Broadcast applications of urea during the overwinterperiod is a common management practice for drylandwinter wheat production in semi-arid Montana. Thepopularity of this practice is based on the need to spread workloads.Farms in this region commonly have > 1,000 ha underno-till or minimal tillage systems and this places considerablepressure on farmers to complete planting operations over ashort-time span (e.g., 20% of the applied amount) can occurfollowing urea application to cold soils with temperatures< 5°C, including soils covered with a modest snowpack. Themost important fi nding of this research is that the worst-caseconditions for NH3 loss were after urea applications were madeto moist soil surfaces with high moisture content, followed byslow drying with little or no precipitation. In Montana andother neighboring areas of the Northern Great Plains, theseconditions are more likely to occur during the overwinter periodcompared with the spring. Soil surfaces are usually frozen duringthe winter, but still can be moist due to intermittent periodsof shallow surface thawing. The distribution of precipitationis such that winters are typically dry with only 14 to 15% ofthe total annual precipitation (310 to 420 mm) occurring overa 4-month period (Dec 1 to Mar 31; MCO-UM, 2015). As aresult, precipitation events will typically be low in volume, andbroadcast urea applied to the soils is likely to remain near thesurface where it is susceptible to volatilization. Conversely, inthe spring following thaw, temperatures are warmer and dryingof the soil surface is more rapid. Spring precipitation eventsthat occur tend to be larger in volume and surface-applied ureawill more likely be dissolved and infi ltrate further into the soilwhere it is less susceptible to volatile losses.
展开▼
