Exploratory Flux Pulses and Emerging Trade-Offs in a Semi-Arid Lettuce Experiment: Plant and Nitrogen Effects on GHG and NH3 Emissions

Agriculture significantly contributes to greenhouse gas (GHG) emissions, yet fluxes from irrigated semi-arid systems remain poorly quantified. This study investigates CO2, CH4, N2O, and NH3 fluxes in a short-term lettuce experiment under semi-arid conditions. The objective was to quantify flux variability and identify key environmental and management drivers. High-frequency soil gas flux measurements were conducted under three treatments: irrigated soil (I), irrigated soil with plants (IP), and irrigated soil with plants plus NH4NO3 fertilizer (IPF). Environmental factors, including solar radiation, soil temperature, water-filled pore space, and relative projected leaf area, were monitored. A Random Forest model identified main flux determinants. Fluxes varied with plant function, growth, and fertilization. IP exhibited net CO2 uptake through photosynthesis, whereas I and IPF showed net CO2 emissions from soil respiration and fertilizer-induced disruption of plant function, respectively. CH4 uptake occurred across treatments but decreased with plant presence. Fertilization in IPF triggered episodic N2O (EF = 0.1%) and NH3 emissions (EF = 0.97%) linked to nitrogen input. Vegetated semi-arid soils can act as CO2 sinks when nitrogen is optimally managed. Excess or poorly timed nitrogen delays CO2 uptake and increases reactive nitrogen losses. Methanotrophic activity drives CH4 dynamics and is influenced by plants and fertilization. Maintaining crop vigor and applying precision nitrogen management are essential to optimize productivity while mitigating GHG and NH3 emissions in semi-arid lettuce cultivation.