Microbial quality of soil from the Pampa biome in response to different grazing pressures.

Rafael S Vargas,Bruno Lisboa,Renata Bataiolli,Anelise Beneduzi,Luciane Maria P Passaglia,Pedro B Da Costa,Luciano K Vargas

doi:10.1590/s1415-475738138120140230

Abstract

The aim of this study was to evaluate the impact of different grazing pressures on the activity and diversity of soil bacteria. We performed a long-term experiment in Eldorado do Sul, southern Brazil, that assessed three levels of grazing pressure: high pressure (HP), with 4% herbage allowance (HA), moderate pressure (MP), with 12% HA, and low pressure (LP), with 16% HA. Two reference areas were also assessed, one of never-grazed native vegetation (NG) and another of regenerated vegetation after two years of grazing (RG). Soil samples were evaluated for microbial biomass and enzymatic (β-glucosidase, arylsulfatase and urease) activities. The structure of the bacterial community and the population of diazotrophic bacteria were evaluated by RFLP of the 16S rRNA and nifH genes, respectively. The diversity of diazotrophic bacteria was assessed by partial sequencing of the 16S rDNA gene. The presence of grazing animals increased soil microbial biomass in MP and HP. The structures of the bacterial community and the populations of diazotrophic bacteria were altered by the different grazing managements, with a greater diversity of diazotrophic bacteria in the LP treatment. Based on the characteristics evaluated, the MP treatment was the most appropriate for animal production and conservation of the Pampa biome.

Highlights

The Pampa biome is located between latitudes 24° and 35° S and covers an area of 500,000 km2 in Uruguay, northeastern Argentina, southern Brazil and part of Paraguay (Pallarés et al, 2005)
Since European immigrants introduced the first herds in the 17th century (Bilenca and Miñarro, 2004), livestock production has been one of the main economic activities of the region, with the natural grassland serving as the feeding basis for animal production (Carvalho and Batello, 2009)
In view of the importance of microbiological processes for preservation of the Pampa biome while at the same time allowing adequate economic exploitation, in this study we examined the impact of different grazing pressures on the microbial activity and diversity of soil bacteria

Summary

Introduction

The Pampa biome is located between latitudes 24° and 35° S and covers an area of 500,000 km in Uruguay, northeastern Argentina, southern Brazil and part of Paraguay (Pallarés et al, 2005) This biome is composed of herbaceous native plants classified as steppe in the international phytogeographic system (Berreta, 2001) and is recognized for its great species diversity that includes approximately 450 grasses and 200 forage legumes (Boldrini, 2007). Conte et al (2011) claimed that adjustment of the number of animals to herbage allowance is critical to the sustainability of natural pastures These authors found a decrease in labile carbon and an increase in soil density associated with a reduction in herbage allowance in an area of the Pampa biome. In addition to the loss of soil physical quality, often resulting in water erosion and compaction of the surface layers (Bertol et al, 2000), excessive stocking may result in decreased plant diversity (Soares et al, 2003)

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