Higher Soil Nitrogen Levels Research Articles

Ectomycorrhizal fungi are more sensitive to high soil nitrogen levels in forests exposed to nitrogen deposition.

Ectomycorrhizal fungi are essential for nitrogen (N) cycling in many temperate forests and responsive to anthropogenic N addition, which generally decreases host carbon (C) allocation to the fungi. In the boreal region, however, ectomycorrhizal fungal biomass has been found to correlate positively with soil N availability. Still, responses to anthropogenic N input, for instance through atmospheric deposition, are commonly negative. To elucidate whether variation in N supply affects ectomycorrhizal fungi differently depending on geographical context, we investigated ectomycorrhizal fungal communities along fertility gradients located in two nemo-boreal forest regions with similar ranges in soil N : C ratios and inorganic N availability but contrasting rates of N deposition. Ectomycorrhizal biomass and community composition remained relatively stable across the N gradient with low atmospheric N deposition, but biomass decreased and the community changed more drastically with increasing N availability in the gradient subjected to higher rates of N deposition. Moreover, potential activities of enzymes involved in ectomycorrhizal mobilisation of organic N decreased as N availability increased. In forests with low external input, we propose that stabilising feedbacks in tree-fungal interactions maintain ectomycorrhizal fungal biomass and communities even in N-richsoils. By contrast, anthropogenic N input seems to impair ectomycorrhizal functions.

Satellites Reveal Slow Shift of the Entire Boreal Biome

According to a new study, warmer temperatures and high soil nitrogen levels are causing Earth’s largest land biome to advance northward.

TIBA and nitrogen incorporation, senescense and yield in a symbiotically grown vining dry bean (Phaseolus vulgaris L.) genotype

The growth regulator 2, 3, 5 triiodobenzoic add (TIBA) was applied foliarly at the beginning of flowering (R1 stage) to a vining dry bean (Phaseolus vulgaris L.) genotype, grown symbiotically in a low nitrogen containing soil. Profiles of root nodule nitrogenase and leaf nitrate reductase activities, as well as of leaf chlorophyll content throughout the postflowering period, indicate that TIBA reduced the duration of nitrogen incorporation and caused early senescence. An increase in the number of pods per plant as well as in dry matter harvest index, supports the theory that TIBA causes bean plants to shift from vegetative to reproductive growth. No seed yield increase occurred as a result of TIBA application. Symbiotic nitrogen fixation alone may not provide sufficient nitrogen to allow optimal development of an increased number of pods. Further trials under higher soil nitrogen levels are suggested.

Effects of nitrogen on egg-laying inhibition and ovicidal response in planthopper-resistant rice varieties

A series of experiments was set up to examine the effects of nitrogen on rice (Oryza sativa L.) resistance against Nilaparvata lugens (Stål) and Sogatella furcifera (Horváth). Egg laying by N. lugens was reduced on the indica variety IR60. Nymph biomass (N. lugens and S. furcifera) was also lower on IR60: this was associated with low honeydew production and a high proportion of xylem-derived honeydew in N. lugens but not in S. furcifera. Nitrogen increased egg-laying by S. furcifera and increased N. lugens nymph biomass on all varieties tested. Oviposition and egg mortality in both planthopper species were examined on plants at 15, 30 and 45 days after sowing (DAS). Sogatella furcifera laid more eggs on plants at 15 DAS, but laid few eggs during darkness; N. lugens continued to lay eggs on older rice plants (30 DAS) and during darkness. Egg mortality was high on cv. Asiminori, highest at 45 DAS, and higher for S. furcifera than for N. lugens. Mortality of S. furcifera eggs was associated with lesions around the egg clusters. These were more common around clusters laid during the day and suggested induction by Asiminori of an ovicidal response. Egg mortality declined under higher soil nitrogen levels. Results are discussed in the light of improving rice resistance against planthoppers and reducing rates of planthopper adaptation to resistance genes.

Nitrogen‐Enriched Eutrophication Promotes the Invasion of Spartina alterniflora in Coastal China

AbstractPollution‐driven eutrophication is a significant by‐product of rapid economic growth in China. Such environmental changes have huge consequences on local biodiversity and ecosystem functions through the spread of invasive introduced species. Spartina alterniflora has been spreading quickly and extensively throughout China since the mid 1980s, soon after its intentional introduction from the eastern United States. The range expansion of S. alterniflora coincides with a significant increase in the per unit standing biomass of the species in the Chinese coastal wetlands from the 1980s to 2000s. Here we showed that the long term per m2 biomass of S. alternifolia changes are significantly correlated with human‐induced increases of the inorganic nitrogen level in the Chinese coastal water. We established this causal relationship by showing that reproduction, growth, and biomass of individual S. alterniflora, grown in experimental tanks, increased with higher levels of soil inorganic nitrogen. Competition experiments indicated that S. alterniflora out competed Phragmites australis at both low and high soil nitrogen levels. These field and experimental based results support the hypothesis that nitrogen‐enriched eutrophication contributed significantly to the successful invasion of S. alterniflora in China.

Responses of Native and Introduced Plant Species to Sucrose Addition in Puget Lowland Prairies

Nitrogen enrichment has often been demonstrated to enhance the success of introduced plant species at the expense of native species. In the south Puget lowland prairies of Washington State, invasion by Scotch broom (Cytisus scoparius), a nitrogenfixing legume, is associated with elevated soil nitrogen levels. After broom removal, the higher soil nitrogen levels may have facilitated the secondary invasion of the prairies by numerous non-native species, particularly rhizomatous pasture grasses that can interfere with native plant seedling establishment. Numerous studies have shown the potential for carbon addition to immobilize soil nitrogen and reduce the success of introduced species relative to native species. We compared the available soil nitrate, the cover of native and introduced species between sugar-addition (1000 g C m-2) and control plots on two Puget lowland prairies. Sugar treatment initially immobilized nitrate and reduced cover of introduced species compared to that on control plots, but these effects dissipated within two years. Moreover, after four years, cover of introduced species, especially that of Agrostis capillaris and Hypochaeris radicata, had rebounded to become higher in sugar-addition than in control plots. In contrast, native species showed no negative responses to sugar treatment, suggesting that where sugar or other carbon treatment is economically feasible, combining carbon with the establishment of a high density of native species might limit the potential for introduced species to rebound.

Belowground microbial symbiont enhances plant susceptibility to a spider mite through change in soybean leaf quality

AbstractTo examine how rhizobia affect the chemical and nutrient status in leaves of soybean (Glycine max L.), and how rhizobia change plant susceptibility to a generalist spider mite (Tetranycus urticae), we cultivated root‐nodulating soybeans (R+) and their non‐nodulating mutant (R−) in a common garden. We experimentally fertilized the plants with nitrogen to examine effects of rhizobia on the plant traits and plant susceptibility to spider mites at different nitrogen levels. R+ plants produced more leaves containing greater nitrogen and less total phenolics than R− plants. Spider mites fed on R+ leaves produced more eggs than those fed on R− leaves. The positive effect of rhizobia on spider mite fecundity could be due to an increase in foliar N content and/or to a decrease in concentration of phenolics. Although root nodule mass did not differ among different nitrogen levels, ureide‐N, an indicator of nitrogen provided by rhizobia, in xylem sap decreased at moderate and high soil nitrogen levels. Therefore, we expected that rhizobia effects on egg production of the spider mite would decrease in high soil nitrogen conditions. However, the effect of rhizobia was still maintained even at high soil nitrogen levels. Thus, soil nitrogen and rhizobia may independently affect the reproductive performance of the spider mite.

Importance of edaphic, spatial and management factors for plant communities of field boundaries

Plant communities of 57 field boundaries, in four regions of Finland, were sampled for this study focusing on factors affecting species diversity and community composition. All the boundaries were buffer strips established following the guidance of an Agri-Environmental Support Scheme. Data on edaphic factors, boundary management practices and spatial coordinates were used as explanatory variables in the data analyses using variation partitioning by Redundancy Analysis and univariate statistics. The variation of the plant species composition and diversity was found to be strongly spatially structured. On the variation of species composition, spatial variables alone explained 35.0% and jointly with environmental factors 32.4% of the variation in species composition. Moreover, mowing, soil pH and P content and boundary width were shown to be important factors determining species composition. The species diversity was the lowest where the land use for agricultural production was most intensive and classification of species traits indicated high soil nitrogen levels. On the basis of this study, biodiversity in boundaries could be enhanced by appropriate management techniques and greater boundary width. Regional differences in agricultural land use should also be taken into account when planning practices for improving biodiversity.

Patterns of exotic plant invasions in Pennsylvania’s Allegheny National Forest using intensive Forest Inventory and Analysis plots

Intensive Forest Inventory and Analysis (FIA) plot data collected in the Allegheny National Forest (ANF), Pennsylvania, between 1999 and 2006 were evaluated for their ability to predict ANF’s vulnerability to invasion by exotic plants. A total of 26 variables classified by biotic, abiotic, or disturbance characteristics were examined. Likelihood of colonization by invasive exotic and non-invasive exotic plants was analyzed using a logistic regression model. Approximately, 11% of the 449 species documented in these plots were exotic, which is higher than has been found in other northeastern forested plots. Only 1% of the ANF flora was invasive exotic plants and these were at low abundance, confirming that most invasions are still at an early stage of establishment. Sites richer in native or non-invasive exotic plants and with more alkaline soils were more likely to be invaded. Younger forests, forests with non-forest patches present, and forests rich in native species were more likely to be colonized by exotic (invasive or non-invasive) plants. Frangula alnus, which is starting to spread locally, differed from the other invasive exotic species in terms of its association with high sapling density to tree density ratios, high soil nitrogen levels, and the presence of fire. Variables representing mortality due to beech bark disease and distance to the nearest exotic planting manifested counterintuitive results. In both cases, the combined occurrence of mortality due to beech bark disease or a close (less than 500 m away) known propagule source and the presence of an invasive or non-invasive exotic plant was rare. We encourage increased use of intensive sampling for FIA in the U.S.A. and similar monitoring programs in other countries, but suggest adding a step to the plot selection phase that would allow forest-wide or regional stratified sampling of typically coarse-scale variables, such as historic or predicted defoliation or fire events, and forest or land type. A more accurate picture of the importance of disturbance variables in defining forest vulnerability to plant invasion may be achieved.

Fungal Endophytes: Common Host Plant Symbionts but Uncommon Mutualists

Fungal endophytes are extremely common and highly diverse microorganisms that live within plant tissues, but usually remain asymptomatic. Endophytes traditionally have been considered plant mutualists, mainly by reducing herbivory via production of mycotoxins, such as alkaloids. However, the vast majority of endophytes, especially horizontally-transmitted ones commonly found in woody plants, apparently have little or no effect on herbivores. For the systemic, vertically-transmitted endophytes of grasses, mutualistic interactions via increased resistance to herbivores and pathogens are more common, as predicted by evolutionary theory. However, even in these obligate symbioses, endophytes are often neutral or even pathogenic to the host grass, depending on endophyte and plant genotype and environmental conditions.We present a graphical model based upon variation in nitrogen flux in the host plant. Nitrogen is a common currency in endophyte/host and plant/herbivore interactions in terms of limitations to host plant growth, enhanced uptake by endophytes, demand for synthesis of nitrogen-rich alkaloids, and herbivore preference and performance. Our graphical model predicts that low alkaloid-producing endophytes should persist in populations when soil nutrients and herbivory are low. Alternatively, high alkaloid endophytes are favored under increasing herbivory and increasing soil nitrogen, at least to some point. At very high soil nitrogen levels, uninfected plants may be favored over either type of infected plants. These predictions are supported by patterns of infection and alkaloid production in nature, as well by a manipulative field experiment. However, plant genotype and other environmental factors, such as available water, interact with the presence of the endophyte to influence host plant performance.

Facilitative effects of a sand dune shrub on species growing beneath the shrub canopy.

Only recently has the importance of positive interactions among plant species in structuring natural communities been supported by experimental evidence. Most studies have focused on interactions between a pair of species at a single life-history stage. In this study positive interactions between a woody nitrogen-fixing shrub (Myrica pensylvanica) and two herbaceous sand dune species (Solidago sempervirens, Ammophila breviligulata) which frequently grow beneath shrub canopies are examined throughout the life cycles of the herbaceous species. Comparisons of S. sempervirens and A. breviligulata growing beneath and outside M. pensylvanica shrubs show that plants growing in association with shrubs are larger, are more likely to flower, produce greater numbers of flowers and seeds, have higher midday xylem water potentials, have higher tissue nitrogen concentrations, and have higher photosynthetic efficiencies. Measurements of environmental conditions show that areas beneath shrubs are more shaded, have lower soil temperatures, and have higher soil nitrogen levels. The results from experimental manipulations designed to test the effects of Myrica shrubs on understory species suggest that the observed differences in plant performance are strongly influenced by canopy shading and soil nutrient enrichment associated with the shrubs. The results demonstrate that M. pensylvanica facilitates growth, reproduction, and recruitment of S. sempervirens and A. breviligulata growing beneath it. This study, one of the few to examine positive interactions at different life-history stages, supports previous predictions that positive interactions may be particularly important in plant communities characterized by physiologically stressful conditions.

The flora of a cultural landscape: environmental determinants of change revealed using archival sources

In a novel approach to the use of archive biological information, qualitative vascular plant abundance categories from the 1930 Flora of Northamptonshire were matched with quantitative distribution data from the 1995 Flora in order to reveal the species that have changed in commonness relative to their pre-1930 populations. Factors thought to be responsible for these changes were analysed by investigating differences in habitat preference, dispersal ability and climate change indicators, using information from published sources. Changes in status were most dramatic for plant species associated with arable, wetland and woodland habitats. A highly significant factor was the trophic status of preferred habitats: burgeoning species were associated with higher soil nitrogen levels. In addition, evidence for the effects of habitat fragmentation were detected both in terms of changes in abundance being correlated with dispersal-related ecological characteristics and to the number of habitats with which particular species were associated. The discussion concerns the changing land use and farming practices in the county of Northamptonshire, which are widely held to be linked with a deterioration in plant biodiversity since 1930.

Methods of establishing tall fescue and ryegrass in a dryland environment

Optimal systems for establishing new pastures were considered at trial sites in southern Hawkes Bay and Manawatu. Systems of pasture establishment, before drilling new pasture in autumn, were: (a) spring sown barley, (b) summer fallow by cultivation, (c) summer fallow by glyphosate spraying, and (d) pasture during summer then a single glyphosate spray at drilling. Each establishment treatment was sown with white/sub clover, and either tall fescue (an example of a dryland grass species) or perennial ryegrass. Methods (a) and (b) used a roller drill to sow pastures into a cultivated seedbed, and methods (a), (c) and (d) used a direct drill. A very moist cool summer resulted in no advantage to the fallow treatments in terms of better soil moisture at sowing, but generally these treatments had the best establishment, possibly due to low weed contents and higher soil nitrogen levels. A single spray with glyphosate before direct drilling gave the poorest results for both grass species. The relative advantages and disadvantages of each treatment are discussed. Establishment and yields of tall fescue were lower than for ryegrass, although tall fescue had higher white/sub clover contents and less dead matter than ryegrass pastures. A low sowing rate and colder-than-average temperature at sowing may have depressed the establishment of tall fescue. Ryegrass quickly recovered from lower levels of establishment (within 3-6 months) but tall fescue took 9-12 months to show any improvement in contribution to pasture yields. Keywords: dryland, Festuca arundinacea, L.&urn perenne, pasture establishment

WINTER-KILLED LEGUMINOUS COVER CROPS FOR SWEET CORN

Field research was conducted in Deerfield, Mass. to study the effects of leguminous cover crops on sweet corn yield. Oat was planted alone and in combination with four leguminous cover crops August 8, 1990. Cover crop residue was disked once and sweet corn seeded April 23, 1991. Each cover crop combination had three rates of nitrogen added in two applications. Sweet corn seeded into stands of hairy vetch (Vicia villosa) yielded the highest of the cover crop combinations. All leguminous cover crop treatments yielded higher than oat alone or no cover crop when no synthetic nitrogen was added. Cover crop combinations were seeded again in the same field plots August 12, 1991. Oat biomass in November was greater where there had been leguminous cover crops or high rates of synthetic nitrogen. Legume growth was retarded in the plots that had previously received high nitrogen. It is thought that legume growth was reduced in the high nitrogen treatments due to increased oat growth and higher soil nitrogen levels which could inhibit root nodulation.

Temperature, Water Availability, and Nutrient Levels at Various Soil Depths -- Consequences for Shallow-Rooted Desert Succulents, Including Nurse Plant Effects

Soil conditions were evaluated over the rooting depths for Agave deserti and Ferocactus acanthodes from the northwestern Sonoran Desert. These succulents have mean root depths of only 10 cm when adults and even shallower distribution when seedlings, which often occur is association with the nurse plant Hilaria rigida, which also has shallow roots. Maximum soil temperatures in the 2 cm beneath bare ground were predicted to exceed 65 C, which is lethal to the roots of A. deserti and F. acanthodes, whereas H. rigida reduced the maximum surface temperatures by over 10 C, providing a microhabitat suitable for seedling establishment. Water Availability was defined as the soil-to-plant drop in water potential, for periods when the plants could take up water, integrated over time. Below 4 cm under bare ground, simulated Water Availability increased slightly with depth (to 35 cm) for a wet year, was fairly constant for an average year, and decreased for a dry year, indicating that the shallow rooting habit is more advantageous in drier years. Water uptake by H. rigida substantially reduced Water Availability for seedlings associated with this nurse plant. On the other hand, a 66-90% higher soil nitrogen level occurred under H. rigida, possibly representingmore » its harvesting of this macronutrient from a wide ground area. Phosphorus was slightly less abundant in the soil under H. rigida compared with under bare ground, the potassium level was substantially higher, and the sodium level was substantially lower. All four elements varied greatly with depth, N and K decreasing and P and Na increasing. Based on the known growth responses of A. deserti and F. acanthodes to these four elements, growth was predicted to be higher for plants in soil from the shallower layers, most of the differences being due to nitrogen.« less

Effect of Nurse Plants on the Microhabitat and Growth of Cacti

(1) Seedlings of the cactus Carnegiea gigantea at two sites in the Sonoran Desert were found only in sheltered microhabitats, 89% occurring under the canopy of Ambrosia deltoidea and Cercidium microphyllum. In contrast, 29% of the seedlings of Ferocactus acanthodes, which tolerates higher temperatures than C. gigantea, occurred in unsheltered microhabitats, where maximum soil surface temperatures reached 71?C. Most (70%) of the sheltered seedlings of F. acanthodes occurred under the canopy of the perennial bunchgrass Hilaria rigida. (2) Shading by nurse plants reduced the total daily photosynthetically active radiation (PAR) available for their associated seedlings. Near an equinox, a seedling of C. gigantea located at the centre of an A. deltoidea shrub received 77% less total daily PAR, which reduces its predicted net CO2 uptake by 90% compared with an unshaded seedling. Similarly, a seedling of F. acanthodes located at the centre of an H. rigida plant received 64% less total daily PAR and could fix 65% less CO2 than could an unshaded seedling. (3) H. rigida and F. acanthodes have overlapping shallow root systems (mean root depth of 0 08 m for H. rigida and 0 05 m for seedlings of F. acanthodes), which accentuates competition for water. A water uptake model, which closely predicted the soil water potential in the root zone of H. rigida, indicated that a seedling of F. acanthodes located at the centre of the bunchgrass took up 32% less water than did an exposed seedling. (4) Assuming that the effects of temperatures, PAR, and soil water on net CO2 uptake are multiplicative, the predicted net CO2 uptake of a seedling of F. acanthodes under the canopy of H. rigida was only 36% of that of an exposed seedling. However, areole production by seedlings of F. acanthodes located under the nurse plant was 68% of that measured in exposed areas. This discrepancy probably reflects the 60% higher soil nitrogen levels under H. rigida. (5) Therefore, nurse plants facilitate seedling establishment by reducing high temperatures near the soil surface and provide a microhabitat with a higher soil nitrogen level. However, shading and competition for water with the nurse plants markedly reduce seedling growth. The magnitude of the reduction depends on seedling size and location under the nurse plant.

Interactions Between Seedlings of Agave Deserti and the Nurse Plant Hilaria Rigida

Seedlings of the succulent crassulacean acid metabolism (CAM plant Agave deserti in the northwestern Sonoran Desert were found only in sheltered microhabitats, nearly all occurring under the canopy of a desert bunchgrass, Hilaria rigida. Apparently because soil surface temperatures can reach 71°C in exposed areas, seedlings were generally located near the center or on the northern side of this nurse plant. Both species have shallow root systems, about half of the roots of H. rigida and all those for seedlings of A. deserti occurring above soil depths of 0.08 m. To examine competition for water between the nurse plant and an associated seedling, a three—dimensional model for root water uptake was developed. The model divided the soil into 17 conentric sheaths each subdivided radially into eight wedges and vertically into nine layers. Predicted pre—dawn soil water potentials at the mean root depth and total shoot transpiration agreed well with field measurements. Simulated annual water uptake by a seedling of A. deserti was reduced °50% when the seedling was moved from an exposed location to the center of the nurse plant. Shading by the nurse plant reduced total daily photosynthetically active radiation (PAR) by up to 74% compared with an exposed seedling. On the other hand, soil nitrogen under the canopy of H. rigida was 60%higher than in exposed locations. Assuming that the effects of nitrogen, temperature, PAR, and soil water on net CO2 uptake are multiplicative, the predicted net CO2 uptake by a seedling of A. deserti under the nurse plant was only °45% of that for an exposed seedling. Thus, although the nurse plant facilitates seedling establishment by reducing maximum soil surface temperatures and provides a microhabitat with higher soil nitrogen levels, its shading and competition for water reduce seedling growth.

Chihuahuan Desert Annuals: Importance of Water and Nitrogen.

We examined the effects of water supplementation and nitrogen amendment on biomass, cover, and density of annual plants on a termite-free and a termite-present area in the Chihuahuan Desert. Soil moisture was higher in the termite than in the termite-free plots, and in the watered than in the unwatered plots during the spring and summer. There were no differences in soil moisture among plots during the winter. Soil nitrogen was higher in the termite-free than in the termite plots. There were no differences in total plant biomass produced in termite and termite-free areas. There were significant differences in relative abundances of species among treatments. natural rainfall was sufficient for maximum spring-annual biomass development on all plots except for the termite-free unfertilized, unwatered plots. These were the driest plots but had high soil nitrogen. Most of the herbaceous species responded to the water amendments by lengthening growing seasons, increasing density, or increasing biomass. When there was sufficient water for most of the spring annuals, high soil nitrogen levels favored increased densities and biomasses of Descurainia pinnata and Lepidium lasiocarpum. The absence of C4 summer annuals in the high-nitrogen plots suggests that relatively high soil nitrogen adversely affected the summer annuals. Termite-free watered plots had higher soil moisture than the termite-unwatered plots, but summer annuals were relatively abundant on the latter. Water amendments had a greater effect on the species abundances in the termite-free area than in the one with termites. In the area with termites, nitrogen amendments had a greater effect on species abundances. Species diversity and richness were affected by fertility as was species composition. This study demonstrates that we must understand patterns of soil nitrogen availability and processes affecting nitrogen availability in addition to water availability, in order to understand productivity and species composition of Chihuahuan Desert annual plants.

The accumulation of nitrate in Kikuyu grass (Pennisetum clandestinum Hochst)

A study was made of some of the factors responsible for the accumulation of excessive amounts of nitrate in certain Kikuyu pastures. Accumulation, mainly in stem tissue, is the nett result of several contributing processes. Climatic conditions causing a decline in the requirements of the plant for assimilated nitrogenous substances, stimulate accumulation. High fertilizer-N applications and the re-cycling of nitrogenous substances in the pasture seem to be of considerable significance. These high soil nitrogen levels in conjunction with high potassium levels in the soil, aiding nitrate absorption, translocation and accumulation, seem to be major factors responsible for the accumulation of nitrate, especially in old Kikuyu pastures.

Effects of density and soil nitrogen levels on growth of Galinsoga quadriradiata and G. parviflora in pure and mixed stands

The responses of two sympatric annual weeds, Galinsoga quadriradiata Ruiz & Pavon (G. ciliata (Raf.) Blake) and G. parviflora Cav., which occur abundantly in croplands and early secondary successional communities of hill regions in northeast India, were studied in relation to population density, soil nitrogen, and nature of the stand (pure versus mixed stands). Both weeds exhibited density-dependent mortality which increased at a higher nitrogen level. In the case of G. quadriradiata more individuals survived in a mixture than in monoculture, whereas G. parviflora showed the reverse trend. Growth of both weeds was better at a higher soil nitrogen level. Increased mortality and suppression of growth caused by an increase in density was also minimized in G. quadriradiata at the higher level of soil nitrogen. With an increase in population density, per-plant biomass and capitula production in both weeds showed an increase in positive skewness which, however, decreased at an increased nitrogen level. Galinsoga quadriradiata was less sensitive to density than G. parviflora regardless of soil nitrogen. At lower densities, both weeds grew better in a mixture than in pure stands, but at higher densities G. quadriradiata suppressed the growth of G. parviflora. Although the crude reproductive effort of the two weeds was independent of soil nitrogen, G. quadriradiata, in contrast to G. parviflora, exhibited a greater crude reproductive effort when grown either at higher densities or in a mixture.