The effect of preferential flow of dairy cow urine and simulated rainfall on movement of potassium through undisturbed topsoil cores

Abstract

Cores of undisturbed topsoil (15 cm in depth and diameter) were taken from a range of soil types to a controlled climate chamber. Urine from dairy cows was spiked with tritiated water (3H2O) and then applied to these cores which had moisture contents near field capacity. Liquid draining from these cores, as a direct consequence of urine application, contained up to 72% of the tritiated water, 74% of the potassium (K), 62% of the nitrogen (N) and 80% of the chloride (Cl) applied in the urine, indicating that urine could flow preferentially beyond the 15 cm depth in these soils. The activity of tritium and the concentrations of K, N and Cl in the effluent indicated that this preferential movement of urine occurred too quickly for sorption reactions to occur between the soil surface and the majority of solutes in the urine. After preferential flow had ceased, the amounts of K leached by subsequent simulated rain events were much smaller than losses immediately following the urine application. Leaching losses were particularly small (accounting for 3-15% of the applied urine K) when the majority of the rain water moved preferentially through the soil cores, thus bypassing the urine K which was in soil micropores or which had been sorbed by the soil. Overall, these results suggest that substantial movement of K (and N) through the topsoil of grazed pastures may occur following a urination event due to preferential flow of dairy cow urine through the soil profile, at least when soils are near field capacity at the time of urine deposition. The extent of this movement through topsoil will be more dependent upon soil structural characteristics rather than soil chemical characteristics.