Nitrogen fixation varies spatially and seasonally in linked stream-lake ecosystems

We performed surveys of nitrogen (N₂)-fixation in three oligotrophic lake-stream systems in the Sawtooth Mountains of central Idaho to address two questions: (1) Which habitat types within linked lake-stream systems (lake pelagic, lake benthic, and stream) exhibit the highest rates of N₂ fixation?, and (2) How does N₂ fixation compare to the hydrologic flux of nitrogen? A seasonal survey showed that N₂ fixation in a single lake and its outlet stream peaked in late summer, when hydrologic N fluxes were lowest. Benthic lake N₂-fixation rates by epiphytes were highest at mid-lake depths, where their percent cover was highest, while rates by epipelon were greatest at shallow lake depths. Pelagic N₂ fixation was below detection. Stream N₂-fixation rates were greatest on rock substrates and in the lake outlet stream. These patterns were supported by a baseflow survey (late July) in three lake-stream ecosystems which confirmed that N₂-fixation rates peaked in the lake benthos at shallow depths and on rock substrates in outlet streams. Scaling N₂-fixation rates to whole lake and stream areas revealed that N₂ fixation could exceed the nitrate, and sometimes the total dissolved nitrogen flux during baseflow in lakes and outlet streams. Despite low rates, total N₂-fixation contributions (kg/day) from lakes were greater because they had far larger surface areas than the stream environments. Fixed nitrogen contributions from stream outlets were also relatively high because of high N₂-fixation rates and despite low surface areas. This study suggests that N₂ fixation could be a seasonally important nitrogen source to nutrient deficient subalpine lake-stream ecosystems. In addition, the frequency and location of lakes could control N₂-fixation contributions to watersheds by providing a large area for within-lake N₂ fixation, and creating conditions favorable for N₂ fixation in outlet streams.     

Selection for improved nitrogen fixation inGlycine max (L.) Merr. andPhaseolus vulgaris L.

Summary While symbiotic nitrogen (N₂) fixation byG. max andP. vulgaris reduces their need for combined N, N₂ fixation under field conditions is rarely maximized. This paper reviews constraints to N₂ fixation in these species, then examines the genetic variability recorded for traits affecting N₂ fixation and the further work needed in this area. It considers emerging programs for the improvement of N₂ fixation inG. max andP. vulgaris and pays particular attention to methodological considerations.Scientific Journal Series, Minnesota Agri. Exp. Station. No 14190.     

Delivery of N2 to bacteroids in simulated soybean nodule cells: consideration of gradients of concentration of dissolved N2 in cell walls,cytoplasm, and symbiosomes

Summary The previously published simulation of physiological functions occurring in infected cells of soybean nodules has been extended to include consideration of the diffusion of N₂ from the outside of a nodule to the nitrogen-fixing bacteroids, in relation to published values for the apparentK _m(N₂) for nitrogen fixation in the soybean nodule system. Nitrogen fixation is driven by bacteroid respiration, so increases in the average relative oxygenation (Y) of cytoplasmic leghaemoglobin lead to increased bacteroid respiration, increased nitrogen fixation, and greater differences in concentration of dissolved N₂ between the cell surface and the innermost bacteroids (d[N₂]). Over the range ofY considered, values for d[N₂] were from 5.2- to 6.2-fold greater than the corresponding values for d[O₂], because of facilitation of O₂ flux by cytoplasmic leghaemoglobin. Gradients of [N₂] within symbiosomes are small relative to cytoplasmic values and at the symbiosome surface [N₂] was greater than 0.4 mol/m³ at the greatest rates of nitrogen fixation calculated. Therefore, it is unlikely that values for [N₂] anywhere in the infected cell are low enough to affect rates of nitrogen fixation significantly, unless low external atmospheric N₂ pressures are used experimentally.Abbreviations Lb leghaemoglobin - LbO₂ oxyleghaemoglobin - [O₂], [N₂ concentrations of free, dissolved oxygen and nitrogen - Y fractional oxygenation of leghaemoglobin     

Isotopic analysis of cyanobacterial nitrogen fixation associated with subarctic lichen and bryophyte species

Dinitrogen fixation by cyanobacteria is of particular importance for the nutrient economy of cold biomes, constituting the main pathway for new N supplies to tundra ecosystems. It is prevalent in cyanobacterial colonies on bryophytes and in obligate associations within cyanolichens. Recent studies, applying interspecific variation in plant functional traits to upscale species effects on ecosystems, have all but neglected cryptogams and their association with cyanobacteria. Here we looked for species-specific patterns that determine cryptogam-mediated rates of N₂ fixation in the Subarctic. We hypothesised a contrast in N₂ fixation rates (1) between the structurally and physiologically different lichens and bryophytes, and (2) within bryophytes based on their respective plant functional types. Throughout the survey we supplied ¹⁵N-labelled N₂ gas to quantify fixation rates for monospecific moss, liverwort and lichen turfs. We sampled fifteen species in a design that captures spatial and temporal variations during the growing season in Abisko region, Sweden. We measured N₂ fixation potential of each turf in a common environment and in its field sampling site, in order to embrace both comparativeness and realism. Cyanolichens and bryophytes differed significantly in their cyanobacterial N₂ fixation capacity, which was not driven by microhabitat characteristics, but rather by morphology and physiology. Cyanolichens were much more prominent fixers than bryophytes per unit dry weight, but not per unit area due to their low specific thallus weight. Mosses did not exhibit consistent differences in N₂ fixation rates across species and functional types. Liverworts did not fix detectable amounts of N₂. Despite the very high rates of N₂ fixation associated with cyanolichens, large cover of mosses per unit area at the landscape scale compensates for their lower fixation rates, thereby probably making them the primary regional atmospheric nitrogen sink.     

Effects of modified nutrient concentrations and ratios on the structure and function of the native phytoplankton community in the Neuse River Estuary, North Carolina, USA

A variety of analyses were used to assess the structure (community composition) and function (assimilation number, nitrogen fixation) of phytoplankton in the Neuse River Estuary (NRE), NC under ambient and modified nutrient concentrations. Dilution bioassays were employed to reduce the concentration of nitrogen (N) or both N and phosphorus (P) and thus compare varied DIN:DIP ratios. Experimental manipulations created conditions that may result from mandated N load reductions to the estuary. We hypothesized that unilateral reduction of N loading to the NRE would increase the activity, abundance and diversity of N₂ fixing cyanobacteria. Changes in phytoplankton primary productivity, N₂ fixation (nitrogenase activity), genetic potential for N₂ fixation (presence of nifH), phytoplankton taxonomic composition (diagnostic photopigment concentration) and abundances of N₂ fixing cyanobacteria (microscopy) were determined. Decreasing ambient DIN:DIP ratios in NRE samples resulted in increased rates of N₂ fixation when seed populations were present and environmental conditions were amenable. Decreasing the DIN:DIP ratio did not lead to an increase in the abundance or diversity of N₂ fixing cyanobacteria. Because N₂ fixing cyanobacteria were only actively fixing nitrogen during periods of low riverine N discharge (summer and early autumn), lowering nutrient ratios may not have a major impact on the NRE. However, the maximum potential amount of N from N₂ fixation was calculated using rates from this study and was found to be approximately 3% of total riverine loading of N to the NRE. Because N₂ fixation occurs farther downstream and later in the year than riverine N loading to the NRE, there is potential for N₂ fixation to modify N dynamics. Analyses of the phytoplankton community as a whole in these relatively short term experiments indicated that reduced DIN:DIP may not have a major impact on their structure and function.     

Comparative adaptations of Aphanizomenon and Anabaena for nitrogen fixation under weak irradiance

1. In situ measurements of nitrogen fixation rates for Aphanizomenon in fertile Colorado lakes with low inorganic nitrogen concentrations demonstrated high efficiency of nitrogen fixation at low irradiance. 2. For study populations, rates of N₂ fixation in darkness and with alternating exposure to light and darkness were a higher percentage of light‐saturated rates for Aphanizomenon than for Anabaena, suggesting storage of reduced metabolites at high irradiance that are used subsequently by Aphanizomenon when cells are forced by mixing into zones of low irradiance. Also, saturation of N₂ fixation occurred over a lower range of irradiance for Aphanizomenon than for Anabaena. 3. High efficiency of N₂ fixation in Aphanizomenon at low or fluctuating irradiance is complementary to its previously demonstrated high efficiency of photosynthesis at low irradiance. Nitrogen fixation rate was also strongly related to DIN concentration; fixation was highest at low DIN (maximum < 5 μg L^?1) but was also most vulnerable to photoinhibition under such conditions. 4. The fixation capabilities of Aphanizomenon under weak or varying irradiance could explain its commonly observed domination over Anabaena when transparency is low and available nitrogen is scarce.     

DIAZOTROPHIC GROWTH OF THE MARINE CYANOBACTERIUM TRICHODESMIUM IMS101 IN CONTINUOUS CULTURE: EFFECTS OF GROWTH RATE ON N2‐FIXATION RATE,BIOMASS, AND C:N:P STOICHIOMETRY1

Trichodesmium N₂ fixation has been studied for decades in situ and, recently, in controlled laboratory conditions; yet N₂‐fixation rate estimates still vary widely. This variance has made it difficult to accurately estimate the input of new nitrogen (N) by Trichodesmium to the oligotrophic gyres of the world ocean. Field and culture studies demonstrate that trace metal limitation, phosphate availability, the preferential uptake of combined N, light intensity, and temperature may all affect N₂ fixation, but the interactions between growth rate and N₂ fixation have not been well characterized in this marine diazotroph. To determine the effects of growth rate on N₂ fixation, we established phosphorus (P)–limited continuous cultures of Trichodesmium, which we maintained at nine steady‐state growth rates ranging from 0.27 to 0.67 d^?1. As growth rate increased, biomass (measured as particulate N) decreased, and N₂‐fixation rate increased linearly. The carbon to nitrogen ratio (C:N) varied from 5.5 to 6.2, with a mean of 5.8 ± 0.2 (mean ± SD, N = 9), and decreased significantly with growth rate. The N:P ratio varied from 23.4 to 45.9, with a mean of 30.5 ± 6.6 (mean ± SD, N = 9), and remained relatively constant over the range of growth rates studied. Relative constancy of C:N:P ratios suggests a tight coupling between the uptake of these three macronutrients and steady‐state growth across the range of growth rates. Our work demonstrates that growth rate must be considered when planning studies of the effects of environmental factors on N₂ fixation and when modeling the impact of Trichodesmium as a source of new N to oligotrophic regions of the ocean.     

Optimising biological N2 fixation by legumes in farming systems

Whether grown as pulses for grain, as green manure, as pastures or as the tree components of agro-forestry systems, the value of leguminous crops lies in their ability to fix atmospheric N₂, so reducing the use of expensive fertiliser-N and enhancing soil fertility. N₂ fixing legumes provide the basis for developing sustainable farming systems that incorporate integrated nutrient management. By exploiting the stable nitrogen isotope ¹⁵N, it has been possible to reliably measure rates of N₂ fixation in a wide range of agro-ecological field situations involving many leguminous species. The accumulated data demonstrate that there is a wealth of genetic diversity among legumes and their Rhizobium symbionts which can be used to enhance N₂ fixation. Practical agronomic and microbiological means to maximise N inputs by legumes have also been identified.     

Nitrogen fixation rates in algal turf communities of a degraded versus less degraded coral reef

Algal turf communities are ubiquitous on coral reefs in the Caribbean and are often dominated by N₂-fixing cyanobacteria. However, it is largely unknown (1) how much N₂ is actually fixed by turf communities and (2) which factors affect their N₂ fixation rates. Therefore, we compared N₂ fixation activity by turf communities at different depths and during day and night-time on a degraded versus a less degraded coral reef site on the island of Curaçao. N₂ fixation rates measured with the acetylene reduction assay were slightly higher in shallow (5–10-m depth) than in deep turf communities (30-m depth), and N₂ fixation rates during the daytime significantly exceeded those during the night. N₂ fixation rates by the turf communities did not differ between the degraded and less degraded reef. Both our study and a literature survey of earlier studies indicated that turf communities tend to have lower N₂ fixation rates than cyanobacterial mats. However, at least in our study area, turf communities were more abundant than cyanobacterial mats. Our results therefore suggest that turf communities play an important role in the nitrogen cycle of coral reefs. N₂ fixation by turfs may contribute to an undesirable positive feedback that promotes the proliferation of algal turf communities while accelerating coral reef degradation.     

Cultivar differences in assimilate partitioning and capacity to maintain N2 fixation rate in pea during pod-filling

Effects of plant development on the rate of N₂ fixation and assimilate partitioning in pea were investigated. In growth cabinets, N₂ fixation declined with the onset of pod-filling in a small, determinant cultivar of field pea (cv. Express). In contrast, in a larger, indeterminant variety (cv. Century) N₂ fixation rate did not peak until several weeks into the pod-filling period. The smaller cultivar, Express, fixed 66% less nitrogen than the cultivar Century. Dry matter and nitrogen content increased during pod-filling in nodules but declined, or held steady in leaves, stems, and roots for Century. This indicates that nodules could compete successfully with pods for assimilates during pod-filling. In contrast, dry matter and nitrogen content did not increase in all non-reproductive plant parts (including nodules) for the smaller cultivar, Express. Under field conditions, rates of N₂ fixation declined severely for cv. Century with the onset of pod-filling. It is proposed that maintenance of the rate of N₂ fixation with the onset of pod-filling is dependent on genetic and environmental factors which influence the source-to-sink ratio of carbon in the plant at the start of pod-filling. This hypothesis is incorporated into a proposed scheme of how to maximize nitrogen accumulation by a legume in a growing season.     

Effect of plant genotype and nitrogen fertilizer on symbiotic nitrogen fixation by soybean cultivars

Summary Isotopic as well as non-isotopic methods were used to assess symbiotic nitrogen fixation within eight soybean [Glycine max (L.) Merr.] cultivars grown at 20 and 100 kg N/ha levels of nitrogen fertilizer under field conditions.The¹⁵N methodology revealed large differences between soybean cultivars in their abilities to support nitrogen fixation. In almost all cases, the application of 100 kg N/ha resulted in lower N₂ fixed in soybean than at 20 kg N/ha in the first year of the study. However, N₂ fixed in one cultivar, Dunadja, was not significantly affected by the higher rate of N fertilizer application. These results were confirmed by measurements of acetylene reduction activity, nodule dry weight and N₂ fixed as measured by the difference method. Further proof of differences in N₂ fixed within soybean cultivars and the ability of Dunadja to fix similar amounts of N₂ at 20 and 100 kg N/ha was obtained during a second year experiment. Dunadja yield was affected by N fertilizer and produced larger yield at 100 kg N/ha than at 20 kg N/ha. This type of cultivar could be particularly useful in situations where soil N levels are high or where there is need to apply high amounts of N fertilizer.The present study reveals the great variability between legume germplasms in the ability to fix N₂ at different inorganic N levels, and also the potential that exists in breeding for nitrogen fixation associative traits. The¹⁵N methodology offers a unique tool to evaluate germplasms directly in the field for their N₂ fixation abilities at different N fertilizer levels.     

Time course of N2 fixation in common bean (Phaseolus vulgaris L.)

Field and greenhouse experiments were conducted to assess the nitrogen fixation rates of four cultivars of common bean (Phaseolus vulgaris L.) at different growth stages. The ¹⁵N isotope dilution technique was used to quantify biological nitrogen fixation. In the greenhouse, cultivars M4403 and Kallmet accumulated 301 and 189 mg N plant^–1, respectively, up to 63 days after planting (DAP) of which 57 and 43% was derived from atmosphere. Under field conditions, cultivars Bayocel and Flor de Mayo RMC accumulated in 77 DAP, 147 and 135 kg N ha^–1, respectively, of which approximately one-half was derived from the atmosphere. The rates of N₂ fixation determined at different growth stages increased as the plants developed, and reached a maximum during the reproductive stage both under field and greenhouse conditions. Differences in translocation of N were observed between the cultivars tested, particularly under field conditions. Thus, the fixed N harvest index was 93 and 60 for cultivars Flor de Mayo and Bayocel, respectively. In early stages of growth, the total content of ureides in the plants correlated with the N fixation rates. The findings reported in the present paper can be used to build a strategy for enhancing biological N₂ fixation in common bean.     

The Effect of Source—Sink Manipulations on Nitrogen Fixation in Peas

The effect on nitrogen fixation of excising leaves or pods in pea (Pisum sativum L. cv. Alaska) was determined over a 60-day period. Flower buds or their subtending leaves were removed, and C₂H₂ reduction, H₂ evolution and N accumulation were measured at weekly intervals. Highest percentage nitrogen content in all treatments coincided with time of maximal C₂H₂-reduction rates. Nitrogen fixation, calculated from C₂H₂ reduction and H₂- evolution data, was significantly lower in the partially defoliated and generally higher in the depodded plants than in the controls. Total N accumulation was greatest in the depodded plants and least in the defoliated ones. Percentage nitrogen content and N₂-fixation rates in the depodded plants were maximized approximately 10 days later than in the defoliated or control plants. The absolute rates of C₂H₂ reduction and H₂ evolution were significantly altered by plant organ removal, but the relative rates were proportional. As a result the ratios of H₂/C₂H₄ production and the related relative efficiency of N₂ fixation in the treatments were not significantly different from the controls.     

Nitrogen fixation in soybean as influenced by cultivar and Rhizobium strain

Summary The¹⁵N-substratum labeling technique and other indirect methods were used to compare nitrogen (N₂) fixation in soybean varieties grown in the field in Greece and Romania. Significant variation in the amount (Ndfa) and proportion of N derived from fixation (% Ndfa) was found in different varieties. With 20 kg N/ha applied to soil, N₂ fixed ranged from 22 to 236 kg N/ha in Greece and from 17 to 132 kg N/ha in Romania. In general, varieties or treatments with higher dry matter yield supported greater fixation. Also, varieties with high Ndfa had high % Ndfa andvice versa. Breeding N₂-fixing legumes for high yields at low soil N levels therefore appears to be a reasonable strategy for enhancing N₂ fixation. Heavy applications of inorganic N fertilizer severely depressed N₂ fixation in two out of the three varieties used in Romania. One variety, F 74–412, however, derived slightly higher amounts of N₂ from fixation at 100 kg N/ha rate than when fertilized with 20 kg N/ha. In Greece, Chippewa, Williams and Amsoy-71 inoculated with a Nitragin inoculant fixed similar amounts of N₂ at both 20 and 100 kg N/ha fertilizer rates. However, when Chippewa and Williams were inoculated with amother, locally-isolated Rhizobium strain, N₂ fixation was substantially depressed at the higher N rate.     

Dinitrogen fixation in the world's oceans

The surface water of themarine environment has traditionally beenviewed as a nitrogen (N) limited habitat, andthis has guided the development of conceptualbiogeochemical models focusing largely on thereservoir of nitrate as the critical source ofN to sustain primary productivity. However,selected groups of Bacteria, includingcyanobacteria, and Archaea canutilize dinitrogen (N₂) as an alternativeN source. In the marine environment, thesemicroorganisms can have profound effects on netcommunity production processes and can impactthe coupling of C-N-P cycles as well as the netoceanic sequestration of atmospheric carbondioxide. As one component of an integrated Nitrogen Transport and Transformations project, we have begun to re-assess ourunderstanding of (1) the biotic sources andrates of N₂ fixation in the world'soceans, (2) the major controls on rates ofoceanic N₂ fixation, (3) the significanceof this N₂ fixation for the global carboncycle and (4) the role of human activities inthe alteration of oceanic N₂ fixation. Preliminary results indicate that rates ofN₂ fixation, especially in subtropical andtropical open ocean habitats, have a major rolein the global marine N budget. Iron (Fe)bioavailability appears to be an importantcontrol and is, therefore, critical inextrapolation to global rates of N₂fixation. Anthropogenic perturbations mayalter N₂ fixation in coastal environmentsthrough habitat destruction and eutrophication,and open ocean N₂ fixation may be enhancedby warming and increased stratification of theupper water column. Global anthropogenic andclimatic changes may also affect N₂fixation rates, for example by altering dustinputs (i.e. Fe) or by expansion ofsubtropical boundaries. Some recent estimatesof global ocean N₂ fixation are in therange of 100–200 Tg N (1–2 × 10¹⁴ g N)yr^–1, but have large uncertainties. Theseestimates are nearly an order of magnitudegreater than historical, pre-1980 estimates,but approach modern estimates of oceanicdenitrification.     

Co-occurrence of in-stream nitrogen fixation and denitrification across a nitrogen gradient in a western U.S. watershed

It is frequently assumed that nitrogen (N₂) fixation and denitrification do not co-occur in streams because each process should be favored under different concentrations of dissolved inorganic nitrogen (DIN), and therefore these processes are rarely quantified together. We asked if these processes could co-exist by conducting a spatial survey of N₂ fixation using acetylene reduction and denitrification using acetylene block [with and without amendments of carbon (C) as glucose and nitrogen (N) as nitrate]. Rates were measured on rocks and sediment in 8 southeastern Idaho streams encompassing a DIN gradient of 26–615 µg L^?1. Sampling at each site was repeated in summer 2015 and 2016. We found that both denitrification and N₂ fixation occurred across the gradient of DIN concentrations, with N₂ fixation occurring primarily on rocks and denitrification occurring in sediment. N₂ fixation rates on rocks significantly decreased 100× across the DIN gradient in 1 year of the study, and amended (with N and C) denitrification rates increased 10× across the DIN gradient in both years. Multiple linear regression and partial least squares models with environmental characteristics measured at the scale of entire stream reaches showed that C and phosphorus were positive predictors of amended and unamended denitrification rates, but no significant model could explain N₂ fixation rates across all streams and years. This, coupled with the observation that detectable rates of N₂ fixation occurred primarily on rocks and denitrification occurred primarily on sediment, suggests that microhabitat scale factors may better predict the co-occurrence of these processes within stream reaches. Overlooking the potential co-occurrence of N₂ fixation and denitrification in stream ecosystems will impede understanding by oversimplifying the contribution of each process to the N cycle.     

Nitrogen fixation and assimilation efficiency in Ethiopian and German pea varieties

Dinitrogen (N₂) fixation and assimilation efficiency in a German and two Ethiopian varieties of Pisum sativum L. was studied in a pot experiment during vegetative and reproductive growth. The objective of the study was to assess whether genotypes having contrasting growth habits showed differences in physiological processes that affect the efficiency of N₂ fixation and assimilation. Dry matter formation, nodulation and nitrogen assimilation were compared between two treatments where one depended solely on N₂ fixation while the other was nourished with nitrate. Moreover, carbon (C) costs of N₂ fixation and the capacity of different respiratory chains in roots and nodules were determined at vegetative and reproductive growth. As compared to the Ethiopian cultivars, the German variety displayed a more rapid vegetative growth with intensive N₂ fixation and assimilation and highly efficient individual nodules. However, during reproductive growth, N₂ fixation in the German variety declined sharply, while continuing in the Ethiopian varieties. Lowest C costs of N₂ fixation coincided with most efficient individual nodules in both growth intervals. C costs of N₂ fixation were lower during reproductive growth in all varieties which was accompanied by a shift in root/nodule respiratory capacity towards more C efficient respiratory pathways. The results provide further evidence that unreliable nitrogen fixation rates during reproductive growth of pea can be connected with restricted C supply to nodules. One strategy of pea plants to adapt to critical C availability is an increase in capacity of more C efficient root/nodule respiration.     

Ecosystem nitrogen fixation throughout the snow‐free period in subarctic tundra: effects of willow and birch litter addition and warming

Nitrogen (N) fixation in moss‐associated cyanobacteria is one of the main sources of available N for N‐limited ecosystems such as subarctic tundra. Yet, N₂ fixation in mosses is strongly influenced by soil moisture and temperature. Thus, temporal scaling up of low‐frequency in situ measurements to several weeks, months or even the entire growing season without taking into account changes in abiotic conditions cannot capture the variation in moss‐associated N₂ fixation. We therefore aimed to estimate moss‐associated N₂ fixation throughout the snow‐free period in subarctic tundra in field experiments simulating climate change: willow (Salix myrsinifolia) and birch (Betula pubescens spp. tortuosa) litter addition, and warming. To achieve this, we established relationships between measured in situ N₂ fixation rates and soil moisture and soil temperature and used high‐resolution measurements of soil moisture and soil temperature (hourly from May to October) to model N₂ fixation. The modelled N₂ fixation rates were highest in the warmed (2.8 ± 0.3 kg N ha^?1) and birch litter addition plots (2.8 ± 0.2 kg N ha^?1), and lowest in the plots receiving willow litter (1.6 ± 0.2 kg N ha^?1). The control plots had intermediate rates (2.2 ± 0.2 kg N ha^?1). Further, N₂ fixation was highest during the summer in the warmed plots, but was lowest in the litter addition plots during the same period. The temperature and moisture dependence of N₂ fixation was different between the climate change treatments, indicating a shift in the N₂ fixer community. Our findings, using a combined empirical and modelling approach, suggest that a longer snow‐free period and increased temperatures in a future climate will likely lead to higher N₂ fixation rates in mosses. Yet, the consequences of increased litter fall on moss‐associated N₂ fixation due to shrub expansion in the Arctic will depend on the shrub species’ litter traits.     

Nitrogen fixation in a desert stream ecosystem

Few measurements of nitrogen fixation exist for streams. Desertstreams are warm, well lighted, and often supportabundant cyanobacterial populations; thus N₂ fixationmay be significant in these N-poor ecosystems. N₂fixation was measured in situ by acetylene reductionfor two patch types (Anabaena mat and anepilithic assemblage). Patch-specific rates were highcompared with published values (maximum 775 µgN₂ [83 µmol C₂H₄]mg chl a ^-1 h^-1or 51 mg N₂ [5.4 mmol C₂H₄] m_-2 h_-1).Daytime fixation was higher than nighttimefixation, and temperature, light and inorganic Nconcentration explained 52% of variance in hourlyrates over all dates. Diel input-output budgets wereconstructed on five dates when cyanobacteria werepresent in the stream. Diel N₂ fixation rates weremeasured for comparison with reach-scale diel nitrogenretention, to assess the importance of this vector to Neconomy of the stream. Fixation accounted for up to85% of net N flux to the benthos, but its importancevaried seasonally. Finally, we applied biomass-specificfixation rates to 1992 and 1993 biomass data to obtainseasonal and annual N₂ fixation estimates.Cyanobacteria were absent or rare during winter andspring, thus most of the annual N₂ fixation occurredduring summer and autumn. Annual rates of nitrogenfixation for 1992 and 1993 (8.0 g/m₂ and 12.5g/m₂) were very high compared to other streams,and moderately high compared to other ecosystems.Like other phenomena in this disturbance-proneecosystem, nitrogen fixation is strongly influenced bythe number and temporal distribution of flood events.     

A NEW NITROGEN-FIXING NON-LEGUME: CHAMAEBATIA FOLIOLOSA (ROSACEAE)

Specimens of Chamaebatia foliolosa Benth. with nodule structures on their roots fix atmospheric nitrogen. The nodules are similar to those of other non-legumes in gross morphology and structure, containing hyphal strands, some with club-shaped vesicles at their ends. A fixation rate of 130 nmoles N₂ per g fresh weight per hr is reported by using ¹⁵N₂ as a tracer. Equivalent rates of acetylene reduction were observed.