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

Abstract

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.

Fig. 1. Greenhouse gas fluxes, soil sensing, and satellite imagery.

Daily mean (± standard error) (a) carbon dioxide (g CO₂ m⁻² d⁻¹), b methane (mg CH₄ m⁻² d⁻¹), and c nitrous oxide (mg N₂O m⁻² d⁻¹) fluxes (n = approximately 80 per day, with a total of 108,638, 103,013, and 102,997 flux measurements of CO₂, N₂O, and CH₄, respectively). Black circles represent mean daily flux measurements. Daily mean (± standard error) (d) soil temperature (°C), e soil oxygen (O₂), f daily near-infrared reflectance of vegetation (NIRv), and (g) volumetric soil moisture (m³ m⁻³) over the soil sensor measurement period and available daily satellite imagery (n = 96 measurements per day except for NIRv). For (d) soil temperature, (e) O_2, and (g) moisture, depth values are labeled as squares (10 cm), circles (30 cm), and triangles (50 cm).

Fig. 2. Diel greenhouse gas fluxes.

Hourly mean (± standard error) (a) air temperature (°C), (b) carbon dioxide (CO₂)fluxes (mg CO₂ m⁻² h⁻¹), (c) methane (CH₄) fluxes (µg CH₄ m⁻² h⁻¹), and (d) nitrous oxide (N₂O) fluxes (µg N₂O m⁻² h⁻¹), grouped by season (Fall = squares, Spring = open circles, Summer = triangles, and Winter = diamonds) over the entire measurement period (Fall: n ≥ 1220 measurements per hour, Spring: n ≥ 848 measurements per hour, Summer: n ≥ 956 measurements per hour, Winter: n ≥ 1060 measurements per hour).

Fig. 3. Weekly soil nitrogen and pH.

Weekly mean (± standard error) (a) soil nitrate (µg NO₃^--N g soil⁻¹), b soil ammonium (µg NH₄⁺-N g soil⁻¹), and c soil pH (n = 10 per week for 52 weeks). Manual soil measurements (0–10 cm depth) were conducted weekly from May 2018–May 2019.