A dominance shift in arid savanna: An herbaceous legume outcompetes local C4 grasses

The characteristic vegetation structure of arid savannas with a dominant layer of perennial grass is maintained by the putative competitive superiority of the C₄ grasses. When this competitive balance is disturbed by weakening the grasses or favoring the recruitment of other species, trees, shrubs, single grass, or forb species can increase and initiate sudden dominance shifts. Such shifts involving woody species, often termed “shrub encroachment”, or the mass spreading of so‐called increaser species have been extensively researched, but studies on similar processes without obvious preceding disturbance are rare. In Namibia, the native herbaceous legume Crotalaria podocarpa has recently encroached parts of the escarpment region, seriously affecting the productivity of local fodder grasses. Here, we studied the interaction between seedlings of the legume and the dominant local fodder grass (Stipagrostis ciliata). We used a pot experiment to test seedling survival and to investigate the growth of Crotalaria in competition with Stipagrostis. Additional field observations were conducted to quantify the interactive effect. We found germination and growth of the legume seedlings to be facilitated by inactive (dead or dormant) grass tussocks and unhindered by active ones. Seedling survival was three times higher in inactive tussocks and Crotalaria grew taller. In the field, high densities of the legume had a clear negative effect on productivity of the grass. The C₄ grass was unable to limit the recruitment and spread of the legume, and Crotalaria did outcompete the putative more competitive grass. Hence, the legume is able to spread and establish itself in large numbers and initiate a dominance shift in savannas, similar to shrub encroachment.     

Reducing Competitive Suppression of a Rare Annual Forb by Restoring Native California Perennial Grasslands

Populations of the rare annual forb Amsinckia grandiflora may be declining because of competitive suppression by exotic annual grasses, and may perform better in a matrix of native perennial bunchgrasses. We conducted a field competition experiment in which Amsinckia seedlings were transplanted into forty 0.64‐m² experimental plots of exotic annual grassland or restored perennial grassland. The perennial grassland plots were restored using mature 3 cm‐diameter plants of the native perennial bunchgrass Poa secunda planted in three densities. The exotic annual grassland plots were established in four densities through manual removal of existing plants. Both grass types reduced soil water potential with increasing biomass, but this reduction was not significantly different between grass types. Both grass types significantly reduced the production of Amsinckia inflorescences. At low and intermediate densities (dry biomass per unit area of 20–80 g/m²), the exotic annual grasses reduced Amsinckia inflorescence number to a greater extent than did Poa, although at high densities (>90 g/m²) both grass types reduced the number of Amsinckia inflorescences to the same extent. The response of Amsinckia inflorescence number to Poa biomass was linear, whereas the same response to the annual grass biomass is logarithmic, and appeared to be related to graminoid cover. This may be because of the different growth forms exhibited by the two grass types. Results of this research suggest that restored native perennial grasslands at intermediate densities have a high habitat value for the potential establishment of the native annual A. grandiflora.     

Effects of long-term grazing on the morphological and functional traits of Leymus chinensis in the semiarid grassland of Inner Mongolia, China

Livestock grazing represents a major human disturbance to grasslands throughout the world. We evaluated the effects of long-term grazing (>20 years) on a dominant perennial grass species, Leymus chinensis (Trin.) Tzvel., in the semiarid grassland of Inner Mongolia, China, in terms of its morphological and functional responses. L. chinensis, one of the most abundant species in semiarid grassland, had significantly smaller leaf area, fewer vegetative tillers and shorter shoot internodes length, but significantly greater specific leaf area for the individuals in the grazing plot than those in the exclosure (grazing-free) plot. Long-term grazing also altered the relative distribution of biomass to leaves, roots and rhizomes. The biomass, coverage and the number of species were lower in the grazing plot by 50–70% in comparison with those in the grazing-free plot. In addition, the long-term grazing substantively decreased the physiological capacities of this grass species, including significantly lower net photosynthetic rate, apparent quantum efficiency, photochemical efficiency of PSII and water use efficiency. Significantly higher stable oxygen isotope ratios (δ ¹⁸O) of the soil water in the grazing plot than those in the grazing-free plot indicated a much higher soil evaporation in the grazing plot because of less litter coverage. Seasonal patterns in the δ ¹⁸O values of the stem water of L. chinensis and three other common species in the grazing and grazing-free plots indicated that plants in the grazing and grazing-free plots might shift their water sources differently from a dry month (June) to a wet month (August). This study illustrated the importance of using different approaches to study the possible responses of grass species in arid regions to human disturbances, such as long-term grazing.     

Effects of kangaroo rat exclusion on vegetation structure and plant species diversity in the Chihuahuan Desert

Long-term (1977–90) experimental exclusion of three species of kangaroo rats from study plots in the Chihuahuan Desert resulted in significant increases in abundance of a tall annual grass (Aristida adscensionis) and a perennial bunch grass (Eragrostis lehmanniana). This change in the vegetative cover affected use of these plots by several other rodent species and by foraging birds. The mechanism producing this change probably involves a combination of decreased soil disturbance and reduced predation on large-sized seeds when kangaroo rats are absent. Species diversity of summer annual dicots was greater on plots where kangaroo rats were present, as predicted by keystone predator models. However, it is not clear whether this was caused directly by activities of the kangaroo rats or indirectly as a consequence of the increase in grass cover. No experimental effect on species diversity of winter annual dicots was detected. Our study site was located in a natural transition between desert scrub and grassland, where abiotic conditions and the effects of organisms may be particularly influential in determining the structure and composition of vegetation. Under these conditions kangaroo rats have a dramatic effect on plant cover and species composition.     

Plant traits and phenotypic variability effect on the phytomass production of Stipagrostis ciliata (Desf.) De Winter

A process of continuous degradation of plant communities, due mainly to long-term overgrazing has been revealed by most ecological studies in North African arid climate. Notably, this degradation appeared across the depletion of perennial grass species exhibiting low density in the majority of range ecosystems. This study aimed to examine the phenology and the aboveground phytomass production of Stipagrostis ciliata (Desf.) De Winter accessions, a perennial grass, growing under the same environment but coming from different climates of Tunisia. Additionally, the extent of genetic variation in phenological parameters, root and shoot phytomass productivity and the correlations among these parameters were also analyzed. Significant differences in all morphological parameters of S. ciliata accessions were revealed by ANOVA test and were corroborated with significant and positive correlation indicated by Pearson’s correlation analysis. Plant diameter, biovolume, root biomass with protective sleeve and spike number exhibited significant differences and high distinctiveness between S. ciliata accessions. Tukey’s HDS tests indicated the presence of three groups of accessions. Principal component analysis (PCA) applied on a table with eight observations and 13 variables, and dispersion of S. ciliata accessions on the first two axes of PCA confirmed the presence of three groups of accessions. Trait variability in the field for the five accessions is more likely to be the result of phenotypic plasticity rather than of genetic differentiation between accessions. Overall, the characterization of S. ciliata accessions exhibited significant differences in terms of morphological and biomass productivity.     

Long- and short-term effects of crop residues on aluminum toxicity,phosphorus availability and growth of pearl millet in an acid sandy soil

Pasture swards containing perennial ryegrass (Lolium perenne L.) alone or with one of five different white clover (Trifolium repens L.) cultivars were examined for production and transfer of fixed nitrogen (N) to grass under dairy cow grazing. Grass-only swards produced 21% less than mixed clover-grass swards during the second year after sowing. Production from grass-only plots under a mowing and clipping removal regime was 44% less than from grass-only plots under grazing. Much of this difference could be attributed to N transfer. In swards without clover, the ryegrass component also decreased in favour of other grasses.The average amount of fixed N in herbage from all clover cultivars was 269 kg N ha^–1 yr^–1. Above-ground transfer of fixed N to grasses (via cow excreta) was estimated at 60 kg N ha^–1 yr^–1. Below-ground transfer of fixed N to grasses was estimated at 70 kg N ha^–1 yr^–1 by ¹⁵N dilution and was similar for all clover cultivars. Thus, about 50% of grass N was met by transfer of fixed N from white clover during the measurement year. Short-term measurements using a ¹⁵N foliar-labelling method indicated that below-ground N transfer was largest during dry summer conditions.     

Association with mature plants protects seedlings from predation in an arid grassland shrub,Gutierrezia microcephala

Summary Survivorship of Gutierrezia microcephala (Compositae) seedlings was studied in an undisturbed arid grassland and in experimental plots where various components of the natural vegetation were removed following seed germination. The major causes of seedling mortality were herbivore damage from the specialist grasshopper, Hesperotettix viridis, and drought stress associated with competition from established plants. The relative intensity of these mortality factors varied strongly with seedling size. Large seedlings had higher overall survivorship but were most likely to be killed by defoliation; most small seedlings died of drought stress.In plots where all perennial grasses were removed (leaving established G. microcephala plants), seedling survivorship was 5 times greater than in undisturbed vegetation. Surviorship in plots where both grasses and mature G. microcephala were removed was slightly poorer than in undisturbed vegetation due to a large increase in mortality from defoliation. From May–August, when the herbivore H. viridis was abundant, seedling survivorship was better in the immediate vicinity of mature conspecific plants than in plots lacking mature G. microcephala, both in the presence and absence of perennial grasses. These results provide a counter example to theories predicting that the impact of specialist herbivores on seedling recruitment is greatest in the vicinity of parent plants.     

Herbivory and local genetic differentiation in natural populations of Arabidopsis thaliana (Brassicaceae)

To explore genetic variation in defence against the natural herbivores of Arabidopsis thaliana, we transplanted genotypes between a dune habitat and inland habitat in both of which A. thaliana occurred naturally. In previous years we had observed that the specialist weevils Ceutorhynchus atomus and C. contractus (Curculionidae) fed conspicuously on flowers and fruits of A. thaliana in the dunes, while these weevils were always rare in inland habitats. Taking all plants together, total fruit damage was indeed much higher in our experimental plots in the dune habitat (59.7%) relative to the inland garden habitat (18.9%). Within a habitat, additional differences existed between plants of different origins, pointing to genetic differences in ecologically relevant characters; plants of inland origin flowered a week earlier, grew better and produced more fruits than plants of dune origin. However, plants of inland origin experienced more total fruit damage by the specialist weevils (75.4%) than plants of dune origin (44.0%) when the two types grew side by side in the dune habitat. Escape from herbivory gives dune genotypes an advantage in their native habitat, whereas stronger growth and higher survival gives inland genotypes an edge under garden conditions.     

Within-generation and transgenerational plasticity in growth and regeneration of a subordinate annual grass in a rainfall experiment

Precipitation changes may induce shifts in plant species or life form dominance in ecosystems, making some previously subordinate species abundant. The plasticity of certain plant functional traits of these expanding subordinate species may be one possible mechanism behind their success. In this study, we tested if the subordinate winter annual grass Secale sylvestre shows plasticity in growth and reproduction in response to altered environment associated with field-scale rainfall manipulations (severe drought, moderate drought, and watering) in a semiarid grassland, and whether the maternal environment influences offspring germination or growth in a subsequent pot experiment. Compared to control plots, S. sylvestre plants grew 38% taller, and produced 32% more seeds in severe drought plots, while plants in watered plots were 17% shorter, and had 22% less seeds. Seed mass was greatest in severe drought plots. Plants growing in drought plots had offspring with enhanced juvenile shoot growth compared to the progeny whose mother plants grew in watered plots. These responses are most likely explained by the decreased cover of previously dominant perennial grasses in severe drought plots, which resulted in wetter soil compared to control and watered plots during the peak growth of S. sylvestre. We conclude that the plasticity of this subordinate annual species in response to changing environment may help to gain dominance with recurring droughts that suppress perennial grasses. Our results highlight that exploring both within-generation and transgenerational plasticity of subordinate species may lead to a better prediction of changes in plant species dominance under climate change.

     

Rhizosphere and root-infecting fungi and the design of ecological field experiments

As part of a wider study into the role of soil fungi in the ecology of the winter annual grass, Vulpia ciliata ssp. ambigua (Le Gall) Stace & Auquier, we applied the fungicides benomyl and prochloraz to three natural populations of the grass growing in East anglia, United Kingdom. The rhizosphere and rootinfecting fungi associated with the three populations were analysed each month between February and May 1992 when plants set seed. There were marked differences between the fungal floras associated with each of the three populations of V. ciliata, despite the fact that associated plant species and soil nutrient status were broadly similar between sites. This was attributed to wide differences in soil pH between the three populations. Prochloraz did not affect fungal abundance, but benomyl decreased the isolation frequencies of Fusarium oxysporum from roots and the frequencies of Penicillium and Trichoderma spp. isolated from rhizosphere soil, and increased the frequency of isolation of Mucor hiemalis from the rhizosphere of V. ciliata. There were also significant increases in the isolation frequencies of F. oxysporum from roots and M. hiemalis, Trichoderma spp. and Phoma fimeti from the rhizosphere of V. ciliata as plants matured. The significance of these results for the design of ecological field experiments are discussed in light of a previous study which has shown that asymptomatic root-infecting fungi can affect plant fecundity and hence abundance in natural populations of V. ciliata. We propose that differences in microbial communities between sites, controlled in part by soil chemistry, are a major factor determining plant performance under field conditions.     

Ants and the enigmatic Namibian fairy circles – cause and effect?

1. Parts of the Namibian landscape show extensive surface perturbation in the form of long‐lived, yet dynamic ‘fairy circles'. While exerting profound ecological effects on 7.3% of the land surface, the origin and nature of these large bare discs embedded in an arid grassland matrix remains unresolved. 2. We found no evidence to support the current hypothesis of a termite origin for fairy circles but instead observed a strong spatial association between fairy circles and large nests of the ant Black pugnacious ant Anoplolepis steingroeveri Forel, with much higher ant abundances on the circles compared with the matrix. 3. Aggression trials showed that different colonies of A. steingroeveri were located on different circles, and that the species was polydomous. 4. Fairy circles and Pogonomyrmex ant nests both have a bare disc surrounding the nest, are overdispersed (evenly spaced), and are associated with elevated soil moisture. Fairy circle soils exhibited a five‐fold increase in soil moisture when compared with the matrix. 5. Senescent Stipagrostis obtusa (Delile) Nees seedlings were only observed on the circles and not in the matrix, and were found to have a reduction in both root length and number of roots. 6. Anoplolepis steingroeveri excavated the root system of both S. obtusa seedlings on the disc and Stipagrostis ciliata (Desf.) de Winter grasses on the perimeter of the circles, where they tended honeydew‐secreting Meenoplidae bugs that fed on grass roots and culms. The bugs occurred almost exclusively on grasses associated with the circles. This ant–bug interaction is a possible mechanism for the observed reduction in root length and number of senescent grass seedlings on the circles.     

Nutrient losses in runoff from grasslandand shrubland habitats in southern New Mexico: II. Field plots

Losses of dissolved nutrients (N, P, K, Ca, Mg, Na,Cl, and SO₄) in runoff were measured on grasslandand shrubland plots in the Chihuahuan desert ofsouthern New Mexico. Runoff began at a lowerthreshold of rainfall in shrublands than ingrasslands, and the runoff coefficient averaged 18.6%in shrubland plots over a 7-year period. In contrast,grassland plots lost 5.0 to 6.3% of incidentprecipitation in runoff during a 5.5-year period. Nutrient losses from shrubland plots were greater thanfrom grassland plots, with nitrogen losses averaging0.33 kg ha^–1 yr^–1 vs0.15 kg ha^–1 yr^–1, respectively, during a 3-year period. Thegreater nutrient losses in shrublands were due tohigher runoff, rather than higher nutrientconcentrations in runoff. In spite of these nutrientlosses in runoff, all plots showed net accumulationsof most elements due to inputs from atmosphericdeposition. Therefore, loss of soil nutrients byhillslope runoff cannot, by itself, account for thedepletion of soil fertility associated withdesertification in the Chihuahuan desert.     

Response of aboveground grassland biomass and soil moisture to moderate long-term CO2 enrichment

Rising atmospheric CO₂ concentrations may alter C cycling and community composition, however, long-term studies in (semi-)natural ecosystems are still rare. In May 1998, the Giessen FACE (Free Air Carbon dioxide Enrichment) experiment started in a grassland ecosystem near Giessen, Germany, consisting of three enrichment (E plots) and three ambient control rings (A plots). Carbon dioxide concentrations were raised to +20% above ambient all-year-round during daylight hours. The wet grassland (Arrhenatheretum elatioris Br.-Bl.; not ploughed for >100 years) has been fertilized with 40 kg ha⁻¹ yr⁻¹ N, and mown two times each year for decades. Since 1993, the biomass has been monitored and since 1997 it was divided into grasses, legumes and non-leguminous forbs.During the 5 years prior to CO₂ enrichment, the annual biomass yield from the A plots was non-significantly higher (3%) than the later E plots yield. Under CO₂ enrichment, the biomass increased significantly from the third enrichment year on by 9.8%, 7.7% and 11.2% in the years 2000–2002, respectively. The increase was surprisingly high considering the moderate CO₂ enrichment regime of only +20% and sub-optimal N supply, possibly suggesting a non-linear response of temperate grassland ecosystems to rising atmospheric CO₂ levels.The leaf area index did not change significantly under elevated CO₂, nor did the soil moisture in the top 15 cm increase. No correlation existed between the magnitude of the yield stimulation under elevated CO₂ and the precipitation sums preceding the respective harvests. The grass biomass increased significantly under FACE, while the forb biomass declined strongly in the fourth and fifth year. The legume fraction was mostly below 1% of the total yield, and did not respond to CO₂ enrichment. These findings are in contrast to other grassland results and possible reasons are discussed.     

不同干扰程度下沙生植物异翅独尾草的种群结构与动态特征

不断加剧的人类活动导致古尔班通古特沙漠南缘异翅独尾草(Eremurus anisopterus(Kar.et Kir.)Regel)生存生境片段化,形成许多大小不一的斑块种群。为深刻理解在不同程度破碎化斑块中异翅独尾草种群的生存现状,共选取19个样点,分析其龄级结构,编制静态生命表,绘制存活曲线和死亡曲线,并引入4个生存分析函数。结果表明:古尔班通古特沙漠南缘不同样地中异翅独尾草种群动态因人类干扰与生境破碎化程度的差异,呈现为不同的结构特征及变化趋势,各样地异翅独尾草种群龄级完整性均不同,破碎化程度高的样地中种群的龄级有残缺或断代现象;人类干扰程度中、弱的b类型、c类型斑块的种群年龄结构分别属于稳定至衰退型和增长型,而受干扰最强的a类型斑块中的种群结构表现出较强的波动性,种群趋于衰退的风险较高;存活曲线与4个生存函数曲线表明,a类型种群前、中期稳定,后期衰退;b类型种群前期衰退,中、后期稳定;c类型种群稳定增长。说明异翅独尾草种群的衰退可能是其生境破碎化引起的,因此,对于人类干扰程度强的衰退型种群应亟需减少人为干扰,依据不同生境中的干扰因素及种群生存现状,制定科学与切实可行的保护、恢复策略。     

Invasion resistance and persistence: established plants win,even with disturbance and high propagule pressure

Disturbances and propagule pressure are key mechanisms in plant community resistance to invasion, as well as persistence of invasions. Few studies, however, have experimentally tested the interaction of these two mechanisms. We initiated a study in a southwestern ponderosa pine (Pinus ponderosa Laws.)/bunch grass system to determine the susceptibility of remnant native plant communities to cheatgrass (Bromus tectorum L.) invasion, and persistence of cheatgrass in invaded areas. We used a 2 × 2 factorial design consisting of two levels of aboveground biomass removal and two levels of reciprocal seeding. We seeded cheatgrass seeds in native plots and a native seed mixture in cheatgrass plots. Two biomass removal disturbances and sowing seeds over 3 years did not reverse cheatgrass dominance in invaded plots or native grass dominance in non-invaded native plots. Our results suggest that two factors dictated the persistence of the resident communities. First, bottlebrush squirreltail (Elymus elymoides (Raf.) Swezey) was the dominant native herbaceous species on the study site. This species is typically a poor competitor with cheatgrass as a seedling, but is a strong competitor when mature. Second, differences in pretreatment levels of plant-available soil nitrogen and phosphorus may have favored the dominant species in each community. Annual species typically require higher levels of plant-available soil nutrients than perennial plants. This trend was observed in the annual cheatgrass community and perennial native community. Our study shows that established plants and soil properties can buffer the influences of disturbance and elevated propagule pressure on cheatgrass invasion.     

Density-dependent mortality in Pinus sylvestris caused by the snow blight pathogen Phacidium infestans

Summary Relationships between disease incidence and the density of host plant populations were investigated in the Pinus sylvestris-Phacidium infestans host-fungal pathogen association, in which the season of death of plants killed up to 3 years previously could be accurately determined. Significant (P<0.05), positive density-dependent relationships between the proportion of plants dying in the winters of 1987–1988, 1988–1989 or 1989–1990 and the original stand density were detected in 12 of 26 comparisons. Of the remaining comparisons, all but three had positive regression coefficients for the same association. Plants killed up to 2 years previously contributed to inoculum production. The use of standing dead as a predictor in the analyses showed that the proportion of plants dying in the winters of 1988–1989 or 1989–1990 was generally better correlated with standing dead in the previous summer than with the density of the original population. Significant (P<0.05), positive density-dependent associations were also found between the proportion of living plants in 1990 infected with P. infestans and the number of standing dead plants in all nine comparisons. In contrast, only four of the nine associations between these proportions of infected plants and population density were significant. The strength of the density-dependent relationships varied substantially within and between sites. Much of this variation appears to be due to differences in the stage of development of the epidemics occurring at different sites.     

Vegetation analysis in colonies of an endangered rodent, the Cabrera vole (Microtus cabrerae), in southern Portugal

The Cabrera vole (Microtus cabrerae) is a threatened rodent endemic in the Iberian Peninsula with a patchy distribution and specific microhabitat requirements. This aim of this study was to document the composition of plant communities in habitats of Cabrera vole colonies in southern Portugal. Differences observed in plant species composition were also compared with vegetation structure, taxonomic and life form groups, species and group diversity, disturbance, topography and soil properties. Vegetation was sampled between March and July 2004, in 26 colonies occurring in five geographical areas. Grasses were the most abundant, common and diverse family in the colonies, and the perennial grass Agrostis castellana was present in 92% of colonies, with a mean cover of 16% of the site. Other frequently occurring species were Briza maxima (85%), Vulpia myuros (85%), Gaudinia fragilis (81%), Leontodon spp. (81%), Avena barbata (77%), Bromus hordeaceus (77%) and Tolpis barbata (77%). Colonies were classified in eight vegetation groups that included meadows, tall perennial grasslands, manured meadows with tall sedges, annual grasslands and ruderal and nitrophilous grasslands. Main gradients associated with composition differences were grass richness, annual and perennial grass cover, vegetation structure (herbaceous vegetation height), soil properties (texture and moisture), disturbance (ruderal species) and colony dimensions (area). Results suggest that the Cabrera vole is able to exploit a wide variety of grasslands, with a varying degree of ecological disturbance. Meadows and perennial grassland communities seem to be higher-quality microhabitats for voles.     

Nitrogen fixation in perennial forage legumes in the field

Nitrogen acquisition is one of the most important factors for plant production, and N contribution from biological N₂ fixation can reduce the need for industrial N fertilizers. Perennial forages are widespread in temperate and boreal areas, where much of the agriculture is based on livestock production. Due to the symbiosis with N₂-fixing rhizobia, perennial forage legumes have great potential to increase sustainability in such grassland farming systems. The present work is a summary of a large number of studies investigating N₂ fixation in three perennial forage legumes primarily relating to ungrazed northern temperate/boreal areas. Reported rates of N₂ fixation in above-ground plant tissues were in the range of up to 373 kg N ha^–1 year^–1 in red clover (Trifolium pratense L.), 545 kg N ha^–1 year^–1 in white clover (T. repens L.) and 350 kg N ha^–1 year^–1 in alfalfa (Medicago sativa L.). When grown in mixtures with grasses, these species took a large fraction of their nitrogen from N₂ fixation (average around 80%), regardless of management, dry matter yield and location. There was a large variation in N₂ fixation data and part of this variation was ascribed to differences in plant production between years. Studies with experiments at more than one site showed that also geographic location was an important source of variation. On the other hand, when all data were plotted against latitude, there was no simple correlation. Climatic conditions seem therefore to give as high N₂ fixation per ha and year in northern areas (around 60°N) as in areas with a milder climate (around 40°N). Analyzing whole plants or just above-ground plant parts influenced the estimate of N₂ fixation, and most reported values were underestimated since roots were not included. Despite large differences in environmental conditions, such as N fertilization and geographic location, N₂ fixation (Nfix; kg N per ha and year) was significantly (P<0.001) correlated to legume dry matter yield (DM; kg per ha and year). Very rough, but nevertheless valuable estimations of Nfix in legume/grass mixtures (roots not considered) are given by Nfix = 0.026DM + 7 for T. pratense, Nfix = 0.031DM + 24 for T. repens, and Nfix = 0.021DM + 17 for M. sativa.     

阿拉善干旱荒漠区不同植被类型土壤种子库研究

[1]兰州大学草地农业科技学院,兰州730020 [2]甘肃草原生态研究所,兰州730020 [3]内蒙古阿拉善盟草原站,巴彦浩特750360     

Inorganic N turnover and availability in annual- and perennial-dominated soils in a northern Utah shrub-steppe ecosystem

The exotic annual grass Bromus tectorum has replaced thousands of hectares of native perennial vegetation in semi-arid ecosystems of the western United States. Inorganic N availability and production were compared in soil from monodominant patches of Bromus tectorum, the perennial bunchgrass Elymus elymoides, and the shrub Artemisia tridentata, in Curlew Valley, a salt-desert shrub site in Northern Utah. Bromus-dominated soil had greater %N in the top 10 cm than Artemisia or Elymus-dominated soils. As determined by spring isotope-dilution assays, gross mineralization and nitrification rates were higher in Bromus-dominated than Artemisia-dominated soils, but gross rates of NH₄ ⁺ and NO₃ ^– consumption were also higher. Litterbags had greater mass loss and N mineralization when buried in Bromus stands than in Artemisia stands, indicating the soil environment under the annual grass promotes decomposition. As determined by nitrification potential assays, nitrifier populations were higher under Bromus than under Artemisia and Elymus. Soil inorganic N concentrations were similar among vegetation types in the spring, but NO₃ ^– accumulated under Bromus once it had senesced. An in situ net mineralization assay conducted in autumn indicated that germinating Bromus seedlings are a strong sink for soil NO₃ ^–, and that net nitrification is inherently low in soils under Artemisia and Elymus. Results of the study suggest that differences in plant uptake and the soil environment promote greater inorganic N availability under Bromus than under perennial species at the site.