Time-Depth Profiles of NO and N2O Concentrations In Andisols As Affected by Urea Placement: Comparison Among Field, Laboratory and Numerical Experiments.

To evaluate the effects of urea placement on the concentration profiles of soil gases (NO and N₂O) and mineral nitrogen, we conducted an experiment on a Chinese cabbage field in Tsukuba, Japan for over one cultivation season. Soil gas and mineral nitrogen concentrations at different depths (0.05, 0.1, 0.15, 0.2, 0.3, 0.45, 0.6 m) were measured 1–2 times per week in experimental plots fertilized either by urea incorporation (U-I; uniformly spread over the soil surface and incorporated down to about 0.15–0.2 m) or by urea deep band (U-DB; placed in 0.12-m-deep trenches cut at intervals of 0.6 m).  Considerable NO was observed in the top 0.2-m soils of U-I treatment during the first 2 weeks after fertilization (hereafter, 2 WAF), while in the soil of U-DB, that was observed only at 0.1 and 0.15-m depths for around 6 WAF. In U-I, the maximum soil N₂O concentration was observed 5 days after fertilization (hereafter, 5 DAF). Significantly high N₂O concentration in soil of U-DB was observed 2 WAF, and lasted longer than that in UI (4 vs. 2 weeks). NH₄⁺ concentrations in U-I exceeded background levels during the first 2 WAF, with a maximum 5 DAF. The high NH₄⁺ concentrations in soil of U-DB mainly located within 0.05–0.15-m soil zone, and lasted around 6 weeks, with a maximum on 9 DAF. NO₃^- concentrations in both U-I and U-DB increased shortly after the application of urea, but more in U-I than in U-DB. These time-depth series of concentration profiles clearly demonstrated the effect of urea placement on NO and N₂O production and transportation in soil, providing a better understanding of the emission dynamics of these gases reported previously. In addition, the comparison of NO and N₂O emissions among field, laboratory, and numerical experiments firmly suggests that deep urea placement is highly effective for reducing NO emissions, with less effect on N₂O emissions from Andisols.