Soil Microbial and Biogeochemical Responses to Differential Carbon and Nitrogen Inputs

Sandy soil is characterized by low organic carbon content and meager fertility. Cover crop inclusion increases the organic inputs, making it a promising practice to improve soil health. However, cover crop species differ in biomass production and chemical composition, which may affect the soil's microbial communities and change the desired outcomes. My dissertation research explored how continual differential organic inputs impact soil microbial community composition, microbial diversity and associated soil functions. In a pilot study, a sandy soil was incubated with two factors: crop residue amendment (with and without) and mineral nitrogen addition (with and without) in a fully crossed factorial design. The laboratory study highlighted that organic carbon inputs primed microbial decomposition of soil organic carbon; however, nitrogen addition reduced such priming effect by increasing microbial carbon use efficiency. The importance of nitrogen in preserving organic carbon was demonstrated in sandy soils In a follow-up field experiment, eight gradients of organic carbon/nitrogen inputs, created by integrating three cover crop monoculture and their two species and three species mixtures, were tested in a cotton-corn rotation system. Soil samples (0-15 cm) were collected before main crop planting (March/May) and at harvest (November), while field greenhouse gas emissions were monitored on a weekly to biweekly basis from September 2021 to September 2023. We observed that cereal rye (grass species) increased the fungal and arbuscular mycorrhizal fungi abundance and β-glucosidase (C-cycling enzyme) activity, whereas clover (legume species) increased the bacteria and ammonia-oxidizing bacteria abundance and N-acetyl-β-glucosaminidase and leucine aminopeptidase (N-cycling enzymes) activities. Additionally, vetch and clover (legume species) improved the soil’s organic nitrogen mineralization and inorganic nitrogen pool. Cover crops impacted soil microbes, modulating their community composition and abundance, but did not change their alpha and beta diversity. Furthermore, changes in microbial communities were positively correlated with soil health parameters. Five years of continual cover crop residue inputs stimulated carbon dioxide emission, especially during the cover crop growing season. The stimulatory effects varied depending on the species and their biomass inputs. Integrating cover crops did not change the annual nitrous oxide (N2O) emissions, which were likely compromised by nitrogen fertilization. The global warming potential and greenhouse gas intensity differed between cover crop treatments. The rye and clover mixtures reduced the global warming potential and greenhouse gas intensity without any yield penalty. In contrast, the clover and vetch mixtures increased the corn yield with the tradeoff of higher global warming potential and greenhouse gas intensity.