<Emphasis Type="Italic"> Gluconacetobacter diazotrophicus</Emphasis>: a natural endophytic diazotroph of Nile Delta sugarcane capable of establishing an endophytic association with wheat

Gluconacetobacter- like diazotrophs were encountered as dense populations inside the root and stem tissues of sugarcane cultivated in ancient agricultural fields of the Nile Delta. Counts of >10⁵ cells g^-1 were recorded in root and stem samples. The leaves contained a smaller population (<10³ g^-1). The typical dark-orange colonies which developed on LGIP agar plates were purified. Identification was performed with the API microtube systems: API 20E for Enterobacteriaceae and API 20NE for non-Enterobacteriaceae. API profiles of the local isolates were closely related to those of the type culture Gluconacetobacter diazotrophicus (ATTC 49037). The isolates successfully reduced C₂H₂ and produced appreciable amounts of ethylene in the presence of cane juice. This suggested that the local isolates are closely related to the type strain G. diazotrophicus. Wheat seedlings were inoculated with a number of isolates under gnotobiotic conditions. Both optical and scanning electron microscopy showed that endophytic Gluconacetobacter spp. were present in all the samples tested. They were observed in apparently intact and enlarged epidermal root cells, and also in stem tissues, indicating that the bacterium was able to migrate upward into the shoot tissues. Although Gluconacetobacter inoculation did not stimulate the growth of the cereal plant, the results obtained are particularly interesting because this bacterial species was capable of colonizing the internal tissues of wheat, not considered a natural host until now.     

Exploration of bacterial N2-fixation systems in association with soil-grown sugarcane,sweet potato,and paddy rice: a review and synthesis

N₂ fixation systems in the nonleguminous crops and bacteria associations have been intensively studied over the last 50 years. Their structure and regulation have been investigated to explore the enhancement of N acquisition in these ecosystems leading to crop-growth with minimum chemical fertilizers. Several lines of important evidence have been accumulated indicating that the magnitudes of associative (nonsymbiotic) N₂ fixation in sugarcane (Saccharum spp.), sweet potato (Ipomoea batatas L.), and paddy rice (Oryza sativa L.) are agronomically significant. In these three crops, unique bacterial N₂-fixation systems may function in addition to the low-level activity (due to the competition in carbon/energy use) of the commonly occurring rhizosphere-associated system by free-living bacteria such as Beijerinckia, Azotobacter, and Klebsiella. Active expressions of the dinitrogenase reductase-encoded gene (nifH) phylogenetically similar to those of Bradyrhizobium spp. and Azorhizobium sp. were abundantly found in the N₂-fixing sugarcane stems, sweet potato stems, and storage tubers. These rhizobia micro-aerobically fix N₂ in the carbon compounds-rich apoplasts. Gluconacetobacter diazotrophicus and Herbaspirillum spp. were previously isolated from inside the sugarcane stems, as the candidates of endophytic N₂ fixers. However, the current molecular and physiological investigations suggest that their major role is production of phytohormonal substances. In paddy rice fields, methane is produced from organic compounds in anoxia and oxidized by contacting with oxygen gas. An active N₂-fixation by methane-oxidizing methanotrophs such as Methylosinus sp. takes place in the root tissues (aerenchyma) and also in the surface soil. This methanotrophic N₂-fixation supports the sustainability of soil fertility although the N₂-fixation and soil fertility are affected by chemical fertilizers. Finally, we discuss the ecological implications of the newly identified rhizobia and methanotroph systems in the N nutrition in nonlegumes and N reservation in field environments.     

Infection and colonization of aseptically micropropagated sugarcane seedlings by nitrogen-fixing endophytic bacterium, Herbaspirillum sp. B501gfp1

In this study, we used Herbaspirillum sp. B501gfp1 (B501gfp1), an isolate from wild rice, to investigate the interaction between a non-host nitrogen-fixing endophytic bacterium and micropropagated sugarcane plants under aseptic condition. Two Japanese sugarcane plants (Saccharum sp.) cultivars (cvs) NiF8 and Ni15 were inoculated using B501gfp1 in two inoculum doses of 10⁸ and 10² bacterial-cells-per-milliliter suspension. The results showed that bacterial cells colonized both the root and stem tissues, and colonization was apparent in the intercellular spaces. Higher bacterial numbers were detected in plant tissues inoculated with the higher inoculum concentration treatment. Bacterial numbers also varied between the two cultivars, with the higher values determined in cv Ni15. This study provides evidence that Herbaspirillum sp. B501gfp1, a rice isolate, could colonize sugarcane tissues, suggesting non-specificity of host plant among endophytes.     

Inoculation of rice plants with the endophytic diazotrophs Herbaspirillum seropedicae and Burkholderia spp.

 Four experiments were performed under gnotobiotic conditions to select strains of the endophytic diazotrophs Herbaspirillum seropedicae and Burkholderia spp. as inocula of rice plants. Eighty strains of H. seropedicae originally isolated from rice, sorghum and maize plants, were tested in test tube cultures with N-free agar as the substrate. Rice plants showed medium and high increases in their fresh weight in response to inoculation with nineteen strains. These strains were tested again, and six strains were then selected to evaluate their contribution to the N of the plant via biological N₂ fixation (BNF) using an agar growth medium containing 5 mg N l^–1of ¹⁵N-labelled (NH₄)₂SO₄. The contribution of the strains to plant N via BNF varied from 54% when rice plants were inoculated with strain ZAE94, to 31% when strain ZAE67 was used. These results were confirmed in the fourth gnotobiotic experiment, which also included strains of the new N-fixing bacteria belonging to the genus Burkholderia, isolated from rice, as well as a strain of Burkholderia vietnamiensis, isolated from rice rhizosphere. Burkholderia spp. strains showed similar effects to those observed for H. seropedicae strains, while B. vietnamiensis fixed only 19% of plant total N. The best four strains were tested in a pot experiment where pre-germinated, inoculated rice seedlings were grown in soil labelled with ¹⁵N. The results confirmed the gnotobiotic experiments, although the levels of N in the rice plants derived from BNF of the selected H. seropedicae and Burkholderia spp. strains were lower. Nevertheless, there was an increase in N content in grains of inoculated plants, and the results showed that the method used for strain selection is very useful and can be applied to other strains of N₂-fixing bacteria and plants. Received: 4 May 1999     

Synergistic effects of vesicular-arbuscular mycorrhizal fungi and diazotrophic bacteria on nutrition and growth of sweet potato (Ipomoea batatas)

Summary Sweet potatoes were micropropagated and then transplanted from axnic conditions to fumigated soil in pots in the greenhouse. Spores of Glomus clarum were obtained from Brachiaria decumbens or from sweet potatoes grown in soil infected with this fungus and with an enrichment culture of Acetobacter diazotrophicus. Three experiments were carried out to measure the beneficial effects of vesicular-arbuscular mycorrhizal (VAM) fungi-diazotroph interactions on growth, nutrition, and infection of sweet potato by A. diazotrophicus and other diazotrophs obtained from sweet potato roots. In two of these experiments the soils had been mixed with ¹⁵N-containing organic matter. The greatest effects of mycorrhizal inoculation were observed with co-inoculation of A. diazotrophicus and/or mixed cultures of diazotrophs containing A. diazotrophicus and Klebsiella sp. The tuber production was dependent on mycorrhization, and total N and P accumulation were increased when diazotrophs and G. clarum were applied together with VAM fungal spores. A. diazotrophicus infected aerial plant parts only when inoculated together with VAM fungi or when present within G. clarum spores. More pronounced effects on root colonization and intraradical sporulation of G. clarum were observed when A. diazotrophicus was co-inoculated. In non-fumigated soil, dual inoculation effects, however, were of lower magnitude. ¹⁵N analysis of the aerial parts and roots and tubers at the early growth stage (70 days) showed no statistical differences between treatments except for the VAM+Klebsiella sp. treatment. This indicates that the effects of A. diazotrophicus and other diazotrophs on sweet potato growth were caused by enhanced mycorrhization and, consequently, a more efficient assimilation of nutrients from the soil than by N₂ fixation. The possible interactions between these effects are discussed.     

Dinitrogen fixation activity of Azorhizobium caulinodans in the rice (Oryza sativa L.) rhizosphere assessed by nitrogen balance and nitrogen-15 dilution methods

 Azorhizobium caulinodans was directly inoculated onto rice plants in three short-term pot trials. Addition of increasing amounts of sucrose (23, 46, 92 kg ha^–1) did not influence the N economy of the A. caulinodans-rice association during the early vegetative growth stage. A. caulinodans inoculation alone and in combination with the highest amount of sucrose had a significantly positive effect on the N balance, with small but significant N gains in the system. Application of 60 kg urea-N ha^–1 had a negative impact on the N economy of the inoculated treatments. N losses increased and the amount of atmospheric N₂ fixed and incorporated decreased significantly as compared to the amounts under the 20 kg urea-N ha^–1 regime. However, N losses were low – a maximum of 8% – at the early vegetative growth stage under the conditions of the experiments. C limitation does not seem to be a limiting factor for the incorporation of fixed N₂ in this bacteria-plant association. Biological N₂ fixation caused by A. caulinodans inoculation was responsible for 14% of the plant N at the vegetative growth stage and under low N conditions. Received: 30 January 2000     

Isolation and partial characterization of endophytic diazotrophs associated with Japanese sugarcane cultivar

Endophytic bacteria were isolated from the juice of a 6 month-old sugarcane cv. NiF-8, which was collected from Miyako, Okinawa and planted in a field lysimeter in Tsukuba, Ibaraki, Japan. The most probable number of N₂-fixing endophytes was 4.5 × 10⁵ cells per gram of fresh weight sample and 21 isolates have a positive reaction for ARA in an N₂-free semi-solid medium with 10% crystallized sugarcane sugar and 0.5% sugarcane juice. Analyses of some of the biochemical properties of the N₂-fixing isolates indicated that 13 isolates were putative strains of Acetobacter diazotrophicus, 4 isolates showed similar characteristics to those of Herbaspirillum seropedicae, and 4 isolates consisted of Herbaspirillum rubribalbicans-like strains. This study confirmed the existence of N₂-fixing endophytic bacteria in the Japanese sugarcane cv. NiF-8.     

Soil mineral nitrogen dynamics under bare fallow and wheat in vertisols of semi-arid Mediterranean Morocco

 The evoluion of NH₄ ⁺-N and NO₃ ^–-N was monitored during three growing seasons, 1992–1993, 1993–1994, 1994–1995 in the soil profile (0–60 or 0–90 cm) under bare fallow and wheat on a vertisol site of the Sais plateau, Morocco. The aim of this study was to relate the soil mineral N dynamics to crop N uptake and soil N transformation processes. The efficacy of the current N fertilisation rate (100 kg N ha^–1) for wheat production in the region was evaluated. The high level of residual mineral N in the soil profile resulted from a low N plant uptake relative to the soil N supply and N fertilisation, and masked the effect of N fertilisation on dry matter accumulation. NH₄ ⁺-N was present in considerable amounts, suggesting a low nitrification rate under the given pedo-climatic conditions. An artefact due to the sampling procedure was encountered shortly after the application of N fertiliser. Losses through leaching and denitrification occurred after heavy rainfall, but were limited. At least part of the exchangeable NH₄ ⁺-N seemed to be barely taken up by the crop. NO₃ ^–-N was therefore considered to be a better indicator of plant-available N than total mineral N for this type of soil. The low N fertiliser use efficiencies demonstrated clearly that the current fertilisation rate (100 kg N ha^–1) for wheat production in this region is unsustainable. The maximum N uptake ranged from 40 kg N ha^–1 to 180 kg N ha^–1. The estimation of the seasonal production potential is considered to be the main prerequisite for the determination of the best rates and timing of N fertiliser application in this region. Received: 9 December 1997     

Crop residues and fertilizer nitrogen influence residue decomposition and nitrous oxide emission from a Vertisol

Crop residues with high C/N ratio immobilize N released during decomposition in soil, thus reducing N losses through leaching, denitrification, and nitrous oxide (N₂O) emission. A laboratory incubation experiment was conducted for 84 days under controlled conditions (24°C and moisture content 55% of water-holding capacity) to study the influence of sugarcane, maize, sorghum, cotton and lucerne residues, and mineral N addition, on N mineralization–immobilization and N₂O emission. Residues were added at the rate of 3 t C ha⁻¹ to soil with, and without, 150 kg urea N ha⁻¹. The addition of sugarcane, maize, and sorghum residues without N fertilizer resulted in a significant immobilization of soil N. Amended soil had significantly (P < 0.05) lower NO₃⁻–N, which reached minimum values of 2.8 mg N kg⁻¹ for sugarcane (at day 28), 10.3 mg N kg⁻¹ for maize (day 7), and 5.9 mg N kg⁻¹ for sorghum (day 7), compared to 22.7 mg N kg⁻¹ for the unamended soil (day 7). During 84 days of incubation, the total mineral N in the residues + N treatments were decreased by 45 mg N kg⁻¹ in sugarcane, 34 mg kg⁻¹ in maize, 29 mg kg⁻¹ in sorghum, and 16 mg kg⁻¹ in cotton amended soil compared to soil + N fertilizer, although soil NO₃⁻–N increased by 7 mg kg⁻¹ in lucerne amended soil. The addition of residues also significantly increased amended soil microbial biomass C and N. Maximum emissions of N₂O from crop residue amended soils occurred in the first 4–5 days of incubation. Overall, after 84 days of incubation, the cumulative N₂O emission was 25% lower with cotton + N fertilizer, compared to soil + N fertilizer. The cumulative N₂O emission was significantly and positively correlated with NO₃⁻–N (r = 0.92, P < 0.01) and total mineral N (r = 0.93, P < 0.01) after 84 days of incubation, and had a weak but significant positive correlation with cumulative CO₂ in the first 3 and 5 days of incubation (r = 0.59, P < 0.05).     

Recycling of Organic Wastes Amended with Trichoderma and Gluconacetobacter for Sustenance in Soil Health and Sugarcane Ratoon Yield in Udic Ustochrept

A field experiment was conducted with the objectives to relate the changes in the physical properties, soil organic carbon (SOC), nutrient availability, and uptake and output input ratios for sustaining sugarcane ratoon growth and yield in an Udic ustochrept. Eight combinations of trash and farmyard manure (FYM) with and without Trichoderma viride and Gluconacetobacter diazotrophicus were applied in two sugarcane ratoon (first and second ratoon in succession) crops. Application of Trichoderma-enriched trash showed the lowest bulk density (1.36 Mg m^?3) and the greatest infiltration rate (4.5 mm h^?1). Greater rate of increase in SOC was observed under inoculation of Trichoderma with FYM compared to trash mulch. The output/input ratios were greater in plots having trash-based treatments compared to FYM and inorganic fertilizers. Bioagent-inoculated FYM produced greater mean sugar yield (8.89 t ha^?1) compared to bioagent-inoculated trash (7.97 t ha^?1).     

Effects of inoculation of cyanobacteria on nitrogen status and nutrition of rice (Oryza sativa L.) in an Entisol amended with chemical and organic sources of nitrogen

A field experiment was conducted with wetland rice (Oryza sativa cv. IR-36) in a sandy clay loam soil (Entisol) to study the effect of inoculation with a soil-based mixed culture of four diazotrophic cyanobacteria,Aulosira fertilissima, Nostoc muscorum, N. commune andAnabaena spp., on the N-flux in inorganic NH₄ ⁺+NO₃ ^–+ NO₂ ^–), easily oxidizable, hydrolysable and non-hydrolysable forms of N in soil during vegetative growth periods of the crop. Effects on grain and straw yield and N uptake by the crop were estimated. The effects of applying urea N and N as organic sources, viz.Sesbania aculeata, Neem (Azardirachta indica) cake and FYM, each at the rate of 40 kg N ha^–1, to the soil were also evaluated. Inoculation significantly increased the release of inorganic N, evidenced by its increased concentrations either in soil or in soil solution. However, such increases rarely exceeded even 4% of total N gained in different froms in the soil system by inoculation during the vegetative growth stages of the rice plant, when the nutritional requirement of the plants is at a maximum. Most of the N₂ fixed by cyanobacteria remained in the soil as the hydrolysable form (about 85%) during this period. Inoculation caused an insignificant increase in grain (8%) and straw (11%) yield, which was, however, accompanied by a significant increase in N uptake by the grain (30%) and an increase in total uptake of 15.3 kg N ha 1. Such beneficial effects of inoculation varied in magnitude with the application of organic sources, with farmyard manure (FYM) being the most effective. Application of urea N, on the other hand, markedly reduced such an effect.     

Effect of phosphorus fertilization on blue-green algal inoculum production and nitrogen yield under field conditions

Summary Field experiments were conducted to assess the effects of the application of P on growth and N yield of inoculated and indigenous blue-green algae (BGA). Addition of 17.4 kg P ha^–1 in split applications led to the highest BGA biomass and N yield, 162 kg dry weight ha^–1 and 6 kg N ha^–1 per 15 days, respectively. When inoculum of Aulosira spp., Aphanothece spp., Gloeotrichia spp. were compared separately, Gloeotrichia spp. grew faster but Aulosira spp. fixed more N. The growth rate and N yield of Aulosira spp. decreased with high P applications, although growth continued until the application of 34.8 kg P ha^–1. The effects of P on inoculum production by local species compared with those collected from other states showed the superiority of the local culture. Applications of P also enhanced the growth and N yield of indigenous BGA, with Wollea spp. showing the best results.     

Effects of nitrogen input and grazing on methane fluxes of extensively and intensively managed grasslands in the Netherlands

 Generally, grasslands are considered as sinks for atmospheric CH₄, and N input as a factor which reduces CH₄ uptake by soils. We aimed to assess the short- and long-term effects of a wide range of N inputs, and of grazing versus mowing, on net CH₄ emissions of grasslands in the Netherlands. These grasslands are mostly intensively managed with a total N input via fertilisation and atmospheric deposition in the range of 300–500 kg N ha^–1 year^–1. Net CH₄ emissions were measured with vented, closed flux chambers at four contrasting sites, which were chosen to represent a range of N inputs. There were no significant effects of grazing versus mowing, stocking density, and withholding N fertilisation for 3–9 years, on net CH₄ emissions. When the ground-water level was close to the soil surface, the injection of cattle slurry resulted in a significant net CH₄ production. The highest atmospheric CH₄ uptake was found at the site with the lowest N input and the lowest ground-water level, with an annual CH₄ uptake of 1.1 kg CH₄ ha^–1 year^–1. This is assumed to be the upper limit of CH₄ uptake by grasslands in the Netherlands. We conclude that grasslands in the Netherlands are a net sink of CH₄, with an estimated CH₄ uptake of 0.5 Gg CH₄ year^–1. At the current rates of total N input, the overall effect of N fertilisation on net CH₄ emissions from grasslands is thought to be small or negligible. Received: 27 January 1998     

Response of CH<Subscript>4</Subscript> oxidation and methanotrophic diversity to NH<Subscript>4</Subscript><Superscript>+</Superscript> and CH<Subscript>4</Subscript> mixing ratios

Methane oxidising activity and community structure of 11, specifically targeted, methanotrophic species have been examined in an arable soil. Soils were sampled from three different field plots, receiving no fertilisation (C), compost (G) and mineral fertiliser (M), respectively. Incubation experiments were carried out with and without pre-incubation at elevated CH₄ mixing ratios (100 ml CH₄ l⁻¹) and with and without ammonium (100 mg N kg⁻¹) pre-incubation. Four months after fertilisation, plots C, G and M did not show significant differences in physicochemical properties and CH₄ oxidising activity. The total number of methanotrophs (determined as the sum the 11 specifically targeted methanotrophs) in the fresh soils was 17.0×10⁶, 13.7×10⁶ and 15.5×10⁶ cells g⁻¹ for treatment C, G and M, respectively. This corresponded to 0.11 to 0.32% of the total bacterial number. The CH₄ oxidising activity increased 10⁵-fold (20–26 mg CH₄ g⁻¹ h⁻¹), the total number of methanotrophs doubled (28–76×10⁶ cells g⁻¹) and the methanotrophic diversity markedly increased in treatments with a pre-incubation at elevated CH₄ concentrations. In all soils and treatments, type II methanotrophs (62–91%) outnumbered type I methanotrophs (9–38%). Methylocystis and Methylosinus species were always most abundant. After pre-incubation with ammonium, CH₄ oxidation was completely inhibited; however, no change in the methanotrophic community structure could be detected.     

Colonization of sugarcane plantlets by mixed inoculations with diazotrophic bacteria

Micropropagated sugarcane plants have been used in Brazil for almost three decades. Besides the improvement in plant health, micropropagated sugarcane carries no endophytic plant growth-promoting bacteria. The Brazilian inoculation technology to reintroduce diazotrophic bacteria in micropropagated sugarcane plantlets revealed a synergistic-like effect in PGP-bacteria mixed inoculations. The infection model of single diazotrophic bacteria species in sugarcane was studied in detail, but still many questions remain open. In this study we used a combined fluorescence in situ hybridization (FISH) and a cultivation based approach (MPN) to evaluate the colonization of sugarcane plantlets by mixed inocula. The highest colonization for three out of the five species studied was obtained with a mixed inoculum, when the Azospirillum amazonense showed an increase by almost 100 times in colonization and Herbaspirillum spp. and Burkholderia tropica was determined at 10⁷ cells per gram root fresh weight. All of the inoculated bacterial species could be detected using the FISH probes 12 h after bacterial inoculation. The FISH results confirmed the MPN counts and showed differences in the population numbers and colonization behavior of particular bacterial inoculum strains in the different mixed inocula. A putative antagonistic effect among the inoculated H. seropedicae and H. rubrisubalbicans strains was observed using FISH, as well as the better competitiveness of B. tropica as compared to the A. amazonense strain. The observed data probably reflect also specific interactions with the sugarcane variety used in this particular inoculation system, and may not be generalized as a rule. This is the first study about the competition for sugarcane colonization in a mixed bacterial inoculum.     

Influence of nitrogen fertilisation on pasture culturable rhizobacteria occurrence and the role of environmental factors on their potential PGPR activities

Plant growth-promoting rhizobacteria (PGPR) naturally occur in the rhizospheres of pasture, but still little is understood regarding how soil agricultural practices affect them. Here, we examined the effects of long-term nitrogen (N) fertilisation on the occurrence of potential culturable PGPR in rhizosphere soils from pastures grown in Chilean Andisols. We also evaluated in vitro the effects of organic acids (citric, malic and oxalic acids), metals (Al and Mn) and N supply (urea and ammonium sulphate) on indole acetic acid (IAA) production and phosphorus (P) liberation by selected strains. Compared with non-N-fertilised pasture, N fertilisation significantly increased (30%) the occurrence of culturable phosphobacteria but decreased (7%) the occurrence of IAA-producing rhizobacteria. Most efficient IAA-producing phosphobacteria were identified as Bacillus, Enterobacter, Pseudomonas and Serratia. At low pH (4.8), the assays showed that the IAA production by Serratia sp. N0-10LB was increased (31–74%) by organic acids. On the other hand, the IAA production by Pseudomonas sp. N1-55PA was increased two- to fivefold by metals. In all strains, the growth and IAA production were significant decreased by 500 μM of Al, except Serratia sp. N0-10LB, suggesting its potential as PGPR for Chilean Andisols. When urea was added as main N source, the bacterial growth and P utilisation significantly increased compared with ammonium sulphate. The influence of environmental factors that are typical of Chilean Andisols on rhizobacterial communities will provide better management practices to enhance their PGPR functions as well as a better selection biofertilisers to be used in Chilean Andisols.     

Organic acid and sugar contents in sugarcane stem apoplast solution and their role as carbon source for endophytic diazotrophs

We have identified aconitate, malate, and citrate organic acids as well as sucrose, glucose, and fructose sugar compounds from the apoplast solution of the stem of the sugarcane cultivars NiF-8 and RK86-129. The content of all the compounds varied with the cultivars, growth stages, and internode portions. Aconitate showed the highest content among the organic acids and sucrose was the predominant sugar compound. Moreover, Gluconoacetobacter diazotrophicus ATTCC 49037 and Herbaspirillum rubrisubalbicans PA18 differed in their ability to utilize sucrose or aconitate as sole carbon source. The G. diazotrophicus strain preferred sucrose to aconitate while H. rubrisubalbicans preferred aconitate to sucrose as carbon source.     

<Emphasis Type="Italic">Trichoderma</Emphasis> inoculation and trash management effects on soil microbial biomass,soil respiration,nutrient uptake and yield of ratoon sugarcane under subtropical conditions

A field experiment was conducted during 2003–2005 and 2004–2006 at the Indian Institute of Sugarcane Research, Lucknow, India to study the effect of Trichoderma viride inoculation in ratoon sugarcane with three trash management practices, i.e. trash mulching, trash burning and trash removal. Trichoderma inoculation with trash mulch increased soil organic carbon and phosphorus (P) content by 5.08 Mg ha⁻¹ and 11.7 kg ha⁻¹ over their initial contents of 15.75 Mg ha⁻¹ and 12.5 kg ha⁻¹, respectively. Soil compaction evaluated as bulk density in 0- to 15-cm soil layer, increased from 1.48 Mg m⁻³ at ratoon initiation (in April) to 1.53 Mg m⁻³ at harvest (in December) due to trash burning and from 1.42 Mg m⁻³ at ratoon initiation (in April) to 1.48 Mg m⁻³ at harvest (in December) due to trash mulching. The soil basal respiration was the highest during tillering phase and then decreased gradually, thereafter with the advancement of crop growth. On an average, at all the stages of crop growth, Trichoderma inoculation increased the soil basal respiration over no inoculation. Soil microbial biomass increased in all plots except in the plots of trash burning/removal without Trichoderma inoculation. The maximum increase (40 mg C kg⁻¹ soil) in soil microbial biomass C, however, was observed in the plots of trash mulch with Trichoderma inoculation treatment which also recorded the highest uptake of nutrient and cane yield. On an average, Trichoderma inoculation with trash mulch increased N, P and K uptake by 15.9, 4.68 and 23.6 kg ha⁻¹, respectively, over uninoculated condition. The cane yield was increased by 12.8 Mg ha⁻¹ with trash mulch + Trichoderma over trash removal without Trichoderma. Upon degradation, trash mulch served as a source of energy for enhanced multiplication of soil bacteria and fungi and provided suitable niche for plant–microbe interaction.     

Boundary Lines Used to Determine Sugarcane Production Limits at Leaf Nutrient Concentrations Less than Optimum

Foliar nutrient analysis is a useful diagnostic tool to complement soil testing as a best‐management practice with sugarcane (Saccharum spp.). This study was conducted to determine sugarcane production limits at leaf nutrient concentrations less than optimum. Eight Florida sugarcane growers participated in a survey of leaf nutrient values in 2004, 2005, and 2006. A total of 412 leaf samples were collected from individual commercial sugarcane fields, from which there were 389 harvest data/leaf data combinations. Fields were selected to be representative of plant cane, first ratoon, and second ratoon crops; mineral and organic soils of the area; and major commercial sugarcane cultivars. Leaf silicon (Si), magnesium (Mg), and manganese (Mn) concentrations had the strongest correlations with tons sugarcane ha^?1 on organic soils, and leaf nitrogen (N), Mg, and Si concentrations had the strongest correlations with tons sugarcane ha^?1 on mineral soils. Boundary lines were used to define practical limits of tons sugarcane ha^?1 for leaf nutrient concentrations less than optimum. A table was developed that provides approximate leaf concentrations of nine nutrients at which 10 and 25% losses in relative tons sugarcane ha^?1 were estimated. Boundary‐line analysis indicated that sugarcane production was most limited nutritionally in survey fields by insufficient Mg, iron, N, and Si on mineral soils and by insufficient Si and Mn on organic soils.