ENVIRONMENTAL-FACTORS CONTROLLING N-2 FIXATION IN MEDITERRANEAN RICE FIELDS

The objectives of this study were to analyze the environmental control s on N-2 fixation in Spanish rice fields. Nitrogenase activity, measur ed as the acetylene-reducing activity (ARA), was estimated in situ dur ing different intervals of the cropping period. At the same time, phys ical and chemical variables and cyanobacterial occurrence were determi ned in water and soil. Nitrogen fixation was measurable at all samplin g sites, being higher in July and lower in Tune after a short dry peri od. The ARA values ranged from 0.23 to 75.5 kg N Na-1 year(-1). Becaus e blooms or other conspicuous cyanobacterial forms were not included i n the measurements, maximum rates of nitrogen fixation may have been h igher. Environmental variables that correlated with ARA varied on a se asonal basis. Water properties such as calcium, hardness, or conductiv ity, and soil properties such as conductivity and sodium correlated po sitively with N-2 fixation; however, nutrient parameters such as disso lved inorganic nitrogen or soluble reactive phosphorus were negatively correlated Cyanobacterial abundance, in general, did not correlate wi th ARA. The overall conclusion is that nitrogen fixation may be an imp ortant N input in the N cycle of rice fields, and could lessen polluti on problems by lowering the demand for chemical fertilizers.