Root‐growth characteristics contributing to genotypic variation in nitrogen efficiency of oilseed rape

A 3‐year field experiment was carried out to determine the significance of root‐growth characteristics contributing to N‐uptake efficiency of two oilseed rape (Brassica napus L.) cultivars differing in N efficiency. Two N treatments were applied, and the core and minirhizotron techniques were used to study root‐length density and number of living roots, respectively. Fertilizer‐N supply increased shoot dry matter, grain yield, total N uptake, and total soil N_min contents particularly in the top soil. Although significant differences occurred in all parameters between years, the interactions between years and cultivars were mostly not significant. Compared to cv. Capitol, the N‐efficient cv. Apex was characterized by a higher grain yield at N0 and a higher N uptake during reproductive growth. This genotype also had a higher root‐length density and more living fine roots particularly in the topsoil layer. Root growth of this genotype was especially high from beginning of shooting to beginning of flowering, while shoot growth and N uptake during vegetative growth were comparatively low. Our results suggest that N‐efficient cultivars can be characterized by a high investment in root growth during the vegetative stage with a comparatively slow shoot growth and N‐uptake rate until beginning of flowering, which, however, continues during reproductive growth. High root production only during reproductive growth seems to be less effective to achieve high N efficiency, because this may lead to a shortage of assimilates for seed filling. High root‐length density at vegetative stages may thus be advantageous for N uptake and reproductive growth and could be a useful morphological character for the selection and breeding of N‐efficient cultivars.     

Legume residue influence arbuscular mycorrhizal colonisation and P uptake by wheat

Legumes have been shown to increase growth and P uptake of the following cereal. This could, in part, be due to nutrients released by the decomposing legume residues. To investigate the effect of P added with legume residues on wheat growth, P uptake and arbuscular mycorrhizal (AM) colonisation, a number of experiments were conducted with different legume residues added to a soil with low P availability under conditions in which N was not limiting. Young and mature faba bean shoots (FYS, FMS) and mature chickpea shoots (CP) were added to soil at different rates (0.5–2%, w/w) with the P concentration being the greatest in the young faba bean shoots and least in the mature chickpea residues. Other treatments included addition of inorganic P at different rates (0–80 mg P kg⁻¹). Available P, growth and P uptake and AM colonisation of wheat were measured after 6 weeks. As expected, inorganic P addition increased growth and P uptake but decreased AM colonisation. The effect of the residues was more complex. AM colonisation was not correlated with available P in the soil amended with residues, whereas there was significant negative correlation between available P and AM colonisation in the treatments with inorganic P. Addition of FYS increased wheat shoot growth and P uptake and decreased AM colonisation. However, FMS and CP addition not only decreased wheat growth and P uptake but also AM colonisation despite low soil P availability. It is concluded that addition of some legume residues can improve the growth of subsequent cereals, but others have a negative effect on wheat growth and AM colonisation which cannot be explained solely by soil P availability.     

Extensivierung von Niedermoorgrünland nach 30 Jahren intensiver N‐Düngung

Im vorliegenden Beitrag werden Ergebnisse aus einem langjährigen N‐Düngungsversuch auf Niedermoor‐grünland dargestellt. Extensivierte Bewirtschaftung hat reduzierte Erträge, eine Veränderung der pflanz‐lichen Inhaltsstoffe und der Pflanzenbestandszusammensetzung an Gräsern und Kräutern zur Folge. Im 3 Jahr nach Aussetzen der N‐Düngung sind Erträge und Inhaltsstoffe der langjährigen Null‐Variante angeglichen. Die Pflanzenbestandszusammensetzung weist dagegen nach 4 Jahren noch Unterschiede auf.     

SEVERITY OF SALINITY ACCURATELY DETECTED AND CLASSIFIED ON A PADDOCK SCALE WITH HIGH RESOLUTION MULTISPECTRAL SATELLITE IMAGERY

We hypothesised that digital mapping of various forms of salt‐affected soils using high resolution satellite imagery, supported by field studies, would be an efficient method to classify and map salinity, sodicity or both at paddock level, particularly in areas where salt‐affected patches are small and the effort to map these by field‐based soil survey methods alone would be inordinately time consuming. To test this hypothesis, QuickBird satellite data (pan‐sharpened four band multispectral imagery) was used to map various forms of surface‐expressed salinity in an agricultural area of South Australia. Ground‐truthing was performed by collecting 160 soil samples over the study area of 159 km². Unsupervised classification of the imagery covering the study area allowed differentiation of severity levels of salt‐affected soils, but these levels did not match those based on measured electrical conductivity (EC) and sodium adsorption ratio (SAR) of the soil samples, primarily because the expression of salinity was strongly influenced by paddock‐level variations in crop type, growth and prior land management. Segmentation of the whole image into 450 paddocks and unsupervised classification using a paddock‐by‐paddock approach resulted in a more accurate discrimination of salinity and sodicity levels that was correlated with EC and SAR. Image‐based classes discriminating severity levels of salt‐affected soils were significantly related with EC but not with SAR. Of the spectral bands, bands 2 (green, 520–600 nm) and 4 (near‐infrared, 760–900 nm) explained the majority of the variation (99 per cent) in the spectral values. Thus, paddock‐by‐paddock classification of QuickBird imagery has the potential to accurately delineate salinity at farm level, which will allow more informed decisions about sustainable agricultural management of soils. Copyright © 2011 John Wiley & Sons, Ltd.     

Growth rate and carbohydrate metabolism of potato tubers exposed to high temperatures

Summary The effect of high tuber temperature (30 °C) on the growth rate and carbohydrate metabolism of tubers was studied with 3 cultivars differing in heat tolerance. Subjecting individual tubers to 30°C for 6 days caused cessation of tuber growth. During this period, non-treated tubers (20°C) on the same plant increased in volume. At 30°C, incorporation of¹⁴C-labelled assimilates into the ethanol-insoluble fraction (mainly starch) as well as the starch content was significantly reduced. In contrast, the incorporation of¹⁴C-labelled assimilates into the sugar fraction was not affected by high tuber temperature. At 30°C the activity of some of the enzymes involved in starch metabolism was depressed. Varietal differences in responses of tuber growth rate and carbohydrate metabolism to high tuber temperature were insignificant. From these preliminary results it is concluded that high soil temperatures may decrease starch content of potato tubers directly by inhibition of the conversion of sugars into starch.     

Effects of reducing leaf area and tuber number on the growth rates of tubers on individual potato plants

Summary Potato plants (Solanum tuberosum L. cv. Ostara) were grown in nutrient solution culture in a controlled environment. Growth rates of individual tubers were determined daily by measuring the increase in their volume. Decreasing the source:sink ratio of individual plants by removing half of the leaf area, decreased tuber growth rate per plant by 50%. Increasing the source: sink ratio of individual plants by cooling to 8°C, or by removing individual tubers of known growth rate, increased the growth rate of non-cooled, remaining tubers within 2–4 days. The potential of the tubers to increase their growth rates was not related to the date of tuber removal after tuber initiation. It is concluded that during the phase of linear tuber bulking, tuber growth is neither limited by pathway nor sink, but by the source capacity of the plant.

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Zusammenfassung Kartoffelpflanzen (Solanum tuberosum L. cv. Ostara) wuchsen in N?hrl?sung unter kontrollierten Umweltbedingungen (14 h Tagesl?nge, 22°C/18°C Tag/Nachttemperatur, 60–80% relative Luftfeuchtigkeit, Licht-intensit?t in Pflanzenh?he ≈380 μE m⁻² s⁻¹). Die Wachstumsraten der einzelnen Knollen wurden t?glich aus ihrer Volumenzunahme bestimmt. Die Auswirkungen von Source- und Sink-Manipulationen auf die Wachstumsraten der Knollen wurden w?hrend der Phase linearen Knollenwachstums untersucht. Eine Reduzierung der Source-St?rke einzelner Pflanzen durch Entfernen jedes 2. Fiederblattes oder eines von 2 gleichwertigen hauptstengeln pro Pflanze bewirkte eine sofortige Abnahme des Knollenwachstums pro Pflanze um 50% (Abb. 4). In den meisten F?llen nahm auch die Wachtstumsrate der einzelnen Knollen an den Pflanzen um etwa 50% ab (Abb. 5). Durch Kühlung einzelner Knollen auf +8°C wurden deren Wachstumsraten im Durchschnitt von 4,5 auf 1,6 cm³ d⁻¹ gesenkt, w?hrend gleichzeitig die Wachstumsraten der ungekühlten Knollen von 2,8 auf 5,2 cm³ d⁻¹ anstiegen (Tab. 1). Das Entfernen einzelner Knollen, die bis dahin durch-schnittlich zu 60% am Gesamtknollenwachstum pro Pflanze beteiligt waren, führte innerhalb von 2–4 Tagen zu einem mehr als doppelt so schnellen Wachstum der an den Pflanzen verbliebenen Knollen, so dass das Gesamtknollenwachstum pro Pflanze nicht signifikant ver?ndert wurde (Abb. 1). Hierbei wurden die Wachstumsraten aller an der Pflanze verbliebenen Knollen erh?ht, wobei Wachstumsraten von mehr als 8 cm³ pro Einzelknolle und Tag erreicht wurden (Abb. 3). Die F?higkeit der Knollen, ihre Wachstumsrate dem jeweiligen Assimilatangebot anzupassen, war nicht davon abh?ngig, zu welchem Zeitpunkt nach der Knolleninduktion (10–30 d) einzelne Knollen entfernt wurden (Abb. 2). Aus den Untersuchungen wird gefolgert, dass in der phase des linearen Knollenwachstums eine Limitierung des Knollenwachstums weder durch den transportweg noch durch die Speicherf?higkeit der Knollen bewirkt wird, sondern durch die F?higkeit des oberirdischen Sprosses, Assimilate zur Verfügung zu stellen.

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Résumé Des plantes de pommes de terre (Solanum tuberosum L. cv. Ostara) sont cultivées dans une solution nutritive sous atmosphère contr?lée (longueur de jours 14 h; températures jour/nuit 22°C/18°C; humidité relative de l'air 60 à 80%; intensité du rayonnement photosynthétique à hauteur de la plante ≃380 μF m⁻² s⁻¹). Le grossissement des tubercules est déterminé chaque jour en mesurant l'augmentation de leur volume. Les effets de la source d'assimilation et du puits d'absorption sont analysés pendant la phase linéaire de grossissement des tubercules. Une diminution du rapport source: puits d'absorption est obtenue en supprimant la moitié de la surface foliaire de chaque plante. Le grossissement des tubercules diminue en conséquence de 50% (figure 4). Mis à part quelques exceptions, les taux de croissance des tubercules individuels baissent également de 50% (figure 5). Le refroidissement de certains tubercules d'une même plante à 8°C entra?ne une diminution moyenne de leur grossissement de 4,5 à 1,6 cm³/jour. Dans le même temps, les autres tubercules voient leur croissance augmenter de 2,8 à 5,2 cm³/jour (tableau 1). La suppression de certains tubercules qui avaient contribué pour 60% au grossissement total obtenu par plante, entr?ne une augmentation de croissance de plus de 100% des tubercules restants, ceci en moins de 2 à 4 jours. De ce fait, la croissance totale, par plante n'est pas significativement inférieure (figure 1). Après suppression de certains tubercules, on constate que pour tous les tubercules restants leur grossissement augmente et le taux maximum de croissance atteint 8 cm³/jour (figure 3). Les tubercules ajustent leur grossissement en fonction de l'apport effectif de photosynthèse sans liaison avec la date de suppression de certains tubercules après leur initiation (figure 2). En conclusion, pendant la phase linéaire de grossisement, la croissance des tubercules n'est pas limitée par le puits d'absorption, mais par la capacité de la source d'assimilation de la plante.

     

Effect of mono- and divalent cations on sorption of water-extractable organic carbon and microbial activity

Biology and Fertility of Soils - Sorption is an important process for retention of organic carbon (C) in soils. The effect of Na+ and Ca2+ on sorption of organic C has been studied in salt-affected...     

Roughness of biopores and cracks in Bt‐horizons assessed by confocal laser scanning microscopy

In structured soils, water and reactive solutes can preferentially move through larger inter‐aggregate pores, cracks, and biopores. The surface roughness of such macropores is crucial for describing microbial habitats and the exchange of water and solutes between macropores and the soil matrix together with other properties. The objective of this study was to compare the roughness of intact structural surfaces from the Bt‐horizons of five Luvisols developed on loess and glacial till and to test the applicability of confocal laser scanning microscopy. Samples of 5 to 10 cm edge length with intact structural surfaces including cracks with and without clay‐organic coatings, earthworm burrow walls, and root channels were prepared manually. The surface roughness of these structures was determined with a confocal laser scanning microscope of the type Keyence VK‐X100K. The root‐mean‐squared roughness (R_q) the curvature (R_cu) and the ratio between surface area and base area (R_A) were calculated from selected surface regions of interest of 0.342 mm² with an elevation resolution of 0.02 µm. The roughness was smaller for coated as compared to uncoated cracks and earthworm burrows of the Bt‐horizons. This reduction of roughness by the illuviation of clayey material was similar for the structural surfaces of the coarser textured till‐Bt and the finer‐textured loess‐Bt. This similarity suggested a dominant effect of pedogenesis and a minor effect of the parent material on the roughness levels of structural surfaces in the Bt‐horizons. An expected “smoothing” effect of burrow wall surfaces by earthworm activity was not reflected in the roughness values compared to those of uncoated cracks at the chosen spatial scale. However, for root channel walls from one loess‐Bt, the roughness was reduced as compared to that of other structures. These results suggest that the surface roughness of the structural surface types should separately be considered when describing preferential flow and macropore‐matrix exchange or analysing root growth, microbial habitats, and colloidal transport in structured soils. The confocal laser scanning microscopy technique was found useful for characterizing the roughness of intact structural surfaces.     

Effect of zinc deficiency in wheat on the release of zinc and iron mobilizing root exudates

The effect of Zn deficiency in wheat (Triticum aestivum L. cv. Ares) on the release of Zn mobilizing root exudates was studied in nutrient solution. Compared to Zn sufficient plants, Zn deficient plants had higher root and lower shoot dry weights. After visual Zn deficiency symptoms in leaves appeared (15–17 day old plants) there was a severalfold increase in the release of root exudates efficient at mobilizing Zn from either a selective cation exchanger (Zn-chelite) or a calcareous soil. The release of these root exudates by Zn deficient plants followed a distinct diurnal rhythm with a maximum between 2 and 8 h after the onset of light. Re-supply of Zn to deficient plants depressed the release of Zn mobilizing root exudates within 12 h to about 50%-, and after 72 h to the level of the control plants (Zn sufficient plants). The root exudates of Zn deficient wheat plants were equally effective at mobilizing Fe from freshly precipitated Fe^III hydroxide as Zn from Zn-chelite. Furthermore, root exudates from Fe deficient wheat plants mobilized Zn from Zn-chelite, as well as Fe from Fe^III hydroxide. Purification of the root exudates and identification by HPLC indicated that under Zn as well as under Fe deficiency, wheat roots of the cv. Ares released the phytosiderophore 2′-deoxymugineic acid. Additional experiments with barley (Hordeum vulgare L. cv. Europa) showed that in this species another phytosiderophore (epi-3-hydroxymugineic acid) was released under both Zn and Fe deficiencies. These results demonstrate that the enhanced release of phytosiderophores by roots of grasses is not a response mechanism specific for Fe deficiency, but also occurs under Zn deficiency. The ecological relevance of enhanced release of phytosiderophore also under Zn deficiency is discussed.     

Long-term nitrogen balance for pearl millet (Pennisetum glaucum L.) in an acid sandy soil of Niger

On acid sandy soils of Niger (West Africa) fertilizer N recovery by pearl millet (Pennisetum glaucum L.) is often more than 100 per cent in years with normal or above average rainfall. Biological nitrogen fixation (BNF) by N₂-fixing bacteria may contribute to the N supply in pearl millet cropping systems. For a long-term field experiment comprising treatments with and without mineral fertilizer (F) and with and without crop residue application (CR) a N balance sheet was calculated over a period of six years (1983-1988). After six years of successive millet cropping total N uptake (36-77 kg N ha^?1 yr^?1) was distinctly higher than the amount of fertilizer N applied (30 kg N ha^?1 yr^?1). The atmospheric input of NH₄-N and NO₃-N in the rainwater was about 2 kg N ha_?1 yr^?1, 70 % in the form of NH₄-N. Gaseous NH₃ losses from urea (broadcast, incorporated) were estimated from other experiments to amount to 36 % of the fertilizer N applied. Nitrogen losses by leaching (15 to > 25 kg N ha^?1 yr^?1) were dependent on the treatment and on the quantity and distribution of single rainfall events (>50 mm). Decline in total soil N content (0-60 cm) ranged from 15 to 48 kg N ha^?1 yr^?1. The long-term N balance (1983-1988) indicated an annual net gain between 6 (+CR-F) and 13 (+CR+F) kg N ha^?1 yr^?1. For the control (-CR-F) the long-term N balance was negative (10 kg N ha^?1 yr^?1). In the treatment with crop residues only, the N balance was mainly determined by leaching losses, whereas in treatments with mineral fertilizer application the N balance depended primarily on N removal by the millet crop. The annual net gain in the N balance increased from 7 kg ha^?1 with mineral fertilizer to 13 kg ha^?1 in the combination mineral fertilizer plus crop residues. In both the rhizosphere and the bulk soil (0-15 cm), between 9 and 45% of the total bacterial population were N₂-fixing (diazotrophic) bacteria. The increased N gain upon crop residue application was positively correlated with an increase in the number of diazotrophic and total bacteria. The data on bacterial numbers suggest that the gain of N in the longterm N balance is most likely due to an N input by biological nitrogen fixation. In addition, evidence exists from related studies that the proliferation of diazotrophs and total bacteria in the rhizosphere due to crop residue application stimulated root growth of pearl millet, and thus improved the phosphorus (P) acquisition in the P deficient soil.