Summary
(1) N added to decomposing organic matter often has no effect or a negative effect on microbial activity, at least in the long term. More than 60 papers are cited in support of this statement.

(2) The negative effect of N is mainly found with recalcitrant organic matter with a high C/N ratio (straw, wood, etc.), whereas a positive effect of N is common for easily degradable organic material with low C/N ratio.

(3) The negative effect of N could be explained by: (i) N disturbs the outcome of competition between potent and less potent decomposers; (ii) through ‘ammonia metabolite repression’, N blocks production of certain enzymes, at least in basidiomycetes, and enhances breakdown of the most available cellulose, whereby recalcitrant lignocellulose accumulates; (iii) amino compounds condense with polyphenols and other decomposition products, forming ‘browning precursors’ which are toxic or inhibitory.

(4) The effect of adding N may depend on the microflora present.

(5) There are indications that some microorganisms have a ‘luxury uptake’ of N when it is present in sufficient amounts, thereby delaying N mineralization.

(6) The addition of N seems to increase the formation of water-soluble, brown, recalcitrant compounds, but to decrease the amount of humus formed.

The effect of added nitrogen on the rate of decomposition of organic matter

Biological conversion of lignocellulosic biomass to ethanol

Recent process developments in solid-state fermentation

Oxygen relations of nitrogen fixation in cyanobacteria.

summary
N2 fixation is an extremely O2-sensitive process. N2-fixing bacteria (diazotrophys) have therefore evolved a variety of strategies by which they maintain an active nitrogenase in the presence of atmospheric O2 and, in some cases, of O2 generated photosynthetically. In this review, the effects of O2 on nitrogenase activity and synthesis are described, as are the mechanisms by which diazotrophs limit O2damage to nitrogenase. These mechanisms are classified as behavioural strategies, physical barriers or physiological and biochemical strategies. Individual diazotrophs frequently empoly a combination of these. In addition, the particular problems faced by the O2-evolving cyanobacteria are discussed.

Reconciling the incompatible: N2 fixation And O2

Anabaena flos-aquae was grown in chemostats with phosphate-limiting growth and dilution rate of 0.015–0.03 h-1. The yields of cells were dependent on dilution rate and a two-fold increase obtained by growth in the presence of 15 mM KNO3. Alkaline phosphatase activity varied 20-fold, lowest activity with excess phosphate light-limited cells and the highest activity with cells grown in the presence of 15 mM KNO3. There was no correlation between hot water soluble phosphate of cells and alkaline phosphatase activity.

Relationship between phosphates and alkaline phosphatase of Anabaena flos-aquae in continuous culture.

The development of insect cell suspension culture techniques for the production of insect pathogenic viruses and recombinant proteins has been reviewed, with an emphasis on process scale-up and reactor design considerations. The problems of culture media cost and insect cell shear sensitivity have also been addressed.

Engineering aspects of insect cell suspension culture: a review

Anabaena flos-aquae is grown in chemostats under phosphate and urea-limited conditions. Nitrogenase activity in phosphate-limited cells has a maximum activity at a dilution rate of 0.025 h-1 and is repressed 24-fold by 15 mM KNO3. Cultures growing on 1.5 mM nitrate obtain 1/2–2/3 of cell nitrogen from N2. Cells form inducible nitrite assimilating enzymes when grown on nitrate. Algae growing under A or He on limiting urea or phosphate-limited with nitrate have active nitrogenase. The ratio of nitrogenase activity to heterocyst numbers varied 90-fold depending on source of nitrogen, 15 mM KNO3 gave the smallest ratio. The regulatory mechanisms controlling the activity of nitrogenase in blue-green algae is discussed.

Nitrogenase activity and nitrogen assimilation in Anabaena flos-aquae growing in continuous culture.

Seventee white-rot and brown-rot fungi were screened for their ability to fractionate the lignocellulose structure of oat straw through the preferential attack of lignin or cellulose. Fermentations were carried out under solid-state conditions with 25 g quantities of straw. The fermented straw was analyzed for weight loss, Klason lignin loss and cellulase digestion. All the fungi attacked both lignin and carbohydrate fractions causing 3–28% weight losses and 26–34 g/100 g enzymatic digestibility. Polyporustulipiferae, Phanerochaetechrysosporium and Polyporus sp. were tested for the effects of various nitrogen, phosphate and carbon levels, incubation temperatures and incubation time. The three fungi had different responses to these factors.

Solid state fermentation and fractionation of oat straw by Basidiomycetes

Nitrate reduction by denitrifying bacteria in single and two stage continuous flow reactors

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