Preferential Flow and Colloids: Their Influence on the Transport of Phosphorus

Although strongly adsorbing ions are relatively immobile within soil, ion transport can be enhanced by preferential flow and by adsorption of ions to mobile colloids. The primary objective of this research was to determine the influence of preferential flow paths on the transportation of phosphorus (P) through a soil profile. Secondary objectives were to determine the effect of fertilizer type (inorganic vs. organic) and colloids on the transport of P.

Eight soil monoliths with a diameter of 0.3 m and a length of 0.75 m were collected from the Water Quality field site at the Ames Plantation. Four monoliths were treated with poultry litter, and four were treated with diammonium phosphate (DAP) and urea. Fertilizer application rate was based on the recommended nitrogen rate for corn in west Tennessee at 168 kg ha^-1 N. Artificial rainfall was applied to the tops of the monoliths in two Phases:

(i) Phase I- 5 mm d^-1, with a bromide tracer and (ii) Phase II- 20 mm every fourth day, with a chloride tracer. Leachate was collected at the base of the monolith and concentrations of total P (P_tot), inorganic P (P_ino), Cl^‾ and Br^‾ were measured. The relative amount of P transported preferentially versus through the soil matrix was compared by constructing breakthrough curves (BTC) of the effluent from the base of the monoliths. At the base of the monoliths, a brass barb collected the water that drained from the monoliths under zero tension. The water from the soil matrix was sampled by four fiberglass wicks inserted into the soil at the base of each monolith. Sub-samples of the effluent were analyzed after being filtered with a 0.45- and/or 0.1-μm nylon filters to evaluate the influence of P transport by colloids.

Two monoliths had an early Br‾ detection and elevated P concentrations following the application of fertilizer during Phase I. The total, unfiltered P (P_tot,uf)concentrations from the zero tension samples was significantly different (Ptot,uf concentrations from the zero tension samples and wick samples were significantly different (Ptot,fil) signifying that colloids assist in the transport of P. The mass of P_ino and P_org transported by the monoliths treated with poultry litter did not differ significantly (P>0.05) from the monoliths treated with DAP and urea, suggesting that fertilizer type did not influence P transport in this study.

Phase II resulted in a much higher concentration of P in the leachate. The only significant difference between P_tot,uf concentrations from the zero tension samples and wick samples was detected in monoliths 2 and 3 (P0.05). Also, Fertilizer type did not significantly influence the transport of P through the monoliths (P>0.05).

Preferential flow paths influenced the transport of P through the soil monoliths. Higher P concentrations were noted under high intensity rainfall following fertilizer application. Colloids contributed 30% of P transported during low flow, and were insignificant during high flow. Fertilizer type (inorganic/organic) did not affect the transport of P.