A Look at Amazon Basin Seasonal Dynamics with the Biophysical ...

Structure of Microbial Communities in Native Areas and in a Pasture inBrazilian Savannas (Cerrado) of Central BrazilViana, L.T. 1 ; Molina, M. 2 ; Pinto, A.S. 3 ; Bustamante, M.C. 4 ; Kisselle, W.K. 5 ; Zepp, R.G. 61,3,4 Departamento de Ecologia, Universidade de Brasília, Brasília, DF2,5,6US Environmental Protection Agency, Georgia, USA1 SQN412 Bl.N Apt. 305 CEP: 70867-140 Brasília-DF (mailing adress)1lviana@unb.br; 2 molina.marirosa@epa.gov; 3 aspinto@unb.br; 4 mercedes@unb.br;5 kisselle.keith@epa.gov; 6 zepp.richard@epa.govThe Brazilian savannas (Cerrado) covers an area of 2 million Km 2 and represents the secondmajor biome in Brazil. It presents a dry season from April to September and a rainy seasonfrom October to March and different vegetation covers that varies in woody species density.The region has suffered drastic changes in land use with conversion of native areas toagriculture and burning practices are common during the dry season. The microorganismshave an important role in nutrient cycling and ecosystem functioning and the effects of landusechanges on microbial community structure and function are not well understood. As partof an effort to understand the effect of vegetation changes and fire regimes on the nutrientdynamics and trace gas emissions, the structure and dynamic of soil microbial communitieswere studied using phospholipid fatty acid (PLFA) analysis. Soil samples (0-5 cm) werecollected from June/00 to June/01 in two native vegetation types (cerrado sensu stricto ecampo sujo) subjected to different fire regimes (2 plots protected from fire since 1992 and 2plots submitted to prescribed fires) and in a 20 year-old active pasture (Brachiaria brizantha).Principal component analysis (PCA) separated microbial communities by vegetation type(native vs. pasture) and seasonality (wet vs. dry), explaining 45,8% and 25,6%, respectively,of the total PLFA variability. Differences between burned and unburned sites were observedalthough it was less significant than vegetation cover and seasonality effects. Gram-negativebacteria (16:1ω7c, 16:1ω5, 18:1ω7c) were in higher concentrations in the pasture than innative areas, which showed more abundance of lipids from eukaryotic microorganisms andGram-positive bacteria. Most of the variability, during the dry season, were explained by 16:0(general prokaryotic lipid) and cy19:0 (Gram-negative lipid) and, during the wet season, byGram-positive bacteria lipids (i16:0, br18:0, 15:0). Total PLFA varied from 7,1 to 41,0 µg/gdry weight of soil with higher values observed during the wet season.