Nitrous oxide emissions and microbial communities during the transition to conservation agriculture using N-enhanced efficiency fertilisers in a semiarid climate

The transition year from tillage to no tillage in semiarid areas and its effects on nitrous oxide (N2O) emissions and related microbial communities, as well as the potential interaction with N management, including enhanced-efficiency fertilisers, are not well studied despite their economic and environmental implications. In tilled and nontilled plots, the effectiveness of the double DMPSA NBPT inhibitor (applied with urea at basal fertilisation) and that of DMPSA (applied with calcium ammonium nitrate at top-dressing) in the mitigation of N2O emissions were evaluated in a rainfed barley (Hordeum vulgare L.) crop in central Spain. Crop yield, nitrogen (N) uptake, the abundances of key genes involved in nitrification and denitrification processes and meteorological conditions and soil ancillary properties were monitored. In addition, the composition of bacterial communities was determined by sequencing the 16S rRNA gene. Fertilisers with inhibitors decreased cumulative N2O emissions and yield-scaled N2O emissions by 53% and 56%, respectively, with respect to those without inhibitors, which coincided with a trend of increasing grain and biomass yield and above-ground N uptake (by 11.3%, 9.2% and 7.2%, respectively). The highest N2O emissions were measured 49 days after harvest (immediately after a rainfall event that like reactivated soil microorganisms), reaching 15 mg N m(-2) d(-1) for the treatment with fertiliser without inhibitor combined with tillage. This peak was linked to a remarkable increase in the abundance of denitrifiers. The abundance of nitrifiers and denitrifiers successfully explained the N2O dynamics observed after basal fertilisation (i.e. an increase in the amoA/nosZ ratio in fertilised plots with inhibitors, where the highest emissions were observed). Our results also showed a reduction in the abundance of the phylum Nitrospirae throughout the cropping period in the plots that received inhibitors. No tillage led to a higher abundance of Cyanobacteria, Verrucomicrobia and Bacteroidetes and resulted in better implantation of the crop and higher plant density compared with tillage, thus increasing yields and N use efficiency and decreasing N2O emissions. Under the conditions of our study, shifting from conventional tillage to no tillage enhanced the balance between N use efficiency and yield-scaled N2O emissions in the first year of conversion, particularity with the use of the double inhibitor with urea at basal fertilisation and DMPSA with CAN at dressing.