Environmental Factors Controlling N2 Fixation in Mediterranean Rice Fields

Abstract The objectives of this study were to analyze the environmental controls on N₂ fixation in Spanish rice fields. Nitrogenase activity, measured as the acetylene-reducing activity (ARA), was estimated in situ during different intervals of the cropping period. At the same time, physical and chemical variables and cyanobacterial occurrence were determined in water and soil. Nitrogen fixation was measurable at all sampling sites, being higher in July and lower in June after a short dry period. The ARA values ranged from 0.23 to 75.5 kg N Na⁻¹ year⁻¹. Because blooms or other conspicuous cyanobacterial forms were not included in the measurements, maximum rates of nitrogen fixation may have been higher. Environmental variables that correlated with ARA varied on a seasonal basis. Water properties such as calcium, hardness, or conductivity, and soil properties such as conductivity and sodium correlated positively with N₂ fixation; however, nutrient parameters such as dissolved inorganic nitrogen or soluble reactive phosphorus were negatively correlated. Cyanobacterial abundance, in general, did not correlate with ARA. The overall conclusion is that nitrogen fixation may be an important N input in the N cycle of rice fields, and could lessen pollution problems by lowering the demand for chemical fertilizers. Received 24 January 1996; Accepted: 11 June 1996