm was still significantly greater in the soil area in which SF had been banded. This may suggest that an early placement of an adequate amount of N and P fertilizer is effective for the efficient growth and positioning of the young root system in cool temperate regions, with the effect lasting until flowering. That roots proliferate in soil zones with favorable conditions and do not grow well in unfavorable environments was suggested as a compensatory adjustment (Bingham and Bengough, 2003; Russell, 1977). Some experiments under semicontrolled conditions have shown that maize roots proliferate in areas with localized application of N or P fertilizer (e.g., Zhang and Barber, 1992, 1993). There are several reports that fertilizer banding increased root growth in the banded area at early (Kaspar et al., 1991) and later (Durieux et al., 1994) growth stages of maize in field trials. As well as RLD, SF also enhanced the MD in 1997. Our results are confirmed by some studies under controlled conditions that showed thicker roots in a localized nutrient area, i.e., N (Drew et al., 1973) and P (Chassot and Richner, 2002; Veen and Boone, 1981). Furthermore, it was reported that plants tend to have finer roots when the content of nutrients in the soil is low and the mobility of ions is limited (Fitter, 1996). In contrast, some experiments under controlled conditions (Zhang and Barber, 1992) or under field conditions (Anderson, 1987; Kaspar et al., 1991) showed that the application of fertilizer resulted in finer maize roots, similar to our finding in Schafisheim in 1999.
The effect of BSF on root growth was quite different between the two investigated years. In 1999, the effect was weak and even opposite to that in 1997. The reasons for the different effects of the BSF on the root growth at anthesis between years might be greatly related to the growing conditions of the maize seedlings. The relatively warm temperature in May of 1999 (15.3–15.6°C) (Qin et al., 2004) ensured a good growth condition for maize seedlings. As a consequence, the positive effect of SF on root growth decreased. On the other hand, the less optimum growth conditions in the spring of 1997 (about 13.3°C) (Qin et al., 2004) could have provided a base for a significant effect BSF. Richner et al. (1996) indicated that a relatively small decrease of 2 to 3°C in the topsoil temperatures around 15°C may markedly reduce root development in chilling-sensitive maize seedlings under field conditions.
The effect of BSF on the RLD was somewhat more pronounced in the NT plots in all environments. The crop residues, which were kept on the soil surface in NT, can decrease the availability of soil N (Blevins and Frye, 1993; Carter, 1994). This could explain, in addition to the hampered root growth in untilled soil, the stronger effect of SF in the NT plots.
The spatial pattern of root distribution differed, depending on whether or not SF was applied, because of an imbalance in nutrient distribution on both sides of the row. On the side enriched by fertilizer, the N and P contents in the 0- to 10-cm soil layer were increased, and thus, there were more roots in this layer. Therefore, the RLD was greatest at the soil surface and decreased gradually to 25-cm depth. In contrast, in the area without SF, the maximum RLD was found deeper in the soil (between 5- to 10-cm soil depths). Similar to our results, Barber and Kovar (1991) found the greatest RLD between 5- to 15-cm soil depth in the zone without fertilizer enrichment, and Liedgens and Richner (2001), using minirhizotrons in lysimeters, found a maximum RLD of maize at 25-cm depth.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Similarly to the effect of tillage on overall RLD, the grain yield and shoot biomass of NT maize in this study were on average significantly lower (by 7%) than those of CT maize; differences in these shoot parameters between tillage systems also strongly depended on the environment (Rieger, 2001). However, it is an open question whether and how the reduced root growth and modified root distribution in NT compared with CT had a direct negative effect on shoot growth and yield of maize, or whether indirect tillage effects on the availability of soil nutrients or on early shoot growth (via reduced topsoil temperatures) were more important for the reduced shoot performance in NT.
The BSF resulted in a significantly greater RLD and MD at anthesis in the fertilizer band area in 1997 while it was weak or even opposite in 1999. It was found in all NT plots, especially in the top 5 cm of soil, that BSF improved RLD. It can be concluded that an adequate placement of SF will be effective for the buildup of the early root system of maize, with a long-lasting effect until anthesis, especially at suboptimal temperatures for maize growth. However, it needs to be further investigated whether and how reliably the enhanced root growth by the BSF results in an improved shoot biomass and grain yield of maize in cool temperate area.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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