Imprint of management on microbial communities in arable soil

Arable soils are a major source of the greenhouse gas nitrous oxide (N2O), emissions of which are directly linked to increased use of N fertilizers. Microbial communities that drive N cycle processes in soil ultimately determine the fate of reactive N inputs and control N2O emissions. Under anoxic conditions, two processes compete for nitrate (NO3-): denitrification, the stepwise reduction of NO3- to N2O or atmospheric nitrogen, and dissimilatory nitrate reduction to ammonium (DNRA), in which NO3- is reduced to ammonium (NH4+). The reduction of N2O by denitrifiers and non-denitrifying N2O reducers is the only known biological sink for N2O. \nThe aim of this thesis was to investigate how edaphic factors that are altered by soil management practices influence the diversity, structure and activity of the soil microbiota regulating anaerobic N cycling, and thereby N2O emissions. We hypothesised that N replete conditions and long-term fertilization promote incomplete denitrifiers, whereas high C content increases the abundance of DNRA bacteria and N2O reducing organisms, enhancing soil N2O sink capacity. Inoculation of soil microcosms with a non-denitrifying N2O reducing strain confirmed that increased abundance of these organisms can mitigate soil N2O emissions. A survey of long-term fertilization trials showed a consistent increase in the relative abundance of taxonomic groups previously inferred from genomic evidence to produce or consume N2O in fertilized soil. Nevertheless, the abundance of organisms comprising a truncated denitrification remained dominant, concomitant with increased potential N2O emissions. Another field study including fertilization and different crop rotations suggested that changes in soil C/N ratio due to cropping system influenced the competition between DNRA and denitrification, with higher C/N promoting DNRA and N2O reducing community abundance and activity. This was confirmed in controlled manipulations of C/N in a microcosm study, suggesting that soils covering higher C/N ratios sustain a greater abundance of DNRA and N2O reducing bacteria and therefore have a lower N2O emission potential.