Effect of urea placement on the time-depth profiles of NO, N2O and mineral nitrogen concentrations in an Andisol during a Chinese cabbage growing season

Abstract

To evaluate the effects of urea placement on the concentration profiles of soil gases (NO and N2O) and mineral nitrogen, we conducted an experiment in a Chinese cabbage field in Tsukuba, Japan, 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 and 0.6 m) were measured 1–2 times per week in experimental plots fertilized by either urea incorporation (U-I; uniformly spread over the soil surface and incorporated down to approximately 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 of soil in the U-I treatment during the first 2 weeks after fertilization (WAF), whereas in soil in the U-DB treatment NO was observed only at depths of 0.1 and 0.15 m for approximately 6 WAF. In U-I, the maximum soil N2O concentration was observed 5 days after fertilization (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). The NH4 + concentrations in U-I exceeded background levels during the first 2 WAF, with a maximum 5 DAF. The high NH4 + concentrations in soil in the U-DB treatment, mainly located within the 0.05–0.15-m soil zone, lasted approximately 6 weeks, with a maximum at 9 DAF. The NO3 − concentrations in both U-I and U-DB increased shortly after the application of urea; the increases were higher in U-I than in U-DB. These time-depth series of concentration profiles clearly demonstrate the effect of urea placement on NO and N2O production and transportation in soil and provide a better understanding of the emission dynamics of these gases than previous reports. In addition, comparison of NO and N2O emissions among field, laboratory and numerical experiments firmly suggests that deep urea placement is highly effective in reducing NO emissions, with less effect on N2O emissions from Andisols.