Gross N transformation rates and related N 2 O emissions in Chinese and UK agricultural soils.

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 (N ₂ O) emissions. For this reason, we undertook a study of gross N transformations and related N ₂ O emissions in contrasting agricultural soils from China and the UK. Seven Chinese and three UK agricultural soils were collected for study using a ¹⁵ N 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 (NH ₄ ⁺ ) to nitrate (NO ₃ ^- ) (O _NH4 ), and mineralization of organic N to NH ₄ ⁺ . 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 N ₂ O emissions, indicating that soil pH was an important factor regulating autotrophic nitrification and N ₂ O emissions. There was also a significant positive correlation between the gross autotrophic nitrification rate and cumulative N ₂ O emissions, highlighting the importance of this process for producing N ₂ O emissions in these agricultural soils under aerobic conditions. Gross NH ₄ ⁺ 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 N ₂ O emissions were controlled by soil pH irrespectively of the soil's origin in these agricultural soils.
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