Alfalfa (*Medicago sativa*) is a globally important perennial forage crop, known for its high productivity and nitrogen-fixing capabilities. Its extensive root system suggests significant carbon sequestration potential, making it a seemingly climate-friendly feedstock. However, the high nitrogen inputs from symbiotic fixation can lead to increased nitrous oxide (N₂O) emissions through nitrification and denitrification processes. These emissions, potent greenhouse gases, could offset the carbon sequestration benefits of alfalfa. Previous studies have often lacked the continuous monitoring necessary to fully understand the annual greenhouse gas budgets of continuous alfalfa agroecosystems, potentially underestimating the role of N₂O. This research aimed to address this gap by using a comprehensive approach combining long-term automated measurements of greenhouse gas fluxes with satellite imagery to quantify the net climate impact of continuous alfalfa cultivation in a California agroecosystem. This is crucial for accurate carbon accounting and evaluating the true climate change mitigation potential of alfalfa, a crop with growing global demand driven by increasing livestock production.

Existing research highlights the dual nature of alfalfa's impact on climate change. While recognized as a potential carbon sink due to its deep roots and perennial nature, concerns exist regarding its potential for N₂O emissions. Studies have examined either carbon sequestration or N₂O emissions individually, but few have combined both in a comprehensive annual CO₂-equivalent budget for continuous alfalfa systems. The dynamic and often unpredictable nature of N₂O emissions, characterized by ‘hot moments’ of intense fluxes, has also posed a challenge for accurate quantification. These hot moments, though short-lived, can contribute significantly to total annual emissions, making infrequent sampling inadequate. Existing literature suggests that soil moisture, temperature, and oxygen availability are key drivers of N₂O production, alongside plant activity and soil pH. Understanding the interplay of these factors in continuous alfalfa systems is crucial for accurate climate impact assessment.

This study was conducted in the Sacramento-San Joaquin Delta region of California, on a conventional perennial alfalfa field (>5 years). The site was characterized by flood irrigation during the growing season and featured Ryde soils, common in the region. The research employed a multifaceted approach that integrated several data sources: 1. **Automated Chamber Measurements:** Surface fluxes of N₂O, CH₄, and CO₂ were continuously measured using an automated chamber system (eosAC, Eosense) connected to a cavity ring-down spectrometer (Picarro G2508). Nine chambers were deployed in a grid pattern, measuring both plant rows and inter-plant areas. Data filtering removed erroneous readings, ensuring data quality. 2. **Eddy Covariance Measurements:** Annual net ecosystem exchange (NEE) of CO₂, CH₄, and energy fluxes were measured using a nearby Ameriflux tower, providing ecosystem-scale CO₂ uptake data. 3. **Soil Sensor Measurements:** Soil sensors (Apogee Instruments) measured soil oxygen (O₂), temperature, and volumetric moisture at three depths (10, 30, and 50 cm) to understand the underlying biogeochemical processes. 4. **Weekly Soil Measurements:** Weekly soil samples (0-15 cm) were collected and analyzed for gravimetric soil moisture, soil pH, nitrate (NO₃⁻), and ammonium (NH₄⁺) concentrations. 5. **Satellite Imagery:** Daily satellite imagery from Planet Labs provided near-infrared reflectance of vegetation (NIRv), serving as a proxy for plant photosynthetic activity and potential soil C inputs. Data analysis involved the use of statistical techniques such as one-way repeated measures ANOVAs and wavelet coherence analyses to assess relationships between different variables across various timescales (hourly, daily, weekly, monthly, and annual). Hot moments were defined as flux measurements exceeding four standard deviations from the mean.

The key findings of the study demonstrate that the continuous alfalfa system functioned as a significant N₂O source, reducing the overall carbon sink potential. Specific findings include: * **Large N₂O Source:** Annual N₂O emissions averaged 624 ± 28 mg N₂O m⁻² y⁻¹, representing a substantial greenhouse gas flux. * **Significant Offset of Carbon Sink:** N₂O emissions offset the ecosystem carbon sink (accounting for CO₂ and CH₄) by up to 14% annually. * **Dominant Role of Hot Moments:** Short-term hot moments, while representing only ≤1% of the total measurement periods, contributed to 44.4 ± 6.3% of annual N₂O fluxes. These hot moments were strongly correlated with rainfall and irrigation events. * **Drivers of Hot Moments:** Rainfall and irrigation events were the primary drivers of hot moments, leading to short periods of anaerobiosis that favored N₂O production. Acidic soil conditions may have further exacerbated the effect. * **Influence of Plant Activity:** Background, low-magnitude N₂O emissions showed a positive correlation with periods of high gross primary productivity (GPP) as indicated by NIRv from satellite data. This suggests plant-derived carbon or NH₄⁺ availability plays a role in regulating background N₂O emissions. * **Small CH₄ Sink:** Soils acted as a net CH₄ sink, but this effect was negligible compared to the N₂O source. * **High Soil CO₂ Emissions:** Soil CO₂ emissions were greater than those reported in other alfalfa studies, potentially due to high plant productivity, high soil C content, and warm temperatures. * **Lagged Responses:** Lagged relationships were observed between NIRv and both soil CO₂ fluxes and N₂O emissions, suggesting that plant inputs likely played a role in regulating these fluxes.

This study provides compelling evidence that the significant N₂O emissions from continuous alfalfa cultivation offset a substantial portion of its carbon sequestration benefits. The findings highlight the importance of considering both carbon sequestration and N₂O emissions when evaluating the climate impact of agricultural systems. The substantial contribution of short-term hot moments to annual N₂O fluxes underscores the limitations of infrequent sampling methods in accurately estimating greenhouse gas budgets. The observed correlation between plant activity and background N₂O fluxes suggests potential avenues for management interventions. Understanding and manipulating the factors influencing these background fluxes could offer strategies to reduce N₂O emissions from alfalfa systems. Future research could focus on further elucidating the complex interplay between plant physiology, soil biogeochemistry, and management practices to optimize both carbon sequestration and N₂O mitigation in alfalfa production.

This study comprehensively assessed the greenhouse gas balance of a continuous alfalfa agroecosystem, revealing a substantial N₂O emission source that significantly diminishes its overall carbon sink capacity. Short-term, high-intensity emission events (“hot moments”) played a disproportionate role in annual emissions, emphasizing the need for continuous monitoring. The influence of plant activity on background N₂O fluxes suggests potential for management strategies to reduce emissions. Further research should focus on optimizing alfalfa management practices to mitigate N₂O emissions while maintaining high carbon sequestration and agricultural productivity.

While this study provides valuable insights, certain limitations should be considered. The study focused on a single site in the Sacramento-San Joaquin Delta; the findings may not be fully generalizable to other regions with different soil types, climate conditions, and management practices. While the study used a combination of field measurements and satellite imagery, there are still limitations to the spatial resolution of the measurements and the ability to extrapolate findings to larger scales. The fate of harvested alfalfa biomass (used as feed for livestock) was not included in the life cycle assessment, potentially underestimating the overall greenhouse gas impact.