Effects of emitter flow rates of drip irrigation on methane uptake, diffusion, transformation in the soil and potato yield

Methane is a powerful greenhouse gas with a global warming potential 34 times greater than carbon dioxide. The emitter flow rate, as a fundamental parameter of drip irrigation systems, controls the horizontal and vertical distribution of soil water, resulting in distinct soil wetting patterns. At the same time, the soil wetting pattern influences soil temperature, aeration, and the movement and transformation of nitrogen, all of which affect soil microbial activity and consequently methane uptake. Understanding the role of the emitter flow rate of drip irrigation in regulating the diffusion, oxidation and surface uptake of methane in the soil is crucial to increasing the potential of methane uptake in drip-irrigated croplands to mitigate global climate change. In this study, a two-year field experiment combining static chamber gas chromatography with the concentration gradient method was conducted in 2021 and 2022 in Wuwei, Gansu of China. Three drip irrigation flow rates (F1: 1.3 L/h, F2: 2.0 L/h, F3: 3.0 L/h) were set to investigate the characteristics of methane diffusion, absorption, and surface net uptake in different soil layers and their responses to soil environmental factors. The results showed that soil methane concentrations decreased with increasing soil depth. There was no significant difference in the average surface methane concentration between treatments, and the average methane concentration in the 0-60 cm soil layer showed F3 > F2 > F1. The value in the F3 treatment was significantly higher than F1 by 11.9% in the 0-20 cm soil layer. The average methane concentration of F3 treatment was significantly higher than that of F2 and F1 treatments by 8.5%, 26.8% (>20-40 cm soil layer) and 10.2%, 19.1% (>40-60 cm soil layer), respectively. Average surface methane uptake fluxes were 85.4, 80.2 and 78.5 μg g/(m⁻² h) for F1, F2 and F3 treatments, respectively, and were significantly increased by 6.4% and 8.9% for F1 treatment compared to F2 and F3 treatments (P < 0.05). The average methane diffusion flux in the 0-20 cm and >20-40 cm soil layers was significantly higher in the F1 treatment than in the F2 and F3 treatments, while the average methane diffusion flux in the 40-60 cm soil layer was significantly lower in the F1 treatment than in the F2 and F3 treatments. The cumulative methane uptake in the F1 treatment increased by 5.9%-13.8% compared to the F2 and F3 treatments, respectively. Methane uptake primarily occurred in the 0-20 cm soil layer, accounting for 72.1%-82.5% of the total uptake. Emitter flow rate had significant effects on soil water-filled pore space, soil oxygen concentration and nitrogen content (P < 0.05). Among various soil environmental factors, soil water-filled pore space had the most significant influence on soil methane concentration. Cumulative surface methane uptake fluxes were significantly and positively correlated with cumulative methane diffusion fluxes (downward) in the 0-20 and >20-40 cm soil layers, and cumulative methane diffusion fluxes in the >20-40 cm soil layer were significantly positively correlated with the values in the 0-20 cm soil layer. Cumulative methane transformation fluxes in the 0-20 cm soil layer were significantly and positively correlated with cumulative methane diffusion fluxes in the 0-20 cm soil layer, and cumulative transformation fluxes in the >20-40 cm soil layer were significantly and positively correlated with cumulative diffusion fluxes in the 0-20 and >20-40 cm soil layers. The lower values of soil water-filled pore space in the surface soil of the F1 treatment facilitated the entry of methane from the atmosphere into the soil and its diffusion into deeper layers, thereby increasing methane uptake. In addition, the potato yield and irrigation water use efficiency under F1 treatment were relatively highest. Soil methane uptake, diffusion, and transformation were influenced by soil water distribution. Therefore, it should be considered to regulate the soil wetting area through an appropriate emitter flow rate to enhance the soil methane sink capacity and potato yield in the drip irrigation management. The results can provide a valuable information for soil carbon sinks and potato yield improvement in drip irrigated soils.