Ammonia volatilization loss from surface-broadcast urea: comparison of vented- and closed-chamber methods and loss in winter wheat-summer maize rotation in North China Plain

Ammonia (NH3) volatilization is an important pathway for fertilizer nitrogen (N) loss from soil and is also a major source of air and environmental pollution. On calcareous soils in North China Plain, application of N fertilizer in the form of urea under intensive cropping with winter wheat (Triticum aestvum L.) and summer maize (Zea mays L.) rotation can lead to serious NH3 loss. The objective of this study was to compare a modified vented-chamber method with the traditional closed-chamber method to measure NH3 volatilization loss under laboratory and field conditions and to determine in situ NH3 volatilization in the field from surface broadcast urea at 0, 120, 240, and 360 kg N ha(-1) rates to each crop (for winter wheat, one-half at sowing and the other half at the elongation growth stage; for summer maize, one-half at the 3-leaf and the other half at the 10-leaf growth stage) in a winter wheat and summer maize rotation at the northern edge of North China Plain from October 1998 to September 1999. Urea was surface applied after irrigation before sowing, or prior to irrigation during the growing season. Compared to the closed chamber method, the vented chamber method was found to be simpler in structure, easier to operate, more suitable for in situ determination of NH3 volatilization in the field, and had higher recovery of emitted NH3 (99.5 vs. 70.8%). For surface broadcast urea after first irrigation prior to sowing of winter wheat, the NH3 volatilization rate reached, a maximum on the second to fifth day after application. Total NH3 loss from soil during the October 8 to November 18, 1998, period was 2.9, 4.8, 10.5, and 35.7 kg N ha(-1) at the 0, 60, 120, and 180 kg N ha(-1) rates, respectively. When urea was top-dressed prior to irrigation at the elongation growth stage of the winter wheat in midspring, NH3 loss was relatively low (i.e., 1.5, 2.1, 2.4, and 2.7 kg N ha(-1) at 0, 60, 120, and 180 kg N ha(-1) rates, respectively). The NH3 loss from urea for the entire winter wheat-growing season accounted for 2.1, 3.6, and 9.5% of the applied N at 120, 240, and 360 kg N ha(-1) rates, respectively. For summer maize where urea was top-dressed at the 3-leaf and 10-leaf growth stages, the NH3 volatilization rate increased more quickly and maximized on the first or second day after N application. During the summer maize-growing season, total NH3 loss from urea accounted for 5.6, 4.8, and 4.9% of the applied N at 120, 240, and 360 kg N ha(-1) rates, respectively. Based on the results of this study, total NH3 loss was estimated to range from 163,000 to 477, 200 Mg of N from soil in a winter wheat-summer maize rotation in North China Plain. Of these, about 68,400 to 382,600 Mg of N loss were from the N fertilizer surface applied as urea.