Remote sensing, mobile, and isotopic measurements of methane emissions and nitrous oxide from dairy manure management in the Central Valley of California

Dairy farming accounts for a substantial amount of methane (CH4) and nitrous oxide (N2O) emissions in the state of California1. Specifically, the Central Valley is home to the nation’s leading dairy industry, with over 80% of dairy farms and about 90% of dairy . cows of the state2. Previous measurements have shown that the Central Valley contributes to substantially elevated levels of CH43. This warrants attention given that CH4 is a powerful radiative forcer and ozone precursor, with a global warming potential (GWP) 25 times greater than carbon dioxide (CO2) over 100 years4. Dairy operations produce CH4 via enteric fermentation and manure management, which is thought to account for 27% and 25% of statewide CH4 emissions, respectively1. As such, California has set forth ambitious legislation to reduce greenhouse gas (GHG) emissions by 40% below 1990 levels by 2030 (Senate Bill 32 of 2016, Governor’s Executive Order B-30-15) and targets short-lived climate pollutants such as CH4 (i.e., Senate Bill 1383 of 2016). The CH4 budget, however, remains uncertain, with studies showing that CH4 enhancements in California are likely about 1.2 - 1.8 times larger than reported5. In particular, there is growing evidence that CH4 emissions from manure lagoons are underestimated. Although N2O is not yet targeted by the state, it is also a strong climate forcer with a GWP close to 300 times higher than CO2 in a 100-year time horizon4. Like CH4, N2O emissions from dairies also arise from manure management systems, and are generated from some of the same sources of dairy CH4 emissions. Field data is still variable, but previous studies attributed the majority of N2O emissions to barns, unlike dairy CH4 emissions, which are mostly expected from manure lagoons and slurry systems6.

The next largest emitter of N2O from dairy manure management is estimated to come from corrals and solid manure piles6. Interestingly, previous observations from manure lagoons and slurry stores also show a relatively large N2O flux6–11. Crop fields fertilized by manure and other synthetic fertilizers near dairies can also emit N2O12. However, it remains challenging to measure and determine the isotopic signature of N2O from different sources and microbial production pathways given that there are twelve possible molecules of N2O derived from two isotopes of N and three isotopes of O13. Consequently, the global N2O budget remains uncertain in part because very little is known about the isotopic signature of distinct N2O sources, and because there is large variation between isotope measurements of sources14. The primary goal of my research will be to study CH4 emissions from wet manure management in the Central Valley. Specifically, I will investigate the magnitude and spatial distribution of CH4 emissions from manure lagoons and examine their primary physical drivers. I also propose to study N2O isotopic molecules, in conjunction with CH4 emissions, across the landscape of a large dairy operation in California.