Seasonal variation of nitrous oxide flux and aeration/water stress at different water table levels

Patterns of nitrous oxide (N₂O) emissions related to water-filled pore space and gas diffusivity have been described as evidence of denitrification activity. However, the complexity of factor combinations in managed agroecosystems calls for comprehensive studies relating to N₂O emissions and water content dynamics, both spatially and temporally. This study examined the impact of two water table levels and free drainage (60 cm, 120 cm and free drainage, WT60, WT120 and FD) in combination with land management (conservation and conventional agriculture) on the seasonal variation of surface N₂O flux and aeration/water stress on the soil profile. N₂O emissions and volumetric water content dynamics were measured on a lysimeter experiment over three years (2018-2020). Preliminary results show that N₂O emissions were driven by fertilization over time irrespective of water table level and land management. In the topsoil, WT60 and WT120 promoted longer periods of aeration stress under conservation agriculture compared to conventional, whereas FD increased water stress days under conventional agriculture. At 30 and 60 cm depth, water content under FD remained mostly within the range of nonlimiting for plant growth, and under WT60 and WT120 was generally above the aeration limit in-season and over time. Correspondently, calculated relative gas diffusivity was limiting for conservation agriculture in the topsoil compared to conventional agriculture, and below the anoxia threshold for both land management at 30 and 60 cm depth. This suggests that the subsoil could become a potential hotspot for N₂O production under shallow water levels. Cumulative surface N₂O appears to be related to the cumulative number of aeration stress days derived from the nonlimiting water range, with variation across years and land management.