Land Application of Dairy Manure Anaerobic Digestate Has Location and Temperature-Dependent Effects on CO2, N2O, & NH3 Emissions from California Dairy Soils

Abstract:

In California, manure management and fertilizer applications are two major sources of greenhouse gas (GHG) and ammonia (NH₃) emissions. As a response to climate regulations, California dairies are implementing anaerobic digestion of manure to reduce manure methane (CH₄) emissions. The byproduct of this process, anaerobic digestate (AD), can be applied as an organic fertilizer because of its lower carbon to nitrogen (C:N) ratio which may reduce soil carbon dioxide (CO₂) emissions and sequester carbon but, may lead to larger soil nitrous oxide (N₂O) and ammonia (NH₃) emissions due to increased NH₄⁺ loading. However, CO₂, N₂O, & NH₃ emissions may vary depending on temperature and soil properties where manure amendments are applied. The objective of this research is to determine the impact of dairy manure treatment, location, and temperature on soil CO₂, N₂O, & NH₃ emissions. In a 38-day laboratory incubation experiment, soils collected from dairies in Tulare and Marin counties of California were amended with either solid dairy manure, a mixture of solid manure & dairy slurry anaerobic digestate (AD), or no amendment (control) at both ambient (23°C) and elevated (28°C) temperature. We measured CO₂ & N₂O fluxes in a closed chamber system and NH₃ emission in a dynamic chamber system. Application of manure + AD significantly increased NH₃ emissions in both soils. This effect was particularly pronounced in Tulare soils, with NH₃ emissions 5.2 times larger than Marin soil, likely due to higher soil pH. Application of manure + AD increased soil N₂O emissions by 44% in Marin soil and 34% in Tulare soil compared to manure only, but results were not statistically different. There was no temperature effect on NH₃ or N₂O emissions. Application of solid manure + AD significantly reduced CO₂ emissions by 23% relative to manure alone at ambient temperature but increased emissions by 15% at elevated temperature relative to manure alone. Application of solid manure + AD had no significant effect on Marin soil CO₂ emissions compared to solid manure alone. Our results suggest that field application of dairy manure anaerobic digestate may allow opportunity for California dairies to meet climate regulations while moving toward sustainable production, though the actual reductions depend on site location, manure treatment, and temperature.