ARS | Publication request: Soil carbon and nitrogen across a chronosequence of woody plant expansion in North Dakota

Submitted to: Ecosystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 20, 2009
Publication Date: August 1, 2010
Citation: Springsteen, A., Loya, W., Liebig, M.A., Hendrickson, J.R. 2010. Soil carbon and nitrogen across a chronosequence of woody plant expansion in North Dakota. Plant and Soil. 328: 369-379.

Interpretive Summary: Overgrazing and fire suppression have contributed to an increased abundance of shrubs within grasslands throughout the world. Increases in shrubs have come at the expense of grasses, and little is known about how this shift in vegetation dominance will affect the structure and function of grassland ecosystems. A study was undertaken to understand the influence of shrub expansion on soil carbon, nitrogen, and roots in a northern mixed-grass prairie grassland near Mandan, North Dakota. Total carbon and nitrogen was found to be greater under established shrubs as compared to grassland in the surface 15 cm of soil. Soil carbon and nitrogen under shrubs was estimated to be accumulating at a rate of 16 g C and 1.9 g N/sq. meter/yr, respectively. Results of this study suggest shrub expansion is altering semiarid northern grasslands similarly to arid grasslands in the southwestern U.S.

Technical Abstract: Woody plant expansion has been documented on grasslands worldwide as a result of overgrazing and fire suppression, but the 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 at this site in the top 5 cm of soil. We found total soil C increased (P<0.05) across the chronosequence from grassland (1700 +/- 80 g C/sq. meter) to woodland (2000 +/- 140 g C/sq. meter). Total soil N also increased from grassland to woodland (140 +/- 10 to 190 +/- 10 g N/sq. meter, P<0.05). In contrast, coarse particulate organic matter decreased from woodland to grassland (940 +/- 100 to 600 +/- 35 g C/sq meter, 70 +/- 10 to 35 +/- 1 g N/sq. meter). At the ecosystem-level, we estimate 67 Mg C and 7.8 Mg N were added to the top 5 cm of soil across the study site, accumulating at a rate of approximately 16 g C and 1.9 g N/sq. meter/yr. Results of this study suggest woody plant expansion is altering semi-arid northern grasslands similarly to arid grasslands in the southwestern U.S.