Enriched 15 N-N 2 O Tracing Reveals the Importance of Soil Moisture and Bulk Density for N 2 O Reduction.

During denitrification or nitrifier-denitrification the greenhouse gas nitrous oxide (N₂O) is an obligatory intermediate during the reduction of nitrate or nitrite to dinitrogen gas (N₂). Topsoil fluxes of N₂O can occur as a result of incomplete denitrification, and are strongly influenced by soil physical properties. However, quantitative insights into the relationship between N₂O reduction and soil physical properties influencing this are still limited.

Here we used artificially enriched ¹⁵N-N₂O to measure N₂O reduction in the soil profile as function of both soil bulk density and moisture content. Columns were repacked with a sieved sandy soil, using three moisture contents (60, 75 and 90% water-filled pore space; WFPS), to one of three bulk densities (1.02, 0.96 and 0.89 g cm^-3). At the bottom of the columns, a silicon sheet, (permeable to N₂O) was installed. The reservoir below the sheet simulated a subsoil from which N₂O diffused upwards. After a pre-incubation phase of 9 days, ¹⁵N-N₂O was injected into the reservoir every 12 hours and topsoil N₂O emissions were subsequently measured for 20 days.

Preliminary results indicate that 90% WFPS resulted in low and 60% WFPS in high N₂O emission. However, the higher N₂O flux at 60% WFPS was the result of an increased diffusion rate rather than enhanced production of N₂O. At both high and low soil moisture contents, bulk density did not affect N₂O emissions. However, at 75% WFPS, higher bulk density decreased N₂O emissions by decreasing the diffusion rate and thereby increasing complete denitrification to N₂. Our results show that N₂O profile dynamics (diffusion characteristics and reduction of nitrous oxide) should be taken into account when linking soil characteristics to N₂O emissions. A wet soil can either lead to higher or lower emissions when compared to a drier soil, depending on the situation in the subsoil.