Showing papers in "Plant and Soil in 1998"

Organic acids in the rhizosphere: a critical review

Abstract: Organic acids, such as malate, citrate and oxalate, have been proposed to be involved in many processes operating in the rhizosphere, including nutrient acquisition and metal detoxification, alleviation of anaerobic stress in roots, mineral weathering and pathogen attraction. A full assessment of their role in these processes, however, cannot be determined unless the exact mechanisms of plant organic acid release and the fate of these compounds in the soil are more fully understood. This review therefore includes information on organic acid levels in plants (concentrations, compartmentalisation, spatial aspects, synthesis), plant efflux (passive versus active transport, theoretical versus experimental considerations), soil reactions (soil solution concentrations, sorption) and microbial considerations (mineralization). In summary, the release of organic acids from roots can operate by multiple mechanisms in response to a number of well-defined environmental stresses (e.g., Al, P and Fe stress, anoxia): These responses, however, are highly stress- and plant-species specific. In addition, this review indicates that the sorption of organic acids to the mineral phase and mineralisation by the soil's microbial biomass are critical to determining the effectiveness of organic acids in most rhizosphere processes.

A leaf-height-seed (LHS) plant ecology strategy scheme

Abstract: A leaf-height-seed (LHS) plant ecology strategy scheme is proposed. The axes would be specific leaf area SLA (light-capturing area deployed per dry mass allocated), height of the plant's canopy at maturity, and seed mass. All axes would be log-scaled. The strategy of a species would be described by its position in the volume formed by the three axes. The advantages of the LHS scheme can be understood by comparing it to Grime's CSR scheme, which has Competitors, Stress-tolerators and Ruderals at the corners of a triangle. The CSR triangle is widely cited as expressing important strategic variation between species. The C–S axis reflects variation in responsiveness to opportunities for rapid growth; in the LHS scheme, SLA reflects the same type of variation. The R axis reflects coping with disturbance; in the LHS scheme, height and seed mass reflect separate aspects of coping with disturbance. A plant ecology strategy scheme that permitted any species worldwide to be readily positioned within the scheme could bring substantial benefits for improved meta-analysis of experimental results, for placing detailed ecophysiology in context, and for coping with questions posed by global change. In the CSR triangle the axes are defined by reference to concepts, there is no simple protocol for positioning species beyond the reference datasets within the scheme, and consequently benefits of worldwide comparison have not materialized. LHS does permit any vascular land plant species to be positioned within the scheme, without time-consuming measurement of metabolic rates or of field performance relative to other species. The merits of the LHS scheme reside (it is argued) in this potential for worldwide comparison, more than in superior explanatory power within any particular vegetation region. The LHS scheme avoids also two other difficulties with the CSR scheme: (a) It does not prejudge that there are no viable strategies under high stress and high disturbance (the missing quadrant in the CSR triangle compared to a two-axis rectangle); (b) It separates out two distinct aspects of the response to disturbance, height at maturity expressing the amount of growth attempted between disturbances, and seed mass (inverse of seed output per unit reproductive effort) expressing the capacity to colonize growth opportunities at a distance. The advantage of LHS axes defined through a single readily-measured variable needs to be weighed against the disadvantage that single plant traits may not capture as much strategy variation as CSR's multi-trait axes. It is argued that the benefits of potential worldwide comparison do actually outweigh any decrease in the proportion of meaningful variation between species that is captured. Further, the LHS scheme opens the path to quantifying what proportion of variation in any other ecologically-relevant trait is correlated with the LHS axes. This quantification could help us to move forward from unprofitable debates of the past 30 years, where CSR opponents have emphasized patterns that were not accommodated within the scheme, while CSR proponents have emphasized patterns that the scheme did account for.

A survey of soils for aggregate stability and glomalin, a glycoprotein produced by hyphae of arbuscular mycorrhizal fungi

Abstract: Understanding the contributions of soil microorganisms to soil stabilization at the molecular level will lead to ways to enhance inputs for sustainable agricultural systems. Recent discoveries of copious production of glycoprotein (glomalin) by arbuscular mycorrhizal (AM) fungi and the apparent recalcitrance of this material in soils led to the comparison between concentration of glomalin and aggregate stability. Stability was measured on air-dried aggregates rewetted by capillary action and then subjected to wet sieving for 10 min. Thirty-seven samples from four geographic areas of the U.S. and one area of Scotland were tested. The monoclonal antibody used to discover glomalin on AM hyphae was employed to assess immunoreactive glomalin on aggregate surfaces by immunofluorescence and in extracts from aggregates by enzyme-linked immunosorbent assay (ELISA). Immunofluorescence was observed on at least some surfaces of aggregates from all soils examined, but was most evident on aggregates with high glomalin concentrations. Easily extractable glomalin (EEG) was solubilized by 20 mM citrate, pH 7.0 at 121 °C for 30 min, and total glomalin (TG) was solubilized with 50 mM citrate, pH 8.0 at 121 °C for 90 to 450 min. Some soils required up to seven sequential extractions to remove all of the glomalin. Aggregate stability was linearly correlated (p < 0.001) with all measures of glomalin (mg/g of aggregates) in these soils. The best predictor of aggregate stability (AS) was immunoreactive easily extractable glomalin (IREEG) according to the following relationship: AS = 42.7 +61.3 × log10 IREEG (r2 = 0.86; p <0.001, n = 37).

Potential of Rhizobium and Bradyrhizobium species as plant growth promoting rhizobacteria on non-legumes: Effect on radishes (Raphanus sativus L.)

Abstract: Bradyrhizobia and rhizobia are symbiotic bacterial partners forming nitrogen fixing nodules on legumes. These bacteria share characteristics with plant growth promoting rhizobacteria (PGPR). Nodule inducing bacteria, like other PGPR, are capable of colonizing the roots of non-legumes and produce phytohormones, siderophores and HCN. They also exhibit antagonistic effects towards many plant pathogenic fungi. The potential of nodule inducing bacteria to function as PGPR, was examined by using radish as a model plant. Three percent of the 266 strains tested were found to be cyanogens, while a majority (83%) produced siderophores. Fifty eight percent of the strains produced indole 3-acetic acid (IAA) and 54% solubilized phosphorus. Some of the bacterial species examined were found to have a deleterious effect while others were neutral or displayed a stimulatory effect on radishes. Bradyrizobium japonicum strain Soy 213 was found to have the highest stimulatory effect (60%), and an arctic strain (N44) was the most deleterious, causing a 44% reduction in radish dry matter yield. A second plant inoculation test, performed in growth cabinets, revealed that only strain Tal 629 of B. japonicum significantly increased (15%) the dry matter yield of radish. This indicates that specific bradyrhizobia have the potential to be used as PGPR on non-legumes.

Analysis of some direct and indirect methods for estimating root biomass and production of forests at an ecosystem level

Abstract: The relationship of global climate change to plant growth and the role of forests as sites of carbon sequestration have encouraged the refinement of the estimates of root biomass and production However, tremendous controversy exists in the literature as to which is the best method to determine fine root biomass and production This lack of consensus makes it difficult for researchers to determine which methods are most appropriate for their system The sequential root coring method was the most commonly used method to collect root biomass data in the past and is still commonly used But within the last decade the use of minirhizotrons has become a favorite method of many researchers In addition, due to the high labor-intensive requirements of many of the direct approaches to determine root biomass, there has been a shift to develop indirect methods that would allow fine root biomass and production to be predicted using data on easily monitored variables that are highly correlated to root dynamics Discussions occur as to which method should be used but without gathering data from the same site using different methods, these discussions can be futile This paper discusses and compares the results of the most commonly used direct and indirect methods of determining root biomass and production: sequential root coring, ingrowth cores, minirhizotrons, carbon fluxes approach, nitrogen budget approach and correlations with abiotic resources No consistent relationships were apparent when comparing several sites where at least one of the indirect and direct methods were used on the same site Until the different root methods can be compared to some independently derived root biomass value obtained from total carbon budgets for systems, one root method cannot be stated to be the best and the method of choice will be determined from researcher's personal preference, experiences, equipment, and/or finances

Relationship between soil chemical factors and grassland diversity

Abstract: Many studies carried out during these last few years have focused on the factors influencing plant diversity in species-rich grasslands. This is due to the fact that these ecosystems, among the most diversified in temperate climates, are extremely threatened; in some areas, they have almost disappeared. The re-establishment of these habitats implies to know the living conditions of the associations to be recreated. Very often, the typical species of these communities have become so rarefied that the seed bank or the seed rain are not sufficient to recreate the plant community. Most of the time, to achieve the restoration of these communities, they have to be totally recreated by sowing. For the restoration or the maintenance of the community, the soil chemical characteristics have also to be appropriate or if not modified. This research tends to establish a relation between some soil chemical factors and the plant diversity of a great number of stations. This research has illuminated the relationship between soil extractable phosphorus and potassium and plant diversity. Over 5 mg of phosphorus per 100 g of dry soil (acetate + EDTA extraction), no station containing more than 20 species per 100 m(2) has been found. The highest number of species is found below the optimum content of the soil for plant nutrition (5-8 mg P/100 g). Concerning the potassium, the highest number of species is found at 20 mg/100, a value corresponcing to an optimum content of the soil for plant nutrition. High potassium contents, in opposition to phosphorus contents, are thus compatible with high values of diversity. Other factors (i.e. pH, organic matter, total nitrogen and calcium) do not show so clearly a relation with plant diversity. Excess of N-NO3 is known for its negative effect on the diversity of plant communities. In these environments, apart from the atmospheric deposits which can be important in some areas, N-NO3 is derived mainly from the symbiotic fixation of atmospheric nitrogen by legumes as well as from the mineralization of the organic matter of the soil. It is possible that, when in small quantities, the available soil phosphorus could be a limiting factor of the N-NO3 supply by these two sources. In this hypothesis, nitrogen would remain the main element limitating plant diversity but its availability would be controlled by phosphorus.

Biofumigation potential of brassicas

Abstract: Isothiocyanates (ITCs) released from Brassica crops or seed meal amendments incorporated into soil have the potential to suppress pest and disease organisms in soil. We investigated in vitro toxicity of six ITCs to the mycelial growth of five cereal root pathogens (Gaeumannomyces graminis var. tritici, Rhizoctonia solani, Fusarium graminearum, Bipolaris sorokiniana, Pythium irregulare) by either adding them to the headspace above, or dissolving them in the growing media. Four alkenyl aliphatic ITCs (methyl-ITC, propenyl-ITC, butenyl-ITC, pentenyl-ITC) and two aromatic ITCs (benzyl-ITC and 2-phenylethyl-ITC) were tested. Aromatic ITCs were less toxic in the headspace experiments due to their lower volatility which reduced the headspace concentration, but were more toxic than the aliphatic ITCs when dissolved in the agar. In both experimental methods, the toxicity of the aliphatic ITCs decreased with increasing length of the side chain although there was little difference between methyl-ITC and propenyl-ITC in the headspace experiment. The fungi differed in sensitivity to the ITCs. Gaeumannomyces was the most sensitive, Rhizoctonia and Fusarium intermediate and Bipolaris and Pythium the least sensitive. Pythium was 2–16 times more resistant than the other fungi to the ITCs dissolved in agar and, in contrast to the other fungi, was more sensitive to the aliphatic ITCs than the aromatic ITCs. Suppression of some fungi by propenyl ITC and 2-phenylethyl ITC, principal products of glucosinolate hydrolysis in Brassica tissue, was superior to that of the synthetic fumigant methyl-ITC, suggesting an important role for these compounds in the pest suppression potential of brassicas. The variation in toxicity of different ITCs to the fungi (up to 7 fold) suggests there is significant scope to enhance the biofumigation potential of brassicas by selecting those which produce large quantities of GSL precursors to the most toxic ITCs for the target organism.

The potential of Thlaspi caerulescens for phytoremediation of contaminated soils

Abstract: Uptake of Cd, Zn, Pb and Mn by the hyperaccumulator Thlaspi caerulescens was studied by pot trials in plant growth units and in populations of wild plants growing over Pb/Zn base-metal mine wastes at Les Malines in the south of France. The pot trials utilised metal-contaminated soils from Auby in the Lille area. Zinc and Cd concentrations in wild plants averaged 1.16% and 0.16% (dry weight) respectively. The unfertilised biomass of the plants was 2.6 t/ha. A single fertilised crop with the above metal content could remove 60 kg of Zn and 8.4 kg Cd per hectare. Experiments with pot-grown and wild plants showed that metal concentrations (dry weight basis) were up to 1% Zn (4% Zn in the soil) and just over 0.1% Cd (0.02% Cd in the soil). The metal content of the plants was correlated strongly with the plant-available fraction in the soils as measured by extraction with ammonium acetate and was inversely correlated with pH. Bioaccumulation coefficients (plant/soil metal concentration quotients) were in general higher for Cd than for Zn except at low metal concentrations in the soil. There was a tendency for these coefficients to increase with decreasing metal concentrations in the soil. It is proposed that phytoremediation using Thlaspi caerulescens would be entirely feasible for low levels of Cd where only a single crop would be needed to halve a Cd content of 10 μg/g in the soil. It will never be possible to remediate elevated Zn concentrations within an economic time frame (<10 yr) because of the lower bioaccumulation coefficient for this element coupled with the much higher Zn content of the soils.

Soil phosphorus fractions and adsorption as affected by organic and inorganic sources

Abstract: The effect of organic and inorganic sources of phosphorus (P) on soil P fractions and P adsorption was studied in a field without plant growth on a Kandiudalf in western Kenya. A high-quality organic source, Tithonia diversifolia (Hemsley) A. Gray leaves, and a low-quality source, maize (Zea mays L.) stover, were applied alone or in combination with triple superphosphate (TSP). The P rate was kept constant at 15 kg P ha-1. Soil extractable P (resin, bicarbonate and sodium hydroxide), microbial biomass P and C and P adsorption isotherms were determined during 16 weeks after application of treatments. Application of tithonia either alone or with TSP increased resin P, bicarbonate P, microbial P, and sodium hydroxide inorganic P. Tithonia alone reduced P adsorption at 2–16 weeks. Maize stover had no effect on any of the P fractions or P adsorption. At 8 weeks, the application of tithonia reduced microbial C-to-P ratio (20) as compared to maize stover, TSP and the control (31–34). The reduction in P adsorption by tithonia was accompanied by increases in all measured P fractions, the sum of P in those fractions (resin, bicarbonate and sodium hydroxide) being larger than the P added. The reduction in P adsorption apparently resulted from competition for adsorption sites, probably by organic anions produced during decomposition of the high quality tithonia. Integration of inorganic P (TSP) with organic materials had little added benefit compared to sole application of TSP, except that combination of tithonia with TSP increased microbial biomass. The results indicate that a high quality organic input can be comparable to or more effective than inorganic P in increasing P availability in the soil.

Iron-dependent oxygen free radical generation in plants subjected to environmental stress: toxicity and antioxidant protection

Abstract: Iron has a pivotal and dual role in free radical chemistry in all organisms. On the one hand, free Fe can participate in Fenton reactions and catalyze (‘catalytic Fe’) the generation of hydroxyl radical and other toxic oxygen species. On the other hand, Fe is a constituent of the antioxidant enzymes catalase, ascorbate peroxidase, guaiacol peroxidase, and ferro-superoxide dismutase. Protein Fe is Fenton inactive but can be released from proteins upon attack by activated oxygen. Healthy, unstressed plants avoid the interaction of catalytic Fe and peroxides by disposing of Fe in vacuoles and apoplast, by sequestering Fe in ferritin, and by having high levels of antioxidant enzymes and metabolites in most subcellular compartments. However, when plants are exposed to a variety of adverse conditions, including chilling, high light, drought and paraquat, oxidative stress ensues due primarily to the decrease in antioxidant defenses but also to the increase in free radical production mediated by catalytic Fe. The latter accumulates in many stressed plant tissues. Oxidative stress may lead to metabolic dysfunction and ultimately to plant cell death, so it needs to be estimated conveniently by quantifying the oxidation products of lipids (malondialdehyde and other cytotoxic aldehydes), proteins (total carbonyls, methionine sulfoxide, 2-oxohistidine), and DNA (8-hydroxyguanine, 5-hydroxycytosine). Protein oxidation appears to be a more sensitive and precocious marker than is lipid peroxidation, and DNA damage may also prove to be a useful marker for stress studies in plants.

Biofumigation potential of brassicas. I. Variation in glucosinolate profiles of diverse field-grown brassicas

Abstract: Biofumigation refers to the suppression of soil-borne pests and pathogens by biocidal compounds released in soil when glucosinolates (GSL) in Brassica green manure or rotation crops are hydrolysed. We investigated the potential to enhance biofumigation by considering the variation in GSL production in the roots and shoots of 76 entries from 13 Brassica and related weed species grown in the field. Total plant GSL production on a ground area basis at mid-flowering ranged from 0.8 to 45.3 mmol m -2 . The variation derived equally from differences in biomass and GSL concentration, which were not correlated in either root or shoot tissues. Roots (0-0.15 m) contributed an average of 23.6% (range 2-81%) of the total plant GSLs, their contribution limited by low biomass rather than GSL concentration, which was usually similar or higher than that of shoots. The GSL concentrations in root and shoot tissues did not correlate significantly with seed levels in any of the species, so selection for higher plant GSL production to enhance biofumigation potential cannot be based on seed GSL levels. The types of GSLs present in the tissues varied considerably between species but were consistent within species. In contrast, the concentration of individual and total GSLs in both root and shoot tissues varied four to ten-fold both between and within all species. Shoots contained predominately aliphatic GSLs, while aromatic GSLs, particularly 2-phenylethyl GSL, were dominant in the roots of all entries. Indolyl GSLs were present in all tissues but at low concentrations (< 1 μmol g -1 ). The variation in the biomass, GSL profiles and concentrations in both roots and shoots provide significant scope to select or develop brassicas with enhanced biofumigation potential. Further studies on the efficacy of the various GSL hydrolysis products to suppress target organisms in soil are required to fully exploit biofumigation as a part of integrated pest management.

Cadmium toxicity effects on growth, mineral and chlorophyll contents, and activities of stress related enzymes in young maize plants (Zea mays L.)

Abstract: Plants were cultivated in a nutrient solution containing increasing cadmium concentrations (i.e. 0.001–25 µM), under strictly controlled growth conditions. Changes in both growth parameters and enzyme activities, directly or indirectly related to the cellular free radical scavenging systems, were studied in roots and leaves of 14-day-old maize plants (Zea mays L., cv. Volga) as a result of Cd uptake. A decrease in both shoot length and leaf dry biomass was found to be significant only when growing on 25 µM Cd, whereas concentrations of chlorophyll pigments in the 4th leaf decreased from 1.7 µM Cd on. Changes in enzyme activities occurred at lower Cd concentrations in solution leading to lower threshold values for Cd contents in plants than those observed for growth parameters. Peroxidase (POD; E.C. 1.11.1.7) activity increased in the 3rd and 4th leaf, but not in roots. In contrast, glucose-6-phosphate dehydrogenase (G6PDH; E.C. 1.1.1.49), isocitrate dehydrogenase (ICDH; E.C. 1.1.1.42) and malic enzyme (ME; E.C. 1.1.1.40) activities decreased in the 3rd leaf. According to the relationship between the POD activity and the Cd content, a toxic critical value was set at 3 mg Cd per kg dry matter in the 3rd leaf and 5 mg Cd per kg dry matter in the 4th. Anionic POD were determined both in root and leaf protein extracts; however, no changes in the isoperoxidase pattern were detected in case of Cd toxicity. Results show that in contrast with growth parameters, the measurement of enzyme activities may be included as early biomarkers in a plant bioassay to assess the phytotoxicity of Cd-contaminated soils on maize plants.

Long-term effects of experimental nitrogen additions on foliar litter decay and humus formation in forest ecosystems

Abstract: Decomposition rates and N dynamics of foliar litter from 4 tree species were measured over a 72 month period on the Chronic Nitrogen Addition plots at the Harvard Forest, Petersham MA, beginning in November 1988. Plots received nitrogen additions of 0, 5 and 15 g N m-2yr-1 in two different stand types: red pine and mixed hardwood. Bags were collected in August and November of each year and litter analysed for mass remaining, nitrogen, cellulose and lignin content. Mass remaining was significantly greater for litter in nitrogen treated plots than in control plots after 48 months. Lignin content of litter was significantly higher with nitrogen treatments but there was little effect of treatment on cellulose content. N concentration was similar between treatments, but greater mass remaining in treated plots resulted in a higher total amount of N in humus produced in the high N plot. This mechanism could be a sink for up to 1.5 g N·m-2yr-1 of the 1.5 g N·m-2yr-1 added annually to the high N plots. Reduced decomposition rates in conjunction with increased lignin accumulation could impact global carbon sequestration as well.

Soil CO2 efflux and its spatial variation in a Florida slash pine plantation.

Abstract: The efflux of CO2 from the soil surface can vary markedly in magnitude both in time and space and its correct determination is crucial in many ecological studies. In this paper, we report results of field measurements, using an open-top dynamic chamber, of soil CO2 efflux in a mature Florida slash pine (Pinus elliottii Engelm. var.elliottii) plantation. The daily average efflux was 0.217 mg CO2 m-2s-1 in the autumn and 0.087 mg CO2 m-2s-1 in the winter. Soil temperature, which accounts for most of the temporal variability in CO2 efflux, is by far the most influential factor controlling soil respiration rate and its temporal variation. The CO2 efflux in the slash pine plantation is highly spatially variable and effluxes from the soil under palmetto is significantly higher than that from the open floor. The CO2 efflux generally increases with increase in soil fine root biomass, litter and humus amount on the forest floor but is inversely related to the amount of organic matter in the mineral soil. The spatial variation in CO2 efflux can be well characterised by a simple multiple regression model incorporating live and dead biomass and soil total porosity as predictor variables. Understorey plants, mostly Serenoa repens, are an important component of the C cycle and the major contributor to the spatial heterogeneity of soil CO2 efflux. The influence of understorey plants on soil respiration is probably via two approaches: increasing litterfall and root metabolism, both consequently stimulating microbial activity in the mineral soil.

Low-molecular-weight organic acids in rhizosphere soils of durum wheat and their effect on cadmium bioaccumulation

Abstract: Cadmium (Cd) accumulation has been found to vary between cultivars of durum wheat (Triticum turgidum var. durum), and it is hypothesized that low-molecular-weight organic acids (LMWOAs) produced at the soil-root interface (rhizosphere) may play an important role in the availability and uptake of Cd by these plants. The objective of this study, therefore, was to (1) investigate the nature and quantity of LMWOAs present in the rhizosphere of durum wheat cultivars Arcola (low Cd accumulator) and Kyle (high Cd accumulator) grown in three different soils: Yorkton, Sutherland and Waitville, and (2) determine the relationship between Cd accumulation in these plants and LMWOAs present in the rhizosphere. Plants were grown for two weeks in pot-cultures under growth chamber conditions. Oxalic, fumaric, succinic, L-malic, tartaric, citric, acetic, propionic and butyric acids were found and quantified in the water extracts of rhizosphere soil, with acetic and succinic acids being predominant. No water extractable LMWOAs were identified in the bulk soil. Total amount of LMWOAs in the rhizosphere soil of the high Cd accumulator (Kyle) was significantly higher than that for the low Cd accumulator (Arcola) in all three soils. Furthermore, large differences in amounts of LMWOAs were found in the rhizosphere soil for the same cultivars grown in different soils and followed the pattern: Sutherland > Waitville > Yorkton. Extractable soil Cd (M NH4Cl) and Cd accumulation in the plants also followed the same soil sequence as LMWOA production. Cadmium accumulation by the high and low Cd accumulating cultivars was proportional to the levels of LMWOAs found in the rhizosphere soil of each cultivar. These results suggest that the differing levels of LMWOAs present in the rhizosphere soil played an important role in the solubilization of particulate-bound Cd into soil solution and its subsequent phytoaccumulation by the high and low Cd accumulating cultivars.

The dynamics of carbon in particle-size fractions of soil in a forest-cultivation sequence

Abstract: Cultivation of forest and grassland soils induces heavy changes in soil organic matter (SOM) dynamics. To better predict the effect of cultivation, there is a need to describe which organic pools are affected and to which extent. We used a chronosequence of thick humic forest soils converted to maize cultivation for 40 yr in southwest France. The dynamics of soil carbon was investigated through particle-size fractionation and the use of 13C allowed to distinguish forest-derived organic matter and new crop-derived organic matter. This partitioning of soil carbon by size on one hand and by age on the other provided a precise description of carbon turnover. The level towards which tend the organic pools under cultivation showed that the decay rates of soil carbon were one order of magnitude higher under cultivation than under forest. SOM can thus be considered as deprotected under cultivation. All size fractions appeared to be deprotected to the same extent. A progressive transfer of silt-sized C to clay-sized C was nevertheless suspected and attributed to the decreasing stability of fine silt-sized microaggregates with cultivation. SOM furthermore contained some very stable C present as silt-sized and possibly clay-sized particles. The turnover times of maize-derived organic matter was the same as that observed in similar soils cultivated for centuries. This indicated that the new conditions induced by cultivation were reached in the very first years after forest clearing and that the high initial SOM content and high mineralization rate of initial organic matter did not affect the dynamics of newly incorporated carbon.

Transient three-dimensional modeling of soil water and solute transport with simultaneous root growth, root water and nutrient uptake

Abstract: A three-dimensional solute transport model was developed and linked to a three-dimensional transient model for soil water flow and root growth. The simulation domain is discretized into a grid of finite elements by which the soil physical properties are spatially distributed. Solute transport modeling includes passive and active nutrient uptake by roots as well as zero- and first-order source/sink terms. Root water uptake modeling accounts for matric and osmotic potential effects on water and passive nutrient uptake. Root age effects on root water and nutrient uptake activity have been included, as well as the influence of nutrient deficiency and ion toxicity on root growth. Examples illustrate simulations with different levels of model complexity, depending on the amount of information available to the user. At the simplest level, root growth is simulated as a function of mechanical soil strength only. Application of the intermediate level with root water and nutrient uptake simulates the influence of timing and amount of NO3 application on leaching. The most comprehensive level includes simulation of root and shoot growth as influenced by soil water and nutrient status, temperature, and dynamic allocation of assimilate to root and shoot.

Water-use efficiency and transpiration efficiency of wheat under rain-fed conditions and supplemental irrigation in a Mediterranean-type environment

Abstract: Growth and water use were measured in wheat (Triticum aestivum L.) grown in northern Syria in a typical Mediterranean climate over five seasons 1991/92–1995/96. Water use was partitioned into transpiration (T) and soil evaporation (Es) using Ritchie's model, and water-use efficiency (WUE) and transpiration efficiency (TE) were calculated. The aim of the study was to examine the influence of irrigation and nitrogen on water use, WUE and TE. By addition of 100 kg N ha-1, Es was reduced from 120 mm to 101 mm under rain-fed conditions and from 143 mm to 110 mm under irrigated conditions, and T was increased from 153 mm to 193 mm under rain-fed conditions and from 215 mm to 310 mm under irrigated conditions. Under rain-fed conditions, about 35% of evapotranspiration (ET) may be lost from the soil surface for the fertilized crops and 44% of ET for the unfertilized crops. Transpiration accounted for 65% of ET for the fertilized crops and 56% for the unfertilized crops under rain-fed. As a result of this, WUE was increased by 44% for dry matter and 29% for grain yield under rain-fed conditions, and by 60% for dry matter and 57% for grain yield under irrigated conditions. Transpiration efficiency for the fertilized crops was 43.8 kg ha-1 mm-1 for dry matter and 15 kg ha-1 mm-1 for grain yield, while TE for the unfertilized crops was 33.6 kg ha-1 mm-1 and 12.2 kg ha-1 mm-1 for dry matter and grain yield, respectively. Supplemental irrigation significantly increased post-anthesis water use, transpiration, dry matter and grain yield. Water-use efficiency for grain yield was increased from 9.7 to 11.0 kg ha-1 mm-1 by supplemental irrigation, although WUE for dry matter was not affected by it. Irrigation did not affect transpiration efficiency for grain yield, but decreased transpiration efficiency for dry matter by 16%. This was associated with higher harvest index as a result of good water supply in the post-anthesis period and increased transpiration under irrigated conditions.

Mobilization of Soil and Fertilizer Phosphate by Cover Crops

Abstract: Nine tropical cover crops, which had produced positive residual effects on following year maize yield in field experiments in Northern Nigeria on a luvisol low in available P, and maize were grown in a pot experiment using the same soil. The effect of plant growth on pH, organic acids, soil P fractions and phosphatase activity in bulk and rhizosphere soil was studied. All plant species raised the pH in the rhizosphere, differed widely in acid phosphatase activity, and derived most of their P from the resin and bicarbonate-extractable inorganic P (Pi). Organic P (Po) accumulated especially in the rhizosphere in all plant species. There was a negative correlation between the species-specific rhizosphere phosphatase activity and Po accumulation. Pigeonpea (Cajanus cajan) appeared to be more P-efficient than other plant species because it was less reduced in biomass production at low compared to adequate P supply and the highest rhizosphere acid phosphatase activity. The results of this ongoing research indicate that inclusion of cover crops into cropping systems can contribute to more efficient use of soil and fertilizer P by less P-efficient crops such as maize. However, further work is necessary to get a better quantitative understanding of the mechanisms involved.

Inhibition of nitrification in forest soil by monoterpenes

Abstract: Nitrate production was detected in untreated soil of a Norway spruce (Picea abies L.) stand only after clear-cutting the stand. The aim of this study was to determine whether allelochemical inhibition of nitrification by monoterpenes played any role in inhibiting nitrification in the stand. Therefore, soils from a clear-cut plot and from a forest plot were studied. In the field, monoterpenes (mostly α- and β-pinenes), measured by soil microair diffusive samplers, were intensively produced in the forest plot, but not in the clear-cut plot. In the laboratory, soil samples taken from the forest plot produced only small amounts of monoterpenes, indicating that monoterpenes were mainly produced by the roots and not to great extent by the soil microbial population. The effect of a mixture of monoterpenes (seven major monoterpenes detected in the field) on net nitrification, net N mineralization and denitrification activities of soil from the clear cut plot, and on carbon mineralization of soils from both the forest and clear-cut plots, was studied in the laboratory. In both aerobic incubation experiments and in soil suspensions with excess NH4-N, nitrification was inhibited by exposure to the vapours of monoterpenes at similar concentrations at which they had been detected in forest plot. This indicates direct inhibition of nitrification by monoterpenes. Exposure to monoterpenes did not affect denitrification. However, it increased respiration activity of both soils. This could also indicate indirect inhibition of nitrification by monoterpenes, due to immobilization of mineral N. Thus it seems that monoterpenes could play a role in inhibiting nitrification in the forest soil.

Nitrogen uptake and use of two contrasting maize hybrids differing in leaf senescence

Abstract: In eastern Canada, the use of fertilizer N has been identified as the most energy-consuming component of maize (Zea mays L.) grain production. As the economic and environmental costs of excessive N fertilization rise, there is an increased emphasis on selection of hybrids with greater N use efficiency (NUE; defined as the ratio of the amount of 15N recovered in grain or stover dry matter to the amount of fertilizer 15N applied to the soil in this study). Using an 15N-labelling approach, a field study was conducted on a tile-drained Brandon loam soil (Typic Endoaquoll) on the Central Experimental Farm at Ottawa, Canada (45°22′ N, 75°43′ W) in 1993 and 1994. Fertilizer N uptake and partitioning within the plant in relation to dry matter changes were monitored during development of a current stay-green maize hybrid and an older early-senescing hybrid grown with three fertilizer N levels (0, 100, 200 kg N ha-1). Dry matter, N concentration and15 N atom% enrichment of plant components were determined at five growth stages. The current stay-green hybrid, ‘Pioneer 3902’ had greater NUE than the old early-senescing hybrid, ‘Pride 5’, which was associated with 24% more dry matter production and 20% more N uptake during grain fill for Pioneer 3902. There was no indication of greater allocation of N to the grain in Pioneer 3902. Our data suggest that prolonged maintenance of green leaf area for photosynthate production during grain fill and the ability to take up available soil N later in grain filling are characteristics of maize hybrids with greater NUE.

The influence of arsenic chemical form and concentration on Spartina patens and Spartina alterniflora growth and tissue arsenic concentration

Abstract: Arsenic (As) uptake by two perennial coastal marsh grasses growing in hydroponic conditions was studied in relation to the chemical form and concentration of As added to nutrient solution A 4×3×2 factorial experiment was conducted with treatments consisting of four As chemical forms [arsenite, As(III); arsenate, As(V); monomethyl arsonic acid, MMAA; and dimethyl arsinic acid, DMAA], three As concentrations (02, 08, and 20 mg As L-1) and two plant species (Spartina patens and Spartina alterniflora) Arsenic phytoavailability and phytotoxicity were primarily determined by the As chemical form present in the nutrient solution, though As concentration also influenced both As availability and toxicity Application of As(V) increased root, shoot and total dry matter production; this positive plant growth response may be linked with P nutrition Organic arsenicals and As(III) were the most phytotoxic species to both marsh grasses when plant growth was considered Arsenic uptake and transport in plant were species-specific Phytoavailability of As followed the trend DMAA ≪MMAA ≅ As(V) < As(III) Root and shoot As concentrations significantly increased with increasing As application rates to the rooting medium, regardless of the As chemical form Upon absorption, inorganic arsenicals and MMAA were mainly accumulated in the root system, while DMAA was readily translocated to the shoot

Soil aggregation status and rhizobacteria in the mycorrhizosphere

Abstract: Soil aggregation is a dynamic process in which plants and the soil microbiota play a major role. This experiment was conducted to determine whether the effects of mycorrhizae on the stability of water-stable soil aggregates (WSA) and on selected groups of soil microorganisms are interrelated. Soil containers consisting of four compartments were utilized. Two compartments on each side of a solid barrier were separated by a 43 μm screen that permitted the passage of hyphae, but not of roots. The roots of Sorghum bicolor plants were split over the center barrier, and the roots on one side were inoculated with an arbuscular-mycorrhizal (AM) fungus. This design produced mycorrhizosphere soils (M) by AM roots or hyphosphere (H) soils by AM hyphae in the two compartments on the one side of the barrier, and rhizosphere soils (R) by nonAM roots or root- and hypha-free bulk soil (S) in the two compartments on the other side. At harvest (10 wk), there were significant differences in WSA between soils in the order: M>R>H>S, and WSA stability was significantly correlated with root or hyphal length. Numbers of colony-forming units of the microflora (total bacteria, actinomycetes, anaerobes, P solubilizers, and nonAM fungi) were in general not correlated with root or hyphal length, but in some cases were significantly correlated with WSA. Bacteria isolated from the water-stable soil-aggregate fraction tended to be more numerous than from the unstable fraction. The difference was significant in the M soil for total bacteria and P solubilizing bacteria. NonAM fungi were more numerous in the unstable fraction of the M soil. The data show that the root and fungal components of mycorrhizae enhance WSA stability individually and additively in concert, and suggest that they affect microorganism numbers indirectly by providing a favorable and protective habitat through the creation of habitable pore space in the WSA.

Elevated CO2, rhizosphere processes, and soil organic matter decomposition

Abstract: The rhizosphere is one of the key fine-scale components of C cycles This study was undertaken to improve understanding of the potential effects of atmospheric CO2 increase on rhizosphere processes Using C isotope techniques, we found that elevated atmospheric CO2 significantly increased wheat plant growth, dry mass accumulation, rhizosphere respiration, and soluble C concentrations in the rhizosphere When plants were grown under elevated CO2 concentration, soluble C concentration in the rhizosphere increased by approximately 60% The degree of elevated CO2 enhancement on rhizosphere respiration was much higher than on root biomass Averaged between the two nitrogen treatments and compared with the ambient CO2 treatment, wheat rhizosphere respiration rate increased 60% and root biomass only increased 26% under the elevated CO2 treatment These results indicated that elevated atmospheric CO2 in a wheat-soil system significantly increased substrate input to the rhizosphere due to both increased root growth and increased root activities per unit of roots Nitrogen treatments changed the effect of elevated CO2 on soil organic matter decomposition Elevated CO2 increased soil organic matter decomposition (22%) in the nitrogen-added treatment but decreased soil organic matter decomposition (18%) without nitrogen addition Soil nitrogen status was therefore found to be important in determining the directions of the effect of elevated CO2 on soil organic matter decomposition

Direct evidence on participation of root hairs in phosphorus ( 32 P) uptake from soil

Abstract: Root hairs substantially extend root surface for ion uptake. Although many reports suggest a relationship between root hairs and phosphorus (P) uptake of plants, the role of root hairs in phosphorus uptake from soils is still debated. We measured uptake of phosphorus from soil directly via root hairs. Root hairs only were allowed to penetrate through a tightly stretched nylon screen (53 µm) glued to the bottom of a PVC tube. The penetrating root hairs grew for 2 and 4 days in soil labelled with radioisotope phosphorus (P) tracer 32P (185 kBq g-1 dry soil) filled in another PVC tube. Transparent plastic rings of thickness ranging from 0.25 mm to 2.0 mm were inserted between the two PVC tubes. This provided slit width for microscopic observations in situ, which confirmed that only root hairs were growing into the 32P labelled soil. In some cases no rings were inserted (slit width = 0) where both root hairs and root surface were in contact with the labelled soil (total 32P uptake). The uptake of32 P from soil via the root hairs only was quantified by measuring activity of 32P in the plant shoot (32P uptake only via root hairs). The results showed that when 70 percent of the root hairs grew into the labelled soil, they contributed to 63 percent of the total P uptake. With decreasing number of root hairs growing into the 32P labelled soil, the quantity of 32P in the plant shoot decreased. In this study, P uptake via root hairs was measured in a soil-based system, where root hairs were the only pathway of 32P from soil to the plant shoot. Therefore, this study provides a strong evidence on the substantial participation of root hairs in uptake of phosphorus from soil.

Root growth and water uptake during water deficit and recovering in wheat

Abstract: Root growth and soil water content were measured in a field experiment with wheat subjected to two periods of water deficit The first period was induced early in the season between the early vegetative stage (22 DAS) and late terminal spikelet (50 DAS), the second period at mid-season between terminal spikelet (42 DAS) and anthesis (74 DAS) Total root growth was reduced under water deficit by a reduction in the top 30 cm, while the root system continued to grow in the deeper soil profile between 30 and 60 cm Shortly after rewatering, the growth pattern reverted to fastest root growth rates in the shallow soil layers In relative terms, the total root system increased in relation to the above ground dry matter under water shortage The early-, the mid-season water deficit treatments, and the control treatment had total root length of 274, 194 and 306 km m-2, respectively, about 2 wk before maturity Evapotranspiration declined under water deficit, but water uptake in deeper layers increased Water uptake per unit root length was reduced with water deficit and was still low shortly after rewatering Remarkable was the increase in water uptake at 2–3 weeks after rewatering, both deficit treatments exceeded the control by almost 100% This increase in water uptake followed the burst of new root growth in the upper regions of the soil However, water uptake rates subsequently declined towards maturity, being between 015 L km-1 d-1 and 017 L km-1 d-1 for the early and mid-season water deficit treatments, slightly higher than the control, 012 L km-1 d-1 The results showed that the crop subjected to early water deficit could compensate for some of the reductions in root growth during subsequent rewatering, but the impact of the mid-season water deficit treatment was more severe and permanent

Nitrogen mineralization by bacterial-feeding nematodes: verification and measurement

Abstract: Bacterial feeding nematodes excrete N assimilated in excess of that required for growth. Because metabolic and developmental rates differ among nematode species, we hypothesized that their contribution to N mineralization in soil would differ. Sand-column microcosms amended with an organic substrate, bacteria, and with or without bacterial-feeding nematodes, were leached at 3-d intervals. Cumulative N, as NH 4 + or NO 3 - , leached from columns containing nematodes was consistently greater than from columns without nematodes. Maximum N-mineralization rates for populations of rhabditid nematodes, which predominated in field soils early in the summer were at lower temperatures than those for cephalobid nematodes, which predominated later in the summer. For an organic substrate with C-to-N ratio of 11:1, rates of N mineralization among species of different body size were similar, ranging between 0.0012 and 0.0058 μg-N nematode-1 d-1, mainly as NH 4 + . Smaller nematodes mineralized more N per unit of body weight than larger nematodes. We hypothesized that at low C-to-N ratios of the organic substrate, bacterial growth is C-limited and N-immobilization will be minimal; at high C-to-N ratios bacterial growth will be N-limited and there may be rapid immobilization of newly-mineralized N. Consequently, net N mineralization in the presence of nematodes will be lower when the organic substrate has a high C-to-N ratio. In experiments with different nematode species, net mineralization and the nematode contribution to mineralization generally decreased with increasing C-to-N ratio, consistent with the hypothesis; however, there were exceptions.

Rare earth elements in soil and in soil-grown plants

Abstract: Concentrations of the rare earth elements (REEs) La, Ce, Nd, Sm, Eu, Gd, Tb, Yb and Lu were determined in leaves of 6 plant species (Norway spruce, silver fir, maple, ivy, blackberry, and wood fern), and in pertinent soils and soil extracts, also taken from the same site. The distribution of the individual REEs in plants showed little or no agreement with that in the soil or the soil extracts. Ce had a negative anomaly with respect to the soil in all plants. The REE distribution patterns of fir and spruce were almost identical, but differed profoundly from that of the other species. In most cases, concentration ratios between species were a smooth function of the atomic number of the REE. Very similar results were obtained at 2 additional sites.

Interactions of an acid tolerant strain of phosphate solubilizing bacteria with a few acid tolerant crops

Abstract: Phosphate solubilizing bacteria (PSB) were isolated from sixty soil samples of various soil classes and cropping histories in Himalayan regions of Uttar Pradesh, India by enrichment culture techniques. Phosphate solubilization and acid tolerance of each strain was estimated. A strain (PAS-2) isolated froma pasture and waste land of pH 4.8, organic matter 2.6% available N 265kg ha-1, available P2O5(Bray's II) 2.3kg ha-1 and available K2O 353 kg ha-1 had the highest P-solubilization (45 µg P per mL per day) and also highest acid tolerance rating 42. The strain was identified as Bacillus sp. Seed inoculation of this bacterial strain resulted in significant increases in grain and vegetative yield of fingermillet (Elosine coracana), maize (Zea mays), amaranth (Amaranthus hypochondriacus), buckwheat (Fagopyrium esculentum), frenchbean (Phaseolus vulgaris) with or without added P sources. The significant grain yield (quintol ha-1) with phosphate and seed inoculation ranged from 33.85 in maize, 26.33 in frenchbean, 22.41 in buckwheat, 20.71 in amaranth and 19.19 in fingermillet as compared to controls. The highest response was observed with frenchbean followed by fingermillet, buckwheat, amaranth and maize. Phosphate use efficiency was highest in frenchbean followed by maize and lowest and almost at par in buckwheat, amaranth and fingermillet. Available phosphate was also highest in frenchbean cultivated plot followed by amaranth, fingermillet, buckwheat and maize. The MPN count of phosphate solubilizing bacteria were also influenced by seed inoculation of strain PAS-2. Frenchbean exerted greaterrhizosphere effect followed by pseudocereals and cereals. Likewise, phosphate nutrition of crops were also improved through seed inoculation irrespective of added P sources. The study thus demonstrated that selection of efficient strain of PSB from acid soil and its seed inoculation in selected crop genotype is beneficial in boosting up crop yield in low productive hill soil. Seed inoculation also created greater rhizosphere effect over uninoculation which improved P-nutrition of crops and also available soil P.

Effect of iron plaque outside roots on nutrient uptake by rice (Oryza sativa L.). Zinc uptake by Fe-deficient rice

Abstract: This solution culture study examined the effect of the deposition of iron plaque on zinc uptake by Fe-deficient rice plants. Different amounts of iron plaque were induced by adding Fe(OH)3 at 0, 10, 20, 30, and 50 mg Fe/L in the nutrient solution. After 24 h of growth, the amount of iron plaque was correlated positively with the Fe(OH)3 addition to the nutrient solution. Increasing iron plaque up to 12.1 g/kg root dry weight increased zinc concentration in shoots by 42% compared to that at 0.16 g/kg root dry weight. Increasing the amount of iron plaque further decreased zinc concentration. When the amounts of iron plaque reached 24.9 g/kg root dry weight, zinc concentration in shoots was lower than that in shoots without iron plaque, implying that the plaque became a barrier for zinc uptake. While rice plants were pre-cultured in −Fe and +Fe nutrient solution in order to produce the Fe-deficient and Fe-sufficient plants and then Fe(OH)3 was added at 20, 30, and 50 mg Fe/L in nutrient solution, zinc concentrations in shoots of Fe-deficient plants were 54, 48, and 43 mg/kg, respectively, in contrast to 32, 35, and 40 mg/kg zinc in shoots of Fe-sufficient rice plants. Furthermore, Fe(OH)3 addition at 20 mg Fe/L and increasing zinc concentration from 0.065 to 0.65 mg Zn/L in nutrient solution increased zinc uptake more in Fe-deficient plants than in Fe-sufficient plant. The results suggested that root exudates of Fe-deficient plants, especially phytosiderophores, could enhance zinc uptake by rice plants with iron plaque up to a particular amount of Fe.