, Volume 16, Issue 2, pp 347–357 | Cite as

Increases of Soil C, N, and P Pools Along an Acacia Tree Density Gradient and Their Effects on Trees and Grasses

  • Judith SittersEmail author
  • Peter J. Edwards
  • Harry Olde Venterink


Nitrogen (N) fixing trees including many species of Acacia are an important though variable component of savanna ecosystems. It is known that these trees enrich the soil with carbon (C) and N, but their effect on the combined C:N:P stoichiometry in soil is less well understood. Theory suggests that they might reduce available phosphorus (P), creating a shift from more N-limited conditions in grass-dominated to more P-limited conditions in tree-dominated sites, which in turn could feed back negatively on the trees’ capacity to fix N. We studied the effects of Acacia zanzibarica tree density upon soil and foliar N:P stoichiometry, and the N2-fixation rates of trees and leguminous herbs in a humid Tanzanian savanna. Foliar N:P ratios and N2-fixation rates of trees remained constant across the density gradient, whereas soil C, N and organic P pools increased. In contrast, the N:P ratio of grasses increased and N2-fixation rates of leguminous herbs decreased with increasing tree density, indicating a shift towards more P-limited conditions for the understory vegetation. These contrasting responses suggest that trees and grasses have access to different sources of N and P, with trees being able to access P from deeper soil layers and perhaps also utilizing organic forms more efficiently.


carbon sequestration legume nitrogen-fixing trees nutrient limitation N:P stoichiometry plant–soil feedback phosphorus savanna tree–grass interactions woody encroachment 



We thank Stefanie Karrer, Marc-Jacques Mächler, and Anneke Valk for their help in the field and Sabine Güsewell for her statistical advice. We acknowledge Britta Jahn-Humphrey, Marilyn Gaschen, and Adolphe Munyangabe for their help in the lab and Stefano Bernasconi for carrying out isotopic analyses. We thank Annette Stähli and the authorities of Saadani National Park for their support in Tanzania, and John Williams and Benjamin Donald for their assistance in the field. Helpful comments of Werner Suter, Cory Cleveland and two anonymous reviewers significantly improved the manuscript. Research was conducted with permission and support given by the Tanzanian Wildlife Research Institute (TAWIRI), Commission of Science and Technology (COSTECH) and Tanzania National Parks (TANAPA). This study was financed by the Swiss National Science Foundation Grant No. 2-77321-08.

Supplementary material

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Supplementary material 1 (DOC 1743 kb)


  1. Amarger N, Mariotti A, Mariotti F, Durr JC, Bourguignon C, Lagacherie B. 1979. Estimate of symbiotically fixed nitrogen in field-grown soybeans using variations in 15N natural abundance. Plant Soil 52:269–80.CrossRefGoogle Scholar
  2. Archer S, Schimel DS, Holland EA. 1995. Mechanism of shrubland expansion: land use, climate or CO2? Climatic Change 29:91–9.CrossRefGoogle Scholar
  3. Baldus RD, Roettcher K, Broska D. 2001. Saadani: an introduction to Tanzania’s Future 13th National Park. In: Baldus RD, Siege L, Eds. Tanzania Wildlife Discussion Paper No. 30. Dar Es Salaam: GTZ Wildlife Programme in Tanzania.Google Scholar
  4. Belsky AJ, Amundson RG, Duxbury JM, Riha SJ, Ali AR, Mwonga SM. 1989. The effects of trees on their physical, chemical, and biological environments in a semi-arid savanna in Kenya. J Appl Ecol 26:1005–24.CrossRefGoogle Scholar
  5. Belsky AJ, Mwonga SM, Amundson RG, Duxbury JM, Ali AR. 1993. Comparative effects of isolated trees on their undercanopy environments in high-rainfall and low-rainfall savannas. J Appl Ecol 30:143–55.CrossRefGoogle Scholar
  6. Binkley D. 2005. How nitrogen-fixing trees change soil carbon. In: Binkley D, Menyailo O, Eds. Tree species effects on soil: implications for global change. Dordrecht: Springer. p 155–64.CrossRefGoogle Scholar
  7. Binkley D, Giardina C, Bashkin MA. 2000. Soil phosphorus pools and supply under the influence of Eucalyptus saligna and nitrogen-fixing Albizia facaltaria. For Ecol Manage 128:241–7.CrossRefGoogle Scholar
  8. Binkley D, Senock R, Cromack K. 2003. Phosphorus limitation on nitrogen fixation by Facaltaria seedlings. For Ecol Manage 186:171–6.CrossRefGoogle Scholar
  9. Boddey RM, Polidoro JC, Resende AS, Alves BJR, Urquiaga S. 2001. Use of the 15N natural abundance technique for the quantification of the contribution of N2 fixation to sugar cane and other grasses. Aust J Plant Physiol 28:889–95.Google Scholar
  10. Bond WJ. 2008. What Limits Trees in C4 Grasslands and Savannas? Annu Rev Ecol Evol Syst 39:641–59.CrossRefGoogle Scholar
  11. Bond WJ, Midgley GF, Woodward FI. 2003. The importance of low atmospheric CO2 and fire in promoting the spread of grasslands and savannas. Glob Change Biol 9:973–82.CrossRefGoogle Scholar
  12. Bond WJ, Woodward FI, Midgley GF. 2005. The global distribution of ecosystems in a world without fire. New Phytol 165:525–37.PubMedCrossRefGoogle Scholar
  13. Boutton TW, Liao JD. 2010. Changes in soil nitrogen storage and δ15N with woody plant encroachment in a subtropical savanna parkland landscape. J Geophys Res 115:G03019.CrossRefGoogle Scholar
  14. Boutton TW, Archer SR, Midwood AJ, Zitzer SF, Bol R. 1998. δ13C values of soil organic carbon and their use in documenting vegetation change in a subtropical savanna ecosystem. Geoderma 82:5–41.CrossRefGoogle Scholar
  15. Cech PG. 2008. Impact of fire, large herbivores and N2-fixation on nutrient cycling in humid savanna, Tanzania. PhD Thesis, Swiss Federal Institute of Technology, Zürich.Google Scholar
  16. Cech PG, Kuster T, Edwards PJ, Olde Venterink H. 2008. Effects of herbivory, fire and N2-fixation on nutrient limitation in a humid African savanna. Ecosystems 11:991–1004.CrossRefGoogle Scholar
  17. Cech PG, Olde Venterink H, Edwards PJ. 2010. N and P Cycling in Tanzanian humid savanna: influence of herbivores, fire, and N2-fixation. Ecosystems 13:1079–96.CrossRefGoogle Scholar
  18. Cochard R. 2004. Patterns and dynamics of secondary Acacia zanzibarica woodlands at Mkwaja Ranch, Tanzania. PhD Thesis, Swiss Federal Institute of Technology, Zürich.Google Scholar
  19. Cochard R, Edwards PJ. 2011. Structure and biomass along an Acacia zanzibarica woodland-savanna gradient in a former ranching area in coastal Tanzania. J Veg Sci 22:475–89.CrossRefGoogle Scholar
  20. Crews TE. 1993. Phosphorus regulation of nitrogen fixation in a traditional Mexican agroecosystem. Biogeochemistry 21:141–66.CrossRefGoogle Scholar
  21. Crews TE, Kitayama K, Fownes JH, Riley RH, Herbert DA, Mueller-Dombois D, Vitousek PM. 1995. Changes in soil phosphorus fractions and ecosystem dynamics across a long chronosequence in Hawaii. Ecology 76:1407–24.CrossRefGoogle Scholar
  22. Duponnois R, Founoune H, Masse D, Pontanier R. 2005. Inoculation of Acacia holosericea with ectomycorrhizal fungi in a semiarid site in Senegal: growth response and influences on the mycorrhizal soil infectivity after 2 years plantation. For Ecol Manage 207:351–62.CrossRefGoogle Scholar
  23. Geesing D, Felker P, Bingham RL. 2000. Influence of mesquite (Prosopis glandulosa) on soil nitrogen and carbon development: implications for global carbon sequestration. J Arid Environ 46:157–80.CrossRefGoogle Scholar
  24. Göransson H, Fransson AM, Jönsson-Belyazid U. 2007. Do oaks have different strategies for uptake of N, K and P depending on soil depth? Plant Soil 297:119–25.CrossRefGoogle Scholar
  25. Grubb PJ, Key BA. 1975. Clearance of scrub and reestablishment of chalk grassland on the Devil’s Dyke. Nature in Cambridgeshire 18:18–23.Google Scholar
  26. Hagos MG, Smit GN. 2005. Soil enrichment by Acacia mellifera subsp. detinens on nutrient poor sandy soil in a semi-arid southern African savanna. J Arid Environ 61:47–59.CrossRefGoogle Scholar
  27. Hansen JP, Vinther FP. 2001. Spatial variability of symbiotic N2 fixation in grass-white clover pastures estimated by the 15N isotope dilution method and the natural 15N abundance method. Plant Soil 230:257–66.CrossRefGoogle Scholar
  28. Hibbard KA, Archer S, Schimel DS, Valentine DW. 2001. Biogeochemical changes accompanying woody plant encroachment in a subtropical savanna. Ecology 82:1999–2011.CrossRefGoogle Scholar
  29. Hobbie EA, Ouimette AP. 2009. Controls of nitrogen isotope patterns in soil profiles. Biogeochemistry 95:355–71.CrossRefGoogle Scholar
  30. Houlton BZ, Wang YP, Vitousek PM, Field CB. 2008. A unifying framework for dinitrogen fixation in the terrestrial biosphere. Nature 454:327–34.PubMedCrossRefGoogle Scholar
  31. Hungate BA, Dukes JS, Shaw MR, Luo YQ, Field CB. 2003. Nitrogen and climate change. Science 302:1512–13.PubMedCrossRefGoogle Scholar
  32. Isaac ME, Harmand JM, Lesueur D, Lelon J. 2011. Tree age and soil phosphorus conditions influence N2-fixation rates and soil N dynamics in natural populations of Acacia senegal. For Ecol Manage 261:582–8.CrossRefGoogle Scholar
  33. Jackson RB, Banner JL, Jobbágy EG, Pockman WT, Wall DH. 2002. Ecosystem carbon loss with woody plant invasion of grasslands. Nature 418:623–6.PubMedCrossRefGoogle Scholar
  34. Jobbágy EG, Jackson RB. 2001. The distribution of soil nutrients with depth: global patterns and the imprint of plants. Biogeochemistry 53:51–77.CrossRefGoogle Scholar
  35. Ludwig F, de Kroon H, Berendse F, Prins HHT. 2004. The influence of savanna trees on nutrient, water and light availability and the understorey vegetation. Plant Ecol 170:93–105.CrossRefGoogle Scholar
  36. Marsh AS, Arnone JA, Bormann BT, Gordon JC. 2000. The role of Equisetum in nutrient cycling in an Alaskan shrub wetland. J Ecol 88:999–1011.CrossRefGoogle Scholar
  37. McCulley RL, Jobbágy EG, Pockman WT, Jackson RB. 2004. Nutrient uptake as a contributing explanation for deep rooting in arid and semi-arid ecosystems. Oecologia 141:620–8.PubMedCrossRefGoogle Scholar
  38. Murphy J, Riley JP. 1962. A modified single solution method for determination of phosphate in natural waters. Anal Chim Acta 26:31–6.CrossRefGoogle Scholar
  39. Odee DW, Sutherland JM, Kimiti JM, Sprent JI. 1995. Natural rhizobial populations and nodulation status of woody legumes growing in diverse Kenyan conditions. Plant Soil 173:211–24.CrossRefGoogle Scholar
  40. O’Halloran IP, Cade-Menun BJ. 2008. Total and organic phosphorus. In: Carter MR, Gregorich EG, Eds. Soil sampling and methods of analysis. Boca Raton: Taylor & Francis Group. p 265–91.Google Scholar
  41. Oliveira RS, Bezerra L, Davidson EA, Pinto F, Klink CA, Nepstad DC, Moreira A. 2005. Deep root function in soil water dynamics in cerrado savannas of central Brazil. Funct Ecol 19:574–81.CrossRefGoogle Scholar
  42. Pearson HL, Vitousek PM. 2001. Stand dynamics, nitrogen accumulation, and symbiotic nitrogen fixation in regenerating stands of Acacia koa. Ecol Appl 11:1381–94.CrossRefGoogle Scholar
  43. Pearson HL, Vitousek PM. 2002. Soil phosphorus fractions and symbiotic nitrogen fixation across a substrate-age gradient in Hawaii. Ecosystems 5:587–96.CrossRefGoogle Scholar
  44. Perreijn K. 2002. Symbiotic nitrogen fixation by leguminous trees in tropical rain forest in Guyana. PhD Thesis, Tropenbos, Wageningen.Google Scholar
  45. R Development Core Team. 2011. R: A language and environment for statistical computing. Vienna: R Foundation for Statistical Computing.Google Scholar
  46. Rao IM, Ayarza MA, Thomas RJ. 1994. The use of carbon isotope ratios to evaluate legume contribution to soil enhancement in tropical pastures. Plant Soil 162:177–82.CrossRefGoogle Scholar
  47. Riginos C, Grace JB, Augustine DJ, Young TP. 2009. Local versus landscape-scale effects of savanna trees on grasses. J Ecol 97:1337–45.CrossRefGoogle Scholar
  48. Sankaran M, Hanan NP, Scholes RJ, Ratnam J, Augustine DJ, Cade BS, Gignoux J, Higgins SI, Le Roux X, Ludwig F, Ardo J, Banyikwa F, Bronn A, Bucini G, Caylor KK, Coughenour MB, Diouf A, Ekaya W, Feral CJ, February EC, Frost PGH, Hiernaux P, Hrabar H, Metzger KL, Prins HHT, Ringrose S, Sea W, Tews J, Worden J, Zambatis N. 2005. Determinants of woody cover in African savannas. Nature 438:846–9.PubMedCrossRefGoogle Scholar
  49. Scholes RJ, Archer SR. 1997. Tree–grass interactions in savannas. Annu Rev Ecol Syst 28:517–44.CrossRefGoogle Scholar
  50. Sims JT. 2000. Soil test phosphorus: Mechlich 1. In: Pierzynski GM, Ed. Methods of phosphorus analysis for soils, sediments, residuals and waters. Raleigh: North Carolina State University. p 15–16.Google Scholar
  51. Tobler MW, Cochard R, Edwards PJ. 2003. The impact of cattle ranching on large-scale vegetation patterns in a coastal savanna in Tanzania. J Appl Ecol 40:430–44.CrossRefGoogle Scholar
  52. Treydte AC, Edwards PJ, Suter W. 2005. Shifts in native ungulate communities on a former cattle ranch in Tanzania. Afr J Ecol 43:302–11.CrossRefGoogle Scholar
  53. Van Auken OW. 2009. Causes and consequences of woody plant encroachment into western North American grasslands. J Environ Manage 90:2931–42.PubMedCrossRefGoogle Scholar
  54. Vitousek PM, Cassman K, Cleveland C, Crews T, Field CB, Grimm NB, Howarth RW, Marino R, Martinelli L, Rastetter EB, Sprent JI. 2002. Towards an ecological understanding of biological nitrogen fixation. Biogeochemistry 57:1–45.CrossRefGoogle Scholar
  55. Walker TW, Syers JK. 1976. The fate of phosphorus during pedogenesis. Geoderma 15:1–19.CrossRefGoogle Scholar

Copyright information

© Springer Science+Business Media New York 2012

Authors and Affiliations

  • Judith Sitters
    • 1
    • 2
    Email author
  • Peter J. Edwards
    • 1
  • Harry Olde Venterink
    • 1
  1. 1.Institute of Integrative BiologyETH ZurichZurichSwitzerland
  2. 2.Department of Ecology and Environmental ScienceUmeå UniversityUmeåSweden

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