Aquifer lithology affects shallow groundwater quality more than nitrogen fertilizer form and placement method in an Iowa agricultural field

Abstract

Excessive nutrient loss threatens local and regional water resources, and many midwestern U.S. states are adopting nutrient reduction strategies to reduce export of N and P. A common practice to reduce N loss is improved fertilizer management. In this study, we used a strip trial design to assess the effects of split N application form (urea and urea ammonium nitrate [UAN]) and placement method (broadcast, coulter, Ydrop) on corn (Zea mays L.) yields and shallow groundwater quality at an agricultural field at Kirkwood Community College in Cedar Rapids, IA. Twelve shallow monitoring wells were installed within the production field and sampled 14 times across a corn-soybean [Glycine max (L.) Merr.] rotation. Results showed that split application of UAN applied with either coulter injection or Y-drop method produced approximately 8-11% higher corn yields than broadcast urea, but no statistically significant relation was found between groundwater quality and N form and placement method. Instead, we report that groundwater quality and levels were significantly influenced by variations in aquifer lithology. Groundwater within fine-textured glacial till had significantly higher dissolved reactive P, SO 4 , and specific conductivity, whereas groundwater within the sand aquifer had a deeper water table and had higher NO 3 -N and dissolved oxygen. Study results suggest aquifer lithology can play a much larger role than varying the N form and application method on shallow groundwater quality in agricultural fields.

Effects of cattle manure and soil parent material on shallow groundwater quality

“…In our study, the coarse-grained colluvial soils showed the highest levels of NO 3 -N. Therefore, practices targeting these soils may be of primary benefit compared to the finer textured tills. Our study confirms the work of Schilling et al (2021) by emphasizing the dominating influence of soil parent material on groundwater quality in agricultural fields. Future nutrient reduction strategies in Iowa and across the Midwest should enhance mapping techniques by delineating parent material defined areas in order to guide fertilizer management plans that will ultimately improve water quality.…”

“…(2003) observed an ideal root density and pattern (which implies an ideal plant type) for N uptake in specific textured soils in western Australia, which supports our interpretation that N storage and use is highly dependent upon the physical properties of the soil parent material. Patterns identified in our study were very similar to those observed during a previous study at the same site where N fertilizer rates and application methods were tested in strips that intersected both parent material types (Schilling et al., 2021) and further supports observations made at a floodplain site in eastern Iowa where groundwater quality was highly dependent upon lithologic variations (Schilling et al., 2015). Our study continues to solidify the dominating influence of parent material on shallow groundwater quality and provides further evidence that accurate surveys of subsurface sediments may be of critical importance to assessing environmental sustainability and agricultural efficiency within cropped fields.…”

“…However, we did observe significant changes in groundwater properties the year following corn when no fertilizer was applied which provides some evidence that groundwater quality responded seasonally to management decisions at the study site. A similar response was reported during an initial investigation at this site when types of commercial N application were assessed (Schilling et al, 2021). While we cannot guarantee that lateral groundwater flow did not affect monitoring results of the current study thereby diluting or magnifying the influence of soil parent material or fertilizer type, since groundwater flow is slow, it is very unlikely that significant interaction between well locations occurred.…”

“…While we cannot guarantee that lateral groundwater flow did not affect monitoring results of the current study thereby diluting or magnifying the influence of soil parent material or fertilizer type, since groundwater flow is slow, it is very unlikely that significant interaction between well locations occurred. Therefore, we assumed that properties measured at each sampling location represented groundwater from localized recharge similar to previous studies (Schilling et al., 2021; Schilling, Jacobson et al., 2020).…”

“…Higher ECa readings are associated with finer soil texture and glacial parent materials whereas lower ECa relates to coarser soil texture and colluvial/alluvial parent materials. A conductivity of 40 mS/m was selected as a “break line” ECa value to differentiate between the two parent materials based on previous research at the site (Schilling et al., 2021). Based on the EM survey results, we bisected the east and west fields approximately perpendicular to the surface contour to create four areas of the field that contained each representative geologic condition (i.e., till, colluviated loess covered bench terrace, and colluvium).…”