Impacts of Elevated Atmospheric CO2 and N Fertilization on N2O Emissions and Dynamics of Associated Soil Labile C Components and Mineral N in a Maize Field in the North China Plain

The elevated atmospheric CO2 concentration (eCO(2)) is expected to increase the labile C input to the soil, which may stimulate microbial activity and soil N2O emissions derived from nitrification and denitrification. However, few studies studied the effect of eCO(2) on N2O emissions from maize field under the free-air CO2 enrichment (FACE) conditions in the warm temperate zone. Here, we report a study conducted during the 12th summer maize season under long-term eCO(2), aiming to investigate the effect of eCO(2) on N2O emissions. Moreover, we tested zero and conventional N fertilization treatments, with maize being grown under either eCO(2) or ambient CO2 (aCO(2)). We hypothesized that N2O emissions would be increased under eCO(2) due to changes in soil labile C and mineral N derived from C-deposition, and that the increase would be larger when eCO(2) was combined with conventional N fertilization. We also measured the activities of some soil extracellular enzymes, which could reflect soil C status. The results showed that, under eCO(2), seasonal N2O and CO2 emissions increased by 12.4-15.6% (p < 0.1) and 13.8-18.5% (p < 0.05), respectively. N fertilization significantly increased the seasonal emissions of N2O and CO2 by 33.1-36.9% and 17.1-21.8%, respectively. Furthermore, the combination of eCO(2) and N fertilization increased the intensity of soil N2O and CO2 emissions. The marginal significant increase in N2O emissions under eCO(2) was mostly due to the lower soil water regime after fertilization in the study year. Dissolved organic C (DOC) and microbial biomass C (MBC) concentration showed a significant increase at most major stages, particularly at the tasseling stage during the summer maize growth period under eCO(2). In contrast, soil mineral N showed a significant decrease under eCO(2) particularly in the rhizospheric soils. The activities of C-related soil extracellular enzymes were significantly higher under eCO(2), particularly at the tasseling stage, which coincided with concurrent increased DOC and MBC under eCO(2). We conclude that eCO(2) increases N2O emissions, and causes a higher increase when combined with N fertilization, but the increase extent of N2O emissions was influenced by environmental factors, especially by soil water, to a great extent. We highlighted the urgent need to monitor long-term N2O emissions and N2O production pathways in various hydrothermal regimes under eCO(2).