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Ecosystems

, Volume 12, Issue 4, pp 686–697 | Cite as

Sheep Grazing Decreases Organic Carbon and Nitrogen Pools in the Patagonian Steppe: Combination of Direct and Indirect Effects

  • Rodolfo A. Golluscio
  • Amy T. Austin
  • Guillermo C. García Martínez
  • Marina Gonzalez-Polo
  • Osvaldo E. Sala
  • Robert B. Jackson
Article

Abstract

We explored the net effects of grazing on soil C and N pools in a Patagonian shrub–grass steppe (temperate South America). Net effects result from the combination of direct impacts of grazing on biogeochemical characteristics of microsites with indirect effects on relative cover of vegetated and unvegetated microsites. Within five independent areas, we sampled surface soils in sites subjected to three grazing intensities: (1) ungrazed sites inside grazing exclosures, (2) moderately grazed sites adjacent to them, and (3) intensely grazed sites within the same paddock. Grazing significantly reduced soil C and N pools, although this pattern was clearest in intensely grazed sites. This net effect was due to the combination of a direct reduction of soil N content in bare soil patches, and indirect effects mediated by the increase of the cover of bare soil microsites, with lower C and N content than either grass or shrub microsites. This increase in bare soil cover was accompanied by a reduction in cover of preferred grass species and standing dead material. Finally, stable isotope signatures varied significantly among grazed and ungrazed sites, with δ15N and δ13C significantly depleted in intensely grazed sites, suggesting reduced mineralization with increased grazing intensity. In the Patagonian steppe, grazing appears to exert a negative effect on soil C and N cycles; sound management practices must incorporate the importance of species shifts within life form, and the critical role of standing dead material in maintaining soil C and N stocks and biogeochemical processes.

Key words

δ15δ13stable isotopes semiarid ecosystems biogeochemistry shrub–grass steppe Argentina desertification life forms 

Notes

Acknowledgements

We thank Ana Srur and Fernando Cavagnaro, who helped us with the field and laboratory work. John Karr performed the laboratory analyses for C, N, and stable isotopes in the Duke University laboratory. Special thanks to INTA, for permission to work in the Río Mayo experimental station, and three anonymous reviewers who allowed us to improve the manuscript. We acknowledge ANPCyT (PICTs 15124/03, 21247/04, 31970/05, and 00463/08), University of Buenos Aires (G044, G062, G090 and G812), Fundación Antorchas of Antorchas (Early Career Award, ATA) and CONICET (PIP 5963/04) for financial support during this study.

Supplementary material

10021_2009_9252_MOESM1_ESM.docx (14 kb)
Supplementary material 1 (DOCX 14 kb)

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Copyright information

© Springer Science+Business Media, LLC 2009

Authors and Affiliations

  • Rodolfo A. Golluscio
    • 1
    • 4
  • Amy T. Austin
    • 1
  • Guillermo C. García Martínez
    • 1
  • Marina Gonzalez-Polo
    • 1
  • Osvaldo E. Sala
    • 2
  • Robert B. Jackson
    • 3
  1. 1.Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura (IFEVA) and Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Facultad de AgronomíaUniversidad de Buenos AiresBuenos AiresArgentina
  2. 2.Department of Ecology and Evolutionary BiologyBrown UniversityProvidenceUSA
  3. 3.Department of Biology and Nicholas School of the EnvironmentDuke UniversityDurhamUSA
  4. 4.Department of Animal Production, and IFEVA (Facultad de Agronomía, UBA-CONICET)Buenos AiresArgentina

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