Algal nitrogen fixation in Californian streams: diel cycles and nocturnal fixation

Thirty-six hour diurnal studies of Ng-fixation by Nostoc in a rocky-bedded stream were carried out during the peak of the seasonal cycle of growth on clear and cloudy days in 1971 and 1972. On both occasions an unexpected pattern of N₂-fixation occurred with maximum fixation rates in the light but also in the dark portion of the day, with lowest fixation periods in the early evening. I postulate that competition for reductant between nitrogenase and other processes, especially photorespiration, controls this unusual diel cycle rather than variations in the intracellular N-pool. N₂-fixation rates on a cloudless May day in 1971 ranged from 0.2 to 4.8 nmoles C₂H₄ cm⁻² h⁻¹ and from 0.3 to 3.3 nmoles C₂H₄ mg⁻¹ h⁻¹ dry weight of Nostoc, depending on time of day and favourableness of site. On the same site on a cloudy, rainy May day in 1972 fixation ranged from 0.5 to 3.1 nmoles C₂H₄ mg⁻¹ h⁻¹ dry weight, and from 1 to 4.5 nmoles C2H4 mg⁻¹ h⁻¹ ash-free dry weight of Nostoc. Since Nostoc is most abundant in unshaded areas, and since one-third of each day's nitrogen i s fixed in the dark, future studies should take dark fixation into account.     

N2(C2H2)ASE ACTIVITY BY ALNUS INCANA SSP. RUGOSA (BETULACEAE) IN THE NORTHERN HARDWOOD FOREST

Field assays of N₂(C₂H₂)ase activity were performed with intact nodules from a pure alder site (alder) and a mixed alder-aspen site (aspen). Assays were performed between 12 June and 12 August 1980 and in May 1981. N₂(C₂H₂)ase rates are expressed as g N g nodule oven-dry wt⁻¹ hr⁻¹ (g N g⁻¹ hr⁻¹). Diurnal N₂(C₂H₂)ase activity showed an increase in both sites between 0600 and midday, then decreased to a low by 1800. Nighttime activity in the May 1981 assay was approximately 25% of the daytime peak. Mean (±SE) 1200 hr N₂(C₂H₂)ase activity (μg N g⁻¹ hr⁻¹) for all sizes in the alder stand rose from 24.56 ± 6.56 on 12 June to 73.96 ± 28.37 on 26 June and declined to 9.20 ± 2.56 by 12 August. In the aspen stand activity decreased from the 12 June rate of 21.81 ± 4.59 to 3.64 ± 1.87 on 24 July but then increased to 30.00 ± 7.39 by 12 August. Based on diurnal assays, the seasonal mean N influx (μg N g⁻¹ hr⁻¹) is statistically higher (P 0.05) in the alder stand with a value of 26.70 compared to 14.63 in the aspen stand. Small size class shrubs had significantly higher (P < 0.05) N₂(C₂H₂)ase activity (μg N g⁻¹ hr⁻¹) in diurnal assays than medium or large class shrubs. The estimated mean (±SE) N₂(C₂H₂)ase activity (mg N g⁻¹ season⁻¹) for all sizes was 44.4 ± 18.6 in the alder stand compared to 16.2 ± 5.2 in the aspen stand. Nodule excavations showed the g shrub⁻¹ in the alder stand to be 16.48 ± 10.29, 38.57 ± 12.34 and 29.11 ± 7.15 for small, medium and large size shrubs and 12.73 ± 3.23, 28.21 ± 4.36 and 56.45 ± 16.23 for respective sizes in the aspen stand. Seasonal N influx was 4.69 kg ha⁻¹ in the alder stand and 0.84 kg ha⁻¹ in the aspen stand, representing 17.9% of the alder stand. Nitrogen feedback inhibition from uric acid-N influx and allelochemic interference from aspen are discussed as explanations for the differences in N influx in the two stands.     

Spatial variation of N2-fixation in field pea (Pisum sativum L.) at the field scale determined by the 15N natural abundance method

Grain legumes such as field pea are known to have high variability of yield and dinitrogen (N₂) fixation between seasons, but less is known about the yearly spatial variability within a field. The objective of this study was to improve the understanding of spatial field scale variability of field pea dry matter (DM) yield and nitrogen (N) acquisition from fixation and soil within a 10 ha farmer’s field. A 42 m systematic random grid providing 56 plant sampling locations across 10 ha supplemented by soil data provided from an existing database were used to determine whether the observed spatial variability could be explained by the variability in selected abiotic soil properties. All measured soil variables showed substantial variability across the field and the pea dry matter production ranged between 4.9 and 13.8 Mg ha^?1 at maturity. The percent of total N derived from the atmosphere (%Ndfa) at flowering, estimated using the ¹⁵N natural abundance method, ranged from 65% to 92% with quantitative N₂-fixation estimates from 93 kg to 202 kg N ha^?1. At maturity %Ndfa ranged from 26% to 81% with quantitative N₂-fixation estimates from 48 kg to 167 kg N ha^?1. Significant correlations were found between pea dry matter production and humus content, potassium content (collinear with humus) and total N in the 0–25 cm topsoil. No correlation was found between any individual soil property and %Ndfa or kg N fixed ha^?1. It was not possible to create a satisfactory global multi-regression model for the field dry matter production and N₂-fixation. A number of other models were tested, but the best was only able to explain less than 40% of the variance in %Ndfa using seven soil properties. Together with the use of interpolated soil data, high spatial variation of soil ¹⁵N natural abundance, a mean increase in pea ¹⁵N natural abundance of 1 δ unit between flowering and maturity and a reference crop decline of 1.3 δ¹⁵N unit over the same period increased noise of derived variables, making modeling of N₂-fixation difficult. Furthermore, complex interactions with other soil variables and biotic stresses not measured in this study may have contributed significantly to the variability of fixation and yield of pea within the field. Pea N₂-fixation obtained from two additional 10 ha farmer fields was in agreement with the other findings highlighting that N₂-fixation takes place under a range of physical and chemical soil properties and is controlled by local site specific conditions. In future studies addressing field scale variability we recommend that soil variables wherever possible should be measured in the same plots as the sampled crop. Sampling designs that optimize the use of a priori information about the field soil and landscape properties for positioning plots and that facilitate estimates of local variances should be considered.     

Isolation and identification of root associated diazotrophs

Diazotrophs have been isolated from the rhizosphere or roots of plants by many workers. To recognize a certain diazotroph as the most abundant bacterium at a certain site or as the principal agent responsible for N₂-fixation is much more difficult. It is probable that many diazotrophs, including possibly the most efficient ones, have not been identified yet. The use of proper selective media which simulate the environment of the various diazotrophsin situ has led to the discovery of 10 new root-associated diazotrophs, three of them during 1986/1987 (Azospirillum halopraeferans, Herbaspirillum seropedicae and the recently proposedAcetobacter diazotrophicus). The importance of using a variety of carbon substrates in the growth media with pH indicators, and the use of N-free semi-solid media, is discussed. Recognition of plant-bacteria interactions requires, in addition to the identification of the bacteria, the demonstration of effects of the plant on the bacteria and of the bacteria on the plant. Confirmation of the identity of diazotrophs responsible for response of plants to inoculation must be made in experiments with strains labelled with antibiotic resistance or other markers. If establishment of the inoculated strain is demonstrated in plants grown in¹⁵N-labelled soil, the¹⁵N enrichment of the plants will reveal if any observed responses in N yield are due to N₂-fixation or increased soil/fertilizer-N uptake.     

Cyanobacterial (unicellular and heterocystous) biofertilization to wetland rice influenced by nitrogenous agrochemical

Comparative growth and N₂-fixation of cyanobacteria, namely Aphanothece sp. (unicellular) and Gloeotrichia sp. (heterocystous, filamentous), were studied after their inoculation to rice crop in the absence and presence of urea nitrogen fertilizer. In the absence of N-fertilizer application (control), inoculation of both cyanobacterial species showed significant increase in growth and acetylene reduction activity (ARA), but gradual reduction in these parameters was observed at 30 and 60 kg N ha^?1 of urea application. In inoculation of Gloeotrichia sp. at control, 30 and 60 kg N ha^?1 increased grain yield significantly over uninoculated control in both wet and dry seasons, but grain yield with Aphanothece sp. inoculation was statistically similar to the control at N levels during both seasons. The inoculation study showed that heterocystous cyanobacteria contributed better than unicellular ones, and application of N-fertilizer adversely affected both growth and N₂-fixation of native as well of inoculated cyanobacteria.     

Terrestrial nitrogen cycle simulation with a dynamic global vegetation model

A global scale Dynamic Nitrogen scheme (DyN) has been developed and incorporated into the Lund–Posdam–Jena (LPJ) dynamic global vegetation model (DGVM). The DyN is a comprehensive process‐based model of the cycling of N through and within terrestrial ecosystems, with fully interactive coupling to vegetation and C dynamics. The model represents the uptake, allocation and turnover of N in plants, and soil N transformations including mineralization, N₂ fixation, nitrification and denitrification, NH₃ volatilization, N leaching, and N₂, N₂O and NO production and emission. Modelled global patterns of site‐scale nitrogen fluxes and reservoirs are highly correlated to observations reported from different biomes. The simulation of site‐scale net primary production and soil carbon content was improved relative to the original LPJ, which lacked an interactive N cycle, especially in the temporal and boreal regions. Annual N uptake by global natural vegetation was simulated as 1.084 Pg N yr⁻¹, with lowest values <1 g N m⁻² yr⁻¹ (polar desert) and highest values in the range 24–36.5 g N m⁻² yr⁻¹ (tropical forests). Simulated global patterns of annual N uptake are consistent with previous model results by Melillo et al. The model estimates global total nitrogen storage potentials in vegetation (5.3 Pg N), litter (4.6 Pg N) and soil (≥67 Pg as organic N and 0.94 Pg as inorganic N). Simulated global patterns of soil N storage are consistent with the analysis by Post et al. although total simulated N storage is less. Deserts were simulated to store 460 Tg N (up to 0.262 kg N m⁻²) as NO₃⁻, contributing 80% of the global total NO₃⁻ inventory of 580 Tg N. This model result is in agreement with the findings of a large NO₃⁻ pool beneath deserts. Globally, inorganic soil N is a small reservoir, comprising only 1.6% of the global soil N content to 1.5 m soil depth, but the ratio has a very high spatial variability and in hot desert regions, inorganic NO₃⁻ is estimated to be the dominant form of stored N in the soil.     

Effect of stand age on greenhouse gas fluxes from a Sitka spruce [Picea sitchensis (Bong.) Carr.] chronosequence on a peaty gley soil

The influence of forest stand age in a Picea sitchensis plantation on (1) soil fluxes of three greenhouse gases (GHGs – CO₂, CH₄ and N₂O) and (2) overall net ecosystem global warming potential (GWP), was investigated in a 2‐year study. The objective was to isolate the effect of forest stand age on soil edaphic characteristics (temperature, water table and volumetric moisture) and the consequent influence of these characteristics on the GHG fluxes. Fluxes were measured in a chronosequence in Harwood, England, with sites comprising 30‐ and 20‐year‐old second rotation forest and a site clearfelled (CF) some 18 months before measurement. Adjoining unforested grassland (UN) acted as a control. Comparisons were made between flux data, soil temperature and moisture data and, at the 30‐year‐old and CF sites, eddy covariance data for net ecosystem carbon (C) exchange (NEE). The main findings were: firstly, integrated CO₂ efflux was the dominant influence on the GHG budget, contributing 93–94% of the total GHG flux across the chronosequence compared with 6–7% from CH₄ and N₂O combined. Secondly, there were clear links between the trends in edaphic factors as the forest matured, or after clearfelling, and the emission of GHGs. In the chronosequence sites, annual fluxes of CO₂ were lower at the 20‐year‐old (20y) site than at the 30‐year‐old (30y) and CF sites, with soil temperature the dominant control. CH₄ efflux was highest at the CF site, with peak flux 491±54.5 μg m⁻² h⁻¹ and maximum annual flux 18.0±1.1 kg CH₄ ha⁻¹ yr⁻¹. No consistent uptake of CH₄ was noted at any site. A linear relationship was found between log CH₄ flux and the closeness of the water table to the soil surface across all sites. N₂O efflux was highest in the 30y site, reaching 108±38.3 μg N₂O‐N m⁻² h⁻¹ (171 μg N₂O m⁻² h⁻¹) in midsummer and a maximum annual flux of 4.7±1.2 kg N₂O ha⁻¹ yr⁻¹ in 2001. Automatic chamber data showed a positive exponential relationship between N₂O flux and soil temperature at this site. The relationship between N₂O emission and soil volumetric moisture indicated an optimum moisture content for N₂O flux of 40–50% by volume. The relationship between C : N ratio data and integrated N₂O flux was consistent with a pattern previously noted across temperate and boreal forest soils.     

Soil nitrate accumulation explains the nonlinear responses of soil CO2 and CH4 fluxes to nitrogen addition in a temperate needle-broadleaved mixed forest

The responses of soil-atmosphere carbon (C) exchange fluxes to growing atmospheric nitrogen (N) deposition are controversial, leading to large uncertainty in the estimated C sink of global forest ecosystems experiencing substantial N inputs. However, it is challenging to quantify critical load of N input for the alteration of the soil C fluxes, and what factors controlled the changes in soil CO₂ and CH₄ fluxes under N enrichment. Nine levels of urea addition experiment (0, 10, 20, 40, 60, 80, 100, 120, 140 kg N ha⁻¹ yr⁻¹) were conducted in the needle-broadleaved mixed forest in Changbai Mountain, Northeast China. Soil CO₂ and CH₄ fluxes were monitored weekly using the static chamber and gas chromatograph technique. Environmental variables (soil temperature and moisture in the 0–10 cm depth) and dissolved N (NH₄⁺-N, NO₃⁻-N, total dissolved N (TDN), and dissolved organic N (DON)) in the organic layer and the 0–10 cm mineral soil layer were simultaneously measured. High rates of N addition (≥60 kg N ha⁻¹ yr⁻¹) significantly increased soil NO₃⁻-N contents in the organic layer and the mineral layer by 120%-180% and 56.4%-84.6%, respectively. However, N application did not lead to a significant accumulation of soil NH₄⁺-N contents in the two soil layers except for a few treatments. N addition at a low rate of 10 kg N ha⁻¹ yr⁻¹ significantly stimulated, whereas high rate of N addition (140 kg N ha⁻¹ yr⁻¹) significantly inhibited soil CO₂ emission and CH₄ uptake. Significant negative relationships were observed between changes in soil CO₂ emission and CH₄ uptake and changes in soil NO₃⁻-N and moisture contents under N enrichment. These results suggest that soil nitrification and NO₃⁻-N accumulation could be important regulators of soil CO₂ emission and CH₄ uptake in the temperate needle-broadleaved mixed forest. The nonlinear responses to exogenous N inputs and the critical level of N in terms of soil C fluxes should be considered in the ecological process models and ecosystem management.     

Exchange of N-gases at the Höglwald Forest – A summary

During 4 years continuous measurements of N-trace gas exchange were carried out at the forest floor-atmosphere interface at the Höglwald Forest that is highly affected by atmospheric N-deposition. The measurements included spruce control, spruce limed and beech sites. Based on these field measurements and on intensive laboratory measurements of N₂-emissions from the soils of the beech and spruce control sites, a total balance of N-gas emissions was calculated. NO₂-deposition was in a range of –1.6 –2.9 kg N ha^–1 yr^–1 and no huge differences between the different sites could be demonstrated. In contrast to NO₂-deposition, NO- and N₂O-emissions showed a huge variability among the different sites. NO emissions were highest at the spruce control site (6.4–9.1 kg N ha^–1 yr^–1), lowest at the beech site (2.3–3.5 kg N ha^–1 yr^–1) and intermediate at the limed spruce site (3.4–5.4 kg N ha^–1 yr^–1). With regard to N₂O-emissions, the following ranking between the sites was found: beech (1.6–6.6 kg N ha^–1 yr^–1) >> spruce limed (0.7–4.0 kg N ha^–1 yr^–1) > spruce control (0.4–3.1 kg N ha^–1 yr^–1). Average N-trace gas emissions (NO, NO₂, N₂O) for the years 1994–1997 were 6.8 kg N ha^–1 yr^–1 at the spruce control site, 3.6 kg N ha^–1 yr^–1 at the limed spruce site and 4.5 kg N ha^–1 yr^–1 at the beech site. Considering N₂-losses, which were significantly higher at the beech (12.4 kg N ha^–1 yr^–1) than at the spruce control site (7.2 kg N ha^–1 yr^–1), the magnitude of total gaseous N losses, i.e. N₂-N + NO-N + NO₂-N + N₂O-N, could be calculated for the first time for a forest ecosystem. Total gaseous N-losses were 14.0 kg N ha^–1 yr^–1 at the spruce control site and 15.5 kg N ha^–1 yr^–1 at the beech site, respectively. In view of the huge interannual variability of N-trace gas fluxes and the pronounced site differences in N-gas emissions it is concluded that more research is needed in order to fully understand patterns of microbial N-cycling and N-gas production/emission in forest ecosystems and mechanisms of reactions of forest ecosystems to the ecological stress factor of atmospheric N-input.     

Nitrogen balance in the Trachypogon grasslands of Central Venezuela

The nitrogen balance of a Trachypogon grassland in Calabozo, Venezuela, is calculated for average conditions using biomass accumulation, nitrogen content, and turnover rates of organic matter. Burning Trachypogon grasslands results in losses of 8.5 kg N ha^?1 yr^?1, while rainfall inputs average 2.6 kg N ha^?1 yr^?1. Uptake of N by vegetation is 14.8 kg N ha^?1 yr^?1, but the total N required to build new tissue during a growing season is about 30 kg N ha^?1 yr^?1, so that about 50% of the nitrogen in the vegetation is recycled internally. Nitrogen lossesvia fire are probably balanced by biological N₂-fixation, but no data are available for N-fixation in these savannas. The calculations presented in this paper are based on few data and more measurements are needed to develop a conclusive picture of the N-balance of Trachypogon grasslands.     

Evaluation of N2-fixation and nitrogen economy of a maize/cowpea intercrop system using15N dilution methods

Yields of above ground biomass and total N were determined in summer-grown maize and cowpea as sole crops or intercrops, with or without supplementary N fertilizer (25 kg N ha⁻¹, urea) at an irrigated site in Waroona, Western Australia over the period 1982–1985. Good agreement was obtained between estimates of N₂ fixation of sole or intercrop cowpea (1984/85 season) based on the¹⁵N natural abundance and¹⁵N fertilizer dilution techniques, both in the field and in a glasshouse pot study. Field-grown cowpea was estimated to have received 53–69% of its N supply from N₂-fixation, with N₂-fixation onlyslightly affected by intercropping or N fertilizer application. Proportional reliance on N₂-fixation of cowpea in glasshouse culture was lower (36–66%) than in the field study and more affected by applied N. Budgets for N were drawn up for the field intercrops, based on above-ground seed yields, return of crop residues, inputs of fixed N and fertilizer N. No account was taken of possible losses of N through volatilization, denitrification and leaching or gains of N in the soil from root biomass. N₂-fixation was estimated tobe 59 kg N ha⁻¹ in the plots receiving no fertilizer N, and 73 kg N ha⁻¹ in plots receiving 25 kg N ha⁻¹ as urea. Comparable fixation by sole cowpea was higher (87 and 82 kg N ha⁻¹ respectively) but this advantage was outweighed by greater land use efficiency by the intercrop than sole crops.     

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.     

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.     

Nitrogen loss from grassland on peat soils through nitrous oxide production

Nitrous oxide (N₂O) in soils is produced through nitrification and denitrification. The N₂O produced is considered as a nitrogen (N) loss because it will most likely escape from the soil to the atmosphere as N₂O or N₂. Aim of the study was to quantify N₂O production in grassland on peat soils in relation to N input and to determine the relative contribution of nitrification and denitrification to N₂O production. Measurements were carried out on a weekly basis in 2 grasslands on peat soil (Peat I and Peat II) for 2 years (1993 and 1994) using intact soil core incubations. In additional experiments distinction between N₂O from nitrification and denitrification was made by use of the gaseous nitrification inhibitor methyl fluoride (CH₃F).Nitrous oxide production over the 2 year period was on average 34 kg N ha^-1 yr^-1 for mown treatments that received no N fertiliser and 44 kg N ha^-1 yr^-1 for mown and N fertilised treatments. Grazing by dairy cattle on Peat I caused additional N₂O production to reach 81 kg N ha^-1 yr^-1. The sub soil (20–40 cm) contributed 25 to 40% of the total N₂O production in the 0–40 cm layer. The N₂O production:denitrification ratio was on average about 1 in the top soil and 2 in the sub soil indicating that N₂O production through nitrification was important. Experiments showed that when ratios were larger than l, nitrification was the major source of N₂O. In conclusion, N₂O production is a significant N loss mechanism in grassland on peat soil with nitrification as an important N₂O producing process.     

Nitrogenase activity associated with some wisconsin prairie grasses

Summary Yearly rates of nitrogen fixation associated with seven species of grass were measured on two artificially-established prairies. The C₂H₂ reduction method was used to measure the activity of soil cores taken within the stands of grass. Nitrogenase activity was specifically associated with Panicum virgatum and Sporobolus heterolepis, which had activities estimated at 3.6 and 2.9 kg N ha^-1 yr^-1. Fixation in stands of the other grasses ranged between 0.2 and 1.8 kg N ha^-1 yr^-1; free-living organisms might have fixed the N₂ without specific association with the grasses, which were Andropogon gerardi, Andropogon scoparius, Spartina pectinata, Stipa spartea, and Poa pratensis. Three relic prairies were also examined, but the rates of fixation were no higher, except for S. heterolepis, which at one relic prairie had rates that extrapolated to 9 kg N ha^-1 yr^-1. The choices made for core location, size, depth, length of C₂H₂ incubation, and the time of day of sampling did not appear to have a substantial effect on the accuracy of the measurements. The organisms associated with S. heterolepis required O₂ for N₂ fixation, and they were located in the soil or on the smaller roots which remained when the major roots were removed from the soil. re]19750217     

Nitrogen fixation by non-legumes in tropical agriculture with special reference to wetland rice

Summary Of the 143 million hectares of cultivated rice land in the world, 75% are planted to wetland rice. Wet or flooded conditions favour biological nitrogen fixation by providing (1) photic-oxic floodwater and surface soil for phototrophic, free-living or symbiotic blue-green algae (BGA), and (2) aphotic-anoxic soil for anaerobic or microaerobic, heterotrophic bacteria. TheAzolla-Anabaena symbiosis can accumulate as much as 200 kg N ha^–1 in biomass. In tropical flooded fields, biomass production from a singleAzolla crop is about 15 t fresh weight ha^–1 or 35 kg N ha^–1. Low tolerance for high temperature, insect damage, phosphorus requirement, and maintenance of inoculum, limit application in the tropics. Basic work on taxonomy, sporulation, and breeding ofAzolla is needed. Although there are many reports of the positive effect of BGA inoculation on rice yield, the mechanisms of yield increase are not known. Efficient ways to increase N₂-fixation by field-grown BGA are not well exploited. Studies on the ecology of floodwater communities are needed to understand the principles of manipulating BGA. Bacteria associated with rice roots and the basal portion of the shoot also fix nitrogen. The system is known as a rhizocoenosis. N₂-fixation in rhizocoenosis in wetland rice is lower than that ofAzolla or BGA. Ways of manipulating this process are not known. Screening rice varieties that greatly stimulate N₂-fixation may be the most efficient way of manipulating the rhizocoenosis. Stimulation of N₂-fixation by bacterial inoculation needs to be quantified.     

Response to inoculation and N fertilization for increased yield and biological nitrogen fixation of common bean (Phaseolus vulgaris L.)

Common bean (Phaseolus vulgaris L.) is able to fix 20–60 kg N ha^–1 under tropical environments in Brazil, but these amounts are inadequate to meet the N requirement for economically attractive seed yields. When the plant is supplemented with N fertilizer, N₂ fixation by Rhizobium can be suppressed even at low rates of N. Using the ¹⁵N enriched method, two field experiments were conducted to compare the effect of foliar and soil applications of N-urea on N₂ fixation traits and seed yield. All treatments received a similar fertilization including 10 kg N ha^–1 at sowing. Increasing rates of N (10, 30 and 50 kg N ha^–1) were applied for both methods. Foliar application significantly enhanced nodulation, N₂ fixation (acetylene reduction activity) and yield at low N level (10 kg N ha^–1). Foliar nitrogen was less suppressive to nodulation, even at higher N levels, than soil N treatments. In the site where established Rhizobium was in low numbers, inoculation contributed substantially to increased N₂ fixation traits and yield. Both foliar and soil methods inhibited nodulation at high N rates and did not significantly increase bean yield, when comparing low (10 kg N ha^–1) and high (50 kg N ha^–1) rates applied after emergence. In both experiments, up to 30 kg N ha^–1 of biologically fixed N₂ were obtained when low rates of N were applied onto the leaves.     

Greenhouse gas fluxes from an Australian subtropical cropland under long‐term contrasting management regimes

The long‐term effects of conservation management practices on greenhouse gas fluxes from tropical/subtropical croplands remain to be uncertain. Using both manual and automatic sampling chambers, we measured N₂O and CH₄ fluxes at a long‐term experimental site (1968–present) in Queensland, Australia from 2006 to 2009. Annual net greenhouse gas fluxes (NGGF) were calculated from the 3‐year mean N₂O and CH₄ fluxes and the long‐term soil organic carbon changes. N₂O emissions exhibited clear daily, seasonal and interannual variations, highlighting the importance of whole‐year measurement over multiple years for obtaining temporally representative annual emissions. Averaged over 3 years, annual N₂O emissions from the unfertilized and fertilized soils (90 kg N ha^?1 yr^?1 as urea) amounted to 138 and 902 g N ha^?1, respectively. The average annual N₂O emissions from the fertilized soil were 388 g N ha^?1 lower under no‐till (NT) than under conventional tillage (CT) and 259 g N ha^?1 higher under stubble retention (SR) than under stubble burning (SB). Annual N₂O emissions from the unfertilized soil were similar between the contrasting tillage and stubble management practices. The average emission factors of fertilizer N were 0.91%, 1.20%, 0.52% and 0.77% for the CT‐SB, CT‐SR, NT‐SB and NT‐SR treatments, respectively. Annual CH₄ fluxes from the soil were very small (?200–300 g CH₄ ha^?1 yr^?1) with no significant difference between treatments. The NGGF were 277–350 kg CO₂‐e ha^?1 yr^?1 for the unfertilized treatments and 401–710 kg CO₂‐e ha^?1 yr^?1 for the fertilized treatments. Among the fertilized treatments, N₂O emissions accounted for 52–97% of NGGF and NT‐SR resulted in the lowest NGGF (401 kg CO₂‐e ha^?1 yr^?1 or 140 kg CO₂‐e t^?1 grain). Therefore, NT‐SR with improved N fertilizer management practices was considered the most promising management regime for simultaneously achieving maximal yield and minimal NGGF.     

Algal nitrogen fixation in Californian streams: seasonal cycles

Using the acetylene reduction technique, nitrogen fixation was measured in Rocky Creek, a small seasonally dry Californian stream. In the 3 years since 1970 nitrogen fixation varied seasonally and spatially, being highest in the early stages of colony growth in shallow, clear regions where there was little shade. The annual rate of N₂-fixation was similar to that found recently for Arctic tundra, but was greater than rates for Antarctic rivulets and less than rates for temperate rocky shores. A hetero-cystous Nostoc appeared to be the sole organism responsible for this fixation. Nostoc occurred in variously sized gelatinous clumps on the stable boulders on which the co-dominant alga, Ulothrix zonata, also grew. Maximum nitrogenase activity yielded 70 nmoles C₂H₄ mg^-1 drywt day^-1 and 123 nmoles C₂H₄cm^-2 day^-1. Estimated annual amounts of nitrogen fixed in 1971 ranged from 42 in shade to 360 mg N fixed m^-2y^-1 in the most favourable areas. Nostoc biomass reached 33 g dry wt m^-2 and peeled off each year in June-July. Because Nostoc was confined to stable rocks the biomass per unit area of the stream as a whole was much less, ranging from 0.054 to 1.26 g dry wt m^-2 in the most favourable site. Nostoc was common throughout the length of Rocky Creek and also plentiful in eight out of ten adjacent streams but not in the main river (Eel River South Fork), probably due to high turbulence and turbidity. N₂-fixation in these streams makes a significant, but probably small contribution to the nitrogen income of the nitrogen-deficient Eel River system. Nostoc colony establishment appears to be controlled by a combination of reduced turbulence and firmer attachment of the colonies to the substrate. Nostoc colony disappearance in June is probably due to nutrient depletion.     

Nitrogen fixation by white clover in pastures grazed by dairy cows: Temporal variation and effects of nitrogen fertilization

Effects of rate of nitrogen (N) fertilizer and stocking rate on production and N₂ fixation by white clover (Trifolium repens L.) grown with perennial ryegrass (Lolium perenne L.) were determined over 5 years in farmlets near Hamilton, New Zealand. Three farmlets carried 3.3 dairy cows ha^–1 and received urea at 0, 200 or 400 kg N ha^–1 yr^–1 in 8–10 split applications. A fourth farmlet received 400 kg N ha^–1 yr^–1 and had 4.4 cows ha^–1.There was large variation in annual clover production and total N₂ fixation, which in the 0 N treatment ranged from 9 to 20% clover content in pasture and from 79 to 212 kg N fixed ha^–1 yr^–1. Despite this variation, total pasture production in the 0 N treatment remained at 75–85% of that in the 400 N treatments in all years, due in part to the moderating effect of carry-over of fixed N between years.Fertilizer N application decreased the average proportion of clover N derived from N₂ fixation (P_N; estimated by ¹⁵N dilution) from 77% in the 0 N treatment to 43–48% in the 400 N treatments. The corresponding average total N₂ fixation decreased from 154 kg N ha^–1 yr^–1 to 39–53 kg N ha^–1 yr^–1. This includes N₂ fixation in clover tissue below grazing height estimated at 70% of N₂ fixation in above grazing height tissue, based on associated measurements, and confirmed by field N balance calculations. Effects of N fertilizer on clover growth and N₂ fixation were greatest in spring and summer. In autumn, the 200 N treatment grew more clover than the 0 N treatment and N₂ fixation was the same. This was attributed to more severe grazing during summer in the 0 N treatment, resulting in higher surface soil temperatures and a deleterious effect on clover stolons.In the 400 N treatments, a 33% increase in cow stocking rate tended to decrease P_N from 48 to 43% due to more N cycling in excreta, but resulted in up to 2-fold more clover dry matter and N₂ fixation because lower pasture mass reduced grass competition, particularly during spring.