Soil Microbial, Physical, and Chemical Response to Cattle Grazing Management in the Northern Great Plains
Creators: K C, Upama
Publisher: name: University of Alberta Library
Year: 2023

<p>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. 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. Thus, small changes in grazing management that lead to reductions in greenhouse gases through carbon sequestration in soil could lead to large offsets due to the spatial extent of the grassland ecosystem. <br>
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. <br>
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 AMP 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. We used soil phospholipid fatty acid 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 (250 µm)] and density fractions [light (&gt;1.6 g cm-3) and heavy (&lt;1.6 g cm-3)]. 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. <br>
I found AMP grazing promotes bacterial to fungal ratios by enhancing labile 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 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, 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, SOC (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. <br>
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. Thus, proper implementation of management metrics is equally important to enhance benefits. Moreover, soil properties like pH, texture, and aridity were important factors in structuring soil microbial communities. Maintaining AMP grazing is crucial for grassland stability, sustainability, and SOC in the prairie regions 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.</p>