Hydrothermal Modeling of Seedling Emergence Timing across Topography and Soil Depth

The soil environment is an essential determinant of microsite conditions required to model weed seedling emergence timing. An experiment was established across topography within an annually cropped field in south-central Manitoba to determine the effect of hillslope position (summit, backslope, toeslope), soil residue (native, added) and soil depth (three 25-mm layers) on the microsite environment and the emergence timing of spring wheat as a surrogate weed. Soil temperature decreased with soil depth whereas soil temperature fluctuation decreased with soil depth and lower hillslope position. Soil water potential was lowest at the summit hillslope position and the upper soil layer. Soil temperature and water potential were combined into hydrothermal time using water potential minimum thresholds (-2.1, -1.3, -0.7, and -0.1 MPa). Thermal accumulation was greatest at the soil surface, whereas hydrothermal accumulation using a water potential minimum threshold of -0.7 or -0.1 MPa was greatest in the 25- to 50-mm and 50- to 75-mm soil depths. Seedling emergence occurred earliest from the 0- to 25- and 25- to 50-mm soil depths and latest from the soil surface. No differences occurred in seedling emergence timing across levels of hillslope or soil residue. This study identifies recruitment depth as an important microsite variable that influences seedling emergence timing. Weed seedling recruitment models should be based on hydrothermal time and depth of recruitment to reflect the spatial and temporal dynamics of the recruitment zone environment.