Showing papers in "Plant and Soil in 1993"
TL;DR: In this paper, a conceptual model describing the influence of elevated atmospheric CO2 on plant production, soil microorganisms, and the cycling of C and N in the plant-soil system was proposed.
Abstract: We tested a conceptual model describing the influence of elevated atmospheric CO2 on plant production, soil microorganisms, and the cycling of C and N in the plant-soil system. Our model is based on the observation that in nutrient-poor soils, plants (C3) grown in an elevated CO2 atmosphere often increase production and allocation to belowground structures. We predicted that greater belowground C inputs at elevated CO2 should elicit an increase in soil microbial biomass and increased rates of organic matter turnover and nitrogen availability. We measured photosynthesis, biomass production, and C allocation of Populus grandidentata Michx. grown in nutrient-poor soil for one field season at ambient and twice-ambient (i.e., elevated) atmospheric CO2 concentrations. Plants were grown in a sandy subsurface soil i) at ambient CO2 with no open top chamber, ii) at ambient CO2 in an open top chamber, and iii) at twice-ambient CO2 in an open top chamber. Plants were fertilized with 4.5 g N m−2 over a 47 d period midway through the growing season. Following 152 d of growth, we quantified microbial biomass and the availabilities of C and N in rhizosphere and bulk soil. We tested for a significant CO2 effect on plant growth and soil C and N dynamics by comparing the means of the chambered ambient and chambered elevated CO2 treatments.
625 citations
TL;DR: Arsenic absorption by rice (Oryza sativa, L.) in relation to As chemical form present in soil solution was examined in this paper, where rice plants were grown in soil suspensions equilibrated under selected conditions of redox and pH, affecting arsenic solubility and speciation.
Abstract: Arsenic absorption by rice (Oryza sativa, L.) in relation to As chemical form present in soil solution was examined. Rice plants were grown in soil suspensions equilibrated under selected conditions of redox and pH, affecting arsenic solubility and speciation. A decrease in pH led to higher dissolved arsenic concentrations. When the soil redox potential dropped below 0 mV, most of the arsenic was present as As(III). Under more oxidizing conditions both As(III) and As(V) are present. Chemical speciation of As in the watersoluble fraction affected its phytoavailability. Most indigenous arsenic taken up by the plants remained in the root. Plant arsenic availability increased with increasing arsenic concentration in solution (lower soil pH) and with increasing amounts of soluble As(III) (lower soil redox). We also studied the uptake of monomethyl arsenic acid (MMAA), a widely used defoliant and herbicide, as affected by soil redox-pH condition. Amended MMAA was approximately two times more phytoavailable than the indigenous inorganic As forms and increased with decreasing pH and redox.
285 citations
TL;DR: In this article, the authors present data from a variety of studies that show that soil organic carbon in managed systems can be lower, the same as, or greater than mature tropical forests and that SOC can increase rapidly after the abandonment of agricultural fields.
Abstract: The prevailing paradigm for anticipating changes in soil organic carbon (SOC) with changes in land use postulates reductions in SOC in managed systems (agriculture and tree plantations) relative to mature tropical forests. Variations of this notion are used in carbon models to predict the role of tropical soils in the global carbon cycle. Invariably these models show tropical soils as sources of atmospheric carbon. We present data from a variety of studies that show that SOC in managed systems can be lower, the same as, or greater than mature tropical forests and that SOC can increase rapidly after the abandonment of agricultural fields. History of land use affects the comparison of SOC in managed and natural ecosystems. Our review of the literature also highlights the need for greater precautions when comparing SOC in mature tropical forests with that of managed ecosystems. Information on previous land use, bulk density, and consistency in sampling depth are some of the most common omissions in published studies. From comparable SOC data from a variety of tropical land uses we estimate that tropical soils can accumulate between 168 and 553 Tg C/yr. The greatest potential for carbon sequestration in tropical soils is in the forest fallows which cover some 250 million hectares. Increased attention to SOC by land managers can result in greater rates of carbon sequestration than predicted by current SOC models.
254 citations
TL;DR: Considering that A. diazotrophicus is found within the plant tissue, the biosynthesis of IAA suggests that the bacteria could promote rooting and improve sugarcane growth by direct effects on metabolic processes, in addition to their role in N2 fixation.
Abstract: Thirteen cane cultivars grown on fields in Mexico were sampled to assess the occurrence of Acetobacter diazotrophicus, a recently identified N2-fixing bacterium. Results showed that the isolation frequencies extended over a broad range (1.1 to 67%), likely to be related to the nitrogen fertilization level. The lowest isolation frequencies (1.1 to 2.5%) were obtained from plants growing at high nitrogen doses (275–300 kg ha-1) and the highest values (10–67%) from plants cultivated with 120 kg N ha-1. All eighteen strains of A. diazotrophicus produced indoleacetic acid (IAA) in defined culture medium. Estimates obtained from HPLC analyses revealed that A. diazotrophicus strains produced from 0.14 to 2.42 μg IAA mL-1 in culture medium. Considering that A. diazotrophicus is found within the plant tissue, the biosynthesis of IAA suggests that the bacteria could promote rooting and improve sugarcane growth by direct effects on metabolic processes, in addition to their role in N2 fixation.
240 citations
TL;DR: Nine crop species were grown in P-sufficient and P-deficient nutrient solutions and the activity of acid phosphatase secreted by the roots of lupin increased most remarkably, and many isozymes existed in the roots and the leaves, but only one was secreted into the rhizosphere in a large amount.
Abstract: Nine crop species were grown in P-sufficient and P-deficient nutrient solutions. The activity of acid phosphatase secreted by the roots increased under P-deficient conditions in all the species examined. That of lupin increased most remarkably. The properties of the enzyme secreted by the roots of lupin was investigated. Many isozymes existed in the roots and the leaves, but only one of them was secreted into the rhizosphere in a large amount. The molecular weight of the purified enzyme secreted was estimated to be 72 KD by SDS-PAGE and 140 KD by gel filtration; it was assumed to be a homo-dimer. The iso-electric point of the enzyme was 4.7 and the pH for optimum activity 4.3. When the enzyme was mixed with aqueous solution extracted from a P-deficient soil, its activity declined to 55% of its original activity after 14 days and to 9% after 21 days.
215 citations
TL;DR: Bacteria, yeasts and filamentous fungi colonizing immature, mature and senescing primary leaves of field grown Beta vulgaris were analysed over a complete growing season and the communities found to have greatest diversity during the summer months.
Abstract: Bacteria, yeasts and filamentous fungi colonizing immature, mature and senescing primary leaves of field grown Beta vulgaris (sugar beet) were analysed over a complete growing season. Greatest microbial numbers were detected on senescing primary leaves and these numbers increased over most of the season. The number of colonizers detected on mature leaves was found to be stable over most of the study. Filamentous fungi and yeasts were identified to the genus level and the communities found to have greatest diversity during the summer months. There was no consistent pattern of diversity according to leaf type. Two genera of filamentous fungi, Cladosporium and Alternaria and two yeast genera, Cryptococcus and Sporobolomyces were the most numerous fungal populations isolated. Only 8 filamentous fungi and 3 yeast genera were commonly isolated on PDA (potato dextrose agar). Bacterial strains (1236) were isolated on Tryptic Soy Broth (TSB) agar and identified to species, or in some cases sub-species level, by analysis of their fatty acid methyl ester (FAME) profiles. Isolated bacteria were grouped into 78 named and 37 unnamed species clusters. Greatest number of bacterial species were isolated from young plants and leaves, sampled during the autumn months. Bacterial community diversity was lowest in mid-summer and winter months. Pseudomonas was the most commonly isolated genus and Erwinia herbicola the most common species. P. aureofaciens was the only species isolated from soil that was also isolated from the phyllosphere of B. vulgaris throughout the season.
213 citations
TL;DR: Despite extensive experimental evidence of root-mediated changes to the physical, chemical and biological status of rhizosphere soil, the quantitative significance of these changes for mineral nutrition has not been assessed.
Abstract: The role of the rhizosphere in relation to mineral nutrition is discussed within a quantitative framework using the Barber-Cushman model as a starting point. The uptake or release of nutrients by roots growing in soil leads to concentration gradients forming in the soil: the zone so affected is termed the rhizosphere. The nature of these gradients depends on three factors: the rate of uptake/release; the mobility of the nutrient in soil; and the rate of conversion between available and unavailable forms. The interplay between these factors determines the amount of mineral nutrients acquired by the plant and it is the complexity of the interplay which demands the use of mathematical models in order to understand which factors most limit uptake. Despite extensive experimental evidence of root-mediated changes to the physical, chemical and biological status of rhizosphere soil, the quantitative significance of these changes for mineral nutrition has not been assessed. The problems of making this quantitative transition are reviewed.
209 citations
TL;DR: In this paper, a food web model was applied to a set of food webs from different sites and from different arable farming systems, and the results showed that the model could simulate N mineralization rates close to the rates obtained from in situ measurements, from nitrogen budget analyses, or from a decomposition based model.
Abstract: In agricultural practices in which the use of inorganic fertilizer is being reduced in favour of the use of organic manure, the availability of nitrogen (N) in soil for plant growth depends increasingly on N mineralization. In simulation models, N mineralization is frequently described in relation to the decomposition of organic matter, making a distinction in the quality of the chemical components available as substrate for soil microbes. A different way to model N mineralization is to derive N mineralization from the trophic interactions among the groups of organisms constituting the soil food web. In the present study a food web model was applied to a set of food webs from different sites and from different arable farming systems. The results showed that the model could simulate N mineralization rates close to the rates obtained from in situ measurements, from nitrogen budget analyses, or from a decomposition based model. The outcome of the model suggested that the contribution of the various groups of organisms to N mineralization varied strongly among the different sites and farming systems.
202 citations
TL;DR: The classification of the maize inbred lines and the near equal whole-plant Cd uptake between the two groups demonstrates that internal distribution rather than uptake is causing the genotypic differences in shoot Cd concentration of maize inbreeding lines.
Abstract: Genotypic variation in uptake and distribution of cadmium (Cd) was studied in 19 inbred lines of maize (Zea mays L.). The inbred lines were grown for 27 days on an in situ Cd-contaminated sandy soil or for 20 days on nutrient solution culture with 10 µg Cd L-1. The Cd concentrations in the shoots showed large genotypic variation, ranging from 0.9 to 9.9 µg g-1 dry wt. for the Cd-contaminated soil and from 2.5 to 56.9 µg g-1 dry wt. for the nutrient solution culture. The inbred lines showed a similar ranking for the Cd concentrations in the shoots for both growth media (r2=0.89). Two main groups of inbreds were distinguished: a group with low shoot, but high root Cd concentrations (shoot: 7.4±5.3 µg g-1 dry wt.; root: 206.0±71.2 µg g-1 dry wt.; ‘shoot Cd excluder’) and a group with similar shoot and root Cd concentrations (shoot: 54.2±3.4 µg g-1 dry wt.; root: 75.6±11.2 µg g-1 dry wt.; ‘non-shoot Cd excluder’). The classification of the maize inbred lines and the near equal whole-plant Cd uptake between the two groups demonstrates that internal distribution rather than uptake is causing the genotypic differences in shoot Cd concentration of maize inbred lines. Zinc (Zn), a micronutrient chemically related to Cd, showed an almost similar distribution pattern for all maize inbred lines. The discrepancy in the internal distribution between Cd and Zn emphasizes the specificity of the Cd distribution in maize inbred lines.
202 citations
TL;DR: Spores of arbuscular mycorrhizal fungi isolated from two heavy-metal polluted soils in France via trap culture with leek may be considered as metal-tolerant ecotypes and results indicate a potential adaptation of AM fungi to elevated metal concentrations in soil.
Abstract: Spores of arbuscular mycorrhizal (AM) fungi were isolated from two heavy-metal polluted soils in France via trap culture with leek (Allium porrum L.). Preliminary identification showed that the predominant spore type of both cultures (P2 and Cd40) belongs to the Glomus mosseae group. Their sensitivity to cadmium was compared to a laboratory reference strain (G. mosseae) by in vitro germination tests with cadmium nitrate solutions at a range of concentrations (0 to 100 mg L−1) as well as extracts from a metal-polluted and unpolluted soils. Both cultures of AM fungi from heavy-metal polluted soils were more tolerant to cadmium than the G. mosseae reference strain. The graphically estimated EC50 was 0.8 mg L−1 Cd (concentration added to the test device) for G. mosseae and 7 mg L−1 for P2 culture, corresponding to effective Cd concentrations of approximately 50–70 μg L−1 and 200–500 μg L−1, respectively. The extract of the metal-polluted soil P2 decreased germination of spores from the reference G. mosseae but not from P2 culture. However, the extracts of two unpolluted soils with different physico-chemical characteristics did not affect G. mosseae, whereas germination of P2 spores was markedly decreased in the presence of one of the extracts. These results indicate a potential adaptation of AM fungi to elevated metal concentrations in soil. The tested spores may be considered as metal-tolerant ecotypes. Spore germination results in presence of soil extracts show the difficulty of assessing the ecotoxic effect of metals on AM fungi without considering other soil factors that may interfere in spore germination and hyphal extension.
186 citations
TL;DR: In this article, changes in δ13 C ratio have been used to measure C turnover in systems which include C3 and C4 species, and the relative size of soil C fractions oxidised by 333mM KMnO4 declined with cropping.
Abstract: The decline in soil organic matter with cropping is a major factor affecting the sustainability of cropping systems. Changes in total C levels are relatively insensitive as a sustainability measure. Oxidation with different strength KMnO4 has been shown to be a more sensitive indicator of change. The relative size of soil C fractions oxidised by 333mM KMnO4 declined with cropping, whilst the relative size of the unoxidised fraction increased. Changes in δ13 C ratio have been used to measure C turnover in systems which include C3 and C4 species.
TL;DR: In this paper, the authors used Ca status of tomato fruit to blossom-end rot (BER) incidence and found that the lowest %Ca was found in the distal placenta and locular tissues, where BER first develops.
Abstract: Studies of Ca uptake and distribution in relation to environmental variables were used to relate Ca status of tomato fruit to blossom-end rot (BER) incidence. Ca uptake was highly correlated with solar radiation and root temperature. The rate of Ca uptake decreased linearly with increasing salinity. High humidity reduced Ca import by the leaves but increased that by the fruit. While total plant dry weight was reduced more than fruit dry weight by salinity, total Ca uptake and the Ca content of the fruit were decreased similarly. Thus, the concentration of calcium in the fruit was substantially reduced by salinity. The distal half of the fruit contained less Ca than the proximal half. The lowest %Ca was found in the distal placenta and locular tissues, where BER first develops. The incidence of BER was often stimulated more by high salinity achieved with the addition of major nutrients than with NaCl. The cause of BER is usually an interaction between the effects of irradiance and ambient temperature on fruit growth and the effects of environmental stress on calcium uptake and distribution within the whole plant.
TL;DR: In this article, a direct correlation was found between fractional losses of added N and the change in δ15N during 19 years in an experiment with annual additions of N at three rates to a Scots pine forest in northern Sweden.
Abstract: A direct correlation was found between fractional losses of added N and the change in δ15N‰ during 19 years in an experiment with annual additions of N at three rates to a Scots pine (Pinus sylvestris L.) forest in northern Sweden. This confirms that processes leading to losses of N discriminate against15N, and opens possibilities to conduct retrospective studies of the N balance in forests.
TL;DR: In this paper, the exudation of soluble carbon compounds from Zea mays roots was investigated over a 10-day growth period under sterile and non-sterile solution culture conditions.
Abstract: The exudation of soluble carbon compounds from Zea mays roots was investigated over a 10 day growth period under sterile and non-sterile solution culture conditions. The results showed that plants grown in sterile static solution culture, where C was allowed to accumulate, released 8 times less C than plants grown under culture conditions in which the solutions were replaced daily. The increased C loss from plant cultures in which exudates were removed daily was attributable to, (a) the reduced potential for root re-sorption of previously lost C, and (b), increasing diffusion gradients between the root and the surrounding bathing solution increasing passive leakage of exudates from the roots [...]
TL;DR: The results revealed that seed-coating with antagonistic Streptomyces spp.
Abstract: This investigation was designed to explore the potential of microbial antagonism in the control of some tomato diseases including bacterial, Fusarium and Verticillium wilts; early blight; bacterial canker. Three Streptomyces spp. were used: S. pulcher, S. canescens and S. citreofluorescens. The in vitro studies showed that an 80% concentration of the culture filtrate of either S. pulcher or S. canescens significantly inhibited spore germination, mycelial growth and spotulation of Fusarium oxysporum f.sp. lycopersici, Verticillium albo-atrum and Alternaria solani. The same concentration of filtrate of either S. pulcher or S. citreofluorescens was detrimental to the bacterial populations of Clavibacter michiganensis subsp. michiganensis and Pseudomonas solanacearum. The in vivo studies involved different treatments: soaking tomato seeds in filtrate of the antagonist prior to sowing, inoculation of the soil with the antagonist 7 days before sowing, and coating of tomato seeds with spores of the antagonist before sowing. The seed-coating treatment was the most effective in controlling all the pathogens at 42 and 63 days after sowing. Soil inoculation with the antagonist 7 days prior to sowing was less effective in controlling the tomato pathogens as compared to seed-coating. The seed-soaking treatment was the least effective in controlling the diseases concerned. The results also revealed that seed-coating with antagonistic Streptomyces spp. significantly improved tomato growth.
TL;DR: In this paper, a field studies on irrigated rice at the IRRI research farm indicate efficient use of fertilizer-N based on plant uptake of applied N, (estimated by N difference), and utilization of acquired N for increased grain yield.
Abstract: Recent field studies on irrigated rice at the IRRI research farm indicate efficient use of fertilizer-N based on plant uptake of applied N, (estimated by N difference), and utilization of acquired N for increased grain yield. These findings contrast with 15N uptake in microplot studies which underestimate the actual increase in plant N from added fertiliser. Constraints other than uptake efficiency, however, may govern fertiliser-N efficiency in farmers fields. In a study of 44 farmers’ fields in Central Luzon, rice yields ranged from 2.5 to 6.2 t ha-1 and N uptake from 35 to 95 kg N ha-1 in plots without fertiliser-N addition. Farmers applied from 35 to 240 kg N ha-1, but there was no relationship between the N rate used by each farmer and the effective soil N supply. Mean N uptake efficiency from fertiliser by N difference was only 36%. We conclude that improved fertiliser-N efficiency by farmers will require a more information-intensive management strategy that makes N fertiliser inputs better fitted to the seasonal pattern of crop N demand and soil N supply.
TL;DR: An effecient, reproducible and simple mass screening technique for the selection of salt tolerant rice lines has been developed in this article, where fourteen-day old seedlings raised in silica gravel culture were transplanted to foam-plugged holes in polystyrene (thermopal) sheets floated over 100 dm3 of nutrient solution in painted galvanised-iron growth tanks lined with plastic.
Abstract: An effecient, reproducible and simple mass screening technique for the selection of salt tolerant rice lines has been developed. Fourteen-day old seedlings raised in silica gravel culture were transplanted to foam-plugged holes in polystyrene (thermopal) sheets floated over 100 dm3 of nutrient solution in painted galvanised-iron growth tanks lined with plastic (120×90×30cm). Three days after transplanting, NaCl was added to salinize the medium in increments, at the rate of 25 mol m-3 per 24 hours, up to the desired salinity levels which ranged from 50–200 mol m-3 NaCl. Six plants of each line were transplanted and allowed to grow for 15 days after the maximum desired stress level was achieved in each case. Absolute shoot fresh and dry weights, as well as percent mortality, were used as criteria for assessing relative salt tolerance. Related studies were also conducted to standardize the technique. The validity of this technique was tested by conducting experiments in salinised soil (pot culture) and in salt-affected field where 9 rice lines were grown up to maturity and absolute paddy yield was considered as the criterion for salt tolerance. Salt tolerance behaviour of cultivars based on different selection criteria was compared. Good reproducibility of results among the three solution culture experiments and their close association with the results of pot culture and of salt-affected field study, authenticated the validity of this technique for practical purposes.
TL;DR: Some possibilities available for scientists interested in enhancing symbiotic nitrogen fixation in grain legumes are discussed, including work on the legume/Rhizobium symbiosis combining experience from various disciplines in inter-disciplinary research programmes.
Abstract: Biological nitrogen fixation of leguminous crops is becoming increasingly important in attempts to develop sustainable agricultural production. However, these crops are quite variable in their effectiveness in fixing nitrogen. By the use of the 15N isotope dilution method some species have been found to fix large proportions of their nitrogen, while others like common bean have been considered rather inefficient. Methods for increasing N2 fixation are therefore of great importance in any legume work. Attempts to enhance nitrogen fixation of grain legumes has been mainly the domain of microbiologists who have selected rhizobial strains with superior effectiveness or competitive ability. Few projects have focused on the plant symbiont with the objective of improving N2 fixation as done in the FAO/IAEA Co-ordinated Research Programme which is being reported in this volume. The objective of the present paper is to discuss some possibilities available for scientists interested in enhancing symbiotic nitrogen fixation in grain legumes. Examples will be presented on work performed using agronomic methods, as well as work on the plant and microbial symbionts. There are several methods available to scientists working on enhancement of N2 fixation. No one approach is better than the others; rather work on the legume/Rhizobium symbiosis combining experience from various disciplines in inter-disciplinary research programmes should be pursued.
TL;DR: In this paper, the effects of applying pruning of three woody species (Acioa barteri, Gliricidia sepium and Leucaena leucocephala), maize (Zea mays L.) stover and rice (Oryza sativa L.) straw as mulch on maize were studied on an Alfisol in southern Nigeria in 1990 and 1991.
Abstract: Effects of application of prunings of three woody species (Acioa barteri, Gliricidia sepium and Leucaena leucocephala), maize (Zea mays L.) stover and rice (Oryza sativa L.) straw as mulch on maize were studied on an Alfisol in southern Nigeria in 1990 and 1991. Maize dry matter and grain yield were higher with applications of plant residues and N fertilizer in both years. Addition of Leucaena prunings gave the highest maize grain yield in both years. Compared to the 1990 results, Acioa showed the least grain yield decline among the mulch treatments in 1991. Nutrient uptake was enhanced by applications of plant residues. Leucaena prunings had the highest effect in both years and increased the mean N, P, and Mg uptake by 96%, 84%, and 50%, respectively, over the control. Addition of Acioa prunings increased K and Ca uptake by 59% and 92%, respectively, over the control. ‘High quality’ (low C/N ratio and lignin level) plant residues enhance crop performance through direct nutritional contributions, whereas ‘low quality’ (high C/N ratio and lignin level) plant residues do so through mulching effects on the microclimate. ‘Intermediate quality’ plant residues have no clear effects on crop performance.
TL;DR: In this paper, the decomposition rates, initial chemical composition, and the relationship between initial chemistry and mass loss of fine roots and foliage were determined for two woody tropical species, Prestoea montana and Dacryodes excelsa, over a gradient of sites in two watersheds in the Luquillo Experimental Forest, Puerto Rico.
Abstract: Decomposition rates, initial chemical composition, and the relationship between initial chemistry and mass loss of fine roots and foliage were determined for two woody tropical species, Prestoea montana and Dacryodes excelsa, over a gradient of sites in two watersheds in the Luquillo Experimental Forest, Puerto Rico. At all locations, fine roots decayed significantly more slowly than foliage during the initial 6 months.
TL;DR: In this paper, the authors have discussed some of the criteria to adopt in selecting reference plants, e.g. how to ensure that the reference plant is not fixing N2, is absorbing most of its N from the same zone as the fixing plant, and in the same pattern with time, etc.
Abstract: The 15N methods are potentially accurate for measuring N2 fixation in plants. The only problem with those methods is, how to ensure that the15N/ 14N ratio in the plant accurately reflects the integrated 15N/ 14N ratio (R) in soil which is variable in time and with soil depth. However, the consequences of using an inappropriate reference plant vary with the level of N2 fixation and the conditions under which the study was made. For example, the errors introduced into the values of N2 fixation are higher at low levels of fixation, and decrease with increasing rates of fixation. At very high N2 fixation rates, the errors are often insignificant. Also, the magnitude of error is proportional to the rate of decline of the 15N/14N ratio with time. Since N2 fixation in most plants would be expected to below 60%, the question of how to select a good reference plant is still pertinent. In this paper, we have discussed some of the criteria to adopt in selecting reference plants, e.g. how to ensure that the reference plant is not fixing N2, is absorbing most of its N from the same zone as the fixing plant, and in the same pattern with time, etc. In addition, we have discussed 15N labelling materials and methods that are likely to minimize any errors even when the fixing and reference plants don’t match well in certain important criteria. The use of slow release 15N fertilizer or 15N labelled plant materials results in slow changes in the 15N/ 14N ratio of soil, and is strongly recommended. Where 15N inorganic fertilizers are used, the application of the fertilizer in small splits at various intervals is recommended over a one-time application. The problem with the reference crop, which has sometimes discouraged potential users of the 15N methods, is surmountable, as discussed in this paper.
TL;DR: Breeding lines capable of fixing enough atmospheric N2 to support seed yields of 1000–2000 kg ha−1 have been identified and new cultivars with high N2 fixation potential are being released.
Abstract: Common bean (Phaseolus vulgaris L.), which is an important food crop in the Americas, Africa and Asia, usually is thought to fix only small amounts of atmospheric nitrogen. However, field data indicate considerable genetic variability for total N2 fixation and traits associated with fixation. Studies have shown that selection to increase N2 fixation will be successful if: (1) discriminating traits (selection criteria) are measured precisely, (2) variability in germplasm is heritable, (3) selected parents are also agronomically suitable, (4) units of selection facilitate quantification of selection criteria, and (5) a breeding procedure that allows maximum genetic gain for N2 fixation and recombination with essential agronomic traits is chosen. Breeding lines capable of fixing enough atmospheric N2 to support seed yields of 1000-2000 kg ha-1 have been identified and new cultivars with high N2 fixation potential are being released.
TL;DR: In this paper, a recirculating column system was used to replace organic and inorganic P in a sterile and non-sterile grassland soil amended with 0 and 235 kg P ha−1 for 13 consecutive years.
Abstract: Replenishment of soil solution organic and inorganic P in a sterile and nonsterile grassland soil amended with 0 and 235 kg P ha−1 for 13 consecutive years was investigated in a recirculating column system. In sterilized treatments, P liberated from soil biomass, initially increased solution organic and inorganic P concentrations to about 0.3 and 0.6 μg P cm−3 in the 0 and 235 kg P treatment, respectively. Sterilization effects were larger than the residual fertilizer effect. Subsequently, in sterilized treatments were microbial activity was lacking, removal of solution P over the duration of the experiment reduced organic P concentration to the detection limit (0.001 μg P cm−3). Organic P concentrations in the nonsterile treatment were maintained at about 0.015 μg P cm−3 which was higher than inorganic P concentration. Inorganic P concentrations were about 0.002 and 0.008 μg P cm−3 in the nonfertilized and the fertilized treatment, respectively. Inorganic P buffer power was greater in the nonsterile treatments, but abiotic buffering alone could not account for the measured inorganic P concentrations found during desorption. It was concluded that biomass P is a major factor controlling organic and inorganic P solution concentrations in this systems.
TL;DR: The interactions between the mycorrhizal fungusGlomus mosseae, the plant pathogenPythium ultimum, and a pathogen-antagonistTrichoderma aureoviride in the rhizosphere of Tagetes erecta (marigold) were studied for their effects on plant growth in a peat-perlite substrate.
Abstract: The interactions between the mycorrhizal fungusGlomus mosseae, the plant pathogenPythium ultimum, and a pathogen-antagonistTrichoderma aureoviride in the rhizosphere ofTagetes erecta (marigold) were studied for their effects on plant growth in a peat-perlite substrate. Mycorrhizal fungus inoculation protected the plant againstP. ultimum, since both phytomass production and foliar development were higher in mycorrhizal plants.T. aureoviride had no effect on nonmycorrhizal plants in the presence or absence ofP. ultimum. However, more biomass was produced by mycorrhizal plants whenT. aureoviride was present, whether or not soil was infested withP. ultimum. ei]R Rodriguez-Kabana
TL;DR: Investigation of how barley cultivars which are different in dry matter yield at low phosphorus (P) supply and agronomic P efficiency respond to mycorrhizal infection found that the response to my corollary infection was positively correlated with response to P application.
Abstract: The purpose of this study was to investigate how barley cultivars which are different in dry matter yield at low phosphorus (P) supply (i.e. they differ in agronomic P efficiency) respond to mycorrhizal infection. In a preliminary experiment, six mycorrhizal fungi were tested for their ability to colonize barley (Hordeum vulgare L.) roots at a soil temperature of 15°C.Glomus etunicatum was the most effective species and was used in the main experiment. The main experiment was conducted under glasshouse conditions in which soil temperature was maintained at 15°C. Treatments consisted of a factorial arrangement of 8 barley cultivars, 2 mycorrhiza (inoculated and non-inoculated), and 3 rates of P (0, 10 and 20 mg kg-1). P utilization efficiency (dry matter yield per unit of P taken up) and agronomic P efficiency among the barley cultivars was significantly negatively correlated with mycorrhizal responses. However, the response to mycorrhizal infection was positively correlated with response to P application. Poor correlation was observed between P concentration when neither mycorrhiza nor P were supplied and the percentage of root length infected. The extent of mycorrhizal infection among the barley cultivars in soil without P amendment varied from 8.6 to 28.6%. Significant interactions between cultivar and P addition, and between mycorrhiza and P addition were observed for shoot dry weight but not root dry weight.
TL;DR: In this article, a 15N labelling technique was used to measure N2O and N2 emissions from an undisturbed grassland soil treated with cow urine and held at 30 cm water tension and 20°C in a laboratory.
Abstract: A 15N labelling technique was used to measure N2O and N2 emissions from an undisturbed grassland soil treated with cow urine and held at 30 cm water tension and 20°C in a laboratory. Large emissions of dinitrogen were detected immediately following urine application to pasture. These coincided with a rapid and large increase in soil water-soluble carbon levels, some of this increase being attributed to solubilization of soil organic matter by high pH and ammonia concentrations. Emissions of nitrous oxide generally increased with time in contrast to dinitrogen fluxes which decreased as time progressed. Estimated losses of N2O and N2 over a 30 day period were between 1 to 5% and 30 to 65% of the urine N applied plus N mineralized from soil organic matter, respectively. Most of the N2 and N2O originated from denitrification with nitrification-denitrification being of minor significance as a source of N2O. Comparisons of the 15N enrichments in the soil mineral N pools and the evolved N2O suggested that much of the N2O was produced in the 5–8 cm zone of the soil. It is concluded that established grassland soils contain large amounts of readily-oxidizable organic carbon which may be used by soil denitrifying organisms when nitrate is non-limiting and soil redox potential is lowered due to high rates of biological activity and high soil moisture contents. ei]{gnR}{fnMerckx}
TL;DR: In this paper, the effect of long-term applications of crop residues (millet straw, 2.4 t ha-1 yr−1) and/or mineral fertilizer (30 kg N, 13 kg P and 25 kg K ha 1 yr-1) on uptake of phosphorus (P) and other nutrients, root growth and mycorrhizal colonization of pearl millet (Pennisetum glaucum L.) was examined for two seasons (1987 and 1988) on an acid sandy soil in Niger.
Abstract: The effect of long-term (1983–1988) applications of crop residues (millet straw, 2–4 t ha-1 yr−1) and/or mineral fertilizer (30 kg N, 13 kg P and 25 kg K ha-1 yr-1) on uptake of phosphorus (P) and other nutrients, root growth and mycorrhizal colonization of pearl millet (Pennisetum glaucum L.) was examined for two seasons (1987 and 1988) on an acid sandy soil in Niger. Treatments of the long-term field experiment were: control (−CR−F), mineral fertilizer only (−CR+F), crop residues only (+CR−F), and crop residues plus mineral fertilizer (+CR+F).
TL;DR: In this article, the responses of five tomato cultivars (L. esculentum mill) of different degrees of salt tolerance were examined over a range of 0 to 140 mM NaCl applied for 3 and 10 weeks.
Abstract: The responses of five tomato cultivars (L. esculentum Mill) of different degrees of salt tolerance were examined over a range of 0 to 140 mM NaCl applied for 3 and 10 weeks. Judged by both Na and Cl accumulations and maintenance of K, Ca and Mg contents with increasing salinity, the most tolerant cultivars (Pera and GC-72) showed different responses. The greater salt tolerance of cv Pera was associated with a higher Cl and Na accumulation and a lower K content in the shoot than those found in the other cultivars, typical of a halophytic response to salinity. However, the greater salt tolerance of cv GC-72 was associated with a retention of Na and Cl in the root, restriction of their translocation to the shoot and maintenance of potassium selectivity under saline conditions. The salt tolerance mechanisms that operated in the remaining cultivars were similar to that of cv GC-72, as at first they excluded Na and Cl from the shoots, accumulating them in the roots; with longer treatment, the ability to regulate Na and Cl concentrations in the plant was lost only in the most salt sensitive cultivar (Volgogradskij), resulting in a massive influx of both ions into the shoot. The salt sensitivity of some tomato cultivars to salinity could be due to both the toxic effect of Na and Cl ions and nutritional imbalance induced by salinity, as plant growth was inversely correlated with Na and Cl contents and directly correlated with K and Ca contents. This study displays that there is not a single salt tolerance mechanism, since different physiological responses among tomato cultivars have been found.
TL;DR: In this article, a phytotron was used to investigate the effect of K (0, 25, 50 and 75 mg K kg−1 soil) application on their salt tolerance.
Abstract: A salt-tolerant (Pokkali) and a salt-sensitive (IR28) variety of rice (Oryza sativa L.) were grown in a phytotron to investigate the effect of K (0, 25, 50 and 75 mg K kg−1 soil) application on their salt tolerance. Potassium application significantly increased potential photosynthetic activity (Rfd value), percentage of filled spikelets, yield and K concentration in straw. At the same time, it also significantly reduced Na and Mg concentrations and consequently improved the K/Na, K/Mg and K/Ca ratios. IR28 responded better to K application than Pokkali. Split application of K failed to exert any beneficial effect over basal application.
TL;DR: Mucilages from the root tips of axenically-grown maize and from a bacterium isolated from the rhizosheaths of field-grown roots, were immobilized by drying onto nylon blotting membrane, and it was concluded that mucilages of both plant and bacterial origin can contribute to the adhesion and cohesion of maize rhizosphere, but each by a different mechanism.
Abstract: Mucilages from the root tips of axenically-grown maize and from a bacterium (Cytophaga sp.) isolated from the rhizosheaths of field-grown roots, were immobilized by drying onto nylon blotting membrane. The mucilage plaques remained in place through repeated rewettings and histochemical treatments. Staining of the plaques showed that both mucilages included acidic groups, and 1,2 diols (the latter notably fewer in bacterial mucilage). Bacterial mucilage plaques stained strongly for protein, plant mucilage was unstained. Plaques of both mucilages bound soil particles strongly if soil was applied to wet mucilage and then dried. Bound soil was not lost with rewetting. Dry weight and densitometer measurements showed that bacterial mucilage bound about 10% more soil than the same surface area of root-cap mucilage. Pretreatment of plaques with periodate oxidation eliminated most soil binding by root-cap mucilage but this was completely reversible by reduction with borohydride. Soil binding to bacterial mucilage was unaffected by periodate but much diminished by borohydride pretreatment (partially restored by subsequent oxidation). Neither pretreatment with cationic dyes nor preincubation in pectinase, pectin methylesterase or protease affected subsequent soil binding by the mucilage plaques. Pretreatment of root-cap mucilage plaques with lectins specific for component sugars also did not alter soil binding. It is concluded that mucilages of both plant and bacterial origin can contribute to the adhesion and cohesion of maize rhizosheaths, but each by a different mechanism. Binding by root-cap mucilage depends on 1,2 diol groups of component sugars, that of bacterial mucilage does not, and is likely to be protein mediated. ei]Section editor: R O D Dixon