Biological Nitrogen Fixation Inputs Research Articles

Nutrient interactions and salinity effects on plant uptake of phosphorus from waste-based fertilisers

Many organically managed farms in Europe have low levels of soil phosphorus (P). Arable farms that rely strongly on biological nitrogen fixation (BNF) have been shown to have rather low outputs and a tendency to deplete soil P and potassium (K) compared with arable farms that have a lower reliance on BNF and higher external inputs. Therefore, research focusing on providing a balanced input of nitrogen (N), P, K and sulphur (S) from alternative sources is of interest to organically managed farms in Europe. The aim of this study was to quantify P availability from different organic wastes applied alone or in combination to improve the mixtures’ N:P:K:S ratio. P availability was measured by P uptake and recovery in ryegrass grown in pots. The isotope dilution approach was used in which a non-labelled fertiliser is added to a soil that has been pre-incubated and equilibrated with labelled 33P. The P recovery of the different organic wastes varied significantly (10–20 %). Manure and anaerobically digested manure mixed with ash from straw had the lowest P recovery. All the organic waste treatments had higher plant growth and P uptake compared with the negative control, but none of them reached the values observed after application of mineral P. Mixing digested manure with ash increased soil pH at the end of the experiment, which may explain the lower P availability. The highest P recovery was found in digested products, either manure alone or mixed with municipal waste or the industrial waste product Fertigro®. However, the mixture of digested manure and Fertigro® led to lower dry matter production, whereas Fertigro® used alone resulted in high leaf P concentrations but depressed shoot and root growth, presumably due to salinity effects and a decrease in soil pH. Anaerobic digestion increased the availability of P, which may be explained by the lower immobilisation potential of the remaining organic matter in the digestate. This study highlights the potential challenges when attempting to improve the N:P:K:S ratios of waste-based fertilisers through mixing due to material interactions. However, such effects are likely to be overexpressed in pot trials that have a limited soil volume. Field trials are therefore needed to quantify such effects in practice.

A spatially explicit, empirical estimate of tree-based biological nitrogen fixation in forests of the United States.

Quantifying human impacts on the N cycle and investigating natural ecosystem N cycling depend on the magnitude of inputs from natural biological nitrogen fixation (BNF). Here, we present two bottom-up approaches to quantify tree-based symbiotic BNF based on forest inventory data across the coterminous US plus SE Alaska. For all major N-fixing tree genera, we quantify BNF inputs using (1) ecosystem N accretion rates (kg N ha-1 yr-1) scaled with spatial data on tree abundance and (2) percent of N derived from fixation (%Ndfa) scaled with tree N demand (from tree growth rates and stoichiometry). We estimate that trees fix 0.30-0.88 Tg N yr-1 across the study area (1.4-3.4 kg N ha-1 yr-1). Tree-based N fixation displays distinct spatial variation that is dominated by two genera, Robinia (64% of tree-associated BNF) and Alnus (24%). The third most important genus, Prosopis, accounted for 5%. Compared to published estimates of other N fluxes, tree-associated BNF accounted for 0.59 Tg N yr-1, similar to asymbiotic (0.37 Tg N yr-1) and understory symbiotic BNF (0.48 Tg N yr-1), while N deposition contributed 1.68 Tg N yr-1 and rock weathering 0.37 Tg N yr-1. Overall, our results reveal previously uncharacterized spatial patterns in tree BNF that can inform large-scale N assessments and serve as a model for improving tree-based BNF estimates worldwide. This updated, lower BNF estimate indicates a greater ratio of anthropogenic to natural N inputs, suggesting an even greater human impact on the N cycle.

Regional constraints to biological nitrogen fixation in post‐fire forest communities

Biological nitrogen fixation (BNF) is a key ecological process that can restore nitrogen (N) lost in wildfire and shape the pace and pattern of post-fire forest recovery. To date, there is limited information on how climate and soil fertility interact to influence different pathways of BNF in early forest succession. We studied asymbiotic (forest floor and soil) and symbiotic (the shrub Ceanothus integerrimus) BNF rates across six sites in the Klamath National Forest, California, USA. We used combined gradient and experimental phosphorus (P) fertilization studies to explore cross-site variation in BNF rates and then related these rates to abiotic and biotic variables. We estimate that our measured BNF rates 22 years after wildfire (6.1-12.1 kg N x ha(-1) x yr(-1)) are unlikely to fully replace wildfire N losses. We found that asymbiotic BNF is P limited, although this is not the case for symbiotic BNF in Ceanothus. In contrast, Ceanothus BNF is largely driven by competition from other vegetation: in high-productivity sites with high potential evapotranspiration (Et), shrub biomass is suppressed as tree biomass increases. Because shrub biomass governed cross-site variation in Ceanothus BNF, this competitive interaction led to lower BNF in sites with high productivity and Et. Overall, these results suggest that the effects of nutrients play a larger role in driving asymbiotic than symbiotic fixation across our post-fire sites. However, because symbiotic BNF is 8-90x greater than asymbiotic BNF, it is interspecific plant competition that governs overall BNF inputs in these forests.

Opportunities and constraints to legume diversification for sustainable maize production on smallholder farms in Malawi

AbstractSustainable intensification of smallholder farms in Africa is highly dependent on enhancing biological nitrogen fixation (BNF). Legume diversification of maize-based systems is a core example of sustainable intensification, with the food security of millions of farm families at stake. This study highlights the constraints and opportunities associated with the adoption of legumes by smallholder farmers in southern Africa. A two-part survey of households and farm fields (n=88) was conducted in the Ekwendeni watershed of northern Malawi. Participatory research and education activities have been underway for over a decade in this region, resulting in expanded uptake of a range of legume species as intercrops and in rotation with the staple maize crop. Farmer adoption has occurred to a varying extent for soybean (Glycine max), pigeon pea (Cajanus cajan), velvet bean (Mucuna pruriens) and fish bean (Tephrosia vogelii). Farmers, working with the project valued pigeon pea and other legumes for soil fertility purposes to a greater extent than farmers not working with the project. Legumes were valued for a wide range of purposes beyond soil cover and fertility enhancement, notably for infant nutrition (at least for soybean), insect control, and vegetable and grain production for both market and home consumption. Literature values for BNF in tropical legumes range up to 170 kg N ha−1for grain and 300 kg N ha−1for green manure species; however, our field interviews illustrated the extent of constraints imposed by soil properties on smallholder fields in Malawi. The key edaphic constraints observed were very deficient to moderate phosphorus levels (range 4–142, average 33 mg kg−1), and moderately acid soils (range pH 5.1–7.9, average 6.2). The per farm hectarage devoted to legume production relative to maize production was also low (0.15 versus 0.35 ha), a surprising find in an area with demonstrated interest in novel legume species. Further, farmers showed a strong preference for legumes that produced edible grain, regardless of the associated nutrient removal in the harvested grain, and did not sow large areas to legume crops. These farm-level decisions act as constraints to BNF inputs in maize-based smallholder cropping systems. Overall, we found that legume productivity could be enhanced. We documented the value of policies and educational efforts that support farmers gaining access to high-quality seeds, amendments for phosphorus-deficient soils, and promotion of multipurpose legumes that build soils through leafy residues and roots, as well as providing grain for food security and sales.

Nitrogen fixation in different biogeochemical niches along a 120 000‐year chronosequence in New Zealand

Biological nitrogen fixation (BNF) is the major nitrogen (N) input in many terrestrial ecosystems, yet we know little about the mechanisms and feedbacks that control this process in natural ecosystems. We here examine BNF in four taxonomically and ecologically different groups over the course of forest ecosystem development. At nine sites along the Franz Josef soil chronosequence (South Westland, New Zealand) that range in age from 7 to 120000 yr old, we quantified BNF from the symbiotic plant Coriaria arborea, cyanolichens (primarily Pseudocyphellaria spp.), bryophytes (many species), and heterotrophic bacteria in leaf litter. We specifically examined whether these groups could act as "nitrostats" at the ecosystem level, turning BNF on when N is scarce (early in primary succession) and off when N is plentiful (later in succession and retrogression). Coriaria was abundant and actively fixing (approximately 11 kg N x ha(-1) x yr(-1)) in the youngest and most N-poor site (7 yr old), consistent with nitrostat dynamics. Coriaria maintained high BNF rates independent of soil N availability, however, until it was excluded from the community after a single generation. We infer that Coriaria is an obligate N fixer and that the nitrostat feedback is mechanistically governed by species replacement at the community level, rather than down-regulation of BNF at the physiological scale. Biological nitrogen fixation inputs from lichens (means of 0-2 kg N x ha(-1) x yr(-1)), bryophytes (0.7-10 kg N x ha(-1) x yr(-1)), and litter (1-2 kg N x ha(-1) x yr(-1)) were driven primarily by changes in density, which peaked at intermediate-aged sites (and increased with soil N availability) for both lichens and bryophytes, and grew monotonically with soil age (but did not change with soil N) for litter. This non-nitrostatic link between soil N availability and lichen/bryophyte BNF likely stems from increased tree biomass in more fertile sites, which increases epiphytic moisture conditions and habitable surface area. This apparent positive feedback could produce N-rich conditions.

Bradyrhizobium strain and the 15N natural abundance quantification of biological N2 fixation in soybean

In commercial plantations of soybean in both the Southern and the Cerrado regions, contributions from biological nitrogen fixation (BNF) are generally proportionately high. When using the 15N natural abundance technique to quantify BNF inputs, it is essential to determine, with accuracy, the 15N abundance of the N derived from BNF (the 'B' value). This study aimed to determine the effect of four recommended strains of Bradyrhizobium spp. (two B. japonicum and two B. elkanii) on the 'B' value of soybean grown in pots in an open field using an equation based on the determination of δ15N natural abundance in a non-labelled soil, and estimate of the contribution of BNF derived from the use of 15N-isotope dilution in soils enriched with 15N. To evaluate N2 fixation by soybean, three non-N2-fixing reference crops were grown under the same conditions. Regardless of Bradyrhizobium strain, no differences were observed in dry matter, nodule weight and total N between labelled and non-labelled soil. The N2 fixation of the soybeans grown in the two soil conditions were similar. The mean 'B' values of the soybeans inoculated with the B. japonicum strains were -1.84 ‰ and -0.50 ‰, while those inoculated with B. elkanii were -3.67 ‰ and -1.0 ‰, for the shoot tissue and the whole plant, respectively. Finally, the 'B' value for the soybean crop varied considerably in function of the inoculated Bradyrhizobium strain, being most important when only the shoot tissue was utilised to estimate the proportion of N in the plant derived from N2 fixation.

Long-term Effects of Pre-harvest Burning and Nitrogen and Vinasse Applications on Yield of Sugar Cane and Soil Carbon and Nitrogen Stocks on a Plantation in Pernambuco, N.E. Brazil

Since the 1970s the area under sugarcane in Brazil has increased from 2 million to over 5 million ha (M ha), and it is expected to pass the 7 M ha mark in 2007. More than half of the cane is harvested to produce bioethanol as a fuel for light vehicles. The distilleries produce approximately 13 L of distillery waste (vinasse) for each litre of ethanol produced. In the 1980s there was considerable concern over the long-term effects of the disposal of this material (containing about 1% carbon and high in K) on cane yields if it was applied to the field. At the same time there was a growing movement to abandon the practice of pre-harvest burning and some research was showing that some Brazilian varieties of sugar cane were able to obtain significant contributions of N from plant-associated biological nitrogen fixation (BNF). For these reasons an experiment was installed on a cane plantation in the state of Pernambuco, NE Brazil to investigate the long-term effects of vinasse and N fertiliser additions and the practice of pre-harvest burning on crop and sugar yield, soil fertility parameters, N balance and soil C stocks. The results showed that over a 16-year period, trash conservation (abandonment of burning) increased cane yields by 25% from a mean of 46 to 58 Mg ha−1. Vinasse applications (80 m3 ha−1 crop−1) increased mean cane and sugar yield by 12 to 13% and the application of 80 kg N ha−1 as urea increased cane yields by 9%, but total sugar yield by less than 6% (from 7.0 to 7.4 Mg ha−1 crop−1). The total N balance for the soil/plant system when only the surface 20 cm of the soil was considered was positive in plots where no N fertiliser was added. However, the data indicated that during the 16 years of the study considerable quantities of soil organic matter were accumulated below 20 cm depth such that the N balance considering the soil to 60 cm depth was strongly positive, except where N fertiliser was added. The data indicated that there were considerable BNF inputs to the system, which was consistent with its low response to N fertiliser and low N fertiliser-use-efficiency. There were no significant effects of vinasse or urea addition, or trash conservation on soil C stocks, although the higher yields proportioned by trash conservation had potentially significant benefits for increased mitigation of CO2 emissions where the main use of the cane was for bioethanol production.

The success of BNF in soybean in Brazil

Approximately forty years after commercial cropping of soybean in Brazil began, the total area under this crop has reached over 13 M ha with a mean productivity of 2400 kg ha−1. Soybean varieties introduced from the USA and varieties rescued from early introductions in Brazilian territory were part of the Brazilian soybean-breeding programme which spread the crop from high to low latitudes. Disease-resistance, pest-resistance, tolerance to low fertility soils, as well as production of plants with pods sufficiently high above the ground for efficient mechanical harvesting, were all aims of the programme. Although BNF was not explicitly considered as a trait for selection in the breeding/selection programme, maximisation of biological nitrogen fixation (BNF) was favoured by conducting selection and breeding trials on soils low in N, in which the seeds were inoculated with efficient Bradyrhizobium inoculants but without N fertiliser application. Several efficient imported Bradyrhizobium strains were found to be unable to compete with native soil micro-flora and other previously-introduced Bradyrhizobium strains. Surprisingly, after being in the soil for many years one or two of these strains had become more competitive while maintaining their high BNF capacity. Today, these strains are included amongst the recommended Brazilian inoculants and have promoted significant improvements in grain yields. The breeding of soybeans in conditions that made grain yield highly dependent on BNF, and the continuous attention paid to the selection of Bradyrhizobium strains appropriate for the newly released varieties, have been the main contributors to today's high yields and their great benefit to the Brazilian economy. There seems to be no reason why this ongoing research programme should not serve as an appropriate model to improve BNF inputs to grain legumes in other countries of the world.

Use of the15N natural abundance technique for the quantification of the contribution of N2 fixation to sugar cane and other grasses

This paper originates from an address at the 8th International Symposium on Nitrogen Fixation with Non-Legumes, Sydney, NSW, December 2000 The use of the 15N natural abundance technique to quantify contribution of biological nitrogen fixation (BNF) to any plant is based on the observation that N derived from soil is generally slightly different [usually higher in 15N abundance (δ15N‰)] than that of the air. Plants or micro-organisms growing solely on BNF generally accumulate N with 15N isotopic abundance lower than that of the air (i.e. δ15N‰ is negative), while plants obtaining all N from the soil generally show a positive δ15N‰ signal. The technique is applied by estimating the 15N abundance of the putative ‘N2-fixing’ crop and analysing the 15N abundance of neighbouring non-N2-fixing reference plants. However, often there are such large variations in the N derived from the soil by different non-N2-fixing plants that in natural ecosystems it is often impossible to even distinguish plants that are benefiting form BNF, let alone quantity this contribution. The reasons why soil derived N can vary so widely, especially in natural ecosystems, are briefly discussed and a sampling strategy is described to assess possible BNF inputs to sugar cane plants in commercial plantations in Brazil. The results suggest that in nine of the 11 sites studied, BNF inputs were significant ranging from 25 to 60% of N assimilated.

Biological nitrogen fixation associated with tropical pasture grasses

This paper originates from an address at the 8th International Symposium on Nitrogen Fixation with Non-Legumes, Sydney, NSW, December 2000 The semi-humid or humid tropics are ideal for the production of large quantities of biomass from fast-growing C4 grasses, but high yields normally require large quantities of fertiliser, especially N, which has a very high input from fossil fuels (natural gas). A program has been started recently to use elephant grass (Pennisetum purpureum Schum.) to substitute firewood as a fuel and also to make charcoal for iron production. In this case, any large N fertiliser additions would mean that the yield of bio fuel per unit of fossil fuel invested would be detrimentally affected. In this study, we report on the potential for the selection of genotypes of fast-growing C4 tropical grasses of the genera Pennisetum and Brachiaria for their capacity to obtain N inputs from plant-associated biological nitrogen fixation (BNF). Fourteen genotypes each of Brachiaria and Pennisetum were screened for BNF contributions by growing them in 15N-labelled soil. In the case of the Pennisetum, after a suitable cutting height for the crop had been selected, there were large differences in dry matter production, N accumulation and 15N enrichment. The differences in 15N enrichment between genotypes were statistically significant and BNF inputs were estimated as high as 41% of accumulated N. In the study on Brachiaria genotypes, potential inputs of BNF seemed lower. Only one or two genotypes of B. brizantha and B. ruziziensis obtained more then 20% of their N from BNF. The N2-fixing bacteria that were most commonly associated with shoots and roots the Pennisetum genotypes were of the genus Herbaspirillum, but predominantly of a recently described new species. The Brachiaria spp. from three different sites (Rio de Janeiro, Goânia, Bahia) were predominately colonised by Azospirillum spp., most of the isolates being of the species Azospirillum amazonense. Very few Herbaspirilla were isolated from these plants.

Biological Dinitrogen Fixation in Gramineae and Palm Trees

Biological nitrogen fixation (BNF) in the Gramineae family has been well documented, but a complete understanding of this issue is needed to turn the research into a practical approach. The literature has a long and diverse list of diazotrophic bacteria found colonizing several plant tissues, such as roots, stems, leaves, and trash as well as the rhizosphere. However, only a limited amount of research has focussed on existing associations of N2-fixing microorganisms with grasses or cereal, especially for BNF inputs and ecological studies under field conditions. The recent discovery of the endophytic diazotroph bacteria such as Acetobacter diazotrophicus, Herbaspirillum spp. and Azoarcus spp. colonizing the interior of sugarcane, rice, Kallar grass (Leptochloa fusca (L.) Kunth), respectively, and other species of grasses as well as cereals has led to a considerable interest in exploring these novel associations. There is a general consensus that plant genotype is a key factor to higher contributions of BNF together with the selection of more efficient bacterial strains. This review summarizes the present data on this field and introduces the discovery of a new group of diazotrophic bacteria colonizing palm trees and therefore opening a future perspective for using these plants, especially African oil palm, to replace diesel as a fuel.

Biological Dinitrogen Fixation in Gramineae and Palm Trees

Biological nitrogen fixation (BNF) in the Gramineae family has been well documented, but a complete understanding of this issue is needed to turn the research into a practical approach. The literature has a long and diverse list of diazotrophic bacteria found colonizing several plant tissues, such as roots, stems, leaves, and trash as well as the rhizosphere. However, only a limited amount of research has focussed on existing associations of N2-fixing microorganisms with grasses or cereal, especially for BNF inputs and ecological studies under field conditions. The recent discovery of the endophytic diazotroph bacteria such as Acetobacter diazotrophicus, Herbaspirillum spp. and Azoarcus spp. colonizing the interior of sugarcane, rice, Kallar grass (Leptochloa fusca (L.) Kunth), respectively, and other species of grasses as well as cereals has led to a considerable interest in exploring these novel associations. There is a general consensus that plant genotype is a key factor to higher contributions of BNF...

Estimation of the contribution of biological N2 fixation to twoPhaseolus vulgaris genotypes using simulation of plant nitrogen uptake from15N-labelled soil

A technique for the application of the15N isotope dilution technique for the quantification of plant associated biological nitrogen fixation (BNF) was tested and applied to quantify the BNF contribution to two genotypes ofPhaseolus vulgaris. The technique makes use of sequential measurements of the15N enrichment of soil mineral N, and the uptake of labelled N by the “N2-fixing” plant, to simulate its uptake of soil N (the “soil to plant simulation” technique). The test was made with two non-N2-fixing crops (non-nodulating beans and wheat) and two bean genotypes (PR 923450 and Puebla 152), at two levels of N fertilizer addition (10 and 40 kg N ha−1), to compare the actual N uptake with that simulated from the soil and crop15N data. The simulation of the soil N uptake by the non-nod bean crop using this “soil to plant simulation” technique underestimated by 20 to 30% the true N uptake, suggesting that the mineral N extracted from soil samples taken from the 0–15cm layer had a higher15N enrichment than that N sampled by the roots of this crop. In the case of the wheat crop the simulation resulted in a much greater underestimation of actual N uptake. In general the results using this technique suggested that BNF inputs to the bean cultivars was higher than would be expected from the nodulation and acetylene reduction data, except for the early PR beans in the 40 kg N ha−1 treatment. In this case the total N and simulated soil N accumulation were well matched suggesting no BNF inputs. An allied technique (the “plant to plant simulation technique”) was proposed where the15N enrichrnent of soil mineral N was simulated from the data for total N and labelled N accumulation taken from sequential harvests of either of the non-N2 -fixing control crops. This was then utilized in combination with the labelled N uptake data of the other crop to simulate its soil N uptake. However, the results using either technique indicated that the wheat and non-nod or nodulating beans exploited pools of N in the soil with completely different15N enrichments probably due to differences in exploitation of the soil N with depth.