Influence of the shallow groundwater table on the groundwater NO and direct NO emissions in summer maize field in the North China Plain.

Agriculture is an important NO emissions source. Water cycle and nitrogen cycles have important effects on NO in farmland ecosystems. The changes in the groundwater table can lead to changes in farmland the water and nitrogen cycle processes. However, how this such changes will affect N2O emissions from farmland remains unclear. In this study, a two-year volume lysimeter experiment (2019-2020), including four controlled groundwater tables (i.e., 40, 70, 110, and 150 cm), was performed to monitor the variations in the NO and NO concentrations in shallow groundwater as well as the direct NO emissions due to surface soil and groundwater evaporation. Our results showed that NO emissions during fertilization accounted for 80%-90% of the total NO emissions throughout the maize growing period. Direct NO emissions increase with the increase in the groundwater table. The total NO emissions in 2020 were 96.44, 9.75, 6.46, and 6.22 kg ha y at a groundwater table of 40, 70, 110, and 150 cm, respectively. The high water-filled pore space (WFPS) value resulting from the elevated groundwater table increased the groundwater-atmosphere connectivity, leading to significantly increased NO emissions after fertilization. Increased precipitation (454.90 mm in 2020 vs. 180.30 mm in 2019) accelerated the hydrological processes in agroecology, reducing the retention time of NO (6 weeks in 2020 vs. 7.5 weeks in 2019) and NO (6.75 weeks in 2020 vs. 7.25 weeks in 2019) in shallow groundwater. Studying the influence of shallow groundwater tables on direct NO emissions will provide insights into the interaction between the water and nitrogen cycles in agroecosystems. The results of this study suggest that direct NO emissions can be effectively reduced by controlling the groundwater table in agricultural fields in the North China Plain.