It is necessary to obtain more detailed information on the processes involved in the N cycle of the soil-plant system. Thus, the fixing ability of several legumes was quantified and the N balance on Winter wheat, at Elves (Portugal) was evaluated, using "1"5#NU# isotope. Fababeans presented the highest fixing capacity (53 - 90% N derived from the fertilizer N dfa), even without inoculation, and under drought stress. Chickpea also showed a high fixing rate (44-80% N dfa), but total N_2 fixed was low (20 Kg N ha"-"1). Symbiosis with peas was depressed by water deficit and by Mo deficiency. Under favourable conditions, fixation rate reached 88%. Soybeans and green showed low fixation rates, probably due to inefficient rhizobium strains and/or inadequate plant genotypes. Sub clover mixed with non-legumes fixed 50% of its N in the above ground plant material, but no significant N transfer was observed to the non-legumes. The fertilizer use efficiency by Winter wheat was 19% - 44%. The fertilizer N recovered in the soil varied from 15% to 66%. Unaccounted N reached a maximum value of 55% of the applied N. Leaching of NO_3_- was the main loss, but denitrification and volatilization also occurredAvailable from Fundacao para a Ciencia e a Tecnologia, Servico de Informacao e Documentacao, Av. D. Carlos I, 126, 1200 Lisboa / FCT - Fundação para o Ciência e a TecnologiaSIGLEPTPortuga
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