SGS-LTER Effects of nitrogen availability on the growth of native grasses and exotic weeds: Plant density on the Central Plains Experimental Range, Nunn, Colorado, USA 1997-2008

It was hypothesized that decreasing nitrogen availability would create a disadvantage for the dominant exotic species and provide an advantage for the native species, returning the community to a vegetative structure more characteristic of undisturbed sites. experiment on a historic study site. Prior researchers added treatments to this site consisting of control, water, nitrogen, and water plus nitrogen from 1971 to 1975 (Lauenroth et al. 1978). The historic experiment was a factorial combination of four treatments with two replicates, with each experimental unit being 1 ha in size. Researchers sampled the experimental plots for plant density, cover, and biomass production through 1974, and then again from 1982 to 1991. The initial sampling from 1970 to 1974 showed that the water and water plus nitrogen treatments had the strongest effect on plant community structure, both treatments increased biomass, and exotic weed species were noted on the water plus nitrogen treatment. Later sampling from 1982 to 1991 showed a ten-fold increase in exotic weed species on the water plus nitrogen plots as compared to the controls (Milchunas and Lauenroth 1995), a community change that has persisted on this site due to a chronic elevation of soil nitrogen caused by a plant tissue/soil organic matter feedback mechanism (Vinton and Burke 1995). Six new treatments were superimposed on the historic study site. The six new treatments were: control, sugar, lignin, sawdust, lignin and sugar, and sawdust and sugar. The additions provided 350 g carbon m-2 yr-1, resulting in 1,061 g m-2 of lignin, 777 g m-2 sawdust, and 833 g m-2 sugar being added to the study plots in from 1998 to 2004. This study has a split-plot completely randomized design. Three randomly transects are located within each 1 ha historic treatment, each one of these three transects was divided evenly into six 9 m2 plots, to which one of the six new treatments was randomly assigned. The entire experiment consisted of two blocks of four historic treatments, and 3 transects of six new treatments for a total of 144 sample plots. Transect plots within each historic plot are statistically psuedoreplicates, resulting in two true replicates of each new treatment, one in each of the two historic replicate plots. Density and basal cover by species within a centrally located 1m2 plot within each 9 m2 new treatment plot, was recorded since 1997 and canopy cover was collected since 2007.