Fixa？？o biológica de nitrogênio em pastagens com diferentes intensidades de corte

the correct management of pastures and the use of biological nitrogen fixation (bnf) therein should be sustainable. as part of an investigation into how best to use bnf in pastures three experiments were performed at the experimental station of empresa pernambucana de pesquisa agropecuária, itambé, pernambuco. these experiments evaluated the production of dry matter and n accumulation and bnf under various cutting regimes. a random experimental design with 5 replications was used. for brachiaria decumbens and b. humidicola the treatments consisted of cutting intensities of 5, 15 and 25 cm, and 25 cm + 80 kg n ha-1 , and for pennisetum purpureum cutting intensities 5, 25 and 50 cm and 50 cm + 80 kg n ha-1 . the periods in which the pastures were evaluated were the transition of the dry/wet season and the wet season of 2001. accumulation of n via bnf was estimated by the natural abundance (δ15n) method. drastic cutting (5 cm) in the dry/wet season transition resulted in high accumulation of dry matter and n in b. decumbens, but there was no effect of cutting intensity on bnf by any of the grasses in either of the seasons. the conclusion is that the bnf contribution varied with season, and that high cutting did not have any influence on n accumulation, thus confirming that the three pasture grasses are tolerant to drastic cutting.