Influences of shallow groundwater depth on N 2 O diffusion along the soil profile of summer maize fields in North China Plain.

The emissions of nitrous oxide (N2 O) from agricultural fields are a significant contribution to global warming. Understanding the mechanisms of N2 O emissions from agricultural fields is essential for the development of N2 O emission mitigation strategies. Currently, there are extensive studies on N2 O emissions on the surface of agricultural soils, while studies on N2 O fluxes at the interface between the saturated and unsaturated zones (ISU) are limited. Uncertainties exist regarding N2 O emissions from the soil-shallow groundwater systems in agricultural fields. In this study, a three-year lysimeter experiment (2019-2020, 2022) was conducted to simulate the soil-shallow groundwater systems under four controlled shallow groundwater depth (SGD) (i.e., SGD = 40, 70, 110, and 150 cm) conditions in North China Plain (NCP). Weekly continuous monitoring of N2 O emissions from soil surface, N2 O concentration in the shallow groundwater and the upper 10 cm of pores at the ISU, and nitrogen cycling-related parameters in the soil and groundwater was conducted. The results showed that soil surface N2 O emissions increased with decreased shallow groundwater depth, and the highest emissions of 96.44 kg ha-1 and 104.32 kg ha-1 were observed at G2 (SGD = 40 cm) in 2020 and 2022. During the observation period of one maize growing season, shallow groundwater acted as a sink for the unsaturated zone when the groundwater depth was 40 cm, 70 cm, and 110 cm. However, when SGD was 150 cm, shallow groundwater became a source for the unsaturated zone. After fertilization, the groundwater in all treatment plots behaved as a sink for the unsaturated zone, and the diffusion intensity decreased with increasing SGD. The results would provide a theoretical basis for cropland water management to reduce N2 O emissions.