Date of Award

5-2018

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Agronomy

Committee Chair

Shalamar D. Armstrong

Committee Member 1

Cindy H. Nakatsu

Committee Member 2

Lori A. Hoagland

Committee Member 3

Ronald F. Turco

Abstract

Across the Midwest United States, there is an increasing number of agriculturalists incorporating cover crops and reduced tillage residue management into sustainable nutrient management plans. However, the impact of these treatments on the soil microbial community, and subsequently, on soil nutrient cycling and inorganic nitrogen (N) availability, is largely unknown. Therefore, this research had two objectives: i) to determine the impact of cover crop species on soil and litter enzyme activity and soil nitrogen supply during the cover crop decomposition period, and ii) to evaluate the impact of cover crops and reduced tillage on the soil microbial community diversity and composition during the cover crop decomposition period. Enzyme activity was measured through β-glucosidase and urease activity assessments on soil and cover crop litter. Soil microbial communities were characterized using the small subunit (16S) rRNA gene sequences determined using the Illumina MiSeq system and analyzed with the QIIME bioinformatics pipeline. Results indicate that cover crops and tillage treatment significantly influenced soil and litter enzyme activity and inorganic N availability during the decomposition period. Specifically, soil β-glucosidase activity peaked in all cover crop treatments in 2016 and 2017 within six decomposition degree-days (DCDs) following cover crop termination. Both seasons, peak inorganic N availability occurred within seven DCDs from peak β-glucosidase activity. In addition, this study revealed that the structure of the soil microbiome changed during the decomposition period. This was evident through changes in microbial community ɑ- and β-diversity, and these differences were greater as the decomposition period progressed, most notably after DCD 2.9 when a majority of the cover crop decomposition occurred. Residue management treatment was also a significant determinant of the soil microbial community structure, and the impact of these treatments was greatest at DCD 6.4. The variation of the soil microbiome structure at the beginning and end of the decomposition period was greater than the variation during peak cover crop decomposition. Overall, it was concluded that DCD, cover crop treatment, and residue management treatment were important variables in explaining the variance of the genus level observations. This study increases our understanding of how the soil microbial community responds to carbon inputs during the decomposition of winter cover crops, leading to the identification of bacteria that are highly responsive to management practices during the cash crop growing season and having the largest effect in differentiating cover crop species. Understanding the abundance patterns of the microbiota that are influenced by management practices has the potential to help optimize agriculture management practices to promote beneficial microorganisms as the adoption of management practices such as cover cropping and reduced tillage residue management treatments continues.

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