Soil carbon and nitrogen across a chronosequence of woody plant expansion in North Dakota
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Woody plant expansion has been documented on grasslands worldwide as a result of overgrazing and fire suppression, but changes in ecosystem structure and function accompanying this phenomenon have yet to be extensively studied in the temperate semi-arid grasslands of North America. The primary objectives of this study were to determine the influence of woody plant expansion on soil carbon (C), soil nitrogen (N), and roots to a depth of 15 cm along a 42-year (1963–2005) chronosequence encompassing grassland, woodland, and transition zones in a northern Great Plains grassland. From these data, we also estimated ecosystem-level soil C and N changes associated with woody plant expansion in the top 0–15 cm of soil. We found total soil C increased across the chronosequence from grassland (5,070 ± 250 g C m−2) to woodland (6,370 ± 390 g C m−2) (P < 0.05) at 0–15 cm soil depth. Total soil N also increased from grassland to woodland (425 ± 16 to 556 ± 30 g N m−2) (P < 0.05) at 0–15 cm soil depth. Coarse particulate organic matter C and N increased from grassland to woodland (940 ± 100 to 598 ± 35 g C m−2, 70 ± 10 to 35 ± 1 g N m−2) at 0–5 cm soil depth. At the ecosystem-level, we estimate C and N accumulations at 0–15 cm soil depth are occurring at a rate of 18 and 1.7 g m−2 year-1, respectively. Results of this study suggest soil resources, namely soil C and N, in the northern Great Plains are changing following woody plant expansion.
KeywordsMicrobial biomass Coarse particulate organic matter Grasslands
We thank Dr. Ofer Beeri for sharing his analysis of shrub expansion which was the basis for this study. Dr. Rod Hanley and Dr. Brett Goodwin provided additional guidance and help in statistical matters. Support from personnel at the USDA-ARS Northern Great Plains Research Laboratory is gratefully acknowledged, particularly Mary Kay Tokach for her assistance with vegetation identification. This work was funded through a grant by the Northern Great Plains Center for People and the Environment and a Specific Cooperative Agreement between USDA-ARS and the Upper Midwest Aerospace Consortium.
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