Results

Soil inorganic nitrogen levels differed significantly between the treated and control plots both during the treatment and one month after the treatment, but the differences were not statistically significant two months after treatment (paired t-test, P < 0.05 except in last collection; Figure 1). Ammonium levels were initially substantially lower in the treated plots than in the control plots; these differences diminished after carbon additions ceased. Nitrate levels also were initially lower in the treatment plots than in the control plots, and the difference again became less pronounced after carbon additions were stopped. In addition to the soil nitrogen decrease, the soil analyses showed a significant increase in soil moisture in the treated plots during the first soil collection (P < 0.05) which was not present during the subsequent soil analyses.

The plant census showed that these plots had a high percentage of invasive species. I compared the species list from the study plots to a collection of exotic species listed by Mack (1989), and I found that 62% of the species in the plots were non-native species.

The census data on the native species Western wheat (Agropyron smithii) and the two invasive species alyssum (Alyssum minus), and diffuse knapweed (Centaurea diffusa) showed no significant treatment effect (Table 1). There was no significant change in native species density between the control and the treatment plots (P = 0.68). In addition, density of the two invasive species did not change significantly between the control and the treatment plots (P = 0.34 for diffuse knapweed and P = 0.44 for alyssum).

An overall census, including both control and experimental plots, showed an average of 4.78 invasive species and 2.47 non-invasive species per plot. There was no difference in plant species number or ratio of invasive species to non-invasive species between the treated and control plots (Table 2). There was also no significant change in species diversity correlating with how soon the treatment was started after disturbance. The 1993 disturbance site was first treated with carbon amendments right after disturbance and reseeding and averaged 7.50 plant species/m². The 1991 disturbance site was first treated two years after disturbance and averaged 7.13 species/m².

Both the non-knapweed biomass and the mature knapweed biomass were significantly lower in the treated plots than in the control plots (Table 3); (P < 0.05 for both). Knapweed was present in nine of the treatment plots and five of the control plots and weighed an average of 81.72 g/m2 in each of the nine treatment plots compared to an average of 236.26 g/m2 in each of the five control plots. When the zero values for all other plots are included in the analysis, average biomass of knapweed in the treated plots was about 2/3 that of knapweed in the control plots. The total biomass amount of all plants was also significantly lower in the treatment plots than in the control plots (P < 0.05).

The knapweed seed count showed no significant difference in seed production per gram of tissue between the knapweed in the control plots and that in the treatment plots (P > 0.05). The average number of seeds per seed head in the control plots was 8.2 while the average number in the treatment plots was 6.2.

Methods	Home	Discussion