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Title: Surface and Subsurface Solute Transport Properties at Row and Inter-Row Positions

item Jaynes, Dan
item Ochsner, Tyson

Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/1/2007
Publication Date: 6/6/2007
Citation: Gaur, A., Jaynes, D.B., Horton, R., Ochsner, T.E. 2007. Surface and Subsurface Solute Transport Properties at Row and Inter-Row Positions. Soil Science. 172(6):419-431.

Interpretive Summary: Understanding how water and chemicals move through soil is required to better predict the impact of current and future farming practices on water quality. Little is known about how chemical transport within cropped rows is different than between rows, or if transport may be affected by crop type or soil compaction from farm machinery. We measured these differences by leaching the common salt calcium chloride into a field soil previously cropped to corn, soybean, and oat and measuring the movement of chloride using two different methods. The first method was developed in earlier work by us and used measurements over time of the top few inches of soil to determine chloride movement. This was compared with a more traditional method that measured the distribution of the chloride within the top four feet of the soil profile after several days of leaching with water. We found that the speed or rate of chemical movement measured near the soil surface was not affected by crop or row position. This was in contrast to measurements made deeper in the profile that showed that chemical movement was faster under crop rows than between crop rows. We also showed that the chloride tended to spread out more within the soil between rows than under rows of corn or soybean. These differences mean that agrichemicals applied to fields move differently under crop rows versus between rows and that the difference in movement depends somewhat on the crop. Thus, scientists need to consider these factors when predicting chemical transport in soils. These findings will be useful to soil scientists and other environmental scientists interested in the movement of chemicals through soil and their possible contamination of water resources.

Technical Abstract: Although numerous studies have investigated the effects of crop production practices on soil water dynamics, not much information is available on the impact of row position on solute transport. A field experiment was carried out to evaluate surface and subsurface solute transport properties in plant-row, non-trafficked inter-row, and trafficked inter-row positions. For this purpose, a plot of 14- by 14-m in a strip-cropped field with soybean (Glycine max L. Merr), corn (Zea mays L.), and oat (Avena L.) was selected. After harvesting the crops, surface (top 2 cm) electrical conductivity measurements were made by time domain reflectometry (TDR) at 45 locations during a chloride pulse leaching experiment. At the conclusion of the pulse leaching experiment, 120-cm deep soil cores were collected at the 45 locations in order to measure the soil profile chemical distributions. No crop or row position effects were observed for surface determined pore water velocities (v), while profile determined v was greater in plant row vs. inter-row positions when averaged over all crops. Overall, the profile determined v was slightly greater than the surface determined v, probably because of lower effective or mobile water contents. The profile determined dispersion coefficient (D), was smaller in row positions than inter-row positions in soybean and corn, perhaps because of surface ponding in the inter-row positions of the crops resulting in macropore flow. Profile determined D was greater in the inter-row positions of soybean than oat, again reflecting possible macropore flow. Overall, the mean soil profile dispersivity (2.97 cm) was larger than the surface soil (1.02 cm). The local surface solute transport varied by row positions while profile solute transport was affected by both row position and crop, perhaps due to surface ponding producing macropore flow in the trafficked and non-trafficked inter-rows of soybean and the trafficked inter-rows of corn. Thus, a one-dimensional solute transport model with a spatially distributed flux or potential controlled upper boundary condition must be used to model this system.