Carbon-sink potential of continuous alfalfa agriculture lowered by short-term nitrous oxide emission events.

Alfalfa is the most widely grown forage crop worldwide and is thought to be a significant carbon sink due to high productivity, extensive root systems, and nitrogen-fixation. However, these conditions may increase nitrous oxide (N 2 O) emissions thus lowering the climate change mitigation potential. We used a suite of long-term automated instrumentation and satellite imagery to quantify patterns and drivers of greenhouse gas fluxes in a continuous alfalfa agroecosystem in California. We show that this continuous alfalfa system was a large N 2 O source (624 ± 28 mg N 2 O m 2 y -1 ), offsetting the ecosystem carbon (carbon dioxide (CO 2 ) and methane (CH 4 )) sink by up to 14% annually. Short-term N 2 O emissions events (i.e., hot moments) accounted for ≤1% of measurements but up to 57% of annual emissions. Seasonal and daily trends in rainfall and irrigation were the primary drivers of hot moments of N 2 O emissions. Significant coherence between satellite-derived photosynthetic activity and N 2 O fluxes suggested plant activity was an important driver of background emissions. Combined data show annual N 2 O emissions can significantly lower the carbon-sink potential of continuous alfalfa agriculture.