Effects of urease and nitrification inhibitors on nitrous oxide emissions and nitrifying/denitrifying microbial communities in a rainfed maize soil: A 6-year field observation

Application of the inhibitor-amended nitrogen fertilizers is a recommended method for reducing agricultural nitrous oxide (N2O) emission. However, the potential impacts of these inhibitors on soil environments still needs to be evaluated using long-term experiments. Through a 6-year field observation, the effects of combined application of a nitrification inhibitor (dicyandiamide, DCD) and a urease inhibitor (hydroquinone, HQ) on N2O emission, as well as soil ammonia oxidizers and denitrifiers in a maize (Zea mays L.) field in Northeast China were investigated. The results showed that annual soil N2O emissions were 0.466, 1.021 and 0.874 kg N2O-N ha(-1) for N0 fertilizer treatment (CK), Urea treatment (U) and Urea + DCD + HQ treatment (UDH), respectively. A significant linear correlation was found between the N2O accumulation in the first month after fertilizer application and the short-term precipitation (i.e., a period from 10 days before to 20 days after fertilizer application). The N2O emissions in the freeze-thaw period accounted for up to 42.5% of the year-round N2O emissions. The remarkable fluctuations of annual N2O emissions were observed (their coefficients of variation were 68.3%, 77.7% and 71.2% for CK, U and UDH treatments, respectively); these fluctuations were mainly attributed to the precipitation. The averaged N2O emission factors (EF) (0.308% and 0.227% for U and UDH treatments, respectively) were far less than the default mean EF of 1% proposed by (IPCC, 2006). An averaged N2O mitigation of 26.4% was fulfilled by UDH application. The results of quantitative PCR for soil nitrification and denitrification gene copy numbers measurement showed that UDH treatment significantly decreased the ammonia oxidation bacteria (AOB) amoA gene copy numbers by 74% on the 10th day after UDH application. No significant effects of combined application of DCD and HQ on microbial denitrification functional gene abundance were observed.