Effects of introduced cicer milkvetch on plant growth, carbon and nitrogen cycling, and soil microbial communities and activities on the Canadian dry mixedgrass prairie
Tran, M.E. 2025.
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 affecting 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 iii
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 suggests that cicer milkvetch causes negative plant-soil feedbacks,
which decrease 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..
Key Words
Cicer milkvetch, soil microbiome, carbon cycling, nitrogen fixation, extracellular enzyme activity, introduced species, grasslands, plant-soil feedback
