Nitrogen fixation from pasture legumes is a fundamental process that contributes to the profitability and sustainability of dryland agricultural systems. The aim of this research was to determine whether well-managed pastures, based on aerial-seeding pasture legumes, could partially or wholly meet the nitrogen (N) requirements of subsequent grain crops in an annual rotation. Fifteen experiments were conducted in Western Australia with wheat, barley or canola crops grown in a rotation that included the pasture legume species French serradella (Ornithopus sativus), biserrula (Biserrula pelecinus), bladder clover (Trifolium spumosum), annual medics (Medicago spp.) and the non-aerial seeded subterranean clover (Trifolium subterraneum). After the pasture phase, five rates of inorganic N fertilizer (Urea, applied at 0, 23, 46, 69 and 92 kg/ha) were applied to subsequent cereal and oil seed crops. The yields of wheat grown after serradella, biserrula and bladder clover, without the use of applied N fertilizer, were consistent with the target yields for growing conditions of the trials (2.3 to 5.4 t/ha). Crop yields after phases of these pasture legume species were similar or higher than those following subterranean clover or annual medics. The results of this study suggest a single season of a legume-dominant pasture may provide sufficient organic N in the soil to grow at least one crop, without the need for inorganic N fertilizer application. This has implications for reducing inorganic N requirements and the carbon footprint of cropping in dryland agricultural systems.

Pigeonpea is an important grain legume. It contributes to the improvement of soil fertility through biological nitrogen (N) fixation. However, the symbiotic efficiency of pigeonpea with native soil rhizobia has not been determined adequately. This study was designed to determine the variation in the N fixation ability of pigeonpea inoculated with the native rhizobia. Forty soil samples were collected from diverse locations across South Africa and used for inoculating pigeonpea seed. Each pigeonpea genotype was inoculated separately with each soil sample and raised in a nitrogen-depleted growth medium in the greenhouse. A split-plot experimental design was used in the study. Several N fixation variables of pigeonpea were measured. There was >40.0% difference in the number of nodules between genotypes ‘Ex-PP-MD-321’ and ‘Mpuma-B-Spot’ but the nodule dry weight between the two genotypes was >80.0%. In contrast, the heaviest dry shoots (0.4513 g), weighed 52.0% heavier than those that were observed for ‘Mpuma-B-Spot’. Pigeonpea showed differential N fixation ability with the nodules, suggesting that there was potential to select for optimum host × rhizobial isolate combinations for the process and to expand the production area of the crop.

Climbing beans play a central role in food security of rural households in the densely populated highlands of East and Central Africa. Soil fertility degradation and the lack of nutrient inputs are major limitations to yield of beans and other crops. We conducted field trials in Northern Rwanda in Kinoni and Muko villages to evaluate the effect of mineral N, P, and K fertilizers (both alone and in combination) and farmyard manure on nitrogen fixation and grain yields of climbing bean in smallholder farmers’ fields. The trials were laid down in a randomized complete block design with seven replicate blocks in each village. Manure and fertilizer application led to greater yields in all fields, and the largest yields were achieved when manure was combined with NPK. Large variability in yield between fields was observed. Application of fertilizer together with manure increased the grain yield from 1.5 to 3.9 t ha−1 in Kinoni and from 2.6 to 5.4 t ha−1 in Muko. Fertilizer and/or manure increased stover yield from 0.8 to 2.3 t ha−1 in Kinoni and from 1.5 to 3.4 t ha−1 in Muko. Application of 30 kg P ha−1 and 5 t manure ha−1 led to increased N and P uptake (from 49 to 106 kg N ha−1 and from 6.1 to 12.4 kg P ha−1 in Kinoni and from 46 to 128 kg N ha−1 and from 5.3 to 17.9 kg P ha−1 in Muko). There was no clear relationship between soil fertility characteristics and the response of climbing bean to applied inputs at Muko site. However, at Kinoni site, limited response to manure and NPK application was observed in plots where soil available P and soil exchangeable K were relatively low. Our results show the benefits of using manure along with mineral fertilizers for increased climbing bean yields and nutrient uptake in smallholder farming systems.

Nitrogen-fixing plants provide critical nitrogen inputs that support the high productivity of tropical forests, but our understanding of the ecology of nitrogen fixers – and especially their interactions with herbivores – remains incomplete. Herbivores may interact differently with nitrogen fixers vs. non-fixers due to differences in leaf nitrogen content and herbivore defence strategies. To examine these potential differences, our study compared leaf carbon, nitrogen, toughness, chemical defence and herbivory for four nitrogen-fixing tree species (Inga oerstediana, Inga sapindoides, Inga thibaudiana and Pentaclethra macroloba) and three non-fixing species (Anaxagorea crassipetala, Casearia arborea and Dipteryx panamensis) in a lowland tropical rain forest. Leaf chemical defence, not nutritional content, was the primary driver of herbivore damage among our species. Even though nitrogen fixers exhibited 21.1% higher leaf nitrogen content, 20.1% lower C:N ratios and 15.4% lower leaf toughness than non-fixers, we found no differences in herbivory or chemical defence between these two plant groups. Our results do not support the common hypotheses that nitrogen fixers experience preferential herbivory or that they produce more nitrogen-rich defensive compounds than non-fixers. Rather, these findings suggest strong species-specific differences in plant–herbivore relationships among both nitrogen-fixing and non-fixing tropical trees.

The effect of chlorsulfuron on growth, infective ability, and symbiotic performance of legume bacteria was investigated. Bacterial growth in pure culture was unaffected by the addition of 0.55 and 5.5 μM chlorsulfuron. Early root hair infections of alfalfa by bacteria were inhibited by chlorsulfuron at 0.28 pM but not at 0.0028 pM in the root media. Inhibition of infection resulted from herbicidal effects on the root hair development. When inoculated red clover and alfalfa plants were grown in aseptic cultures in the presence of 0, 5.5, or 55 μM of chlorsulfuron, only 55 μM of herbicide inhibited the development and nodulation of plants. Early emergence and growth of alfalfa plants in soil supplemented with 2 × 10-6, 2 × 10-3, 2, 4, and 8 g/ha of chlorsulfuron were unaffected. However, 5 to 8 days after emergence plants grown in soil supplemented with 4 or 8 g/ha of chlorsulfuron were severely damaged, with no nodules developed. Nodulation occurred in plants grown in soil containing 2 × 10-3, 3 × 10-6, and 2 g/ha chlorsulfuron, but the nitrogenase activity of the nodules of these plants was less than for control plants. Plants grown in soil containing 2 g/ha of chlorsulfuron developed normally only after an initial growth inhibition of 5 to 6 weeks. The inhibition of nodulation and nitrogenase activity of nodules grown in the presence of chlorsulfuron is probably due to adverse effects of the herbicide on plant growth and development rather than on the rhizobia.

Two strains of crownvetch (Coronilla varia L. # CZRVA) rhizobia were cultured in vitro with various rates of atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine] and bifenox [methyl 5-(2,4-dichlorophenoxy)-2-nitrobenzoate]. Growth, measured turbidimetrically over 48 h, was similar for both strains. Atrazine and bifenox significantly reduced bacterial growth after 14 and 36 h, respectively, only at the highest concentrations tested (463 μM atrazine and 292 μM bifenox). Since growth of crownvetch rhizobia was apparently not affected by rates of atrazine or bifenox above reasonable soil solution concentrations, it is likely that herbicidal effects on nodulation were due to toxicity to the host plant rather than toxicity to these bacteria. In a growth chamber experiment, total nodule activity (TNA) and carbon dioxide exchange rate (CER) were measured simultaneously in an effort to distinguish direct atrazine effects on nodule function from indirect effects due to inhibition of photosynthesis and a resulting decrease in photosynthate supply to nodules. When 5 and 50 mg atrazine per kg soil were applied to intact plants, CER was severely reduced within 24 h, but similar reductions in TNA were not observed until 48 h after treatment. Total nodule activity was reduced similarly by atrazine and defoliation; the application of atrazine to defoliated plants did not inhibit TNA more than did defoliation alone. The data indicate that reductions in crownvetch nodule activity by atrazine are due to inhibition of photosynthesis or other processes rather than direct toxicity to N fixation.

Aqueous extracts of quackgrass [Agropyron repens (L.) Beauv. # AGRRE] shoots and rhizomes inhibited seed germination and root growth of alfalfa (Medicago sativa L. ‘Vernal’), soybean (Glycine max (L.) Merr. ‘Corsoy 79’], navybean (Phaseolus vulgaris L. ‘Seafarer’), and curly cress (Lepidium sativum L.) at concentrations of less than 2.5 mg dried extract/ml. Extracts of quackgrass shoots were generally more inhibitory than extracts of rhizomes. Root and shoot dry weights of snapbeans (Phaseolus vulgaris L. ‘Bush Blue Lake’) grown under sterile conditions were reduced by aqueous extracts of shoots. Root systems were stunted and necrotic and lacked root hairs. The growth of Rhizobium species was not influenced by the presence of 40 or 80 mg/ml concentrations of extracts of shoots or rhizomes. Quackgrass may inhibit indirectly the legume-Rhizobium symbiosis by inhibiting root hair formation rather than directly inhibiting Rhizobium growth. The presence of soil microorganisms was not necessary for the development of quackgrass toxicity in soil or agar. Soil microorganisms reduced toxicity of quackgrass residues in soil.

The survival of two Rhizobium japonicum strains was determined in soils treated with 13 herbicides used in soybeans [Glycine max (L.) Merr.]. Survival in soils treated with concentrations equivalent to or 10-fold greater than those from field applications at recommended rates was compared to survival in untreated soil under controlled temperature and moisture conditions. Nine of the herbicides had transitory or no effects, regardless of rate. Acifluorfen {5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid}, bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide], fluchloralin [N-(2-chloroethyl)-2,6-dinitro-N-propyl-4-(trifluoromethyl)benzenamine], and dinoseb [2-(1-methylpropyl)-4,6-dinitrophenol] consistently reduced survival of one or both strains at rates 10-fold greater than recommended. At rates of application equivalent to suggested field use rates, effects on survival were reduced or eliminated. Reductions in survival attributable to herbicides were in some cases comparable to those obtained by a weekly wetting and drying of the soil, indicating that herbicide effects on R. japonicum may be equivalent in magnitude to effects due to environmental stress. The herbicides were not sufficiently toxic to reduce R. japonicum populations in soil to levels that would be likely to affect nodulation.

Studies were conducted to determine whether photosynthesis-inhibiting herbicides atrazine or bifenox, and shade affect the number, weight, senescence, and N fixation activity of nodules on the roots of the perennial legume crownvetch. Atrazine and bifenox were applied at rates of 2.24 kg ai/ha to shaded and unshaded plots. The shade was varied during the growing season to simulate changing irradiance levels beneath a corn canopy. Atrazine and bifenox treatments reduced nodule number to 13 and 42% of the untreated control, respectively, in 1980, and 18 and 35% in 1981. Shade treatments reduced nodule number to a low of 40% of the control in 1980 and 50% in 1981. Combined effects of herbicides and shade on nodule numbers were more than additive. Nodule fresh weights were reduced an average of 37% by herbicide treatments and 39% by shade treatments. Sloughed nodule numbers decreased in the herbicide and shaded treatments, suggesting that the reduction in nodule numbers was due to fewer nodules being produced. Nodule numbers were reduced a greater percentage by herbicides and shade than was herbage dry matter production. Specific nodule activity (SNA) did not differ in nodules from the atrazine, bifenox, or shade treatments on the six sampling dates in 1980 or on two of three sampling dates in 1981.

Of 15 pesticides evaluated in a screening test with respect to their effects on growth and nitrogen assimilation of the Azolla mexicana (Presl) – Anabaena azollae (Strasburger) symbiosis, the bipyridilium and phenolic herbicides at 0.1 ppmw were the most detrimental, causing up to a 75% reduction in nitrogen fixation and nitrate reduction with little or no effect on growth. Chloramben [3-amino-2,5-dichlorobenzoic acid] at 1.0 ppmw, and dicamba [3,6-dichloro-o-anisic acid], and benomyl [methyl-1-(butylcarbamoyl)-2-benzimidazolyl carbamate] at 10.0 ppmw caused an 84 to 99% reduction in nitrogen fixation without affecting nitrate reduction or growth. Simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] at 10.0 ppmw stimulated nitrate reduction 20 fold, causing a 99% reduction in nitrogen fixation. Growth and nitrogen assimilation were reduced at similar concentrations between 0.1 and 10 ppmw for each of the other benzoic, triazine, dinitroanaline, and urea herbicides tested. Naptalam [N-1-naphthylphthalamic acid] was the only pesticide tested that had no effect on growth or nitrogen assimilation at 10.0 ppmw.

The objective of this study was to evaluate the enzymatic activity of homogenates of insects fed on grain of cowpea, Vigna unguiculata (L.), cultivars grown with different nitrogen sources. For the experiment we used aliquots of the homogenate of 100 unsexed adult insects, emerged from 10 g of grain obtained from four cowpea cultivars: ‘BRS Acauã’, ‘BRS Carijó’, ‘BRS Pujante’, and ‘BRS Tapaihum’ grown under different regimes of nitrogen sources: mineral fertilizer, inoculation with strains of diazotrophs (BR 3267, BR 3262, BR 3299; INPA 03–11B, 03–84 UFLA, as well as the control (with soil nitrogen). The parameters evaluated were enzymatic activities of insect protease, amylase and lipase and the starch content of the grains. There were differences in the enzymatic activity of amylase, lipase and protease of insect homogenate according to the food source. A lower activity of the enzyme amylase from C. maculatus homogenate was observed when insects were fed grain of the cultivar BRS Carijó. A lower activity of lipase enzyme from C. maculatus homogenate was observed when the insects fed on grain from the interaction of the cultivar Tapaihum inoculated with BR 3262 diazotrophs. The lowest proteolytic activity was observed in homogenate of insects fed on interaction of ‘BRS Carijó’ inoculated with BR 3262 diazotrophs. Starch content correlated positively with the amylase activity of C. maculatus homogenate. The cultivar BRS Carijó had a different behavior from the other cultivars, according to the cluster analysis.

Pulse crop management can increase pulse yields and N fixation, but the effects of previous pulse crop management on subsequent crop performance is poorly understood. Field studies were conducted at three locations, in the Parkland region of Alberta, Canada, between 2004 and 2006. Tannin-free faba bean, narrowleaf lupin, and field pea were planted at 0.5, 1.0, 1.5, and 2.0 times the recommended pulse planting density (PPD), with or without barley as a model weed. Faba bean produced the highest seed yields in higher precipitation environments, whereas pea produced the highest seed yields in lower precipitation environments. Lupin seed yields were consistently low. In the absence of weed interference, faba bean, pea, and lupin N-fixation yields ranged from 70 to 223, 78 to 147, and 46 to 173 kg N ha−1, respectively. On average, faba bean produced the highest N-fixation yield. The absence of weed interference and a high PPD increased pulse seed and N-fixation yields. Quality wheat crops were grown on pulse stubble without additional N fertilizer in some site–years. Management practices that increased N fixation resulted in only marginal subsequent wheat yield increases. Subsequent wheat seed yield was primarily influenced by pulse species. Pea stubble produced 11% higher wheat yields than lupin stubble but only 2% higher wheat yields than faba bean stubble. Consistently high wheat yields on pea stubble may be attributed to synchronized N release from decomposing pea residues with subsequent crop N demand and superior non-N rotational benefits.

Wood-boring bivalves (Bivalvia, family Teredinidae), also known as shipworms, host dinitrogen-fixing and cellulolytic symbiotic bacteria in gill bacteriocytes, which may be a necessary adaptation to a wooden diet. Although oxygen (O2) inhibits nitrogenase in other species, symbionts are able to fix nitrogen (N) within the gill tissue and provide newly fixed N to the host shipworm. The recent direct evidence of new N incorporation into the host tissue indicates that there are potentially complex nutrient cycles in this symbiosis and uninvestigated controls upon these cycles.To elucidate the mechanisms of this unique N2-fixing symbiosis and determine whether symbionts can excrete newly fixed N, we measured rates of growth, N2-fixation, respiration, and inorganic N content for the cultivated symbiont Teredinibacter turnerae (γ-proteobacteria, strain T7901) under a range of headspace O2 conditions. In all conditions, headspace O2 did not affect maximum specific N2-fixation and respiration activity, but did influence the rate and timing of growth. These results are consistent with the development of microaerobic conditions through an oxygen gradient in the culture medium, which facilitates N2-fixation and growth. The medium accumulated a small amount of NH4+, which represented 0.5–2.5% of the total N fixed by the culture. We constructed a simple N budget for T. turnerae to assess the role of the major known N sources and sinks. The N budget was not closed, indicating that new N is allocated to currently unidentified sinks, which may include excreted dissolved organic nitrogen.

Mars' total atmospheric nitrogen content is 0.2 mbar. One-dimensional (1D) photochemical simulations of Mars' atmosphere show that nitric acid (HNO3(g)), the most soluble nitrogen oxide, is the principal reservoir species for nitrogen in its lower atmosphere, which amounts to a steady-state value of 6×10−2 kg or 4 moles, conditions of severe nitrogen deficiency. Mars could, however, support ∼1015 kg of biomass (∼1 kg N m−2) from its current atmospheric nitrogen inventory. The terrestrial mass ratio of nitrogen in biomass to that in the atmosphere is ∼10−5; applying this ratio to Mars yields ∼1010 kg of total biomass – also, conditions of severe nitrogen deficiency. These amounts, however, are lower limits as the maximum surface-sink of atmospheric nitrogen is 2.8 mbar (9×1015 kg of N), which indicates, in contradistinction to the Klingler et al. (1989), that biological metabolism would not be inhibited in the subsurface of Mars. Within this context, we explore HNO3 deposition on Mars' surface (i.e. soil and ice-covered regions) on pure water metastable thin liquid films. We show for the first time that the negative change in Gibbs free energy increases with decreasing HNO3(g) (NO3−(aq)) in metastable thin liquid films that may exist on Mars' surface. We also show that additional reaction pathways are exergonic and may proceed spontaneously, thus providing an ample source of energy for nitrogen fixation on Mars. Lastly, we explore the dissociation of HNO3(g) to form NO3−(aq) in metastable thin liquid films on the Martian surface via condensed phase simulations. These simulations show that photochemically produced fixed nitrogen species are not only released from the Martian surface to the gas-phase, but more importantly, transported to lower depths from the Martian surface in transient thin liquid films. A putative biotic layer at 10 m depth would produce HNO3 and N2 sinks of −54 and −5×1012 molecules cm−2 s−1, respectively, which is an ample supply of available nitrogen that can be efficiently transported to the subsurface. The downward transport as well as the release to the atmosphere of photochemically produced fixed nitrogen species (e.g. NO2−, NO and NO2) suggests the existence of a transient but active nitrogen cycle on Mars.

Low phosphorus (P) supply frequently has been shown to limit the abundance and activity of nitrogen (N)-fixing organisms, potentially constraining N inputs to ecosystems. Previous research in a montane Hawaiian forest has shown that ground-level P-fertilization led to significant increases in the population size of epiphytic N-fixing lichens (cyanolichens), as well as a shift in community composition from crustose to leafy species. In this study, we ask whether these changes in the cyanolichen community have resulted in increased N inputs to the forest, and also whether the very high levels of P in the canopy of P-fertilized forest stimulate individual lichen fixation rates over those of lichens from a nearby unfertilized reference forest. We used acetylene reduction (AR) assays to measure the fixation rates of 14 cyanolichen species from P-fertilized forest, and calibrated these rates by measuring 15N2 fixation incorporation in four species. We found that the ratio of acetylene reduced to N fixed ranged from 2.4 ± 0.4 in Pseudocyphellaria crocata to 9.3 ± 2.4 in Leptogium denticulatum. Nitrogen fixation rates in the P-fertilized forest ranged from 0.64 ± 0.05 nmol N cm−2 h−1 in Nephroma helveticum to 3.97 ± 1.48 nmol N cm−2 h−1 in Parmeliella nigrocincta. Fixation rates did not vary greatly among species from P-fertilized forest. We compared these P-fertilized rates to those of 10 species from the reference forest, and found that mass-based fixation rates of P-fertilized lichens were not greater than those of lichens from the unfertilized forest. Using the measured AR rates, we estimate that the P additions increase cyanolichen N inputs to the forest 30-fold, from ~0.3 kg N ha−1 y−1 to ~9 kg N ha−1 y−1. These results suggest that P additions to this ecosystem increase N inputs primarily by increasing the abundance of cyanolichens, and that shifts in cyanolichen community composition and changes in individual fixation rate were of lesser importance in determining ecosystem N inputs.

Nitrogen stable isotope composition (δ15N) of particulate organic matter (POM) has been used to infer dominant nitrogen cycling processes in lakes. However, very few studies have compared the isotope variations in lakes along trophic state and other biogeochemical gradients. Here we report an analysis of δ15NPOM and selected environmental variables from 96 subtropical lakes to assess the patterns and controls of isotope variations. Results indicate that δ15NPOM values varied from −2.8 to 13.2‰ and were not significantly correlated with total phosphorus (TP), total nitrogen (TN) and chlorophyll a (Chl a). The 15N depletion in POM was found across the entire trophic gradient and likely reflected contributions from planktonic nitrogen fixation. The 15N enrichment was attributed to high primary production and the contributions of anthropogenic wastes in eutrophic lakes, and the presence of microzooplankton in the water samples of the oligotrophic lakes. The δ15NPOM was negatively correlated with water color and positively correlated with pH. Because water color is indicative of light availability which affects phytoplankton growth and pH is significantly correlated with Chl a concentration in the study lakes, the close relationship between water color, pH and δ15NPOM therefore suggests primary productivity-driven isotope fractionation. Results from this study did not reveal the importance of trophic state, nitrogen concentration and surface area to the variations of δ15NPOM and points to the complex interactions of nitrogen cycling processes in lakes with diverse biogeochemical features.

We have studied a number of related processes of the nitrogen cycle in a Brazilian floodplain lake to identify the major pools and pathways over a short time period. The study was centred on the littoral zone dominated by the floating plant Eichhornia azurea, which has a large epiphyte algal community of which heterocystous cyanobacteria were the major components. The water column was continuously undersaturated with oxygen although some elevated values (to 60% saturation) were recorded in the macrophyte beds in the afternoon. Marked diel temperature changes were documented. NH4-N dominated the dissolved N component in the water with maximal values (60 mg m−3) at lowest O2, concentrations early in the morning. Nitrogen fixation (acetylene reduction) of the epiphyte community showed marked diel changes with daily values of 5 mg N fixed m−2 day−1 (based on 3:1 C2H4:N2 ratio). Macrophyte NH4-N uptake rates (in situ incubations) were 93 mg N m−2 day−1. The activities of nitrifying bacteria could not be detected with the nitrapyrin block on dark CO2 fixation but denitrification (acetylene block technique) was recorded in the sediments when enhanced with NO-3. The major pathways of aquatic nitrogen involved macrophyte uptake and sediment release of NH4-N.

As a consequence of its relatively high productivity in primary successional sites on Mauna Loa, Hawai'i, the mat-forming fern Dicranopteris linearis can influence attributes of soil detrital pools. Decomposition, nutrient release and rates of asymbiotic N fixation in Dicranopteris litter were determined over an elevational range of oligotrophic sites. ‘Hot spots’ of nitrogen fixation occurred in Dicranopteris litter, as evidenced by acetylene reduction rates as high as 22 nmol g−1 h−1. However, potential N fixation rates for the entire litter mass were 0–0.1 g m−2 y−1, less than other N inputs such as rainfall. Dicranopteris' decomposition rates were low compared to other tropical species, even under high temperature and rainfall conditions, with ≥50% of the original leaf and >77%of the stem mass remaining after 2 y of decomposition. Slow decomposition was related to high ligninrnitrogen ratios (56–129) in litter and above-ground positioning of unabscised, decomposing litter. As a result of its slow decomposition rates, Dicranopteris is an major contributor to soil detrital pools. Aggradation of the detrital pool is an important process whereby nutrients are accrued within these ecosystems. Consequently, Dicranopteris exerts an important influence on soil genesis and ecosystem development during primary succession on pahoehoe lava.