Upama K C | ALES Graduate Seminar

Date(s) - 10/05/2023
9:00 am - 10:00 am

A graduate exam seminar is a presentation of the student’s final research project for their degree.
This is an ALES PhD Final Exam Seminar by Upama K C. This seminar is open to the general public to attend.

Meeting ID: 993 4742 4125 | Passcode: 254024

Thesis Topic: Soil Microbial, Physical, and Chemical Response to Cattle Grazing Management in the Northern Great Plains

PhD with Dr. Cameron Carlyle

Seminar Abstract:

Globally, 25% of the terrestrial surface is covered by grasslands, 40% of which is used for grazing livestock and is estimated to hold 30% of global soil carbon. Native grasslands in the Canadian prairie cover 12 million hectares and are used extensively for grazing cattle. Livestock grazing behaviors such as feeding, trampling, and fouling lead to changes in soil characteristics, which will directly affect the innate soil microbial communities. Soil microbes play a major role in carbon and nitrogen cycling and contribute significantly to the production and consumption of greenhouse gases. Furthermore, grazing may also alter the stability of carbon pools through changes in soil fractions and the carbon held within them, as soil microbes play a critical role in carbon pools and stability in soil. Changes in soil microbial communities through cattle grazing affect key ecosystem functions such as nutrient cycling, soil organic carbon (SOC) storage and stability which implicate with climate change. Thus, small changes in grazing management that lead to reductions in GHG through carbon sequestration in soil could lead to large offsets due to the spatial extent of the grassland ecosystem.
Among different grazing management systems, a specialized form of rotational grazing known as adaptive multi-paddock grazing (AMP) is considered a regenerative grazing management practice that can improve soil carbon sequestration, productivity, and sustainability of grasslands by altering soil microbial community. However, it is not known how AMP grazing affects microbial communities or subsequent effects on SOC pools in the Northern Great Plains. The overall objective of this thesis research is to understand the effects of grazing systems and specific management metrics on soil microbial community and soil carbon pools.

In this study, soil samples were collected from 19 ranch pairs (38 ranches in total) located across the Canadian prairie, where, in each pair, one ranch practiced multi-paddock grazing while the other practiced conventional grazing (varying from continuous to slow to fast rotational grazing). Grazing management metrics such as stocking rate, stocking density, and rest periods were calculated based on management information data obtained from each landowner. Various soil microbial, molecular, and soil analytical techniques were applied to quantify soil microbial biomass, diversity, co-occurrence pattern, and soil properties. We used soil phospholipid fatty acid (PLFA) profiles and chloroform fumigation/extraction to quantify microbial functional groups, and measured microbial biomass carbon. Gene abundance of total bacteria and total fungi were enumerated by targeting the 16S rRNA, and ITS through quantitative real-time PCR; alpha, beta diversity and co-occurrence pattern were assessed using 16S/ITS amplicon sequencing. Further, different SOC pools (i.e., labile to recalcitrant carbon) was quantified through different soil particle size [fine (<53 µm), medium (53-250 µm), and coarse (>250 µm)] and density fractions [light (>1.6 g cm3) and heavy (<1.6 g cm3)]. To understand AMP grazing effects we applied two step statistical approaches 1) direct comparison of AMP vs. conventional grazing and 2) analysis of management metrics effect.

I found AMP grazing promotes bacterial to fungal ratios by enhancing liable nutrients (water-soluble organic carbon). Additionally, we found that AMP grazing increased fungal diversity and evenness and led to more complex microbial associations by reducing soil pH. In general, I found the largest SOC stock was in the fine fraction, which was 1.1 times and 1.7 times higher than in the coarse and medium size fractions, respectively in AMP grazing. In contrast, within soils under conventional grazing, the coarse fraction held the largest SOC stock, which was more than 2.1 and 1.6 times higher than that in the medium and fine fractions on average. Furthermore, soil organic carbon (concentration and stock) were significantly higher in the fine soil fractions from AMP grassland than conventionally grazed grasslands in association with clay particles and corresponding fungi: bacterial (F:B) ratios.

My findings also highlighted stocking density and rest periods are equally important management metrics besides stocking rate in structuring resilient microbial communities and carbon storage in soil. I found that stocking rates play a key role in bacterial and fungal richness while stocking density plays a major role in F: B ratio and soil aggregate distribution. Rest periods were vital for fungal richness and diversity; where long rest periods boost fungal richness and diversity. Moreover, soil properties like pH, texture, and climate (AHM) were equally important factors in structuring soil microbial communities and these communities were influenced by aridity. Thus, proper implementation of management metrics is equally important to enhance benefits. Maintaining AMP grazing is crucial for grassland stability, sustainability, and SOC in the prairie regions, in particular, with long rest periods and low to medium stocking rate and stocking density. Thus, my results support the idea that AMP grazing is generally beneficial for increased soil carbon storage.