FIELD STUDIES OF INORGANIC NITROGEN ADDED TO SEMIARID SOILS BY RAINFALL AND BLUE-GREEN ALGAE

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Abstract
N fixed by atomospheric reactions and by blue-green algae provides an important source of nutrient to semiarid soils. The amount of N2 fixed in an area of west Texas [USA] was measured. Field studies estimated the amount of inorganic N added to the soil by nitrates in rainfall and N fixed by blue-green algae. Colorimetric analysis of rainwater, using phenoldisulfonic acid reagent, indicated that a 1 cm rainfall brought an average of 39 g NO3-N/ha to the soil. The acetylene-reduction technique was used to measure the N-fixation capacity in the field by crusts of blue-green algae and mold. With an estimated 15% surface coverage by algal crusts, up to 1.3 g N2/ha per h may be added to the soil by blue-green algae for each of the daylight hours during the first 24 h after a rainfall. Frequent showers caused more fixed N to be added to the soil (from atmospheric reactions and blue-green algae) than the same amount of rainfall occurring at one time.