Impact of soil nitrogen concentration on Glomus spp.-Sinorhizobium interactions as affecting growth, nitrate reductase activity and protein content of Medicago sativa

Our objective was to evaluate how increasing levels of N in the medium (0, 4, 8 and 16 mmol N added kg^-1 soil) affect the interaction between Sinorhizobium and arbuscular mycorrhiza (AM) fungi in the tripartite symbiosis with Medicago sativa. Growth response, nutrient acquisition, protein content, and nitrate reductase (NR) activity were measured both in plant shoots and roots. Results showed that N levels in soil did not affect mycorrhizal colonization but they strongly influenced nodulation, particularly of mycorrhizal plants. Mycorrhizal colonization was required for a proper nodulation when no N was applied to soil. In contrast, the addition of 4 mmol N kg^-1 soil reduced nodulation only in mycorrhizal plants and 8 mmol N added kg^-1 soil allowed nodule formation only in non-mycorrhizal plants. Nodulation was totally inhibited in all treatments with the addition of 16 mmol N added kg^-1 soil. N addition enhanced NR activity in all the treatments, while AM colonization increased the proportion of NR allocated to roots. This effect was more pronounced under the lowest N levels in the medium. The two AM fungal species showed different distribution pattern of enzymatic activities in plant tissues indicating specific physiological traits. Protein content as well as the relative proportion of protein in roots were greatly increased after mycorrhizal colonization. Glomus intraradices-colonized plants had the highest protein content in shoot and root. Mycorrhizal effects on growth, N acquisition and biochemical variables cannot be interpreted as an indirect P-mediated effect since P content was lower in mycorrhizal plants than in those which were P fertilized. Mycorrhizal colonization increased the N content in plants irrespective of the N level, but the effectiveness of AM fungi on plant N acquisition depended on the AM fungus involved, G. intraradices being the most effective, particularly at the highest N rate. N₂ fixation, enhanced by AM colonization, contributed to N acquisition when a moderate N quantity was available in the soil. Nevertheless, under a high N amount the nodulating process and/or fixing capacity by Sinorhizobium was reduced in AM plants. In contrast, the AM fungal mycelium from a particular mycorrhizal fungus may continue to contribute efficiently to the N uptake from the soil even at high N levels. These results demonstrate the particular sensitivity of AM fungal species in terms of their growth and/or function to increasing N amounts in the medium. A selection of AM fungi used to address specific environmental conditions, such as N fertilization regimes comparable to those used in agronomic practices, is required for a better use of N applied to soil.     

Effects of mycorrhizal inoculation of shrubs from Mediterranean ecosystems and composted residue application on transplant performance and mycorrhizal developments in a desertified soil

Arbuscular mycorrhizal inoculation and composted residue application are being assayed to aid restoration of desertified areas under Mediterranean climate. The particular objective of the present study was to assess the short-term (8 months) effects on the initial stages of plant performance and on mycorrhizal propagule release, key factors to decide further developments in the restoration process. Mycorrhizal inoculation, with Glomus intraradices, was practised during nursery production of representative shrub species from Mediterranean ecosystems and composted residues were added to soil before transplanting to a desertified area in southern Spain. Pistacia lentiscus, Rhamnus lycioides, Olea europaea subsp. sylvestris and Retama sphaerocarpa, key species from the natural succession in the target area, were the test plants. Mycorrhizal inoculation, and in some cases compost addition, improved the ability for nutrient acquisition by plants upon transplanting in the field. The number of "infective" mycorrhizal propagules was higher in soil around mycorrhiza-inoculated shrubs than that around the corresponding non-inoculated controls. The organic amendment significantly increased propagule production in the rhizosphere of mycorrhiza-inoculated plants. The number of mycorrhizal spores was relatively low in soil around transplants, being hardly affected by treatments. Only three distinguishable glomalean spore morphotypes were found, belonging to the species Glomus geosporum, G. contrictum and Scutellospora calospora, with very few unidentified spores, corroborating the low diversity in degraded ecosystems. An increased development of the extramatrical AM mycelium was found in soil around the roots of the four mycorrhiza-inoculated test plants, probably the main source of AM fungal propagules in the ecosystem at this stage of plant development. In conclusion, the tailored AM inoculation assayed was functioning under field conditions to enhance nutrient acquisition by the target indigenous shrubs and, in interaction with organic amendments, promoted mycorrhizal propagule production in soil, critical factors to benefit further stages of the revegetation process.     

Arbuscular mycorrhizal fungi influence tomato competition with bahiagrass

A strip-tillage production system for tomatoes (Lycopersicon esculentum Mill.) is impacted by nutrient competition from bahiagrass (Paspalum notatum Flügge). Tomato and bahiagrass differ in mycorrhizal responsiveness and our objective was to evaluate the influence of arbuscular mycorrhizal (AM) fungi on the competitive pressure of bahiagrass on growth of tomato. The first experiment evaluated the effect of bahiagrass competition, soil pasteurization, and AM fungal inoculation on tomato growth, P content, and root colonization in a low-P soil. Tomato grown alone was very responsive to mycorrhizal colonization - shoot dry mass of inoculated plants was up to 243% greater than that of noninoculated plants. Tomato grown with bahiagrass had reduced root and shoot growth across all treatments compared with tomato grown alone, but there was an increase in shoot mass following AM fungal inoculation across both pasteurized and nonpasteurized treatments resulting in a >50% increase in shoot dry mass of tomato compared to noninoculated controls. A second experiment was conducted to test bahiagrass competition, soil pasteurization, AM fungal inoculation, and P amendment on tomato growth in a moderate-P soil. With bahiagrass competition and no P addition, inoculation increased root mass by 115% and shoot mass by 133% in pasteurized soil; however, with the application of 32 mg P kg^-1 the trend was reversed and inoculated plants were smaller than noninoculated controls. We conclude that the role of mycorrhizae in plant competition for nutrients is markedly impacted by soil nutrient status and reduced P application may allow tomatoes to take advantage of their inherent responsiveness to mycorrhizae in a low to moderate soil-P environment.     

Effect of mixing soil saprophytic fungi with organic residues on the response of Solanum lycopersicum to arbuscular mycorrhizal fungi

The effect of the dual inoculation with arbuscular mycorrhizal (AM) and saprophytic fungi and a combination of wheat straw and sewage sludge residues were studied by determining their effect on dry weight of tomato and on chemical and biochemical properties of soil. Incubation of organic residue (sewage sludge combined with wheat straw) with saprophytic fungi and plant inoculation with mycorrhizal fungi was essential to study plant growth promotion. Soil application of organic residues increased the dry weight of tomato inoculated with Rhizophagus irregularis. The greatest shoot dry mass was obtained when the organic residues were incubated with Trichoderma harzianum and applied to AM plants. However, the greatest percentage of root length colonized with AM in the presence of the organic residues was obtained with inoculation with Coriolopsis rigida. The relative chlorophyll was greatest in mycorrhizal plants regardless of the presence of either saprophytic fungus. The presence of the saprophytic fungi increased soil pH as the incubation time increased. Soil nitrogen and phosphorus contents and acid phosphatase were stimulated by the addition of organic residues, and contents of N and P. Total N and P content in soil increased when the organic residue was incubated with saprobe fungi, but this effect decreased as the incubation period of the residue with saprobe fungi increased. The same trend was observed for soil β‐glucosidase and fluorescein diacetate activities. The application of organic residues in the presence of AM and saprophytic fungi seems to be an interesting option as a biofertilizer to improve plant growth and biochemical parameters of soils.     

Arbuscular-mycorrhizal inoculation of five tropical fodder crops and inoculum production in marginal soil amended with organic matter

Five fodder crops, Zea mays, Medicago sativa, Trifolium alexandrinum, Avena sativa, and Sorghum vulgare were inoculated with a consortia of indigenous arbuscular mycorrhizal (AM) fungi in non-sterile PO₄^3- deficient sandy loam soil amended with organic matter under field conditions. Shoot and root dry weights and total uptake of P and N of all the test plants were significantly increased by AM inoculation. Mycorrhizal inoculation increased yield in terms of shoot dry weight by 257% in T. alexandrinum followed by 50% in A. sativa, 28% in Z. mays, 20% in M. sativa and 6% in S. vulgare. Variation in dependence on mycorrhiza was observed among the fodder crops. T. alexandrinum showed a maximum dependence of 72% in contrast to 5.7% dependency in S. vulgare. Plant species showed differences in percentage AM colonization, with a high root infection recorded in Z. mays (76%). Spore production and infectious propagules (IP) were as high as 78 spores/IP g^-1 and 103 spores/IP g^-1 in S. vulgare. This study clearly indicates the potential of using indigenous AM inoculations in fodder crops grown in marginal soils along with in situ large-scale production of AM inocula.     

Soil and weed management for enhancing arbuscular mycorrhiza colonization of wheat

Tillage and weed control are critical components of cropping systems that need to be combined such that crops benefit from reduced competition. However, weeds may also contribute to the biological diversity within the agro‐environment. This greenhouse study investigated whether common weeds of arable cropping systems were suitable host plants for arbuscular mycorrhizal fungi (AMF), allowing the development of extraradical mycelium (ERM) that can contribute to the early colonization of a following wheat crop, especially in the absence of soil disturbance. Weeds were allowed to grow for up to 2 months before being controlled by soil disturbance or herbicide application (glyphosate or paraquat). Pregerminated wheat seeds were then planted. Chemical control of the weeds prior to sowing enhanced the early arbuscular mycorrhiza (AM) colonization rate of wheat roots, whereas mechanical disturbance was less acceptable as a method of weed control for rapid AM colonization. The type of herbicide (contact or systemic) had no impact on colonization of the wheat crop. Enhanced AM colonization promoted early P acquisition and growth of the crop. Appropriate management of weeds emerging between two consecutive cropping seasons coupled with no‐till soil management could ensure a quick and efficient AM colonization of the following wheat plants.     

Comparison of the decomposition and N and P mineralization of canola, pea and wheat residues

Insight into nutrient cycling is gained by understanding the dynamics and quantifying nutrient mineralization from decomposing crop residues. Since wheat (Triticum aestivum L.), canola (Brassica napus L.) and pulse crops such as pea (Pisum sativum L.) are commonly grown in rotation, our objectives were to: (1) compare, using the mesh bag technique, the dry matter (DM) loss and release of N and P of straw and root residues of those crops in the 10-11 months following harvest, and (2) determine the influence of N fertilizer on residue decomposition and nutrient release. The no-tillage study started in autumn 1997 when straw residues were placed on the soil surface and root residues were buried in the soil, and sampled periodically through the 1998 growing season. Wheat was grown in 1998 and received 0 or 60 kg N ha^-1. The study was repeated in 1998/1999. Wheat straw decomposed more slowly than canola or pea straw (losing an average of 12%, 24% and 25%, respectively, of initial DM in 10-11 months), however, the converse was noted for root residues (42%, 26% and 19% of initial DM). Average net N mineralization from wheat, canola and pea straw was essentially 0, 0.7 and 5.6 kg N ha^-1, respectively. Phosphorus released from straw ranged from 0.5 kg ha^-1 for pea to 0.75 kg ha^-1 for canola. Net N and P mineralization from root varied little between crop species: 0.9-1.6 kg N ha^-1 and 0.1-0.3 kg P ha^-1. Nitrogen fertilization increased DM loss, and N and P release from straw residues.     

The mycorrhizal fungus Glomus intraradices and rock phosphate amendment influence plant growth and microbial activity in the rhizosphere of Acacia holosericea

Plants inoculated with arbuscular mycorrhizal (AM) fungi utilize more soluble phosphorus from soil mineral phosphate than non-inoculated plants. However, there is no information on the response of soil microflora to mineral phosphate weathering by AM fungi and, in particular, on the catabolic diversity of soil microbial communities.The AM fungus, Glomus intraradices was examined for (i) its effect on the growth of Acacia holosericea, (ii) plant-available phosphate and (iii) soil microbial activity with and without added rock phosphate.After 4-months culture, AM fungal inoculation significantly increased the plant biomasses (by 1.78× and 2.23× for shoot and root biomasses, respectively), while mineral phosphate amendment had no effect in a sterilized soil. After 12-months culture, the biomasses of A. holosericea plants growing in a non-sterilized soil amended with mineral phosphate were significantly higher than those recorded in the control treatment (by 2.5× and 5× for shoot and root biomasses, respectively). The fungal inoculation also significantly stimulated plant growth, which was significantly higher than that measured in the mineral phosphate treatment. When G. intraradices and mineral phosphate were added together to the soil, shoot growth were significantly stimulated over the single treatments (inoculation or amendment) (1.45×). The P leaf mineral content was also higher in the G. intraradices+mineral phosphate treatment than in G. intraradices or rock phosphate amendment. Moreover, the number of fluorescent pseudomonads has been significantly increased when G. intraradices and/or mineral phosphate were added to the soil. By using a specific type of multivariate analysis (co-inertia analysis), it has been shown that plant growth was positively correlated to the metabolization of ketoglutaric acid, and negatively linked to the metabolisation of phenylalanine and other substrates, which shows that microbial activity is also affected.G. intraradices inoculation is highly beneficial to the growth of A. holosericea plants in controlled conditions. This AM symbiosis optimises the P solubilization from the mineral phosphate and affects microbial activity in the hyphosphere of A. holosericea plants.     

Phosphorus fractions in Andisol and Ultisol inoculated with Bacillus thuringiensis and phosphorus uptake by wheat

Abstract

Only a small portion of soil phosphorus (P) is available to plants on a short-term basis, and therefore, P taken up by crops in one growing season is small compared to total P (Pt) content of the soil. A group of soil microorganisms capable of transforming insoluble P into soluble and plant accessible forms. The objective of this study was to evaluate the changes in soil P-pools and P uptake by wheat crops as influenced by inoculation with Bacillus thuringiensis in two soils (Andisol and Ultisol), the experiment was conducted in pots under greenhouse conditions using a completely randomized design. Wheat plants were inoculated and re-inoculated at 20 and 46?days after sowing, respectively, with B. thuringiensis; and, plant sampling were performed after 46, 66 and 87 and soil at 87?days based on the Zadoks growth scale, and the soil was submitted by Hedley’s P fractions. The inoculation with B. thuringiensis affected significantly some P organic P (P_o) and inorganic P (P_i) fractions in both soils (Andisol and Ultisol), improved P uptake by wheat crop in (Ultisol) and decreased significantly in (Andisol). The positive effect was more consistent in Ultisol than in Andisol, this strain can be solubilized P fraction extracted with conc. HCl-P_o and HCl 1?mol.     

Effect of inoculation with wild type Azospirillum brasilense and A. irakense strains on development and nitrogen uptake of spring wheat and grain maize

Under the controlled conditions of the greenhouse and by varying some biotic and abiotic factors, we tried to identify some of the factors critical to obtain successful Azospirillum inoculation. Spring wheat and grain maize were inoculated with different concentrations of the wild type strains A. brasilense Sp245 and A. irakense KBC1, and grown in a substrate with varying concentrations of organic matter (OM) and N fertiliser. The inoculum concentration was one of the factors that influenced most the outcome of an inoculation experiment on wheat, with lower inoculum concentrations (10⁵-10⁶ cfu plant^-1) stimulating root development and plant dry weight and higher inoculum concentrations (10⁷-10⁸ cfu plant^-1) having no effect or sometimes even inhibiting root development. The effect of inoculation was most pronounced at low to intermediate N fertilisation levels, while the OM content of the substrate had no effect. Inoculation was found to affect early plant and root development, plant and root dry weight, grain yield and the N-uptake efficiency of plants. However, inoculation did not change the N concentration in plants or grains. In addition, a difference in the ability of both strains to stimulate plant growth and N uptake of wheat and maize was observed, with A. brasilense Sp245 having most effect on spring wheat and A. irakense KBC1 being more effective on grain maize. The significance of the obtained results for agriculture is discussed.     

Population size and specific potential of P-mineralizing and -solubilizing bacteria under long-term P-deficiency fertilization in a sandy loam soil

The effects of organic manure, mineral fertilizer (NPK), and P-deficiency fertilization (NK) on the individual biomass of young wheat plants, arbuscular mycorrhizal (AM) colonization in wheat root systems, population sizes of soil organic phosphorus mineralizing bacteria (OPMB) and inorganic phosphate solubilizing bacteria (IPSB) as well as soil P-mineralization and -solubilization potential were investigated in a long-term (18-year) fertilizer experiment. The experiment included five treatments: organic manure, an equal mixture of organic manure and mineral fertilizer, fertilizer NPK, fertilizer NK, and the control (without fertilization). Plant biomass, population sizes of soil OPMB and IPSB were greatly increased (P<0.05) by the application of organic manure and slightly increased by the balanced application of mineral fertilizer, while undiminished AM colonization in wheat root system was only observed in the case of the NK treatment. Compared to balanced fertilization, P-deficiency fertilization resulted in a significant increase (P<0.05) of OPMB-specific mineralization potential (soil P-mineralization potential per OPMB cell) and highest IPSB-specific solubilization potential (soil P-solubilization potential per IPSB cell), suggesting that OPMB and IPSB are likely more metabolically active in P-deficiency fertilized soils after long-term fertilizer management, and mycorrhizal plants are more dependent on AM in P-poor soils than in P-fertilized soils. Our results also showed the different effects of mineral fertilizer versus organic manure on soil P-mineralization and -solubilization potentials, as well as specific potentials of OPMB and IPSB in arable soils.     

Effects of Arbuscular Mycorrhizal Inoculation and Phosphorus Application on Yield and Nutrient Uptake of Yam

To be sustainable, production in the traditional yam cropping system, faced with declining soil fertility, could benefit from yam–arbuscular mycorrhizal (AM) symbiosis, which can improve nutrient uptake, disease resistance, and drought tolerance in plants. However, only limited information exists about AM colonization of yam. A pot experiment was conducted to collect information on the response of two genotypes (Dioscorea rotundata accession TDr 97/00903 and D. alata accession TDa 297) to AM inoculation (with and without) and phosphorus (P) (0, 0.05, 0.5, and 5 mg P kg^–1 soil). Factorial combinations of the treatments were arranged in a completely randomized design with four replicates. The percentage of AM colonization was significantly lowered at 5 mg P kg^–1 soil rate in mycorrhizal plants of both genotypes. TDr 97/00903 showed more responsiveness to AM inoculation than TDa 297. The greatest AM responsiveness for tuber yield (52%) was obtained at 0.5 mg P kg^–1 soil rate for TDr 97/00903. Mycorrhizal inoculation significantly increased root dry weight and tuber yield of TDr 97/00903 with the greatest values obtained at the 0.5 mg P kg^–1 soil rate. Arbuscular mycorrhizal inoculation did not lead to significant (P < 0.05) changes in root length and area. Phosphorus application significantly increased the shoot dry weight and root diameter of TDa 297. Uptake of P was greatest at 0.5 mg P kg^–1 soil in both genotypes and was significantly influenced by AM inoculation. Nitrogen (N) and potassium (K) uptake were greatest in mycorrhizal plants at 0.05 mg P kg^–1 soil for TDr 97/00903 but at 0.5 mg P kg^–1 soil of nonmycorrhizal plants of TDa 297. The increased tuber yield and nutrient uptake observed in the mycorrhizal plants indicate the potential for the improvement of nutrient acquisition and tuber yield through AM symbiosis.     

Denitrification, N2O and CO2 fluxes in rice-wheat cropping system as affected by crop residues, fertilizer N and legume green manure

Use of renewable N and C sources such as green manure (GM) and crop residues in rice-wheat cropping systems of South Asia may lead to higher crop productivity and C sequestration. However, information on measurements of gaseous N losses (N₂O+N₂) via denitrification and environmental problems such as N₂O and CO₂ production in rice-wheat cropping systems is not available. An acetylene inhibition-intact soil core technique was employed for direct measurement of denitrification losses, N₂O and CO₂ production, in an irrigated field planted to rice (Oryza sativa L.) and wheat (Triticum aestivum L.) in an annual rotation. The soil was a coarse-textured Tolewal sandy loam soil (Typic Ustochrept) and the site a semi-arid subtropical Punjab region of India. Wheat residue (WR, C:N=94) was incorporated at 6 t ha^-1 and sesbania (Sesbania aculeata L.) was grown as GM crop for 60 days during the pre-rice fallow period. Fresh biomass of GM (C:N.=18) at 20 or 40 t ha^-1 was incorporated into the soil 2 days before transplanting rice. Results of this study reveal that (1) denitrification is a significant N loss process under wetland rice amounting to 33% of the prescribed dose of 120 kg N ha^-1 applied as fertilizer urea-N (FN); (2) integrated management of 6 t WR ha^-1 and 20 t GM ha^-1 supplying 88 kg N ha^-1 and 32 kg FN ha^-1 significantly reduced cumulative gaseous N losses to 51.6 kg N ha^-1 as compared with 58.2 kg N ha^-1 for 120 kg FN ha^-1 alone; (3) application of excessive N and C through applying 40 t GM ha^-1 (176 kg N ha^-1) resulted in the highest gaseous losses of 70 kg N ha^-1; (4) the gaseous N losses under wheat were 0.6% to 2% of the applied 120 kg FN ha^-1 and were eight- to tenfold lower (5-8 kg N ha^-1) than those preceding rice; (5) an interplay between the availability of NO₃^- and organic C largely controlled denitrification and N₂O flux during summer-grown flooded rice whereas temperature and soil aeration status were the primary regulators of the nitrification-denitrification processes and gaseous N losses during winter-grown upland wheat; (6) the irrigated rice-wheat system is a significant source of N₂O as it emits around 15 kg N₂O-N ha^-1 year^-1; (7) incorporation of WR in rice and rice residue (C:N=63) in wheat increased soil respiration, and increased CO₂ production in WR- and GM-amended soils under anaerobic wetland rice coincided with enhanced rates of denitrification; and (8) with adequate soil moisture, most of the decomposable C fraction of added residues was mineralized within one crop-growing season and application of FN and GM further accelerated this process.     

Microbial and chemical sources of phosphorus supply modulate the yield and chemical composition of essential oil of rose-scented geranium (Pelargonium species) in sodic soils

Rose-scented geranium (Pelargonium sp.) is a highly valued aromatic crop. Its growth is limited by soil salinity and sodicity stress. Arbuscular mycorrhizal (AM) fungus, phosphate-solubilizing bacteria (PSB), and P fertilizers may enhance the growth and secondary metabolism in geranium plants. In this context, a pot experiment was conducted to study the effects of PSB, AM fungi (Glomus intraradices), and P fertilizer on the yield, chemical composition of essential oil, and mineral element acquisition of geranium. The dry matter yield of shoot and essential oil yield, and mineral element (P, K, Ca, Mg, Na, Fe, Cu, and Zn) uptake in shoot tissues of geranium were significantly increased by the inoculation with AM fungi, co-inoculation with AM fungi and PSB, and P fertilization as compared to control. While the co-inoculation of geranium with AM fungi and PSB significantly enhanced the content of the monoterpenes such as citronellol, geraniol, geranial, and a sesquiterpene (10-epi-γ eudesmol), the P fertilization only enhanced the content of a sesquiterpene, 10-epi-γ eudesmol in the volatile oil. We conclude that the co-inoculation of PSB and AM fungi could be the best natural alternative to phosphate fertilizers to enhance the yield and quality of essential oil from geranium plants grown in sodic soils.     

Influence of nitrogen source on the viability,functionality and persistence of Glomus etunicatum fungal propagules in an Andisol

This study investigated how two different N sources used as fertilizer (NO₃⁻ or NH₄⁺) interact with an inoculated arbuscular mycorrhizal (AM) fungus (Glomus etunicatum) in an Andisol from southern Chile. The effect of NO₃⁻ or NH₄⁺ on mycorrhizal and non-mycorrhizal wheat plants was measured on key root–soil interface activities: pH, acid phosphatase (P-ase) activity and P availability. Root AM colonization, extraradical mycelium length and spore number were also examined at three stages of AM symbiosis development (120, 150 and 240 days after sowing, DAS). The effect of N-source on AM propagule formation was used as an index of the quality and vigor of AM colonization. Mycorrhizal root length was greater with NO₃⁻ than with NH₄⁺ at all times. The NO₃⁻ source also improved extraradical mycelium density, which reached its maximum at 150 DAS. At each harvest the spore number in the rhizosphere soil was also greater with NO₃⁻ fertilization. This NO₃⁻ effect on spore formation ranged from 20% at a 120 DAS to 287% at a 240 DAS increase, compared with NH₄⁺. Extraradical mycelium and AM efficiency for P acquisition appeared to be related. The particular fungus/plant metabolism as affected by N sources (NO₃⁻ or NH₄⁺) applied did not result in differential plant growth or in changes in N plant acquisition, but affected AM development and activity. Differences in soil pH, available P or P-ase activity in soil seems not to be responsible for the improved physiological status of mycorrhizal development in NO₃⁻ fed plants. Mycorrhizal propagule formation in this soil and the high persistence of extraradical mycelium are important factors which may have a strong influence on the next crop, and thus, this aspects should be considered when a cropping system is designed. The influence of N sources on AM performance is of ecological and practical interest in volcanic soils when conventional management is used.     

Wheat plants invest more in mycorrhizae and receive more benefits from them under adverse than favorable soil conditions

Soil chemistry and biota heavily influence crop plant growth and mineral nutrition. The stress-severity and optimal resource allocation hypotheses predict mutualistic symbiotic benefits to increase with the degree of metabolic imbalance and environmental stress. Using two cross-factorial pot experiments with the same biologically active calcareous soil, one time highly saline and nutrient-deficient, and the other time partially desalinated and amended with mineral soil fertilizer, we explored whether these general predictions hold true for zinc (Zn) nutrition of bread wheat in mycorrhizal symbiosis. Increased arbuscular mycorrhizal (AM) fungal root colonization positively correlated with plant Zn nutrition, but only when plants were impaired in growth due to salinity and nutrient-deficiency; this was particularly so in a cultivar-responsive to application of mineral Zn fertilizer. Evidence for direct involvement of AM fungi were positive correlations between Zn uptake from soil and frequency of fungal symbiotic nutrient exchange organelles, as well as the quantitative abundance of AM fungi of the genera Funneliformis and Rhizophagus, but not Claroideoglomus. Combined partial soil desalination and fertilization swapped the dominance ranking from Claroideoglomus spp. to Funneliformis spp. Positive growth, nitrogen, and Zn uptake responses to mycorrhization were contingent on moderate soil fertilization with ZnSO₄. In agreement with the predictions of the stress-severity and optimal resource allocation hypotheses, plants limited in growth due to chemically adverse soil conditions invested relatively more into AM fungi, as evident from heavier root colonization, and took up relatively more Zn and nitrogen in response to mycorrhization, than better growing and less mycorrhized plants. It thus appears that crop plant cultivar-dependent mycorrhization and Zn fertilizer-responsiveness may reinforce each other, provided that there is bioavailable Zn in soil and plant growth is impaired by suboptimal chemical soil conditions.     

Short-term patterns of carbon and nitrogen mineralisation in a fallow field amended with green manures from agroforestry trees

The mineralisation of green manure from agroforestry trees was monitored with the objective to compare the temporal dynamics of mineralisation of litter from different species. Green manures from five agroforestry tree species were used on a fallow field during the long rainy season of 1997 (March-August) and from two species in the following short rainy season (September-January) in western Kenya. Different methods, i.e. measurements of isotopic ratios of C in respired CO₂ and of soil organic matter (SOM) fractions, soil inorganic N and mass loss from litterbags, were used in the field to study decomposition and C and N mineralisation. Soil respiration, with the separation of added C from old soil C by using the isotopic ratio of ¹³C/¹²C in the respired CO₂, correlated well with extractable NH₄⁺ in the soil. Mineralisation was high and very rapid from residues of Sesbania sesban of high quality [e.g. low ratio of (polyphenol+lignin)/N] and low and slow from low quality residues of Grevillea robusta. Ten days after application, 37% and 8% of the added C had been respired from Sesbania and Grevillea, respectively. Apparently, as much as 70-90% of the added C was respired in 40 days from high quality green manure. Weight losses of around 80%, from high quality residues in litterbags, also indicate substantial C losses and that a build-up of SOM is unlikely. For immediate effects on soil fertility, application of high quality green manure may, however, be a viable management option. To achieve synchrony with crop demand, caution is needed in management as large amounts of N are mineralised within a few days after application.     

Impact of Arbuscular-Mycorrhizal Fungi on Phosphorus Efficiency of Wheat,Rye, and Triticale

Genotypic variation and mycorrhiza play an important role in plant uptake of phosphorus (P). A pot experiment was conducted with three cereals, wheat (Triticum aestivum L. cv. PBW-34), rye (Secale cereale L. cv. R-308), and triticale (Triticale octoploide L. cv. DT-46), a hybrid of wheat and rye, to examine the genetic variation in the degree of arbuscular-mycorrhizal (AM) infection and its inheritability from parents (wheat and rye) to their progeny (triticale). The soil used for pot culture was low in available P (7.8 mg P kg^?1soil). Inoculation with AM fungi showed a significant increase in extent of root colonization for all three cereals (average 70%) compared with their performance without AM (average 19.1%). However, among the three cereals, this increase was significantly greater in rye than in the other two crops, while wheat and triticale did not differ significantly. Mycorrhizal infection resulted in 1.6, 1.7, and 1.8-fold increases in shoot, root, and total plant dry matter, respectively, compared with the un-inoculated treatment. Among the three cereals, rye recorded maximum shoot, root, and total plant dry mass and P content with AM inoculation. The P uptake by wheat, rye, and triticale was 10%, 64%, and 35%, respectively, higher with rather than without mycorrhizal infection. Rye was most responsive to AM inoculation, with mycorrhizal dependency of 193%; here again, triticale followed wheat, with similar mycorrhizal dependency. Rye showed an increase in P utilization efficiency (PUE) without AM inoculation while the PUE of triticale was intermediate between wheat and rye. High efficiency of AM symbiosis in terms of P uptake exists in rye and most of these traits in triticale seem to be inherited from wheat rather than rye.     

Variable response of growth and arbuscular mycorrhizal colonization of maize plants to preceding crops in various types of soils

The effects of the preceding crops, sunflower (mycorrhizal host) and mustard (nonhost), on arbuscular mycorrhizal (AM) colonization and growth of succeeding maize were examined in 17 soils in an attempt to clarify the influence of soil characteristics on the effects of preceding crops. Shoot weight and P uptake of maize planted after sunflower were much higher than those after mustard in 14 soils, although the preceding crop had little effect on soil-P availability. AM colonization of maize after sunflower was much higher than that after mustard. The effect of the preceding crop was eliminated by soil sterilization. These results suggested that the differences in maize growth were caused by differences in the AM colonization. Correlation analysis of the effect of the preceding crop and soil properties showed that the difference in the effects on maize growth could not be explained by soil chemical properties, but only by the AM colonization of the preceding sunflower crop. In one of the 17 soils, however, the effect was not evident despite the higher AM colonization of sunflower. This soil was sterilized, and the effect of inoculation by AM fungi (AMF) on maize was examined. However, it was found that the inoculation increased AM colonization but did not improve maize growth at any P level, suggesting that the effect of AMF was unusually inhibited in this soil by unknown soil physicochemical properties. In most soils, however, the preceding mycorrhizal host crop, sunflower, improved the growth and AM colonization of maize depending on the AM colonization of sunflower.     

Nitrous oxide emissions from the wheat-growing season in eighteen Chinese paddy soils: an outdoor pot experiment

To identify the key soil parameters influencing N₂O emission from the wheat-growing season, an outdoor pot experiment with a total of 18 fertilized Chinese soils planted with wheat was conducted in Nanjing, China during the 2000/2001 wheat-growing season. Average seasonal N₂O-N emission for all 18 soils was 610 mg m^-2, ranging from 193 to 1,204 mg m^-2, approximately a 6.2-fold difference between the maximum and the minimum. Correlation analysis indicated that the seasonal N₂O emission was negatively correlated with soil organic C (r²=0.5567, P<0.001), soil total N (r²=0.4684, P<0.01) and the C:N ratio (r²=0.4530, P<0.01), respectively. A positive dependence of N₂O emission on the soil pH (r²=0.3525, P<0.01) was also observed. No clear relationships existed between N₂O emission and soil texture, soil trace elements of Fe, Cu and Mg, and above-ground biomass of the wheat crop at harvest. A further investigation suggested that the seasonal N₂O-N emission (E, mg m^-2) can be quantitatively explained by E=1005-34.2SOC+4.1S_a (R²=0.7703, n=18, P=0.0000). SOC and S_a represent the soil organic C (g kg^-1) and available S (mg kg^-1), respectively.