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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 (N2O) 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 N2O source (624 ± 28 mg N2O m2
y-1), offsetting the ecosystem carbon (carbon dioxide (CO2) and methane
(CH4)) sink by up to 14% annually. Short-term N2O 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 N2O emissions. Significant
coherence between satellite-derived photosynthetic activity and N2O fluxes
suggested plant activity was an important driver of background emissions.
Combined data show annual N2O emissions can significantly lower the
carbon-sink potential of continuous alfalfa agriculture.
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