UC Davis

Dairy farms produce vast amounts of manure, which can be stored in anaerobic lagoons, facilitating the production of potent greenhouse gasses. Manure can be diverted to the field for use as fertilizer to aid in forage crop production. The viability of this practice depends on the breakdown of organic matter, making it available to the plant and soil microbial community. Microorganisms rest at the heart of this process, and assessments of how their structure and function changes throughout cropping cycles remain limited. In this thesis research, a field trial was established using three rates of manure application: high (100% of plant nitrogen stemmed from manure), low (50% plant nitrogen was provided by manure and 50% from chemical fertilizer), and zero (100% plant nitrogen came from chemical fertilizer) to grow forage crops over two years. Plots were sampled regularly, resulting in 22 sampling dates spanning agricultural milestones such as fertilizer application, plant growing season, and harvest. Here I found manure changes the soil microbial community, increasing microbial diversity, microbial biomass-carbon (MBC) and dissolved organic carbon (DOC) following application. Furthermore, manure associated microbial groups appear to be most responsive to application, increasing following application before diminishing throughout the growing season. Manure application also improves carbon use efficiency (CUE) and stable isotope probing (SIP) revealed microbial carbon incorporators temporarily increase following application before diminishing in the growing season. Assessment of functional genes in manure applied vs. zero manure soils revealed a greater abundance of genes involved in carbon metabolism and degradation in manure soils. Together, effective manure management can play a profound role in improving the structure and function of the soil microbial community, curating a more robust and efficient carbon cycle.