Walter Richner

Root development and heavy metal phytoextraction efficiency: comparison of different plant species in the field

Heavy metal phytoextraction is a soil remediation technique which implies the optimal use of plants to remove contamination from soil. Plants must thus be tolerant to heavy metals, adapted to soil and climate characteristics and able to take up large amounts of heavy metals. Their roots must also fit the spatial distribution of pollution. Their different root systems allow plants to adapt to their environment and be more or less efficient in element uptake. To assess the impact of the root system on phytoextraction efficiency in the field, we have studied the uptake and root systems (root length and root size) of various high biomass plants (Brassica juncea, Nicotiana tabacum, Zea mays and Salix viminalis) and one hyperaccumulator (Thlaspi caerulescens) grown in a Zn, Cu and Cd contaminated soil and compared them with total heavy metal distribution in the soil. Changes from year to year have been studied for an annual (Zea mays) and a perennial plant (Salix viminalis) to assess the impact of the climate on root systems and the evolution of efficiency with time and growth. In spite of a small biomass, T. caerulescens was the most efficient plant for Cd and Zn removal because of very high concentrations in the shoots. The second most efficient were plants combining high metal concentrations and high biomass (willows for Cd and Zn and tobacco for Cu and Cd). A large cumulative root density/aboveground biomass ratio (LA/B), together with a relative larger proportion of fine roots compared to other plants seemed to be additional favourable characteristics for increased heavy metal uptake by T. caerulescens. In general, for all plants correlations were found between L A/B and heavy metal concentrations in shoots (r=0.758***, r=0.594***, r=0.798*** (P<0.001) for Cd, Cu and Zn concentrations resp.). Differences between years were significant because of variations in climatic conditions for annual plants or because of growth for perennial plants. The plants exhibited also different root distributions along the soil profile: T. caerulescens had a shallow root system and was thus best suited for shallow contamination (0.2 m) whereas maize and willows were the most efficient in colonising the soil at depth and thus more applicable for deep contamination (0.7 m). In the field situation, no plant was able to fit the contamination properly due to heterogeneity in soil contamination. This points out to the importance and the difficulty of choosing plant species according to depth and heterogeneity of localisation of the pollution.

Root distribution and morphology of maize seedlings as affected by tillage and fertilizer placement

Suboptimal soil conditions are known to result in poor early growth of maize (Zea mays L.) in no-tillage (NT) systems in contrast with conventional tillage (CT) systems. However, most studies have generally focused on maize roots at later growth stages and/or do not give details on root morphology. In a 2-year field study at two locations (silt loam and loam soils) in the Swiss midlands, we investigated the impacts of tillage intensity, NT vs. CT, and NP-fertilizer sidebanding on the morphology, vertical and horizontal distribution, and nutrient uptake of maize roots at the V6 growth stage. The length density (RLD) and the length per diameter-class distribution (LDD) of the roots were determined from soil cores taken to a depth of 0.5 m and at distances of 0.05 and 0.15 m from both sides of the maize row. The temperature of the topsoil was lower, and the bulk density and penetration resistance were greater in the topsoil of NT compared with CT. The growth and the development of the shoot were slower in NT. RLD was greater and the mean root diameter smaller in CT than in NT, while the vertical and horizontal distribution of roots did not differ between CT and NT. RLD increased in the zone enriched by the sidebanded fertilizer, independent of the tillage system, but LDD did not change. The poorer growth of the roots and shoots of maize seedlings was presumably caused by the lower topsoil temperature in NT rather than by mechanical impedance. The placement of a starter fertilizer at planting under NT is emphasized.

The effect of tillage intensity and time of herbicide application on weed communities and populations in maize in central Europe

Abstract Soil-conserving cropping systems aim at reducing tillage intensity to decrease soil erosion, leaching of nitrate and pesticides, and production costs. They are, therefore, likely to change the efficiency of weed control and hence weed populations. There is a lack of information on how reduced tillage systems, especially no-tillage (NT), affect the development of weed populations and the efficiency of weed control of maize crops under the humid, temperate climate of large parts of Europe. An experiment was conducted at two sites of the Swiss midlands to investigate the impact of the time of chemical weed control on weed populations in different tillage systems. Pre- and post-emergence herbicides were applied in a conventional tillage (CT) system with a moldboard plough, in a minimum tillage (MT) system with a chisel plough, and in a NT system in a winter wheat (Triticum aestivum L.)–oilseed rape (Brassica napus L.)–winter wheat–maize (Zea mays L.) crop rotation. Crop residues were left on the field and stubble tillage was not carried out. The density of weeds in treatments without herbicide tended to be lower in NT than in MT and CT. An analysis of variance and canonical discriminant analysis (CDA) showed that perennial weeds such as Epilobium spp. L. and Sonchus arvensis L. were related to NT, and annual broad-leaved species were associated with MT and CT. In general, post-emergence weed control was more efficient than pre-emergence weed control regardless of the tillage system. However, NT systems may be adopted successfully when weed control measures are adapted to changing weed populations.

A rhizolysimeter facility for studying the dynamics of crop and soil processes: description and evaluation

An experimental facility was designed to study simultaneously soil and shoot processes in agricultural crop systems. The facility is composed of 48 drainage lysimeters, from which 24 can be in use at the same time. These were equipped with horizontal minirhizotrons (for non-destructive root observation), suction cups (to sample the soil solution), thermistors (for the control and monitoring of the temperature), time-domain reflectometry (TDR) probes (to control and measure the volumetric soil water content) and leachate samplers (to measure the leachate and to monitor nutrient leaching). Together with non-destructive shoot data measurements, the instrumentation can be used to study the dynamics of above- and below-ground crop growth, as well as the leaching of nutrients (like nitrate) and the water budget. The results of a 1-year maize (Zea mays L.) experiment, in which shoot growth and development were compared to root growth, nitrogen and water dynamics, are presented. Maximum leaf area and maximum root density were well synchronized in the upper soil horizons, while in deeper horizons time of maximum root density was delayed. Nitrate leaching was high throughout the season, always exceeding the ‘safe limit’ for drinking water (10 mg l-1). It was especially high during early season, exceeding the rather tolerant ‘EC limit’ for drinking water (50 mg l-1). As a consequence, 90% of the nitrogen leaching losses were observed within 50 days after planting. Intensive water percolation followed high precipitation early in the crop season. At the end of the crop season, water percolation lagged behind precipitation, as soil water content replenished. The intensive growth of the shoot up to tasseling is reflected by the extensive exploitation of soil water reserves. The coincidence of minimum soil water storage and maximum leaf area, as well as the maximum rooting density in the upper soil layers, is remarkable and demonstrates the close relationship between demand (shoot activity), supply (root activity) and the exploration of soil water reserves. The facility was demonstrated to be suitable for the investigation of complex interactions between two plant components (shoots and roots) and between the plants and the environment, as are expected to occur during the growth of an agricultural crop. It will be most useful to evaluate present and alternative agronomic strategies in relation to their environmental feasibility.

Responses of roots to low temperatures and nitrogen forms

Throughout its life cycle, the development of the root system is finely tuned to the requirements of the whole plant. This is only possible because roots adapt to physical impedance, availability of nitrogen, phosphorus and water, and to thermal conditions. This paper concentrates on cereals during the early establishment of the plant.

Accumulation of dry matter and nitrogen by minimum-tillage silage maize planted into winter cover crop residues

The use of winter cover crops in conjunction with minimum tillage may eliminate or at least mitigate the environmental problems associated with traditional maize tillage. The main goal of the present research was to study the accumulation of nitrogen and dry matter in the tops of silage maize under three cropping systems: (1) PLOUGH (=maize sown into an autumn-ploughed soil), (2) NW/MT (maize sown into frost-killed residues of the non-winterhardy phacelia and white mustard cover crops, and (3) RYE/MT (=maize sown into a stubble of forage rye whose above-ground phytomass was removed from the field shortly before maize planting). The experiments were conducted in the Swiss midlands and in the Jura range. Averaged across the five environments (=site × year combinations) tested, dry matter and nitrogen yields of maize were highest under PLOUGH and lowest under RYE/MT. These differences occurred as early as the 3rd leaf stage and remained until the end of the growing season of maize, but there were significant (P <0.05) interactions between environment and cropping system. Total yields of dry matter and nitrogen (maize plus rye) of the RYE/MT system tended to be higher than the dry matter and nitrogen yields of maize in the other systems. The effect of method of seedbed preparation (rototilling vs. band rotary hoeing) on the yields of dry matter and nitrogen was not significant; there were no interactions between maize cropping system and manner of seedbed preparation. Under RYE/MT, both the mineral nitrogen content of the soil (0–90 cm depth) prior to maize sowing and the nitrogen concentration in the maize tops throughout the growing season of maize were relatively low, indicating that the rye cover crop reduced the nitrogen supply to the succeeding maize crop through pre-emptive competition.