Living Root System Research Articles

Selective successional transport of bacterial populations from rooted agricultural topsoil to deeper layers upon extreme precipitation events

Substantial amounts of organic matter are mobilized from upper soil layers during extreme precipitation events. This results in considerable fluxes of carbon from plant-associated topsoil to deeper mineral soil and to groundwater. Microbes constitute an important part of this mobile organic matter (MOM) pool. Previous work has shown that specific bacteria associated with the rhizosphere of decaying maize roots were selectively transported with seepage water upon snowmelt in winter. However, effective mechanisms of mobilization and also possible distinctions to microbial transport for living root systems remain poorly understood. In the present study, bacteria in seepage water were sampled from lysimeters at an experimental maize field after extreme rain events in summer. We show that a distinctive subset of rhizoplane-associated bacterial populations was mobilized after summer rain, especially including abundant members of the Bacteroidetes, representing a microbial conduit for fresh plant-derived carbon inputs into deeper soil layers. Marked distinctions of seepage communities were not observed between lysimeters with a different relative contribution of preferential vs. matrix flow. Time-resolved analyses of seepage water during an artificial rain event revealed temporal patterns in the mobilization of certain lineages, with members of the Chitinophagaceae, Sphingomonadaceae, and Bradyrhizobiaceae preferentially mobilized in early and late seepage fractions, and members of the candidate phyla Parcubacteria and Microgenomates mobilized mostly in intermediate fractions. While average bacterial cell counts were at ∼107 ml−1 in seepage water, the recovery of amended fluorescently labeled cells of Arthrobacter globiformis was low (0.2–0.6%) over seepage events. Still, mobilized bacteria clearly have the potential to influence bacterial activities and communities in subsoils. These findings demonstrate that dynamic hydraulic events must be considered for a better understanding of the connectivities between microbial populations and communities in soil, as well as of the links between distinct carbon pools over depth.

The effect of roots and easily available carbon on the decomposition of soil organic matter fractions in boreal forest soil

The priming effect induced by carbon (C) that is easily available to microbes has been shown to increase the mineralization of soil organic matter (SOM) that is resistant to decomposition, but the combined effects of easily available carbon and the living root system have rarely been studied. The aim of this research was to study the decomposition of SOM fractions of different solubility in water and their 13C, 14C and 15N abundance with and without the presence of a living root system and easily available carbohydrate in the form of glucose. The SOM collected from the organic horizon of a boreal forest soil in Hyytiälä, southern Finland (61°51′N, 24°17′E), and exposed to laboratory incubations with and without the presence of Pinus sylvestris L. seedlings and glucose, was separated into three chemical fractions with accelerated solvent (ASE) and pressurized hot water extractions (PHWE). Changes in the natural abundance of 13C, 14C and 15N, spectral properties assessed by Fourier transform infrared spectroscopy (FTIR) and the C and N pools of SOM fractions were studied after incubation for 6 months. The extractions separated SOM into fractions with distinctive isotopic composition. The most easily soluble SOM fraction showed the largest abundance of 15N and 14C, and the living root system induced changes in the abundance of 15N and FTIR spectra. Our research suggests that plant roots may induce SOM degradation and N uptake from soluble SOM fractions, but 13C, 14C, 15N or FTIR alone cannot be used to describe the recalcitrance of SOM and its accessibility to microorganisms. It is better to use several methods in parallel to study the decomposability of SOM.HighlightsWe studied the effects of tree roots and addition of glucose on the solubility of soil organic matter (SOM). SOM can be separated into pools with distinctive isotopic concentrations with pressurized hot water. Roots may induce organic matter degradation and N uptake from soluble SOM fractions. Larger 14C abundance in soluble SOM indicated it contained older C than the recently assimilated C.

Contrasting effects of increased carbon input on boreal SOM decomposition with and without presence of living root system of Pinus sylvestris L.

We studied how the availability of carbon affects the decomposition of soil organic matter (SOM) in the presence of living root system compared to bare SOM treatments without roots. We measured the effect of living Pinus sylvestris L. root systems on SOM C and N budgets, on the age of carbon in CO2 efflux, and on SOM protease enzyme activity. We examined the possible explanatory factors for increased SOM mineralization such as plant biomass, photosynthesis, microbial C, and protease enzyme activity in the soil. The age of carbon respired during the decomposition of soil organic matter was significantly older in the presence of living root systems than in treatments lacking roots. If plants were present, glucose added directly in the SOM accelerated the C and N loss from the bulk material and decreased the rate of photosynthesis. We conclude that the priming effect of SOM decomposition was affected by the presence of living root system. Our results stress the importance of including the plant–soil interactions in the SOM decomposition models used in climate change studies.

Electrical capacitance of roots in relation to plant electrodes, measuring frequency and root media

The electrical capacitance method was applied for the examination of living root systems in a pot experiment. The measured root capacitances gave an unambiguous indication of the development of root mass and length. The root capacitances measured using needle and clamp plant electrodes were closely similar when the roots of whole plants were placed in water, while increasing differences were observed with a decrease in soil water saturation. The difference in capacitance between the plant electrodes is outlined by interpreting the action mechanism of the clamp electrode. The capacitance and electrical impedance spectra (30 Hz-1 MHz) were determined for roots in soil, for pieces of roots washed free of soil, and for the soil itself. The root capacitance was smaller than that of the soil and higher than that of root pieces at 1 kHz, while the capacitance of the soil became equal to that of roots in soil at about 2 kHz. This calls attention to the importance of the measuring frequency when determining root capacitance. A capacitor model with two dielectric media is proposed besides Dalton's model in order to interpret the behaviour of root and soil capacitances. However, its validity requires further verification.

Fine roots of trees - a new perspective.

Fine roots of trees - a new perspective.

Root renewal of sugar beet as a mechanism of P uptake efficiency

Sugar beet (Beta vulgaris L.) was grown in two different long-term P fertilization experiments on a sandy and a loamy soil. The P supply levels of the soils were ”low”, ”sufficient”, and ”high”, according to the German recommendation scheme. The low P level decreased shoot and storage root yield only on the loam soil, where the recovery of the P-deficient plants after a drought period was slower than at a sufficient P supply. The size of the living root system, as determined by a conventional auger sampling method, peaked at early July and decreased until harvest on the sandy soil without any influence of the P level. On loam, the living root systems were more constant and larger at P shortage. Total root production, as determined by the ingrowth core method, was about 120 km m—2 in the well P supplied loam treatments and 200 km m—2 at P deficiency, which was 3—4 times and 5 times higher than the average size of the living root systems, respectively. Hence, a rapid root renewal took place. On sand, where no P deficiency occurred, total root production was not different between the P supply levels but higher than in the well-supplied loam treatments. Modelling P uptake revealed that this root turnover and the concomitant better exploitation of the soil facilitates P uptake at a low P level in soil, but is of no advantage at a sufficient P supply. The increase of root production at P shortage increased calculated P uptake by 25% compared to a calculation with the ”usual” root production at a sufficient supply.

Activity and Persistence of Sorgoleone, a Long-Chain Hydroquinone Produced by Sorghum bicolor

Summary Sorghum bicolor (L.) Moensch and other related Sorghum species produce sorgoleone-a long chain hydroquinone compound (MW = 358), which is exuded by living root systems. This compound exhibits potent activity as a photosynthetic inhibitor. A diverse group of sorghum germplasm was evaluated for sorgoleone production. Production was quite variable, with certain accessions producing up to 15 mg sorgoleone/g fresh root weight. The root exudate composition among accessions was less variable, with sorgoleone being the major constituent (76-99%) within the extract. The potential binding of sorgoleone to the D1 protein of the PSII complex was evaluated in triazine resistant and susceptible redroot pigweed (Amaranthus retroflexus L.) thylakoids. Sorgoleone, metribuzin and diuron exhibited competitive binding with atrazine in susceptible thylakoids, while no competition was evident in resistant thylakoids. Sorgoleone has an intermediate affinity between that of diuron and metribuzin from estimated binding co...

The effects of ammonium sulphate deposition and root sinks on soil solution chemistry in coniferous forest soils

The effects of enhanced (NH4 2SO4deposition on soil solution cation and anion concentrations and annualionic fluxes were followed using a standardised experimental protocolin six European coniferous forests with contrasting soil types, pollutioninputs and climate. Native soil cores containing a ceramic suction cupwere installed in the field, roofed and watered every two weeks withlocal throughfall or local throughfall with added(NH4)2SO4 at 75 kgNH4 +-N ha-1 a-1. Livingroot systems were established in half of the lysimeters.Untreated throughfall NH4 +-N deposition at thesites ranged from 3.7 to 29 kg ha-1 a-1Soil leachates were collected at two weekly intervalsover 12 months and analysed for volume, andconcentrations of major anions and cations. Increasesin soil solution NO3 - concentrations inresponse to N additions were observed after 4–9months at three sites, whilst one sandy soil with highC:N ratio failed to nitrify under any of thetreatments. Changes in NO3 - concentrationsin soil solution controlled soil solution cationconcentrations in the five nitrifying soils, withAl3+ being the dominant cation in the more acidsoils with low base saturation. The acidification responses ofthe soils to the (NH4 2SO4additions were primarily related to the ability of thesoils to nitrify the added NH4 +. pH and soiltexture seemed important in controllingNH4 + leaching in response to the treatments,with two less acidic, clay/clay loam sites showingalmost total retention of added NH4 +, whilstnearly 75% of the added N was leached asNH4 + at the acid sandy soils. The presenceof living roots significantly reduced soil solutionNO3 - and associated cation concentrations attwo of the six sites. The very different responses of the sixsoils to increased (NH4)2SO4deposition emphasise that the establishment of N critical loadsfor forest soils need to allow for differences in N storagecapacity and nitrification potential.

A PROCEDURE FOR ISOLATION OF SINGLE‐SPORE CULTURES OF CERTAIN ENDOMYCORRHIZAL FUNGI

SummaryA procedure for the isolation of single‐spore cultures of endomycorrhizal fungi is described. A single spore is placed on plant roots with a Pasteur pipette by use of a binocular microscope, and the plant is transplanted into sand in a 50 ml growth tube. Plants are incubated for 6 to 8 weeks in a greenhouse and are continuously subirrigated by capillarity with dilute (5 to 10%) Hoagland's solution. Forty to 80% of plants inoculated by this technique with single spores of Glomus macrocarpus, G. etunicatus and G. caledonius became infected. Cultures of G. constrictus and G. microcarpus were not obtained. Hoagland's solutions of up to 25% were equally effective for isolating G. macrocarpus, but limiting plant size by using only 5 or 10% Hoagland's solution was advantageous. The presence of spores on living root systems was the most reliable indicator of fungal development. Non‐inoculated control plants incubated in a common Hoagland's solution reservoir with inoculated plants never became mycorrhizal. The procedure uses readily available materials, is easy to use in the laboratory and conserves greenhouse space and labour.

The Production by Roots of Calluna Vulgaris of a Factor Inhibitory to Growth of some Mycorrhizal Fungi

It is well established that tree species, such as birch, Betula pendula, and spruce, Picea abies, frequently fail to develop in association with heather, Calluna vulgaris. The affected trees can remain in a state of 'check' for many years, exhibiting no pathological symptoms but making little measurable growth. This lack of growth is characteristic of both roots and shoots, and, in addition, the latter fail to form any mycorrhizal associations. The poor development of uninfected roots is, in part, due to the presence of phytotoxic metabolites in the Calluna raw humus (Robinson 1971), but compounds released by decaying litter cannot be implicated with the absence of mycorrhiza. Various attempts have been made to explain this latter phenomenon (Rayner & Neilson-Jones 1944; Brian, Hemming & McGowan 1945), but it was Handley (1963) who first put forward the hypothesis that the paucity of mycorrhizal associations was due to a fungitoxic factor produced by the root system of living C. vulgaris. This idea received a large measure of support from field observations, in that it was noticed that any factor which restricted the growth of C. vulgaris was conducive to the formation of mycorrhizas by the 'checked' trees. However, although it was demonstrated that water extracts of raw humus from beneath living C. vulgaris contained a factor inhibitory to the growth of certain mycorrhizal fungi, the exact origin of the factor was not established. It is the purpose of this paper to show that a fungitoxic factor with the same characteristics as that isolated by Handley is, in fact, produced by the living root system of heather. Evidence in favour of this view was obtained in two ways: firstly, by showing that decaying litter of C. vulgaris did not give rise to such a factor and, secondly, by demonstrating that under certain experimental conditions a material is produced by the root system of C. vulgaris which is toxic to some fungi. Evidence relating to the first of these two points has been presented elsewhere (Robinson 1971), and is not dealt with further.

Living Root System distinguished by the Use of Carbon-14

THERE is no adequate method to distinguish a functional root from a non-functional or presumably dead root. An electrical technique has been used1 to diagnose the vitality of portions of roots of some weeds by measuring the resistance and capacitance of the tissue. Vital staining, in many cases with triphenyl tetrazolium chloride, has also been used for seedlings of grasses such as rice2. These methods, however, can only be used when the root is thick enough to allow insertion of an electrical probe into tissues or when the root is so young that a stained portion may be easily seen.