Root channel macropores enhance downward movement of water in a Mojave Desert ecosystem

Macropores formed by the root systems of woody shrubs may be an important conduit for downward water movement in desert soils. An experiment with 32 1·0-m2plots, 16 with a previous Larrea tridentata (creosotebush) plant and 16 without, was designed to investigate the influence of dead root systems on the downward movement of water and chemical tracers in a Mojave Desert soil. The experiment also examined the influence of water application amounts, frequency of application, and initial soil profile moisture status on downward water flux. Half of the plots received a pre-application of 15 cm of water; 48 h later either 15 cm or 30 cm of water with dye and bromide (KBr) tracers (each applied at 1·0 gl−1) were applied to the plots in either a single pulse or in five equal volumes over a 5-day period. Each plot was then excavated in 10 cm depth increments 48 h after the final water and tracers were applied, and nine soil samples were taken at each depth using a 3×3 grid for gravimetric water and bromide contents. Gravimetric water and bromide concentration profiles with depth confirmed that water penetrated more deeply in the plots with a previous root system, and photos of the dye stained soil at each 10 cm depth confirmed that a significant percentage of the cross-sectional area was not contributing to flow at the same rate. Multiple regression analysis further indicated that the deepest depth of dye and bromide penetration, and the depth at which mean gravimetric water content was above 0·10, included the aboveground biomass of the previous Larrea shrub as a significant factor accounting for plot-to-plot variability. These results suggest that (1) shrub root systems act to increase downward water flux in desert soils via root channel preferential flow to deeper soil depths, and (2) this effect becomes more pronounced as shrubs and plant root systems become larger