Spatial and Temporal Variation in Economically Optimum Nitrogen Rate for Corn

rally affected, it would suggest that the benefits associated with a nitrification inhibitor may also be spatially The economically optimum N rate (EONR) required for corn (Zea and temporally influenced. mays L.) may vary spatially due to variation in soil characteristics and temporally due to the interactions of environmental factors. The The potential economic and environmental benefits objectives of this research were to quantify the impact of field variabilassociated with site-specific N rate management will ity on the yield response of corn to N fertilization and to evaluate depend on the ability to predict and/or define the magnithe temporal stability of these response functions. A production field tude of these dynamic soil and crop process. Therefore, near Revere, MN, was cropped with corn in 1995, 1997, and 1999 in the objectives of this research were to quantify the imrotation with soybean [Glycine max (L.) Merr.]. Four replications of pact of field variability on the yield response of corn to seven treatments were established in a split-plot arrangement of a N fertilization (EONR and profitability) for a location randomized complete block design. Main plots consisted of three N in southwest Minnesota and to evaluate the temporal rates (0, 67, 134, and 202 kg ha 1) while the split plots were two stability of these response functions across separate rates (0 and 0.56 kg ha 1) of nitrapyrin [2-chloro-6 (trichloromethyl)growing seasons. pyridine]. Each replication was divided into subblocks to estimate spatial patterns in yield N response and EONR. Spatial analysis indicated that only half of the field responded to N. Uniform application MATERIALS AND METHODS recommendation of 145 kg N ha 1 for the whole field overfertilized these areas while other areas were underfertilized. Variable-rate N Site and Experimental Design applications according to the EONR would have resulted in 69 and An experiment was established on a production field near 75 kg ha 1 less N being applied than the uniform N rate in 1997 and Revere, MN (44 14 N, 95 21 W), beginning in 1994. The soils 1999, respectively. Potential economic benefits were $8 and $23 ha 1 at the site belong to the Canisteo-Ves association and are higher than the uniform N rate in 1997 and 1999, respectively. Approxnearly level to gently sloping. The three major soils present imately 60% of the field responded in a similar manner in both 1997 at the site are the Ves loam (fine-loamy, mixed, mesic Udic and 1999, suggesting that temporal variations must also be considered Haplustolls) with 27 g kg 1 organic matter (OM), the Webster with site-specific N management. clay loam (fine-loamy, mixed, mesic Typic Haplaquolls) with 40 g kg 1 OM, and the Normania loam (fine-loamy, mixed, mesic Aquic Haplustolls) with 33 g kg 1 OM. Normal annual N fertilizer needs of corn may vary both precipitation is about 635 mm and is adequate for soybean, corn, and small grains because 80% of annual rainfall occurs between fields (Bundy and Andraski, 1995; during the growing season from April to September. Growing Schmitt and Randall, 1994) and within fields (Blackmer season monthly precipitation and temperature at the experiand White, 1998; Malzer et al., 1996). When uniform mental site for 1995, 1997, and 1999 are presented in Fig. 1. application rates of N are made across a field with variThe crop sequence was corn–soybean alternating each year. able soil and plant N relations, the results will be overFour replications of seven treatments were established in a fertilization in some areas and underfertilization in othsplit-plot arrangement of a randomized complete block design. ers (Fiez et al., 1995; Pan et al., 1997). Overapplication Main-plot treatments consisted of four N rates (0, 67, 134, of N increases the probability of NO3–N leaching below and 202 kg ha ) while the split plots were two rates (0 and the root zone (Meisinger and Randall, 1991) while un0.56 kg ha ) of nitrapyrin applied with each N rate. Each derfertilization limits yields (Pan et al., 1997) and may replication was further divided into 15 subblocks in space that was 15 m long and 42.7 m wide (Fig. 2) to facilitate spatial restrict economic returns (Scharf and Lory, 2000). interpretation of yield response to applied N. In 1995, only Differential responses to fertilizer N both between three N rates (0, 67, and 134 kg N ha ) were used. All treatand within fields are due to both spatial and temporal ments were applied in strips 225 m long and 6.1 m wide in variations in crop demand (Fiez et al., 1995) and soil N the fall before the corn year; however, in 1995, two rates (67 supply and losses (Hergert et al., 1995). Spatial soil and 134 kg N ha ) were also applied in the spring. Each strip variability may influence yield potential, N requireconsisted of eight corn rows planted at 0.76-m spacing. The ments, mineralization of organic N, and available soil treatments remained in the same strips all three years, and N N (Fiez et al., 1994) while temporal variability may carryover effect was assumed to be negligible because soybean influence the expression of spatial variability (Eghball was cropped in alternate years with corn. Anhydrous ammonia and Varvel, 1997). Because N losses from soils via leachwas used as the N source and applied with a radar-controlled variable-rate applicator to compensate for variations in speed ing and denitrification may be both spatially and tempoand to ensure a constant rate of N within each strip. In 1997 and 1999, preplant [Double Play—670 g L 1 EPTC (S-ethyl M. Mamo, Dep. of Agron. and Hortic., Univ. of Nebraska, Lincoln, NE 68583-0915; G.L. Malzer, D.J. Mulla, and J. Strock, Dep. of Soil, dipropylthiocarbamate) plus 170 g L 1 acetochlor (2-chloroWater, and Climate, Univ. of Minnesota, 1991 Upper Buford Circle, N-ethoxymethyl-6 -ethylacet-o-toluidide)] and postemergence St. Paul, MN 55108; and D.R. Huggins, USDA-ARS, Washington State {Pursuit—240 g L 1 imazethapyr [( )-2-[4,5-dihydro-4-methylUniv., Pullman, WA 22222. Contrib. from the Dep. of Soil, Water, 4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridineand Climate, Univ. of Minnesota, and the Minnesota Agric. Exp. Stn. Received 4 June 2002. *Corresponding author ([email protected]). Abbreviations: EONR, economically optimum nitrogen rate; OM, organic matter; YEONR, yield at economically optimum nitrogen rate. Published in Agron. J. 95:958–964 (2003).