Woody plant expansion, in which shrub abundance increases at the expense of grasses, is a threat to grasslands worldwide as a result of overgrazing and fire suppression. Across the Great Plains, this shift in plant life form dominance has increased aboveground heterogeneity and nutrient pools, but belowground changes have not been documented as consistently; and the majority of studies have been done in the southern and eastern Great Plains. Because few studies on woody plant expansion have been done in northern, semi-arid grasslands, there is uncertainty if these grassland ecosystems are being similarly altered. The objective of this study was to assess the differences in belowground carbon and nitrogen pools associated with woody plant expansion over a 42-year chronosequence in a semi-arid grassland in the northern Great Plains. Soil samples were collected in Mandan, North Dakota within a reserve rangeland owned by the Northern Great Plains Research Laboratory of the USDA-ARS. Samples were analyzed for total carbon and nitrogen at three depths along a 42-year chronosequence of woody shrub encroachment. Total soil organic carbon was found to be significantly different across the chronosequence and by depths (p<0.05); it decreased from woodland to grassland (2000±140 - 1700±80 g/m2). Total soil nitrogen displayed the same significant decrease (190±10 Ð 140±10 g/m2), while total C:N increased (11± 0.1 -- 12±0.1). Course particulate organic matter also significantly changed from woodland to grassland (940±100 Ð 600±35 C g/m2, 70±10 -- 35±1 N g/m2), although it was not a linear decrease across the chronosequence. However, despite these changes in nutrient pools, microbial biomass was not different between plant life forms. These results show that woody plant expansion is altering northern grasslands both above- and below-ground.
