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Biogeochemistry

, Volume 112, Issue 1–3, pp 409–422 | Cite as

Density fractionation and 13C reveal changes in soil carbon following woody encroachment in a desert ecosystem

  • Heather L. ThroopEmail author
  • Kate Lajtha
  • Marc Kramer
Article

Abstract

Woody encroachment has dramatically changed land cover patterns in arid and semiarid systems (drylands) worldwide over the past 150 years. This change is known to influence bulk soil carbon (C) pools, but the implications for dynamics and stability of these pools are not well understood. Working in a Chihuahuan Desert C4 grassland encroached by C3 creosote bush (Larrea tridentata), we used two density fractionation techniques (2 and 7 pool density fractionations) and isotopic analysis to quantify changes in C pools and dynamics among vegetation microsites typical of an encroachment scenario (remnant intact grassland, shrub subcanopies, and in shrub intercanopy spaces within a shrub-encroached area). The C concentration of bulk soils varied with microsite, with almost twice the C in shrub subcanopies as in intercanopy spaces or remnant grasslands. Estimated SOC accumulation rates from Larrea encroachment (4.79 g C m−2 year−1 under canopies and 1.75 g C m−2 year−1 when intercanopy losses were taken into account) were lower than reported for higher productivity Prosopis systems, but still represent a potentially large regional C sink. The composition of soil C varied among microsites, with the shrub subcanopy C composed of proportionally more light fraction C (<1.85 g cm−3) and C that was soluble in sodium polytungstate. Grassland soils contained very little carbonate C compared to shrub subcanopies or shrub intercanopy spaces. Stable isotope analyses indicate that inputs from C3 shrubs were incorporated into all density fractions, even in heavy fractions in which shrub inputs did not change overall C concentration. The seven density fractionation yielded unexpected δ13C patterns, where the two heaviest fractions were strongly depleted in 13C, indicating strong fractionation following organic matter inputs. These results suggest that the utility of isotope mixing models for determining input sources may be limited in systems with similar fractionation patterns. We propose a five pool model for dryland soil C that includes a relatively dynamic light fraction, aggregate and heavy fractions that are stable in size but that reflect dynamic inputs and outputs, a potentially large and seasonally dynamic pool of soluble C, and a large pool of carbonate C. Combined, these results suggest that dryland soil C pools may be more dynamic than previously recognized.

Keywords

Larrea Bouteloua Soil organic matter Dryland Carbonates Soil carbon stabilization Woody encroachment 

Notes

Acknowledgments

We greatly appreciate advice, ideas, and laboratory expertise from P. Sollins and assistance from C. Laney, D. Hewins, H. Lee, J. Smith, J. Throop, and J. Wigg. Thoughtful comments from two reviewers improved this manuscript. Financial support for this work was provided by the National Science Foundation (DEB 0817064 to KL, DEB 0953864 to HT) and USDA-NRI Managed Ecosystems (2005-35101-15408 to S. Archer, HT, and M. McClaran). The Jornada Basin LTER is supported by NSF DEB 0618210.

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© Springer Science+Business Media B.V. 2012

Authors and Affiliations

  1. 1.Department of BiologyNew Mexico State UniversityLas CrucesUSA
  2. 2.Department of Crop and Soil ScienceOregon State UniversityCorvallisUSA
  3. 3.Department of GeologyPortland State UniversityPortlandUSA

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