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Published online 29 June 2007
Published in Soil Sci Soc Am J 71:1343-1351 (2007)
DOI: 10.2136/sssaj2006.0262
© 2007 Soil Science Society of America
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SOIL & WATER MANAGEMENT & CONSERVATION

Soil Profile Nitrate Response to Nitrogen Fertilization of Winter Triticale

Carasella D. Nancea, Lance R. Gibsona,* and Douglas L. Karlenb

a Dep. of Agronomy, Iowa State Univ., Ames, IA 50011
b USDA-ARS, National Soil Tilth Lab., Ames, IA 50011

* Corresponding author (lgibson{at}iastate.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Growing triticale (xTriticosecale Wittmack) as a winter crop has the potential to utilize residual NO3–N from previous crops, thus reducing its availability for leaching. Our objectives were to quantify N capture and changes in soil NO3–N levels in response to N fertilization of triticale grown following either silage corn (Zea mays L.) or soybean [Glycine max (L.) Merr.]. Field studies were conducted in 2003–2004 and 2004–2005 near Ames and Lewis, IA. Soil samples to a depth of 120 cm were collected after the corn and soybean were harvested and again after growing triticale with four rates of N fertilizer (0, 33, 66, and 99 kg N ha–1). Partial N budgets were computed using profile NO3–N before triticale planting, N fertilizer applications, plant uptake, and profile NO3–N after triticale harvest. Nitrogen capture by triticale at physiological maturity was 44 to 93 kg N ha–1 when no N was applied and was as high as 164 kg N ha–1 with addition of 99 kg N ha–1. Growing winter triticale reduced profile NO3–N by an average of 33 to 53 kg ha–1 at Ames and 46 to 53 kg ha–1 at Lewis. Winter triticale dry matter and grain yields were maximized while simultaneously capturing and efficiently utilizing soil N left from previous silage corn and soybean crops.


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The current corn and soybean rotation used in the Upper Midwest may not be economically sustainable without producing tile drainage water that exceeds the maximum contamination level for NO3–N (David et al., 1997; Jaynes et al., 2001; Tomer et al., 2003). Nitrogen inputs are necessary for maintaining the productivity of intensive agricultural systems (Follett and Delgado, 2002), but those inputs can be a major source of nonpoint-source pollution. Several factors including changes in cropping systems, increased artificial drainage, high rates of N fertilization, and lack of synchronization between crop growth and soil NO3–N availability (Dinnes et al., 2002; Donner et al., 2004) are collectively contributing to increased NO3–N leaching. They have subsequently been identified as factors causing higher NO3–N concentrations in surface waters of the region (Balkcom et al., 2003; Jaynes et al., 2001) and contributing to hypoxia in the Gulf of Mexico (Donner et al., 2004; Rabalais et al., 2002). Differences measured in NO3–N leaching from corn and soybean crops were generally small (Baker and Melvin, 1994; Jaynes et al., 1999; Zhu and Fox, 2003) and overall, fertilized crops may account for almost 900f the NO3–N leached to the Mississippi River system within the U.S. Corn Belt, despite representing only 200f the watershed area (Donner et al., 2004).

Most of the NO3–N leaching from continuous corn and corn–soybean rotations occurs during late autumn and spring months when the soil is fallow and water is percolating through the profile and being discharged into subsurface tile drains (Owens et al., 1995; Karlen et al., 1998; Kladivko et al., 2004). Most of the water and NO3–N flows in tile lines occur during April, May, and June (Jaynes et al., 1999; Tomer et al., 2003). Corn and soybean are planted beginning in late April or early May; therefore, the crop is either not present or not fully established during most of this period. Tile flow decreases after June because evapotranspiration increases as the corn and soybean crops become more fully established and late summer tends to be dry (Power et al., 1998; Tomer et al., 2003). Drainage through the tile lines typically stops in late July to early August and starts again in late autumn or the following spring, depending on the amount of precipitation received during the summer and autumn months (Karlen et al., 1998; Strock et al., 2004).

If the concept of sustainable agricultural production systems includes both water quality and productivity, dramatic changes in management practices are required to make current agricultural systems in the U.S. Corn Belt sustainable (Jaynes et al., 2001). Nitrate is the dominant form of N present in soil water (Baker et al., 1975; Kladivko et al., 1991; Jacinthe et al., 1999). Therefore, the most successful N management strategies for reducing NO3 loss after corn and soybean are ones that minimize the amount of residual NO3–N in the soil (Karlen et al., 1998) and limit water movement through the soil profile and into tile drains (Baker and Melvin, 1994; Randall et al., 1997; Dinnes et al., 2002). Incorporating crops that capture excess NO3–N and limit its movement out of crop fields is one option, and winter triticale may fit this need. Other winter cereal grains, most notably rye (Secale cereale L.) and wheat (Triticum aestivum L.), have proven successful as NO3–N scavenging cover crops (Kessavalou and Walters 1997, 1999; Raun and Johnson, 1995; Strock et al., 2004). The limited growth of cover crops planted between successive summer annual crops, however, may restrict their usefulness for capturing NO3–N.

Winter triticale can be incorporated into current rotations as a cover crop because it grows in the autumn and spring and provides positive environmental benefits lacking in most cropping systems currently used in the U.S. Corn Belt. Winter triticale may be even more useful than short-term cover crops if it is placed in the rotation as a forage or grain crop. It can be planted after corn silage or soybean (Schwarte et al., 2006) in the central Corn Belt and provides valuable forage (Schwarte et al., 2005) or grain for feeding swine (Hale et al., 1985; Myer et al., 1990) and cattle (Hill and Utley, 1989; Smith et al., 1994), and straw for either bedding or possibly bioenergy production. As a forage crop between successive corn silage crops, winter triticale has already been adopted by some beef and dairy producers in the region.

Incorporating winter triticale into a cropping system may have the potential to capture significant amounts of soil N (Schwarte et al., 2005) and buffer against excess residual soil N (Raun and Johnson, 1995). Our objectives were to quantify N capture and changes in soil NO3–N levels in response to N fertilization of triticale grown following either silage corn or soybean. With this information, farmers and policymakers can make better decisions regarding cropping system changes that can reduce NO3–N loss from crop fields.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The response of winter triticale to four N fertilizer rates applied following corn silage or soybean was evaluated during 2003–2004 and 2004–2005 at two Iowa locations. Trials were conducted in central Iowa at the Iowa State University (ISU) Bruner Farm near Ames (42.0°N, 93.6°W, 291 m) and in southwest Iowa at the ISU Armstrong Research and Demonstration Farm near Lewis (41.2°N, 95.1°W, 370 m). The predominate soil types were Clarion loam (fine-loamy, mixed, mesic Typic Hapludolls) at the Bruner Farm in both years, Marshall silty clay loam (fine-silty, mixed, mesic Typic Hapludolls) at Lewis in 2003–2004, and Exira silty clay loam (fine-silty, mixed, mesic Typic Hapludolls) at Lewis in 2004–2005.

Crop Culture and Nitrogen Application
Corn silage and soybean crops were grown before winter triticale at both sites. Fields in Ames were prepared for corn and soybean planting with one pass of a field cultivator. No preplant tillage was used at Lewis. Corn and soybean were planted in alternating strips, 9.15 m wide (12 rows with 0.762 m between rows) and 21.3 m long. An early maturity group soybean was grown at both sites to ensure an optimum triticale planting date (Schwarte et al., 2005). Corn was harvested as silage and soybean was harvested as grain with the residue returned to the field. Dates of important field operations and sampling activities are listed in Table 1.


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  		Table 1. Timeline for field activities.

 
Soil tests of the surface 15 cm at Ames indicated 40 mg kg–1 P, 250 mg kg–1 K, and pH 6.5 in October 2001 and 27 mg kg–1 P, 160 mg kg–1 K, and pH 6.9 in October 2004. Soil tests of the top 15 cm of the profile at Lewis indicated 23 mg kg–1 P, 207 mg kg–1 K, and pH 6.5 in October 2003 and 25 mg kg–1 P, 167 mg kg–1 K, and pH 7.3 in October 2004.

At Ames in 2003, corn (Dekalb DKC64–11 RR, 114-d relative maturity) was planted at 79500 seeds ha–1 and soybean (Dekalb DKB17–51 RR, 1.7 relative maturity) was planted at 395000 seeds ha–1 on 20 May. For Ames in 2004, the same corn hybrid and soybean cultivar were planted at the same densities on 5 May. Nitrogen fertilizer was applied to corn at 134 kg ha–1 in the form of urea on 9 June 2003 and in the form of injected 32% (320 g kg–1) urea–NH4NO3 solution on 16 June 2004. The corn was cultivated between the rows on 9 June 2003. Roundup Ultramax (glyphosate [N-(phosphonomethyl) glycine]) was applied to the soybean and corn at 1.9 L ha–1 on 16 June 2003. Roundup Weathermax (glyphosate) was applied to the soybean and corn at 1.8 L ha–1 on 15 June 2004.

At Lewis in 2003, corn (Channel 7699C, 109-d relative maturity) was planted at 79000 seeds ha–1 on 27 April and soybean (Pioneer 92B05 RR, 1.9 relative maturity) was planted at 395000 seeds ha–1 on 13 May. In 2004, corn (Nutrident C 1153 ND, 115-d relative maturity) was planted on 19 April and soybean (Pioneer 92B05 RR, 1.9 relative maturity) was planted on 23 April. The same planting densities were used in 2003 and 2004. Weed management in 2003 consisted of 55 mL ha–1 Steadfast (nicosulfuron (2-[[(4,6-dimethoxypyrimidin-2-yl)aminocarbonyl]aminosulfonyl]-N,N-dimethyl-3-pyridinecarboxamide) and rimsulfuron [N((4,6-dimethoxypyrimidin-2-yl) aminocarbonyl)-3-(ethylsulfonyl)-2-pyridinesulfonamide]), 7.7 L ha–1 Callisto [mesotrione (2-[4-(methylsulfonyl)-2-nitrobenzoyl]-1,3-cyclohexanedione)], and 1.1 kg ha–1 atrazine [6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine] applied to corn on 7 June and 1.6 L ha–1 Roundup Weathermax applied to soybean on 20 June. Weed management in 2004 consisted of interrow cultivation of corn on 15 June and 1.5 L ha–1 Roundup Weathermax applied to soybean on 15 June.

Winter triticale (DANKO ‘Presto’ in 2003, NE426GT in 2004) was seeded at 320 seeds m–2 with a Tye Model 2007 no-till drill (AGCO Corp., Lockney, TX). The row spacing was 20.3 cm. No tillage was performed between corn or soybean harvest and triticale planting at either site.

Four N fertilizer rates (0, 33, 66, or 99 kg N ha–1) were randomly assigned to each pair of triticale plots growing on the harvested corn silage and soybean strips. The N fertilizer was applied at Ames using a Gandy Model 1010T-TBM (Gandy Co., Owatonna, MN) spreader with a 3-m width and at Lewis with a Gandy Model 6500 spreader with a 1.5-m width. Ammonium nitrate was used as the N fertilizer source at both locations.

Plant Measurements
Corn silage was harvested with a forage chopper and weighed. Silage yield was calculated for the entire site and adjusted to 650 g kg–1 moisture content. Soybean was combine harvested and weighed with on-board scales. Soybean grain yield was calculated for the entire site and adjusted to 130 g kg–1 moisture content.

Triticale dry matter production was determined in late autumn, after the crop became dormant, and early July, shortly after physiological maturity (Table 1). Two 48.3-cm lengths of row were harvested from two randomly selected areas within each plot for the fall samples. One 48.3-cm length of row was harvested from two randomly selected areas within each plot for the spring samples. The two samples for each plot were combined, oven dried at 65°C for at least 48 h, and weighed. The dried samples were ground to pass a 2- mm screen using a Thomas-Wiley mill (Model 4, Thomas Scientific, Swedesboro, NJ). The samples were ground for a second time using an Udy Cyclone sample mill (Udy Corp., Ft. Collins, CO) to pass a 0.5-mm screen. The ground samples were analyzed by dry combustion in a Fison NA 1500 elemental analyzer (Fison Instruments SpA, Milan, Italy) to determine total N concentration of the harvested dry matter (AOAC International, 2000).

Triticale grain was harvested with combines equipped with electronic weighing systems. The harvested area in each plot was 3.66 m wide by 21.34 m long at Ames and 4.57 m wide by 21.34 m long at Lewis. Grain subsamples (approximately 2 kg) were collected to determine moisture concentration. Crop residue and other debris were removed from the grain samples with a seed cleaner (Office Model Clipper, Ferrel Ross, Bluffton, IN). Moisture content was determined on the cleaned grain using a grain analysis computer (Model GAC2100, Dickey-John, Auburn, IL). Final grain yields were adjusted to 135 g kg–1 moisture content. The triticale grain was ground to a fine powder using a Magic Mill III Plus grain mill (K-Tec, Orem, UT) and analyzed for moisture using AACC Method 44–15A (American Association of Cereal Chemists, 2003).

Soil Nitrate Measurements
Soil profile NO3–N concentrations were determined on soil cores collected before triticale planting and after triticale grain harvest at each production site (Table 1). Cores were collected to a depth of 120 cm using a truck-mounted Giddings hydraulic sampling probe (Model 10-SC, Giddings Machine Co., Windsor, CO). Two soil cores (3.8-cm diameter) were taken from each plot and divided into 0- to 15-, 15- to 30-, 30- to 60-, 60- to 90-, and 90- to 120-cm depth increments. Soil moisture was determined by drying a 15-g subsample at 105°C for 18 h in a forced-air oven. Bulk density for each soil depth increment was determined as the dry weight of the soil per unit volume. Samples from similar depths in each plot were combined, mixed, pushed to pass an 8-mm screen, air dried, and crushed. A 20-g subsample of the soil for each depth increment in a plot was extracted with 100 mL of 2 M KCl and analyzed colorimetrically for NO3–N (Keeney and Nelson, 1982) using flow injection analysis (Latchat Instruments, Milwaukee, WI). The NO3–N concentration was multiplied by bulk density to determine the quantity of NO3–N throughout the soil profile. Partial N budgets were calculated using the equation:

Formula

The sampling depth for profile soil NO3–N was 0 to 120 cm for both production years at Ames and for 2003–2004 at Lewis. A profile depth of 0 to 90 cm was used for the 2004–2005 N budget for Lewis because dry soil prevented the probe from going deeper than 90 cm in five plots during the autumn of 2004 and seven plots in the summer of 2005.

Weather Data
Daily minimum temperature, maximum temperature, and rainfall were recorded for 2003, 2004, and 2005 using weather stations at each location. The mean weather conditions for each site were determined using means from 1951 to 2005 from the Iowa Environmental Mesonet (2006). Daily rainfall measurements did not include frozen precipitation, which was not measured.

Statistical Design and Analysis
The statistical design for these field experiments was a randomized complete block with separate analyses for each location and previous crop combination. Each nitrogen rate treatment was randomly assigned to paired corn silage–soybean strips within a block and the treatments were replicated four times at each site in each year. The corn silage and soybean strips that preceded the triticale crops were not randomized because of the difficulty it would have created for managing those crops. This lack of randomization meant that the statistical analysis for winter triticale response to N fertilizer rates required separate analysis for each previous crop. The variance for each factor measured was stabilized through transformation according to the procedure of Box and Cox (1964). Analysis of variance was performed on the transformed data using the GLM procedure of SAS (SAS Institute, Cary, NC). Means were calculated using the least squares method. A combined analysis was performed for years. Main effects of N rate and the N rate x year interaction were analyzed using an F test. The F test for N rate was calculated using the mean squares for the year x N rate interaction. The F test for the N rate x year interaction was calculated using the error mean square. Too few years were included in the experiment to test it as a main effect (Gomez and Gomez, 1984, p. 328–332). Tukey's test was used to make mean comparisons at the P ≤ 0.05 level (Steel and Torrie, 1980).


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Previous Crop Yields
Low rainfall in July and August 2003 (Fig. 1 and 2) stressed soybean, decreasing grain yield below the 5-yr county averages at both sites (Table 2). The low rainfall also resulted in below-average corn silage yield at Lewis in 2003. Rainfall and temperatures in 2004 supported above-average corn silage and soybean yields at both sites.


Figure 1
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  		Fig. 1. Temperature and rainfall conditions at the Iowa State University Bruner Farm near Ames in 2003, 2004, and 2005.

 

Figure 2
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  		Fig. 2. Temperature and rainfall conditions at the Iowa State University Armstrong Farm near Lewis in 2003, 2004, and 2005.

 

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  		Table 2. Previous crop yield, fall dry matter production, and N capture by winter triticale at two Iowa locations for two growing seasons.

 
Autumn Dry Matter Production and Nitrogen Accumulation
Climatic conditions between planting of the winter triticale in late September and dormancy in mid-November were suitable for establishment of intended plant stands at the two sites in both growing seasons. Autumn dry matter production was between 0.13 and 0.99 Mg ha–1 and resulted in the capture of between 5 and 42 kg N ha–1 (Table 2). Nitrogen content of winter rye cover crops have been close to the reduction in residual soil NO3–N (Kessavalou and Walters, 1999) and N leaching (Schröder et al., 1996). Therefore, up to 40 kg ha–1 of soil NO3–N was probably removed from the soil by the winter triticale in our study, making it unavailable for leaching during late autumn, winter, and early spring.

Below-average air temperatures in October and November limited dry matter production and N capture in 2003; above-average air temperatures in October and November promoted dry matter production and N capture in 2004. Warm autumn temperatures have also been associated with vigorous growth and N capture with winter rye (Strock et al., 2004; Kessavalou and Walters, 1997). The relationships among increased dry matter production, increased N uptake, and reductions in soil NO3–N suggest that soil NO3–N uptake between planting and winter dormancy would be greatest with early (before 1 October in Iowa) triticale planting (Schwarte et al., 2005) and above-average autumn temperatures.

Triticale Productivity
There was no N rate x year interaction for dry matter or grain yield at either Ames or Lewis. Applying 33 kg N ha–1 to winter triticale grown after corn silage and soybean at Ames increased both total dry matter and grain yield when compared with 0 N (Table 3). Nitrogen applications of 66 and 99 kg ha–1 to winter triticale grown after corn silage in Ames produced total dry matter and grain yields similar to 33 kg ha–1. Likewise, 66 and 99 kg N ha–1 applied to triticale grown after soybean in Ames produced total dry matter yields similar to 33 kg ha–1. There was no difference in grain yield for 0, 66, and 99 kg ha–1 or 33, 66, and 99 kg ha–1 after soybean at Ames. Dry matter production and grain yield of winter triticale grown after corn silage or soybean at Lewis did not respond to N application.


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  		Table 3. Productivity of winter triticale at Ames and Lewis, IA, following corn or soybean averaged for the 2003–2004 and 2004–2005 growing seasons. Data used for the analysis was transformed using the natural logarithm to stabilize variance.

 
When averaged across the two growing seasons and N rates producing maximum dry matter, triticale dry matter was 11.9 Mg ha–1 after corn silage at Ames, 13.0 Mg ha–1 after soybean at Ames, 12.9 Mg ha–1 after corn silage at Lewis, and 13.4 Mg ha–1 after soybean at Lewis. The 2003–2004 triticale crop was affected by septoria leaf blotch (Septoria spp.) at both sites due to the cool, moist conditions during June and July (Wiese, 1987, p. 43–45) resulting in low grain yields in 2004 (data not shown). Triticale grain yields averaged across the two growing seasons and optimum N rates were 3.5 Mg ha–1 after corn silage at Ames, 4.1 Mg ha–1 after soybean at Ames, 3.1 Mg ha–1 after corn silage at Lewis, and 3.6 Mg ha–1 after soybean at Lewis.

Soil Nitrate and Nitrogen Loss
There was a substantial amount of residual soil NO3 available to the triticale crop regardless of the previous crop (Table 4). Average preplant soil NO3–N to a depth of 120 cm was 48 kg ha–1 after corn silage and 70 kg ha–1 after soybean at Ames and 76 kg ha–1 after corn silage and 80 kg ha–1 after soybean at Lewis. The residual soil NO3–N levels were within the range of those reported for corn and soybean in the U.S. Corn Belt. Strock et al. (2004) reported autumn NO3–N levels in southwest Minnesota of 60 to 108 kg ha–1 after corn and 58 to 91 kg ha–1 after soybean. Residual soil NO3–N measured in early May in Nebraska was 133 to 202 kg ha–1 after corn and 91 to 187 kg ha–1 after soybean (Kessavalou and Walters, 1999). Karlen et al. (1998) measured 62 to 153 kg NO3–N ha–1 in the spring in a continuous corn system on four watersheds in southwest Iowa. Soil NO3–N after corn harvest in mid to late October in Illinois was 14 to 16 kg N ha–1 (Ruffo et al., 2004).


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  		Table 4. Soil NO3–N by soil depth for winter triticale grown after corn silage or soybean at Ames and Lewis, IA, averaged for 2003–2004 and 2004–2005 and N fertilizer rates of 0, 33, 66, and 99 kg ha–1.

 
More than half of the residual soil NO3–N after corn silage or soybean in our study was found in the top 15 cm of the soil profile and about three-fourths was found in the top 30 cm. In contrast, Karlen et al. (1998) reported that NO3–N was fairly uniform throughout the top 90 cm of the profile when sampled in April in a continuous corn system. They suggested that residual and mineralized NO3–N was distributed through the soil profile when the root zone water content was being recharged from October through April. Since the greatest proportion of NO3–N was near the soil surface in September, it was available for uptake by winter triticale plants soon after planting and again when plants began spring regrowth in late March. This potentially limited the movement of NO3–N to deeper in the soil profile and the loss of NO3–N below the rooting zone.

With two exceptions, post-harvest soil NO3–N levels were generally not affected by changes in the fertilizer N rates applied to the triticale (data not shown). One exception occurred at the 15- to 30-cm depth after corn silage at Lewis where the soil NO3–N was 7.9 kg ha–1 for the 66 kg N ha–1 rate compared with 4.8 kg ha–1 for the other three N fertilizer rates. Similarly, for 90 to 120 cm after corn silage at Lewis, the soil NO3–N for the 66 kg N ha–1 rate was 2.7 kg ha–1 compared with an average of 0.7 kg ha–1 for the other three N fertilizer rates. Soil NO3–N after growing triticale was lower than it was after the previous crop at all depths (Tables 4 and 5). Total reductions in soil NO3–N to a depth of 120 cm, as estimated by the difference in soil NO3–N before and after growing triticale, averaged 33 kg ha–1 after corn silage and 53 kg ha–1 after soybean at Ames and 45 kg ha–1 after corn silage and 52 kg ha–1 after soybean at Lewis. The 15 to 31 kg ha–1 soil NO3–N left after triticale harvest (Table 4) was considerably less than the 56 to 72 kg ha–1 NO3–N found after a rye cover crop in Minnesota (Strock et al., 2004). There was less than 3 kg ha–1 of NO3– N found at soil depths deeper than 60 cm after triticale harvest in July in our study.


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  		Table 5. Root-zone N budgets for winter triticale following corn or soybean at Ames and Lewis, IA, averaged for the 2003–2004 and 2004–2005 growing seasons. Soil sampling depth was 0 to 120 cm for 2003–2004 and 2004–2005 at Ames and 2003–2004 at Lewis. Soil sampling depth was 0 to 90 cm at Lewis in 2004–2005.

 
Nitrogen removal by the triticale crop at Ames increased with N fertilizer rates up to 66 kg ha–1. Similar trends occurred for triticale grown after corn and soybean at Lewis, but P values for the F test of N rate were >0.05 due to high plot-to-plot variability. There was also a trend toward greater estimated N loss with increasing N rates after both corn silage and soybean at the two sites, but differences among N rates were only significant at the P ≤ 0.05 level for triticale grown after soybean at Ames. There was no N rate x year interaction for N removal by the crop or estimated N loss. Average estimated N loss for triticale grown after corn silage at Ames was zero. The greatest estimated N loss was 37 kg ha–1 for triticale grown with 99 kg ha–1 after soybean at Ames. Estimated N loss was negative at all N fertilizer rates when triticale was grown after corn silage or soybean at Lewis. Estimated N losses of –44 to 6 kg ha–1 (Table 5) from growing triticale using the minimum N rates needed to maximize dry matter and grain production (Table 3) were much less than the 21 to 180 kg ha–1 annual losses of NO3–N from the upper 90 cm of the soil with continuous corn in Iowa (Karlen et al., 1998).

Our partial N budget (Table 5) does not include several potential N sources and losses. Nitrogen inputs not accounted for were NO3–N in precipitation (Hatfield et al., 1996) and N mineralization from the soil mineral fraction, organic matter, and crop residues (Schepers and Mosier, 1991). Nitrogen losses not included in the budget were surface runoff, denitrification (Parkin and Meisinger, 1989), NO3–N leaching below 120 cm in the soil, N in plant roots, loss from the plant through volatilization (Francis et al., 1993, 1997), and incorporation, cycling, and stabilization within the soil organic matter.

Based on observations by Hatfield et al. (1996) of 1.5 mg L–1 NO3–N concentrations in central Iowa precipitation, there would have been approximately 7 kg ha–1 addition of NO3–N from precipitation between the preplant and post-harvest soil samples in our study. Inorganic N mineralized from soil and crop residues in southwest Iowa was recently estimated as 75 kg N ha–1 yr–1 (Burkart et al., 2006). Nitrogen mineralization in our study may have been less than these annual estimates because there were only 300 d between sampling dates and mineralization rates increase with warmer temperatures (Jenkinson, 1990; Van Veen and Paul, 1981). Less N mineralization would be expected for late fall, winter, and spring months (periods included in our study) than late July, August, and early September (periods not included in our analysis).

The methods and field sites used in this study would have limited losses from surface runoff and denitrification (Dinnes et al., 2002). There are no literature reports of N capture by triticale roots; however, N content of wheat roots has been reported as 10 to 500f the shoot N content (Turpin et al., 2002; Andersson and Johansson, 2006; Youssef et al., 2006). These values should be viewed with caution, since accurate measurement of roots is particularly difficult and the amount of N in roots relative to shoots changes with soil conditions including changes in soil N status (Hoad et al., 2001). In a field study from Australia, Turpin et al. (2002) reported 48 kg N ha–1 in roots of wheat fertilized at 0 to 100 kg N ha–1 and producing grain yields of 1.6 to 4.8 Mg ha–1. Average plant N loss through volatilization among six winter wheat cultivars grown in Oklahoma was 8 to 16 kg ha–1 with 0 kg ha–1 N fertilizer and 13 to 26 kg ha–1 with the addition of 60 to 90 kg ha–1 N fertilizer (Kanampiu et al., 1997).

In research with winter rye cover crops, N removal was close to the reductions in residual soil NO3–N (Kessavalou and Walters, 1999). In our study, this appeared to be the case for winter triticale grown without N after corn silage or soybean at Ames. Nitrogen removal by the triticale grown without N at Lewis, however, was greater than the soil NO3–N reductions. This suggested that winter triticale captured considerable amounts of additional NO3–N mineralized from the soil during the growing season at Lewis. The 44 to 93 kg N ha–1 captured by winter triticale beyond that applied as fertilizer was equal to or greater than the 42 to 48 kg N ha–1 captured by a winter rye cover crop in Nebraska (Kessavalou and Walters, 1999).

Vyn et al. (1999) reported that aboveground biomass and N capture by cover crops as well as soil NO3–N levels in the spring were rarely increased by applying more fertilizer N to a small-grain crop in the previous season, but they could not determine the reasons for the lack of fertilizer response by the small grain. Our study may help explain their results. The absence of significant differences in soil NO3–N after winter triticale harvest in our study suggest that soil N status following a small-grain crop may be similar regardless of N rate. Dry matter and grain yields were maximized with 33 kg N fertilizer ha–1 at Ames and without additions of N fertilizer at Lewis. Nitrogen fertilizer in excess of that needed to maximize yields was removed by the crop, resulting in similar soil NO3–N status among N fertilization rates between 0 and 99 kg ha–1.

In winter wheat, increased N uptake has been identified as a mechanism limiting soil profile N accumulation when N rates exceed what is required for maximum grain yield (Raun and Johnson, 1995; Johnson and Raun, 1995). Excess N fertilizer can continue to be assimilated by the plant, resulting in increased grain protein, increased straw N, and plant N loss as volatilized NH3. These act as buffering mechanisms against accumulation of excess soil NO3–N (Raun and Johnson, 1995). This mechanism was clearly evident with winter triticale in our study and could be exploited to deplete residual soil NO3–N in the season after a corn silage or soybean crop. Soil NO3–N was not increased by N rates at least 66 kg ha–1 greater than needed to maximize grain yield and 33 kg ha–1 greater than needed to maximize dry matter production.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Growing winter triticale resulted in a low amount of residual NO3–N in the soil even when N fertilization rates were 66 kg N ha–1 greater than necessary for maximum triticale grain and dry matter production. The 15 to 31 kg ha–1 residual NO3–N after triticale was considerably less than the 48 kg ha–1 to 80 kg ha–1 after corn silage or soybean in our study and the 58 to 108 kg ha–1 after corn and soybean in southwest Minnesota (Strock et al., 2004). Managing for grain production would provide the greatest potential for N capture when compared with cover cropping or harvest as forage. Between 60 (Schwarte et al., 2005) and 100% (Gibson et al., 2007) of the N captured by triticale at the two Iowa sites used in our study was taken up, however, by the second week of May. This indicated that killing or harvesting triticale in mid-May would result in a considerable amount of NO3–N removal from the soil.

Our research demonstrated a depletion of soil NO3–N levels at N fertilization rates above those needed to maximize winter triticale production, but did not answer many questions related to soil NO3–N status during triticale growth. It does appear that N availability within the soil and plant demand for N may be better synchronized when growing winter triticale than when growing corn. This was especially true at Lewis, in southwest Iowa, where maximum triticale productivity was obtained with little or no addition of N fertilizer and estimated N loss was negative even with N fertilization of 99 kg ha–1. The average increase in soil NO3–N during the first 8 wk after fertilizer application to corn in southwest Iowa was approximately 100 kg N ha–1 (Karlen et al., 1998), indicating poor synchronization between N availability and uptake by corn plants during the period with the greatest potential for NO3–N loss from leaching.

Surplus and potentially leachable NO3–N levels for 26 watersheds in western Iowa, including the watershed containing the Lewis site in the current study, have been estimated at 18 to 43 kg N ha–1 yr–1 (Burkart et al., 2005, 2006). The greatest amounts of potentially leachable N were attributed to corn and soybean production on soils with high soil organic matter (Burkart et al., 2005). In continuous corn and corn–soybean rotations, most of the NO3–N leaching occurs during periods of high rainfall in the spring, when water percolates through the soil profile and is discharged into subsurface tile drains (Owens et al., 1995; Karlen et al., 1998; Kladivko et al., 2004). Drainage through tile lines typically stops in late July to early August (Karlen et al., 1998; Strock et al., 2004). The vigorous growth of winter triticale during April, May, and June has the potential to limit NO3–N loss through both N capture and water uptake by the plant. Assessment of soil NO3–N status, tile flow, and loss of NO3–N through drainage during April, May, and June was beyond the scope of the current study, but warrants further study. This would provide a more complete analysis of winter triticale's ability to reduce NO3–N leaching from crop fields of the U.S. Corn Belt.

The potential of intercropping triticale with red clover (Trifolium pretense L.) for minimizing soil NO3–N levels is another possibility for future study. The clover could provide N to subsequent crops when killed (Blaser et al., 2006; Vyn et al., 1999, 2000) and extend the time between successive corn and soybean crops while providing up to 19 mo of ground cover and soil NO3–N capture. This cropping system change may further limit N loss during production of the corn crop because N released from red clover crops is well synchronized with the N uptake pattern of corn (Stute and Posner, 1995).

Currently, winter triticale is at a disadvantage in U.S. farm policies when compared with corn and soybean because it is included only as a grazing crop. While grazing of winter cereal grains is commonly practiced in the southern Great Plains, it is not common in the Corn Belt. Production of triticale grain is not currently included in U.S. price support or crop insurance programs. Other major limitations to winter triticale production in the Corn Belt are susceptibility to fusarium head blight (Fusarium graminearum), a shortage of cultivars specifically adapted to the region, and a lack of established best management practices. To help solve these problems, future research and policy decisions should encourage the study and development of winter triticale to reduce soil NO3–N loss from U.S. cropping systems.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication July 18, 2006.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 




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