Biological N2 fixation by heterotrophic and phototrophic bacteria in association with straw

Abstract

IntroductionAsymbiotic diazotrophs Straw components and their direct utilisation by diazotrophs Aerobic and anaerobic cellulose degradation N2 fixation in soils by bacteria using products of straw decomposition N2 fixation in flooded soils N2 fixation in dryland soils Factors affecting N2 fixation associated with straw Ways to enhance straw-associated N2 fixation Conclusions References AbstractMuch of the crop residues, including cereal straw, that are produced worldwide are lost by burning. Plant residues, and in particular straw, contain large amounts of carbon (cellulose and hemicellulose) which can serve as substrates for the production of microbial biomass and for biological N2 fixation by a range of free-living, diazotrophic bacteria. Microorganisms with the dual ability to utilise cellulose and fix N2 are rare, but some strains that utilize hemicellulose and fix N2 have been found. Generally, cellulolysis and diazotrophy are carried out by a mixed microbial community in which N2-fixing bacteria utilise cellobiose and glucose produced from straw by cellulolytic microorganisms. N2-fixing bacteria include heterotrophic and phototrophic organisms and the latter are 212 apparently more prominent in flooded soils such as rice paddies than in dryland soils. The relative contributions of N2 fixed by heterotrophic diazotrophic bacteria compared with cyanobacteria and other phototrophic bacteria depend on the availability of substrates from straw decomposition and on environmental pressures. Measurements of asymbiotic N2 fixation are limited and variable but, in rice paddy systems, rates of 25 kg N ha-1 over 30 days have been found, whereas in dryland systems with wheat straw, in situ measurements have indicated up to 12 kg N ha-l over 22 days. Straw-associated N2 fixation is directly affected by environmental factors such as temperature, moisture, oxygen concentration, soil pH and clay content as well as farm management practices. Modification of managements and use of inoculants offer ways of improving asymbiotic N2 fixation.In laboratory culture systems, inoculation of straws with cellulolytic and diazotrophic microorganisms has resulted in significant increases in N2 fixation in comparison to uninoculated controls and gains of N of up to 72 mg N fixed g-~ straw consumed have been obtained, indicating the potential of inoculation to improve N gains in composts that can then be used as biofertilisers. Soils, on the other hand, contain established, indigenous microbial populations which tend to exclude inoculant microorganisms by competition. As a consequence, improvements in straw-associated N2 fixation in soils have been achieved mostly by specific straw-management practices which encourage microbial activity by straw-decomposing and N2-fixing microorganisms.Further research is needed to quantify more accurately the contribution of asymbiotic N2 fixation to cropping systems. New strains of inoculants, including those capable of both cellulolytic and N2-fixing activity, are needed to improve the N content of biofe...

Seasonal Changes in N2 Fixation Activity and N Enrichment in Paddy Soils as Affected by Soil Management in the Northern Area of Japan

“…In rice straw incorporated into the soil in spring, a peak of ARA was detected in the very early period in the first cropping season, which was probably associated with the vigorous decomposition of readily decomposable substrates such as soluble sugars and hemicellulose 11,16 . On the other hand, the second slight increase of ARA of rice straw during the drainage period in April before transplanting in the following season was possibly related to the presence of microaerophilic or aerobic-anaerobic interfacial conditions that may stimulate BNF 2,4 .…”

Effects of Temperature, Water Regime, Light, and Soil Properties on 15N2 Fixation Associated with Decomposition of Organic Matter in Paddy Soils

“…As most of other microbial activities, BNF activity in cellulosic substrates is generally enhanced by higher temperatures and the optimum temperature seemed to range from 20 to 45ºC 6,13 . The results obtained in this study indicated that heterotrophic BNF is maintained to an extent similar to or higher than the decomposition rate of organic matter at a temperature as low as 15ºC.…”

Evaluation of rice–legume–rice cropping system on grain yield, nutrient uptake, nitrogen fixation, and chemical, physical, and biological properties of soil

“…Yes, it is cited in the text as follows p. 17, Line 422: "….observation by earlier workers (Roper and Ladha 1995), we also found that…" Each value on the line graph represents mean nitrogenase activity in roots of 12 plants from four replicated plots. Values that differ significantly (one-way ANOVA, P<0.01) on each line graph are followed by different letters.…”