Fontes e doses de adubação nitrogenada na atividade microbiana e fertilidade do solo cultivado com Brachiaria brizantha

Sources and levels of nitrogen fertilization on microbial activity and fertility in soil cropped with Brachiaria brizantha. Pastures usually lose their yield potential and re-growth energy in a few years, most often due to the lack of nitrogen fertilization. The objective of this study was to verify the effect of sources and levels of nitrogen (N) fertilization, under microbial activity (carbon of microbial biomass and carbon released as C[O.sup.2]) and fertility of soil cropped with Brachiaria brizantha cv. Xaraes. The experimental design was randomized, organized under a 3 x 4 factorial scheme, with three nitrogen sources (urea, ammonium sulfate and Ajifer-L40) and four nitrogen levels (0, 100, 200 and 400 kg [ha.sup.-1]), analyzed on two depths (0 - 0,10 e 0,10-0,20 m), with 3 replications. The higher levels of N doses, the 0 - 0.10 m depths, reduced the values of pH, MO, [K.sup.+], [Ca.sup.2+], [Mg.sup.2+], SB and V%. Ajifer and urea resulted in higher values of [Ca.sup.2+], in the depths of 0.10 - 0.20 m, and the values of [Mg.sup.2+] and SB exhibited high values with the doses of 100 and 200 kg ha-1 levels of N. The higher levels of ammonium sulfate resulted in soil acidification. Soil carbon loss, due to the high microbial activity, could be the result of metabolic stress caused by soil acidification. The application of 100 kg [ha.sup.-1] of nitrogen was the best fertilizing level, considering the smaller loss of soil carbon by microbial activity.