Natural disturbances such as lava flows, landslides and glacial moraines, and human-damaged sites such as pavement, road edges and mine wastes often leave little or no soil or biological legacy. This 2003 book provided the first comprehensive summary of how plant, animal and microbial communities develop under the harsh conditions following such dramatic disturbances. The authors examine the basic principles that determine ecosystem development and apply the general rules to the urgent practical need for promoting the reclamation of damaged lands. Written for ecologists concerned with disturbance, landscape dynamics, restoration, life histories, invasions, modeling, soil formation and community or population dynamics, this book will also serve as an authoritative text for graduate students and a valuable reference for professionals involved in land management.

/pdf/primary-succession-and-ecosystem-rehabilitation-46xfnlnrj7.pdf

Primary Succession and Ecosystem Rehabilitation

The Internal Transcribed Spacer (ITS) regions of fungal ribosomal DNA (rDNA) are highly variable sequences of great importance in distinguishing fungal species by PCR analysis. Previously published PCR primers available for amplifying these sequences from environmental samples provide varying degrees of success at discriminating against plant DNA while maintaining a broad range of compatibility. Typically, it has been necessary to use multiple primer sets to accommodate the range of fungi under study, potentially creating artificial distinctions for fungal sequences that amplify with more than one primer set. Numerous sequences for PCR primers were tested to develop PCR assays with a wide range of fungal compatibility and high discrimination from plant DNA. A nested set of 4 primers was developed that reflected these criteria and performed well amplifying ITS regions of fungal rDNA. Primers in the 5.8S sequence were also developed that would permit separate amplifications of ITS1 and ITS2. A range of basidiomycete fruiting bodies and ascomycete cultures were analyzed with the nested set of primers and Restriction Fragment Length Polymorphism (RFLP) fingerprinting to demonstrate the specificity of the assay. Single ectomycorrhizal root tips were similarly analyzed. These primers have also been successfully applied to Quantitative PCR (QPCR), Length Heterogeneity PCR (LH-PCR) and Terminal Restriction Fragment Length Polymorphism (T-RFLP) analyses of fungi. A set of wide-range plant-specific primers were developed at positions corresponding to one pair of the fungal primers. These were used to verify that the host plant DNA was not being amplified with the fungal primers. These plant primers have been successfully applied to PCR-RFLP analyses of forest plant tissues from above- and below-ground samples and work well at distinguishing a selection of plants to the species level. The complete set of primers was developed with an emphasis on discrimination between plant and fungal sequences and should be particularly useful for studies of fungi where samples also contain high levels of background plant DNA, such as verifying ectomycorrhizal morphotypes or characterizing phylosphere communities.

/pdf/fungal-specific-pcr-primers-developed-for-analysis-of-the-1hvts7gia4.pdf

Fungal-specific PCR primers developed for analysis of the ITS region of environmental DNA extracts

Root development is remarkably sensitive to variations in the supply and distribution of inorganic nutrients in the soil. Here we review examples of the ways in which nutrients such as N, P, K and Fe can affect developmental processes such as root branching, root hair production, root diameter, root growth angle, nodulation and proteoid root formation. The nutrient supply can affect root development either directly, as a result of changes in the external concentration of the nutrient, or indirectly through changes in the internal nutrient status of the plant. The direct pathway results in developmental responses that are localized to the part of the root exposed to the nutrient supply; the indirect pathway produces systemic responses and seems to depend on long-distance signals arising in the shoot. We propose the term ‘trophomorphogenesis’ to describe the changes in plant morphology that arise from variations in the availability or distribution of nutrients in the environment. We discuss what is currently known about the mechanisms of external and internal nutrient sensing, the possible nature of the long-distance signals and the role of hormones in the trophomorphogenic response.

/pdf/the-nutritional-control-of-root-development-1umt46sf92.pdf

The nutritional control of root development

Soils chronosequences are valuable tools for investigating rates and directions of soil and landscape evolution. Post-incisive chronosequences are the most common type of chronosequence. They are found in many landscapes, including sand dunes, glacial moraines, landslide scars, old pasture, burnt landscape patches, old mining areas, lava flows, alluvial fans, floodplains, river terraces, and marine terraces. They register pedogenic change over time-scales ranging from years to millions of years. Soil chronosequences help in testing rival theories of pedogenesis. Traditional soil formation theory sees a soil developing progressively under the influence of the environmental state factors until it is in equilibrium with prevailing environmental conditions. This developmental view of pedogenesis is supported by the classic soil chronosequence studies. A new evolutionary view of pedogenesis, which was prompted by the omnipresent inconstancy of environmental conditions and the notions of multidirectional changes and multiple steady states (as predicted by non-linear dynamics), proposes that environmental inconstancy and non-linear behaviour in soil-landscapes lead to soil evolution, rather than to soil development. Soils `evolve' through continual creation and destruction at all scales, and may progress, stay the same, or retrogress, depending on the environmental circumstances. Some recent soil and vegetation chronosequence investigations support an evolutionary view of pedogenesis. It is concluded that soil chronosequences are still potent instruments for pedological investigations and that they have a starring role to play in the testing of pedological theories.

Soil chronosequences, soil development, and soil evolution: a critical review

The natural abundance of the nitrogen isotope 15, δ15N, was analysed in leaves of 23 subarctic vascular plant species and two lichens from a tree-line heath at 450 m altitude and a fellfield at 1150 m altitude close to Abisko in N. Sweden, as well as in soil, rain and snow. The aim was to reveal if plant species with different types of mycorrhizal fungi also differ in their use of the various soil N sources. The dwarf shrubs and the shrubs, which in combination formed more than 65% of the total above-ground biomass at both sites, were colonized by ericoid or ectomycorrhizal fungi. Their leaf δ15N was between−8.8 and−5.5‰ at the heath and between−6.1 and −3.3‰ at the fellfield. The leaf δ15N of non- or arbuscular mycorrhizal species was markedly different, ranging from −4.1 to −0.4‰ at the heath, and from −3.4 to+2.2‰ at the fellfield. We conclude that ericoid and ectomycorrhizal dwarf shrubs and shrubs utilize a distinct N source, most likely a fraction of the organic N in fresh litter, and not complexed N in recalcitrant organic matter. The latter is the largest component of soil total N, which had a δ15N of −0.7‰ at the heath and +0.5‰ at the fellfield. Our field-based data thus support earlier controlled-environment studies and studies on the N uptake of excised roots, which have demonstrated protease activity and amino acid uptake by ericoid and ectomycorrhizal tundra species. The leaves of ectomycorrhizal plants had slightly higher δ15N (fellfield) and N concentration than leaves of the ericoids, and Betula nana, Dryas octopetala and Salix spp. also showed NO
 inf3
 sup-
 reductase activity. These species may depend more on soil inorganic N than the ericoids. The δ15N of non- or arbuscular mycorrhizal species indicates that the δ15N of inorganic N available to these plants was higher than that of average fresh litter, probably due to high microbial immobilization of inorganic N. The δ15N of NH
 inf4
 sup+
 -N was +12.3‰ in winter snow and +1.9‰ in summer rain. Precipitation N might be a major contributer in species with poorly developed root systems, e.g. Lycopodium selago. Our results show that coexisting plant species under severe nutrient limitation may tap several different N sources: NH
 inf4
 sup+
 , NO
 inf3
 sup-
 and organic N from the soil, atmospheric N2, and N in precipitation. Ericoid and ectomycorrhizal fungi are of major importance for plant N uptake in tundra ecosystems, and mycorrhizal fungi probably exert a major control on plant δ15N in organic soils.

Leaf 15N abundance of subarctic plants provides field evidence that ericoid, ectomycorrhizal and non-and arbuscular mycorrhizal species access different sources of soil nitrogen.

The response ofAlnus glutinosa, Casuarina cunninghamiana, Elaeagnus angustifolia andMyrica cerifera to a range of substrate nitrogen levels in solution, in relation to plant growth, infection, nodulation and root fine structure was studied. Nine concentrations of potassium nitrate ranging from 0.05 to 3.0 mM, were tested on each of the species. Plants were inoculated withFrankia pure cultures after a two week exposure to one of the nine levels of added nitrate. After six more weeks with constant exposure to nitrate, plants were harvested and assayed. With the exception of Myrica, regression analyses of whole plant dry weights as a function of added nitrate were highly significant. There was a tendency for nodulated plants grown at intermediate levels of added nitrate to exhibit higher relative growth rates, probably due to the additive effect of substrate nitrogen and fixation of atmospheric nitrogen. The mean numbers of nodules per plant were, with the exception of Alnus, significantly higher at intermediate levels of added nitrate, as were mean nodule dry weights. A highly significant inverse relationship between nodule weight as a percentage of whole plant weight was found in Elaeagnus and Myrica. The observed response of Elaeagnus to added nitrate compared to other actinorhizal plants appears to demonstrate that root hair infected plants are much more sensitive to the inhibitory effects of added nitrate than plants infected by intercellular penetration. A sharp reduction in the presence of root hairs at high concentrations of nitrate was observed. This indicates that the inhibition of nodulation in some actinorhizal plant species results from nitrate induced root hair suppression.

Effects of substrate nitrate concentration on symbiotic nodule formation in actinorhizal plants

The role of both actinorhizal and leguminous N 2 -flxing plants during primary succession within glacial forelands has received much attention, but there are few estimates of the contribution of fixed N to neo-glacial substrates. The main objectives of our work were (i) to assess the N 2 -fixing ability of three Dryas taxa across a chronosequence of ca 135 yr of post neo-glacial recession, and (ii) to establish whether Dryas serves as a source of N for non-N 2 -fixing species. The mineral N pool was highly limited across the sere and increased from 0.3 to 1.3 μg g −1 of soil over the chronosequence. The vascular non-N 2 -fixing species had negative mean δ 15 N values over the sere, ranging from −6.4 ± 0.4 to −3.3 ± 0.4. Dryas drummondii , present over the entire chronosequence, showed δ 15 N values ranging from −6.0 ± 0.5 to 0.32 ± 0.4, and appeared not to fix N 2 until the mid to late stages of the sere. Mean estimated values of percent N derived from the atmosphere for D. drummondii ranged from 81 to 89%. The observed δ 15 N mean values for D. octopetala and D. integrifolia were −3.5 ± 0.5 and −4.9 ± 0.3, respectively. Those values were close to the δ 15 N values of the non-N 2 -fixing species, therefore, at this site, these species did not appear to fix N 2 . Four non-N 2 -fixing woody vascular taxa that grew adjacent to N 2 -fixing plants had significantly greater foliar N contents and higher (less negative) δ 15 N values than non-N 2 -fixing woody vascular taxa grown at a distance from the N 2 -fixing Dryas . The δ 15 N values from non-N 2 -fixing plants that grew adjacent to N 2 -fixing D. drummondii appear to contradict the widely held hypothesis that pioneer N 2 -fixing species facilitate the establishment of later successional species during primary succession. The absence of N 2 -fixing activity by D. drummondii during the early stages of the sere, the higher concentration of N, and the less negative δ 15 N values for foliar N content in plants adjacent to this taxon, suggest an alternative mechanism for the spatial and temporal vegetation change during primary succession. These observations confirm and refine a hypothesis suggested earlier that N 2 -fixing plants stimulate, through the process of N transfer, adjacent non-N 2 -fixing plants that are already established.

Assessment of N2 fixation and n cycling by Dryas along a chronosequence within the forelands of the athabasca glacier, Canada

Nodulation and nitrogenase activity of inoculated Casuarina cunninghamiana Miq. were evaluated after exposing parts of root systems of intact plants to varied concentrations of potassium nitrate using split-root, solution culture systems. In a first experiment, plants were supplied with either: low concentrations of nitrate (0.05 m m ) on both sides of the root system (L: L), high concentrations of nitrate (3.00 m m ) on both sides of the root system (H:H), or low concentrations of nitrate on one side and high concentrations on the other side of the root system (L:H). In a second experiment, L:L and H:H treatments were retested using an apparatus which continuously supplied nutrient solutions to each side of the split-root system. In this experiment, the L:H treatment was substituted by a L:VH (6.00m m ) treatment to evaluate the effects of very high nitrate concentrations. In both experiments, all plants were exposed to nitrate treatments 2 weeks prior to inoculation with Frankia strain CcI3 (HFP 020203). In the first experiment, root hair development was suppressed and no nodules formed on root systems directly exposed to 3.00 m m nitrate, but root hairs and nodules were observed on all root systems exposed to 0.05 m m nitrate. The number of nodules and nodule biomass on the low side of L:H plants were not significantly different from those observed on either side of L:L plants. In the second experiment, root hair development was not suppressed by any of the nitrate treatments (6.00, 3.00 and 0.05 m m ), however, the number of nodules formed per side and their biomass were generally inversely related to the nitrate concentration on that side. Specific nitrogenase activities measured on the low side of L:VH plants were significantly lower than those measured on either side of L:L plants. The results of both experiments showed that: (1) nitrate inhibited nodule initiation strictly locally, (2) nodule development appeared to be inhibited locally, and (3) specific nitrogenase activity was inhibited systemically.

Nitrate effects on nodulation and nitrogenase activity of actinorhizal Casuarina studied in split-root systems

Root nodules of the actinorhizal shrub Ceanothus were collected from seven sites from its native range. DNA was extracted from individual nodule lobes using a cetyl trimethyl ammonium bromide (CTAB) extraction procedure. Three DNA probes were used in combination with two restriction endonucleases to evaluate the extent of restriction fragment length polymorphism (RFLP) diversity within and among populations of Ceanothus endophytes. We observed some diversity using a nif DH gene probe; however there was no correlation of RFLP pattern and geographic site. Using two random Frankia probes, we observed more diversity among the Ceanothus endophytes than with the nif probe. Differences in RFLP patterns were observed among plants at a single geographic site and between geographical sites. The results demonstrated that considerable diversity exists among Frankia strains symbiotic with Ceanothus, as has been shown for pure-cultured Frankia strains isolated from other actinorhizal genera. We have also demonstrated the usefulness of this method for the study of Frankia ecology in planta.

Diversity of frankia nodule endophytes of the actinorhizal shrub ceanothus as assessed by rflp patterns from single nodule lobes

Biomass productivity and nitrogen fixation were studied for combinations of four Casuarina cultivars and four Frankia strains. Seedlings were planted in pots containing a Frankia-free, low-nutrient soil. To evaluate nitrogen fixation over the duration of the experiment, a small amount of 15N-enriched KNO3 was supplied to the pots on a weekly basis. After 56 weeks, all of the plants were harvested and dry weights determined. Plant tissue was analyzed for N concentration and 15N (atom % excess). Significant differences were observed in biomass productivity among the various host-strain concentrations. Differences were also observed in the proportion of N derived from nitrogen fixation (pNdfa) among the various host-strain combinations, and these were correlated with biomass productivity. From the results of our experiment, it is clear that performance and nitrogen fixation are related to the interaction between the host plant and the Frankia strain. If Casuarina-Frankia strain combinations are to be used, it is appropriate to conduct trials of symbiont compatibility to improve yield.

Dwight D. Baker

Papers

Effects of substrate nitrate concentration on symbiotic nodule formation in actinorhizal plants

Assessment of N2 fixation and n cycling by Dryas along a chronosequence within the forelands of the athabasca glacier, Canada

Nitrate effects on nodulation and nitrogenase activity of actinorhizal Casuarina studied in split-root systems

Diversity of frankia nodule endophytes of the actinorhizal shrub ceanothus as assessed by rflp patterns from single nodule lobes

Selection trials for effective N2-fixing Casuarina-Frankia combinations in Egypt