Effect of antecedent soil moisture conditions on emissions and isotopologue distribution of N2O during denitrification

The present study determined the influence of initial moisture conditions on the production and consumption of nitrous oxide (N₂O) during denitrification and on the isotopic fingerprint of soil-emitted N₂O. Sieved arable soil was pre-incubated at two different moisture contents: pre-wet at 75% and pre-dry at 20% water-filled pore space. After wetting to 90% water-filled pore space the soils were amended with glucose (400 kg C ha⁻¹) and KNO₃ (80 kg N ha⁻¹) and incubated for 10 days under a He/O₂-atmosphere. Antecedent moisture conditions affected denitrification. N₂ + N₂O fluxes and the N₂O-to-N₂ ratio were higher in soils which were pre-incubated under dry conditions, probably because mobilization of organic C during the pre-treatment enhanced denitrification. Gaseous N fluxes showed similar time patterns of production and reduction of N₂O in both treatments, where N₂O fluxes were initially increasing and maximised 3-4 days after fertilizer application, and N₂ fluxes were delayed by 1-2 days. Time courses of δ¹⁵N^bulk-N₂O and δ¹⁸O-N₂O exhibited in both treatments increasing trends until maximum N₂ fluxes occurred, reflecting isotope fractionation during intense NO₃⁻ reduction. Later this trend slowed down in the pre-dry treatment, while δ¹⁸O-N₂O was constant and δ¹⁵N^bulk-N₂O decreased in the pre-wet treatment. We explain these time patterns by non-homogenous distribution of NO₃⁻ and denitrification activity, resulting from application of NO₃⁻ and glucose to the surface of the soil. We assume that several process zones were thus created, which affected differently the isotopic signature of N₂O and the N₂O and N₂ fluxes during the different stages of the process. We modelled the δ¹⁵N^bulk-N₂O using process rates and associated fractionation factors for the pre-treated soils, which confirmed our hypothesis. The site preference (SP) initially decreased while N₂O reduction was absent, which we could not explain by the N-flux pattern. During the subsequent increase in N₂ flux, SP and δ¹⁸O-N₂O increased concurrently, confirming that this isotope pattern is indicative for N₂O reduction to N₂. The possible effect of the antecedent moisture conditions of the soil on N₂O emissions was shown to be important.