Dynamics of inorganic nitrogen cycling, redox conditions, and microbial community composition in a floodplain soil induced by a heavy rainfall after summer drought

Abstract. Climate change increases the frequency of droughts and heavy rainfall events in temperate climates of Central Europe. This study examined the impacts of such extreme weather events on the downward transport of water and inorganic nitrogen species, the main processes governing inorganic nitrogen turnover and the dynamics of the soil microbial community in loamy floodplain soils. To this end, we conducted a field experiment simulating a heavy rainfall event applying isotopically (²H) enriched water on a plot of arable land with fine-textured soil after a prolonged dry period. Subsequently the experimental plots were irrigated with non-labelled water. 50 cm deep soil cores were collected from irrigated and dry control plots at different time points (days 0, 6/8 and 14) and analysed for soil nitrate and ammonium content, soil water content, ²H-enrichment of soil water, microbial community composition based on DNA and RNA, as well as extracellular enzyme activities. Furthermore, redox potentials were recorded in situ along the soil profile on dry and irrigated plots. Whereas the irrigation water of the simulated heavy rainfall event penetrated at least 50 cm deep into the soil along preferential flow paths after only two hours, ammonium and nitrate values remained constant at 50 cm depth despite a significant decrease of nitrate in the topsoil after irrigation. This suggests that some nitrate might have been transported rapidly below 50 cm depth in contrast to ammonium, which is rather immobile in soils due to strong sorption to clay minerals. Ammonium behaved very conservatively and only increased significantly in the topsoil (0–5 cm) in response to the wetting of the dry soil (“Birch effect”). At day 6 and day 14 of the experiment, the non-labelled irrigation water of the daily recurrent moderate irrigation doses resided in the topsoil layer (0–5 cm depth) and only a small fraction penetrated to the subsoil, because the shrinkage cracks present at the beginning of the experiment had closed. In parallel, the in situ redox potential dropped from > +400 mV to below -100 mV relative to the standard hydrogen electrode. Soil nitrate contents decreased when redox potentials were below +100 mV and vice versa. This implies that nitrate formation and transformation after the initial rewetting was governed by soil redox conditions instead of processes induced by the rewetting after drought. Microbial community composition analysis showed no significant differences between the active fraction of the community (RNA) on irrigated and dry plots. Diversity indices on the RNA-level were somewhat higher on dry than on wet plots, although not significantly, which suggests that drying and the following rewetting selected for more adapted taxa. In contrast, activities of the extracellular enzymes β-glucosidase and exochitinase showed significantly higher activities on wet plots than on dry plots, whereas the activity of acid phosphatase did not respond to the irrigation treatment. All enzymes decreased in activity with soil depth. Overall, the heavy rainfall event had only a minor effect on the turnover of inorganic nitrogen species. Nitrogen turnover was mainly governed by redox conditions. The presence of pronounced shrinkage cracks formed after a drought period before the heavy rainfall event, however, allows for rapid nitrate dissipation into the subsoil and eventually to the underlying aquifer. Our study also showed that the soil microbial community in temperate climates reacts in terms of activity and may ultimately be reshaped to represent more resilient taxa regarding drying and rewetting cycles.