Long-term fertilization regimes drive the abundance and composition of N-cycling-related prokaryotic groups via soil particle-size differentiation

Even though organic fertilization is known to promote soil nitrogen cycling (N-cycling), it is unclear how this effect is mediated. Here we studied the potential roles of soil physiochemical properties and soil particle-size fraction (PSF) in relation to the diversity and abundance of N-cycling-related prokaryotes by sampling field sites that had been subjected to either mineral or organic fertilization for over 30 years (two independent fields subjected to both fertilization treatments). We found that organic fertilization increased the 63-200 μm sized soil fraction and the abundance of various N-cycling-related prokaryotes including diazotrophs, ammonia-oxidizers and denitrifiers. Moreover, increase in the abundance of different N-cycling-related prokaryotes varied between different size fractions: the abundance of ammonia-oxidizers and nitrate-reducers was the highest in the 63-200 μm size fraction, while the abundance of diazotrophs, ammonifiers and denitrifiers was the highest in the 200-2000 μm, 2-63 μm and 0.1-2 μm size fractions, respectively. According to structural equation modelling (SEM), soil PSF, physiochemical properties and the community structure of N-cycling-related prokaryotes within different PSFs were associated with the abundance of N-cycling-related prokaryotes in non-fractionated bulk soils. In conclusion, these results suggest that organic, but not mineral, fertilization can support the abundance of N-cycling-related prokaryotes and that the strength of the effects depends on their association with specific particle-size fractions.