Irrigation rather than fertilization drives the abundance, community structure and assembly process of soil denitrifiers

Fertilization and irrigation have significant impacts on soil nitrogen cycling and associated microbes, yet we have limited understanding on community structure and assembly process of soil denitrifiers. Here, we examined the abundance, diversity, composition, and assembly process of soil denitrifier communities harboring the nirS, nirK, and nosZ genes in soils under long-term (11 years) fertilization and irrigation managements. Our results indicated that irrigation significantly increased the abundance of nirS, nirK, and nosZ genes by 6.10–18.85, 2.36–2.85, and 4.85–6.16 times in the chemical fertilizer (NPK) treatments and by 1.02–13.68, 0.95–1.45, and 1.49–2.28 times in the manure (M) treatments compared to the corresponding control without irrigation (I0), respectively. Principal co-ordinates analysis (PCoA) and permutational multivariate analysis of variance (PERMANOVA) showed that fertilization had a more important role in altering the nosZ-type denitrifier community structure, while irrigation dominated in influencing the nirS- and nirK-type. Regardless of fertilization, irrigation shifted community assembly of the nirS-type denitrifier from deterministic process to stochastic process, while driving the nosZ-type from stochastic process to deterministic process. The nirK-type denitrifier was determined by stochastic process and was neither affected by fertilization nor irrigation. Network co-occurrence analysis showed that irrigation differently affected the community network complexity under NPK and M. One irrigation (I1) and two irrigations (I2) increased and decreased the network complexity, respectively, in the NPK treatments, while opposite results were found in M. Additionally, soil moisture and pH significantly (p < 0.05) correlated with the gene abundance, community structure, and assembly process of the denitrifiers. Overall, irrigation rather than fertilization greatly induced the changes in abundance, community structure, network structure, and assembly process of the denitrifiers via soil moisture and pH, highlighting the importance of irrigation in regulating soil denitrification associated microbes.