Dynamics of soil prokaryotes catalyzing nitrification and denitrification in response to different fertilizers in a greenhouse experiment with Cynodon dactylon

• Single applications of fertilizers induce plant growth but do not increase the abundance of nitrifiers and denitrifiers.
• In contrast a repeated application stimulates, in addition to plant growth, also the increase of the two functional groups.
• Organic fertilizers effects on soil N cycling bacterial abundances can become detectable upon a second season of application.
• Repeated fertilizers application drives faster ammonium mineralization which stimulates nitrifiers and denitrifiers growth.
• Bacterial amoA and nosZ genes abundance by realtime qPCR showed strong correlation with plant dry matter yield at harvest.

Organic fertilizers are of high interest in agriculture as they promise a retarded release of the nitrogen to soil, which improves uptake efficiency by plants and reduces negative impacts like the formation of nitrate by nitrifiers or N2O by denitrifying microorganisms. In this study we tested the effects of seven different commercially available organic fertilizers of plant-, animal-, or microbial origin in a two-month greenhouse trial, using the perennial grass Cynodon dactylon in pots and monitoring effects on plant growth as well as on the abundance of prokaryotic nitrifiers and denitrifiers by realtime PCR. In most cases a single application of the fertilizers induced plant growth but did not increase the abundance of nitrifiers and denitrifiers. In contrast a repeated application stimulated, in addition to plant growth, also the increase of the two functional groups studied and a faster mobilization of nitrogen from the different fertilizers. Upon analyzing total bacterial DNA extracted from soil nirK abundance was found responsive to plant presence. Bacterial amoA and nosZ gene copies were significantly positively correlated with plant growth and cumulative dry weight at harvest. The latter was responsive as early as 9 days after fertilizer supplementation.