Limiting factors for seedling emergence and establishment of the invasive non-native Rosa rugosa in a coastal dune system

The relative importance of seed, habitat and microhabitat limitation has rarely been investigated for invasive non-native species, although this is critical for their effective management and for predicting future range expansion. Rosa rugosa is an abundant non-native shrub in coastal habitats of NW Europe; it is common along the Baltic coast but more scarce in exposed dunes of the North Sea. To investigate whether invasion of exposed dunes is limited by seed, habitat and microhabitat limitation, seedling emergence and establishment were examined in a factorial sowing, transplant and disturbance experiment. Twenty plots were randomly placed in each of five dune habitats (white dune, Empetrum dune, grey dune, outer dune heath, inner dune heath), and studied over 2 years. Seedling emergence in control subplots was zero in all habitats, whereas 96% and 98% of the undisturbed and disturbed seeded subplots produced seedlings. Disturbance had a positive effect on emergence and subsequent survival in white dune, outer and inner dune heath. Seedling survival and growth, and growth of transplanted seedlings, were markedly lower in grey dune. These findings indicate that establishment of R. rugosa is seed-limited in coastal dune habitats, and that the species is able to establish in both active and fixed dunes once seeds have arrived. Although differential seedling emergence and growth indicate that habitats differ in their degree of invasion susceptibility by R. rugosa, the positive influence of small-scale disturbance suggests microhabitat limitation in some dune habitats as well. Dune management should aim to reduce seed production and dispersal of R. rugosa in near-natural sites, and anthropogenic changes of habitat dynamics should be prevented.     

Utilization of simple phenolics for dinitrogen fixation by soil diazotrophic bacteria

Summary A microaerobic diazotrophic bacterium tentatively identified as aPseudomonas species was isolated from a forest soil. Its nitrogenase (C₂H₂ reduction) activity in liquid medium was significantly supported by phenolic compounds when compared with glucose-, mannitol- or malate-supported activity. The utilization of phenolics was dependent on substrate induction and the appropriate oxygen concentration. At a pO₂ of 0.05 protocatechuate was a better carbon source for N₂ fixation than glucose. In the case ofLignobacter protocatechuate was a better carbon source for N₂ fixation than glucose at pO₂ 0.2 but not at pO₂ 0.05. It is suggested that certain monomeric phenols can support nitrogenase activities in many carbon-limited soil environments.Contribution No. 1484 from the Chemistry and Biology Research Institute, Agriculture Canada, Ottawa, Canada.     

Invasion of coastal dunes by the alien shrub Rosa rugosa is associated with roads,tracks and houses

Mapping the distribution of invasive alien plant species is significant for testing ecological hypotheses and for guiding effective management. Little is known about the distribution of invasive plants at landscape scale, and the factors controlling their dispersal and establishment are still poorly understood. This is the case for Rosa rugosa, an invasive shrub with negative effects on biodiversity in dune ecosystems of NW Europe. The aim of the study was to identify the factors which determine the distribution of R. rugosa in coastal dunes. In a large semi-natural dune area of NW Denmark (2364 ha) all patches of the species (1321, 1.3–59.1 m²) were GPS mapped. Patch distribution was GIS analysed, based on aerial photographs and vegetation maps. The distance of R. rugosa from the most conspicuous landscape elements was compared with randomly placed reference points. The species had invaded 0.35% of the dune landscape, and it was present in all vegetation types and all parts of the study area. Rosa rugosa patches were slightly smaller in grey dunes than in white dunes and brown dunes, and some of the largest patches were found close to villages. The occurrence of the shrub was positively correlated with distance to the coastline, to roads, tracks and houses. We conclude that the distribution of R. rugosa is determined by both natural and anthropogenic factors. It is further discussed how these factors may control dispersal and establishment of the species, and how the results can be used for improved management of coastal dunes.     

Determination of the nitrogen source for arbuscular mycorrhizal fungi by 15N application to soil and plants

The source of nitrogen in the spores of arbuscular mycorrhizal (AM) fungi was quantified by a ¹⁵N-labeling technique. N was applied as coated urea to the soil and in solution to plant shoots. Soil-applied fertilizer had a significant effect on spore % ¹⁵N (P<0.01), with a 24–75% contribution to spore N. Fertilizer applied to either alfalfa shoots or bahia grass shoots had little effect on spore % ¹⁵N, accounting for 0–14% or 1–9% of spore N, respectively. These results indicate that AM fungi obtain spore N mostly from the soil. The small amount of spore N originating from shoot-applied N may have been obtained via root exudation. Accepted: 6 November 2000     

Restoring natural seepage conditions on former agricultural grasslands does not lead to reduction of organic matter decomposition and soil nutrient dynamics

In the central part of the Netherlands, wetland restoration projects involve the rewetting of former agricultural land, where low water levels were artificially maintained (polders). Many of these projects do not result in the expected reduction of nitrogen and phosphorus availability and subsequent re-establishment of a diverse wetland vegetation. The aim of the present study was to investigate which mechanisms are responsible for this lack of success. Thereto, we studied the effect of rewetting of former agricultural grasslands on acidified peat soil (pH = 3.5) on organic matter decomposition, nitrogen cycling and phosphorus availability in the soil for three seasons. To provide an explanation for the observed effects, we simultaneously studied a set of potentially controlling abiotic soil conditions that were expected to change after rewetting. It was found that rewetting of these grasslands with natural, unpolluted seepage water did not affect nitrogen cycling, but raised decomposition rates and almost doubled phosphorus availability. The main cause of these effects is a raise of soil pH to about 7 due to the hydrochemical composition of the soil pore water after rewetting, which reflects groundwater with high amounts of buffering ions. This effect overruled any reduction in process rates by the lowered soil redox potential. The counterintuitive finding of eutrophication after rewetting with natural and unpolluted water is considered to represent a new form of internal eutrophication, triggered by the restoration of natural site conditions of former agricultural land on acid peat soil.     

Responses of Bradford-reactive soil protein to experimental nitrogen addition in three forest communities in northern lower Michigan

Increased use of anthropogenically fixed N and the release of N in combustion products have led to concerns about possible long-term impacts on terrestrial ecosystems. Previous studies demonstrating the potential of atmospheric N deposition to influence forest soil carbon have focused on decomposition processes with much less known about potential impacts on mycorrhiza-derived carbon. Glomalin is a unique glycoprotein produced by arbuscular mycorrhizal (AM) fungi that has been implicated in the formation of soil aggregates and potentially a significant store of soil carbon. To determine the possible impact of experimental N deposition of such stores we examined the operationally defined glomalin-related soil protein (GRSP) levels over two growing seasons in three forest types receiving background N deposition (control) or treated with 80 kg N ha⁻¹ year⁻¹ as NaNO₃. Three sites of each of three forest types, sugar maple-basswood (SMBW), sugar maple-red oak (SMRO), and black oak-white oak (BOWO), in northern Lower Michigan were studied during the 2001 and 2002 growing seasons. GRSP were extracted from air-dried soils with citric acid and measured by the Bradford method. Analysis of variance revealed significant differences related to forest type and sample date in easily extractable Bradford reactive (EE-BRSP) and Bradford-reactive soil protein (BRSP), but failed to detect significant effects of experimental N amendment. EE-BRSP and BRSP varied in a pattern that was consistent with an AM fungal origin; a pattern that reflected the mycorrhizal types of the dominant over and understory plants of each forest ecosystem. SMBW forests dominated by AM plants had the highest levels of protein. BOWO forests with low AM plant cover had the lowest protein levels and SMRO forests were intermediate. Both Bradford-reactive fractions and their ratio varied seasonally, generally being highest in fall samples. Significant correlations observed between BRSP fractions, phosphorous, and soil organic matter were likely related to covariation of soil properties across forest types. While not statistically significant, response patterns of BRSP to N deposition were ecosystem-specific and reflected mycorrhizal types of dominant species. Abundance of these proteins reflected previously observed changes in SOC in the two forest types examined with abundant AM hosts. Specifically, nitrate addition led to BRSP decreases in SMBW and increases in SMRO forests. Changes in BRSP accounted for a small fraction of the changes in SOC; appearing to increase as a fraction of residual SOC consistent with the idea that GRSP are recalcitrant. BRSP remained unchanged at BOWO sites despite a significant increase in SOC at these sites. Our results point to the potential of proteins as contributors to differential, mycorrhizal type-specific responses to changes in soil carbon following N amendment.     

Alleviating soil sickness caused by aerobic monocropping: responses of aerobic rice to soil oven-heating

“Aerobic rice” system is the cultivation of nutrient-responsive cultivars in nonflooded and nonsaturated soil under supplemental irrigation. It is intended for lowland areas with water shortage and for favorable upland areas with access to supplementary irrigation. Yield decline caused by soil sickness has been reported with continuous monocropping of aerobic rice grown under nonflooded conditions. The objective of this study was to determine the growth response of rice plant to oven heating of soil with a monocropping history of aerobic rice. A series of pot experiments was conducted with soils from fields where rice has been grown continuously under aerobic or anaerobic (flooded) conditions. Soil was oven heated at different temperatures and for various durations. Plants of Apo, an upland variety that does relatively well under the aerobic conditions of lowland, were grown aerobically without fertilizer inputs in all six experiments. Plants were sampled during vegetative stage to determine stem number, plant height, leaf area, and total biomass. Heating of soil increased plant growth greatly in soils with an aerobic history but a relatively small increase was observed in soils with a flooded history as these plants nearly reached optimum growth. A growth increase with continuous aerobic soil was already observed with heating at 90°C for 12 h and at 120°C for as short as 3 h. Maximum plant growth response was observed with heating at 120°C for 12 h. Leaf area was most sensitive to soil heating, followed by total biomass and stem number. We conclude that soil heating provides a simple and quick test to determine whether a soil has any sign of sickness that is caused by continuous cropping of aerobic rice.     

Soil modification by invasive plants: effects on native and invasive species of mixed-grass prairies

Invasive plants are capable of modifying attributes of soil to facilitate further invasion by conspecifics and other invasive species. We assessed this capability in three important plant invaders of grasslands in the Great Plains region of North America: leafy spurge (Euphorbia esula), smooth brome (Bromus inermis) and crested wheatgrass (Agropyron cristatum). In a glasshouse, these three invasives or a group of native species were grown separately through three cycles of growth and soil conditioning in both steam-pasteurized and non-pasteurized soils, after which we assessed seedling growth in these soils. Two of the three invasive species, Bromus and Agropyron, exhibited significant self-facilitation via soil modification. Bromus and Agropyron also had significant facilitative effects on other invasives via soil modification, while Euphorbia had significant antagonistic effects on the other invasives. Both Agropyron and Euphorbia consistently suppressed growth of two of three native forbs, while three native grasses were generally less affected. Almost all intra- and interspecific effects of invasive soil conditioning were dependent upon presence of soil biota from field sites where these species were successful invaders. Overall, these results suggest that that invasive modification of soil microbiota can facilitate plant invasion directly or via ‘cross-facilitation’ of other invasive species, and moreover has potential to impede restoration of native communities after removal of an invasive species. However, certain native species that are relatively insensitive to altered soil biota (as we observed in the case of the forb Linum lewisii and the native grasses), may be valuable as ‘nurse’species in restoration efforts.     

Effects of arbuscular mycorrhizal fungi and soil developmental stages on herbaceous plants growing in the early stage of primary succession on Mount Fuji

A pot culture experiment was conducted to examine the effects of arbuscular mycorrhizal (AM) fungi and soil developmental stages on the growth and nutrient absorption of pioneer plants growing in the early stage of primary succession on Mt. Fuji. Four herbaceous plants, Polygonum cuspidatum (Polygonaceae), Miscanthus oligostachyus (Gramineae), Aster ageratoides var. ovatus (Compositae), and Hedysarum vicioides (Leguminosae), were grown from seed in soils collected from two different successional stages, bare ground and an herbaceous plant community. Spores of indigenous AM fungi collected from the herbaceous plant community were used as inoculum. The initial colonizer P. cuspidatum showed very low levels of AM colonization (<0.4%), whereas the average AM colonization levels of M. oligostachyus, A. ageratoides var. ovatus, and H. vicioides were within the range of 13–49%. AM fungi had positive effects on the growth and N acquisition of the leguminous species (H. vicioides) irrespective of soil developmental stages. In contrast, AM colonization did not increase the plant dry weight and N content of the non-leguminous species (P. cuspidatum, M. oligostachyus, and A. ageratoides var. ovatus) in both soil developmental stages. A positive effect of AM colonization on P content was observed in M. oligostachyus, A. ageratoides var. ovatus, and H. vicioides only in soil collected from the herbaceous plant community. P. cuspidatum showed no or a negative response to AM colonization in all cases. These results suggest that the effect of AM fungi on plant growth depends more on the plant species than soil developmental stages in the early stage of primary succession in this volcanic area.     

A method for measuring the transfer of fixed nitrogen from free-living bacteria to higher plants using 15N2

An apparatus is described in which the root systems of up to ten plants can be simultaneously incubated with ¹⁵N₂ gas mixtures, while the aerial environment remains undisturbed. The size of incubation chamber and the number of chambers can be adjusted depending on the size and growth stage of the plants being tested. The apparatus has the facility for the recovery of gas mixtures, sampling and adjustment of gas concentrations, and control of soil moisture status within a sealed system. Possible experimental applications are suggested and some preliminary results presented which demonstrate bacterial nitrogen fixation and uptake into Sorghum seedlings.     

Flooding effects on rapid responses of the invasive plant Alternanthera philoxeroides to defoliation

In experiments under controlled growth conditions it was examined how flooding affected the responses of the invasive plant Alternanthera philoxeroides to defoliation. In drained and flooded conditions, plants were subjected to five defoliation levels: 0, 10, 50, 90% removal of leaf tissue and apex removal (90% leaf tissue plus apical bud removal). Plants were harvested weekly for five weeks. In drained conditions, plant biomasses including total biomass, shoot biomass and root biomass after 50% defoliation rapidly recovered to the control plant level. They were significantly lower for the 90% defoliation and apex removal treatments compared to control plants throughout the experiment. In flooded conditions, total biomass and shoot biomass after 50% defoliation, 90% defoliation, and apex removal treatments could return to control plant levels before the end of the experiment. In 90% defoliation and apex removal treatments root to shoot biomass ratios of both drained and flooded plants were initially much higher than in control plants, but the difference disappeared rapidly. The final biomasses decreased with increased defoliation intensity in drained conditions, but no significant difference was generally found in any of the defoliation treatments in flooded conditions. The rapid re-growth of A. philoxeroides plants after defoliation may partly be responsible for its invasion success. However, defoliation capable of removing 90% of the leaf tissue may be desirable in restricting the growth of this invasive species in drained conditions.     

The cost to plants of different strategies of adaptation to stress and the alleviation of stress by increasing assimilation

Summary The fitter of two species that use different strategies to overcome the same stress may be the one that expends the least resources to cope with this stress. However, this concept has proven difficult to quantify. It is proposed here that the increase in maintenance respiration in response to stress factors such as high temperature, salinity or a high-oxygen atmosphere (one indirect effect of which is nitrogen deficiency) may provide a measure of the cost of adaptation, in terms of expenditure of assimilated carbon. A corrolary to this is that, where it can be shown that an adaptive strategy results in the expenditure of assimilates, adaptation may be enhanced by increasing carbon assimilation. Results are presented supporting the hypothesis and its corrolary.     

Effects of a soil fulvic acid on the growth and nutrient content of cucumber (Cucumis sativus) plants

Summary Cucumber (Cucumis sativus) plants were grown in Hoagland solution to which 20 to 2000 ppm of a soil fulvic acid (FA) were added. The addition of 100 to 300 ppm of FA produced highly significant increases in the growth and development of above and below ground plant parts, in the uptake of nutrient elements (N, P, K, Ca, Mg, Cu, Fe and Zn), and in the formation of numbers of flowers per plant. Effects of adding 500 and more ppm of FA were less beneficial.     

Theoretical study on the mechanism of catalytic reduction of hydrazine to ammonia mediated by vanadium (III) thiolate complexes

DFT calculations on the free energy profile for the catalytic reduction of hydrazine to ammonia, the late stage of nitrogen fixation, mediated by vanadium (III) thiolate complexes VPS3 (1) and VNS3 (7) were carried out. The calculated energy profile revealed that all the reduction steps were exergonic while the protonation steps were endergonic. The generation of first equivalent of ammonia and the reduction of the cationic complex [V-NH₃]⁺ to the neutral V-NH₃ species were found to be the most exergonic of all the steps. Based on the calculated energy profile, both VPS3 and VNS3 were found to be catalytically active for the reduction of hydrazine to ammonia, although some quantitative differences in free energy profile had been observed.     

Decline of soil nitrogen mineralization and nitrification during forest conversion of evergreen broad-leaved forest to plantations in the subtropical area of Eastern China

We examined soil nitrogen (N) mineralization and nitrification rates, and soil and forest floor properties in one native forest: evergreen broad-leaved forest (EBLF), one secondary shrubs (SS), and three adjacent plantation forests: Chinese fir plantation (CFP), bamboo plantation (BP) and waxberry groves (WG) in Tiantong National Forest Park, Eastern China. All forests showed seasonal dynamics of N mineralization and nitrification rates. Soil N mineralization rate was highest in EBLF (1.6 ± 0.3 mg-N kg⁻¹ yr⁻¹) and lowest in CFP (0.4 ± 0.1 mg-N kg⁻¹ yr⁻¹). Soil nitrification rate was also highest in EBLF (0.6 ± 0.1 mg-N kg⁻¹ yr⁻¹), but lowest in SS (0.02 ± 0.01 mg-N kg⁻¹ yr⁻¹). During forest conversion of EBLF to SS, CFP, BP and WG, soil N mineralization rate (10.7%, 73%, 40.3% and 69.8%, respectively), soil nitrification rate (94.9%, 32.2%, 33.9% and 39%, respectively), and soil N concentration (50%, 65.4%, 78.9% and 51.9%, respectively) declined significantly. Annual soil N mineralization was positively correlated with total C and N concentrations of surface soil and total N concentration of forest floor, and negatively correlated with soil bulk density, soil pH and C:N ratio of forest floor across the five forests. Annual soil nitrification was positively correlated with total C concentration of surface soil and N concentration of forest floor, and negatively correlated with soil bulk density and forest floor mass. In contrast, annual soil nitrification was not correlated to pH value, total N concentration, C:N ratio of surface soil and total C concentration and C:N ratio of forest floor.     

Spatial distribution of soil nutrients and ephemeral plants underneath and outside the canopy of Porlieria chilensis shrubs (Zygophyllaceae) in arid coastal Chile

Soil nutrients and density and biomass of annual plants underneath and outside the canopy of Porlieria chilensis shrubs were measured at the end of the growing season in a protected arid coastal site in Chile. Levels of soil nitrogen, phosphorus and organic matter were significantly higher underneath than outside the canopies of shrubs. Almost 4 times as many plants occurred outside than underneath shrubs, but no significant differences in total aboveground biomass were found. Several species had higher densities and/or biomass outside rather than underneath shrubs, whereas others showed the oppsite trend. Species richness was lower underneath P. chilensis canopy. The spatial microdistribution of ephemeral species may be explained by differential water and nutrient requirements. Comparison of the patterns observed in our protected site versus surrounding unprotected areas supports the generalization that man, by removing shrubs and trees, has changed a previous heterogeneous spatial distribution of nutrients to a more homogenous one.This is a contribution of the Program of Arid Zone Studies of the Universidad de la Serena     

甘肃定西华家岭土壤纤毛虫群落对生态恢复的响应

为了解土壤纤毛虫群落对生态恢复下的土壤环境的响应,于2007年9月—2008年8月用活体观察法和"三级十倍环式稀释法"对甘肃定西华家岭5个不同恢复阶段的人工云杉林和1个荒草坡对照点的土壤纤毛虫群落以及季节动态进行了研究,探讨了土壤纤毛虫群落组成对生态恢复下土壤环境的相关性。共鉴定到127种纤毛虫,隶属于3纲9目34科50属。研究发现,各不同恢复年限土样中土壤纤毛虫的物种分布存在明显差异,并随恢复时间的延长,样点间物种相似性减小,群落组成复杂化,丰度、物种数和物种多样性指数均呈上升趋势。优势类群也发生了响应,由I号对照样点和恢复初期II号样点的肾形目变为恢复后期样点的下毛目。结果显示,土壤纤毛虫群落动态很好地响应了生态恢复过程中土壤环境条件的变化。土壤纤毛虫群落可用于陆地生态恢复评价,但要建立有效可行的、具有普遍意义的评价指标体系,尚需大量的和系统的工作。     

Sensitivity of the common bean (Phaseolus vulgaris L.) symbiosis to high soil temperature

Summary Experiments were done to test whether N fixation is more sensitive to high soil temperatures in common bean than in cowpea or soybean. Greenhouse experiments compared nodulation, nitrogenase activity, growth and nitrogen accumulation of several host/strain combinations of common bean with the other grain legumes and with N-fertilization, at various root temperatures. Field experiments compared relative N-accumulation (in symbiotic relative to N-fertilized plants) of common bean with cowpea under different soil thermal regimes. N-fertilized beans were unaffected by the higher temperatures, but nitrogen accumulation by symbiotic beans was always more sensitive to high root temperatures (33°C, 33/28°C, 34/28°C compared with 28°C) than were cowpea and soybean symbiosis. Healthy bean nodules that had developed at low temperatures functioned normally in acetylene reduction tests done at 35°C. High temperatures caused little or no suppression of nodule number. However, bean nodules produced at high temperatures were small and had low specific activity. ForP. vulgaris some tolerance to high temperature was observed among rhizobium strains (e.g., CIAT 899 was tolerant) but not among host cultivars. Heat tolerance ofP. acutifolius andP. lunatus symbioses was similar to that of cowpea and soybean. In the field, high surface soil temperatures did not reduce N accumulation in symbiotic beans more than in cowpea, probably because of compensatory nodulation in the deeper and cooler parts of the soil.     

Annual plant community responses to density of small-scale soil disturbances in the Negev Desert of Israel

We investigated whether plant diversity and productivity in small-scale soil disturbances, which is known to be higher than in undisturbed soil, decreases as the density of the disturbances increases. We studied this in an experiment with soil diggings (15 cm diameter and 15 cm depth) dug at a range of densities, on a north- and a south-facing slope of a watershed in the central Negev Desert of Israel. The diggings were similar to the commonly occurring pits made by porcupines (Hystrix indica) as they forage for below-ground plant parts. We used four levels of digging density, within the naturally occurring range in the region, represented by a rectangular plot with rows of diggings dug at four distances between diggings. The plots were laid out in a blocked design with three replications on both slopes, with each block containing all four levels of digging density. In the spring of 1992, 1994 and 1995 we measured plant density, species richness and plant productivity in the diggings, and in adjacent equal-sized undisturbed control areas (“soil matrix”) and on the mounds made by the removed excess soil. Plant density, species richness and productivity of annual plants were higher in the diggings than in the undisturbed matrix, while these responses were very low on the mounds. Plant density, species richness and productivity in the diggings, but not in the matrix or mounds, decreased as digging density increased. This effect varied slightly with location within a watershed and with annual rainfall. The density of seeds captured in the diggings from outside the digging during the 1995 dispersal season decreased with increasing digging density, but only on one of the slopes. At the highest digging density, plant density and species number in the diggings did not decrease down the slope, as expected if interference between diggings in runoff water capture were the cause of the digging density effect. There was a weak decrease in biomass production in 1994–1995 down the slope. We used a simple mathematical model to estimate whether the distribution of rainfall intensities that occurred during the winter of 1994–1995 could result in differences between digging densities in the amount of water captured by the diggings, and whether this could explain the observed effect of digging density. The model showed that there were four events during which less water was captured by the diggings at high digging densities, except in the topmost row of diggings. Soil moisture measurements, however, showed very little difference between diggings at different digging densities. We explain our findings as the result of the interaction between the properties of the disturbance patch with its surroundings, as the diggings capture resources in the form of runoff water, and seeds moved primarily by wind. The additional resources and seeds captured in diggings increase plant density, species richness and productivity relative to the undisturbed matrix. However, the contrast in plant responses between the disturbed patches and undisturbed soil diminishes at higher digging densities. We explain this as interference among diggings at close proximity. As we did not detect a decrease in plant responses down the slopes, we conclude that interference is due to interception of the wind-driven, non-directional flow of seeds. Interception of the down-slope flow of runoff water by upslope diggings is insufficient to affect plant density, determined at the beginning of the season. Later in the season, runoff interception may become important for biomass production. Received: 16 August 1996 / Accepted: 26 October 1997