Christel Baum

Innovative methods in soil phosphorus research: A review

Phosphorus (P) is an indispensable element for all life on Earth and, during the past decade, concerns about the future of its global supply have stimulated much research on soil P and method development. This review provides an overview of advanced state-of-the-art methods currently used in soil P research. These involve bulk and spatially resolved spectroscopic and spectrometric P speciation methods (1 and 2D NMR, IR, Raman, Q-TOF MS/MS, high resolution-MS, NanoSIMS, XRF, XPS, (µ)XAS) as well as methods for assessing soil P reactions (sorption isotherms, quantum-chemical modeling, microbial biomass P, enzymes activity, DGT, 33P isotopic exchange, 18O isotope ratios). Required experimental set-ups and the potentials and limitations of individual methods present a guide for the selection of most suitable methods or combinations.

Long-term effects of short rotation forestry with willows and poplar on soil properties

Abstract Short rotation forestry is a promising tool for biomass production. In order to determine long-term effects of short rotation forestry on chemical, biochemical and physical soil properties we have investigated two former arable sites with willow and poplar clones (Salix and Populus spp.) in northeast Germany. The organic carbon and the total nitrogen contents of the sandy topsoil increased up to 4.0 g kg−1 and 0.2 g kg−1, respectively after 12 years. The C/N ratio was higher under short rotation forestry caused by the stronger C than N accumulation. The contents and reserves of plant available phosphorus and potassium were decreased significantly from the 6th to the 12th year, but the plant supply was still sufficient without fertilization. The pH in the topsoil was significantly lower under willow clones than under poplar clones. Higher dehydrogenase activity of the soil was measured under poplar and willows than at tree-less control plots. The bulk density decreased and the porosity of the soil increased under short rotation forestry. In conclusion the major long-term effects of short rotation forestry on arable used Cambisols were the accumulation of organic matter followed by slightly improved soil physical conditions and increased soil biological activity.

Associated bacteria increase the phytoextraction of cadmium and zinc from a metal-contaminated soil by mycorrhizal willows.

In order to enhance phytoremediation efficiency, we investigated the effects of dual inoculation with ectomycorrhizal fungi and the ectomycorrhiza associated bacteria Micrococcus luteus and Sphingomonas sp. on the growth and metal accumulation of willows (Salix viminalis x caprea) on contaminated soil. The bacterial strains were previously collected from sporocarps of ectomycorrhizal fungi. The bacteria increased plant growth and the mycorrhizal dependency of willows colonized with the ectomycorrhizal fungus Hebeloma crustuliniforme. The total cadmium (Cd) and zinc (Zn) accumulation in the shoot biomass was increased after inoculation with the fungal strain Hebeloma crustuliniforme in combination with Micrococcus luteus up to 53% and in combination with Sphingomonas sp. up to 62%, respectively. The dual inoculation in combination with Laccaria laccata did not increase the accumulation of Cd and Zn in the willows. We conclude that associated bacteria can enhance the ectomyorrhiza formation and growth of willows and, thereby, the Cd and Zn accumulation in the plant biomass. The results suggest that bacterial support of root growth promoting ectomycorrhizal fungi may be a promising approach to improve the remediation of metal-contaminated soils by using willows.

The Potential of Rhizosphere Microorganisms to Promote the Plant Growth in Disturbed Soils

The significance of rhizosphere microorganisms, especially mycorrhizal fungi and bacteria, in polluted soils can be enormous, since they are able to increase the tolerance of plants against abiotic stress, stimulate plant growth and contribute in this way to an accelerated remediation of disturbed soils. The majority of known higher plant species is associated with mycorrhizal fungi, which can increase the tolerance of plants against abiotic stress, e.g. by an improved nutrient supply or by detoxification of pollutants. Rhizosphere bacteria can strongly promote the growth of plants solely and in interaction with mycorrhizal fungi. They can contribute to the mobilization of nutrients and degradation of organic pollutants. Co-inoculation of plants with mycorrhizal fungi and rhizosphere bacteria is a very promising biotechnological approach for the promotion of plant growth and soil remediation. The application of microbial inoculum for the remediation of disturbed soils was tested with several plant species, e.g., fast growing tree species, but mostly on a small scale. Main reasons for the lack of field applications in a larger scale are the lack of suitable time- and cost-effective strategies for a site-specific selection, preparation and application of microbial inoculum and the strong restriction of information on on-site efficiency of inoculated microbial strains.

The significance of host-fungus combinations in ectomycorrhizal symbioses for the chemical quality of willow foliage

The majority of plants interacts with mycorrhizal fungi, which predominantly provide mutual benefits, but also costs. We tested the hypothesis that specific combinations of host plants (four commercial varieties of Salix spp.) and ectomycorrhizal partners (species of Laccaria, Paxillus, Tricholoma and Hebeloma) differ in their effects on the host foliar chemistry. Twenty specific host - mycorrhiza combinations were pot-grown outdoors under low-N conditions. Foliar concentrations of total phenolics and salicylic acid were decreased by mycorrhizas in S. schwerinii x S. viminalis and S. x dasyclados genotypes, but increased in S. viminalis. Mycorrhiza effects on host biomass production were positive, indifferent or negative, depending on the specific host—mycorrhiza combination. The host plant genotype influenced mainly the direction of mycorrhizal effects on foliar chemistry, whereas the magnitude of mycorrhizal effects varied mostly among the fungal genotypes. The results are relevant with respect to the possible interactions between mycorrhizas, plants and leaf herbivores.

Interactive and Single Effects of Ectomycorrhiza Formation and Bacillus cereus on Metallothionein MT1 Expression and Phytoextraction of Cd and Zn by Willows

Single and joint ectomycorrhizal (+ Hebeloma mesophaeum) and bacterial (+ Bacillus cereus) inoculations of willows (Salix viminalis) were investigated for their potential and mode of action in the promotion of cadmium (Cd) and zinc (Zn) phytoextraction. Dual fungal and bacterial inoculations promoted the biomass production of willows in contaminated soil. Single inoculations either had no effect on the plant growth or inhibited it. All inoculated willows showed increased concentrations of nutritional elements (N, P, K and Zn) and decreased concentrations of Cd in the shoots. The lowest biomass production and concentration of Cd in the willows (+ B. cereus) were combined with the strongest expression of metallothioneins. It seems that biotic stress from bacterial invasion increased the synthesis of these stress proteins, which responded in decreased Cd concentrations. Contents of Cd and Zn in the stems of willows were combination-specific, but were always increased in dual inoculated plants. In conclusion, single inoculations with former mycorrhiza-associated B. cereus strains decreased the phytoextraction efficiency of willows by causing biotic stress. However, their joint inoculation with an ectomycorrhizal fungus is a very promising method for promoting the phytoextraction of Cd and Zn through combined physiological effects on the plant.

Spatial distribution of arsenic and heavy metals in willow roots from a contaminated floodplain soil measured by X-ray fluorescence spectroscopy

Under changing redox conditions some plants create plaques at their root surface, which may affect the mobility and uptake of As and heavy metals but it is unknown to what extent this also holds true for willows in contaminated floodplain soils. Therefore, willow roots were sampled from a phytoremediation trial in the contaminated floodplain of the river Elbe (Germany), cryofixed, freeze-dried, and cross sections were mapped for the distribution of As, Ca, Cu, Fe, K, Mn, Ni, S and Zn by synchrotron based X-ray fluorescence spectroscopy. The elements Ca, Cu, Ni, S and Zn were concentrated in the aerenchymatic tissue, and not associated with Fe and Mn. Mixed Fe-Mn plaques covered the surface of the willow roots and As was accumulated in these plaques. The observed association pattern between As and Fe was explained by the different sorption/desorption properties of As(III) and As(V). The Cu and Zn intensities were not associated with the intensity of Fe in the plaque, which seems to be a willow-specific difference compared to other wetland plants. These results suggested that willows are especially suited to stabilize low-phytoextractable elements like Cu and As in their roots and rhizosphere. Thus, short rotation coppicing of willows may be a practical approach to mitigate the adverse effects of floodplain soil contamination.

Growth response of Populus trichocarpa to inoculation by the ectomycorrhizal fungus Laccaria laccata in a pot and a field experiment

The growth response of Populus trichocarpa cv. Muhle Larsen to inoculation by the ectomycorrhizal (ECM) fungus Laccaria laccata was investigated in a pot and a field experiment on arable soils. Non-sterilized soil from a Cambisol in Wildeshausen (Lower Saxonia, Germany) was taken for the pot experiment. The field experiment was established at a reforestation plantation on Stagnic Gleysols in Leubnitz (Saxonia, Germany). Both arable soils were assumed to be low in indigenous ECM inoculum caused by the lack of host plants. The study was designed to determine (1) the ECM colonization on the poplar clone with or without inoculation in the pot and in the field experiment and (2) the effects of inoculation on plant growth parameters. In the pot experiment, 7% of fine roots of non-inoculated control plants and 18% of inoculated test plants were colonized with ECM fungi after one growing season. In the field experiment, 6% of fine roots were colonized with ECM fungi at the control plot and 20% at the inoculated plot after the first growing season. After the second growing season 50 or 53% were colonized, respectively. The inoculation by L. laccata caused significantly increased shoot lengths and leaf potassium concentrations of the poplar clone in the pot and field experiment after the first growing season. In the second growing season, only the leaf potassium concentrations were further increased compared to the non-inoculated control plants. The density of VAM spores in the soil and the leaf nitrogen, magnesium and calcium concentrations were significantly reduced after inoculation. However, after the second growing season there were no longer were significant differences in the ECM colonization and shoot lengths of inoculated or non-inoculated poplar cuttings. The results indicated that inoculation can be successfully used to increase ECM colonization and growth rates of P. trichocarpa on former arable soils in the first growth period. This could increase the resistance of the cuttings to soil-borne pathogens and their competitiveness for nutrients and space against weeds.

Advances in Understanding Organic Nitrogen Chemistry in Soils Using State-of-the-art Analytical Techniques

Abstract During the past decade, soil and geochemists have adopted a variety of novel chemical–analytical methods to explore the chemistry of soil organic N (N org ). This chapter summarizes some of the more recent developments in the use of wet-chemical and instrumental methods to determine total N org concentrations as well as to speciate the N org in soils. A critical evaluation of 15 N nuclear magnetic resonance (NMR) spectroscopy found the technique to be wanting, in terms of its sensitivity and ability to identify classes of N org compounds in soils. Complementary mass spectrometric techniques are described briefly, and improved data evaluations based on broad applications of high-resolution pyrolysis-field ionization mass spectrometry are presented and discussed. A reassessment of older data sets using the new spectral evaluation algorithms provides strong evidence of fire- and management-induced changes in N org speciation. Isotope-ratio mass spectrometry, Fourier transform ion cyclotron resonance mass spectrometry, and nanoscale secondary ion mass spectrometry (Nano-SIMS) also are discussed, with the latter two techniques having potential to (1) identify N org compounds and (2) provide spatially resolved information on the molecular, elemental and isotopic composition of soil N org . The use of 15 N labeling techniques is discussed both from a methodological standpoint and in terms of tracking the fate of plant-derived (residue or rhizodeposit) N in the soil. Indeed, coupling 15 N labeling with analytical techniques such as 15 N NMR, Nano-SIMS and high- or ultrahigh-resolution mass spectrometry can provide information on how N is incorporated into soil organic matter. Analytical and instrumental innovations have resulted in new insights into the chemistry of N org —together with a revised summary of the relative amounts of the different N org compound classes present in soils (e.g. aliphatic amine and amide N, aromatic heterocyclic N), as well as their ecophysiological functions. Particular emphasis is given to the use of multitechnique analyses and the outstanding molecular–chemical diversity of biogenic heterocyclic N org compounds. Examples are given of the new insights obtained using multi-analytical research approaches to explore microbial utilization of heterocyclic N and organic–mineral interactions, as well as the ability of human and environmental intervention to alter the composition of soil N org . Finally, we examine future challenges and propose analytical approaches to tackle open questions regarding the basic chemistry and cycling of N org in soils, as well as the agronomic and environmental consequences associated with N transformations in agro-ecosystems.

Correspondence of ectomycorrhizal diversity and colonisation of willows (Salix spp.) grown in short rotation coppice on arable sites and adjacent natural stands.

Willows (Salix spp.) are mycorrhizal tree species sometimes cultivated as short rotation coppice (SRC) on arable sites for energy purposes; they are also among the earliest plants colonising primary successional sites in natural stands. The objective of this study was to analyse the degree of colonisation and diversity of ectomycorrhizal (EM) communities on willows grown as SRC in arable soils and their adjacent natural or naturalized stands. Arable sites usually lack ectomycorrhizal host plants before the establishment of SRC, and adjacent natural or naturalized willow stands were hypothesized to be a leading source of ectomycorrhizal inoculum for the SRC. Three test sites including SRC stands (Salix viminalis, Salix dasyclados, and Salix schwerinii) and adjacent natural or naturalized (Salix caprea, Salix fragilis, and Salix × mollissima) stands in central Sweden were investigated on EM colonisation and morphotypes, and the fungal partners of 36 of the total 49 EM fungi morphotypes were identified using molecular tools. The frequency of mycorrhizas in the natural/naturalized stands was higher (two sites) or lower (one site) than in the corresponding cultivated stands. Correspondence analysis revealed that some EM taxa (e.g. Agaricales) were mostly associated with cultivated willows, while others (e.g. Thelephorales) were mostly found in natural/naturalized stands. In conclusion, we found strong effects of sites and willow genotype on EM fungi formation, but poor correspondence between the EM fungi abundance and diversity in SRC and their adjacent natural/naturalized stands. The underlying mechanism might be selective promotion of some EM fungi species by more effective spore dispersal.