Export of nitrogenous compounds due to incomplete cycling within biological soil crusts of arid lands

Second only to water among limiting factors, nitrogen controls the fertility of most arid regions. Where dry and wet depositions are weak, as in the western US deserts, N inputs rely heavily on biological N(2) fixation. Topsoil cyanobacterial communities known as biological soil crusts (BSCs) are major N(2) fixation hot spots in arid lands, but the fate of their fixed N remains controversial. Using a combination of microscale and mesoscale process rate determinations, we found that, in spite of theoretically optimal conditions, denitrification rates in BSCs were paradoxically immaterial for nitrogen cycling. Denitrifier populations within BSCs were extremely low. Because of this absence of denitrification, and because of the limitation of respiration and ammonia oxidation by diffusive O(2) supply, we could demonstrate that BSCs function as net exporters of ammonium, nitrate and organic N to the soils they cover, in approximately stoichiometrically equal proportions. Overall export rates during periods of biological activity are in the range of tens to hundreds of mumol-N m(-2) h(-1), commensurate with those of N(2) fixation. These results explain the long-term dependence of BSCs on N(2) fixation, confirm their role in landscape fertility, and provide a robust argument for conservation of these endangered communities.     

Non-cyanobacterial diazotrophs mediate dinitrogen fixation in biological soil crusts during early crust formation

Biological soil crusts (BSCs) are key components of ecosystem productivity in arid lands and they cover a substantial fraction of the terrestrial surface. In particular, BSC N₂-fixation contributes significantly to the nitrogen (N) budget of arid land ecosystems. In mature crusts, N₂-fixation is largely attributed to heterocystous cyanobacteria; however, early successional crusts possess few N₂-fixing cyanobacteria and this suggests that microorganisms other than cyanobacteria mediate N₂-fixation during the critical early stages of BSC development. DNA stable isotope probing with ¹⁵N₂ revealed that Clostridiaceae and Proteobacteria are the most common microorganisms that assimilate ¹⁵N₂ in early successional crusts. The Clostridiaceae identified are divergent from previously characterized isolates, though N₂-fixation has previously been observed in this family. The Proteobacteria identified share >98.5% small subunit rRNA gene sequence identity with isolates from genera known to possess diazotrophs (for example, Pseudomonas, Klebsiella, Shigella and Ideonella). The low abundance of these heterotrophic diazotrophs in BSCs may explain why they have not been characterized previously. Diazotrophs have a critical role in BSC formation and characterization of these organisms represents a crucial step towards understanding how anthropogenic change will affect the formation and ecological function of BSCs in arid ecosystems.     

Relevance of ammonium oxidation within biological soil crust communities

Thin, vertically structured topsoil communities that become ecologically important in arid regions (biological soil crusts or BSCs) are responsible for much of the nitrogen inputs into pristine arid lands. We studied N(2) fixation and ammonium oxidation (AO) at subcentimetre resolution within BSCs from the Colorado Plateau. Pools of dissolved porewater nitrate/nitrite, ammonium and organic nitrogen in wetted BSCs were high in comparison with those typical of aridosoils. They remained stable during incubations, indicating that input and output processes were of similar magnitude. Areal N(2) fixation rates (6.5-48 micromol C(2)H(2) m(-2) h(-1)) were high, the vertical distribution of N(2) fixation peaking close to the surface if populations of heterocystous cyanobacteria were present, but in the subsurface if they were absent. Areal AO rates (19-46 micromol N m(-2) h(-1)) were commensurate with N(2) fixation inputs. When considering oxygen availability, AO activity invariably peaked 2-3 mm deep and was limited by oxygen (not ammonium) supply. Most probable number (MPN)-enumerated ammonia-oxidizing bacteria (6.7-7.9 x 10(3) cells g(-1) on average) clearly peaked at 2-3 mm depth. Thus, AO (hence nitrification) is a spatially restricted but important process in the nitrogen cycling of BSC, turning much of the biologically fixed nitrogen into oxidized forms, the fate of which remains to be determined.     

Effect of biological soil crusts on soil elemental concentrations: implications for biogeochemistry and as traceable biosignatures of ancient life on land

Biological soil crusts (BSCs) are topsoil biosedimentary structures built by photosynthetic microbes commonly found today on arid soils. They play a role in soil stabilization and the fertility of arid lands, and are considered modern analogues of ancient terrestrial microbial communities. We determined the concentrations of four biogenic and 21 other elements, mostly metals, in surface soils that hosted BSCs, in the soils underneath those crusts, and in proximate but non-crusted surface soils. The samples were from six sites in the Colorado Plateau highlands and the Sonoran Desert lowlands. In spite of the variability in climate and geologic setting, we found statistically significant overall trends of enrichment in biogenic elements and depletion in non-biogenic elements when BSCs were compared with non-crusted soils. The differences between crusted and non-crusted soils were statistically significant at ∼95% confidence for C, N (enrichments) and for Ca, Cr, Mn, Cu, Zn, As, and Zr (depletions). These trends are best explained by the activity of microbes. As expected, no differences in the concentrations of C, N, P, and S were detected between the soils underneath the crusts and the non-crusted soils, but the former showed depletion of non-biogenic elements, indicating that the leaching effect of crust microbes extends downward in the soil. These patterns speak to the need for a sustained input of allochthonous material, possibly dust, to maintain BSC fertility. These elemental patterns can be considered a biosignature that may be preserved in the rock record and might help identify ancient microbial communities on land.     

Patterns of diversity for fungal assemblages of biological soil crusts from the southwestern United States

Molecular methodologies were used to investigate fungal assemblages of biological soil crusts (BSCs) from arid lands in the southwestern United States. Fungal diversity of BSCs was assessed in a broad survey that included the Chihuahuan and Sonoran deserts as well as the Colorado Plateau. At selected sites samples were collected along kilometer-scale transects, and fungal community diversity and composition were assessed based on community rRNA gene fingerprinting using PCR-denaturing gradient gel electrophoresis (DGGE). Individual phylotypes were characterized through band sequencing. The results indicate that a considerable diversity of fungi is present within crusted soils, with higher diversity being recovered from more successionally mature BSCs. The overwhelming majority of crust fungi belong to the Ascomycota, with the Pleosporales being widespread and frequently dominant. Beta diversity patterns of phylotypes putatively representing dominant members of BSC fungal communities suggest that these assemblages are specific to their respective geographic regions of origin.     

Soil microbial carbon and nitrogen transformations at a glacial foreland on Anvers Island, Antarctic Peninsula

Microbial communities can play a critical role in soil development and succession at glacial forelands through their contribution to soil carbon (C) and nitrogen (N) cycling. Using a combination of molecular fingerprinting techniques and metabolic rate measurements, we examined the soil microbial community composition and key transformations in the C and N cycles at a glacial foreland on Anvers Island along the Antarctic Peninsula. Soils were sampled along transects representing a chronosequence of <1 to approximately 10?years since deglaciation. The soil microbial community was active adjacent to the receding edge of the glacier, where soil had been ice-free for <1?year. A survey of the microbial community composition identified typical soil bacterial species such as Arthrobacter and Sphingomonas, as well as known Antarctic heterotrophs, cyanobacteria and fungi. The soil C cycle over this zone was dominated by phototrophic microbial activity, while the N cycle was dominated by heterotrophic N₂-fixation and not cyanobacterial N₂-fixation as found at other recently deglaciated forelands. Other N transformations such as ammonia oxidation and denitrification appeared to be of limited relevance.     

Niche-based assessment of contributions of legumes to the nitrogen economy of Western Kenya smallholder farms

Nitrogen (N) deficiency is a major constraint to the productivity of the African smallholder farming systems. Grain, green manure and forage legumes have the potential to improve the soil N fertility of smallholder farming systems through biological N₂-fixation. The N₂-fixation of bean (Phaseolus vulgaris), soyabean (Glycine max), groundnut (Arachis hypogaea), Lima bean (Phaseolus lunatus), lablab (Lablab purpureus), velvet bean (Mucuna pruriens), crotalaria (Crotalaria ochroleuca), jackbean (Canavalia ensiformis), desmodium (Desmodium uncinatum), stylo (Stylosanthes guianensis) and siratro (Macroptilium atropurpureum) was assessed using the ¹⁵N natural abundance method. The experiments were conducted at three sites in western Kenya, selected on an agro-ecological zone (AEZ) gradient defined by rainfall. On a relative scale, Museno represents high potential AEZ 1, Majengo medium potential AEZ 2 and Ndori low potential AEZ 3. Rainfall in the year of experimentation was highest in AEZ 2, followed by AEZ 1 and AEZ 3. Experimental fields were classified into high, medium and low fertility classes, to assess the influence of soil fertility on N₂-fixation performance. The legumes were planted with triple super phosphate (TSP) at 30 kg P ha^?1, with an extra soyabean plot planted without TSP (soyabean-P), to assess response to P, and no artificial inoculation was done. Legume grain yield, shoot N accumulation, %N derived from N₂-fixation, N₂-fixation and net N inputs differed significantly (P<0.01) with rainfall and soil fertility. Mean grain yield ranged from 0.86 Mg ha^?1, in AEZ 2, to 0.30 Mg ha^?1, in AEZ 3, and from 0.78 Mg ha^?1, in the high fertility field, to 0.48 Mg ha^?1, in the low fertility field. Shoot N accumulation ranged from a maximum of 486 kg N ha^?1 in AEZ 2, to a minimum of 10 kg N ha^?1 in AEZ 3. Based on shoot biomass estimates, the species fixed 25–90% of their N requirements in AEZ 2, 23–90% in AEZ 1, and 7–77% in AEZ 3. Mean N₂-fixation by green manure legumes ranged from 319 kg ha^?1 (velvet bean) in AEZ 2 to 29 kg ha^?1 (jackbean) in AEZ 3. For the forage legumes, mean N₂-fixation ranged from 97 kg N ha^?1 for desmodium in AEZ 2 to 39 kg N ha^?1 for siratro in AEZ 3, while for the grain legumes, the range was from 172 kg N ha^?1 for lablab in AEZ 1 to 3 kg N ha^?1 for soyabean-P in AEZ 3. Lablab and groundnut showed consistently greater N₂-fixation and net N inputs across agro-ecological and soil fertility gradients. The use of maize as reference crop resulted in lower N₂-fixation values than when broad-leaved weed plants were used. The results demonstrate differential contributions of the green manure, forage and grain legume species to soil fertility improvement in different biophysical niches in smallholder farming systems and suggest that appropriate selection is needed to match species with the niches and farmers’ needs.     

黄土高原生物结皮覆盖对风沙土和黄绵土溶质运移的影响

干旱和半干旱地区生物结皮的普遍发育显著改变了表层土壤的结构与养分富集特征,但其对土壤养分迁移和淋失的影响目前尚不明确。本研究针对黄土高原风沙土和黄绵土上发育的藓类生物结皮,以Ca²⁺和Cl^-为示踪离子开展溶质穿透试验,对有无生物结皮层及其覆盖下不同深度土壤的溶质运移特征进行了研究。结果表明: 在0～5 cm土层,生物结皮覆盖延缓了风沙土和黄绵土的溶质穿透过程,其Cl^-的穿透时间比无结皮延长了3.83(风沙土)和2.09倍(黄绵土),而Ca²⁺则分别延长了2.50和2.73倍。生物结皮覆盖条件下,表层0～5 cm土壤溶质完全穿透所对应的孔隙体积数比下层5～10 cm土壤更高,且其穿透历时更长;其中,Cl^-的穿透时间分别增加了67.3%(风沙土)和51.8%(黄绵土),Ca²⁺的穿透时间分别增加了8.0%和33.7%。生物结皮覆盖降低了土壤孔隙水流速(37.5%～70.2%);除风沙土的5～10 cm土层外,生物结皮使溶质弥散系数提高了1.73～6.29倍,使溶质弥散度提高了2.77～20.95倍。置换液完全穿透土壤后,风沙土和黄绵土的生物结皮层Ca²⁺含量显著高于无结皮,其分别比无结皮高4.14和2.58倍。研究证实,生物结皮覆盖能够提高表层土壤对养分的吸附与固持能力,从而减少土壤表聚养分的深层渗漏和流失,对干旱和半干旱地区退化土壤的肥力提升与植被恢复具有重要意义。     

The effects of snowpack properties and plant strategies on litter decomposition during winter in subalpine meadows

Background and aims

Approximately 50 % of belowground organic carbon is present in the northern permafrost region and due to changes in climate there are concerns that this carbon will be rapidly released to the atmosphere. The release of carbon in arctic soils is thought to be intimately linked to the N cycle through the N cycle’s influence on microbial activity. The majority of new N input into arctic systems occurs through N₂-fixation; therefore, N₂-fixation may be the key driver of greenhouse gases from these ecosystems.

Methods

At Alexandra Fjord lowland, Ellesmere Island, Canada concurrent measurements of N₂-fixation, N mineralization and nitrification rates, dissolved organic soil N (DON) and C, inorganic soil N and surface greenhouse gas fluxes (CO₂, N₂O and CH₄) were taken in two ecosystem types (Wet Sedge Meadow and Dryas Heath) over the 2009 growing season (June-August). Using Structural Equation Modelling we evaluated the hypothesis that CO₂, CH₄ and N₂O flux are linked to N₂-fixation via the N cycle.

Results

The soil N cycle was linked to CO₂ flux in the Dryas Heath ecosystem via DON concentrations, but there was no link between the soil N cycle and CO₂ flux in the Wet Sedge Meadow. Methane flux was also not linked to the soil N cycle, nor surface soil temperature or moisture in either ecosystem. The soil N cycle was closely linked to N₂O emissions but via nitrification in the Wet Sedge Meadow and inorganic N in the Dryas Heath, indicating the important role of nitrification in net N₂O flux from arctic ecosystems.

Conclusions

Our results should be interpreted with caution given the high variability in both the rates of the N cycling processes and greenhouse gas flux found in both ecosystems over the growing season. However, while N₂-fixation and other N cycling processes may play a more limited role in instantaneous CO₂ emissions, these processes clearly play an important role in controlling N₂O emissions.     

N2-fixation by methanotrophs sustains carbon and nitrogen accumulation in pristine peatlands

Symbiotic relationships between N₂-fixing prokaryotes and their autotrophic hosts are essential in nitrogen (N)-limited ecosystems, yet the importance of this association in pristine boreal peatlands, which store 25 % of the world’s soil (C), has been overlooked. External inputs of N to bogs are predominantly atmospheric, and given that regions of boreal Canada anchor some of the lowest rates found globally (~1 kg N ha^?1 year^?1), biomass production is thought to be limited primarily by N. Despite historically low N deposition, we show that boreal bogs have accumulated approximately 12–25 times more N than can be explained by atmospheric inputs. Here we demonstrate high rates of biological N₂-fixation in prokaryotes associated with Sphagnum mosses that can fully account for the missing input of N needed to sustain high rates of C sequestration. Additionally, N amendment experiments in the field did not increase Sphagnum production, indicating that mosses are not limited by N. Lastly, by examining the composition and abundance of N₂-fixing prokaryotes by quantifying gene expression of 16S rRNA and nitrogenase-encoding nifH, we show that rates of N₂-fixation are driven by the substantial contribution from methanotrophs, and not from cyanobacteria. We conclude biological N₂-fixation drives high sequestration of C in pristine peatlands, and may play an important role in moderating fluxes of methane, one of the most important greenhouse gases produced in peatlands. Understanding the mechanistic controls on biological N₂-fixation is crucial for assessing the fate of peatland carbon stocks under scenarios of climate change and enhanced anthropogenic N deposition.     

基于CCA的坡面尺度生物结皮空间分布

生物结皮是干旱半干旱地区普遍存在的活地被物,在该区生态系统中具有重要的生态功能。探讨生物结皮的空间分布规律是科学管理该资源的理论基础。选择黄土高原水蚀风蚀交错区六道沟小流域内的典型坡面,通过全面调查并应用GS+和CANOCO统计软件进行分析,探讨了坡面尺度上生物结皮的空间分布特征及其影响因子。结果表明:(1)生物结皮的分布具有明显的空间分异性。沙土区生物结皮以大面积连续分布为主,平均结皮盖度在30%以上;在黄土区则以零星分布为主,结皮盖度大都在20%以下,主要分布在坡的边缘和末端。而生物结皮厚度和抗剪强度的空间变异不大,说明其主要与生物结皮的发育年限有关。(2)典范对应分析(CCA)表明:生物结皮的空间分布与土壤、植被、地表湿度、坡度坡向等有密切关系。其中,土壤类型和生物结皮的空间分布关系最大,可以解释生物结皮空间变异的20%。其次是植被群落类型和地形湿度指数,沙蒿(Artemisia desertorum Spreng.Syst.Veg.)地和小叶杨(Populus simonii Carr.)林地是其最理想的生长环境;其他如坡度、坡向、太阳辐射等也都对生物结皮的分布有一定的影响。总体上,生物结皮具有明显的地形、土壤和植被群落的选择性,偏向于生长在较湿润的沙生植被群落当中。     

Increases of Soil C, N, and P Pools Along an Acacia Tree Density Gradient and Their Effects on Trees and Grasses

Nitrogen (N) fixing trees including many species of Acacia are an important though variable component of savanna ecosystems. It is known that these trees enrich the soil with carbon (C) and N, but their effect on the combined C:N:P stoichiometry in soil is less well understood. Theory suggests that they might reduce available phosphorus (P), creating a shift from more N-limited conditions in grass-dominated to more P-limited conditions in tree-dominated sites, which in turn could feed back negatively on the trees’ capacity to fix N. We studied the effects of Acacia zanzibarica tree density upon soil and foliar N:P stoichiometry, and the N₂-fixation rates of trees and leguminous herbs in a humid Tanzanian savanna. Foliar N:P ratios and N₂-fixation rates of trees remained constant across the density gradient, whereas soil C, N and organic P pools increased. In contrast, the N:P ratio of grasses increased and N₂-fixation rates of leguminous herbs decreased with increasing tree density, indicating a shift towards more P-limited conditions for the understory vegetation. These contrasting responses suggest that trees and grasses have access to different sources of N and P, with trees being able to access P from deeper soil layers and perhaps also utilizing organic forms more efficiently.     

Nitrogen cycling in the soil–plant system along a precipitation gradient in the Kalahari sands

Nitrogen (N) cycling was analyzed in the Kalahari region of southern Africa, where a strong precipitation gradient (from 978 to 230 mm mean annual precipitation) is the main variable affecting vegetation. The region is underlain by a homogeneous soil substrate, the Kalahari sands, and provides the opportunity to analyze climate effects on nutrient cycling. Soil and plant N pools, ¹⁵N natural abundance (δ¹⁵N), and soil NO emissions were measured to indicate patterns of N cycling along a precipitation gradient. The importance of biogenic N₂ fixation associated with vascular plants was estimated with foliar δ¹⁵N and the basal area of leguminous plants. Soil and plant N was more ¹⁵N enriched in arid than in humid areas, and the relation was steeper in samples collected during wet than during dry years. This indicates a strong effect of annual precipitation variability on N cycling. Soil organic carbon and C/N decreased with aridity, and soil N was influenced by plant functional types. Biogenic N₂ fixation associated with vascular plants was more important in humid areas. Nitrogen fixation associated with trees and shrubs was almost absent in arid areas, even though Mimosoideae species dominate. Soil NO emissions increased with temperature and moisture and were therefore estimated to be lower in drier areas. The isotopic pattern observed in the Kalahari (¹⁵N enrichment with aridity) agrees with the lower soil organic matter, soil C/N, and N₂ fixation found in arid areas. However, the estimated NO emissions would cause an opposite pattern in δ¹⁵N, suggesting that other processes, such as internal recycling and ammonia volatilization, may also affect isotopic signatures. This study indicates that spatial, and mainly temporal, variability of precipitation play a key role on N cycling and isotopic signatures in the soil–plant system.     

Planktonic nitrogen fixation in Lake Malawi/Nyasa

Nitrogen (N₂) fixation has been identified as possibly an important source of “new” nitrogen (N) to the epilimnion of Lake Malawi but studies in 1999–2000 and 2002 (September–December) estimated that the contribution of N₂-fixation by heterocystous Anabaena filaments to the N budget of the lake’s epilimnion is only 3–4% of total N input. N₂-fixation rates in Lake Malawi, as estimated by the acetylene reduction assay were higher during the stratified season (September–March) than during the rest of the year. Planktonic N₂-fixation in Lake Malawi can be monitored by measuring heterocyst biovolume concentrations because a significant correlation (r ² = 0.945, P < 0.0001) was observed between the two parameters. Heterocyst density cannot be used to estimate N₂-fixation because heterocyst sizes in the lake change continuously as at least two Anabaena species with different heterocyst dimensions are present. During September–October 2002, a species similar to A. maxima, contained larger heterocysts (16.34 ± 2.46 and 13.25 ± 1.89 μm in cross and apical section dimensions, respectively). In November–December of 2002, A. discoidea dominated and had smaller sized heterocysts (8.92 ± 1.13 and 7.25 ± 0.95 μm in cross and apical section dimensions). Since planktonic N₂-fixation is higher near shore than offshore, its contribution to the N budget in the littoral zone where high densities of grazing benthic fish occur may be more critical to maintaining the high rates of benthic algal productivity observed.     

Bryophyte-cyanobacterial associations as a key factor in N2-fixation across the Canadian Arctic

Nitrogen inputs via biological N₂-fixation are important in arctic environments where N often limits plant productivity. An understanding of the direct and indirect theoretical causal relationships between key intercorrelated variables that drive the process of N₂-fixation is essential to understanding N input. An exploratory multi-group Structural Equation Modeling (SEM) approach was used to examine the direct and indirect effects of soil moisture, plant community functional composition, and bryophyte and lichen abundance on rates of nitrogen fixation at a low arctic ecosystem, two high arctic oases and a high arctic polar desert in the Canadian Arctic. Increasing soil moisture was strongly associated with an increasing presence of bryophytes and increasing bryophyte abundance was a major factor determining higher N₂-fixation rates at all sites. Shrubs had a negative effect on bryophyte abundance at all sites with the exception of the polar desert site at Alexandra Fjord highland. The importance of competition from vascular plants appears to be greater in more productive sites and may increase at lower latitudes. Moisture availability may have an indirect effect on ecosystem development by affecting N input into the system with bryophyte-cyanobacterial associations playing an important intermediary role in the process.     

BIOLOGICAL NITROGEN INFLUX IN AN OHIO RELICT PRAIRIE

The principal contributors of biologically fixed N in natural grassland ecosystems appear to be asymbiotic bacteria and heterocystous cyanobacteria. The environmental factors of light, moisture, and temperature play important roles in the magnitude of the N₂-fixation activity. Biological N₂-fixation was measured in the Elizabeth's Prairie section of the Lynx Prairie Preserve, Adams County, Ohio, during 15 site visits beginning 29 March through 8 November 1980. In situ N₂-fixation activity was measured using the acetylene-reduction technique. The percentage cover of cyanobacterial colonies (Nostoc sp.) was determined using Point-Frame Analysis. Soil and air temperatures and soil water potentials also were measured. Intact soil cores with a surface cover of Nostoc were collected and returned to the laboratory to quantify the effect of decreasing water potential on the N₂(C₂H₂)ase activity of Nostoc. The N₂(C₂H₂)ase activity of Nostoc on the intact soil cores displayed a linear response of approximately 10% decrease in N₂(C₂H₂)ase activity per one bar decrease in soil water potential. The cyanobacteria contributed almost all of the biologically fixed N at the site until late June. From late June through to mid September, heterotrophic diazotrophs played the major role in the N₂-fixation activity. These changes are attributed to fluctuations in Nostoc sp. colony cover, temperature, and soil water potentials. Extrapolation of the measured rates, and assuming an average of 10 hr per day of activity, Nostoc sp. is shown to have contributed 4.60 ± 1.17 kg N ha⁻¹ yr⁻¹. Heterotrophic diazotrophs contributed an estimated 3.19 ± 1.18 kg N ha⁻¹ yr⁻¹. The total biological N₂-fixation for the site was calculated at 8.2 ± 2.55 kg N ha⁻¹ yr⁻¹, from additional measurements which estimated total diazotrophic activity of the site. These rates of N₂-fixation are among the highest reported for temperate grassland habitats.     

Nitrogen fertilisation reduces grass-induced N2 fixation of tree seedlings from semi-arid savannas

Aims

Coexistence of trees and grasses in nutrient-poor arid savannas may result in competition for soil N. While grasses may be more effective than woody plants in acquiring N from the soil, some leguminous woody species rely on N₂ fixation. We assessed the role of N₂ fixation in the N-budget of Acacia mellifera seedlings by varying N supply and grass competition.

Methods

The contribution of N₂ fixation to the N-budget of Acacia mellifera seedlings with varying N supply and grass competition was determined by measuring growth, nutrient concentrations, and ¹⁵N values.

Results

Tree seedlings were 4-fold taller and had 20-fold more biomass in the absence of grass. Tree foliar δ¹⁵N was lower with (?0.25?±?0.2‰, n?=?9) than without grasses (5.2?±?0.1‰, n?=?64). The contribution of N₂-fixation to the N budget decreased with increasing N supply. Greater reliance on N₂-fixation by trees in the presence of grasses did not result in greater biomass accumulation or tissue [N] relative to tree seedlings grown without grass competition. Tree seedlings competing with grass had significantly more negative δ¹³C (?29.5?±?0.6‰) than seedlings without grass competition (?28.8‰?±?0.5‰).

Conclusions

Induction of N₂-fixation by grass may have resulted from competition for nutrients. N₂-fixation enables tree seedlings to compensate for limited soil N and survive grass competition at a critical and vulnerable developmental stage of germination and establishment.     

Short-term Nitrogen Fixation by Legume Seedlings and Resprouts After Fire in Mediterranean Old-fields

Fires may greatly alter the N budget of a plant community. During fire nitrogen is lost to the atmosphere. Although high light availability after fire promotes N₂-fixation, the presumably high soil N availability could limit N₂-fixation activity. The latter limitation might be particularly acute in legume seedlings compared with resprouts, which have immediate access to belowground stored carbon. We wished to learn whether early post-fire conditions were conducive to N₂-fixation in leguminous seedlings and resprouts in two types of grassland and in a shrubland and whether seedlings and resprouts incurred different amounts of N₂-fixation after fire. We set 18 experimental fires in early autumn on 6 plots, subsequently labelling 6 subplots (2 × 2 m²) in each community with ¹⁵NH₄⁺-N (99 atom % excess). For 9 post-fire months we measured net N mineralisation in the top 5 cm of soil and we calculated the fraction of legume N derived from the atmosphere (%Ndfa) in seedlings and resprouts. We used two independent estimates of the amounts of N derived from non-atmospheric sources in potentially N₂-fixing plants: mean soil pool abundance and the ¹⁵N-enrichment of non-legumes. Despite substantial soil net N mineralisation in all burned community types (about 2.6 g Nm⁻² during the first nine months after fire), the %Ndfa of various legume species was 52–99%. Legumes from both grasslands showed slightly higher N₂-fixation values than shrubland legumes. As grassland legumes grew in more belowground dense communities than shrubland legumes, we suggest that higher competition for soil resources in well established grass-resprouting communities may enhance the rate of N₂-fixation after fire. In contrast to our hypothesis, legume seedlings and resprouts from the three plant communities studied, had similar %Ndfa and apparently acquired most of their N from the atmosphere rather than from the soil.     

Spatial and temporal variation in moss-associated dinitrogen fixation in coniferous- and deciduous-dominated Alaskan boreal forests

Dominant canopy tree species have strong effects on the composition and function of understory species, particularly bryophytes. In boreal forests, bryophytes and their associated microbes are a primary source of ecosystem nitrogen (N) inputs, and an important process regulating ecosystem productivity. We investigated how feather moss-associated N₂-fixation rates and contribution to N budgets vary in time and space among coniferous and broadleaf deciduous forests. We measured N₂-fixation rates using stable isotope (¹⁵N₂) labeling in two moss species (Pleurozium schreberi and Hylocomium splendens) in broadleaf deciduous (Alaska paper birch—Betula neoalaskana) and coniferous (black spruce—Picea mariana) stands near Fairbanks, interior Alaska, from 2013 to 2015. N₂-fixation rates showed substantial inter-annual variation among the 3 years. High N₂-fixation was more strongly associated with high precipitation than air temperature or light availability. Overall, contribution of N₂-fixation to N budgets was greater in spruce than in birch stands. Our results enhance the knowledge of the processes that drive N₂-fixation in boreal forests, which is important for predicting ecosystem consequences of changing forest composition.     

Rhizobium-Legume Symbiosis and Nitrogen Fixation under Severe Conditions and in an Arid Climate

Biological N₂ fixation represents the major source of N input in agricultural soils including those in arid regions. The major N₂-fixing systems are the symbiotic systems, which can play a significant role in improving the fertility and productivity of low-N soils. The Rhizobium-legume symbioses have received most attention and have been examined extensively. The behavior of some N₂-fixing systems under severe environmental conditions such as salt stress, drought stress, acidity, alkalinity, nutrient deficiency, fertilizers, heavy metals, and pesticides is reviewed. These major stress factors suppress the growth and symbiotic characteristics of most rhizobia; however, several strains, distributed among various species of rhizobia, are tolerant to stress effects. Some strains of rhizobia form effective (N₂-fixing) symbioses with their host legumes under salt, heat, and acid stresses, and can sometimes do so under the effect of heavy metals. Reclamation and improvement of the fertility of arid lands by application of organic (manure and sewage sludge) and inorganic (synthetic) fertilizers are expensive and can be a source of pollution. The Rhizobium-legume (herb or tree) symbiosis is suggested to be the ideal solution to the improvement of soil fertility and the rehabilitation of arid lands and is an important direction for future research.