The elevated atmospheric CO₂ concentration (eCO₂) 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 eCO₂ 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 eCO₂, aiming to investigate the effect of eCO₂ on N₂O emissions. Moreover, we tested zero and conventional N fertilization treatments, with maize being grown under either eCO₂ or ambient CO₂ (aCO₂). We hypothesized that N₂O emissions would be increased under eCO₂ due to changes in soil labile C and mineral N derived from C-deposition, and that the increase would be larger when eCO₂ 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 eCO₂, 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 eCO₂ and N fertilization increased the intensity of soil N₂O and CO₂ emissions. The marginal significant increase in N₂O emissions under eCO₂ 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 eCO₂. In contrast, soil mineral N showed a significant decrease under eCO₂ particularly in the rhizospheric soils. The activities of C-related soil extracellular enzymes were significantly higher under eCO₂, particularly at the tasseling stage, which coincided with concurrent increased DOC and MBC under eCO₂. We conclude that eCO₂ 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 eCO₂.

Wheat grain quality is equivalent to grain yield in terms of ensuring food security under climate change but has received less attention. Identifying critical meteorological conditions in key phenological periods to account for the variability in grain protein content (GPC) can provide insight into linkages between climate change and wheat quality. The wheat GPC data from different counties of Hebei Province, China during 2006-2018 and corresponding observational meteorological data were used in our study. Through a fitted gradient boosting decision tree model, latitude of the study area, accumulated sunlight hours during the growth season, accumulated temperature and averaged relative humidity from filling to maturity were suggested as the most relevant influencing variables. The relationship between GPC and latitude was distinguished between areas north and south of 38.0° N. GPC decreased with the increasing latitude in areas south of 38.0° N, where at least accumulated temperatures of 515°C from filling to maturity were preferred to maintain high GPC. Besides, averaged relative humidity during the same phenological period exceeding 59% could generate an extra benefit to GPC here. However, GPC increased with increasing latitude in areas north of 38.0° N and was mainly attributed to more than 1500 sunlight hours during the growth season. Our findings that different meteorological factors played a major role in deciding regional wheat quality provided a scientific basis for adopting better regional planning and developing adaptive strategies to minimize climate impacts.