Solute response to changing nutrient loads in soil and walled ceramic cup samplers under continuous extraction.

This report evaluates a vacuum-assisted walled percolation sampler preconditioned in soil, and examines the dynamic response of leachate solutes. The 20-cm walled percolation sampler extracted soil water under continuous tension via a ceramic cup collector embedded in a silica flour layer, whose upper surface interfaced with field soil. In the laboratory, alternating solutions with high and low NO3-N (232 or 3.6 mg L(-1)), molybdate-reactive P (MRP) (1.75 or 0.0 mg L(-1)), K+ (568 or 3.6 mg L(-1)), and Br- (9.6 or 0.0 mg L(-1)) concentrations were delivered directly to the (i) sampler ceramic cup; (ii) silica flour bed surface, or (iii) 12-mm soil layer placed over the silica flour bed. For alternating input solutions delivered to the silica-flour bed surface, (i) solute breakthrough (95% equivalency) occurred in 4 pore volumes and was the same for both the high and low concentration input phases of the application, and (ii) concentrations of NO3-N, Br-, and MRP in cumulative extracted water volumes were within 5% of those in corresponding input volumes. Alternating nutrient loads from high to low levels in the fixed flow rate input waters caused excess MRP (1.6 times that in the high concentration MRP solution) to leach from the calcareous soil. The dynamic character of P transport in K-fertilized soils deserves further study and may have important environmental implications.