Residue cover influences temperature and water gradients in the soil profile. Changes in the physical environment of the soil influence NH3 volatilization from urea‐containing fertilizers. Field and laboratory experiments were conducted to investigate the influence of residue‐cover‐induced changes in soil water and temperature on NH3 volatilization as impacted by urea treatment with a nitrification and urease inhibitor. Fertilizer treatments were urea, urea plus dicyandiamide (DCD), urea plus N‐(n‐butyl)thiophosphoric triamide (NBPT), and urea plus NBPT and DCD. Following fertilizer application, the soil was either left bare or covered with corn (Zea mays L.) residue. Every 3 h over a 4‐d period, water potential, soil temperature, CO2 production, and NH3 volatilization were measured. The influence of fertilizer treatments on soil pH was determined in a laboratory incubation experiment conducted over 8 d under controlled environmental conditions. Treatments were similar to the field experiment, with NH3 volatilization, pH, and CO2 production measured daily. The NH3‐volatilization rate in the field was highest 2 d after urea application at a time that corresponded with daily maximum soil temperature and decreasing soil water content. Residue cover reduced NH3 volatilization. Volatilization of NH3 as a result of urea application was not increased when urea was treated with DCD. Ammonia volatilization as a result of urea treatment with NBPT was reduced by 100 times over untreated urea. During an incubation experiment, soil pH increased from 6.5 to 7.2 in the urea‐NBPT, and from 6.5 to 9.0 in the urea and urea‐DCD treatments. Associated with the pH increase in the urea‐NBPT treatment was a reduction in CO2 production when compared with the untreated soil.

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 variabil-associated 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 Ϫ1) while the split plots were two stability of these response functions across separate rates (0 and 0.56 kg ha Ϫ1) 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 Ϫ1 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 Ϫ1 less N being applied than the uniform N rate in 1997 and Revere, MN (44Њ14Ј N, 95Њ21Ј W), beginning in 1994. The soils 1999, respectively. Potential economic benefits were $8 and $23 ha Ϫ1 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 Ϫ1 organic matter (OM), the Webster with site-specific N management. clay loam (fine-loamy, mixed, mesic Typic Haplaquolls) with 40 g kg Ϫ1 OM, and the Normania loam (fine-loamy, mixed, mesic Aquic Haplustolls) with 33 g kg Ϫ1 OM. Normal annual

Interactions among environmental factors, management decisions, and field characteristics cause temporal and spatial variability in corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] yields. The objectives of this paper are (i) to test whether yield response of corn to N and P and of soybean to P are spatially and temporally stable, and (ii) to evaluate the profitability of a variable rate (VR) N and P fertility management strategy over a 5-yr, corn-soybean rotation using this response information. A field near Windom, MN, USA, was cropped with corn (1997, 1999, and 2001) and soybean (1998, 2000). Three replications of 13 N and P treatments were established in a split-plot arrangement of a randomized complete block design. Treatments were applied at constant rates in strips across the entire field. Fertilizer N treatments were 0, 67, 112, 157, and 202 kg ha 21 and P treatments were 0, 56, and 112 kg P 2 O 5 ha 21. The field was partitioned into sub-blocks for spatial analysis of yield response. Corn and soybean response to these inputs was estimated for each block, each year. Results indicate that spatial variation of crop response to these inputs is significant, and that response of corn and soybean to P is temporally stable in some parts of the field, but not others. Response to N was not temporally stable. Results of an ex post profitability analysis found that average returns over the 5-yr period from the VR N and P management strategy were $28 ha 21 higher than returns from a uniform application strategy.

To optimize management, farmers require quantitative understanding of the factors aff ecting variability in soybean [Glycine max (L.) Merr.] seed yield and quality. Our objectives were to characterize spatial variation in soybean seed yield, oil concentration, and protein concentration in two south-central Minnesota fi elds over 6 yr of a corn [Zea mays L.]-soybean rotation, and to determine the infl uence of fertilizer treatments, soil chemical properties, and topography on soybean yield, oil, and protein. Soil and topographical variables were observed on 0.014-ha cells, and included Bray P1, Olsen P, K, Zn, pH, organic matter, total organic C, NH 4 -N, NO 3 -N, total N, mineralizable N, elevation, slope, curvature, fl ow accumulation, and aspect. Soybean yields consistently exhibited spatial structure. Within fi elds, spatial patterns of soybean yields were highly correlated across years, and we observed consistent relationships between yield and soil variables. Overall, soybean yield related positively to soil P and Zn and negatively to pH at all site-years. Models of soybean yield in relation to soil P and Zn indicate that in high pH soils at these sites, yield is optimized when soil P and Zn levels are higher than current extension recommendations. Protein and oil concentrations exhibited inconsistent spatial structure, and the spatial pattern of protein and oil concentrations diff ered across years. Relationships between soybean quality and soil properties were more consistent between sites within years than across years within sites, indicating that soybean quality is infl uenced by soil-climate interactions that function on a regional basis.

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