Ivan R. Kennedy

Biological nitrogen fixation in non-leguminous field crops : Recent advances

There is strong evidence that non-leguminous field crops sometimes benefit from associations with diazotrophs. Significantly, the potential benefit from N2 fixation is usually gained from spontaneous associations that can rarely be managed as part of agricultural practice. Particularly for dryland systems, these associations appear to be very unreliable as a means of raising the nitrogen status of plants. However, recent technical advances involving the induction of nodular structures on the roots of cereal crops, such as wheat and rice, offer the prospect that dependable symbioses with free-living diazotrophs, such as the azospirilla, or with rhizobia may eventually be achieved.

Prospects and potentials for systems of biological nitrogen fixation in sustainable rice production

The N requirement of rice crops is well known. To overcome acute N deficiency in rice soils, this element is usually supplied to the rice crop as the commercially available fertilizer urea. But unfortunately a substantial amount of the urea-N is lost through different mechanisms causing environmental pollution problems. Utilization of biological N fixation (BNF) technology can decrease the use of urea-N, reducing the environmental problems to a considerable extent. Different BNF systems have different potentials to provide a N supplement, and it is necessary to design appropriate strategies in order to use BNF systems for efficient N supply to a rice crop. Research has been conducted around the world to evaluate the potential of different BNF systems to supply N to rice crops. This paper reviews salient findings of these works to assess all the current information available. This review indicates that the aquatic biota Cyanobacteria and Azolla can supplement the N requirements of plants, replacing 30–50% of the required urea-N. BNF by some diazotrophic bacteria like Azotobacter, Clostridium, Azospirillum, Herbaspirillum and Burkholderia can substitute for urea-N, while Rhizobium can promote the growth physiology or improve the root morphology of the rice plant. Green manure crops can also fix substantial amounts of atmospheric N. Among the green manure crops, Sesbania rostrata has the highest atmospheric N2-fixing potential, and it has the potential to completely substitute for urea-N in rice cultivation.

Azospirillum Genomes Reveal Transition of Bacteria from Aquatic to Terrestrial Environments

Fossil records indicate that life appeared in marine environments ∼3.5 billion years ago (Gyr) and transitioned to terrestrial ecosystems nearly 2.5 Gyr. Sequence analysis suggests that “hydrobacteria” and “terrabacteria” might have diverged as early as 3 Gyr. Bacteria of the genus Azospirillum are associated with roots of terrestrial plants; however, virtually all their close relatives are aquatic. We obtained genome sequences of two Azospirillum species and analyzed their gene origins. While most Azospirillum house-keeping genes have orthologs in its close aquatic relatives, this lineage has obtained nearly half of its genome from terrestrial organisms. The majority of genes encoding functions critical for association with plants are among horizontally transferred genes. Our results show that transition of some aquatic bacteria to terrestrial habitats occurred much later than the suggested initial divergence of hydro- and terrabacterial clades. The birth of the genus Azospirillum approximately coincided with the emergence of vascular plants on land.

Analysis of sulphonamide residues in edible animal products: A review

The methods of analysis for sulphonamide residues in edible animal products are reviewed. Sulphonamides are widely used for therapeutic and prophylactic purposes in both humans and animals, sometimes as growth promoters as additives in animal feed. As a result of their widespread use, there is concern about whether the levels used of these drugs can generate serious problems in human health, e.g., allergic or toxic reactions. Several methods for the determination of sulphonamides have been reported in the literature and this review considers high-performance liquid chromatography (HPLC), liquid chromatography-mass spectrometry (LC/MS), gas chromatography (GC), thin-layer chromatography (TLC), high-performance capillary electrophoresis (HPCE), enzyme-linked immunosorbant assay (ELISA), biosensor immunoassay (BIA) and microbiological methods. Specific aspects of analysing sulphonamides, such as sample handling, chromatographic conditions and detection methods are discussed. Methods for drug residue monitoring should be accurate, simple, economical in both time and cost, and capable of detecting residues below the maximum residue limits (MRL). The current sulphonamide detection technologies are based on chromatographic methods or bacteriological growth inhibition. The instrumental methods such as HPLC and GC are both sensitive and specific, but are laborious and expensive. Because of the labour-intensive processes, only a few cases of GC methods applied to residue analysis have been published. These methods are suitable for confirmation but not for screening of large numbers of samples. Microbiological methods do not require highly specialized and expensive equipment. They also use highly homogeneous cell populations for testing and thus result in better assay precision. Although HPCE has powerful separation ability, the precision is poor and the instrument still needs to be improved. To date, this technique has not been widely applied to routine analysis. Currently, TLC has been almost replaced by other instrumental analysis. A rapid, sensitive and specific assay is required to detect positive samples in routine analysis, which can then be confirmed for the presence of sulphonamides by HPLC. Immunochemical methods such as ELISA can be simple, rapid and cost-effective, with enough sensitivity and specificity to detect small molecules. This review can be considered as a basis for further research aimed at identifying the most efficient approaches.

Nitrogen Fertilizer Losses from Rice Soils and Control of Environmental Pollution Problems

Abstract Nitrogen (N) requirements of rice crop are met from both the soil and fertilizers. Because of acute N deficiency in most rice soils, fertilizer N must be applied to meet the crop demand. N fertilizer applied to rice crops is partially lost through different mechanisms, including ammonia volatilization, denitrification, and leaching. These losses may cause environmental problems such as polluting the atmosphere, aquatic systems, and groundwater. These problems cannot be alleviated completely. However, they can be reduced a considerable extent by various techniques. Research has been conducted around the world to minimize N fertilizer losses. This paper reviews this information on N fertilizer losses, indicating management practices for minimizing these losses from the soil‐water system.

Ecological relative risk (EcoRR): another approach for risk assessment of pesticides in agriculture

A site-specific methodology was developed to assess and compare the ecotoxicological risk that agricultural pesticides pose to ecosystems. The ecological relative risk (EcoRR) is a composite scoring index for comparing relative risks between different plant protection products, and is used to assess the potential ecological impact their residues have after being applied to agricultural systems. The EcoRR model is based on standard frameworks for risk assessment (e.g. PEC/toxicity), but takes account of factors such as persistence of residues and biodiversity of ecosystems. The exposure module considers the environmental concentrations of a substance, its persistence, bioaccumulation and probability of exposure in several environmental compartments (water, sediment, soil, vegetation, air). The toxicity module takes into account the biodiversity of the ecosystems affected, whereby the endpoints used are weighted by the proportional contribution of each taxon in a given environmental compartment. EcoRR scores are calculated independently for each compartment and affected areas, thus enabling pinpointing of where risks will occur. The procedure to calculate EcoRR scores is explained using an example, and a sensitivity analysis of the model is included. A simulated risk assessment of 37 pesticides intended for use in a cotton development is also given as a case study. Exposure data were obtained using fugacity model II in areas previously defined by spray drift models. Toxicity data to vertebrate taxa and crustaceans were obtained from several databases, and biodiversity data from local sources. EcoRR scores were calculated for each compartment both on- and off-farm, during a normal growing season and during a flood, and a comparative relative assessment for all pesticides is discussed. EcoRR scores were also compared to traditional assessments using quotients for some taxa in the aquatic and terrestrial environments, revealing a good correlation between both models in some cases. It is apparent that EcoRR scores reflect adequately the potential risk of those chemicals to ecosystems, though they are less dependent on toxicity to sensitive species than the simple quotient. This methodology can be used either with field measured data or model predicted data, so management options for new chemicals can be tested prior to their application on crops.

The current and potential contribution of asymbiotic nitrogen fixation to nitrogen requirements on farms: a review

Significant levels of biological nitrogen fixation from sources other than nodulated legumes have become a tantalizing prospect for decades. Since the benefit to agriculture of nitrogen fixation from nodulated legumes was established, there have been widespread efforts to promote the use of various asymbiotic diazotrophic bacteria to fix extra nitrogen in soil. Despite much optimism by scientists and farmers, this prospect remains to be realised. Recently, the prospect has been pursued with renewed enthusiasm and several commercialised products have appeared. What are the reasons for this fresh enthusiasm? Are the new products based on realistic assessments of their biological potential? Why has it taken so long to advance to a stage where there is still only limited evidence that verifies hope becoming reality? This review assesses the current contribution from asymbiotic nitrogen fixation and re-assesses the prospects for greater contributions from this source. Among the many aspects of this multi-faceted subject that will be considered are: (i) the range of free-living microbial strains currently contributing to signficant asymbiotic nitrogen fixation; (ii) the significance of nitrogen-fixing microbes naturally associated with plants; (iii) the significance of endophytic systems and their role in sugarcane and other Gramineae; (iv) the possibility of extending this range by introducing new strains or discovering new systems capable of contributing additional nitrogen fixation. The case will be made that conditions providing a sustainable contribution for more than a short time are usually missing in such systems so that spontaneous biological nitrogen fixation is usually transient. It will be argued further that if all the positive factors controlling spontaneity at the biothermodynamic level are exploited, significant biological nitrogen fixation may soon be achieved in some of these systems on farms.

A Transcriptional Regulator of the LuxR-UhpA Family, FlcA, Controls Flocculation and Wheat Root Surface Colonization by Azospirillum brasilense Sp7

Genetic complementation of a spontaneous mutant, impaired in flocculation, Congo red binding, and colonization of root surface, led to the identification of a new regulatory gene in Azospirillum brasilense Sp7, designated flcA. The deduced amino acid sequence of flcA shared high similarity with a family of transcriptional activators of the LuxR-UphA family. The most significant match was with the AgmR protein, an activator for glycerol metabolism in Pseudomonas aeruginosa. Derivatives of Sp7 resulting from site-directed Tn5 mutagenesis in the flcA coding sequence were constructed by marker exchange. Characterization of the resulting mutant strains showed that flcA controls the production of capsular polysaccharides, the flocculation process in culture, and the colonization of the root surface of wheat. This study provides new information on the genetic control of the mechanism of plant root colonization by Azospirillum.

Biodegradation of Endosulfan by a Soil Bacterium

A bacterium capable of metabolizing endosulfan (6,7,8,9,10,10-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,4,3-benzodioxathiepine3-oxide) was isolated from cotton-growing soil and effectively shown to degrade endosulfan into endosulfan sulfate. The bacterium degraded 50% of the compound within 3 days of incubation. Endosulfan sulfate was the only terminal product and no other metabolites were formed during the incubation. Endosulfan and its metabolites were analyzed by gas chromatography. The metabolites formed indicated that the organism follows an oxidative pathway for metabolism of this pesticide. Therefore, the present study, microbial degradation of endosulfan by a soil bacterium, may provide a basis for the development of bioremediation strategies to remediate the pollutants in the environment.

The Fate of Endosulfan in Water

Although the use of endosulfan to control cotton pests has declined, this insecticide still has widespread application in agriculture and can contaminate riverine systems as runoff from soil or by aerial deposition. The degradation of endosulfan in pure water at different pH values of 5, 7 and 9 and in river water from the Namoi and the Hawkesbury rivers of New South Wales (NSW), Australia, was studied in the laboratory. Endosulfan transformation into endosulfan sulfate in river water using artificial mesocosms was also investigated. The results show endosulfan is stable at pH 5, with increasing rates of disappearance at pH 7 and pH 9 by chemical hydrolysis. Incubation of endosulfan with river water at pH 8.3 resulted in the disappearance of endosulfan and the formation of endosulfan diol due to the alkaline pH as well as formation of endosulfan sulfate. Although the degradation of endosulfan by Anabaena, a blue-green alga, did not result in the transformation of endosulfan to endosulfan sulfate, we conclude that other microorganisms catalyzed the formation of the sulfate. Significant conversions of endosulfan into endosulfan sulfate were also reported from associated field studies using artificial mesocoms containing irrigation water from rivers inhabitated by micro-macro fauna. From these results, we conclude that the presence of endosulfan sulfate in river water cannot be used to distinguish contamination by runoff from soil from contamination by aerial drift or redeposition.