03038naa a2200313 a 450000100080000000500110000800800410001910000140006024501760007426000090025052021440025965000110240365000130241465000130242765000090244065000260244965000120247565000160248765000130250370000150251670000220253170000190255370000190257270000170259170000200260870000170262870000180264577300610266315987412007-05-31       2001    bl ---       0--  u    #d1 aTARRE, R.  aThe effect of the presence of a forage legume on nitrogen and carbon levels in soils under Brachiaria pastures in the Atlantic forest region of the South of Bahia, Brazil.  c2001  aThe impact of forest clearance, and its replacement by Brachiaria pasture, on soil carbon reserves has been studied at amny sites in the Brazilian Amazonian, but to date there appear to be no reports of similar studies undertaker in the Atlantic forest region of Brazil. In this study performed in the extreme south of Bahia, the chance in C and N content of the soil were evaluted from the time of establishment  of grass-only B. humidicola and mixed B. humidicola/Desmodium ovolifolium pastures through 9 years of grazing in comparision with the C and N contents of the adjacent secondary forest.  The decline in the content of soil C derived from the forest(C3) vegetation and the accumulation of that derived from the Brachiaria (C4) were  followed by determining the 13C natural abundance of the soil organic matter (SOM). The pastures were established in 1987, 10 years deforestation, and it was estimated that until 1994 there was a loss in forest-derived C in the top 30 cm of soil of aproximately 20% (9.1mg C há-1). After the establishment of the pastures, C derived from Brachiaria accumulated steadily such that at the sampling (1997) it was estimated 13.9 Mg há -1 was derived from this source under the grass-only pasture(0-30 cm). Samples taken from all pastures and forest in 1997 to a depth of 100 cm showed that below 40 cm depth there was no significant contribution of the Brachiaria-derived C and that total C reserves under the gass/legume and the grass/only pastures were slightly higher than under the forest(not significant at p=0.05). The more metailed sampling under the pasture showed that to a depth of 30 cm there was significanly (P-0.050 more C under the mixed pasture than the grass-only pasture. It was estimated that from the time of establishment the apparent rate of C accumulation (0-100 cm depth) under the grass/legume pastures (1.17 Mg há -1 yr -1) was almost double that under the grass-only pastuires(0.66 Mg há-1 yr-1). The data indicated that newly incorporated SOM derived from the Brachiaria had a considerably higher C:N retio than that present under the forest. Nitrogênio N; Carbono C  acarbon  anitrogen  apastures  asoil  aBrachiaria Humidicola  aCarbono  aNitrogênio  aPastagem1 aMACEDO, R.1 aCANTARUTTI, R. B.1 aREZENDE, C. D.1 aPEREIRA, J. M.1 aFERREIRA, E.1 aALVES, B. J. R.1 aURQUIAGA, S.1 aBODDEY, R. M.  tPlant and Soil, Dordrechtgv. 234, n .1, p. 15-26, 2001.