Effects of land use on soil microbial communities in the Cerrado region

1Silva, M. R. S. S., 1Bresolin, J. D., 2Krüger, R. H., 1Bustamante, M. M. C., 3 Reis Jr., F. B. 1 Departamento de Ecologia, Universidade de Brasília, Brasília, DF.

2 Universidade Católica de Brasília, DF. 3 EMBRAPA – Cerrados, Planaltina, DF.

Introduction

The composition and activity of soil microbial communities largely determine biogeochemical cycles, the turnover process of organic matter, and the fertility and quality of soils. The intensification of agricultural activities in the Cerrado region results in the replacement of native vegetation cover, changes in fire regime and soil physical and chemical properties. Biomass burning can affect biogeochemical cycles through direct transformation of the elements during combustion process and through environmental changes that could affect the nutrient dynamics for many years after fire. Our objective was to analyze the diversity of the soil bacterial communities from native Cerrado areas (burned and unburned) and from pasture areas using a molecular technique (PCR-DGGE).

Material and MethodsSoil sampling

The soils are classified as Oxisols, very acidic, with high aluminum saturation and low cation exchange capacity. Soils samples were taken from the Ecological Reserve of the IBGE, Brasilia, Brazil and from Rio de Janeiro farm, Planaltina, Brazil. Soils were taken from 0-5 cm depth in native Cerrado areas (campo sujo and cerrado restricted sense) and in pasture areas {fertilizated pasture, unfertilizated pasture, young pasture, mixed pasture (grass/legume)} . The burned and unburned plots of the two native vegetation types were sampled 2, 10 and 30 days after burning. The samples were collected during the wet and dry season as well as during the transition wet to dry season.

DNA extraction

Soil DNA was obtained by direct extraction with a protocol which included mechanical lyses of cells, phenol and chloroform extractions, a potassium acetate precipitation and a final purification using the Wizard DNA clean up Kit.

PCRThe 16S rDNA primers used in PCR were 968F and 1401R, which are commonly used to analyze bacterial communities.

DGGEDGGE was carried out using a Bio-Rad Dcode Universal Mutation Detection System at 70V and 60° C for 18 h in 0.5x TAE buffer. The 6% (w/v) polyacrilamide gels were made with a denaturing gradient ranging from 40 to 70% and used with PCR products. After electrophoresis, gels were stained for 40 minutes with SYBR green I and photographed on a UV transilumination table with a Kodak digital camera.

DGGE analysisGel pictures were manually converted to 1/10 matrices, which were used for clustering by the unweighted pair group method with mathematical averages (UPGMA; Dice coefficient of similarity), followed by tree inference.

Results

A comparison of the DGGE profiles from soil from burned and unburned native areas, showed no differences in the bacterial community between the areas after the prescribed fire. The differences between burned and unburned plots were detected after the first rain events (Fig.1). The comparison between DGGE profiles from unburned native areas, campo sujo and cerrado restricted sense, showed differences for seasonality and vegetation types (Fig. 2).Analysis from DDGE profiles from soil from pasture showed differences between treatments for pasture restoration. Pasture with grass/legume showed a DDGE profile similar from burned campo sujo native area (Fig. 3).

PF (set/02)

CSNQ (out/02)

FP (set/02)

UCS (out/02)

BC 10d (out/02)

BCS 30d (out/02)

UC (out/02)

BC (out/02)

UCS (set/02)

BC 2d (out/02)

BCS 10d (out/02)

Fig.1. Dendogram of PCR-DGGE analysis of the bacteria communities in Cerrado soils from unburned and burned native areas (campo sujo and cerrado restricted sense) and pasture. Samples FP represent fertilized pasture, UCS – unburned campo sujo, UB – burned campo sujo, BC – burned cerrado, UC – unburned cerrado.

FP (set/02)

UCS (out/02)

UCS (dez/02)

UC (out/02)

UC (dez/02)

UCS (set/02)

UCS (jul/02)

Fig.2. Dendogram of PCR-DGGE analysis of the bacterial communities in Cerrado soils from unburned native area (campo sujo and cerrado restricted sense) and pasture. Samples FP represent fertilized pasture, UCS – unburned campo sujo, UC – unburned cerrado.

Fig.3. Dendogram of PCR-DGGE analysis of the bacterial communities in Cerrado soils from burned native area (campo sujo) and pasture. Samples FP represent fertilized pasture, YP – young pasture, MP – mixed pasture (grass/legume), UP – unfertilized pasture, BCS – burned campo sujo.

FP (set/02)

YP (set/02)

MP (set/02)

BCS (dez/02)

UP (set/02)

BCS (abr/03)

BCS 10d (out/02)

Conclusions

The first rain effects were more important to determine differences in the bacterial communities from native areas than burning.In the wet season, bacterial communities profiles from unburned native areas showed differences between vegetation types.In the pasture areas, time of conversion (native areas to pasture), input of P and consortium grass/legume were relevant to diferentiate bacterial communities.

The differences between burned and unburned plots were detected after the first rain events.

The comparison between DGGE profiles from unburned native areas, campo sujo and cerrado restricted sense, showed differences for seasonality and vegetation types.

Pasture with grass/legume showed a DDGE profile similar from burned campo sujo native area.