Gross N transformation rates and related N2O emissions in Chinese and UK agricultural soils

Highlights: • We examined gross N transformations in Chinese and UK agricultural soils. • Autotrophic nitrification and mineralization were the key N transformation processes. • Autotrophic nitrification was important for N₂O emission under aerobic conditions. • pH was the primary factor controlling autotrophic nitrification and N₂O emissions. • Gross N transformation rates were not related to the origin of soil sampling sites. -- Abstract: The properties of agricultural soils in various regions of the world are variable and can have a significant but poorly understood impact on soil nitrogen (N) transformations and nitrous oxide (N₂O) emissions. For this reason, we undertook a study of gross N transformations and related N₂O emissions in contrasting agricultural soils from China and the UK. Seven Chinese and three UK agricultural soils were collected for study using a ¹⁵N tracing approach. The soil pH ranged from 5.4 to 8.7, with three acidic soils collected from Jinjing, Lishu and Boghall; one neutral soil collected from Changshu, and the other six alkaline soils collected from Quzhou, Zhangye, Changwu, Jinzhong, Boxworth and Stetchworth. Our results showed that the main N transformation processes were oxidation of ammonium (NH₄⁺) to nitrate (NO₃⁻) (O_NH4), and mineralization of organic N to NH₄⁺. The gross autotrophic nitrification rates calculated in the three acidic soils were between 0.25 and 4.15 mg N kg⁻¹ d⁻¹, which were significantly lower (p < 0.05) than those in the remaining neutral and alkaline soils ranging from 6.94 to 14.43 mg N kg⁻¹ d⁻¹. Generally, soil pH was positively correlated (p < 0.001) with gross autotrophic nitrification rate and cumulative N₂O emissions, indicating that soil pH was an important factor regulating autotrophic nitrification and N₂O emissions. There was also a significant positive correlation between the gross autotrophic nitrification rate and cumulative N₂O emissions, highlighting the importance of this process for producing N₂O emissions in these agricultural soils under aerobic conditions. Gross NH₄⁺ immobilization rates were very low in most soils except for the Jinjing soil with the lowest pH. In conclusion, the gross autotrophic nitrification rates and related N₂O emissions were controlled by soil pH irrespectively of the soil's origin in these agricultural soils.