Short-term response of soil N-cycling genes and transcripts to fertilization with nitrification and urease inhibitors, and relationship with field-scale N2O emissions

In agroecosystems, efficient fertilizer use is key to optimizing productivity and reducing nutrient losses that can be detrimental for the environment, such as nitrous oxide (N₂O) emissions. Because microbial communities regulate nitrogen (N) fate in soils, some agrochemicals inhibit specific transformations to reduce N losses. Our study aimed to describe short-term dynamics of N-cycling genes and transcripts and N₂O emissions after fertilization with urea-ammonium nitrate (UAN) with or without the addition of nitrification plus urease inhibitors (NUI). The experiment consisted of 4-ha corn plots located in SE Ontario, Canada, where field-scale N₂O emissions were monitored continuously using micro-meteorological techniques. Soil samples (0–10 cm) were taken 10 days before (baseline) and 2, 6, 9, 13 and 16 days after fertilization, and immediately flash-frozen. We co-extracted DNA and RNA and, using real-time PCR, quantified genes/transcripts targeting total bacteria (16S rRNA) and key N-cycling groups: ureolytic (ureC), ammonia-oxidizers (bacterial/archaeal amoA), nitrite-reducers (nirK/nirS) and N₂O-reducers (clade I/II nosZ). The addition of NUI did not prevent an N₂O flux event but reduced its duration and magnitude by more than 50%, and net cumulative N₂O emissions for the sampling period by ~68%. NUI effects on N-cycling microorganisms were evident on day 9, as a transient reduction (40–56%) of ammonia-oxidizers and denitrifiers. Changes in transcripts were minor and only detectable on ureC (day 2), nirS and clade II nosZ (day 9). NUI did not interfere with temporal fluctuations in nirK and nirS, but it differentially affected nosZ response to a later rainfall event. Unexpectedly, N₂O emissions were negatively associated with the ratio between nitrite-reducers and N₂O-reducers. NUI effects on N-cycling microorganisms were minor and transient but resulted in a field-scale reduction in N₂O emissions, possibly due to a combination of environmental factors and legacy effects from previous years of treatment.