N_2O, NH_3 AND NO_x EMISSIONS AS A FUNCTION OF UREA GRANULE SIZE AND SOIL TYPE UNDER AEROBIC CONDITIONS

摘要

We examined the influence of various urea granule sizes ( < 2,7.0,9.9 and 12.7 mm) applied into a silt loam soil (experiment 1) and soil types (sandy, silt and clay loam) treated with the largest granule (experiment 2) on gaseous N loss (except N_2) at field capacity. The prilled urea (PU) was mixed into the soil whereas the urea granules were point-placed at a 5.0-cm depth. For experiment 1, N_2O emission was enhanced with increasing granule size, ranging from 0.17-0.50% of the added N during the 45-day incubation period. In the case of experiment 2, the sandy loam soil (0.59%) behaved similarly with the silt loam (0.53%) but both showed remarkably lower emissions than were found for the clay loam soil (2.61%). Both nitrification and N_2O emissions were delayed by several days with increasing granule size, and the latter was influenced by mineral N, soil water and pH. By contrast, the NH_3 volatilization decreased with increasing granule size, implying the inhibition of urease activity by urea concentration gradients. Considering both experimental results, the NH_3 loss was highest for the PU-treated (1.73%) and the larger granules regardless of soil type did not emit more than 0.27% of the added N over 22 days, possibly because the high concentrations of either mineral N or NH_4~+ in the soil surface layer (0-2.5 cm) and the high H~+ buffering capacity might regulate the NH_3 emission. Similar to the pattern of NH_3 loss, NO_x emission was noticeably higher for the PU-treated soil (0.97%) than for the larger granule sizes (0.09-0.29%), which were the highest for the sandy and clay loam soils. Positional differences in the concentration of mineral N and nitrification also influenced the NO_x emission. As such, total NH_3 loss was proportional to total NO_x emission, indicating similar influence of soil and environmental conditions on both. Pooled total N_2O, NH_3 and NO_x emission data suggest that the PU-treated soil could induce greater gaseous N loss over larger urea granules, largely in the form of NH_3 and NO_x emissions, whereas a similar increase with the largest granule size was mainly due to the total N_2O flux.