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

Ecosystems volume 12, pages 686–697 (2009)Cite this 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.

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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.

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Authors and Affiliations

  1. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura (IFEVA) and Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Facultad de Agronomía, Universidad de Buenos Aires, Av. San Martín 4453, Buenos Aires, C1417DSE, Argentina

    Rodolfo A. Golluscio, Amy T. Austin, Guillermo C. García Martínez & Marina Gonzalez-Polo

  2. Department of Ecology and Evolutionary Biology, Brown University, Providence, Rhode Island, USA

    Osvaldo E. Sala

  3. Department of Biology and Nicholas School of the Environment, Duke University, Durham, North Carolina, USA

    Robert B. Jackson

  4. Department of Animal Production, and IFEVA (Facultad de Agronomía, UBA-CONICET), Av. San Martín 4453, Buenos Aires, Argentina

    Rodolfo A. Golluscio

Authors
  1. Rodolfo A. Golluscio
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  2. Amy T. Austin
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  3. Guillermo C. García Martínez
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  4. Marina Gonzalez-Polo
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Correspondence to Rodolfo A. Golluscio.

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Author Contributions

RAG designed study, performed research, analyzed data, wrote the paper; ATA designed study, wrote the paper; CGGM designed study, performed research, analyzed data; MGP performed research; OES designed study; RBJ designed study, contributed new methods.

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Golluscio, R.A., Austin, A.T., García Martínez, G.C. et al. Sheep Grazing Decreases Organic Carbon and Nitrogen Pools in the Patagonian Steppe: Combination of Direct and Indirect Effects. Ecosystems 12, 686–697 (2009). https://doi.org/10.1007/s10021-009-9252-6

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  • DOI: https://doi.org/10.1007/s10021-009-9252-6

Key words

  • δ15N
  • δ13C
  • stable isotopes
  • semiarid ecosystems
  • biogeochemistry
  • shrub–grass steppe
  • Argentina
  • desertification
  • life forms