Automated greenhouse gas fluxes and associated drivers from dairy agriculture

Abstract:

Dairy farms are a significant source of greenhouse gases (GHG) to the atmosphere. In California, the largest dairy state in the U.S., dairies account for almost half of the state’s CH₄ inventory. Manure management is a well-known CH₄ source on dairies; less is known about the role of dairy soils in greenhouse gas fluxes. For example, silage fields could be a significant contributor of nitrous oxide (N₂O) due to manure fertilizer application (CARB, 2014). GHG flux estimates from these crop systems have large uncertainty due to low temporal resolution from static chamber methods, and few studies have simultaneous explored potential drivers of the patterns observed. In this study, we measured continuous GHG fluxes in a manure-fertilized wheat silage crop at a northern California organic dairy farm using an automated chamber method coupled with a cavity ring down spectrometer (CRDS). We measured 10-minute soil surface fluxes (9 chambers separated every 15 m) during the growing season (October 2019 to May 2020). We also measured continuous soil temperature, moisture and oxygen from 0-10 cm depth. We capture both “hot spots” and “hot moments” of soil GHG fluxes during the sampling period. Hot spots were found in 2 chambers with consistently higher fluxes than the others. Hot moments of high CH₄ and N₂O fluxes occurred in 22 and 26 out of the 188 evaluated days, respectively. CH₄ and N₂O fluxes ranged between 0.8 to 23 nmol m^-2s^-1 and 1.0 to 38 nmol m^-2s^-1 during these hot moments, which represented more than 95% (CH₄) and 82% (N₂O) of the total cumulative fluxes during the growing season. We also found that these high fluxes were found when volumetric water content (VWC) was between 0.35 to 0.45 for CH₄ whereas for N₂O the range was narrower (0.35 to 0.38; spline fit, r²=0.90 and r²=0.86, p<0.05 for CH₄ and N₂O, respectively). Oxygen content between 6 to 12 % also correlated with high N₂O fluxes. For CH₄, the correlation exhibited a hysteresis pattern with high fluxes occurring primarily between 2 and 10% oxygen concentrations. Temperature did not affect CH₄ and N₂O fluxes during the growing season Our results demonstrate that VWC is an excellent predictor of CH₄ and N₂O fluxes. Soil VWC continuous monitoring could be used to estimate GHG inventories from crop soil at a facility scale, and to implement water management practices to mitigate GHG fluxes.