Soil carbon and nitrogen across a chronosequence of woody plant expansion in North Dakota

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.