Nitrous oxide and carbon dioxide emissions from two soils amended with different manure composts in aerobic incubation tests

Identification of nitrous oxide (N₂O) and carbon dioxide (CO₂) emissions from soils amended with different types of compost is needed for appropriate use of manure in agriculture. This study aimed at investigating the interaction effects of compost type and soil properties and effects of moisture contents on N₂O and CO₂ emissions, with identification of relative abundances of functional ammonia-oxidizing genes. Laboratory tests were conducted using cattle manure compost (CC) or mixed compost (MC) (cattle, poultry, and swine manure) amended Kochi (from a greenhouse) or Ushimado (from a paddy field) soils (3% by weight) with controls (no compost). Initial moisture contents were adjusted to 60% water-holding capacity (WHC) for Kochi soil and 70% WHC for both soils. The samples were aerobically incubated at 25°C. Emissions of N₂O and CO₂ and contents of ammonium N (NH₄⁺-N) and nitrate N in soils were measured continuously until day 42. The abundances of ammonia-oxidizing bacteria (AOB) and archaea genes were estimated to evaluate nitrifying activities. Cumulative N₂O and CO₂ emissions were significantly higher (p < 0.05) in MC than those in CC treatments probably due to higher NH₄⁺-N content and lower C/N ratio, which facilitated faster N mineralization and C decomposition. Emissions of N₂O and CO₂ were higher in compost-amended Kochi soil (70% WHC) with high total C and N, mineral N, and clay contents than those in less fertile Ushimado soil. Interestingly, interactions of compost type and soil properties on N₂O emissions were significant (p < 0.05) only in Kochi soil because the addition of decomposition resistant CC increased N₂O emissions only from this soil with high C and N contents. Higher soil moisture contents increased N₂O and CO₂ emissions significantly (p < 0.05) in Kochi soil. Emissions of N₂O until day 15 were mainly due to activities of AOB amoA genes (R² = 0.91). This study suggests that N₂O emissions are increased by high NH₄⁺-N contents and a low C/N ratio in compost and high total C and N, mineral N, and clay contents in soil. The application of compost with less decomposable C increases N₂O emissions only from nutrient-rich soil.