Degree

Doctor of Philosophy (PhD)

Department

The School of Plant, Environmental and Soil Sciences

Document Type

Dissertation

Abstract

Pastures produce hay for livestock, and they conserve soil, water, and air resources. However, high nitrogen (N) fertilizer demands in pasture production have significant implication on greenhouse gas emission and environmental pollution. Appropriate forage management strategies can reduce environmental N loss and improve soil health, to enhance the long-term pasture productivity. Therefore, this study was carried out to investigate the effects of management strategies on N budgets and soil health parameters in subtropical pastures. Both field and laboratory experiments were conducted to evaluate the effects of N stabilizers including urease inhibitor and nitrification inhibitor on pasture yield, greenhouse gas emission, and N loss through runoff and leaching water. For soil health assessment, management practices including the use of N stabilizers, with or without tillage, and change of vegetation species were evaluated for their effects on soil aggregate stability and microbial community structure. In addition, the characteristics of soil organic C (SOC) within different size-classed aggregates were characterized to investigate the relationships between aggregates formation and the change of managements. Results showed that N nitrification inhibitors dicyandiamide (DCD) and 3,4-dimethyl pyrazole phosphate (DMPP) significantly reduced nitrous oxide (N2O) emissions with urea and legume residues application, while urease inhibitor thiophosphoric triamide (NBPT) did not reduce N2O with legume residue and poultry litter application. DCD and DMPP also reduced nitrate-N loss through runoff and leaching water and improved total microbial biomass with urea application. Growing white clover (Trifolium repens) as winter cover crops covered a part of the N demands of bermudagrass (Cynodon dactylon L.), reduced N loss through runoff water, and increased soil aggregate stability and total microbial biomass under no-till condition. Analysis of Fourier transform infrared (FTIR) Spectroscopy revealed that the components of SOC were heterogeneously distributed in different size aggregate fractions, and the chemical characteristics of SOC were influenced by agroecosystem vegetation types and tillage practices. Overall, this study demonstrated the positive effects of N stabilizers and legume forage species on N budgets and soil health parameters.

Date

7-9-2021

Committee Chair

Wang, Jim J.

Available for download on Friday, July 08, 2022

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