Increases of N2O emissions due to enhanced nitrification in a sandy loam soil under long-term manure application

¹⁵N tracing was carried out on sandy loam soil amended with (i) mineral nitrogen-phosphorus-potassium fertilizer (NPK) alone, (ii) half mineral N and half N from chicken manure (HFC), or (iii) half mineral N and half N from cattle manure (HCM), for 8 years. Cumulative N2O emissions during incubation were 30.2 µg N kg^{− 1} in the NPK treatment, which increased to 37.8 and 51.3 µg N kg^{− 1} in the HFC and HCM treatments, respectively. The majority of N₂O emissions in all the treatments were attributed to nitrification (81.0% in the NPK treatment, 83.0% in the HFC treatment, and 85.1% in the HCM treatment). Compared with NPK, HCM treatment caused a significant increase in the gross rate of nitrification, while HFC treatment slightly enhanced the rate of dissimilatory NO₃⁻ reduction to NH₄⁺. Additionally, HFC treatment achieved higher gross rates of organic N mineralization, and both HFC and HCM treatments had higher NH₄⁺ mineralization-immobilization turnover (MI_AT) rates than NPK treatment. The results suggest that application of cattle or chicken manure increased soil NH₄⁺ availability. The gross rate of NO₃⁻ adsorption in the HCM treatment was greater than that in the NPK treatment, while the release of adsorbed NO₃⁻ in the HFC treatment was slower than that in the NPK treatment, indicating that application of cattle or chicken manure lowered the potential for NO₃⁻ leaching in soil. Overall, combining cattle or chicken manure with mineral fertilizer decreased NO₃⁻ availability but increased NH₄⁺ availability, leading to higher N₂O emissions through nitrification. Our results suggest that organic manures should be applied with nitrification inhibitors in sandy loam soil containing low organic carbon to increase soil fertility and mitigate N₂O emissions.