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

Semi-terrestrial soils such as floodplain soils are considered potential hot spots of nitrous oxide (N 2 O) emis- sions. Microhabitats in the soil - such as within and out- side of aggregates, in the detritusphere, and/or in the rhizo- sphere - are considered to promote and preserve specific re- dox conditions. Yet our understanding of the relative effects of such microhabitats and their interactions on N 2 O produc- tion and consumption in soils is still incomplete. Therefore, we assessed the effect of aggregate size, buried leaf litter, and plant-soil interactions on the occurrence of enhanced N 2 O emissions under simulated flooding/drying conditions in a mesocosm experiment. We used two model soils with equivalent structure and texture, comprising macroaggre- gates (4000-250 μm) or microaggregates ( < 250 μm) from a N-rich floodplain soil. These model soils were planted with basket willow ( Salix viminalis L.), mixed with leaf litter or left unamended. After 48 h of flooding, a period of enhanced N 2 O emissions occurred in all treatments. The unamended model soils with macroaggregates emitted significantly more N 2 O during this period than those with microaggregates. Lit- ter addition modulated the temporal pattern of the N 2 O emis- sion, leading to short-term peaks of high N 2 O fluxes at the beginning of the period of enhanced N 2 O emission. The pres- ence of S. viminalis strongly suppressed the N 2 O emission from the macroaggregate model soil, masking any aggregate- size effect. Integration of the flux data with data on soil bulk density, moisture, redox potential and soil solution composi- tion suggest that macroaggregates provided more favourable conditions for spatially coupled nitrification-denitrification, which are particularly conducive to net N 2 O production. The local increase in organic carbon in the detritusphere appears to first stimulate N 2 O emissions; but ultimately, respiration of the surplus organic matter shifts the system towards redox conditions where N 2 O reduction to N 2 dominates. Similarly, the low emission rates in the planted soils can be best ex- plained by root exudation of low-molecular-weight organic substances supporting complete denitrification in the anoxic zones, but also by the inhibition of denitrification in the zone, where rhizosphere aeration takes place. Together, our exper- iments highlight the importance of microhabitat formation in regulating oxygen (O 2 ) content and the completeness of den- itrification in soils during drying after saturation. Moreover, they will help to better predict the conditions under which hot spots, and "hot moments", of enhanced N 2 O emissions are most likely to occur in hydrologically dynamic soil sys- tems like floodplain soils.