The Effects of Carbon Amendments on Growth and Reproduction of a Prairie Plant (Illinois)

Prairie restorations are often conducted in areas that were once cropland with very high levels of nitrogen, which encourages the growth of nitrophilic weeds. Soil amendments with high carbon-to-nitrogen (C/N) ratios have been suggested to reduce weed emergence, as organic matter stimulates soil microbes causing them to accumulate more nitrogen in their biomass (immobilization), making it less available to plants. Although studies have demonstrated that such carbon amendments can reduce aboveground plant cover and biomass of weedy species and increase the abundance of native species (see review in Alpert 2010, Perry et al. 2010), to our knowledge no study has examined how carbon amendments may affect the growth and reproduction of prairie plants in restorations. Prairie plants have evolved in nutrient-poor soils; however, low nitrogen levels can negatively impact plant growth and reproduction.

Our primary objective was to determine how a variety of carbon amendments affect the growth and reproductive output of black-eyed Susan (Rudbeckia hirta), a short-lived perennial species that is present and often abundant during the early years of a prairie restoration (Camill et al. 2004). We predicted that carbon amendments would reduce nitrogen availability, leading to a reduction in the growth and reproductive output of black-eyed Susan, and that this negative effect would be greater with higher C/N ratios.

In May 2006, we established 30 1-m² plots in an agricultural field at the Midewin National Tallgrass Prairie (Will County, Illinois, USA). In June, we randomly assigned 5 plots to each treatment, which comprised a control and 5 carbon amendments: sawdust and sugar, aged straw, dry soybean hulls, leaf mold, and brewery waste products (see Molano-Flores and Mlynarski 2011 for methodology).

We collected soil samples and determined nitrogen (NO₃ and NH₄) levels before (June) and after (October) treatment. We found that NO₃ levels decreased in the sawdust and sugar, straw, and leaf mold treatments and the control plots and increased in the plots with soybean hulls and brewery waste products treatments, though the last was not significant. The sawdust and sugar treatment showed a 3-fold reduction in NO₃ levels, the largest of any treatment. The levels of NH₄ were significantly higher only for the brewery waste products and leaf mold treatments, which differed from our results in a concurrent study on nitrophilic weed emergence (Molano-Flores and Mlynarski 2011). Here, we present only the growth and reproductive results for black-eyed Susan plants.

Two days after the carbon amendment treatments, we planted 1 black-eyed Susan plug (ca. 13 cm height) in every corner of each plot (n = 120) and watered them for 3 d to ensure establishment. At the end of the growing season, we collected all black-eyed Susan plants. First, we measured plant height from the transition zone (between stem and root) to the top of the highest flower head. Then we placed the roots of each plant in a paper bag and the stem, leaves, and flower heads in another paper bag after counting the number of flower heads. To determine black-eyed Susan biomass, we placed the paper bags in a drying cabinet at 48.8°C for 2 d and then weighed the content of each bag. We also determined the root/shoot ratio. Lastly, we measured reproductive output as total number of flower heads per plant and total seed biomass per plant. We extracted seeds from the flower heads using a Westrup Model LA-H (Hoffman Manufacturing, Inc., Jefferson, OR) brushing machine followed by a Clipper Office Tester and Cleaner machine (Seedburo© Equipment Company, Des Plaines, IL). Because 4 plants were used per plot, we calculated mean values for plant height, aboveground and root plant biomass, root/shoot ratio, total number of flower heads, and seed biomass per plot.

Table 1.

Mean (± SD) growth and reproductive output of black-eyed Susan in 5 carbon amendments and control plots; C = control, SS = sawdust and sugar, S = straw, SH= soybean hulls, LM = leaf mold...