Control of Invasive Plants via addition of Carbon

My research plots show an increased soil nitrogen level that may be due to a soil nitrogen response to disturbance, decomposition of already existing weedy species, atmospheric deposition, or a combination of factors. The invasive species appear to grow best in this disturbed, increased nitrogen environment. The native plants will add additional biomass in response to nitrogen inputs to a point, but are not as well equipped as the invasive species to utilize this increase in nitrogen. These conditions give the invasive plants a competitive edge over the native plants. This would not be a problem in some ecosystems, but the invasive plants I am trying to control are toxic or unpleasant to native herbivores. The two invasive plants I am using as indicator species are knapweed Centaurea diffusa and alyssum Alyssum minus. One potential solution to this problem may be addition of carbon to reduce the soil nitrogen to levels that are more like those found in undisturbed grassland.

Various research projects have shown that addition of a carbon source to the soil increases the populations of soil microbes. These soil microbes need nitrogen, so they bind it up from the soil, taking it away from the plants. The native plants are used to lower nitrogen conditions while the invasive species are not. The native plants are given their own competitive advantage and can fight against encroachment.

I added sugar and sawdust to a one square meter section of a paired plot. The other half of the plot was left untreated to compare soil nitrogen levels, plant species composition, biomass, and seed count. I added 225g of sugar to each meter once a month from February to May. I added 325g of sawdust at two dates during the same time period. The sugar decomposed quickly, in about a week. The sawdust took approximately three months to decompose and showed little evidence of drifting despite strong winds off the mountains. Three soil analyses were done and showed a significant change in both nitrate and ammonium levels. Foliar biomass measurements and knapweed seed counts showed an overall decrease in foliar biomass and a slight decrease in seed production of knapweed in the experimental plots. The decrease in total foliar biomass and in knapweed biomass was significant, but the decrease in seed count was not statistically significant.

Results of soil analysis three show a decrease in the treatment effect. This soil analysis was done two months after the last addition of carbon amendments and may show the results of a gradual loss of treatment effect over time. The trends are still obvious, but the significant error is much larger. The increased significant error may also be due to the small sample size.

Plant biomass data was collected by placing a quadrat in the NW corner of each plot and clipping all of the organic matter within that quadrat. The biomass for each plot was placed into paper bags and allowed to air-dry for three days. The bags were then placed in a 60°C baking oven and allowed to dry for an additional two days. All numbers have been multiplied to give the number of grams per square meter. The control plots average 311.8g and the treated plots average 178.4g, showing a definite decrease in biomass coinciding with the decrease in soil nitrogen level. Plant census data shows no significant difference in the average number of species found in the treated plots and the average number of species found in the control plots.

Seed Count and Biomass Measurements for Knapweed

All the knapweed in the control and experimental plots was harvested and weighed. Ten seed heads were taken from the plants in each plot and the seeds were counted from all of these ten seed heads. There was an average of 8.24 seeds per seed head in the control plots and 6.22 seeds per seed head in the experimental plot. The potential reason for lowered seed production seems to be that we have reduced knapweed's ability to fend off a biocontrol agent, the fly Urophora affinis, whose larvae formed galls in the seed heads of the plants. The plants in the treated plots had a higher occurrence of these galls and larvae and this reduced the number of seeds.

There were 44 knapweed harvested from the experimental plots with a total biomass of 735.49 g. There were only 19 knapweed harvested from the control plots, but these had a combined biomass of 1181.32 g. We were able to stunt growth of the knapweed, just as this treatment lowered total biomass in the plots. There was a significant decrease in knapweed biomass (P < 0.05) as well a significant decrease in total plot biomass (P < 0.05). A count of all the flower heads on three plants shows a constant flower per gram ratio: 15.63 f/g, 15.80 f/g, and 15.74 f/g. Lowering the biomass lowered the seed production from the experimental plots. Following is the data:

Overall the study was able to significantly decrease inorganic soil nitrogen levels and biomass of the weedy species. The species census showed that 62% of the plants in these plots were exotic or invasive species. The decrease in soil nitrogen only lasted for about a month after the last carbon addition. This decrease in effectiveness may have been due to nitrogen deposition balancing out the treatment effect. The decrease in effectivness may also have been due to urea addition from cattle (who wandered the plots for a weekend at about the time of the last carbon amendment treatment). On the other hand, this decrease in effectiveness may have been an indicator that nitrogen decreases by carbon amendments only last as long as addition of carbon is continued, which indicates that this treatment will be less effective in areas of chronic nitrogen deposition. The short-term effectiveness of carbon addition and the lack of significant decrease in seed production invites the conclusion that this treatment is not quite strong enough to significantly influence invasive species populations in areas sufferring from regular nitrogen deposition. While the treatment significantly decreased plant biomass, it did not significantly decrease seed production in the most problematic invader, diffuse knapweed. Yet the treatment is not useless, the unexpected increase in seed predation by Urophora indicates that this treatment may have promise if combined with other treatments. The combination of nitrogen-lowering carbon amendment treatments and seed production-lowering due to release of biocontrol agents (such as Urophora) may provide a far more effective treatment.

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