Effect of nitrogen fertilisers on nitrous oxide emission, nitrifier and denitrifier abundance and bacterial diversity in closed ecological systems

In a 3-year microcosm study the nitrous oxide (N2O) emission from an agricultural soil amended with urea, ammonium sulphate or potassium nitrate were examined. Soil samples were taken every year to determine the soil physicochemical properties, the total abundance of bacteria, archaea, nitrifier and denitrifier communities and the changes in the structure and composition of the bacterial community. Gene abundance and biodiversity were estimated using quantitative PCR and pyrosequencing, respectively. The soils were watered weekly to reach 80% water filled pore space and varied from 80 to 62% during that time. N-fertilisation increased the abundance of bacteria and decreased that of archaea. Soils treated with ammonium or urea emitted more N2O than that amended with potassium nitrate and produced yearly increases in the abundance of nitrification genes. Also, the abundance of the denitrification genes gradually increased during the experimental period. N2O emission decreased on a yearly basis and so did the ratio of the genes involved in N2O production and reduction. A non-metric multidimensional scaling plot showed that N2O emission was positively related with the norB gene and negatively with the nosZ. The Shannon and Simpson diversity indices indicated that N-fertilisation reduced the number of operational taxonomic units (OTUs) and that the bacterial community became dominated by a small group of OTUs, respectively. N-fertilisation reduced the number of OTUs whose relative abundance was lower than 1%. We conclude that potassium nitrate reduces N2O emissions and increases the abundance of the N2O reducers. This work also shows that N-fertilisation decreases soil biodiversity and that its response depends on the type of the N-fertiliser.