Corn Yield and Nitrate Loss in Subsurface Drainage from Midseason Nitrogen Fertilizer Application

Whether in response to remotely sensed plant N status or as a rescue treatment when previously applied N has been lost to denitrification or leaching, there is growing interest in applying N to corn at midseason. While the yield benefits of this practice are variable, little information is available as to the impacts of midseason N application on water quality. We compared grain yields and NO3 losses in drainage water as a result of applying N either once after emergence or equally split between just after emergence and midseason (V16). Nitrogen treatments consisted of 199 (H), 138 (M), and 69 (L) kg ha applied postemergence (V1–V3), and 69 kg ha applied postemergence and again at midseason (R). Grain yield for corn (Zea mays L.) and soybean [Glycine max (L.) Merr.], grown in a 2-yr rotation, and drainage water NO3 concentrations were measured on replicated tile-drained plots in a producer’s field from 2002 through 2005. Midseason application of additional N resulted in 0.9 and 2.5 Mg ha greater yield than the L treatment in 2002 and 2004, respectively; however, yield was greater when the same total amount of N was applied in one application shortly after emergence (M treatment) vs. the split treatment. There was no carryover effect on subsequent soybean yields for any of the N treatments. Annual flow-weighted NO3 concentrations in tile drainage were consistently greater (0.3–1.3 mg L) for the R treatment than the M treatment and significantly greater when averaged across all years. Residual soil NO3 at the end of the year also indicated that some of the midseason N application was not taken up by the crop and was available for leaching. Thus, midseason N application was beneficial for recovering some of the potential yield in corn when initial N applications are insufficient for optimum yield, but the practice did not benefit water quality in this study compared with a single application at emergence. WITHIN the Midwest Corn Belt, NO3 concentrations in surface waters often exceed the 10 mg L maximum contaminant level (MCL) for drinking water set by the USEPA (Jaynes et al., 1999; Mitchell et al., 2000). This has led some cities that rely on surface water for their drinking supply to install denitrification systems to remove NO3, causing increased expense for water treatment (Dinnes et al., 2002). Excessive NO3 in the Mississippi River has also been identified as a leading cause of hypoxia in the northern Gulf of Mexico (Rabalais et al., 1996). Numerous studies at the field and watershed scale (David et al., 1997; Goolsby et al., 1999; Jaynes et al., 1999) have shown thatmuch of theNO3 in surface waters of the Midwest comes from corn– soybean production. These same studies indicate that the primary pathway for this NO3 to enter surface waters is through the discharge of subsurface drains (tiles) that are common across the Midwest Corn Belt (Zucker and Brown, 1998). Thus, it is not surprising that the area within the Mississippi River watershed identified by Goolsby et al. (2001) as the primary source of NO3 to the Gulf is the same area where corn production on artificially drained lands is prevalent. Numerous suggestions have been made on how to reduce NO3 leaching from tile-drained lands in the Midwest (Dinnes et al., 2002). A common strategy is to fine tune N fertilizer application rates to the N need of the crop. Optimum N rates can vary greatly among years based on mineralization rates of soil organic matter and the leaching and denitrification of soil NO3. To compensate for this yearly variation, a reactive strategy has been proposed where soil NO3 measurements made a few weeks after corn emergence are used to determine the properN rate for a side-dress application. Thepresidedress soil NO3 test (Magdoff et al., 1984) and the late spring soil NO3 test (Blackmer et al., 1989) are examples of this approach. SplittingN fertilizer application between planting and early season, with the rate for the second application determined by a soil test, can dramatically reduce NO3 leaching at field (Bjorneberg et al., 1998; Guillard et al., 1999; Mitchell et al., 2000; Bakhsh et al., 2002) and watershed scales (Jaynes et al., 2004). The requirement and cost of soil sampling, however, greatly limits the feasibility of this approach for most farmers. To avoid soil sampling, plant-based monitoring systems have been proposed for determining N content and sufficiency in plants and determining the proper N rate at side-dressing. Most of these systems rely on measuring the chlorophyll content of leaves, which is directly related to N content and can be used to infer N need. Hand-held chlorophyll meters have been shown to be correlated with leaf N content (Schepers et al., 1992) and have been used to determine the N rate for sidedressing (Piekielek and Fox, 1992). Frequently, however, chlorophyll meters could not identify N deficiencies until after the V6 to V12 crop stage (Ritchie et al., 1996), which delays N application until midseason at the earliest (Blackmer and Schepers, 1995; Siambi et al., 1999; Binder et al., 2000). Rather than using hand-held chlorophyll meters,many investigators have shown that spectral sensors mounted on airplanes can be used to measure different levels of N stress in corn (Blackmer et al., 1996; Blackmer and White, 1998; Goel et al., 2003; Hendrickson et al., 2002). USDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable. Received 13 Feb. 2006. *Corresponding author ([email protected]). Published in Agron. J. 98:1479–1487 (2006).