Nitrous oxide and carbon dioxide emissions from two types of soil amended with manure compost at different ammonium nitrogen rates

Ammonium nitrogen (NH₄⁺-N) content in soil is a key factor affecting nitrous oxide (N₂O) emissions due to its role as a primary substrate of nitrification. This study aimed at investigating the effects of different application rates of NH₄⁺-N on N₂O and CO₂ emissions from two different types of manure compost-amended soil, along with analysis of relative abundances of narG and nosZ genes under aerobic conditions. Laboratory experiments were conducted using Kochi and Ushimado soils amended with mixed compost (MC: mixture of cattle, poultry, and swine manure) or cattle manure compost (CC) at 3% (dry weight basis). In no compost- and compost-amended soils, (NH₄)₂SO₄ was added as a solution equivalent to 160, 200, and 400 mg-N kg⁻¹ of soil. Soil samples were aerobically incubated at 70% water-holding capacity (WHC) and 25°C. Emissions of N₂O and CO₂ were measured on days 0, 3, 7, 15, 21, 28, and 42. The abundances of narG and nosZ genes in Kochi (day 7) and Ushimado (day 21) soils were estimated using qPCR tests. Emissions of N₂O and CO₂ were higher in MC-amended soil because of higher mineral N content and lower C/N ratio of MC than those of CC, regardless of NH₄⁺-N rates. Emissions of N₂O and CO₂ were higher in compost-amended Kochi soil due to higher mineral N, total N and C, and clay contents, and possibly because of higher water-filled pore spaces than those in Ushimado soil at the same WHC. In both soils with CC and no compost, raising NH₄⁺-N rate from 160 to 200 increased N₂O emissions due to stimulation of nitrification. In contrast, increasing NH₄⁺-N rate from 200 to 400 decreased N₂O and CO₂ emissions except for N₂O emissions in MC- and CO₂ emissions in CC- and no compost-amended Ushimado soil possibly due to osmotic stress on microorganisms and limited C availability. Emissions of N₂O were positively related to narG gene copy numbers in Kochi soil (R ² =0.78) due to high N and C contents. Our study revealed that NH₄⁺-N rate 400 suppresses N₂O and CO₂ emissions from manure compost-amended soil under aerobic conditions.