Michele Tran | ALES Graduate Seminar

Event details: A graduate exam seminar is a presentation of the student’s final research project for their degree.
This is an ALES MSc Final Exam Seminar by Michele Tran. This seminar is open to the general public to attend.

MSc with Drs. Malinda Thilakarathna and Cameron Carlyle.

Thesis Topic: Effects of introduced cicer milkvetch on plant growth, carbon and nitrogen cycling, and soil microbial communities and activities on the Canadian dry mixedgrass prairie

Abstract:

Grasslands provide valuable ecosystem goods and services, such as livestock forages, habitats for wildlife, and processes such as carbon sequestration and nitrogen cycling, which help offset greenhouse gas emissions. Cicer milkvetch (Astragalus cicer L.) is a perennial forage legume introduced to the Canadian prairies. Previous studies have found that cicer milkvetch can increase forage productivity while simultaneously decreasing soil carbon and vegetation species richness. Negative plant-soil feedback from the growth of cicer milkvetch may suppress the growth of other plant species, where the changes in vegetation may cause changes in the soil microbiome and subsequently affect nutrient cycling. To investigate these effects, two separate experiments were conducted to examine the effects of cicer milkvetch on the soil microbiome and their activities related to carbon and nitrogen cycling, as well as the effects of cicer-conditioned soils on plant growth and carbon and nitrogen contents. The first hypothesis of this research posits that the introduced cicer milkvetch alters soil microbial communities and activities related to nutrient cycling, which was examined through a two-year field study in the Canadian Dry Mixedgrass prairie region near Brooks, Alberta. In this study, plots dominated by cicer milkvetch were compared to cicer-free grassland plots, with or without a nitrogen addition, to simulate the effect of legume nitrogen fixation. Results indicated that cicer milkvetch plots exhibited differences in terms of soil microbial composition, increases in total live shoot and root C and N contents, C and N cycling gene pathway abundances (although not statistically significant), and extracellular enzyme activities related to C cycling, compared to the grassland plots. Findings from the field study suggest that cicer milkvetch impacts microbial diversity, particularly regarding rarer taxa, but the dominant groups remained relatively unchanged. The general increase of carbon cycling pathways, alongside increased enzymatic activity, indicates that cicer milkvetch enhances overall carbon cycling activities in the soil.

A second hypothesis suggest that cicer milkvetch causes negative plant-soil feedbacks, which alter the growth of other plant species growing in cicer-conditioned soil. This hypothesis was tested in a greenhouse study, where the effects of cicer-conditioned soils on the growth and carbon and nitrogen contents of two legumes (Cicer milkvetch Astragalus cicer L. and purple prairie clover Dalea purpurea) and two grass species (timothy Phleum pratense and blue grama Bouteloua gracilis) found in the Canadian Dry Mixedgrass prairie region. Cicer-conditioned soils were compared against cicer-free grassland soils (main factor), and because the presence of cicer milkvetch can increase soil nitrogen, a treatment of nitrogen addition was also included along with a no nitrogen addition treatment (sub-factor) to test whether additional nitrogen fixation affected plant-soil feedbacks. Shoot and root biomass was greater in cicer-conditioned soils for all plant species compared to the grassland soil. Shoot and root total carbon content (per plant) of all species were greater when grown in cicer milkvetch soil compared to the grassland soil. However, the root carbon content of prairie clover decreased with nitrogen addition. Similarly, shoot and root nitrogen contents per plant were increased in cicer milkvetch soil compared to the grassland soil, but the positive impact of nitrogen addition was significant only in timothy shoots. The C/N ratio showed variable results, where cicer shoots, timothy shoots, and prairie clover roots showed interaction effects by soil type and nitrogen, and were generally lowest in nitrogen-supplied treatments. Clover shoots, blue grama shoots, and timothy roots showed significant effects only from nitrogen treatments, while cicer and blue grama roots showed effects from both soil type and nitrogen treatment with no interactions, generally lowering in cicer soils. Legume root parameters (nodule number, length, surface area, average diameter, and volume) were generally increased in cicer soil, except for the root diameter in cicer milkvetch, which was unaffected by any treatment factor. Overall, results from the greenhouse study suggest that cicer-conditioned soil play a significant role, contributing increased growth, nitrogen, and carbon contents compared to the grassland soil, indicating positive plant-soil feedbacks.

Overall, the two studies offer insights into invasive legumes and their impact on nutrient cycling and productivity in grasslands, where other factors such as competition are causing reductions in growth and carbon contents of other plant species.