Bioenergy cropping systems play a crucial role in reducing carbon (C) emissions, and particularly with perennial vegetation, which enhances soil C sequestration. The fundamental principles of the C sequestration involve stabilizing C within the soil matrix and promoting additional C inputs into the matrix, with soil pores having a key role in creating micro-environments that influence the sequestration. However, without an in-depth understanding of the soil’s physical structure and its association with the plant roots and soil C processing, it will be challenging to optimize bioenergy cropping systems to effectively mitigate climate change. The goal of my Ph.D. dissertation research was to explore the complex interactions among plant roots, soil structure, and soil C processing in different perennial vegetation. In Chapter 1, I evaluated the associations among soil texture, pore structure, and C characteristics in two perennial vegetation, monoculture switchgrass and polyculture restored prairie, across diverse soil types in the Upper Midwest of the USA. This study employed X-ray computed micro-tomography (X-ray μCT) and structural equation modeling to assess their interactions, revealing increases in the volume of medium-sized (50-150 μm diameter) pores and microbial biomass as well as their positive implication on soil C accumulation positive particularly in the prairie vegetation. In Chapter 2, I examined the pore structure within the detritusphere of soils under the switchgrass and restored prairie vegetation. By investigating soil texture, minerology, and vegetation influences on soil biopores, particulate organic matter (POM) within biopores, and pore structures in close proximity to the POM, this study highlighted that soil texture and mineralogy played a major, while vegetation a modest, role in defining the pore structure in root detritusphere. In Chapter 3, I focused on the switchgrass, particularly the differences in very fine roots among switchgrass cultivars and their effects on soil pores and C processes. Using flatbed scanner for roots and X-ray μCT for pores, I demonstrated how pore structures altered by very fine roots positively impact increases in soil microbial biomass and C accrual, with their notable variations across different cultivars. In Chapter 4, I explored belowground C and nitrogen (N) transfer between plants and its association with soil C and N inputs and pore structure formation. This study provided insights into the variation in root- and mycorrhizae-based C and N transfers by different plant combinations and their implications for potential soil C storage, emphasizing the fine pore (8-30 μm diameter) formation. In Chapter 5, I investigated how spatial soil N variability, achieved by partial N application or legume planting, alters root distribution and affects belowground C and N transfer and soil pore structure. This study showed that roots grew towards N enriched locations, promoting C and N transfer and their subsequent inputs into soil to those locations, emphasizing the formation of fine pores via root-based transfer mechanism. My dissertation contributed to the understanding of how two bioenergy cropping systems: monoculture switchgrass and polyculture prairie can be optimized for maximum C sequestration. By elucidating the interactions among roots, soil structure, and C processes, this work provides valuable insights for developing sustainable bioenergy cropping that mitigate climate change and enhance soil health.
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