Effect of nitrogen fertilizer amount on N₂O emission from wheat-maize rotation system in lime concretion black soil

Lime concretion black soil is an important medium and low-yield soil in the Huanghuaihai Plain. It is prone to cracks because containing a high clay and sand ginger layer, making it unique in nitrogen transport. This study used the wheat and corn rotation system in lime concretion black soil as the research object, researching the N₂O emission characteristics and key driving factors by static box-gas chromatography methods. The experiment included four treatments: no fertilization (CK), traditional fertilization (TR), optimized fertilization (OPT), and re-optimized fertilization (ZOPT). Results showed that the average emission flux of N₂O in the wheat season ranged from 14.2 to 21.6 μg∙m⁻²∙h⁻¹, and the cumulative emission amount ranged from 0.82 to 1.24 kg(N)∙hm⁻²; the average emission flux of N₂O in the corn season ranged from 14.4 to 24.5 μg∙m⁻²∙h⁻¹, the cumulative emission amount ranged from 0.42 to 0.71 kg(N)∙hm⁻²; the N₂O emission in the wheat season was higher than that in the corn season, and the N₂O emission in the top dressing period of the two seasons was higher than that in the basal fertilizer period, demonstrating that the wheat season and top dressing period were high N₂O emission periods for lime concretion black soil. The correlation analysis results showed that N₂O emission of CK showed a significant multiple linear correlation with soil temperature, water content, and NO₃⁻-N content (P<0.05); whereas that of TR, OPT, and ZOPT only showed a significant multiple linear correlation with soil nitrate (P<0.01). There was no significant correlation between soil temperature and soil water content (except in individual cases), indicating that under fertilization conditions, the level of soil nitrate content was the most critical factor affecting N₂O emissions from the farmland of lime concretion black soil. In addition, the cumulative N₂O emissions of different nitrogen application rates were significantly different (P<0.05), and the N₂O emissions of the TR treatment were the highest, which were 1.24 kg(N)∙hm⁻² and 0.71 kg(N)∙hm⁻², respectively, in the wheat and corn seasons, significantly higher than those of OPT treatment 0.99 kg(N)∙hm⁻² and 0.51 kg(N)∙hm⁻² and ZOPT treatment 0.82 kg(N)∙hm⁻² and 0.42 kg(N)∙hm⁻². The cumulative emissions of N₂O in both the wheat and corn seasons showed an exponentially increasing trend with the increase in nitrogen application, with the correlation coefficients reaching 0.997 and 0.977 (P<0.05), respectively, indicating that the traditional lime concretion black soil nitrogen application had the problem of excessive emissions of N₂O. Overall, compared with other soils, although lime concretion black soil is not a high-emission soil of N₂O, the N₂O emission caused by higher nitrogen application cannot be ignored.