Rhizodeposition of maize: Short‐term carbon budget and composition

The aim of this study was to assess differences in rhizodeposition quantity and composition from maize cropped on soil or on 1:1 (w/w) soil–sand mixture and distribution of recently assimilated C between roots, shoots, soil, soil solution, and CO₂ from root respiration. Maize was labeled in ¹⁴CO₂ atmosphere followed by subsequent simultaneous leaching and air flushing from soil. ¹⁴C was traced after 7.5 h in roots and shoots, soil, soil solution, and soil‐borne CO₂. Rhizodeposits in the leachate of the first 2 h after labeling were identified by high‐pressure liquid chromatography (HPLC) and pyrolysis–field ionization mass spectrometry (Py‐FIMS). Leachate from soil–sand contained more ¹⁴C than from soil (0.6% vs. 0.4%) and more HPLC‐detectable carboxylates (4.36 vs. 2.69 μM), especially acetate and lactate. This is either because of root response to lower nutrient concentrations in the soil–sand mixture or decreasing structural integrity of the root cells during the leaching process, or because carboxylates were more strongly sorbed to the soil compared to carbohydrates and amino acids. In contrast, Py‐FIMS total ion intensity was more than 2 times higher in leachate from soil than from soil–sand, mainly due to signals from lignin monomers. HPLC‐measured concentrations of total amino acids (1.33 μM [soil] vs. 1.03 μM [soil–sand]) and total carbohydrates (0.73 vs. 0.34 μM) and ¹⁴CO₂ from soil agreed with this pattern. Higher leachate concentrations from soil than from soil–sand for HPLC‐measured carbohydrates and amino acids and for the sum of substances detected by Py‐FIMS overcompensated the higher sorption in soil than in sand‐soil. A parallel treatment with blow‐out of the soil air but without leaching indicated that nearly all of the rhizodeposits in the treatment with leaching face decomposition to CO₂. Simultaneous application of three methods—¹⁴C‐labeling and tracing, HPLC, and Py‐FIMS—enabled us to present the budget of rhizodeposition (¹⁴C) and to analyze individual carbohydrates, carboxylates, and amino acids (HPLC) and to scan all dissolved organic substances in soil solution (Py‐FIMS) as dependent on nutrient status.     

Identifying difficulties in rapeseed root penetration in farmers’ fields in northern European conditions

Tap‐rooted rapeseed is generally considered sensitive to soil compaction. We examined 473 Finnish rapeseed fields sown in 2007 to 2009 to both spring turnip rape Brassica rapa L. (dominant crop) and oilseed rape Brassica napus L. Roots were rated late in the growing season according to penetration capacity. Roots in each field were rated from G1 (no problems with root penetration) to G5 (failure to penetrate deeper soil layers). There were significant effects of year, crop species, soil type, special soil properties and soil cultivation methods on frequency of G1 and G4+5 roots. Restricted root penetration was a common phenomenon. Typically, fields had <30% G3 and G4+5 roots, however, some fields had >70% G4+5 roots and were cases for serious concern. Yield loss was linearly related to the incidence of G4+5 plants in 2007, but there was no relationship with the other years. Oilseed rape had a greater proportion of G4+5 roots than turnip rape and twice the proportion of G4+5 roots occurred in direct‐drilled fields than in reduced or fully tilled fields. Rapeseed yield decline may at least partly be related to poor root penetration and it is a useful indicator crop to demonstrate the degree of soil compaction and should be cultivated in fields where sufficient root penetration is possible.     

Soybean root morphology in soils with and without tillage pans in the lower Mississippi river valley 1

During 1980, an extremely dry growing season, soybean (Glycine max L. Merrill) root morphologies were characterized at the R2 growth stage in Steele, Sharkey, Rilla, Calloway, and Stuttgart soils with a tillage pan and with a disrupted tillage pan. Results showed that the presence or absence of tillage pans resulted in dramatic changes in the soybean root morphology. Without pans, classical taproot systems tended to develop. With pans, soybean root systems tended to follow old root channels and fractures through the pan. Below the pan, soil structure manifested strong influences on root morphology. On all soils except the Steele (loamy sand), roots penetrated the tillage pan in numbers comparable to those penetrating the soil horizons with the pan disrupted. Although soil impedance to root penetration increases with drying and the 1980 growing season was exceptionally dry, mechanical impedance to soybean root penetration of tillage pans on silt loam and clay soils did not appear to be a problem. However, on the loamy sand soil mechanical impedance inhibited soybean root penetration and appeared to be a major problem.     

Reduced root water uptake after drying and rewetting

The ability of plants to extract water from soil is controlled by the water‐potential gradient between root and soil, by the hydraulic conductivity of roots, and, as the soil dries, by that of the soil near the roots (rhizosphere). Recent experiments showed that the rhizosphere turned hydrophobic after drying and it remained temporarily dry after rewetting. Our objective was to investigate whether rhizosphere hydrophobicity is associated with a reduction in root water uptake after drying and rewetting. We used neutron radiography to trace the transport of deuterated water (D₂O) in the roots of lupines growing in a sandy soil. The plants were grown in aluminum containers (28 × 28 × 1 cm³) filled with a sandy soil. The soil was initially partitioned into different compartments using a 1‐cm layer of coarse sand (three vertical × three horizontal compartments). We grew plants in relatively moist conditions (0.1 < θ < 0.2). Three weeks after planting, we let the upper left compartment of soil to dry for 2–3 d while we irrigated the rest of the soil. Then, we injected D₂O in this compartment and in the upper right compartment that was kept wet. We monitored D₂O transport in soil and roots with time‐series neutron radiography. From the changes of D₂O concentration inside roots, we estimated the root water uptake. We found that root water uptake in the soil region that was let dry and rewetted was 4–8 times smaller than that in the region that was kept moist. The reduced uptake persisted for > 1–0.5 h. We conclude that a reduction in hydraulic conductivity occurred during drying and persisted after rewetting. This reduction in conductivity could have occurred in roots, in the rhizosphere, or more likely in both of them.     

SoilFlex‐LLWR: linking a soil compaction model with the least limiting water range concept

Soil compaction impacts growing conditions for plants: it increases the mechanical resistance to root growth and modifies the soil pore system and consequently the supply of water and oxygen to the roots. The least limiting water range (LLWR) defines a range of soil water contents within which root growth is minimally limited with regard to water supply, aeration and penetration resistance. The LLWR is a function of soil bulk density (BD), and hence directly affected by soil compaction. In this paper, we present a new model, ‘SoilFlex‐LLWR’, which combines a soil compaction model with the LLWR concept. We simulated the changes in LLWR due to wheeling with a self‐propelled forage harvester on a Swiss clay loam soil (Gleyic Cambisol) using the new SoilFlex‐LLWR model, and compared measurements of the LLWR components as a function of BD with model estimations. SoilFlex‐LLWR allows for predictions of changes in LLWR due to compaction caused by agricultural field traffic and therefore provides a quantitative link between impact of soil loading and soil physical conditions for root growth.     

Influence of soil strength on root growth: experiments and analysis using a critical-state model

Roots grow thicker in compacted soil, even though it requires greater force for a large object to penetrate soil than it does for a small one. We examined the advantage of thickening in terms of the stresses around a root penetrating with constant shape, rather than the stresses around an expanding cylinder or sphere, as has been studied previously. We combined experiments and simulations of the stresses around roots growing in compacted soils. We measured the diameter of pea roots growing in sandy loam and clay loam at four different densities, and the critical‐state properties of the soils. At a penetration resistance of about 1 MPa the diameter of the roots in the sandy loam was about 40% greater than that at 0.7 MPa, and at 2 MPa it was about 60% greater. In the clay loam, there was less thickening – about 10% greater at 1 MPa and about 20% greater at 1.5 MPa. The maximum axial stresses were predicted using a critical‐state finite‐element model to be at the very tip of the root cap. When there was friction between the root and the soil, shear stresses were predicted with smaller values at the tip than just behind the tip. When the interface between the soil and the root was assumed to be frictionless, there were by definition no shear stresses. In the frictionless case the advantage of root thickening on relieving peak stress at the root tip was diminished. The axial and shear stresses were predicted to be smaller in the clay loam than in the sandy loam and may explain why the roots did not thicken in this soil although its resistance to penetration was similar. Our results suggest that the local values of axial and shear stresses experienced by the root near its tip may be as important in constraining root growth as the total penetration resistance.     

Changes in root length at the reproductive stage of maize plants grown in the field and quartz sand

The aims of this work were to investigate possible reasons for root mortality of maize plants at the reproductive stage and relationships between root mortality and internal sugar and external nitrogen (N) supply. Maize (Zea mays L.) plants were grown in the field in fertile soil and in a greenhouse in quartz sand with sufficient or deficient N supply. Deficient N supply reduced plant growth and total N uptake by 38% and 52%, respectively. The lengths of the seminal roots and of the early initiated adventitious roots of the first two whorls declined after reaching their maximum values before silking, no matter whether the plants were grown in the field or in quartz sand in the greenhouse. The lengths of the adventitious roots from higher nodes of plants grown in quartz sand, irrespective of N supply, did not decrease at the reproductive stage despite of decreasing sugar concentrations. In contrast, under field conditions, the length of adventitious roots from higher nodes decreased during grain filling. Total activity of all roots of greenhouse‐grown plants as deduced from translocation of N and cytokinins in the xylem exudate reached peak values at the end of the growing period, whereas in field‐grown plants N translocation decreased and cytokinin translocation did not change toward the end of the growing period. The results indicate that the pattern of root growth and mortality of maize plants in the reproductive stage was not affected by external N supply. Differences between glasshouse‐ and field‐grown plants are possibly due to effects of soil biota, which have to be further studied.     

Minirhizotron quantification of soybean root growth as affected by reduced A horizon in soil

Shallow soil A horizon (topsoil) caused by soil erosion and soil movement from cultivation is known to reduce soil and crop productivity. The reduction may be related to limitation of root growth. A field study was conducted to investigate the effects of topsoil thickness on distributions of root density and growth. Soybeans [Glycine max (L.) Merr.] were grown on plots of Mexico silt loam (fine, montmorillonitic, mesic Mollic Endoaqualfs) with topsoil thicknesses of 0, 12.5, 25.0, and 37.5 cm above the Bt horizons. Root density was measured 60 and 90 days after planting using a minirhizotron video‐camera system. Root density was significantly reduced as topsoil thickness decreased from 37.5 to 0 cm. Mean density and net change of the density across profile between 30 and 60 days of growth had a linear function of topsoil thickness. The reduction and lower activity induced by shallow topsoil were attributed to detrimental properties in the Bt horizons. Root distribution pattern and rooting depth were not significantly affected by topsoil thickness. The roots appeared to be accumulated on the upper layers of the Bt horizons. Roots growing in thicker topsoil were more active than roots growing without topsoil. High soil moisture content during the growing season may mitigate the detrimental effects of shallow topsoil, inhibit root penetration, and enhance root activity.     

Characterization of recently 14C pulse-labelled carbon from roots by fractionation of soil organic matter

The inability of physical and chemical techniques to separate soil organic matter into fractions that have distinct turnover rates has hampered our understanding of carbon (C) and nutrient dynamics in soil. A series of soil organic matter fractionation techniques (chemical and physical) were evaluated for their ability to distinguish a potentially labile C pool, that is ‘recent’ root and root‐derived soil C. ‘Recent’ root and root‐derived C was operationally defined as root and soil C labelled by ¹⁴CO₂ pulse labelling of rye grass–clover pasture growing on undisturbed cores of soil. Most (50–94%) of total soil + root ¹⁴C activity was recovered in roots. Sequential extraction of the soil + roots with resin, 0.1 m NaOH and 1 m NaOH allocated ‘recent’ soil + root ¹⁴C to all fractions including the alkali‐insoluble residual fraction. Approximately 50% was measured in the alkali‐insoluble residue but specific activity was greater in the resin and 1 m NaOH fractions. Hot 0.5 m H₂SO₄ hydrolysed 80% of the ¹⁴C in the alkali‐insoluble residue of soil + roots but this diminished specific activity by recovering much non‐¹⁴C organic matter. Pre‐alkali extraction treatment with 30% H₂O₂ and post‐alkali treatment extractions with hot 1 m HNO₃ removed organic matter with a large ¹⁴C specific activity from the alkali‐insoluble residue. Density separation failed to isolate a significant pool of ‘recent’ root‐derived ¹⁴C. The density separation of ¹⁴C‐labelled roots, and roots remixed with non‐radioactive soil, showed that the adhesion of soil particles to young ¹⁴C‐labelled roots was the likely cause of the greater proportion of ¹⁴C in the heavy fraction. Simple chemical or density fractionations of C appear unsuitable for characterizing ‘recent’ root‐derived C into fractions that can be designated labile C (short turnover time).     

Effects of drought stress on photosynthesis,rhizosphere respiration,and fine‐root characteristics of beech saplings: A rhizotron field study

Soil drought influences the C turnover as well as the fine‐root system of tree saplings. Particularly during the period of establishment, the susceptibility to drought stress of saplings is increased because of incompletely developed root systems and reduced access to soil water. Here, we subjected beech saplings (Fagus sylvatica L.) to different levels of drought stress. Beech saplings were planted in rhizotrons, which were installed in the soil of a Norway spruce forest before bud burst. Soil moisture was manipulated in the following year during May to September. We measured photosynthetic net CO₂ uptake, volume production of fine roots, and rhizosphere respiration during the growing season. Biometric parameters of the fine‐root system, biomass, and nonstructural carbohydrates were analyzed upon harvest in October. Photosynthesis and rhizosphere respiration decreased with increasing drought‐stress dose (cumulated soil water potential), and cumulative rhizosphere respiration was significantly negatively correlated with drought‐stress dose. Fine‐root length and volume production were highest at moderate soil drought, but decreased at severe soil drought. The proportion of fine‐roots diameter < 0.2 mm and the root‐to‐shoot ratio increased whereas the live‐to‐dead ratio of fine roots decreased with increasing drought‐stress dose. We conclude that the belowground C allocation as well as the relative water‐uptake efficiency of beech saplings is increased under drought.     

Sorghum root growth as influenced by soil physical properties

Abstract

Variations in the plant growth media were achieved by combining kaolinite clay (<lμm esd.), silt (2–50μm esd.) and sand (100–250 μm esd.) in various ratios. Peds of different sizes were separated from an Okolona clay soil and used as a growth media. A layer (3 cm thickness) of the sand, silt or clay and their combinations were intercalated between sandy loam soil material in a lucite coated cardboard carton. After 21 days the plants were harvested and analyzed for a number of growth parameters and related to the physical and micromorphology of the central control layer.

Germination and emergence of sorghum seed were delayed in the finer aggregates. An increase in aggregate size increased the root elongation. An examination of thin sections showed that most of the roots in the finer aggregates were grown in interpedal regions whereas in the larger aggregates roots were found in both the intrapedal as well as interpedal regions.

An increase in clay content of the central layer reduced the root growth. Silt also reduced root growth but not to the extent of the clay. Maximum root growth and penetration occurred in the mixture containing about 50 percent sand. Better root growth was observed in a sandy to sandy loam texture than clay to clay loam texture.     

Impact of soil texture on temporal and spatial development of osmotic‐potential gradients between bulk soil and rhizosphere

Solute transport from the bulk soil to the root surface is, apart from changes in soil moisture and plant nutrient uptake, a prerequisite for changes in soil osmotic potential (Ψ_o). According to the convection‐diffusion equation, solute transport depends on a number of parameters (soil moisture–release curve, hydraulic conductivity, tortuosity factor) which are functions of soil texture. It was thus hypothesized that soil texture should have an effect on the formation of Ψ_o gradients between bulk soil and the root surface. The knowledge about such gradients is important to evaluate water availability in the soil‐plant‐atmosphere continuum (SPAC). A linear compartment system with maize grown under controlled conditions in two texture treatments (T1, pure sand; T2, 80% sand, 20% silt) under low and high initial application of salts (S1, S2) was used to measure the development of Ψ_o gradients between bulk soil and the root surface by microscale soil‐solution sampling and TDR sensors. The differences in soil texture had a strong impact on the formation of Ψ_o gradients between bulk soil and the root surface at high and low initial salt application rate. At high initial salt application, a maximum osmotic‐potential gradient (ΔΨ_o) of –340 kPa was observed for the texture treatment T2 compared to ΔΨ_o of –180 in T1. The steeper gradients in osmotic potential in treatment T2 compared to T1 corresponded to higher cumulative water consumption in this treatment which can partly be explained by higher soil hydraulic conductivity in the range of soil matric potentials covered during the duration of the experiments. Differences between texture treatments in Ψ_o at the root surface did not result in differences in plant‐water relations measured as gas‐exchange parameters (transpiration rate, water‐use efficiency) and leaf osmotic potential. If soil osmotic and matric potential are regarded as additive in calculating the driving force for water movement from the soil into the root, the observed differences in water flux between treatments cannot be explained.     

Determination of the extent of rhizosphere soil

Abstract

The extent of the rhizosphere was investigated by using root volume and root length in ten replications. The experiment was conducted using split cylindrical pots, 23 cm long and 7.5 cm in diameter. Sorghum (Sorghum bicolor) plants were grown in a calcareous soil of low phosphorus (P) status. Fertilized soil (750 g soil and 250 g sand) was placed in a closed‐bottom PVC tube. At harvest, plant roots were gently removed from the pots and the roots were shaken five times in order to reduce variation between samples. The soil that was easily shaken from the root surface was assumed to be non‐rhizosphere soil, and the soil adhering to the root segment after a gentle shake was considered to be rhizosphere soil. The rhizosphere thickness was found to have a range of 0.39 to 0.64 mm from the root surface (0.51 mm average thickness). Rhizosphere soil mass was also calculated and found to be on average 22% of the total soil mass.     

Improved root penetration of soil hard layers by a selected genotype

Abstract

Crops can be effectively grown on hardpan soils and water effectively used from deep in the profile if hard layers in soils can be penetrated or if they are broken up by tillage. Addition of gypsum to the soil or exploitation of genetic differences in root penetrability may help improve root penetration through hard layers with less need to depend on the energy requirements of deep tillage. To test this theory, a single‐grained Ap horizon of Norfolk loamy sand soil was compacted into soil columns to compare root penetrability of soybean [Glycine max (L.) Merr.] genotypes Essex and PI 416937 in the presence and absence of gypsum and at two soil compaction levels (columns with uniform compaction at 1.4 g cm^‐1 and columns with increasing compaction with depth from 1.4 to 1.75 g cm^‐1). Compaction treatments were imposed by constructing soil columns composed of 2.5‐cm‐deep, 7.5‐cm‐diameter cylindrical cores compacted to predetermined bulk densities (1.40,1.55,1.65,and 1.75 g cm^.3). Soil penetration resistances were measured on duplicate cores using a 3‐mm‐diameter cone‐tipped penetrometer. Columns were not watered during the study; soybean genotypes were grown in the columns until they died. Both genotypes lived one day longer in columns with lower bulk density and penetration resistance. Although root growth was more abundant for Essex than for PI 416937, root growth of PI 416937 was not decreased by compaction as much as it was for Essex. These results suggest that PI 416937 may possess the genetic capability to produce more root growth in soils with high penetration resistance. This study suggests that genetic improvement for root growth in soils with hard or acidic layers may potentially reduce our dependence on tillage. Gypsum did not affect root growth in this study.     

The maize root system in situ: Evaluation of structure and capability of utilization of phytate and inorganic soil phosphates

• 1 The dependence of the morphology of the maize (Zea mays L.) seminal root system on physical, chemical and biotic parameters was investigated with pot cultures in quartz sand and in a natural loamy sand soil. Low O₂-supply to the soil resulted in a substantially smaller root biomass despite a relative increase in total root length. Reduced N-supply also stimulated root length growth, but also enhanced the formation of laterals. The presence of soil microorganisms, in comparison to sterile cultures, resulted in a reduced length of the main roots, and the production of slender laterals with a decreased root hair density. Generally, the structural variability of laterals in response to different growth conditions was much more pronounced than that of the main roots.
• 2 A major part of the work reported here was dedicated to a detailed study of phosphate (P) acquisition by the maize root system under field conditions. Radioactive labelling of the roots and radioautography of soil cores revealed the in situ distribution pattern of the maize root system. Controlled labelling of the soil with radioactive phosphate allowed the documentation of the development and replenishment of the phosphate depletion zone around roots. Finally, the longevity and phosphate uptake activity of the different parts and tissues of the primary root system of maize was examined by electron microscopy and tracer studies including pulse chase experiments. From these studies the phosphate-acquiring strategy of the maize root system appears as follows: The capability of P uptake decreases in the order: root hairs, 1^st order laterals, 2^nd order laterals, main root. The life-spans of the components of the maize root system increase by the sequence: root hairs, laterals, main root. Inorganic P uptake, therefore, mainly occurs during the first weeks of root development. Dying back of the root occurs in an ordered manner resulting in a relocation of stored P predominantly into the main root cortex. Furthermore, it could be shown that competition for P between roots of the same or of adjacent maize and/or lupin plants virtually does not occur in situ.
• 3 The utilization of phytate-P was studied with ¹⁴C/³²P-labelled Camyo-inositol-hexaphosphate supplied to maize plants grown in sterile quartz sand or in hydroponic cultures. The ratio of P- and C-uptake as well as the incidence of phytate hydrolysis products in the rooting medium indicated the capability of maize roots to acquire P from phytate by enzymatic hydrolysis. This was confirmed by enzyme studies of the root tissues. A specific hydrolyzing enzyme (phytase; molecular weight 51 kD) could be detected in the cell wall of the root, especially in the root tip, which initiates phytate dephosphorylation. Further breakdown is presumably accomplished by monophosphoric phosphohydrolases.

     

Biological nitrogen fixation associated with roots of field-grown barley (Hordeum vulgare L.)

Nitrogenase (C₂H₂) activity was measured in microbial media inoculated with barley root segments or corresponding rhizosphere soil. Three different media were used, Döbereiner's malate medium, a modified Ashby medium, and an acid nitrogen-free medium. Only Döbereiner's medium gave consistently positive results, and cultures inoculated with roots showed higher activity than cultures inoculated with corresponding rhizosphere soil. Similar experiments with roots and rhizosphere soil from wheat gave only negligible nitrogenase activity, whereas the tropical grass, Cynodon dactylon, gave higher activity than barley. Measurements on intact soil cores containing barley root systems showed an initial lag phase followed by a rather stable activity level over a period from 12 h to 48 h, and then the activity again decreased. The activity during the stable period corresponded to fixation of about 100 to 200 g N₂ ha^?1 24 h^?1. Measurements on isolated, washed barley roots showed only negligible nitrogenase activity.     

Microbial CO2 production, CH4 dynamics and nitrogen in a wetland soil (New York State, USA) associated with three plant species (Typha, Lythrum, Phalaris)

Plants furnish soil with organic carbon (OC) compounds that fuel soil microorganisms, but whether individual plant species – or plants with unique traits – do so uniquely is uncertain. We evaluated soil microbial processes within a wetland in which areas dominated by a distinct plant species (cattail –Typha sp.; purple loosestrife –Lythrum salicaria L.; reed canarygrass –Phalaris arundinacea L.) co‐mingled. We also established an experimental plot with plant shoot removal. The Phalaris area had more acidic soil pH (7.08 vs. 7.27–7.57), greater amount of soil organic matter (19.0% vs. 9.0–11.5%), and the slowest production rates of CO₂ (0.10 vs. 0.21–0.46 μmol kg⁻¹ s⁻¹) and CH₄ (0.040 vs. 0.054–0.079 nmol kg⁻¹ s⁻¹). Nitrogen cycling was dominated by net nitrification, with similar rates (17.2–18.9 mg kg⁻¹ 14 days⁻¹) among the four sampling areas. In the second part of the study, we emplaced soil cores that either allowed root in‐growth or excluded roots to evaluate how roots directly affect soil CO₂ and CH₄. The three plant species had similar amounts of root growth (ca 290 g m⁻² year⁻¹). Fungal biomass was similar in soils with root in‐growth versus root exclusion, regardless of dominant plant species. Rates of soil CO₂ production did not differ with root in‐growth versus root exclusion, and added glucose increased CO₂ production rates by only 35%. Root in‐growth did lead to greater rates of CH₄ production; albeit, addition of glucose had much greater effect on CH₄ production (1.24 nmol kg⁻¹ s⁻¹) compared with controls without added glucose (0.058 nmol kg⁻¹ s⁻¹). Our data revealed relatively few subtle differences in soil characteristics and processes associated with different plant species; albeit, roots had little effect, even inhibiting some microbial processes. This research highlights the need for both field and experimental studies in long‐established monocultures of plant species to understand the role of plant biodiversity in soil function.     

Image analysis for non‐destructive and non‐invasive quantification of root growth and soil water content in rhizotrons

Studies aiming at quantification of roots growing in soil are often constrained by the lack of suitable methods for continuous, non‐destructive measurements. A system is presented in which maize (Zea mays L.) seedlings were grown in acrylic containers — rhizotrons — in a soil layer 6‐mm thick. These thin‐layer soil rhizotrons facilitate homogeneous soil preparation and non‐destructive observation of root growth. Rhizotrons with plants were placed in a growth chamber on a rack slanted to a 45° angle to promote growth of roots along the transparent acrylic sheet. At 2‐ to 3‐day intervals, rhizotrons were placed on a flatbed scanner to collect digital images from which root length and root diameters were measured using RMS software. Images taken during the course of the experiment were also analyzed with QUACOS software that measures average pixel color values. Color readings obtained were converted to soil water content using images of reference soils of known soil water contents. To verify that roots observed at the surface of the rhizotrons were representative of the total root system in the rhizotrons, they were compared with destructive samples of roots that were carefully washed from soil and analyzed for total root length and root diameter. A significant positive relation was found between visible and washed out roots. However, the influence of soil water content and soil bulk density was reflected on seminal roots rather than first order laterals that are responsible for more than 80 % of the total root length. Changes in soil water content during plant growth could be quantitifed in the range of 0.04 to 0.26 cm³ cm^—3 if image areas of 500 x 500 pixel were analyzed and averaged. With spatial resolution of 12 x 12 pixel, however, soil water contents could only be discriminated below 0.09 cm³ cm^—3 due to the spatial variation of color readings. Results show that this thin‐layer soil rhizotron system allows researchers to observe and quantify simultaneously the time courses of seedling root development and soil water content without disturbance to the soil or roots.     

Root morphology of wheat genotypes differing in zinc efficiency

The root morphology (root length, diameter) of the three wheat genotypes (Triticum aestivum L. cvs Excalibur and Gatcher, and T. turgidum conv. durum (Desf.) McKay cv Durati) grown in zinc (Zn)‐deficient, sandy soil under controlled conditions has been measured by a root scanner coupled to a computer. Wheat plants were supplied with 0, 0.025, 0.05, 0.1, 0.2, or 0.4 mg Zn/kg soil. Excalibur has previously been identified as the Zn‐efficient genotype which can take up more Zn and has higher yield in soils with low plant‐available Zn. Durati is Zn‐inefficient and Gatcher an intermediate genotype with respect to Zn efficiency. Root and shoot dry matter significantly increased at 0.1 mg Zn/kg soil compared to the 0 Zn level. Zinc content in shoots was lower in Durati than in Excalibur and Gatcher at sufficient supply of Zn. Zinc applications had no significant effect on root morphology at two weeks after sowing. At that time, however, the Zn‐efficient genotype Excalibur developed a longer and thinner roots (greater proportion of fine roots with diameter <0.2 mm) than the less efficient Gatcher and Zn‐inefficient Durati. Hence, growing longer and thinner roots and having a greater proportion of thinner roots in the total root biomass early in the growth period may be the two characters associated with the Zn‐efficient genotypes.     

Quantifying the effects of aggregation, particle size and components on the colour of Mediterranean soils

Aggregation, particle size, and chemical composition affect the colour of the soil. We have attempted to quantify and understand these effects in 12 Mediterranean soils. We measured the CIELAB colour variables h_ab, L*, and C*_ab in aggregated and dispersed soil samples, and also in coarse sand, fine sand, silt, and clay samples before and after sequential removal of organic matter, carbonates, and Fe oxides. Grassmann's colour‐mixing equations adjusted by regression analysis described the colour of the dispersed soil from its particle‐size fractions with an error of 1% for h_ab, 4% for L*, and 9% for C*_ab. This suggests that the contribution of each fraction to the colour of the dispersed soil can be accurately calculated by its colorimetric data weighted by its content and a regression coefficient, which was greatest for clay. We inferred the influence of a component within each fraction by measuring the colour changes after its removal. Iron oxides reduced h_ab of the silicated substrate by 19%, reduced L* by 12%, and increased C*_ab by 64% in all particle‐size fractions. Carbonates and organic matter had little influence: the former because they impart little colour to the silicates and the latter because there was little of it. The CIELAB colour‐difference between dispersed and aggregated soil (mean ΔE*_ab = 15.3) was due mainly to ΔL* (?14.7). Aggregation contributed to diminishing L* of dispersed soil by 34%. Scanning electron microscopy showed that Fe oxides and organic coatings cover the surface of aggregates thereby influencing soil colour.