Pore network and water retention characteristics of volcanic porous media

Aggregate media are often characterized by multi‐porous systems, which have structural and water retention characteristics that depend on the complex interaction between intra‐ and inter‐aggregate pores. Here we investigate the structure and water retention dynamics of rigid aggregate volcanic materials. In particular, we focus on commercially used pumices, lapilli and zeolites. The aim was to estimate the air and water content through complex dual‐porous systems, and thus to evaluate their suitability for vegetation growth. Both inter‐ and intra‐aggregate characteristics were determined by means of mercury intrusion porosimetry, X‐ray microtomography and water retention curves. The wilting point was determined with pressure plates, a dew point hygrometer and the sunflower method to assess their reliability at small matric potentials. Results indicate that aggregate porous media were bimodal and their heterogeneous pore network affected the water retention dynamics because (i) the large inter‐aggregate pores allowed a rapid drainage near saturation and (ii) the intra‐aggregate porosity held water available for root uptake and plant growth. In contrast, volcanic powders were less affected by the inter‐ and intra‐aggregate dual‐porosity. The use of a dew point hygrometer instead of pressure plates for determining small matric potentials is also suggested because pressure plates might over‐estimate the water content because of poor plate and soil conductance. However, the reference potential at wilting point should be set at values greater than ?1471.5 kPa (?784.8 kPa) to consider the interaction between plant roots and porous media with small hydraulic conductivity. Results from this work indicate that aggregate multi‐porous media allow the simultaneous supply of oxygen and available water for plants, although the heterogeneous nature of the pore network involves uncertainties regarding water balance and root–matrix interactions.     

Visual structure assessment and mechanical soil properties of re‐cultivated soils made up of loess

Re‐cultivated soils (previously piled soils used as the final surface cover in renovation of open cast mine sites) are particularly susceptible to compaction, which is why a simple estimate of mechanical strength is necessary for land management. In this study, therefore, precompression stress (?6 kPa matric potential) was determined for a total of 20 soil layers from 9 repeatedly cultivated areas of arable land in North Rhine–Westphalia (Germany), along with the aggregate density/dry bulk density ratio (as a measure of density heterogeneity) and air capacity (as a soil ecological parameter). These results are contrasted with the determination of packing density. Packing density (PD) is an integrated parameter that combines various properties (aggregate size, cohesion of the soil structure, root distribution, biogenic macropores and aggregate arrangement) and is assessed visually in the field. Packing density levels range between 1 (very loose soil) and 5 (very highly compacted). There is a strongly negative relationship between packing density and both the aggregate density/dry bulk density ratio and air capacity. Conversely, mechanical precompression stress increases with packing density. Ranges of the individual parameters can be assigned to each of the packing density levels. Packing density level 3 represents an optimization with regard to mechanical soil stability whilst maintaining minimum air capacity requirements (5–8 Vol.‐%).     

Effects of decreasing soil water content on seminal lateral roots of young maize plants

Soil micropores that contain water at or below field capacity cannot be invaded by seminal or first‐order lateral roots of maize plants because their root diameters are larger than 10 μm. Hence, at soil‐water levels below field capacity plant roots must establish a new pore system by displacement of soil particles in order to access soil water. We investigated how decreasing soil water content (SWC) influences growth and morphology of the root system of young maize plants. Plants were grown in rhizotrons 40 cm wide, 50 cm high, and approximately 0.7 cm thick. Five SWC treatments were established by addition of increasing amounts of water to soil and thorough mixing before filling the rhizotrons. No water was added to treatments 1–4 throughout the experiment. Treatment 5 was watered frequently throughout the experiment to serve as a control. Seminal‐root length and SWC in soil layers 0–10, 10–20, 20–30, 30–40, and 40–50 cm were measured at intervals of 2–3 d on scanner images by image analysis. At 15 d after planting, for treatments 1–4 shoot dry weight and total root length were directly related to the amount of water added to the soil, and for treatments 4 and 5, total root length and shoot dry weights were similar. Length of seminal roots visible at the transparent surface of the rhizotron for all treatments was highest in the uppermost soil layer and decreased with distance from the soil surface. For all layers, seminal‐root elongation rate was at maximum above a SWC of 0.17 cm³ cm^–3, corresponding to a matric potential of –30 kPa. With decreasing SWC, elongation rate decreased, and 20% of maximum seminal root elongation rate was observed below SWC of 0.05 cm³ cm^–3. After destructive harvest for treatment 1–4, number of (root‐) tips per unit length of seminal root was found uninfluenced over the range of initial SWC from 0.10 to 0.26 cm³ cm^–3. However, initial SWC close to the permanent wilting point strongly increased number of tips. Average root length of first‐order lateral (FOL) roots increased as initial SWC increased, and the highest length was found for the frequently watered treatment 5. The results of the study suggest that the ability to produce new FOL roots across a wide range of SWC may give maize an adaptive advantage, because FOL root growth can rapidly adapt to changing soil moisture conditions.     

Einfluß des Bodengefüges auf Wurzelwachstum und Phosphataufnahme von Sommerweizen

Influence of Soil Structure on Root Growth and P Uptake of Spring Wheat The effect of soil structure (fine aggregat, coarse aggregate and compacted structure) on root growth, root morphology, and P availability of spring wheat (Triticum aestivum L.) was studied in pot and split root experiments using three soils (2 × Alfisol-Udalf, Alluvium). CAL-soluble P was 63–90 mg P · kg^?1 soil, indicating a sufficient P supply. Root length, root surface, root fresh weight, shoot weight, and seed yield were decreased due to coarse aggregate and compacted structure. Roots were significantly thickened and roots hairs were longer in the fine aggregate structure than in the compacted and coarse aggregate structures. P concentration in the shoot and P uptake of spring wheat growing in the coarse aggregate and compacted structure were lower because root growth was decreased. In the split root experiment, in contrast to pot experiment, P uptake was lower in the compacted than in the fine aggregate treatment. The results demonstrate that P availability was influenced by soil structure via the influence on root growth and thus access of roots to P.     

Messung und Simulation der anisothermen Feuchtebewegung in benetzungsgehemmten Mineralböden

Measurement and simulation of nonisothermal moisture movement in water-repellent mineral soils Water repellency often occurs in surface soil horizons, where the temperature may have a significant effect on water movement. Relatively few studies have been conducted to measure the effect of temperature gradients on hydraulic processes in water-repellent soils. The objective of this paper is to analyze the simultaneous impact of hydraulic and thermal gradients on water movement in unsaturated soils. Four sandy soils with organic matter contents between 1.0 and 2.4 percent were used in a laboratory column experiment. The soils had similar particle size distribution and contact angles between 25 and 111 degrees. The water repellency was measured with the “sessile drop method”. The experiments were conducted with open laboratory soil columns. A ceramic plate was placed at the upper end of the soil column to allow water movement from a reservoir, the lower end was closed. The matric potential of ?175 hPa at the upper part of the column was held constant. Simultaneously a constant temperature gradient of approximatly 1.1 °C/cm was applied during the nonisothermal runs. Hydraulic properties of the four soils were determined sequentially at different temperatures (5, 20 and 38°C). The time dependent volumetric water content, matric potential and temperature were measured at five positions in the column. It was found that after steady state was established the water content and matric potential profiles of the four soils differed considerably. A computer code based on the theory of Philip and De Vries (1957) was developed to simulate water and heat transport. For three soils with contact angles of 25–35, 93–108 and 109–111 degrees, respectively, the model performed well when the theoretically calculated thermal vapor difivisivity, D_TV, was multiplied by a factor between two and three. For the slightly water repellent soil with a contact angle between 45 and 83 degrees, however, this factor was about seven to eight. This factor could be the result of a combined impact of the moderate water repellency and the micromorphology of the humic substance.     

Contact between root and iron substrate and the mobilization of iron by soybean roots

The question of whether a direct (uninterrupted) contact between plant roots and solid substrate is a prerequisite for the mobilization of sparingly soluble iron (Fe) by plant roots was investigated. Factorial combinations of two types of contact between the roots and rooting medium [direct or interrupted (roots placed inside a dialysis tube (DT)], two rooting media (nutrient solution or vermiculite), and two Fe supply conditions [without soluble Fe (control) or plus Fe‐EDDHA] were investigated. Placing the roots inside DT, with no visible mechanical impedance for the root, reduced the growth of tops and roots in most cases. Iron mobilization, judged by the total amount of Fe in the tops, strongly decreased if roots were enclosed in DT. This was true in plants grown in vermiculite as well as in nutrient solution containing Fe‐EDDHA. It is concluded that a direct contact between the surface of root cell walls and the Fe substrate is a prerequisite for the Fe mobilization by the plant roots.     

Biogeochemical processes and arsenic enrichment around rice roots in paddy soil: results from micro‐focused X‐ray spectroscopy

The spatial distribution and speciation of iron (Fe), manganese (Mn) and arsenic (As) around rice roots grown in an As‐affected paddy field in Bangladesh were investigated on soil sampled after rice harvest. Synchrotron micro‐X‐ray fluorescence spectrometry on soil thin sections revealed that roots influence soil Fe, Mn and As distribution up to 1 mm away from the root–soil interface. Around thick roots (diameter around 500 µm), Mn was concentrated in discrete enrichments close to the root surface without associated As, whereas concentric Fe accumulations formed farther away and were closely correlated with As accumulations. Near thin roots (diameter < 100 µm), in contrast, a pronounced enrichment of Fe and As next to the root surface and a lack of Mn enrichments was observed. X‐ray absorption fine structure spectroscopy suggested that (i) accumulated Fe was mainly contained in a two‐line ferrihydrite‐like phase, (ii) associated As was mostly As(V) and (iii) Mn enrichments consisted of Mn(III/IV) oxyhydroxides. The distinct enrichment patterns can be related to the extent of O₂ release from primary and lateral rice roots and the thermodynamics and kinetics of Fe, Mn and As redox transformations. Our results suggest that in addition to Fe(III) plaque at the root surface, element accumulation and speciation in the surrounding rhizosphere soil must be taken into account when addressing the transfer of nutrients or contaminants into rice roots.     

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.     

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.     

Oxygen and redox potential gradients in the rhizosphere of alfalfa grown on a loamy soil

Oxygen (O₂) supply and the related redox potential (E_H) are important parameters for interactions between roots and microorganisms in the rhizosphere. Rhizosphere extension in terms of the spatial distribution of O₂ concentration and E_H is poorly documented under aerobic soil conditions. We investigated how far O₂ consumption of roots and microorganisms in the rhizosphere is replenished by O₂ diffusion as a function of water/air‐filled porosity. Oxygen concentration and E_H in the rhizosphere were monitored at a mm‐scale by means of electroreductive Clark‐type sensors and miniaturized E_H electrodes under various matric potential ranges. Respiratory activity of roots and microorganisms was calculated from O₂ profiles and diffusion coefficients. pH profiles were determined in thin soil layers sliced near the root surface. Gradients of O₂ concentration and the extent of anoxic zones depended on the respiratory activity near the root surface. Matric potential, reflecting air‐filled porosity, was found to be the most important factor affecting O₂ transport in the rhizosphere. Under water‐saturated conditions and near field capacity up to –200 hPa, O₂ transport was limited, causing a decline in oxygen partial pressures (pO₂) to values between 0 and 3 kPa at the root surface. Aerobic respiration increased by a factor of 100 when comparing the saturated with the driest status. At an air‐filled porosity of 9% to 12%, diffusion of O₂ increased considerably. This was confirmed by E_H around 300 mV under aerated conditions, while E_H decreased to 100 mV on the root surface under near water‐saturated conditions. Gradients of pO₂ and pH from the root surface indicated an extent of the rhizosphere effect of 10–20 mm. In contrast, E_H gradients were observed from 0 to 2 mm from the root surface. We conclude that the rhizosphere extent differs for various parameters (pH, Eh, pO₂) and is strongly dependent on soil moisture.     

Aggregate stability of a silt loam soil as affected by roots of corn,soybeans and wheat

Abstract

A greenhouse experiment was conducted to investigate the effect of root growth and exudation of 3 crop species on soil aggregation. Two plant populations for each of 3 crops (corn, soybeans, and wheat) were grown in a Fincastle silt loam for 5 time periods (7, 14, 21, 28, and 41 days) and compared with fallow controls. Aggregate stability was estimated by the wet‐sieve method on both initially moist and air‐dry samples.

Soil water content of initially moist soil samples varied widely among replicates, crops, and sampling dates. Wet‐sieving using initially moist soil showed that samples with higher initial soil water content had greater aggregate stability. Wet‐sieving performed on initially air‐dry soil samples was used for subsequent interpretation because the water content variable was removed.

The presence of any crop and its roots in the planted soils versus the fallow controls was associated with increases in aggregate stability. No differences in aggregate stability were found among the different crops or over the established range of root length densities. Aggregate stability decreased from the original level during the first 14 to 21 days of the experiment, possibly due to daily watering. After 21 days, as root growth continued to increase, restabilization occurred until the original aggregate stability of the soil was exceeded for all crops. The observed increase in aggregate stability may be due in part to the physical entanglement of aggregates by roots and to the increased production of root exudates resulting from increased root growth.     

Structural differences between bulk and rhizosphere soil

The physical characteristics of the soil at the root–soil interface are crucial because they determine both physical aspects of root function such as water and nutrient uptake and the microbial activity that is most relevant to root growth. Because of this we have studied how root activity modifies the structure and water retention characteristic of soil adjacent to the root for maize, wheat and barley. These plants were grown in pots for a 6‐week growth period, then the soil adjacent to the root (rhizosphere soil) and bulk soil aggregates were harvested. These soil aggregates were then saturated and equilibrated at matric potentials between ?600 kPa and saturation, and the water retention characteristics were measured. From subsamples of these aggregates, thin sections were made and the porosity and pore‐size distributions were studied with image analysis. Both image analysis and estimates of aggregated density showed that the rhizosphere soil and bulk soil had similar porosities. Growing different plants had a small but significant effect on the porosity of the soil aggregates. Image analysis showed that for all the plant species the structure of the rhizosphere soil was different to that of the bulk soil. The rhizosphere soil contained more larger pores. For maize and barley, water retention characteristics indicated that the rhizosphere soil tended to be drier at a given matric potential than bulk soil. This effect was particularly marked at greater matric potentials. The difference between the water retention characteristics of the bulk and rhizosphere soil for wheat was small. We compare the water retention characteristics with the data on pore‐size distribution from image analysis. We suggest that differences in wetting angle and pore connectivity might partly explain the differences in water retention characteristic that we observed. The impact of differences between the water retention properties of the rhizosphere and bulk soil is discussed in terms of the likely impact on root growth.     

Effects of soil bulk density on seminal and lateral roots of young maize plants (Zea mays L.)

It is well established that increasing soil bulk density (SBD) above some threshold value reduces plant root growth and thus may reduce water and nutrient acquisition. However, formation and elongation of maize seminal roots and first order lateral (FOL) roots in various soil layers under the influence of SBD has not been documented. Two studies were conducted on a loamy sand soil at SBD ranging from 1.25 g cm^–3 to 1.66 g cm^–3. Rhizotrons with a soil layer 7 mm thick were used and pre‐germinated plants were grown for 15 days. Over the range of SBD tested, the shoot growth was not influenced whereas total root length was reduced by 30 % with increasing SBD. Absolute growth rate of seminal roots was highest in the top soil layer and decreased with increasing distance from the surface. Increasing SBD amplified this effect by 20 % and 50 % for the top soil layer and lower soil layers, respectively. At the end of the experiment, total seminal roots attributed to approximately 15 % of the total plant root length. Increasing SBD reduced seminal root growth in the lowest soil layer only, whereas FOL root length decreased with SBD in all but the uppermost soil layer. For FOL, there was a positive interaction of SBD with distance from the soil surface. Both, increasing SBD and soil depth reduced root length by a reduction of number of FOL roots formed while the length of individual FOL roots was not influenced. Hence, increasing SBD may reduce spatial access to nutrients and water by (i) reducing seminal root development in deeper soil layers, aggravated by (ii) the reduction of the number of FOL roots that originate from these seminal roots.     

Clay mineralogy differs qualitatively in aggregate‐size classes: clay‐mineral‐based evidence for aggregate hierarchy in temperate soils

Clay minerals have a major role in soil aggregation because of their large specific surface area and surface charges, which stimulate interactions with other mineral particles and organic matter. Soils usually contain a mixture of clay minerals with contrasting surface properties. Although these differences should result in different abilities of clay minerals regarding aggregate formation and stabilization, the role of different clay minerals in aggregation has been seldom evaluated. In this study, we took advantage of the intrinsic mineral heterogeneity of a temperate Luvisol to compare the role of clay minerals in aggregation. First, grassland and tilled soil samples were separated in water into aggregate‐size classes based on the aggregate hierarchy model. Then, clay mineralogy and organic C in the aggregate‐size classes were analysed. Interstratified minerals containing swelling phases accumulated in aggregated fractions compared with free clay fractions under the two land‐uses. The accumulation increased with decreasing aggregate size from large macroaggregates (> 500 µm) to microaggregates (50–250 µm). Carbon content and carbon‐to‐nitrogen ratio followed the opposite trend. This fully supports the aggregate hierarchy model, which postulates an increasing importance of mineral reactivity in smaller aggregates than in larger aggregates in which the cohesion relies mostly on physical enmeshment by fungal hyphae or small roots. Consequently, differences in the proportion of the different 2:1 clay minerals in soils can influence their structure development. Further research on the links between clay mineralogy and aggregation can improve our understanding of mechanisms of soil resistance to erosion and organic matter stabilization.     

Spatially tracking carbon through the root–rhizosphere–soil system using laser ablation‐IRMS

The intimate relationships between plant roots, rhizosphere, and soil are fostered by the release of organic compounds from the plant into soil through various forms of rhizodeposition and the simultaneous harvesting of nutrients from the soil to the plant. Here we present a method to spatially track and map the migration of plant‐derived carbon (C) through roots into the rhizosphere and surrounding soil using laser ablation‐isotope ratio mass spectrometry (LA‐IRMS). We used switchgrass microcosms containing soil from field plots at the Kellogg Biological Station (Hickory Corners, Michigan, USA) which have been cropped with switchgrass since 2008. We used a ¹³CO₂ tracer to isotopically label switchgrass plants for two diel cycles and tracked subsequent movement of labeled C using the spatially specific (< 100 µm resolution) δ¹³C analysis enabled by LA‐IRMS. This approach permitted assessment of variable C flow through different roots and enabled mapping of spatial variability of C allocation to the rhizosphere. Highly ¹³C‐enriched C (consistent with production during the ¹³CO₂ application period) extended ≈ 0.5–1 mm from the root into the soil, suggesting that the majority of recent plant‐derived C was within this distance of the root after 48 h. Tracking the physical extent of root exudation into the rhizosphere can help evaluate the localization of plant‐microbe interactions in highly variable subsurface environments, and the use of the isotopic label can differentiate freshly fixed C (presumably from root exudates) from other types of subsurface C (e.g., plant necromass and microbial turnover). The LA‐IRMS technique may also serve as a valuable screening technique to identify areas of high activity for additional microbial or geochemical assays.     

Tillage effects on shear strength and bulk density of soil aggregates

An experiment was conducted to determine the effect of four tillage systems (moldboard plow, chisel plow, Paraplow and no-till) on soil aggregate shear strength and bulk density. Two soils, a Canisteo clay loam (fine-loamy, mixed (calcareous), mesic, Typic Haplaquoll) and a Haig silt loam (fine, montmorillonitic, mesic, Typic Argiaquoll) were used in this study. Soil samples were collected from the 0.075–0.15-m-depth increment in 1983 and the 0.075–0.15- and 0.225–0.30-m-depth increments in 1985. Shear strength of soil aggregates 0.02–0.03 m in diameter was measured by a fall-cone penetrometer and bulk density of the same aggregates was measured by gamma-ray attenuation. Aggregates were tested at soil water matric potentials (ψ_m) of −0.2, −1.1 and −4.0 kPa in 1983 and at ψ_m of −0.2, −1.1, −4.0 and −7.9 kPa in 1985. Tillage for the 1983 growing season was conducted under very wet conditions, whereas tillage for the 1985 growing season was conducted under much drier conditions. Samples collected in 1983 showed little tillage effect on shear strength or bulk density. In 1985, tillage had an effect on shear strength and bulk density for the Haig soil, but not for the Canisteo soil. Much of the tillage effect on soil aggregate shear strength could be explained by tillage-induced changes in the aggregate bulk density. As bulk density decreased, soil aggregate shear strength decreased.Sampling depth had no effect on soil aggregate shear strength or bulk density. Matric potential had an effect on soil aggregate shear strength and bulk density. As matric potential decreased, both shear strength and bulk density increased.     

Modern wheat cultivars have greater root nitrogen uptake efficiency than old cultivars

The availability of nitrogen (N) contained in crop residues for a following crop may vary with cultivar, depending on root traits and the interaction between roots and soil. We used a pot experiment to investigate the effects of six spring wheat (Triticum aestivum L.) cultivars (three old varieties introduced before mid last century and three modern varieties) and N fertilization on the ability of wheat to acquire N from maize (Zea mays L.) straw added to soil. Wheat was grown in a soil where ¹⁵N‐labeled maize straw had been incorporated with or without N fertilization. Higher grain yield in three modern and one old cultivar was ascribed to preferred allocation of photosynthate to aboveground plant parts and from vegetative organs to grains. Root biomass, root length density and root surface area were all smaller in modern than in old cultivars at both anthesis and maturity. Root mean diameter was generally similar between modern and old cultivars at anthesis but was greater in modern than in old cultivars at maturity. There were cultivar differences in N uptake from incorporated maize straw and the other N sources (soil and fertilizer). However, these differences were not related to variation in the measured root parameters among the six cultivars. At anthesis, total N uptake efficiencies by roots (total N uptake per root weight or root length) were greater in modern than in old cultivars within each fertilization level. At maturity, averaged over fertilization levels, the total N uptake efficiencies by roots were 292?336 mg N g^?1 roots or 3.2?4.0 mg N m^?1 roots for three modern cultivars, in contrast to 132?213 mg N g^?1 roots or 0.93?1.6 mg N m^?1 roots for three old cultivars. Fertilization enhanced the utilization of N from maize straw by all cultivars, but root N uptake efficiencies were less affected. We concluded that modern spring wheat cultivars had higher root N uptake efficiency than old cultivars.     

Quantifying rhizosphere particle movement around mutant maize roots using time‐lapse imaging and particle image velocimetry

Soil surrounding a growing root must be displaced to accommodate the increased root volume. To ease soil penetration, root caps produce border cells and mucilage that lubricate the root surface, decreasing friction at the root‐soil interface. Rhizosphere deformations caused by roots with or without a functional root cap were compared to determine the effects of the root cap on sand displacement and penetration. Intact (KYS wild type) and decapped (agt1_dec mutant) primary maize roots were grown in observation chambers filled with sand. Non‐destructive time‐lapse micro‐imaging combined with particle image velocimetry was used to visualize and quantify sand displacements as small as 0.5 µm caused by growing roots. Decapped (agt1_dec) roots displayed typical responses of mechanically impeded roots at sand densities that did not affect intact KYS roots. Sand displacement decreased exponentially with distance from the root and extended four to eight root radii into the sand. The calculated mean sand density increase and the compressed sand area were doubled by decapping. Maximum density often occurred in front of the apex of decapped roots whereas it occurred along the sides of intact roots. Periodic variation in sand deformation was observed, probably associated with root circumnutation, which may also facilitate soil penetration. Sand particles moved alongside KYS roots more easily than they did alongside agt1_dec roots. A functional exuding cap was therefore essential for efficient rhizosphere deformation and penetration by roots. Manipulating root tip, and specifically root cap, properties is a possible target for improving root penetration in hard soil.     

Phosphatverarmung des wurzelnahen Bodens und Phosphataufnahme von Mais und Raps

Phosphate depletion at the soil — root interface and the phosphate uptake of maize and rape Maize and rape plants were grown in flat containers in a ³3P-labelled sandy soil and the distribution of soil phosphate near roots was determined by using densitometric scans of autoradiographs. The concentration of isotopically exchangeable phosphate at the root surface decreased within a few days by 42 per cent with rape and by 50–65 per cent with maize. Initially the width of the depletion zone is very small. Within six days the depletion zone extended to the final distance from the surface of the root cylinders of about 2 mm for maize and 2.6 mm for rape. The soil within the range of the mean length of root hairs (0.7 mm for maize and 1.3 mm for rape) is almost equally depleted. This indicates that root hairs are very important for P-uptake from soil. This is further supported by higher P-uptake rates per cm root length of rape than of maize. The P-concentration of the soil solution was estimated by means of the phosphate desorption curve. Within the root hair cylinder the P-concentration of the soil solution decreased from 0.8 to 0.03 mg P/l. Changes of the P-depletion profile with time were used to calculate P-uptake rates for roots of different age. The results indicate that for the first 3–5 days P-uptake rates remained near maximum, even though the P-concentration of the soil solution at the root surface had strongly decreased within two days. Phosphate uptake rates per cm root length did not decrease unless the whole root hair cylinder had been depleted.     

Effects of N‐fertilization on shoots and roots of rape (Brassica napus L.) and consequences for the soil matric potential

The underlying question of these investigations asked, how and to which extent rape plants react with transpiration and soil water uptake to different degrees of nitrogen fertilization. Therefore repeated campaigns with concurrent measurements of plant surfaces (leaves, stems, pods), diurnal courses of leaf transpiration and root length density of rape plants growing on heavily (240 kg ha^—1), moderately, (120 kg ha^—1), and nil N‐fertilized plots of an experimental field in northern Germany were performed during two growing seasons. Additionally, matric potentials at different soil depths were measured. In the first year (1994) investigations were concentrated primarily on shoot area development and transpiration, whereas in the subsequent year (1995) root measurements were mainly undertaken. Also, the influence of soil management (ploughing, conservation tillage) was taken into consideration. The plots where the shoot measurements were carried out were ploughed in 1994 and rotovated in 1995. Matric potentials were measured in both years in ploughed soil and, for comparison, also in soils with conservation tillage. Shoot area index, as measure of the transpiratory capacity of the canopy, increased on ploughed soil and reached a maximum before flowering. Thereafter it decreased until harvest when the relative amount of green stems and pods was increasing. Then, the measured transpiration rate per pod surface area was equal to, or higher than, the transpiration rate per leaf surface area. Plant surface area was smaller in plots with conservation tillage and decreased generally with decreasing N‐fertilization. Increasing plant surface area was joined by an increasing density of plant canopy. Light interception was thus highest in the plots receiving 240 kg N ha^—1. Although the shading effect may cause a reduction of transpiration per plant, the total plant mass per area generally resulted in a greater water loss from these plots. Roots reached at least 110 cm depth. Root length density was significantly higher in the upper 10—30 cm of soil than at greater depths. Root mass was smaller in soil with conservation tillage than in ploughed soil. Oscillations of soil matric potentials in the diurnal and long‐term periods were highest in the upper 10 cm of soil. Here, they corresponded well with the cumulative diurnal transpiratory water loss. It is concluded that the soil water dynamics depends largely on the distribution of plant roots. As a result, rape plants did not change their specific transpiration capacity as a response to increased nitrogen fertilization. However, the transpiring plant surface and root length density increased the turnover rate of water by a higher plant density per plot. This effect was more pronounced in ploughed than in rotovated plots.