Solute Leaching in Crop Row vs. Interrow Zones

This was a pilot study to investigate overall differences in solute leaching from soil beneath row vs. interrow zones of a row crop. Overall leaching could be minimized by exploiting such differences. Strontium bromide was uniformly applied to two, 7.2 by 6 m plots on Bosville fine sandy loam (fine, mixed, thermic Albaquic Paleudalf) planted in corn (lea mays L.) and soybean [Glycine max (L.) Merr.]. Within each plot, the surface soil texture varied from fine sandy loam to fine sandy clay loam. Soil water status was monitored with tensiometers. After the vegetative growth stage, soil samples were taken along five transects perpendicular to the crop rows at 0.2-m intervals, to a depth of 0.5 m. Sampling locations corresponded to row, quarterrow, and interrow positions. In the fine sandy loam half of the corn plot, there was significantly more leaching of Br below the 0.5-m depth in the interrow positions than in the row positions. This appears to be related to the soil water status, since soil conditions in the fine sandy loam half of the corn plot were, on the average, drier under row positions than under interrow positions. Overall leaching was less under soybean than under corn. Simplified two-dimensional simulations of solute transport in row-interrow zones further enhanced our understanding of the effects of net flux differences between those zones, overall net flux, and soil and crop type. The simulations indicated that overall leaching could be slowed by enhancing the differences in evapotranspiration between the row and interrow zones. S BEHAVIOR in soil planted in row crops is not fully understood. Knowledge of this behavior will help us better understand the implications of certain management practices, and utilize these to our advantage. For example, proper placement of fertilizer near the crop row, well known to increase availability to plant roots, may also reduce its downward movement. Similarly, the leaching risks of pesticides could be reduced by spatial placements, combined with some cultural practices if necessary. Here, we are seeking a reduction in leaching other than by plant uptake and recycling of solutes (Kung, 1990). A major factor that may alter solute retention and transport in a row-cropped soil is differential, lateral root distribution. The differences in root density between the crop-row and interrow zones will result in differences in water uptake between these zones, and hence differences in downward movement of water and solutes. Depending on the spacing between rows and the type of crop, the lateral differences in root density may occur only during early stages of growth (Bland and Dugas, 1989), or may persist throughout the growth period. These differences are expected to be greater in the topsoil than in the subsoil (Arya et al., 1975). Arya et al. (1975) measured strong lateral soil waterpotential gradients in soybean. The potentials were D.J. Timlin, USDA-ARS Systems Research Lab., 10300 Baltimore Ave., Beltsville, MD 20705; G.C. Heathman, USDA-ARS National Agricultural Water Quality Lab., P.O. Box 1430, Durant, OK 74702; and L.R. Ahuja, USDA-ARS Great Plains Systems Research Unit, P.O. Box E, Ft. Collins, CO 80522. Received 27 Feb. 1991. * Corresponding author. Published in Soil Sci. Soc. Am. J. 56:384-392 (1992). lowest in the rows, and the differences were greatest near the surface early in the season. Zhai et al. (1990) and van Wesenbeeck and Kachanoski (1988) using TDR to measure soil water content, reported lower water contents, for most times, below the row zone of a corn crop than below the interrow zone, within the surface 0.2 m. The average seasonal differences in the soil water content between the row and interrow zones measured in these studies were in the range of 0.01 to 0.02 m m~. The average seasonal water contents were lower in the row zone, even though the soil water contents were often higher in this zone immediately after a rainfall (van Wesenbeeck and Kachanoski, 1988). These differences, which persisted until crop maturity, were obviously due to differences in root uptake between the two zones. Greater root water uptake and lower soil water contents in the crop-row zone will result in smaller downward soil water and solute fluxes below this zone than below the interrow zone. Greater downward fluxes in the interrow zone would hie due to higher antecedent water contents and greater wetting-front advance during a recharge event, as well as greater water movement during the subsequent redistribution. Due to lateral gradients, however, some of the solute from the interrow zone may also move with water to the row zone. Some soil conditions may enhance the above effects of lateral root distribution in reducing leaching of solutes below the row zone. For example, ridging or hilling in the soil near the plant stems will shed rainwater away from the plant-row zone, causing greater infiltration of water and leaching in the interrow zone (Saffigna et al., 1976). Some other factors, such as crop geometry, may have an opposite effect. The crop canopy has been shown to funnel a significant proportion of the rainfall to the soil via stemflow (Quin and Laflen, 1983) causing; relatively greater infiltration in the row zone (van Wesenbeeck and Kachanoski, 1988). As a result of protection from raindrop impact by the crop canopy, the surface soil in the row zone may also have a higher infiltration rate than that of the unprotected interrow zone:. The infiltration of rainwater in the row zone may thus be greater than in the interrow zone even without stemflow. Both these effects of crop canopy, of course, depend very much on the rainfall intensity and amount, hydraulic properties of soil, and the soil's susceptibility to crusting. Under natural field conditions, in corn on a silty clay loam soil, van Wesenbeeck and Kachanoski (1988) measured a preferential recharge of the root zone as a result of the above canopy effects. The differences in plant water uptake between the row and interrow zones, however, still dominated overall, as indicated by the lower soil water contents in the row zone. No Abbreviations: TDR, time domain reflectometry; R, row; IR, interrow; QR, quarter row; ANOVA, analysis of variance; ET, evapotranspiration; AET, actual evapotranspiration; PET, potential evapotranspiration.