Influence of the shallow groundwater table on the groundwater N2O and direct N2O emissions in summer maize field in the North China Plain

Abstract

Agriculture is an important N2O emissions source. Water cycle and nitrogen cycles have important effects on N2O 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 NO3− and N2O concentrations in shallow groundwater as well as the direct N2O emissions due to surface soil and groundwater evaporation. Our results showed that N2O emissions during fertilization accounted for 80%–90% of the total N2O emissions throughout the maize growing period. Direct N2O emissions increase with the increase in the groundwater table. The total N2O emissions in 2020 were 96.44, 9.75, 6.46, and 6.22 kg ha−1 y−1 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 N2O 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 N2O (6 weeks in 2020 vs. 7.5 weeks in 2019) and NO3− (6.75 weeks in 2020 vs. 7.25 weeks in 2019) in shallow groundwater. Studying the influence of shallow groundwater tables on direct N2O emissions will provide insights into the interaction between the water and nitrogen cycles in agroecosystems. The results of this study suggest that direct N2O emissions can be effectively reduced by controlling the groundwater table in agricultural fields in the North China Plain.