DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION Positional and Temporal Changes in Ponded Infiltration in a Corn Field M. A. Prieksat,* T. C. Kaspar, and M. D. Ankeny ABSTRACT Infiltrating rainwater can move agricultural chemicals through soil and may contribute to contamination of drinking water supplies. Ponded infiltration rates were measured in corn (Zea mays L.) fields at four positions relative to plants and to crop rows: center of a trafficked interrow (TRK), center of an untrafficked interrow (UNT), between corn plants in a row (BPIR), and directly over the base of a plant in a row (OPIR). Measurements were taken in chisel-plow plots during 1990 and 1991, and in no-till plots during 1991. A Canisteo silty clay loam (fine-loamy, mixed [calcareous], mesic Typic Haplaquoll) was the predominant soil type in the plots. In chisel-plow plots, infiltration rates for TRK and UNT positions remained relatively constant during both years, with temporary increases after tillage or cultivation. At BPIR and OPIR positions, infiltration rates increased steadily over the growing season. Infiltration rates at the OPIR position increased from 43 to 211 fim s~' in 1990 and from 63 to 257 pm s~' in 1991. At the end of both growing seasons, the OPIR position had the greatest infiltration rate, and the TRK position the lowest. In no-till, infiltra- tion rates at all positions remained relatively constant throughout the 1991 growing season. Infiltration rates at BPIR and OPIR positions were not different from each other, were greater than 200 /tm s~', and were higher than rates at TRK and UNT positions. High potential infiltration rates in the row, especially around the bases of corn plants, have implications for the management of row-banded chemicals. P REFERENTIAL FLOW of chemicals through soil may contribute to contamination of drinking water supplies. White (1985) described preferential flow as the process whereby water movement through a po- rous medium follows favored routes, bypassing other parts of the medium. Leaching of solutes, however, depends on the position of the solute and its affinity for the soil. Nevertheless, preferential flow has the potential to rapidly conduct solutes, such as NO 3 , to soil depths below the root zone, where they are un- available to plants, are an economic and energy loss to farmers, and are potential contaminants of water supplies. Plant roots influence preferential flow because con- tinuous linear macropores can be created when roots desiccate or decompose. Barley (1954) found that de- cayed corn roots increased the permeability of a sandy loam soil. Experiments conducted by Gish and Jury (1981, 1983) showed that wheat roots decayed, in- M.A. Prieksat, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011; T.C. Kaspar, USDA-ARS National Soil Tilth Lab., Ames, IA 50011; and M.D. Ankeny, Daniel B. Stephens & Assoc., Inc., 4415 Hawkins NE, Albuquerque, NM 87109. Joint contribution from USDA-ARS and Iowa State Univ. Journal Paper no. J-15251 of the Iowa Agricultural and Home Economics Exp. Stn., Ames, IA. Project no. 2878. Received 23 Mar. 1993. * Corresponding author. Published in Soil Sci. Soc. Am. J. 58:181-184 (1994). creasing solute dispersion and hastening Cl~ break- through. Meek et al. (1990) concluded that macropores formed by decaying alfalfa roots increased infiltration rates approximately 140 and 240% for treatments with and without harvest traffic, respectively. The effect of living roots on preferential flow is not so clear. Both Barley (1954) and Gish and Jury (1981, 1983) hypothesized that, as living roots grow, they compact soil and obstruct existing macropores, thus decreasing hydraulic conductivity. Other researchers, however, have presented data contradicting this hy- pothesis. Aubertin (1971, p. 33) determined that ma- cropores created by tree roots were a major source of water movement into the soil profile. There was evi- dence of flow not only in decaying root channels, but also along living tree roots. Warner and Young (1991) observed macropore flow directly beneath rows of growing corn plants. After applying water with a rain- fall simulator, they collected 93% of the total infil- trated water directly below the corn rows. Dye tracers used in their study showed that macropore flow was occurring along living roots. Another study conducted by Hino et al. (1987) suggested that living grass roots increased hydraulic conductivity from 1.67 to >27.7 jam s -1 . Living roots also may increase soil macro- porosity indirectly, through their effect on soil water. In some soils, desiccation cracks may form after plant roots have extracted most of the available water from the soil and the soil continues to dry. Positional variability of field soil properties caused by crop-production practices affects both infiltration and root growth. Kaspar et al. (1991) found that bulk density and soil strength of trafficked interrows were greater than those of untrafficked interrows and of rows in both no-till and chisel-plow plots. Corn root growth was much greater in trie row than in either interrow position, but untrafficked interrows had twice as much root growth as did trafficked interrows. An- keny et al. (1990) measured much lower infiltration rates in trafficked interrows than in untrafficked in- terrows and attributed the decrease to destruction of macropores. Corn canopies alter the distribution of rainfall reaching the soil surface. A number of researchers (Haynes, 1940; Glover and Gwynne, 1962; Parkin and Codling, 1990) have shown that corn leaves divert a significant percentage of rain water to the stem and that water flows down the stem (stemflow) to the soil surface at the base of corn plants. Studies conducted by Saffigna et al. (1976) and by Warner and Young Abbreviations: TRK, center of trafficked interrow; UNT, center of untrafficked interrow; BPIR, between corn plants in a row; OPIR, directly over the base of a corn plant in a row; DAP, days after planting. 181