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Revista de la ciencia del suelo y nutrición vegetal

versión On-line ISSN 0718-2791

R.C. Suelo Nutr. Veg. v.8 n.especial Temuco  2008

http://dx.doi.org/10.4067/S0718-27912008000400017 

 

J. Soil Sci. Plant Nutr. v8 no especial 2008 (111-120)

ORAL ABSTRACTS

 

Session 2. Soil-Root-Microbe Interaction & their Effects on the Transformation & Bioavailability of Nutrients

Changes on Soil Phosphorus Fractions by Phosphorus Solubilising Fungi after Rock Phosphate Addition

D. Pinochet* and E. Valenzuela

Facultad de Ciencias Agrarias, Facultad de Ciencias, Universidad Austral de Chile, Valdivia, Chile. *E-maü: dpinoche@uach.cl

Keywords: Soil phosphorus fractions; Aspergillus niger; rock phosphate.

Soils have significant amount of phosphorus which is not immediately available to crops and only a small fraction becomes available during crop season. When rock phosphate is applied to soils a significant P accumulation occurs in the inorganic P fractions extracted with HC1 (Figure 1). The aim of this study was to evaluate changes on soil P fractions after rock P applications due to native solubilising fungi collected previously from an Andisol (Valenzuela et al., 2002). Soil samples of an Andisol, Valdivia soil Series, 15.4% OM, pH 5.6 with 8.5 mg kg-1 P-Olsen were incubated, for 90 days to 25°C and 70% of maximum water holding capacity, with 1500 mg P kg-1 applied as North Caroline rock phosphate. The treatments were native fungi Aspergillus niger 1 (HI) and Aspergillus niger 2 (H2). A Aspergillus niger strain CBS was used as a control (HC) and a sample with no fungi addition (RP). Phosphorus fractions were measured in 1 g of soil samples using three replicates by Tiessen and Moir method (Pinochet et al., 2001). Results showed a decrease on inorganic P fractions extracted with HC1 both diluted and concentrated extractions, which represents mainly P associated with Calcium, indicating that fungi solubilised rock P in soils. Also, fungi did not decrease labile organic P fraction extracted with NaOH and increased organic P extracted with HC1 which represent non labile P associated to fulvic and humic acid. This study shows that Aspergillus niger native and CBS control strains solubilise rock P applied to soil increasing labile inorganic without decreasing labile organic P.

 

References

D. Pinochet, G. Epple and R. MacDonald,. (2001). R C. Suelo Nut. Veg. 1: 58-69.

F.E. Valenzuela D. Pinochet and P. Carias M. (2002). Mycotaxon81: 357-366.

 

 

Oxalate Crystal Formation in Fine Lateral Roots of Eucalyptus sp. Induced by Ectomycorrhizal Fungi

J. Zambrano-Gonzalez1*, M. Dutra-Costa2,1. Ribeiro da Silva3, J. Lima-Neves3, N. Félix de Barros3 and A. Chaer-Borges2

'Laboratorio de Biotecnología. CHEMTEC S.A.E. Capitán Felipe Gómez 1087, Ñemby, Paraguay. 2Departamento de Microbiología, Universidade Federal de Vicosa-UFV. CEP 36571-000 Vicosa, Minas Gerais, Brasil. 3Departamento de Solos, Universidade Federal de Vicosa-UFV, CEP 36571-000 Vicosa, Minas Gerais, Brasil. * E-mail: ja.zambrano@yahoo.com

Keywords: Oxalate; Eucaliptus sp.; ectomycorrhizal fungi.

The accumulation of calcium oxalate crystals (CaOx) was evaluated in fine lateral roots and ectomycorrhizas of Eucalyptus sp. cultivated for 2.5 years in an area with typical topography with concave-convex side in the region of Vicosa, MG. Approximately 2,100 fine lateral roots, mycorrhizal and nonmycorrhizal, randomly collected in the study area, were diaphanized and analyzed by light microscopy for CaOx visualization. Ectomycorrhizal morphotypes were analyzed by scanning electron microscopy for the presence of calcium oxalate crystals on the mantle surface. Seventy percent of the total number of lateral roots observed showed the accumulation of CaOx in the root cortex cells either in the form of druses or grains. The conspicuous presence of CaOx was observed in 56.2% of the ectomycorrhizae and in 17.5% of the nonmycorrhizal lateral fine roots, evidencing the role of the ectomycorrhizal association in the storage of calcium in the roots oí Eucalyptus sp. In the ectomycorrhizae druses were the predominant CaOx forms, while in nonmycorrhizal roots crystalline grains were most frequent. In the topographical positions studied (top, slope, lowland), ten ectomycorrhizal morphotypes were observed which varied as to the content of CaOx in the root cortex, suggesting distinct capacities of each ectomycorrhizal fungal species to supply calcium to the host plant. The analysis of the mantle surface of the different ectomycorrhizal morphotypes by scanning electron microscopy did not evidence the presence of CaOx in this structure, confirming that under the conditions evaluated, the accumulation of calcium crystals in the association is limited to the root cortex. This the first report on the occurrence of CaOx in the ectomycorrhizae of eucalypts under the prevalent conditions of Brazilian soils, suggesting a role for the association in supplying calcium to the plant in areas with low calcium availability.

 

A Single Root Model for the Impact of Root Exudates on the Fate of Phosphorus in Soils

S. Klepsch1'2*, A. Schnepf1, D. Leitner1, J. Santner1, M. Puschenreiter1 and W.W. Wenzel1

; University of Natural Resources and Applied Life Sciences, Vienna, Institute of Soil Research, Peter-Jordan Strasse 82, A-1190 Vienna, Austria.2Austrian Research Centers GmbH - ARC, A-2444 Seibersdorf Austria. *E-mail:saine.klepsch@boku.ac.at

Keywords: Mathematical model, root exudates, phosphorus.

A mechanistic single root model for rhizosphere processes is proposed, which describes the effect of root exudates on the bioavailability phosphorus (P). The model includes reaction kinetics between root exudates, different forms of P, dissolved organic carbon, metal ions, and the respective sorbed species. Equilibrium and kinetic sorption processes, besides complexation, mineralization, dissolution/precipitation, degradation and decay processes are implemented in the model. Interaction between soil microorganisms, P and exudates will additionally be accounted for. Time-dependent boundary conditions imply exudation of organic ligands, and uptake of phosphate ions at the root surface. All dissolved species are subject to diffusive/dispersive processes resulting in a system of coupled 1-dimensional partial differential equations. The model will be applied to study the P nutrition of oil seed rape as affected by root exudation. It will be parameterized with data from literature and databases, but also from experiments within the scope of a project funded by the "Vienna Science and Technology Fund". Experimental approaches include the selection of oil seed rape cultivars with differing exudation pattern, determination of phosphorus exúdate interactions and assessment of the effect of exudation and root architecture on plant phosphorus efficiency. Under certain conditions, plants can express higher amounts of root exudates, leading to an enhanced solute concentration around the root. Thus more appropriate conditions regarding nutrient supply may be created. The proposed model shall help to interpret the complex chemical, physical and biological interactions in the rhizosphere with regard to P and exúdate control mechanisms, and lead to a better understanding of the relevant processes involved.

Studies on Modeling Impact of Pesticides and Photodegradation Products on Soil Microbiota by Microbiological Test-Experiments

D. Virág, Z. Naár, A. Kiss* and Z. Murányi

Eszterházy Károly University, Regional Knowledge Center, Leányka str. 6, H-3300, Eger, Hungary. *E-mail: attkiss@ektf.hu

Keywords: Pesticides; Photodegradation; soil microbiota.

Pesticides might be regarded as one of the most relevant soil microcontaminants, since the use of agrochemicals has been recently significantly increased. The impact of the most frequently applied pesticides on soil microbes is more or less revealed, however degradation products have hot been studied for antimicrobial effect. Thus it is of crucial importance to acquire relevant information of interaction between products formed by pesticides' natural transformation and the soil microbiota. Our study aimed at acquiring information about the biological effect of pesticides and their degradates produced by UV-treatment on microbiological activity. Five photosensitive pesticides (carbendazim, acetochlor, simazine, chlorpyrifos, EPTC) and six representative soil microbes (Bacillus subtilis, Pseudomonas fluorescens, Mycobacterium phlei, Fusarium oxysporum, Penicillium expansum, Trichoderma harzianum) were applied throughout our model experiments. A special, immerseable UV-light source emitting light of 254 nm effected degradation. Subsequent to the irradiation of the sample the isolation of the degradation products was carried out by column-, and gas-chromatography. Identification of degradation species was implemented by mass spectrometry. The antimicrobial effects of the pesticides and their degradates were assessed by means of applying filter paper disk method. The antimicrobial effect of the degradation products exhibited marked differences in terms of pesticide types, irradiation time, and the test organisms. Acetochlor and its photolytic degradation products were found to be more toxic to bacteria than fungi. All the three bacteria proved to be sensitive to the basic compound and its degradation products as well. The end-product of carbendazim was weakly antibacterial against P. fluorescens and B. subtilis but strongly antifungal against T. harzianum. Chlorpyrifos and its end-product inhibited neither test organisms, but the degradates hindered the growth of four of them. The basic compound of EPTC and the degradates of simazine exhibited significant toxicity to the test bacteria. It might be claimed that the pesticide photodegradation may result in significant changes in soil microbiota, as well as formation of biologically harmful degradates.

 

Ectomycorrhizas ... Does Age Matter?

N.V. Fernández1'2*. S.B. Fontenla1, L. Gallo3 and P. Marchelli2'3

1 Laboratorio de Microbiología Aplicada y Biotecnología, CRUB, UNComahue - S.C. de Bariloche, Río Negro, Argentina.2 CONICET.3Unidad de Genética Forestal, INTA - S.C. de Bariloche, Río Negro, Argentina. *E-mail: natifern@yahoo.com.ar

Keywords: Ectomycorrhizas; Nothofagus nervosa; plant growth

Soil microorganisms play a significant role in regulating ecosystem processes, ranging from nutrient cycling to plant health. Studies on the interactions between plants and rhizospheric microbes are important for understanding ecosystem dynamics and because it would be interesting to find out how these microorganisms could be used in agriculture and forestry management. Mycorrhizas are mutualistic associations between soil fungi and plant roots, and form a key component of soil microbial populations. They influence plant growth and nutrient uptake, and provide a greater area for interactions with other soil microbes. In temperate and boreal forests, the establishment, growth and survival of different tree species are usually dependent on colonization by ectomicorrhizas (ECM). Nothofagus nervosa (Rauli) is an ecologically and economically important species of South American temperate forests. In Argentina, this species has a reduced natural distribution area, in part due to its overexplotation in the past because of its high wood quality. This critical situation led to the implementation of conservation and domestication programs. Among the different aspects that should be considered in these programs are the ECM associated with this species, which are of great importance for plant growth and for their subsequent implantation in the field. The general aim of the project is to analyze the abundance and diversity of ECM in N. nervosa, to compare them between native and nursery cultivated individuals and finally to select some ECM strains which would have significant application in domestication processes. In this context, the starting point is the quantification and characterization of ECM in N. nervosa. Hence, the first step was to analyze the percentage of root tips colonized by ECM in N. nervosa individuals and to compare it among plants of different ages belonging to four categories: seedlings, young and old native individuals and young nursery cultivated individuals. During spring 2007, 5 trees of each category were randomly selected from a native forest and from a field trial established in this forest in Patagonia, Argentina. Seedlings with complete root systems were carefully removed with a shovel and the rest of the plants were sampled by taking 3 soil cores per tree. Samples were wrapped in plastic bags and stored at 4°C. Roots were sieved from the soil cores, carefully washed and then ECM were quantified. ECM were observed in every sample. At least 95% of the root tips examined per plant had formed ECM. There were no significant differences in the percentage of root tips colonized by ECM among the four analyzed plant categories or within each of them. These results agree with some authors which have suggested that the development of ECM do not depend on the age of the tree. It might be possible to find different ECM morphotypes in different plant categories, but this fact needs to be further studied. This work constitutes the first description of the influence of tree age on ECM colonization in a forest of Patagonia. Besides, this study constitutes the initial step in the analyses of ECM in N. nervosa and provides the basis for further investigations, such as the interactions of ECM with other soil microbes, their influence in plant fitness and their possible application in conservation and domestication programs.

 

Yeasts from Soil, Rhizosphere and Ectomycorhizosphere of Nothofagus pumilio Forest in Northwestern Patagonia, Argentina

M.C. Mestre1*. D. Libkind1'2, C.A. Rosa3 and S.B. Fontenla1

'MABB, UNComahue, 2CONICET-INIBIOMA, 3UFMG, Argentina. * E-mail :mcmestre@crub. uncoma. edu. ar

Keywords: Ectomycorrhizas; Nothofagus pumilio; forest soils.

Most of the microbial activity occurs in the soil, mainly in the surface layers and in the rhizosphere. The rhizosphere fraction is mainly influenced by root exudates and symbiotic activities, among which mycorrhiza fungi are one of the predominant microorganisms. These are found inside roots, in the rhizosphere (mycorhizosphere) itself and in the bulk soil. In such soil fractions they interact with other members of the soil biota. The interactions may be of various types, both beneficial and deleterious. Some author sustain that for many ectomycorrizal trees in the world (Pinaceae, Fagaceae, Betulaceae) the survival and normal growth is dependent of ectomycorrhizal colonization. The Andean-patagonic forests are dominated by species of genera Nothofagus and conifers like Austrocedrus chilensis and Araucaria araucana. These native forests are characterized by its low antropic impact and minimal atmospheric pollution. High rates of ectomycorrhizal colonization (73-79% of infected tips) in Nothofagus species of the Northwestern region were previously observed. Yeasts are present in soil, but the knowledge of their diversity and function on such complex ecosystem is scarce. There are many unknown aspects of the interactions of yeasts with other biotic soil components, particularly in the mycorhizosphere. A few reports indicated that yeasts have a probable role as colonization helper for mycorrhizal fungi. Moreover, a broad diversity of yeasts associated to roots and spores of mycorrhiza have been reported. In an attempt to understand yeast ecology of forest soils, the diversity of yeasts inhabiting bulk soil, rhizosphere and ectomycorhizosphere of a N. pumilio forest is being investigated. The sampling site was located at the Southeast slope of Cerro Otto in S.C. Bariloche (Parque Nacional Nahuel Huapi, Argentina). Soil samples were collected from surrounding areas trees and were processed for yeast isolation and counting. Each sample was analyzed for ectomycorrhizal occurrence and specimens of the same ectomicorrhizae morphotype were pooled together for yeast isolation and counting. Pure colonies were obtained for yeast isolates and these were then grouped based on morphological and physiological characteristics. All N. pumilio trees presented ectomicorrhizal infection. A total of 134 yeast isolates were obtained: 40 from bulk soil, 49 from rhizosphere and 45 from the six more abundant ectomycorrhizal morphotypes. Yeast counts indicated that N. pumilio bulk soil contained an average of 4.35xlO3 viable yeast cells, while similar values were obtained for the rhizosphere fraction. Ectomycorhizosphere values ranged between 102 and 103, depending on the morphotype. The 134 yeast isolates were organized in 24 groups of which more than the 50% corresponded to soil fraction. Molecular studies are being performed in order to obtain species identification.

Soil Matrix and Plant Nutrition

T.A. Zubkova* and L.O. Karpachevski

Lomonosov Moscow State University, Faculty of Soil Science, Russia. *E-mail: dusy.taz@mail.ru

Keywords: Soil matrix; plant nutrition; podzolic soils.

The Maximum quantity of nutritious elements in soil is concentrated in solid particles, and accessible to plants in a soil solution. However the soil solution does not exist in itself, in dry soil it even is not present. It is result of interaction of atmospheric precipitations with a soil matrix. As morphology the soil matrix represents a superficial layer of solid soil particles. Its functioning is shown in the organization particles, molecules, ions around of itself in the certain order. The matrix is characterized by geometrical, chemical and power heterogeneity. The most active elements on it are the active centers. They take up very insignificant part of all surface (0.1-20%), and define almost all its reactionary ability and orientation of substances around of it, and also formation of an absorbed layer. The matrix is characterized by quantity and force of the active centers. Their number, as a rule, is proportional to the general surface, but the factor of proportionality can be distinguished in different soils and horizons. Chernozems have many "very weak" and "weak" acid centers on which nutritious elements are kept. Soddy-podzolic soils are characterized by rather raised maintenance of the "strong" acid centers. The matrix together with an absorptive layer keeps nutritious elements with different force. A part from them can pass in a solution and be absorbed by roots of plants. The surfaces of a root and a soil matrix have similarities and distinctions in a structure. Similarities are in available active sites. There are active centers on a soil matrix, a root hair on a root surface and compartments in a cellular membrane. All of them occupy an insignificant part of a total surface (0.1-10%). Compartments in a cellular membrane differ of the high specificity. The active centers of a soil matrix are less specific, though differ on the acid-basic and power properties. Potentially accessible volume of nutritious elements in soil is defined by volume of a soil matrix (not of all soil weight) and connected with its sizes, capacity of absorption and an exchange cation capacity. Existence of matrix systems in soil: biological (roots of plants) and abiotic (horizons E, B,C) and a bioinert matrix where microbiota plays a role of the active centers (horizon A), defines special mechanisms of interaction of matrix systems in soil with other elements by means of the active centers and carrying channels. Such mechanisms allow adjusting and organizing more effectively substances in surrounding space. Regulation is shown in buffer properties of a soil matrix and a root surface. Buffer action of a soil matrix is capable to support a solution in a condition necessary for a plant during all vegetation. Potentially accessible nutritious elements are defined by their stocks in a soil matrix. They make the maximal limit of elements accessible to plants.

Isotopic Image Analysis of Soil-Microbe-Root Interactions at the Nano-scale

D. Murphy1*. M. Kilburn2, D. Jones3, E. Stockdale4, P. Clode2, J. Cliff2 and A. Herrmann1'4

; Soil Biology Group, School of Earth and Geographical Sciences, The University of Western Australia, Crawley, WA 6009, Western Australia.2 Centre for Microscopy, Characterization and Analysis, The University of Western Australia, Crawley, WA 6009, Western Australia.3 School of the Environment and Natural Resources, University of Wales, Bangor, Gwynedd, LL57 2UW, UK.4 School of Agriculture, Food and Rural Development, Newcastle University, King George VI Building, Newcastle upon Tyne, NE1 7RU, UK. * E-mail: daniel.murphy@uwa.edu.au

Keywords: Soil-Microbe-Root Interactions; NanoSIMS; Triticum aestivum.

Many microbially-mediated processes exhibit high spatial variability across a wide range of scales (nm to cm) and very little is known about the spatial organization of soil microorganisms and its control. Understanding the link between the heterogeneity of the soil's physical/chemical environment and its impact on biological processes is arguably the next major frontier in soil science. Nano-scale secondary ion mass spectrometry (NanoSIMS) is a novel imaging approach that links isotopic analysis at high sensitivity with high resolution microscopy; levels of spatial analytical resolution are better than 50 nm. Here we have used NanoSIMS for soil-microbe-plant studies to trace and image 15N isotopes into individual bacterial cells and intracellular within root cells. Amino acids are an important source of N for plants and microorganisms and are major factors regulating ecosystem productivity. 15N- and 13C-labelled amino acids are often used to determine the relative competition between plants and microorganisms for dissolved organic N. However, it remains challenging to distinguish between direct and indirect (amino acids first mineralized to NH4+) uptake by roots. Existing methods also lack adequate sensitivity for data collection at appropriate spatial scales. In this study we combined traditional 15N isotopic tracer techniques with NanoSIMS 15N/14N imaging approaches to investigate the competition between roots and microorganisms for amino acids in the rhizosphere. Highly enriched 15N-labelled solutions containing N as either NH4 or amino acid were injected along the zone of root elongation of Wheat (Triticum aestivum L.). Rhizocosms were sampled sequentially over a 24 hour period. Subsamples allowed the traditional determination of 15N/14N isotopic ratios for roots, soluble N pools and residual soil. In addition samples were rapidly fixed and resin embedded so that 15N/14N isotopic ratio image maps (10-30 µm2) of cross-sections of the rhizocosms could be obtained by NanoSIMS. Data indicate differential enrichment of roots cell types and microbes and show clear spatial patterns between the soil physical matrix (assessed as 28Si), soil organic matter (assessed as 12C), microbial cells-15N and plant roots-15N within the rhizocosm. NanoSIMS enabled visualization and quantification of nutrient resource capture between competing plant and microbial cells. The ability to measure 15N enrichment within the rhizosphere at this previously unattainable scale provides the first opportunity to simultaneously quantify and image nutrient flow pathways in complex biological systems at a scale appropriate to the size of the competing organisms.

Comparative Studies on Revealing Relation between Pesticide Accessibility and Major Soil Parameters

Z. Murányi, A. Kiss, K. Szováti* and D. Virág

Eszterházy Károly University, EGERFOOD Regional Knowledge Centre Leányka str. 6, H-3300, Eger, Hungary. *E-mail: szovati.katalin@ektf.hu

Keywords: Pesticides; GC-MS; Triticum aestivum.

Accessibility of pesticides for plants is an issue recently coming into the forefront of interest; however most of achieved results of scientific research have not led to direct agrochemical implications and practical applications so far. Relevance of the concerned research fields is emphasized by the pronounced harmful biological and environmental effect of pesticides, as well as the fact that versatile impacts of environmental conditions on plant uptake and bioavailability have not been lately extensively studied. Comprehensive studies including investigation availability of distinctive pesticides from several soil types at different pH values and organic matter content, extending to consideration of microbiological activity of soil samples have not been performed yet. In addition to filling in this gap, extensive comparative studies of diverse extraction methods have also been carried out by us in order to be capable of precise assessment of pesticide residues in distinctive environmental and biological samples. The objective of this study was to model the way and extent of plant uptake of pesticides in - one of the most important agricultural plant - wheat (Triticum aestivum) samples, in order to acquire information regarding "biologically incorporated" amounts of examined pesticides. On the other hand impacts of environmental conditions, including major soil types and parameters were also assessed in our studies. Applying 5 diverse extraction methods provided solid basis for proper comparison and selection of the best method, as well as led to authentic estimation of pesticide residues. The bioavailability and the extent of plant uptake were investigated for 3 different types of soil (sandy, brown forest and alluvial soil) and 5 pesticides (simazine, chloropyrifos, acetochlore, diuron and pirimicarb) at different pH values, in cases of diverse organic matter contents. Pesticide residues in both soil and plant (roots and aerial parts, separately) samples were analyzed by GC-MS technique. The effect of microbiological activity has also been studied pointing out marked differences between extractable amounts of pesticides from sterilized and non sterilized soil samples. According to the statistical analysis the bioavailable amounts of pesticides were largely conditional on their chemical characteristics, but physical and chemical properties of the soil types might also play a considerably role. The available amounts of pesticides in microbiologically inactive soil samples were found to be 20-60% lower than those that could be gained from air-dry soil samples, where 5 germs (Gram+ and Gram-) and 4 different fungi genus were identified. The extracted pesticide amounts were in inverse proportion with the increase of the organic content in cases of all the three soil types. On the average the aqueous solvents proved to be nearly as effective in terms of extracting pesticides as chloroform. In general it might be stated that natural-like extraction methods provided significantly efficient and excellent models for estimation of bioavailabilty of pesticides. Pesticides were adsorbed in soils having distinctive pH values to different extent in accordance with their chemical characteristics. Pesticides were not equally accumulated in different segments of the plants exhibiting major role of pesticides' chemical feature. Differences in accumulated amounts in terms of the examined soil types were observed.

FTIR as a Tool to Study Interactions between Mineral Surfaces and Microorganisms

R. Giesler1* and P. Persson2

'Climate Impacts Research Centre, Department of Ecology and Environmental Science, llmea University, Box 62, 981 07 Abisko, Sweden. 2 Chemistry Department, llmea University, 901 87 llmea, Sweden.* E-mail: reiner.giesler@emg.umu.se

Keywords: FTIR; mineral surfaces; microorganisms.

Sorption may to affect the bioavailability and biodegradation of surface-bound substrates in the soil. The aim of this presentation is to show how attenuated total reflectance Fourier transform Infrared Spectroscopy (ATR-FTIR) can be used as a tool to follow the fate of different surface bound substrates with a molecular-level examination. We are specifically interested in P containing substrates and their bioavailability. We used the P-containing pesticide glyphosate as a model substrate in our first experimental setup since its sorption characteristics are well documented. We followed the degradation of glyphosate added to a forest soil in a bioassay where we stimulated microbial growth by additions of carbon (C), nitrogen (N) and phosphorus (P) in different combinations. Glyphosate was added in sorbed state to goethite or in a free state without goethite. We followed the effect of glyphosate additions by means of microbial C02 production and FTIR spectroscopy. Additions of glyphosate, in combination with glucose and N, did not change the respiration rate in comparison to the same treatment but without glyphosate. In contrast, glyphosate additions combined with glucose and P decreased microbial growth, whereas the combination with goethite counteracted the negative effect. The different treatments were examined using FTIR; the results suggest that glyphosate was decarboxylated in the sorbed state. Stimulating microbial growth by the addition of glucose and nitrogen resulted in further oxidation of glyphosate and only phosphate was detectable after 13 days incubation. Our results show that sorbed glyphosate is microbially degradable and that it retards microbial activity. The study emphasizes the importance of combining quantitative measurements with a molecular-level examination, to better understand biogeochemical processes. We have now refined the FTIR technique so that we can study the effect of bacterial and fungal interactions with surface bound compounds in situ using FTIR-microscope with the novel array detector technology. Goethite is added directly as a thin film on the IR-transparent crystal in combination with the P-containing substrate we want to study. The set-up allows us to follow bacterial-surface interactions temporally by additions of inoculums directly on the crystal or by in-growth of fungal hyphae from the side of the crystal. Here we will present some preliminary results from the novel experimental set-up.

Cover Crop Composition Affects on Soil Nitrogen Dynamics

B. Singh1*. U. Sainju2, and H. Singh1

'Fort Valley State University, Fort Valley, Georgia, 2USDA - ARS, Sidney, Montana. USA. *E-mail: singhb@Jvsu.edu

Keywords: Cover crops; nitrogen dynamics; C/N ratio.

Cover crops are grouped into two broad categories, viz., leguminous and non-leguminous, to differentiate between nitrogen fixers and non-fixers. How the two types of cover crop residues differ in influencing the nitrogen turnover in the soil will be described based on our own research findings as well as those reported in the literature. Factors affecting the rate of mineralization of cover crops and the role of C/N ratio in this process will be discussed. The extent of variation in the nitrogen mineralization rate that can be expected between legumes and non-legumes and among different species within each group will be given. Additionally, several studies in which we incorporated cover crops in the sustainable crop production system to meet full or partial nitrogen needs of the succeeding crops will be narrated. The need for research geared to developing reliable methods of determining the rate of availability of nitrogen from different cover crop species under varying climate, soil, and tillage practices and procedures for predicting the need, required amount, and the time of application of additional nitrogen from commercial sources to achieve optimum plant growth and yield will be expounded. The relationship of biomass and nitrogen concentration of cover crop residue with the soil organic nitrogen content will be deliberated and various reported estimates of soil organic nitrogen changes due to different cover crops will be provided. The possible advantages of using a blend of legume and nonlegume instead of mono-cropping for nitrogen supply to the succeeding crop and improvement of soil organic nitrogen content will be examined.

 

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