Soil functioning in a mosaic of herbaceous communities of a chalky environment: temporal variations of water availability and N dynamics

Background and aims

Although changes in water and nitrogen (N) supply have been largely used to explain modifications in plant communities, the spatio-temporal variability of those factors has been little studied in chalky environments.

Methods

In this study, we explored for 1?year the temporal variations in soil water content, N inorganic forms and net N-mineralization and nitrification for two horizons in three herbaceous communities (short grasslands, tall grasslands, and encroached grasslands) in the Hénouville Nature Reserve (Upper-Normandy, France). Plant available soil water and permanent wilting points of seven plant species were also characterized.

Results

We found that plant available soil water was lower in short grasslands than in tall grasslands and encroached grasslands. Soil water content was below permanent wilting point during four months in short grasslands and only three months in the other communities. Seasonal patterns for inorganic N content and N-mineralization and nitrification were observed with peaks of NH ₄ ⁺ –N in summer and peaks of N-mineralization in spring.

Conclusions

For the studied year, our data highlight the harsh soil desiccation that vegetation endured during the late spring (active growth period) and summer, and show that water shortage is an ecological factor affecting the N cycling in the three successional herbaceous communities.     

Structure and function of denitrifying and nitrifying bacterial communities in relation to the plant species in a constructed wetland

The community structure and potential activities of nitrifying and denitrifying bacteria were studied in the rhizosphere of Typha latifolia and Phragmites australis present in a free water system constructed wetland (CW). Potential nitrate reduction and nitrification activities were shown to be significantly higher in the rhizosphere when compared with the nonvegetated sediment. Higher rates were generally obtained for P. australis . The community structure of denitrifying bacteria in the rhizosphere differed from that found at the bulk sediment, as revealed by PCR-denaturing gradient gel electrophoresis (DGGE) of the nitrous oxide reductase encoding gene nosZ . Results also show a greater nosZ genotype diversification and suggest a plant species effect in rhizosphere samples obtained during events of low hydraulic retention times. Ammonia-oxidizing communities were less complex on the basis of PCR-DGGE analysis of the 16S rRNA gene. Retrieved sequences were all related to Nitrosomonas marina and Nitrosomonas ureae , being both present in rhizosphere and bulk sediment regardless of environmental changes. The results demonstrate the effect of vegetation on the functioning and structure of bacterial communities involved in the removal of nitrogen in the treatment cells of a CW and point to the use of vegetation coverage to promote nitrification or denitrification in particular areas.     

The effect of single tree species on soil microbial activities related to C and N cycling in the Siberian artificial afforestation experiment

The effects of grassland conversion to forest vegetation and of individual tree species on microbial activity in Siberia are largely unstudied. Here, we examined the effects of the six most commonly dominant tree species in Siberian forests (Scots pine, spruce, Arolla pine, larch, aspen and birch) on soil C and N mineralization, N₂O-reduction and N₂O production during denitrification 30 years after planting. We also documented the effect of grassland conversion to different tree species on microbial activities at different soil depths and their relationships to soil chemical properties. The effects of tree species and grassland conversion were more pronounced on N than on C transformations. Tree species and grassland conversion did significantly alter substrate-induced respiration (SIR) and basal respiration, but the differences were not as large as those observed for N transformations. Variances in SIR and basal respiration within species were markedly lower than those in N transformations. Net N mineralization, net nitrification, and denitrification potential were highest under Arolla pine and larch, intermediate under deciduous aspen and birch, and lowest beneath spruce and Scots pine. Tree species caused similar effects on denitrification potential, net N mineralization, and net nitrification, but effects on N₂O reduction rate were idiosyncratic, indicating a decoupling of N₂O production and reduction. We predict that deciduous species should produce more N₂O in the field than conifers, and that Siberian forests will produce more N₂O if global climate change alters tree species composition. Basal respiration and SIR showed inverse responses to tree species: when basal respiration increased in response to a given tree species, SIR declined. SIR may have been controlled by NH₄ ⁺ availability and related therefore to N mineralization, which was negatively affected by grassland conversion. Basal respiration appeared to be less limited by NH₄ ⁺ and controlled mostly by readily available organic C (DOC), which was higher in concentration under forests than in grassland and therefore basal respiration was higher in forested soils. We conclude that in the Siberian artificial afforestation experiment, soil C mineralization was not limited by N.     

Stimulation of soil nitrification and denitrification by grazing in grasslands: do changes in plant species composition matter?

Stimulation of nitrification and denitrification by long term (from years to decades) grazing has commonly been reported in different grassland ecosystems. However, grazing generally induces important changes in plant species composition, and whether changes in nitrification and denitrification are primarily due to changes in vegetation composition has never been tested. We compared soil nitrification- and denitrification-enzyme activities (NEA and DEA, respectively) between semi-natural grassland sites experiencing intensive (IG) and light (LG) grazing/mowing regimes for 13 years. Mean NEA and DEA (i.e. observed from random soil sampling) were higher in IG than LG sites. The NEA/DEA ratio was higher in IG than LG sites, indicating a higher stimulation of nitrification. Marked changes in plant species composition were observed in response to the grazing/mowing regime. In particular, the specific phytomass volume of Elymus repens was lower in IG than LG sites, whereas the specific volume of Lolium perenne was higher in IG than LG sites. In contrast, the specific volume of Holcus lanatus, Poa trivialis and Arrhenatherum elatius were not significantly different between treatments. Soils sampled beneath grass tussocks of the last three species exhibited higher DEA, NEA and NEA/DEA ratio in IG than LG sites. For a given grazing regime, plant species did not affect significantly soil DEA, NEA and NEA/DEA ratio. The modification of plant species composition is thus not the primary factor driving changes in nitrification and denitrification in semi-natural grassland ecosystems experiencing long term intensive grazing. Factors such as trampling, N returned in animal excreta, and/or modification of N uptake and C exudation by frequently defoliated plants could be responsible for the enhanced microbial activities.     

Effects of drought and N-fertilization on N cycling in two grassland soils

Changes in frequency and intensity of drought events are anticipated in many areas of the world. In pasture, drought effects on soil nitrogen (N) cycling are spatially and temporally heterogeneous due to N redistribution by grazers. We studied soil N cycling responses to simulated summer drought and N deposition by grazers in a 3-year field experiment replicated in two grasslands differing in climate and management. Cattle urine and NH₄NO₃ application increased soil NH₄ ⁺ and NO₃ ^? concentrations, and more so under drought due to reduced plant uptake and reduced nitrification and denitrification. Drought effects were, however, reflected to a minor extent only in potential nitrification, denitrifying enzyme activity (DEA), and the abundance of functional genes characteristic of nitrifying (bacterial and archaeal amoA) and denitrifying (narG, nirS, nirK, nosZ) micro-organisms. N₂O emissions, however, were much reduced under drought, suggesting that this effect was driven by environmental limitations rather than by changes in the activity potential or the size of the respective microbial communities. Cattle urine stimulated nitrification and, to a lesser extent, also DEA, but more so in the absence of drought. In contrast, NH₄NO₃ reduced the activity of nitrifiers and denitrifiers due to top-soil acidification. In summary, our data demonstrate that complex interactions between drought, mineral N availability, soil acidification, and plant nutrient uptake control soil N cycling and associated N₂O emissions. These interactive effects differed between processes of the soil N cycle, suggesting that the spatial heterogeneity in pastures needs to be taken into account when predicting changes in N cycling and associated N₂O emissions in a changing climate.     

Invasive plants decrease microbial capacity to nitrify and denitrify compared to native California grassland communities

Exotic plant invasions are a major driver of global environmental change that can significantly alter the availability of limiting nutrients such as nitrogen (N). Beginning with European colonization of California, native grasslands were replaced almost entirely by annual exotic grasses, many of which are now so ubiquitous that they are considered part of the regional flora (“naturalized”). A new wave of invasive plants, such as Aegilops triuncialis (Barb goatgrass) and Elymus caput-medusae (Medusahead), continue to spread throughout the state today. To determine whether these new-wave invasive plants alter soil N dynamics, we measured inorganic N pools, nitrification and denitrification potentials, and possible mediating factors such as microbial biomass and soil pH in experimental grasslands comprised of A. triuncialis and E. caput-medusae. We compared these measurements with those from experimental grasslands containing: (1) native annuals and perennials and (2) naturalized exotic annuals. We found that A. triuncialis and E. caput-medusae significantly reduced ion-exchange resin estimates of nitrate (NO₃ ^?) availability as well as nitrification and denitrification potentials compared to native communities. Active microbial biomass was also lower in invaded soils. In contrast, potential measurements of nitrification and denitrification were similar between invaded and naturalized communities. These results suggest that invasion by A. triuncialis and E. caput-medusae may significantly alter the capacity for soil microbial communities to nitrify or denitrify, and by extension alter soil N availability and rates of N transformations during invasion of remnant native-dominated sites.     

放牧对草地土壤氮素循环关键过程的影响与机制研究进展

放牧是人类管理利用草地生态系统的最主要途径之一.食草动物的采食、践踏、卧息和排泄物归还等干扰不仅会改变草地地上植物群落,也会对土壤养分循环产生显著的影响.随着人类活动的加剧,放牧强度和频率也在逐渐增加,从而对草地土壤氮素循环关键过程产生重要影响.放牧主要通过改变土壤的物理性质、土壤氮库以及微生物的组成和结构,进而影响氮素在土壤中的迁移与转化.适度放牧会促进土壤氮素的矿化过程和硝化过程,加快氮素的周转,有利于植物吸收可利用氮素,而对于反硝化的影响与草地的水热条件和土壤类型等密切相关.目前,关于放牧强度对土壤氮素循环关键过程影响的研究结果不一致,其影响机制尚不明晰,尤其对于不同类型的草原仍存在较大的差异.本研究在大量查阅国内外已有研究结果的基础上,论述了放牧对土壤氮素循环关键过程的影响效应,总结了放牧对土壤氮素循环的影响机制,指出了目前研究过程中存在的不足,并对未来研究中值得重点关注和深入研究的科学问题进行了探讨与展望,为进一步理解放牧对草地土壤氮素循环的影响提供参考.     

Soil Environmental Conditions and Microbial Build-Up Mediate the Effect of Plant Diversity on Soil Nitrifying and Denitrifying Enzyme Activities in Temperate Grasslands

Random reductions in plant diversity can affect ecosystem functioning, but it is still unclear which components of plant diversity (species number – namely richness, presence of particular plant functional groups, or particular combinations of these) and associated biotic and abiotic drivers explain the observed relationships, particularly for soil processes. We assembled grassland communities including 1 to 16 plant species with a factorial separation of the effects of richness and functional group composition to analyze how plant diversity components influence soil nitrifying and denitrifying enzyme activities (NEA and DEA, respectively), the abundance of nitrifiers (bacterial and archaeal amoA gene number) and denitrifiers (nirK, nirS and nosZ gene number), and key soil environmental conditions. Plant diversity effects were largely due to differences in functional group composition between communities of identical richness (number of sown species), though richness also had an effect per se. NEA was positively related to the percentage of legumes in terms of sown species number, the additional effect of richness at any given legume percentage being negative. DEA was higher in plots with legumes, decreased with increasing percentage of grasses, and increased with richness. No correlation was observed between DEA and denitrifier abundance. NEA increased with the abundance of ammonia oxidizing bacteria. The effect of richness on NEA was entirely due to the build-up of nitrifying organisms, while legume effect was partly linked to modified ammonium availability and nitrifier abundance. Richness effect on DEA was entirely due to changes in soil moisture, while the effects of legumes and grasses were partly due to modified nitrate availability, which influenced the specific activity of denitrifiers. These results suggest that plant diversity-induced changes in microbial specific activity are important for facultative activities such as denitrification, whereas changes in microbial abundance play a major role for non-facultative activities such as nitrification.     

Characterization of microbial community in an aerobic moving bed biofilm reactor applied for simultaneous nitrification and denitrification

A continuous-flow moving bed biofilm reactor (MBBR) under aerobic conditions was established for simultaneous nitrification and denitrification (SND), and microbial communities were investigated by a combination of denaturing gel gradient electrophoresis (DGGE) and fluorescence in situ hybridization (FISH). DGGE analysis has revealed more similar microbial community structures formed in the biofilms with more similar carbon nitrogen (C/N) ratios. FISH analysis shows that the dominance of both Betaproteobacteria ammonia-oxidizing bacteria and Nitrospira-like nitrite-oxidizing bacteria were negatively correlated to C/N ratios. Sequence analysis of DGGE bands has indicated the presence of anoxic denitrifying bacteria Agrobacterium tumefaciens and Rhizobium sp., suggesting that the oxygen gradient inside the biofilm may be responsible for the mechanism of SND in aerobic MBBRs. The study confirms that appropriate control of microbial community structure resulting from optimal C/N ratio is beneficial in improving SND, thus optimizing nitrogen removal in aerobic MBBR. The established SND-based MBBR can save operation space and time in comparison to the traditional nitrogen removal process, and might be very attractive for future practical applications.     

Ungulate stimulation of nitrogen cycling and retention in Yellowstone Park grasslands

We studied how ungulates and a large variation in site conditions influenced grassland nitrogen (N) dynamics in Yellowstone National Park. In contrast to most grassland N studies that have examined one or two soil N processes, we investigated four rates, net N mineralization, nitrification, denitrification, and inorganic N leaching, at seven paired sites inside and outside long-term (33+ year) exclosures. Our focus was how N fluxes were related to one another among highly variable grasslands and how grazers influenced those relationships. In addition, we examined variation in soil δ¹⁵N among grasslands and the relationships between soil ¹⁵N abundance and N processes. Previously, ungulates were reported to facilitate net N mineralization across variable Yellowstone grasslands and denitrification at mesic sites. In this study, we found that herbivores also promoted nitrification among diverse grasslands. Furthermore, net N mineralization, nitrification, and denitrification (kg N ha^–1 year^–1, each variable) were postively and linearly related to one another among all grasslands (grazed and fenced), and grazers reduced the nitrification/net N mineralization and denitrification/net N mineralization ratios, indicating that ungulates inhibited the proportion of available NH₄ ⁺ that was nitrified and denitrified. There was no relationship between net N mineralization or nitrification with leaching (indexed by inorganic N adsorbed to resin buried at the bottom of rooting zones) and leaching was unaffected by grazers. Soil δ¹⁵N was positively and linearly related to in situ net N mineralization and nitrification in ungrazed grasslands; however, there was no relationship between isotopic composition of N and those rates among grazed grasslands. The results suggested that grazers simultaneously increased N availability (stimulated net N mineralization and nitrification per unit area) and N conservation (reduced N loss from the soil per unit net N mineralization) in Yellowstone grasslands. Grazers promoted N retention by stimulating microbial productivity, probably caused by herbivores promoting labile soil C. Process-level evidence for N retention by grazers was supported by soil δ¹⁵N data. Grazed grassland with high rates of N cycling had substantially lower soil δ¹⁵N relative to values expected for ungrazed grassland with comparable net N mineralization and nitrification rates. These soil ¹⁵N results suggest that ungulates inhibited N loss at those sites. Such documented evidence for consumer control of N availability to plants, microbial productivity, and N retention in Yellowstone Park is further testimony for the widespread regulation of grassland processes by large herbivores. Received: 5 May 1999 / Accepted: 1 November 1999     

Effect of water and nitrogen additions on free-living nitrogen fixer populations in desert grass root zones.

In this study we measured changes in population levels of free-living N2-fixing bacteria in the root zones of potted Bouteloua eriopoda and Sporobolus flexuosus plants as well as the photosynthetic indices of the plants in response to added nitrogen, added water, and added water plus nitrogen treatments. In addition, N2 fixer population changes in response to added carbon source and nitrogen were measured in plant-free soil columns. There were significant increases in the numbers of N2 fixers associated with both plant species in the water and the water plus nitrogen treatments. Both treatments increased the photosynthetic index, suggesting that plant exudates were driving N2 fixer population changes. Population increases were greatest in the water plus nitrogen treatments, indicating that added nitrogen was synergistic with added water and suggesting that nitrogen addition spared bacteria the metabolic cost of N2 fixation, allowing greater reproduction. Plant-free column studies demonstrated a synergistic carbon-nitrogen effect when carbon levels were limiting (low malate addition) but not when carbon was abundant (high malate), further supporting this hypothesis. The results of this study indicate the presence of N2 fixer populations which interact with plants and which may play a role in the nitrogen balance of desert grasslands.     

Abundance, diversity and functional gene expression of denitrifier communities in adjacent riparian and agricultural zones

Lands under riparian and agricultural management differ in soil properties, water content, plant species and nutrient content and are therefore expected to influence denitrifier communities, denitrification and nitrous oxide (N(2) O) emissions. Denitrifier community abundance, denitrifier community structure, denitrification gene expression and activity were quantified on three dates in a maize field and adjacent riparian zone. N(2) O emissions were greater in the agricultural zone, whereas complete denitrification to N(2) was greater in the riparian zone. In general, the targeted denitrifier community abundance did not change between agricultural and riparian zones. However, nosZ gene expression was greater in the riparian zone than the agricultural zone. The community structure of nirS-gene-bearing denitrifiers differed in June only, whereas the nirK-gene-bearing community structure differed significantly between the riparian and the agricultural zones at all dates. The nirK-gene-bearing community structure was correlated with soil pH, while no significant correlations were found between nirS-gene-bearing community structure and soil environmental variables or N(2) O emissions, denitrification or denitrifier enzyme activity. The results suggested for the nirK and nirS-gene-bearing communities different factors control abundance vs. community structure. The nirK-gene-bearing community structure was also more responsive than the nirS-gene-bearing community structure to change between the two ecosystems.     

Linkages between N turnover and plant community structure in a tundra landscape

The spatial distribution of organic soil nitrogen (N) in alpine tundra was studied along a natural environmental gradient, covering five plant communities, at the Latnjajaure Field Station, northern Swedish Lapland. The five communities (mesic meadow, meadow snowbed, dry heath, mesic heath, and heath snowbed) are the dominant types in this region and are differentiated by soil pH. Net N mineralization, net ammonification, and net nitrification were measured using 40-day laboratory incubations based on extractable NH₄⁺ and NO₃⁻. Nitrification enzyme activity (NEA), denitrification enzyme activity (DEA), amino acid concentrations, and microbial respiration were measured for soils from each plant community. The results show that net N mineralization rates were more than three times higher in the meadow ecosystems (mesic meadow 0.7 μg N g⁻¹ OM day⁻¹ and meadow snowbed 0.6 μg N g⁻¹ OM day⁻¹) than the heath ecosystems (dry heath 0.2 μg N g⁻¹ OM day⁻¹, mesic heath 0.1 μg N g⁻¹ OM day⁻¹ and heath snowbed 0.2 μg N g⁻¹ OM day⁻¹). The net N mineralization rates were negatively correlated to organic soil C/N ratio (r = −0.652, P < 0.001) and positively correlated to soil pH (r = 0.701, P < 0.001). Net nitrification, inorganic N concentrations, and NEA rates also differed between plant communities; the values for the mesic meadow were at least four times higher than the other plant communities, and the snowbeds formed an intermediate group. Moreover, the results show a different pattern of distribution for individual amino acids across the plant communities, with snowbeds tending to have the highest amino acid N concentrations. The differences between plant communities along this natural gradient also illustrate variations between the dominant mycorrhizal associations in facilitating N capture by the characteristic functional groups of plants. Responsible Editor: Bernard Nicolardot     

Importance of sediment deposition and denitrification for nutrient retention in floodplain wetlands

Questions: Various floodplain communities may differ in their relative abilities to influence water quality through nutrient retention and denitrification. Our main questions were: (1) what is the importance of sediment deposition and denitrification for plant productivity and nutrient retention in floodplains; (2) will rehabilitation of natural floodplain communities (semi‐natural grassland, reedbed, woodland, pond) from agricultural grassland affect nutrient retention? Location: Floodplains of two Rhine distributaries (rivers Ussel and Waal), The Netherlands. Methods: Net sedimentation was measured using mats, denitrification in soil cores by acetylene inhibition and bio‐mass production by clipping above‐ground vegetation in winter and summer. Results: Sediment deposition was a major source of N and P in all floodplain communities. Highest deposition rates were found where water velocity was reduced by vegetation structure (reedbeds) or by a drop in surface elevation (pond). Sediment deposition was not higher in woodlands than in grassland types. Denitrification rates were low in winter but significantly higher in summer. Highest denitrification rates were found in an agricultural grassland (winter and summer) and in the ponds (summer). Plant productivity and nutrient uptake were high in reedbeds, intermediate in agricultural grasslands, ponds and semi‐natural grasslands and very low in woodlands (only understorey). All wetlands were N‐limited, which could be explained by low N:P ratios in sediment. Conclusions: Considering Rhine water quality: only substantial P‐retention is expected because, relative to the annual nutrient loads in the river, the floodplains are important sinks for P, but much less for N. Rehabilitation of agricultural grasslands into ponds or reedbeds will probably be more beneficial for downstream water quality (lower P‐concentrations) than into woodlands or semi‐natural grasslands.     

Niche construction by the invasive Asian knotweeds (species complex <Emphasis Type="Italic">Fallopia</Emphasis>): impact on activity,abundance and community structure of denitrifiers and nitrifiers

Big Asian knotweeds (Fallopia spp.) are among the most invasive plant species in north-western Europe. We suggest that their success is partially explained by biological and chemical niche construction. In this paper, we explored the microbial mechanisms by which the plant modifies the nitrogen cycle. We found that Fallopia spp. decreased potential denitrification enzyme activity (DEA) by reducing soil moisture and reducing denitrifying bacteria density in the soil. The plant also reduced potential ammonia and nitrite oxydizing bacteria enzyme activities (respectively, AOEA and NOEA) in sites with high AOEA and NOEA in uninvaded situation. Modification of AOEA and NOEA were not correlated to modifications of the density of implicated bacteria. AOB and Nitrobacter-like NOB community genetic structures were significantly different in respectively two and three of the four tested sites while the genetic structure of denitrifying bacteria was not affected by invasion in none of the tested sites. Modification of nitrification and denitrification functioning in invaded soils could lead to reduced nitrogen loss from the ecosystem through nitrate leaching or volatilization of nitrous oxides and dinitrogen and could be considered as a niche construction mechanism of Fallopia.     

Comparison of soil microbial communities from two distinct karst areas in Hungary

Karst areas belong to the most exposed terrestrial ecosystems, therefore their study have a priority task in Hungary, as well. The aim of this study was to compare the structure, activity and diversity of soil microbial communities from two distinct Hungarian karst areas (Aggtelek NP and Tapolca-basin). Soil samples were taken three times from 6 distinct sites, from different depths. Soil microbial biomass C (MBC), microbial biomass N (MBN), basal respiration (BRESP) and substrate induced respiration (SIR) were measured. The phylogenetic diversity of bacterial communities was compared by Denaturing Gradient Gel Electrophoresis (DGGE). The highest MBC, MBN, BRESP and SIR values were measured in the rendzina soil from Aggtelek. On the basis of biomass and respiration measurements, microbial communities differentiated mainly according to soil depths whereas DGGE profiles of bacterial communities resulted in groups mainly according to sampling sites.     

Maintenance of soil functioning following erosion of microbial diversity

The paradigm that soil microbial communities, being very diverse, have high functional redundancy levels, so that erosion of microbial diversity is less important for ecosystem functioning than erosion of plant or animal diversity, is often taken for granted. However, this has only been demonstrated for decomposition/respiration functions, performed by a large proportion of the total microbial community, but not for specialized microbial groups. Here, we determined the impact of a decrease in soil microbial diversity on soil ecosystem processes using a removal approach, in which less abundant species were removed preferentially. This was achieved by inoculation of sterile soil microcosms with serial dilutions of a suspension obtained from the same non-sterile soil and subsequent incubation, to enable recovery of community size. The sensitivity to diversity erosion was evaluated for three microbial functional groups with known contrasting taxonomic diversities (ammonia oxidizers < denitrifiers < heterotrophs). Diversity erosion within each functional group was characterized using molecular fingerprinting techniques: ribosomal intergenic spacer analysis (RISA) for the eubacterial community, denaturing gradient gel electrophoresis (DGGE) analysis of nirK genes for denitrifiers, and DGGE analysis of 16S rRNA genes for betaproteobacterial ammonia oxidizers. In addition, we simulated the impact of the removal approach by dilution on the number of soil bacterial species remaining in the inoculum using values of abundance distribution of bacterial species reported in the literature. The reduction of the diversity of the functional groups observed from genetic fingerprints did not impair the associated functioning of these groups, i.e. carbon mineralization, denitrification and nitrification. This was remarkable, because the amplitude of diversity erosion generated by the dilution approach was huge (level of bacterial species loss was estimated to be around 99.99% for the highest dilution). Our results demonstrate that the vast diversity of the soil microbiota makes soil ecosystem functioning largely insensitive to biodiversity erosion even for functions performed by specialized groups.     

Impacts of nitrogen addition on plant biodiversity in mountain grasslands depend on dose,application duration and climate: a systematic review

Although the influence of nitrogen (N) addition on grassland plant communities has been widely studied, it is still unclear whether observed patterns and underlying mechanisms are constant across biomes. In this systematic review, we use meta‐analysis and metaregression to investigate the influence of N addition (here referring mostly to fertilization) upon the biodiversity of temperate mountain grasslands (including montane, subalpine and alpine zones). Forty‐two studies met our criteria of inclusion, resulting in 134 measures of effect size. The main general responses of mountain grasslands to N addition were increases in phytomass and reductions in plant species richness, as observed in lowland grasslands. More specifically, the analysis reveals that negative effects on species richness were exacerbated by dose (ha^?1 year^?1) and duration of N application (years) in an additive manner. Thus, sustained application of low to moderate levels of N over time had effects similar to short‐term application of high N doses. The climatic context also played an important role: the overall effects of N addition on plant species richness and diversity (Shannon index) were less pronounced in mountain grasslands experiencing cool rather than warm summers. Furthermore, the relative negative effect of N addition on species richness was more pronounced in managed communities and was strongly negatively related to N‐induced increases in phytomass, that is the greater the phytomass response to N addition, the greater the decline in richness. Altogether, this review not only establishes that plant biodiversity of mountain grasslands is negatively affected by N addition, but also demonstrates that several local management and abiotic factors interact with N addition to drive plant community changes. This synthesis yields essential information for a more sustainable management of mountain grasslands, emphasizing the importance of preserving and restoring grasslands with both low agricultural N application and limited exposure to N atmospheric deposition.     

Patterns of diversity in plant and soil microbial communities along a productivity gradient in a Michigan old-field

The relationship between plant diversity and productivity has received much attention in ecology, but the relationship of these factors to soil microbial communities has been little explored. The carbon resources that support soil microbial communities are primarily derived from plants, so it is likely that the soil microbial community should respond to changes in plant diversity or productivity, particularly if the plant community affects the quality or quantity of available carbon. We investigated the relationship of plant diversity and productivity to the composition of the soil microbial community along a topographic gradient in a mid-successional old-field in southwestern Michigan. Soil moisture, soil inorganic N, and plant biomass increased from the top to the base of the slope, while light at ground level decreased along this same gradient. We characterized the changes in resource levels along this gradient using an index of productivity that incorporated light levels, soil N, soil moisture, and plant biomass. Average plant species richness declined with this productivity index and there were associated compositional changes in the plant community along the gradient. The plant community shifted from predominantly low-growing perennial forbs at low productivities to perennial grasses at higher productivities. Although there was variation in the structure of the soil microbial community [as indicated by fatty acid methyl ester (FAME) profiles], changes in the composition of the soil microbial community were not correlated with plant productivity or diversity. However, microbial activity [as indicated by Biolog average well color development and substrate-induced respiration (SIR)] was positively correlated with plant productivity. The similarity between patterns of plant biomass and soil microbial activity suggests that either plant productivity is driving microbial productivity or that limiting resources for each of these two communities co-vary.     

Impact of seasonal variations and nutrient inputs on nitrogen cycling and degradation of hexadecane by replicated river biofilms

Biofilm communities cultivated in rotating annular bioreactors using water from the South Saskatchewan River were assessed for the effects of seasonal variations and nutrient (C, N, and P) additions. Confocal laser microscopy revealed that while control biofilms were consistently dominated by bacterial biomass, the addition of nutrients shifted biofilms of summer and fall water samples to phototrophic-dominated communities. In nutrient-amended biofilms, similar patterns of nitrification, denitrification, and hexadecane mineralization rates were observed for winter and spring biofilms; fall biofilms had the highest rates of nitrification and hexadecane mineralization, and summer biofilms had the highest rates of denitrification. Very low rates of all measured activities were detected in control biofilms (without nutrient addition) regardless of season. Nutrient addition caused large increases in hexadecane mineralization and denitrification rates but only modest increases, if any, in nitrification rates, depending upon the season. Generally, both alkB and nirK were more readily PCR amplified from nutrient-amended biofilms. Both genes were amplified from all samples except for nirK from the fall control biofilm. It appears that bacterial production in the South Saskatchewan River water is limited by the availability of nutrients and that biofilm activities and composition vary with nutrient availability and time of year.