Soil nutrient availability as a mechanistic assessment of carbon addition and biological control of spotted knapweed (Centaurea maculosa Lam.)
Brockington, M. R. 2005. M.Sc Thesis. Dept. Land Resources and Environmental Sciences, Montana State University, Bozeman, MT
Abstract
Invasive plant infestations alter indigenous plant communities, and have serious ecological and
economic consequences. Management strategies aim to alter the processes driving plant communities
to favor desired species. The goal of this research was to investigate a mechanistic approach to
large-scale weed problems. Two studies examined: 1) the ability of carbon to reduce soil nutrients and
invasive plant growth, and 2) insect biological control agent impacts on a spotted knapweed population
and associated soil nutrient availability changes. The objective of the first study was to lower soil
nutrients by adding a carbon source. We hypothesized that increasing the amount of sucrose would
reduce soil nutrients and spotted knapweed growth. Sucrose addition reduced spotted knapweed
biomass, yet led to soil nitrogen increases. This result may be attributed to a flush of nutrients after
initial microbial immobilization. Our results suggest that if sucrose is used in research, high amounts
(>70 gC/m2) must be coupled with frequencies of 30 days or less in order to sustain immobilization of
limiting nutrients. The second study investigated the effect of an insect biological control agent,
Cyphocleonous achates Fahr. (Coleoptera: Curculionidae), on soil nutrients and spotted knapweed
(Centaurea maculosa Lam.). We hypothesized that spotted knapweed growth would increase with
N-addition and decrease with sucrose addition, and that soil N would increase with the addition of C. achates where N is most limited. With one year of introduction, spotted knapweed aboveground
biomass increased in response to C achates, suggesting spotted knapweed may have compensated for
insect infection. In a long-term experiment, soil ammonium decreased and biomass increased in
response to C achates. Uninfected individual plants may have responded to available resources
prevented from acquisition by infected plants. This may suggest that sufficient insect density must be
established to infect a majority of plants for an overall growth decline. Natural enemy impact on a
target weed population may potentially be predicted from its influence on soil resources. Prediction
accuracy will likely be improved where either weevil densities or infection intensities are considered.
