Nitrous oxide emissions from cultivated black soil: A case study in Northeast China and global estimates using empirical model

Manure application is effective in promoting soil carbon sequestration, but its impact on N₂O emission is not well understood. A field experiment was conducted in a maize-cultivated black soil in Northeast China with six treatments: inorganic fertilizer (NPK), 75% inorganic fertilizer N plus 25% pig (PM1) or chicken (CM1) manure N, 50% inorganic fertilizer N plus 50% pig (PM2) or chicken (CM2) manure N, and no N fertilizer (CK). Annual N₂O emission significantly increased from 0.34 kg N ha^-1 for CK to 0.86 kg N ha^-1 for NPK and further to 1.65, 1.02, 1.17, and 0.93 kg Nha^-1 for PM1, CM1, PM2, and CM2, respectively. A ¹⁵N tracing study showed that 71-79% of total N₂O was related to nitrification at 30-70% water-filled pore space (WFPS), and heterotrophic nitrification contributed 49% and 25% to total N₂O at 30% and 70% WFPS, respectively. In an incubation, N₂O emission was only stimulated when nitrate and glucose were applied together at 60% WFPS, indicating that denitrification was carbon limited. PM had a stronger effect on denitrification than CM due to higher decomposability, and the lower N₂O emission at higher manure application rate was associated with decreased mineral N supply. After compiling a worldwide database and establishing an empirical model that related N₂O emissions (kg Nha^-1) to precipitation (P_r, m) and fertilizer N application rate (N_r, kg N ha^-1) (N₂O = 1.533P_r + 0.0238P_rN_r), annual N₂O emission from global-cultivated black soil applied with inorganic fertilizer N was estimated as 347 Gg N. Our results suggested that N₂O emission from cultivated black soils in China was low primarily due to low precipitation and labile organic carbon availability, and would be stimulated by manure application; thus, increased N₂O emission should be taken into consideration as applying manure increases soil organic carbon sequestration.