Topsoil Depth at the Centralia Site Optimizing Crop N Use Efficiency Using a Variable Source N Fertilizer Application Strategy Adam J. Noellsch1, Peter P. Motavalli1, Kelly Nelson2, Newell Kitchen3, Stephen H. Anderson1, Peter Scharf4, and Paul Tracy5 1Dept. of Soil, Environmental and Atmospheric Sciences, University of Missouri, Columbia, MO 65211, 2Greenley Research Center, Univ. of Missouri, Novelty, MO 63460, 3USDA-ARS, Univ. of Missouri, Columbia, MO 65211, 4Division of Plant Sciences, University of Missouri, Columbia, MO 65211, 5MFA Incorporated, Columbia, MO 65201 Table 3. Corn grain yield, N uptake and NUE in 2006 and 2007 at the Centralia site. A two-year field trial planted to corn (Zea mays. L) was initiated in 2005 at the Greenley Experiment Station in Northeast Missouri in the central claypan area (Fig. 1A) on a Putnam silt loam soil (fine, smectitic, mesic, Vertic Albaqualfs). The site was mapped for elevation with a total station and apparent electrical conductivity (ECa) using a EM-38 sensor (Fig. 1B). Measurement of soil ECa gives an indication of relative depth to the claypan subsoil layer. The experimental arrangement was a RCB design with 4 reps and treatments consisting of a control and 4 N treatments (168 Kg N ha-1) applied across 3 landscape positions (summit, sideslope and low-lying). Development of improved management practices to increase nitrogen use efficiency (NUE) within agricultural fields is needed to improve crop production and reduce nitrogen (N) loss. Field studies planted to corn (Zea mays L.) were conducted in 2005 and 2006 in the claypan region of north central (Centralia) and northeast Missouri (Greenley Research Center) to determine the effects of landscape position and soil depth to the claypan on crop growth and N uptake, and to examine the use of a variable-source N fertilizer application strategy to optimize crop N fertilizer use. Treatments at the northeast Missouri site consisted of a control and 168 kg N ha-1 of urea, polymer-coated urea (PCU), a 50% urea/50% PCU mixture, or anhydrous ammonia applied in 457 m long strips that included variation in elevation and claypan depth. At the north Centralia site, N fertilizer treatments of 168 kg N ha-1 of urea or PCU were broadcast surface-applied within three different cropping/tillage systems and at different landscape positions representing the summit, sideslope and footslope positions in the field. At the Greenley site, PCU treatments showed a consistent 1535 to 1814 kg ha-1 increase in corn grain yields compared to the urea treatments in 2005 and 2006, respectively, in the low lying area, possibly due to the wetter conditions in the low-lying area affecting the fate of the applied N. Similarly, anhydrous ammonia application resulted in a 1475 and 1659 kg ha-1 yield increase in 2005 and 2006, respectively, in the low-lying area. At the north central Missouri site in 2006, the corn grain yield of the PCU-treated area was 1203 kg ha-1 higher than that of the urea-treated area only at the footslope landscape position in the no-till corn/soybean/wheat cropping system. These results suggest that a variable source N fertilizer application approach based on identifying periodically wet field areas due to their lower elevation may improve NUE and economic returns. Reduced NUE and crop yields due to landscape variability may warrant the use of variable source N (VSN) fertilizer applications. The VSN fertilizer application strategy is when enhanced efficiency N fertilizers are applied in field areas that have a higher risk for N loss and conventional N fertilizer sources are applied in lower risk areas of that same field, possibly through use of a multi-bin fertilizer spreader. Schmidt et al. (2007) showed that economically optimal N rate (EONR) may be affected by within-field soil water content variability during a growing season. Gravimetric moisture content data collected at the Greenley Memorial Research Center in the claypan region of northeast Missouri has shown that soil water content can increase by as much as 22% after a rainfall event from a summit to footslope position with only 1 m difference in elevation (unpublished data). Use of enhanced efficiency fertilizers in low-lying landscape positions may be an effective way of improving NUE. 1. To determine the interactive effects of landscape position and soil depth to the claypan on NUE and agronomic response of corn to conventional and N fertilizer sources. 2. To examine the use of a variable-source strategy to optimize N fertilizer use efficiency and increase economic returns. Figures 2 A & B. (A) Topsoil depth at the Centralia site as derived from ECa data collected with an EM-38 sensor. (B) Map showing elevation and general landscape positions of 2006 & 2007 plots at Centralia. Topsoil Depth at the Centralia Site 2A. 2B. Abstract Materials and Methods 1A. The treatments included: 1. Anhydrous ammonia 2. Conventional urea 3. Polymer coated urea (ESN) 4. Mixture of 50% PCU/50% urea 5. Control with no N Results and Discussion All N treatments were pre-plant broadcast-applied and incorporated immediately after application. The low-lying area in the experimental field frequently had standing water during the growing season (Fig. 1C). Both silage and grain yields were determined and the tissue N content of the silage was used to calculate N uptake and N use efficiency (NUE). Table 1. Initial soil characteristics at the Centralia site. 1B. Summit (Shallow) Introduction As a result of long-term cropping, initial soil properties at Centralia varied by cropping system and landscape position in respect to bulk density, organic matter content, pH, soil test K, and exchangeable Ca and Mg (Table 1). At the Greenley site, PCU treatments showed a consistent 1505 to 1818 kg ha-1 increase in corn grain yields compared to the urea treatments in 2005 and 2006, respectively, in the low lying area, possibly due to the wetter conditions in the low-lying area affecting the fate of the applied N (Table 2). Similarly, anhydrous ammonia application resulted in a 1505 and 1630 kg ha-1 yield increase in 2005 and 2006, respectively, in the low-lying area. Significantly higher grain yields due to N fertilizer treatments were observed at Centralia in 2006 and 2007 in CS# 2 & 3, but PCU had higher grain yields compared to urea only in 2006 in CS# 3 at the footslope position. Both N uptake and NUE at Greenley were higher at the low-lying position compared to urea when PCU and anhydrous were applied in 2005, but this result was not repeated in 2006. In 2006, N uptake was generally higher in the summit and footslope position in CS # 2 & 3 and no consistent differences were observed in N uptake or NUE among the N fertilizer treatments. Using results from Greenley and based on a corn grain price of $0.12 kg-1 and a total application cost of $50 ha-1, the profit from using PCU versus urea in the low-lying area was $175 ha-1. 1C. Sideslope (Deep) Table 2. Corn grain yield, N uptake and NUE in 2005 and 2006 at Greenley. Low-lying The Centralia site in north central Missouri was established in 2006 on a long-term (15 yrs) cropping system experiment with claypan soils. Treatments were arranged in a RCB with 3 reps and consisted of a control and post-plant application of 168 kg N ha-1 of either urea or PCU applied across 3 landscape positions (summit, sideslope, footslope) and 3 cropping systems : (1) minimum till, corn/ soybean rotation, (2) no-till corn/soybean, (3) no-till corn/soybean/wheat. Figures 1 A-C. (A) Location of the Greenley and Centralia sites in the Central Claypan area. (B) ECa map produced with data collected from the Greenley site using an EM-38 sensor. (C) Photo taken at the Greenley site from the sideslope position, showing the low-lying area the day after a rainfall event. Objectives Conclusion Application of multiple N fertilizer sources in fields with claypan soils and low-lying areas may enhance crop yields and increase economic benefits compared to a single application of urea fertilizer across the whole field.