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. Semi-terrestrial soils such as floodplain soils areconsidered potential hot spots of nitrous oxide ( N2O ) emissions. Microhabitatsin the soil – such as within and outside of aggregates, in the detritusphere, and/or inthe rhizosphere – are considered to promote and preserve specific redox conditions. Yetour understanding of the relative effects of such microhabitats and their interactions on N2O production and consumption in soils is still incomplete. Therefore, weassessed the effect of aggregate size, buried leaf litter, and plant–soil interactionson the occurrence of enhanced N2O emissions under simulated flooding/dryingconditions in a mesocosm experiment. We used two model soils with equivalent structureand texture, comprising macroaggregates (4000–250  µ m) or microaggregates( µ m) from a N-rich floodplain soil. These model soils were planted withbasket willow (Salix viminalis L.), mixed with leaf litter or left unamended.After 48 h of flooding, a period of enhanced N2O emissions occurred in alltreatments. The unamended model soils with macroaggregates emitted significantly more N2O during this period than those with microaggregates. Litter additionmodulated the temporal pattern of the N2O emission, leading to short-term peaksof high N2O fluxes at the beginning of the period of enhanced N2O emission. The presence of S. viminalis strongly suppressed the N2O emission from the macroaggregate model soil, masking any aggregate-size effect.Integration of the flux data with data on soil bulk density, moisture, redox potentialand soil solution composition suggest that macroaggregates provided more favourableconditions for spatially coupled nitrification–denitrification, which are particularlyconducive to net N2O production. The local increase inorganic carbon in the detritusphere appears to first stimulate N2O emissions;but ultimately, respiration of the surplus organic matter shifts the system towards redoxconditions where N2O reduction to N2 dominates. Similarly, the lowemission rates in the planted soils can be best explained by root exudation oflow-molecular-weight organic substances supporting complete denitrification in the anoxiczones, but also by the inhibition of denitrification in the zone, where rhizosphereaeration takes place. Together, our experiments highlight the importance of microhabitatformation in regulating oxygen ( O2 ) content and the completeness ofdenitrification in soils during drying after saturation. Moreover, they will help tobetter predict the conditions under which hot spots, and “hotmoments”, of enhanced N2O emissions are most likely to occur in hydrologically dynamic soil systemslike floodplain soils.

Alteration of nitrous oxide emissions from floodplain soils by aggregate size, litter accumulation and plant–soil interactions