Differential effect of afforestation on nitrogen-fixing and denitrifying communities and potential implications for nitrogen cycling

Denitrification and nitrogen (N) fixation are two major processes which drive N transformation in ecosystems. We investigated the effect of afforestation of pasture in New Zealand on denitrifying and N-fixing communities using molecular techniques. We optimised and applied multiplex-terminal restriction fragment length polymorphism for simultaneous analysis of denitrifying and N-fixing communities. Soil samples from four pine (Pinus radiata) forests of various age and adjacent pastures were analysed for microbial communities, denitrification rate and some environmental variables. Analysis of variance revealed that the denitrifying community was mainly influenced by site while the N-fixing community was influenced by both site and land-use types. Among environmental variables, moisture and porosity had the strongest impact on denitrifying communities while C:N ratio explained most of the variability in the N-fixing community. Denitrification rate as measured by nitrous oxide (N2O) flux, was significantly lower in older forest soils compared to adjacent pasture soils. Simple and step-wise regression analyses revealed that the N2O flux rate was strongly linked to the denitrifying community composition, providing evidence for the influence of community composition on denitrification. Moreover, we explored the relationship between denitrifying and N-fixing communities using various statistical approaches. Our results for the first time showed that denitrifying and N-fixing communities are related in situ, and suggest that changes in land-use may have a significant impact on N-transformations and ecosystem processes.