02972naa a2200361 a 450000100080000000500110000800800410001910000180006024501020007826000090018052019690018965000130215865000150217165000260218665000210221265000250223365000090225865000130226765300210228065300120230165300120231365300330232570000150235870000180237370000170239170000230240870000220243170000220245370000190247570000200249470000170251477300790253116246962004-08-09       2004    bl ---       0--  u    #d1 aBODDEY, R. M.  aNitrogen cycling in Brachiaria pasturesbthe key to understanding the process of pasture decline.  c2004  aIn the tropical regions of Brazil there are at leas 80 million ha Mha of pastures planted to grasses introduced from Africa, principaly Brachiaria spp. It is estamated at  least half of theses pastures are degraded, that is, support very low stocking rates, show low plant cover, are invaded by-non-palatable native species and often densely populated with termite mounds. The main causes of the process of pature decline are lack of maintenance fertilisation and excessively high animal  stocking rates. In this study the of increasing stockingrate on the fluxes of nitrogen N though the animal (forage consumiption, production of faeces and urine) and through the plant (growth, senescence) pathways were studied on Brachiaria humidicola pastures grazed by Zebu beef cattle in the Atlantic fores region of south of (Brazil). As stocking rate increased from 2 to 4animals ha -1, live weight gain per animal decreased from 153 to 120 kg per animal per year but overall weight gain per hectare increased from 305 to 360 kg per vear. Nitrogen exported in the animal weight gain only increased from 7.3 to 8.6 kg N ha-1, but the pathways of Ncycling were radically changed. Increasing the stocking rate from 2 to 4 animal ha-1, increased total Nconsumed by the animal from 94 to 158 kb ha-1 per year, and that deposited in plant litter decreased from 170 to 105 kg ha-1.This resulted in increases in N deposited as urine and dung in the paddocks from 50 to 90 and 37 to 59 kg ha-1, respectively. A large proportion of the excretions were deposited in rest ares and around drinking troughs where the grass was so trampled that it coud not take advantage of thisN, and other nutrients. Data from complementary studies showed that N losses from urine could be between 35 and 80%, being much higher in without vegetation. Weconclude that decline is hastened by increasing stocking rates because of these losses of N and decrease in N nutrients available for grass growth.  apastures  aBrachiaria  aBrachiaria Humidicola  aCapim Brachiaria  aCiclo do Nitrogênio  aFeno  aPastagem  aBrcahiaria grass  aCycling  aLitters  aNitrogen cycle in ecosystems1 aMACEDO, R.1 aTARRÉ, R. M.1 aFERREIRA, E.1 aOLIVEIRA, O. C. de1 aREZENDE, C. de P.1 aCANTARUTTI, R. B.1 aPEREIRA, J. M.1 aALVES, B. J. R.1 aURQUIAGA, S.  tAgriculture Ecosystems & Environment, Amsterdamgv. 103, p. 389-403, 2004.