Algal nitrogen fixation on temperate arable fields |
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Authors: | J. F. Witty |
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Affiliation: | (1) Soil Microbiology Department, Rothamsted Experimental Station, Harpenden, Herts, UK;(2) Present address: Luton College of Higher Education, Luton, Beds, England;(3) Present address: I.C.I. Ltd., Jealotts Hill, England;(4) Present address: ICRISAT, Hyderabad, A.P., India |
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Abstract: | Summary N2-fixation by algae on the Broadbalk continuous wheat experiment was measured over a two year period using the acetylene reduction technique. The plots studied receive spring fertilizer treatments including farmyard manure and combinations of nitrochalk and Na, P, K and Mg which have remained much the same since the experiment started in 1843.Nitrogen applied at 196 kg ha–1 in spring suppressed algal N2-fixation until late in the season but at lower levels (48 kg N ha–1) the denser plant canopy increased both surface moisture and fixation. Herbicide treatment decreased fixation on plots of moderate nutritional status early in the season but had little effect on unfertilised plots where weed cover was sparse. On plots where weed and crop cover was very dense herbicide treatment increased fixation in August.Algal N2-ase activity, assayed by C2H2 reduction, continued throughout the night at a rate which averaged 33% of the midday value. Laboratory experiments indicate that dark fixation is very temperature sensitive and this value may represent a maximum. Algal crust in the field dried to 4.5–6.8% H2O content became active 3 1/2 h after rewetting and reached a steady state after 7 h which represented only 6–22% of that at the previous maximum suggesting that many cells had been killed.In a year with average rainfall algae on plots receiving 48 kg N ha–1 were estimated to fix 25–28 kg N ha–1 and plots without fertiliser 13–19 kg N ha–1. Algal fixation appeared to make a substantial contribution to the continuing fertility of unfertilised plots. |
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Keywords: | Acetylene reduction Blue-green algae Dark-fixation Desiccation N2-fixation N-fertiliser Temperate soil Wheat |
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