Arbuscular mycorrhizal colonization and spore density across different land‐use types in a hot and arid ecosystem,Southwest China

We investigated the arbuscular mycorrhizal (AM) colonization and spore density in cropped land, fallow land, and an undisturbed savanna ecosystem under hot and arid climatic conditions in a valley of southwest China. Plants surveyed in the three land‐use types showed heavy arbuscular mycorrhizal colonization, indicating a high mycorrhizal dependency of plants in this environment. One‐way analysis of variance (ANOVA) showed that the colonization of different AM structures and the spore density varied greatly among plant species both within and between different land‐use types. The AM colonization and spore density were higher in undisturbed than in fallow or cropped land. No significant correlation between AM colonization and spore density was observed when land‐use types were either considered separately or together. Cluster analysis based on the similarity in AM status with respect to both colonization and spore density showed similarities between fallow land and the undisturbed savanna. The results indicate that continuous cropping reduces AM colonization and spore density. These parameters appear to nearly fully recover when the land has been left to fallow for 4 years.     

Diversity of arbuscular mycorrhizal fungi in red tangerine (Citrus reticulata Blanco) rootstock rhizospheric soils from hillside citrus orchards

Root colonization, abundance of spores and hyphae, as well as species diversity of arbuscular mycorrhizal (AM) fungi were analyzed in citrus orchards along an altitudinal gradient. The citrus trees were heavily colonized (50.87–77.45%) by native AM fungi. In citrus orchards located at <600 m above sea level (asl), we recorded more extensive hyphal and arbuscular colonization, and higher spore and hyphal length density. AM fungal colonization, spore density, and hyphal length density were closely correlated with edaphic factors such as available phosphorus, pH, and organic matter. A total of 18 AM fungal species belonging to 3 different orders, Archaeosporales (1 species), Diversisporales (7 species) and Glomerales (10 species), were identified on the basis of spore morphological characteristics. In orchards located at higher altitudes (≥700 m asl), we observed a significant decrease in species richness and Shannon–Wiener index values. However, in all of the surveyed orchards, Glomus aggregatum, Funneliformis mosseae and Rhizophagus intraradices were the dominant species. Isolate frequency and relative abundance of AM fungi exhibited clearly distinct distribution patterns among taxonomic families. Canonical correspondence analysis revealed that the AM fungal community structure was significantly influenced by environmental factors, especially altitude, pH, soil moisture, and available nitrogen. Our data indicated that environmental factors are important in determining AM fungal root colonization, propagule numbers, and species diversity in citrus orchards.     

No‐tillage with half‐amount residue retention enhances microbial functional diversity,enzyme activity and glomalin‐related soil protein content within soil aggregates

A 3‐year field tillage and residue management experiment established in North China was used to analyse topsoil (0–15 cm) aggregation, and microbial functional diversity, enzyme activity and glomalin‐related soil protein (GRSP) content within aggregates. Compared with conventional tillage (CT), no‐tillage (NT) alone significantly (P < 0.05) increased organic C contents in 50–250 and <2 μm aggregates and decreased the proportion of C accumulated by 2–50 μm aggregates and microbial functional diversity indices in <2 μm aggregates. Regardless of tillage practice, both half‐amount (C50) and full (C100) residue retention tended to increase organic C and GRSP contents, or dehydrogenase and invertase activities, in certain aggregates. Under CT, a poorer performance of C50 than C100 was observed in maintaining Shannon index (H′) and Simpson index (D) in >250 and <2 μm aggregates, and also McIntosh index (U) in <2 μm aggregates, owing to insufficient residue and possible decreases in the distribution of decomposer micro‐organisms. Under NT, however, C50 was more effective than C100 in maintaining/elevating H′, D and U in all soil aggregates except for 50–250 μm, suggesting that surplus residue may induce worse soil conditions, decreasing heterotrophic microbial activities. Thus, NT with half‐amount residue retention improved soil physical–chemical–biological properties and could be a useful management practice in North China.     

Arbuscular mycorrhizal fungal diversity, external mycelium length, and glomalin-related soil protein content in response to long-term fertilizer management

Purpose

Arbuscular mycorrhizal (AM) fungi are crucial for ecosystem functioning and can contribute to the formation and maintenance of soil aggregates through the exudation of glomalin by extraradical hyphae. Monitoring fertilization effects on AM fungi may help us to develop sound management strategies. The objectives of this study were to investigate the impacts of long-term fertilization on AM fungal parameters and to find out the key factor that affects the diversity and function of AM fungi.

Materials and methods

A long-term fertilization experiment established in a sandy loam soil at northern China has received continuous fertilization treatments for 21 years, including control; mineral fertilizers of NK, PK, NP, and NPK; organic manure (OM); and half organic manure N plus half mineral fertilizer N (1/2 OMN). Top soil samples (0–15 cm) from three individual plots per treatment were collected for the analysis of chemical properties and fungal parameters. The population size of soil AM fungi was determined by real-time PCR, and the community composition was analyzed using PCR-denature gradient gel electrophoresis (DGGE), cloning, and sequencing techniques. The external mycelium of AM fungi was assessed using the grid-line intersect method, and the glomalin-related soil protein (GRSP) was extracted with citrate solution using bovine serum albumin as a standard.

Results and discussion

Long-term fertilization significantly increased (P?<?0.05) soil organic C content, AM fungal population, species richness (R), Shannon–Wiener index (H), and GRSP content, except for the P-deficiency (NK) fertilization treatment. OM had a significantly greater (P?<?0.05) impact on AM fungal population and GRSP content compared to mineral fertilizers but significantly decreased the length of external mycelium compared to the control (P?<?0.05). Fertilization also changed the community composition of AM fungi, and the P-deficiency treatment again had the slightest influence. In addition, most species recovered from the DGGE profiles belonged to three genera, Glomus, Diversispora, and Archaeospora. Redundancy analysis showed that the population size and species richness of AM fungi and the GRSP content all significantly correlated to soil organic C content (P?<?0.05).

Conclusions

Long-term P-containing fertilization, especially the application of OM, greatly increased the population size, species richness, and species diversity of AM fungi, as well as the contents of GRSP and soil organic C, but tended to decrease the length of external mycelium, while the P-deficiency fertilization had no such effect, suggesting that P was the key factor to maintain soil fertility as well as soil AM fungal diversity in this sandy loam soil.     

Role of allophanes in the accumulation of glomalin‐related soil protein in tropical soils (Martinique,French West Indies)

Thermo‐stable, operationally defined soil protein, known as glomalin, may make an important contribution to carbon storage in soils. The term glomalin is used because this putative protein, or group of proteins, was originally thought to be produced only by Glomus fungi. There is currently little information on the glomalin‐related soil protein (GRSP) content of tropical soils, particularly allophanic soils that are known to have different carbon dynamics to temperate climate soils. We have measured the Bradford‐reactive GRSP content of soils sampled from forests and grasslands on the tropical island of Martinique and compared the observations with soil composition. Two operationally defined fractions of GRSP were measured, namely easily‐extractable and total GRSP. The contents of GRSP in moist soils were in the range of 2–36 g kg^?1, accounting for about 8% of soil organic carbon, and were greater in topsoils than in corresponding subsoils. Both the GRSP contents and the fraction of soil organic carbon attributed to GRSP were greater than those reported for temperate climate soils. Both total and easily extractable GRSP contents were positively correlated to soil organic carbon content. The fraction of soil organic carbon that could be attributed to soil protein decreased with increasing allophane content for allophanic soils. No other trends of GRSP content with soil properties or land use were found. GRSP extraction was decreased about seven‐fold by air‐drying of soils, confirming the irreversible change in the soil microstructure of allophanic soils. Total and easily extractable GRSP were correlated and we conclude that both are good probes of thermo‐stable soil protein content for these soils. No attempt was made to verify the fungal origin of the protein detected.     

Glomalin-related soil protein and water relations in mycorrhizal citrus (Citrus tangerina) during soil water deficit

Glomalin-related soil protein (GRSP), a glycoprotein of arbuscular mycorrhizal fungi (AMF) secreted into soil, governs the aggregate stability, but the role of GRSP in soil and plant water is sparsely studied. The 24-week-old red tangerine (Citrus tangerina) inoculated with Glomus etunicatum and G. mosseae were subjected to a soil drying for 12 days as soil water deficit (SWD). Length of SWD significantly reduced mycorrhizal colonization, soil hyphal length, and leaf and soil water potential (Ψ), but increased total GRSP (T-GRSP), easily extractable GRSP (EE-GRSP), and proportion of water-stable aggregates (WSAs) in >0.25 mm size, irrespective of AMF source. The AMF-inoculated seedlings showed significantly higher T-GRSP, EE-GRSP, and leaf/soil Ψ than non-AMF seedlings during SWD. A significantly positive correlation was observed for mycorrhizal colonization versus leaf or soil Ψ, and hyphal length versus leaf Ψ, suggesting that root intra- and extra-radical hyphae participated in water transport. Interestingly, in GRSP fractions, only T-GRSP was significantly positively correlated with 0.25–1 and >0.25 mm WSA and negatively with leaf and soil Ψ. These results revealed a strong glue function of T-GRSP (not EE-GRSP and hyphae) to alter the proportional distribution of WSA size, thereby aiding toward prevention of soil water loss for improving soil–plant water relations.     

Depletion of non‐exchangeable NH$ _4^+ $ at the root–soil and hyphae–soil interface of VA mycorrhizal maize (Zea mays) and parsley (Petroselinum sativum)

Mobilization of non‐exchangeable ammonium (NH ) by hyphae of the vesicular‐arbuscular mycorrhizal (VAM) fungus Glumus mosseae was studied under controlled experimental conditions. Maize (Zea mays) and parsley (Petroselinum sativum) were grown either alone or in symbiosis with Glomus mosseae in containers with separated compartments for roots and hyphal growth. In one experiment, ¹⁵NH was added to the soil to differentiate between the native non‐exchangeable NH and the non‐exchangeable NH derived from N fertilization. Non‐exchangeable NH was mobilized by plant growth. Plant dry weight and N uptake, however, were not significantly influenced by mycorrhizal colonization of the roots. The influence of root infection with mycorrhizal fungus on the mobilization of non‐exchangeable NH was negligible. In the hyphal compartment, hyphal uptake of N resulted in a decrease of NH in the soil solution and of exchangeable NH . However, the NH concentration was still too high to permit the release of non‐exchangeable NH . The results demonstrate that, in contrast to roots, hyphae of VAM fungi are not able to form a non‐exchangeable‐NH depletion zone in the adjacent soil. However, under conditions of a more substantial depletion of the exchangeable NH in the mycorrhizal sphere (e.g., with longer growth), an effect of mycorrhiza on the non‐exchangeable NH might be found.     

Effects of soil conservation on soil properties of citrus orchards in the Three‐Gorges Area,China

Mountain region citrus production systems occupy large parts of the landscape prone to soil erosion in the Three‐Gorges Area of China. Several soil conservation measures, such as terracing hedgerows, are widely implemented in citrus orchards to control soil erosion. However, little is known about the effect of those measures on soil properties. The objective of this work was to analyse the changes of soil properties in citrus orchards subjected to different conservation measures. Experiments were conducted in four plots: control plot (SC, sloping orchard without conservation measures), sloping orchard with contour hedgerows (SCH), terraced orchard (TC) and terraced orchard with grass cover (TCG). Samples of soil surface from 0 to 15 cm were collected to analyse texture, bulk density (BD), aggregate stability, saturated hydraulic conductivity, soil organic matter (SOM) and soil nutrients. The spatial variability of those properties was also evaluated at slope and field scales. The results showed that the three conservation measures significantly increased saturated hydraulic conductivity, aggregate stability, SOM and available N, P, K, but decreased BD. The terracing with grass cover measures were the most effective in improving soil fertility among the three measures. The average SOM, available P, N, K in TCG were 0·27, 0·50, 1·74 and 1·72 and these rates are larger than for the in SC. Compared to SC, silt content was increased the SCH, while clay content was increased with TC and TCG. There was a downslope increase in clay content and total N but a decrease in gravels in the SC plot. However, that trend did not exist in the other plots. At the field scale, the soil aggregate stability and total nitrogen were significantly higher near the hedgerow under SCH, and the available N was significantly higher in the middle position under TC; the other soil properties had no significant trends based on the distance to hedgerow or stone bunds. Results of this study will contribute to a further understanding of the environmental influence of soil conservation measures. Copyright © 2010 John Wiley & Sons, Ltd.     

Image analysis for non‐destructive and non‐invasive quantification of root growth and soil water content in rhizotrons

Studies aiming at quantification of roots growing in soil are often constrained by the lack of suitable methods for continuous, non‐destructive measurements. A system is presented in which maize (Zea mays L.) seedlings were grown in acrylic containers — rhizotrons — in a soil layer 6‐mm thick. These thin‐layer soil rhizotrons facilitate homogeneous soil preparation and non‐destructive observation of root growth. Rhizotrons with plants were placed in a growth chamber on a rack slanted to a 45° angle to promote growth of roots along the transparent acrylic sheet. At 2‐ to 3‐day intervals, rhizotrons were placed on a flatbed scanner to collect digital images from which root length and root diameters were measured using RMS software. Images taken during the course of the experiment were also analyzed with QUACOS software that measures average pixel color values. Color readings obtained were converted to soil water content using images of reference soils of known soil water contents. To verify that roots observed at the surface of the rhizotrons were representative of the total root system in the rhizotrons, they were compared with destructive samples of roots that were carefully washed from soil and analyzed for total root length and root diameter. A significant positive relation was found between visible and washed out roots. However, the influence of soil water content and soil bulk density was reflected on seminal roots rather than first order laterals that are responsible for more than 80 % of the total root length. Changes in soil water content during plant growth could be quantitifed in the range of 0.04 to 0.26 cm³ cm^—3 if image areas of 500 x 500 pixel were analyzed and averaged. With spatial resolution of 12 x 12 pixel, however, soil water contents could only be discriminated below 0.09 cm³ cm^—3 due to the spatial variation of color readings. Results show that this thin‐layer soil rhizotron system allows researchers to observe and quantify simultaneously the time courses of seedling root development and soil water content without disturbance to the soil or roots.     

Morphological and molecular characterization of arbuscular mycorrhizal fungal communities inhabiting the roots and the soil of saffron (Crocus sativus L.) under different agricultural management practices

Crocus sativus L. cultivation is expanding to areas with low soil fertility, where mycorrhizal fungi are supposed to be essential for plants growth and ecosystems functioning. Agricultural practices applied under these conditions should lead to good saffron productivity and quality. Our objective was to study the density and diversity of mycorrhizal fungi populations associated with saffron grown in Taliouine (Morocco) under different agricultural management practices (fertilization type, age and plantation method). Morpho-anatomical studies identified rhizospheric mycorrhizal spores and assessed root colonization by arbuscular mycorrhizal fungi (AMF). Molecular identification of AMF was realized by sequencing the Large Subunit (LSU) rDNA gene region. Among the eleven species of AMF spores identified, Funneliformis and Rhizoglomus species were the most abundant (> 35%). Modern saffron plantation showed higher roots colonization rates (mycorrhization intensity (100%) and frequency (51.6%)), while in traditional plantations lower mycorrhization frequency values were found (17.4%). LSU sequencing identified five AMF genera and three unknown genomic groups, whereas Shannon diversity index indicated that AMF community composition changed significantly according to plantation age and fertilization type. Our results contribute to a better knowledge of saffron AMF communities and open new perspectives for a rational utilization of the agricultural practices for organic saffron production.     

Decomposition and the contribution of glomalin-related soil protein (GRSP) in heavy metal sequestration: Field experiment

Glomalin is a metal-sorbing glycoprotein excreted by arbuscular mycorrhizal fungi (AMF). One method of estimating glomalin in soils is as glomalin-related soil protein (GRSP). In this study the role of GRSP in sequestering Pb and Cd was investigated in an in situ field experiment. The effect of metal sequestration on the subsequent decomposition of GRSP was also investigated. GRSP was determined using the Bradford method as total glomalin-related soil protein (T-GRSP) and as easily extractable glomalin-related soil protein (EE-GRSP). After 140 days, GRSP bound Pb accounted for 0.21–1.78% of the total Pb, and GRSP bound Cd accounted for 0.38–0.98% of the total Cd content in the soil. However when compared on a soil organic matter (SOM) basis, only 4% of the Pb or Cd was bound to the GRSP fraction of the SOM compared with 40–54% of the Pb or Cd bound to the humin and fulvic acids in the SOM fraction. In soils contaminated with the highest levels of Pb and Cd, the T-GRSP (EE-GRSP) decomposition after 140 days was reduced by 8.0 (6.6)% and 7.0 (7.5)%, respectively, when compared with the controls. In the high Pb or Cd treatment groups we found that the fraction of metal bound to GRSP increased even though the total GRSP content declined over time. The mass ratio between Pb and GRSP-carbon changed from 2.3 to 271.4 mg (100 g)⁻¹ in all Pb levels soil, while with the high-Cd treatment group the mass ratio between Cd and GRSP-carbon (0.36 mg (100 g)⁻¹) was higher than the mass ratio seen with Cd-bound humic acid fractions. Our in situ field study shows that while GRSP does bind Pb and Cd, in the soils we investigated, the levels are insignificant compared to soil organic matter such as humic and fulvic acids.     

Surface runoff phosphorus (P) loss in relation to phosphatase activity and soil P fractions in Florida sandy soils under citrus production

Phosphorus losses by surface runoff from agricultural lands have been of public concern due to increasing P contamination to surface waters. Five representative commercial citrus groves (C1-C5) located in South Florida were studied to evaluate the relationships between P fractions in soils, surface runoff P, and soil phosphatase activity. A modified Hedley P sequential fractionation procedure was employed to fractionate soil P. Soil P consisted of mainly organically- and Ca/Mg-bound P fractions. The organically-bound P (biological P, sum of organic P in the water, NaHCO₃ and NaOH extracts) was dominant in the acidic sandy soils from the C2 and C3 sites (18% and 24% of total soil P), whereas the Ca/Mg-bound P (HCl-extractable P) accounted for 45-60% of soil total P in the neutral and alkaline soils (C1, C4 and C5 soils). Plant-available P (sum of water and NaHCO₃ extractable P fractions) ranged from 27 to 61 mg P kg⁻¹ and decreased in the order of C3>C4>C1>C2>C5. The mean total P concentrations (TP) in surface runoff water samples ranged from 0.51 to 2.64 mg L⁻¹. Total P, total dissolved P (TDP), and PO₄³⁻-P in surface runoff were significantly correlated with soil biological P and plant-available P forms (p<0.01), suggesting that surface runoff P was directly derived from soil available P pools, including H₂O- and NaHCO₃- extractable inorganic P, water-soluble organic P, and NaHCO₃- and NaOH-extractable organic P fractions, which are readily mineralized by soil microorganisms and/or enzyme mediated processes. Soil neutral (55-190 mg phenol kg⁻¹ 3 h⁻¹) and natural (measured at soil pH) phosphatase activities (77-295 mg phenol kg⁻¹ 3 h⁻¹) were related to TP, TDP, and PO₄³⁻-P in surface runoff, and plant-available P and biological P forms in soils. These results indicate that there is a potential relationship between soil P availability and phosphatase activities, relating to P loss by surface runoff. Therefore, the neutral and natural phosphatase activities, especially the natural phosphatase activity, may serve as an index of surface runoff P loss potential and soil P availability.     

Additive effects of copper and zinc on cadmium toxicity on phosphatase activities and ATP content of soil as estimated by the ecological dose (ED50)

The ecological dose (ED₅₀) of Cd on alkaline and acid phosphatase activity and the ATP content of three contrasting forest soils was measured with or without Cu and Zn to assess the additive toxic effects of these two metals. Soils polluted with Cu and/or Zn were treated with increasing Cd concentrations to give the following metal combinations: Cd, Cd+Cu, Cd+Zn and Cd+Cu+Zn. Alkaline and acid phosphatase activities and ATP content of the three soils were analysed 4 h, 7 and 28 days after the metal additions. The ED₅₀ values were obtained by interpolating the enzyme activities or ATP data with a kinetic model and the goodness of fit was satisfactory.Generally, the ED₅₀ values of both acid and alkaline phosphatase activities for Cd were lower (higher toxicity) with than without Cu and Zn and the effect of Cu and Zn was particularly adverse when these two metals were both added to soils. The alkaline phosphatase was more sensitive in the acid and neutral soil whereas the acid phosphatase was more sensitive in the alkaline soil. Both phosphatase activities and the ATP content were more sensitive in the sandy than in the finer textured soils. The ATP content was less sensitive to the additive effects. Increasing toxicity was observed during the incubation.Analysis of 1 M NH₄NO₃-extractable Cd, Cu and Zn revealed that Cd competed with Zn for the adsorption sites but not with Cu. However, the lower ED₅₀ values for Cd of the two phosphatase activities and of the ATP content in the presence of heavy metal combinations could be not explained by the heavy metal solubility data. It is concluded that the ED₅₀ may be a sensitive tool for assessing additve toxic effects to soil biochemical parameters.     

Nutrient concentrations and NH4+‐N mineralization under different soil types and fallow forms in southern Cameroon

To evaluate the soil‐fertility sustainability of the fallow systems, nutrient concentrations and NH₄⁺‐N mineralization were determined in different soil and fallow types in the humid forest zone of southern Cameroon. Two experiments were conducted, the first comprised planted leguminous tree Calliandra calothyrsus, planted leguminous Pueraria phaseoloides, and regrowth mainly composed of Chromolaena odorata on the Typic Kandiudult. The second experiment made up of a fallow dominated by C. odorata, a fallow with C. odorata removed, and a P. phaseoloides fallow on the Rhodic Kandiudult, Typic Kandiudult, and Typic Kandiudox. In the first experiment, available P, Ca²⁺, K⁺ concentrations and effective CEC under C. calothyrsus were, respectively, 40%, 22%, 45%, and 15% lower when compared to P. phaseoloides but no differences were found between soils under P. phaseoloides and C. odorata. Mineralization of NH₄⁺‐N was higher under C. calothyrsus than under C. odorata‐ and P. phaseoloides‐fallow types, indicating the impoverishment of organic material under the former. In the second experiment, the beneficial effect of P. phaseoloides was found in the Rhodic Kandiudult in the 0–10 cm layer throughout its low NH₄⁺ release from mineralization. In the Typic Kandiudult, no differences in NH₄⁺‐N mineralization were found between C. odorata and P. phaseoloides fallows. In the Typic Kandiudox, there was no difference in NH₄⁺ mineralization between the three fallow types. According to the nutrient concentrations and NH₄⁺ mineralization, the fertility sustainability of the different fallow types may be ranked as follow: P. phaseoloides ≥ C. odorata > C. calothyrsus > fallow without C. odorata.     

Arbuscular mycorrhizal fungi affect the growth,nutrient uptake and water status of maize (Zea mays L.) grown in two types of coal mine spoils under drought stress

Drought stress greatly affects the growth and development of plants in coal mine spoils located in the Inner Mongolia grassland ecosystem. Arbuscular mycorrhizal fungi (AMF) can increase plant tolerance to drought. However, little is known regarding the contribution of AMF to plants that are grown in different types of coal mine spoils under drought stress. To evaluate the mycorrhizal effects on the drought tolerance of maize (Zea mays L.) grown in weathered (S1) and spontaneously combusted (S2) coal mine spoils, a greenhouse pot experiment was conducted to investigate the effects of inoculation with Rhizophagus intraradices on the growth, nutrient uptake, carbon:nitrogen:phosphorus (C:N:P) stoichiometry and water status of maize under well-watered, moderate and severe drought stress conditions. The results indicated that drought stress increased mycorrhizal colonization and decreased plant dry weights, nutrient contents, leaf moisture percentage of fresh weight (LMP), water use efficiency (WUE) and rehydration rate. A high level of AMF colonization ranging from 65 to 90% was observed, and the mean root colonization rates in S1 were lower than those in S2. In both substrates, inoculation with R. intraradices significantly improved the plant growth, P contents, LMP and WUE and decreased the C:P and N:P ratios of plants under drought stress. In addition, maize grown in S1 and S2 exhibited different wilting properties in response to AMF inoculation, and plant rehydration after drought stress occurred faster in mycorrhizal plants. The results suggested that inoculation with R. intraradices played a more positive role in improving the drought stress resistance of plants grown in S2 than those grown in S1. AMF inoculation has a beneficial effect on plant tolerance to drought and effectively facilitates the development of plants in different coal mine spoils.     

Cadmium‐ and barium‐toxicity effects on growth and antioxidant capacity of soybean (Glycine max L.) plants,grown in two soil types with different physicochemical properties

The pollution of agricultural soils by metals is of growing concern worldwide, and is increasingly subject to regulatory limits. However, the effect of metal pollutants on the responses of plants can vary with soil types. In this study, we examined the growth and antioxidant responses of soybean plants exposed to contrasting soils (Oxisol and Entisol), which were artificially contaminated with cadmium (Cd) or barium (Ba). Cadmium reduced plant growth at concentrations higher than 5.2 mg (kg soil)^–1, while Ba only affected plant growth at 600 mg kg^–1. Such levels are higher than the limits imposed by the Brazilian environmental legislation. Lipid peroxidation was increased only at a Cd concentration of 10.4 mg kg^–1 in the Oxisol, after 30 d of exposure. Twelve superoxide dismutase (SOD; EC 1.15.1.1) isoenzymes were evaluated, most of which were classified as Cu/Zn forms. The SOD activity in the leaves of plants grown in the Oxisol decreased over time, whilst remaining high in the Entisol. Catalase (CAT; EC 1.11.1.6) activity in the leaves exhibited little response to Cd or Ba, but increased over time. Glutathione reductase (GR; EC 1.6.4.2) activity was reduced over time when exposed to the higher Cd concentrations, but increased following Ba exposure in the Oxisol. The enzyme‐activity changes were mainly dependent on soil type, time of exposure and, to a lesser extent, the metal concentration of the soil. Soybean plants grown in a sandy soil with a low buffering capacity, such as Entisol, suffer greater oxidative stress than those grown in a clay soil, such as Oxisol.     

<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.