Effects of water allocation process on greenhouse gas emissions in drip-irrigated apple orchards on the Loess Plateau, China

Apple tree (Malus pumila Mill.) is a perennial and deciduous tree with large water demand in the growth period. Drip irrigation, as a mature and high-efficiency water-saving irrigation technology, has been widely used in apples, grapes and other economic crops around the world, achieving huge economic benefits. However, studies addressing the effects of drip irrigation on soil greenhouse gas emissions from apple orchards on the Loess Plateau in China are limited, and little is known about the potential drivers of the global warming potential based on the water allocation processes in apple orchards under drip irrigation technology. Hence, the goal of this study was to evaluate the impact of water allocation processes on soil greenhouse gas emissions and potential drivers of the global warming potential in apple orchards based on different irrigation water treatments under drip irrigation technology for a two-year consecutive field experiment in 2019 and 2020. The results showed that soil water-filled pore space, daily mean soil surface evaporation and plant transpiration were all significantly increased by drip irrigation. The peaks of carbon dioxide and nitrous oxide fluxes showed a lagged effect, with fluxes proportional to irrigation amount (with emissions occurring throughout the growth period) while methane fluxes were consumptive. Overall, using higher drip irrigation water in apple orchards increased the growth-period mean emissions of carbon dioxide and nitrous oxide and decreased the growth-period mean uptake of methane. Therefore, optimization of drip irrigation water is needed in practice to mitigate greenhouse gas emissions from apple orchard production. Furthermore, the global warming potential was directly driven by soil water-filled pore space, soil surface evaporation, plant transpiration and indirectly driven by rainfall and drip irrigation. The different water allocation processes were closely linked and interacted with each other, thereby affecting the global warming potential directly or indirectly.