Quantifying direct and indirect effects of fungicide on an old-field plant community: an experimental null-community approach

Environmental perturbations can alter the composition of plant communities, either directly, by altering growth of some species more than others, or indirectly, by altering the strength of interspecific interactions among species. The relative importance of direct and indirect effects is not at all well known. We used an experimental approach to quantifying direct and indirect effects of fungicide on the composition of a plant community. To separate the direct and indirect impacts of fungicide we grew plant species in monoculture and mixed communities, and with and without the systemic fungicide benomyl. We predicted that direct effects of fungicide would be important at low but not high nutrient availability, while indirect effects would be more important at high nutrient availability. After 3 years there was little impact of fungicide on arbuscular mycorrhizal fungal colonization, and on soil microbial community composition assessed as the relative abundance of different phospholipid fatty acids. Like fertilizer, fungicide increased plant biomass. However, in contrast to fertilizer, this did not result in a decline in species evenness. Although not significant, the direct effects of fungicide tended to oppose the indirect effects of both fungicide and interspecific interactions on plant community composition. Experiments relying on fungicide treatments must be interpreted extremely cautiously, because the impact of fungicide is potentially the integrated response of plants to multiple factors, including arbuscular mycorrhizal fungi, pathogenic and saprophytic fungi, and nutrient inputs.     

Effects of mycorrhizae on plant growth and dynamics in experimental tall grass prairie microcosms

Experimental microcosms (40 X 52 X 32 cm) containing an assemblage of eight tallgrass prairie grass and forb species in native prairie soil were maintained under mycorrhizal (untreated control) or mycorrhizal-suppressed (fungicide-treated) conditions to examine plant growth, demographic, and community responses to mycorrhizal symbiosis. The fungicide benomyl successfully reduced mycorrhizal root colonization in the fungicide-treated microcosms to only 6.4% (an 83% reduction relative to mycorrhizal controls). Suppression of mycorrhizas resulted in a 31% reduction in total net aboveground plant production and changes in the relative production of C4 and C3 plants. The C4 tallgrasses Andropogon gerardi and Sorghastrum nutans produced less plant biomass in the fungicide-treated microcosms, and had a greater ratio of reproductive to vegetative biomass. Cool-season C3 grasses, Koeleria pyramidata and Poa pratensis accumulated more biomass and were a significantly greater proportion of total community biomass in mycorrhizal-suppressed microcosms. Forbs showed variable responses to mycorrhizal suppression. The two legumes Amorpha canescens and Dalea purpurea had significantly lower survivorship in the fungicide-treated microcosms, relative to the controls. The results confirm the high mycorrhizal dependency and growth responsiveness of dominant prairie grasses, and indicate that differential growth and demographic responses to mycorrhizal colonization among species may significantly affect plant productivity and species relative abundances in tallgrass prairie.     

Effects of mycorrhizae on growth and demography of tallgrass prairie forbs

The effects of arbuscular mycorrhizal (AM) symbiosis on ramet and genet densities, vegetative growth rates, and flowering of three forb species were studied in native tallgrass prairie in northeastern Kansas. Mycorrhizal activity was experimentally suppressed for six growing seasons on replicate plots in an annually burned and an infrequently burned watershed with the fungicide benomyl. Benomyl reduced mycorrhizal root colonization to an average of 4.2%, approximately a two-thirds reduction relative to controls (13.7% colonization). Mycorrhizae influenced the population structure of these forbs. Although mycorrhizal suppression had no long-term effect on genet densities and no effect on ramet survivorship throughout the growing season, the number of ramets per individual was significantly increased such that ramet densities of all three species were approximately doubled in response to long-term mycorrhizal suppression. Effects of mycorrhizae on ramet growth and reproduction varied among species. Ramet growth rates, biomass, and flowering of Salvia azurea were greater in plots with active mycorrhizal symbiosis, whereas mycorrhizae reduced ramet growth rates and biomass of Artemesia ludoviciana. Aster sericeus ramet growth rates and biomass were unaffected by the fungicide applications, but its flowering was reduced.The pattern of responses of these three species to mycorrhizae differed considerably between the two sites of contrasting fire regime, indicating that the interaction of fire-induced shifts in resource availability and mycorrhizal symbiosis together modulates plant responses and the intensity and patterns of interspecific competition between and among tallgrass prairie grass and forb species. Further, the results indicate that effects of mycorrhizae on community structure are a result of interspecific differences in the balance between direct positive effects of the symbiosis on host plant performance and indirect negative effects mediated through altered competitive interactions.     

Topsin-M: the new benomyl for mycorrhizal-suppression experiments

The fungicide benomyl was the most commonly used biocide for both field and greenhouse experiments in which arbuscular mycorrhizal fungal (AMF) suppression is desired. Unfortunately benomyl is no longer manufactured and therefore is not available for experimental use and no fungicide has been proposed as a successful alternative for experimentally suppressing mycorrhizal fungi. In this study we examined the potential for the fungicide Topsin M (topsin) to suppress mycorrhizal symbiosis in both field and greenhouse experiments. Topsin reduced AMF colonization of the obligately mycotrophic, warm-season grass Andropogon gerardii with a large and significant reduction in plant biomass production. Topsin reduced AMF colonization of the facultatively mycotrophic, cool-season grass Pascopyron smithii but did not significantly reduce biomass production. Fertilization with nitrogen and phosphorus was able to compensate for reductions in biomass due to the application of fungicide because biomass production of plants that received topsin fungicide was not significantly different from fertilized controls not receiving topsin. While we are not advocating that topsin fungicide is a universal mechanism for mycorrhizal-suppressed controls, in systems where benomyl was found to be successful topsin appears to be a useful, available and successful alternative.     

Arbuscular mycorrhizal fungi alter plant community composition along a grazing gradient in Inner Mongolia Steppe

Arbuscular mycorrhizal fungi (AMF) have a significant influence on plant productivity and diversity in non-grazing grassland. However, the interactive effects between grazing intensity and AMF on plant community composition in natural grassland communities are not well known. We conducted a field experiment that manipulated AMF colonization and grazing intensity to study the impact of AMF suppression on plant community composition and nutrient status over 2 years (2015–2016) with contrasting rainfall levels. We found that AMF root colonization was significantly reduced by the application of the fungicide benomyl as a soil drench. Grazing intensity regulated plant community composition and aboveground biomass mainly by reducing the growth of Leymus chinensis over 2 years. AMF suppression increased the growth of Chenopodium glaucum, but it did not alter other plant species across all grazing intensities. The effects of AMF suppression on plant community composition changed along a grazing gradient considerably between years: AMF suppression increased the biomass of C. glaucum across all grazing intensities in 2015, but slightly increased it in 2016. Interactions between AMF suppression and grazing intensity altered the phosphorus concentration of Stipa grandis and Cleistogenes squarrosa in 2015 but not in 2016. AMF suppression decreased the shoot phosphorus content of L. chinensis but increased that of C. glaucum across all grazing intensities. Our results indicate that grazing intensity substantially alters aboveground community biomass and affects growth of dominant species; AMF by itself have limited effects on plant communities along a grazing gradient in typical steppe.     

Nitrogen limitation of production in a Californianannual grassland: The contribution of arbuscularmycorrhizae

Nutrient availability limits plant production acrossa wide range of terrestrial ecosystems. In this studyof a Californian annual grassland community, theinfluence of arbuscular mycorrhizal (AM) associationson plant nutrient acquisition was investigated usingfactorial combinations of nitrogen (N), phosphorus (P)and benomyl fungicide. N additions resulted in asignificant increase in shoot biomass demonstratingthat plant productivity in these soils was N-limited. The effect of P additions consistently depended onfungicide treatment. In the absence of benomyl, shootP accumulation was unaltered by P addition. In thepresence of benomyl, P addition significantlyincreased shoot P accumulation and was associated witha consistent trend towards enhanced shoot biomass.The induction of P-deficiency with benomylapplication suggests that effective P acquisition byAM may contribute to the strong N-limitation ofproduction observed in many terrestrial ecosystems.     

Effect of AM symbiosis exclusion on grassland community composition

Experimental data on grassland community response to exclusion of arbuscular mycorrhizal (AM) symbiosis using fungicide benomyl and to phosphate addition were evaluated after four years of continued manipulation using multivariate direct gradient analysis. While the community responded only to phosphate application in the third year (phosphate application suppressing forbs in favour of graminoids), the data from the fourth season indicate relative suppression of some forbs (includingPlantago lanceolata, Cerastium holosteoides) with both phosphate and fungicide applications. Positive response of graminoids to phosphate addition seems to interact with fungicide application, while an important community constituent —Achillea millefolium — seems to profit primarily from fungicide applications. The effect of the fungicide on the AM symbiosis was confirmed on roots ofPlantago lanceolata. The direct evaluation of frequency of AM fungal structures in plant roots also revealed a diverse response of different plant species to phosphate application. Nevertheless, in all species (except non-mycorrhizalLuzula campestris), phosphate decreased the frequency of arbuscules and increased the frequency of non-infected root segments.     

Soil community composition and the regulation of grazed temperate grassland

The effect of the community composition of soil microbes on ecosystem processes has received relatively little attention. Here we examined the variation in soil microbial composition in a Yellowstone National Park grassland and the effect of that variation on the growth, in a greenhouse, of the dominant grass in the community. Plants and their rhizospheric soil were collected from paired, Poa pratensis-dominated grassland plots located inside and outside a 40-year-old exclosure. P. pratensis aboveground, belowground, and whole plant growth were greater in pots with soil communities from grazed grassland compared to fenced grassland, indicating (1) soil microbial communities differed, and (2) this difference influenced the growth of the plant that dominated both grasslands. Treating pots with fungicide (benomyl) suppressed the soil community influence, indicating that different fungal communities caused the soil microbe effect. In addition, two lines of evidence are consistent with the hypothesis that arbuscular mycorrhizal fungal (AMF) species composition affected P. pratensis: (1) a divergence in AMF spore communities in the two field soils, and (2) little evidence of pathogenic fungi. These findings emphasize the need to examine the role that the composition of the soil microbial community plays in controlling terrestrial ecosystems.Due to an error in the citation line, this revised PDF (published in December 2003) deviates from the printed version, and is the correct and authoritative version of the paper.     

Long-term addition of fertilizer,labile carbon,and fungicide alters the biomass of plant functional groups in a subarctic-alpine community

In subarctic ecosystems, plant growth is mostly limited by nutrient availability and harsh climate. Investigating how soil nutrient availability controls the plant community composition may therefore help to understand indirect effects of climate change. The study was conducted in a long-term field experiment on a subarctic-alpine fellfield dominated by woody evergreen shrubs, bryophytes, and lichens. To manipulate nutrient availability additions of NPK fertilizer, labile C, and fungicide (benomyl) were done in a fully factorial design, replicated in six blocks. The treatments were run for 10 years, and the aboveground plant biomass was harvested 4 and 16 years after initiating the experiment. In addition, soil inorganic N and P concentration was analyzed the same years. Increased nutrient availability (NPK fertilizer) largely increased the biomass of graminoids and unexpectedly of bryophytes, but not of other vascular plant groups. Also, limitation of soil nutrient availability caused by labile C addition decreased the relative proportion of green shoots in evergreen shrubs, although these were expected to cope better with the nutrient limitation than the opportunistic graminoids, which, by contrast, were unaffected. Reduced fungal biomass due to benomyl addition was accompanied by increased evergreen shrub and clubmoss biomass. Taken together, the effects of treatments were most pronounced 16 years after initiation of the experiment, but despite changes in biomass the overall plant community composition was resistant to environmental changes.     

Effects of endomycorrhizal infection,artificial herbivory,and parental cross on growth of Lotus corniculatus L.

Summary We examined how combinations of parentage, fungicide application, and artificial herbivory influence growth and shoot phosphorus content in pre-reproductive Lotus corniculatus, using young offspring arising from three parental crosses, two of which had one parent in common. Soil with vesicular-arbuscular mycorrhizal (VAM) fungi was treated with either water or benomyl, an anti-VAM fungicide, and added to trays containing groups of four full siblings. There were two experiments; in the first no plants were clipped while in the second two of the four plants were clipped to simulate herbivory. In both experiments plants of the two related crosses accumulated more biomass and total shoot P than did plants of the third cross. Plants inoculated with watertreated soil had greater shoot mass and P concentration than did fungicide-treated replicates but the extent of increase in P concentration varied among crosses. In Experiment 2, clipping reduced root mass and resulted in higher shoot P concentration. In this experiment there was a significant interaction of fungicide application and clipping: both unclipped and clipped plants grew better in soil not treated with fungicide, but the increase in shoot mass, total mass, and total P was greater in unclipped plants. Significant interaction of fungicide treatment and clipping is most likely due to reduced availability of carbon to the roots of clipped plants, resulting in poorer symbiotic functioning.     

Root proliferation strategies and exploration of soil patchiness in arid communities

Soil patchiness is a key feature of arid rangelands. As root proliferation contributes to soil exploration and resource uptake, it is ecologically relevant to understand how species respond to soil heterogeneity and coexist. Campbell et al.'s influential 1991 hypothesis proposes that dominant species deploy root systems (scale) that maximize soil volume explored. Instead, subordinate species show accurate root systems that exclusively proliferate in nutrient‐rich patches (precision). After many experiments under controlled conditions, the generality of this hypothesis has been questioned but a field perspective is necessary to increase realism in the conceptual framework. We worked with a guild of perennial graminoid species inside a grazing exclosure in an arid Patagonian steppe, a model system for ecological studies in arid rangelands for four decades. We buried root traps in bare ground patches with sieved soil, with or without a pulse of nitrogen addition, to measure specific root biomass and precision at 6 and 18 months after burial. We also estimated scale (root density) in naturally established plants, and root decomposition in litter bags. Several species grew in root traps. Dominant species showed the highest root biomass (in both harvests) and scale. Subordinate species grew more frequently with nitrogen addition and showed lower biomass and scale. Similar total root biomass was found with and without nitrogen addition. Species differed in root decomposition, but correcting species biomass by decomposition did not change our conclusions. We did not find a relation between scale and precision, indicating that Campbell's hypothesis is probably not supported in this Patagonian steppe. Soil resource acquisition differences probably do not utterly explain the coexistence of dominant and subordinate species because the steppe is also affected by large herbivore grazing. We propose that root proliferation in this steppe is the result of the interaction between individual density in the community and specific root growth rates.     

Soil heterogeneity and community composition jointly influence grassland biomass

Question: Does the spatial pattern of nutrient supply modify community biomass responses to changes in both species composition and richness? Location: Duke University Phytotron (Durham, North Carolina, USA). Methods: We conducted a microcosm experiment to evaluate individual plant and whole community responses to species richness, species composition and soil nutrient heterogeneity. The experiment consisted of seven levels of species composition (all possible combinations of Lolium perenne, Poa pratensis and Plantago lanceolata) crossed with three levels of soil nutrient distribution (homogeneous, heterogeneous‐up, and heterogeneous‐down, where up and down indicates the location of a nutrient patch in either the upper or the lower half of the soil column, respectively). Results: Communities containing Plantago and Lolium responded to nutrient heterogeneity by increasing above‐ and below‐ground biomass. Nutrient heterogeneity also increased size inequalities among individuals of these species. Significant species composition X nutrient heterogeneity interactions on community biomass and individual size inequality were observed when nutrient patches were located in the upper 10 cm of the soil columns. However, root proliferation in nutrient patches was equivalent regardless of the vertical placement of the patch. Conclusions: Our results suggest that nutrient heterogeneity may interact with plant species composition to determine community biomass, and that small‐scale vertical differences in the location of nutrient patches affect individual and community responses to this heterogeneity.     

Effects of livestock grazing intensity on soil arbuscular mycorrhizal fungi and glomalin-related soil protein in a mountain forest steppe and a desert steppe of Mongolia

Arbuscular mycorrhizal fungi (AMF) are important components of the grassland ecosystems in terms of plant phosphorus uptake and accumulation of glomalin-related soil protein (GRSP). Though Mongolian grasslands are seriously degraded by livestock grazing, the effects of grazing on soil AMF and GRSP remain unclear. Here, we examined community composition and diversity of AMF as well as amount of GRSP at three different grazing intensities: lightly grazed (LG), moderately grazed (MG), and heavily grazed (HG) under two different types of grassland, mountain forest steppe at Hustai and desert steppe at Mandalgobi. The diversity and biomass of AMF-host and non-AMF plants strongly affected the overall AMF community composition and its diversity. When we separately analyzed the factors affecting soil AMF diversity at Hustai and Mandalgobi, decrease in the shoot biomass of Poaceae plants at Hustai and decreases in the species number and shoot biomass of AMF-host plants at Mandalgobi were significantly correlated with AMF diversity. GRSP decreased with increasing grazing intensity, which was significantly correlated with soil pH and total root biomass at Hustai. The decrease in plant biomass caused by grazing thus led to GRSP reduction. Our results showed that change in soil AMF community caused by livestock grazing were associated with change in the biomass and diversity of functional vegetation groups such as Poaeceae, AMF-host and non-AMF plants, indicating the importance to focus on such functional vegetation groups to evaluate the effect of grazing on AMF.

     

Effect of the fungicide benomyl on spore germination and hyphal length of the arbuscular mycorrhizal fungus Glomus mosseae.

The fungicide benomyl inhibited spore germination and hyphal length of the arbuscular mycorrhizal fungus Glomus mosseae when applied at doses of 21.25 microg/ml (agronomic dose), 10.62 microg/ml and 10 microg/ml. G. mosseae was able to germinate in the presence of 2.12 microg/ml of benomyl, and the percentage of spore germination was unaffected by dosis of 0.1, 0.01 and 0.001 microg/ml of the fungicide. However, all doses of fungicide tested in this study decreased the hyphal length. When ungerminated G. mosseae spores previously exposed to benomyl were transferred to water-agar medium without benomyl, the maximum germination was 16%. Small spores of G. mosseae were more resistant to benomyl than the larger ones. Our results show some of the factors which can explain the variability of the effect of benomyl on arbuscular mycorrhizal fungi.     

Growth and viability of mycorrhizal extraradical mycelia associated with three temperate orchid species

Growth and enzymatic activities of extraradical mycelia (ERM) of native mycorrhizal symbionts associated with three orchid species, Dactylorhiza fuchsii, D. majalis and Platanthera bifolia, were studied. ERM extracted from the mycorrhizosphere of these species showed features typical for fungi that form orchid mycorrhiza. In the first pot experiment, three different treatments were applied on tubers of D. fuchsii transplanted from a natural site: control (no specific treatment), reinoculated (surface-sterilized tubers reinoculated with mycorrhizal fungi-colonised roots), and benomyl (nonsterilized tubers treated with fungicide). However, no significant differences in ERM growth and intensity of root mycorrhizal colonisation at harvest were observed among these treatments. ERM associated with reinoculated D. fuchsii plants showed significantly higher alkaline phosphatase (ALP) enzymatic activity at week 36 than at week 24, but no differences were observed for NADH diaphorase activity. Benomyl application significantly reduced ALP activity in comparison with reinoculated plants at week 36. In the second experiment, plants of all three species were either untreated (control), or repeatedly treated with benomyl. Similarly to the results of the first experiment, benomyl application did not reduce the ERM growth of mycorrhizal symbionts associated with D. majalis and D. fuchsii. The low ERM growth associated with benomyl-treated P. bifolia was probably caused by poor root system development in this treatment. Significantly higher mycorrhizal colonisation was found for D. fuchsii compared to P. bifolia in control treatments at the end of cultivation. The ERM of native symbionts of the three orchid species studied seemed to have a different growth pattern over time and responded differently to fungicide application.     

Arbuscular mycorrhizal infection changes the bacterial 16 S rDNA community composition in the rhizosphere of maize

Mycorrhizal and non-mycorrhizal (NM) maize plants were grown for 4 or 7 weeks in an autoclaved quartz sand-soil mix. Half of the NM plants were supplied with soluble P (NM-HP) while the other half (NM-LP), like the mycorrhizal plants, received poorly soluble Fe and Al phosphate. The mycorrhizal plants were inoculated with Glomus mosseae or G. intraradices. Soil bacteria and those associated with the mycorrhizal inoculum were reintroduced by adding a filtrate of a low P soil and of the inocula. At 4 and 7 weeks, plants were harvested and root samples were taken from the root tip (0-1 cm), the subapical zone (1-2 cm) and the mature root zone at the site of lateral root emergence. DNA was extracted from the roots with adhering soil. At both harvests, the NM-HP plants had higher shoot dry weight than the plants grown on poorly soluble P. Mycorrhizal infection of both fungi ranged between 78% and 93% and had no effect on shoot growth or shoot P content. Eubacterial community compositions were examined by polymerase chain reaction-denaturing gradient gel electrophoresis of 16 S rDNA, digitisation of the band patterns and multivariate analysis. The community composition changed with time and was root zone specific. The differences in bacterial community composition in the rhizosphere between the NM plants and the mycorrhizal plants were greater at 7 than at 4 weeks. The two fungi had similar bacterial communities after 4 weeks, but these differed after 7 weeks. The observed differences are probably due to changes in substrate composition and amount in the rhizosphere.     

Impact of plant functional group, plant species, and sampling time on the composition of nirK-type denitrifier communities in soil

We studied the influence of eight nonleguminous grassland plant species belonging to two functional groups (grasses and forbs) on the composition of soil denitrifier communities in experimental microcosms over two consecutive years. Denitrifier community composition was analyzed by terminal restriction fragment length polymorphism (T-RFLP) of PCR-amplified nirK gene fragments coding for the copper-containing nitrite reductase. The impact of experimental factors (plant functional group, plant species, sampling time, and interactions between them) on the structure of soil denitrifier communities (i.e., T-RFLP patterns) was analyzed by canonical correspondence analysis. While the functional group of a plant did not affect nirK-type denitrifier communities, plant species identity did influence their composition. This effect changed with sampling time, indicating community changes due to seasonal conditions and a development of the plants in the microcosms. Differences in total soil nitrogen and carbon, soil pH, and root biomass were observed at the end of the experiment. However, statistical analysis revealed that the plants affected the nirK-type denitrifier community composition directly, e.g., through root exudates. Assignment of abundant T-RFs to cloned nirK sequences from the soil and subsequent phylogenetic analysis indicated a dominance of yet-unknown nirK genotypes and of genes related to nirK from denitrifiers of the order Rhizobiales. In conclusion, individual species of nonleguminous plants directly influenced the composition of denitrifier communities in soil, but environmental conditions had additional significant effects.     

Effects of below-ground insects, mycorrhizal fungi and soil fertility on the establishment of Vicia in grassland communities

 The effects of, and interactions between, insect root feeders, vesicular-arbuscular mycorrhizal fungi and soil fertility on the establishment, growth and reproduction of Vicia sativa and V. hirsuta (Fabaceae) were investigated in an early-successional grassland community. Seeds of both species were sown into plots where soil insecticide (Dursban 5G), soil fungicide (Rovral) and soil fertiliser (NPK) were applied in a factorial randomised block design. Fertiliser addition reduced growth, longevity and reproduction of both Vicia species, due to the commonly recorded increase in the competitive advantage of the non-nitrogen-fixing species when nitrogen is added to the plant community. However, in plots where fertiliser was not applied, a reduction in root feeders and mycorrhizal infection led to an increase in seedling establishment and fruit production of V. sativa, and to an increase in flower production for both Vicia species. The interaction between all three soil treatments explained much of the variation in growth and longevity of V. sativa. Plants grew larger and survived longer in plots where natural levels of mycorrhizal infection and root feeders were low compared with plots where all the treatments were applied. This suggests that, although soil nutrient availability was a strong determinant of the performance of these two leguminous species, at natural levels of soil fertility biotic factors acting in the soil, such as mycorrhizal fungi and soil-dwelling insects, were important in shaping the competitive interactions between the two Vicia species and the plant community. Our results indicate that non-additive interactions between ecological factors in the soil environment may strongly affect plant performance. Received: 18 July 1995 / Accepted: 14 August 1996     

土壤养分空间异质性与植物根系的觅食反应

植物在长期进化过程中,为了最大限度地获取土壤资源,对养分的空间异质性产生各种可塑性反应．包括形态可塑性、生理可塑性、菌根可塑性等．许多植物种的根系在养分丰富的斑块中大量增生,增生程度种间差异较大,并受斑块属性(斑块大小、养分浓度)、营养元素种类和养分总体供应状况的影响．植物还通过调整富养斑块中细根的直径、分枝角、节问距以及空间构型来实现斑块养分的高效利用．根系的生理可塑性及菌根可塑性可能在一定程度上影响其形态可塑性．生理可塑性表现为处于不同养分斑块上的根系迅速调整其养分吸收速率,从而增加单位根系的养分吸收,对在时间上和空间上变化频繁的空间异质性土壤养分的利用具有重要意义,可在一定程度上弥补根系增生反应的不足．菌根可塑性目前研究较少,一些植物种的菌根代替细根实现在富养斑块中的增生．菌根增生的碳投入养分吸收效率较高、根系增生对增加养分吸收的作用较复杂,取决于养分离子在土壤中的移动性能以及是否存在竞争植物;对植物生长(竞争能力)的作用因种而异,一些敏感种由此获得生长效益,而其它一些植物种受影响较小．植物个体对土壤养分空间异质性反应能力和生长差异,影响其在群落中的地位和命运,最终影响群落组成及其结构．     

Plant species richness, elevated CO2, and atmospheric nitrogen deposition alter soil microbial community composition and function

We determined soil microbial community composition and function in a field experiment in which plant communities of increasing species richness were exposed to factorial elevated CO₂ and nitrogen (N) deposition treatments. Because elevated CO₂ and N deposition increased plant productivity to a greater extent in more diverse plant assemblages, it is plausible that heterotrophic microbial communities would experience greater substrate availability, potentially increasing microbial activity, and accelerating soil carbon (C) and N cycling. We, therefore, hypothesized that the response of microbial communities to elevated CO₂ and N deposition is contingent on the species richness of plant communities. Microbial community composition was determined by phospholipid fatty acid analysis, and function was measured using the activity of key extracellular enzymes involved in litter decomposition. Higher plant species richness, as a main effect, fostered greater microbial biomass, cellulolytic and chitinolytic capacity, as well as the abundance of saprophytic and arbuscular mycorrhizal (AM) fungi. Moreover, the effect of plant species richness on microbial communities was significantly modified by elevated CO₂ and N deposition. For instance, microbial biomass and fungal abundance increased with greater species richness, but only under combinations of elevated CO₂ and ambient N, or ambient CO₂ and N deposition. Cellobiohydrolase activity increased with higher plant species richness, and this trend was amplified by elevated CO₂. In most cases, the effect of plant species richness remained significant even after accounting for the influence of plant biomass. Taken together, our results demonstrate that plant species richness can directly regulate microbial activity and community composition, and that plant species richness is a significant determinant of microbial response to elevated CO₂ and N deposition. The strong positive effect of plant species richness on cellulolytic capacity and microbial biomass indicate that the rates of soil C cycling may decline with decreasing plant species richness.