Root growth dynamics of spring cereals with discontinuation of mouldboard ploughing

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Abstract

A vigorous root system is essential for efficient use of plant nutrients. This paper focuses on root growth and its response to tillage changes in the most fertile soil horizon, 0–40 cm depth. The field experiment was established in 1995 on clay soil, with 45–50% clay and 5.5% organic matter in the topsoil. Three tillage treatments were mouldboard plough to a depth of 20 cm (conventional), field cultivator to a depth of 8 cm, and no primary tillage (conservation). The field had an oat (Avena sativa L.)–barley (Hordeum vulgare L.) crop rotation. In 1997–1998 and 2000, root distribution during the growing season was evaluated by a non-destructive minirhizotron (MR) and video recording method. Root length density and root diameter were also measured once a season (1997 and 1998) by destructive root sampling and image analysis of washed roots. At shoot elongation, root numbers increased more under conventional than conservation tillage, at soil depth of 10–25 cm. The effect was clear for both barley (1997) and oat (2000) with maximum root numbers of 175 and 210 per 100 cm2 by mouldboard ploughing, but 120 and 170 per 100 cm2 under unploughed conditions (in the whole 0–0.4 m region). The suboptimal condition of unploughed soil was also indicated by lower shoot nutrient contents at tillering (studied in 1997) and by higher penetrometer resistance (studied in 1998, 2000) and lower macroporosity (studied in 2000) at 10–25 cm soil depth. Root growth dynamics were similar for both plant species. Root diameter was not significantly affected by the tillage treatments. Discontinuation of mouldboard ploughing reduced root growth (P<0.05) within this clay soil 5 years after the tillage change, although conservation tillage preserved more water for plant use. The data show that a clay soil can be too dense for optimal rooting during the 3rd–6th-years after discontinuation of ploughing.

Introduction

Primary tillage without mouldboard ploughing has been gradually adapted for clay soils on many grain-growing farms in Finland due to environmental EU-support to prevent soil erosion. However, the consequences of conservation tillage on root growth and nutrient use efficiency remain uncertain, and more data are needed on soil responses following conversion to more environmentally friendly crop production. This is important, since altered root development could change the efficiency of water and nutrient use, which determine environmental impacts of agriculture (van Noordwijk et al., 1993).

For optimal nutrient uptake, roots should be distributed evenly in the nutrient-rich topsoil. Discontinuation of mouldboard ploughing significantly changes soil structure in clay soils (Alakukku, 1998). During the transition from mouldboard ploughing to conservation tillage, surface soil may became compacted, thereby inhibiting root growth by increased mechanical impedance (Ehlers et al., 1983, Cannell and Hawes, 1994). In heterogeneous clay soils, high mechanical resistance or poor aeration in dense zones implies large spatial variation in root growth. A compacted soil environment reduces root length and root penetration by buckling and increases root diameter and compensatory growth of laterals into looser zones (Unger and Kaspar, 1994). Roots tend to preferentially colonize soil between aggregates and in biopores (Eissenstat and Caldwell, 1988, Logsdon and Allmaras, 1989). This, in turn, leads to inefficient absorption of water and nutrients, especially immobile nutrients such as phosphorus, the uptake of which is closely related to root morphology and growth (Schenk and Barber, 1980) and root to soil contact (Veen et al., 1992). In temperate climates, surface residues may also result in cooler topsoils (Kaspar et al., 1990), which can delay emergence and nutrient uptake in nutrient-poor soils (Chassot and Richner, 2002).

Besides the soil environment, rainfall can affect root system architecture (Fitter, 1991), but this influence is modified by soil structure, which depends on the tillage system employed. The adverse effects of excessively dry or moist seasons can be buffered by a beneficial soil structure in the seedbed and deeper soil horizons (Håkansson et al., 2002). In Nordic clay areas, rainfall is often too low in the early season and too high after vegetative growth. Soil management should, therefore, promote fast early growth by storing sufficient water for plant use and providing continuous macropores for roots to penetrate the zones of high mechanical impedance (Ehlers et al., 1983) to allow them to benefit from deep soil water reservoirs. Macropores are required for bypass flow of excess water later in the growing season. For efficient nutrient supply, micropores are needed. Under moderately compacted soil conditions, root growth, root to soil contact, absorption efficiency of nutrients, and unsaturated hydraulic conductivity are better than in very loose soils (Lipiec et al., 1992, Veen et al., 1992), as well as unploughed soils, where macropores may dominate root pathways.

The objective of this investigation was to characterize root growth and morphology in the upper soil horizons in response to differences in tillage.

Section snippets

Materials and methods

A field experiment was conducted on a very fine, illitic Typic Cryaquept with 45–50% clay (<2 μm), 25–30% silt (2–20 μm) and 5.5% organic matter in the 0–20 cm depth at Jokioinen (60°49′N, 23°28′E) Finland (Yli-Halla and Mokma, 2001). At a depth of 20–40 cm, the soil consisted of 58% clay, 24% silt and 3% organic matter. Since 1995, the field was either (1) tilled each September with a mouldboard plough (MP) to a depth of 20 cm, (2) tilled each September with a field cultivator (FC) to a depth of 8 

Results and discussion

Discontinuation of ploughing preserved more soil water for plant use during the 3rd, 4th, and 6th years after the tillage change (Fig. 1A–C). Both soils under FC and NPT had more plant-available water, most likely as a consequence of less evaporation due to straw on the soil surface (Steiner, 1989). During plant growth in 1998, less water occurred under MP, because of more intensive water uptake, as the best yield was achieved in MP (Pietola and Tanni, 2003).

Despite better water supply in

Conclusions

Discontinuation of MP reduced root densities in small grains from flowering to ripeness, primarily at a 15–25 cm soil depth. In the same soil depth, discontinuation of MP increased penetrometer resistance and decreased macroporosity. Reduced tillage also decreased N and P concentrations in shoots at tillering, but not later. In unploughed soils, roots may profit from better water supply and more continuous macropores, but these positive effects resulting from a reduction in tillage could not

Acknowledgements

The author wish to thank Risto Tanni and Leena Mäkäräinen at MTT Agrifood Research Finland and students Jonas Laxåback, Teija Marjamäki and Ville Sipilä for technical support during the field experiment, MR video recording and shoot analyses, and Olga Nikolenko and Leila Räty at the University of Helsinki for laboratory analysis of roots. Financial support by the Ministry of Agriculture in Finland is gratefully acknowledged.

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