Effect of Tillage and Anhydrous Ammonia Application on Nitrogen Use Efficiency of Hard Red Winter Wheat

Correspondence: William R. Raun, 044 North Ag Hall, Department of Plant and Soil Sciences, Oklahoma State University, OK 74078; Telephone: (405) 744 6418; Fax: (405) 774 5269; E-mail: wrr@mail.pss.okstate.edu

ABSTRACT

INTRODUCTION

Soil erosion has been a major concern since the beginning of agriculture, but it was not until the Great Dust Bowl of the 1930s that the problem received worldwide attention. With so many deaths caused by black pneumonia and total crop destruction by wind-borne soil in massive volumes, measures were taken to make sure that this would never happen again. Zero tillage or No-till was originally used as a method to stop soil erosion. McGregor et al. (1992) reported that during a 5-year period (1987 through 1991), no-till soybean yielded 44% more than conventional-till yields. Intensive tillage has led to annual sediment discharge of 15.9 Mg ha^-1 in the southern Great Plains (Smith et al., 1991). Work by McGregor et al. (1992) showed increasing soil losses with time under conventional-till and decreasing soil loses with time for no-till. This continued study conducted over 14 years; noted no-till soybean yields exceeded those of conventional-till yields by 800 kg ha^-1 yr^-1 (McGregor et al., 1999). No-till reduced runoff 1 to 35% over conventional-till and reduced soil loss by 23 to 77% compared to conventional-till (McGregor et al., 1999). King et al. (1995) similarly found that long-term no-till practices are effective and practical in reducing rill erodibility and sediment loss.

There have been several other unforeseen advantages of no-till over conventional-till that researchers have discovered over time. Edwards et al. (1990) found that no-till improved soil drainage, while Weersink et al. (1992) stated that no-till reduces labor costs. Aase and Pikul (1995) found that in annual spring wheat production, no-till was the most efficient crop and soil management practice from the standpoint of yield, water use efficiency, soil organic C, and bulk density. However, many researchers have reported that bulk density increases under no-till versus conventional-till (Vyn and Raimbault, 1993), as well as that tillage has no effect on bulk density (Ismail et al., 1994). Alternatively, Lal et al. (1994) and Dao (1993) supported studies that reported no-till reduced bulk density. There have been other controversies comparing no-till to conventional-till systems as well as bulk density, one such argument being pH. Dick (1983) found that pH decreased under no-till as compared to conventional-till as nitrogen rates were increased, but Lal et al. (1994) found no effect of tillage on pH.

Research has indicated that soil organic matter content is related to amount of residue returned to the soil (Eghball et al., 1994; Christensen et al., 1994). Loss of soil organic C and N in the Great Plains has been caused by the use of tillage and summer fallow, which have accelerated organic matter decomposition rates and erosion. Ismail et al. (1994), Lal et al. (1994), and Christensen et al. (1994) reported that soil organic matter was greater under no-till and increased with time in some instances. Bauer and Black (1994) discovered that 1 Mg ha^-1 of soil organic matter contributed the equivalent of 15.6 kg ha^-1 of wheat grain yield. Further comparisons showed that no-till enhanced N immobilization and reduced nitrification rates when compared to conventional-till (Doran, 1980), often resulting in less nitrate leaching and leaving less nitrate in the soil profile (Lamb et al., 1985). Although there are lower nitrate levels in soil profiles in no-till systems, studies have shown that nitrate has been found deeper in the profiles of no-till soils (Eck and Jones, 1992).

The results of a 10-year study showed that N-mineralization rates were higher in annual cropping systems under no-till, than under conventional-till (Wienhold and Halvorson, 1999). Increased N stored as labile organic forms causes this increased mineralization. Increased amounts of organic N will supply more nitrogen to crops, which will result in less N required from fertilizers as well as reduced leaching. Wienhold and Halvorson (1999) also believe that higher N rates will increase immobilization because of the increased plant residue resulting from the higher N rates increasing the C/N ratio of the residue. Other studies have shown that immobilization was higher at lower applied N rates and crop N uptake was less with no-till systems (Smith and Sharpley, 1990; Knowles et al., 1993). Their research has also found evidence that immobilization of surface applied N fertilizers accounts for most of the differences in N response between no-till and conventional-till systems. Their research also shows that no-till systems required more N fertilizer when surface applied at lower rates. However, Fox and Bandel (1986) discovered that no-till increased mineralization compared to conventional-till during the latter part of the growing season. Rodriguez and Giambiagi (1995) found that no-till enhances denitrification, because of the increase in soil water supply commonly occurring in no-till, reducing the amount of aerobic activity in the soil. There are some conflicting views between Wienhold and Halvorson (1999) and the others stated above, but keep in mind that the Wienhold and Halvorson (1999) study was long-term (10 years), while the others were short-term (5 years or less). Wienhold and Halvorson were the only ones to account for the build up of soil organic matter (OM), and it would not be possible for soil OM to be a major factor in a short-term study.

Bonfil et al. (1999) found that no-till management over a 5-year study increased yields 62 to 67% in wheat-fallow rotations and 18 to 75% in continuous wheat over conventional-till in semiarid regions of Israel. Cantero-Martinez et al. (1999) and Peterson et al. (1996) found similar results in Australia and the Great Plains of the United States. No-till increases soil water absorption by reducing evaporation, increasing water infiltration, improving soil structure, which in turn enhances root development (Norwood, 1994; Merrill et al., 1996; Dao, 1993). Winter wheat is now being produced successfully and out-yielding spring wheat in the Northern Great Plains of the United States and Canada without requiring a fallow period, when no-till is used with adequate N fertilization (Halvorson et al., 1999). By increasing stored water in the soil, no-till has reduced the detrimental effects of climate variability on annual winter wheat production (Dao, 1993).

Studies have shown that deep placement of N can minimize volatilization losses and immobilization. Placement of N is a major factor of N utilization and a 20% increase in NUE has been observed with band placement, compared to surface broadcast (Tomar and Soper, 1981). They found that reduced N immobilization and increased N uptake could be achieved by reducing fertilizer contact with the surface residue. Rao and Dao (1996) found that final grain yield and grain N content were not affected by N placement in plowed plots. No-till improved grain yield by 32% for a below the seed row (BL) application and 15% for between the rows (BT) application. Grain N content was increased by 33% for BL and 25% for BT as compared to a surface broadcast application.

Surface applications of ammonium-based N can be lost to the atmosphere up to 70% from volatilization (Hamid and Mahler, 1994) and nitrate more readily leaches from the soil than ammonium; therefore, injected ammonium-based N has the greatest potential to increase NUE in single pre-plant applications. Some researchers have agreed that AA moves more in sandy soils with low CEC and low moisture than finer textured soils with high CEC, but under moist conditions and at depths over 10 cm, high rates of AA can be applied with little or no loss from volatilization (Swart et al., 1971). Swart et al. (1971) found that ammonia losses were reduced considerably when the applications were changed from 102 cm to 41 cm spacing (greater yields and less ammonia loss at 41 cm), but no differences between 15 cm and 41 cm. Swart et al. (1971) went on to report while vertical movement remains constant (4 to 5 cm) regardless of N rate, higher N rates usually cause greater lateral movement. Other research has suggested that AA decreases pH and depletes the amount exchangeable Ca and Mg leading to decreases in yield due to higher levels of aluminum accumulation (Bouman et al., 1995). Since AA is the cheapest form of commercial inorganic N fertilizer and most Oklahoma soils allow for deep injection, it is a widely used N fertilizer in Oklahoma accounting for 21% (24,483 Mg) of all applied N fertilizer in the 2004-05 crop year (Oklahoma Department of Agriculture, 2005, personal communication).

The objectives of this experiment were to determine the effects of tillage and AA application rate and placement on grain yield and NUE of hard red winter wheat.

MATERIALS AND METHODS

Two experimental sites were established in the fall of 2000, one near Stillwater, OK at the Agronomy Research Station (Easpur loam fine-loamy, mixed, superactive, thermic Fluventic Haplustoll), and one in Lahoma, OK at the North-Central Oklahoma Research Station (Grant silt loam fine-silty, mixed, thermic Udic Argiustoll). Initial soil test results are reported in Table 1. The experiment employed an N rate by N method factorial arrangement in a thrice replicated, randomized complete block design within each level of tillage. Individual plots measured 3.0 x 4.6 m.

Anhydrous ammonia (82-0-0) was applied at rates of 0, 61, 123, and 185 kg N ha^-1 using two different methods of injection. A rolling coulter applicator (DMI) with five knives spaced 46 cm apart at a depth of 15 cm, a method commonly used in nitrogen application of winter wheat, was used as one method of AA application. The Noble or undercutting blade (V-Blade), an experimental applicator, was used as the other method of AA application. The V-blade applicator has a single coulter, centered in front of the point of the undercutting blade, where AA was applied in 10 cm bands at a depth of 10 cm and a total width of 1.5 meters.

The winter wheat variety ‘Jagger’ was planted at both sites (planting and fertilizer dates are reported in Table 2). At the Lahoma site, a seeding rate of 95 kg ha^-1 was planted the initial year and increased to 125 kg ha^-1 the second year and was sustained through the third and fourth years in 19 cm rows within wheat stubble from the previous year as well as conventionally worked ground. Triple super phosphate (0-46-0) was applied pre-plant at a rate of 39 kg P ha^-1 the first two years at Lahoma to alleviate possible phosphorous deficiencies, however starting in 2002 at both locations, triple super phosphate was banded with the seed at a rate of 12 kg P ha^-1. At the Efaw site, a seeding rate of 125 kg ha^-1 was planted in 15 cm rows in grain sorghum stubble from the previous summer as well as conventionally worked ground the first two years, but beginning in 2002 the same seeding rate was planted in 18 cm rows within wheat stubble from the previous year as well as conventionally worked ground. In this case, conventional tillage at both sites consisted of disking throughout the summer, and preparing the seedbed with a field cultivator.

Wheat residue cover was measured before and after AA applications on the no-till starting in 2002, using a modified, line-intercept method (Morrison et al., 1997) as a means of determining residue disturbance (residue disturbance = residue cover (%) before AA – residue cover (%) after AA) by AA application method. In addition, plant counts were taken from a 0.54 m² area out of the center of each plot for evaluation of plant stands as a potential independent variable or covariate beginning in 2002. Furthermore, bulk density measurements using a cylinder (7.5 x 15 cm) and soil compaction measurements using an electronic cone penetrometer (SC 900 Soil Compaction Meter, Spectrum Technologies, Inc.) at 2.5 cm intervals, to a depth of 30 cm were taken post-harvest from each plot in 2004.

Wheat grain was harvested with a Massey Ferguson 8XP experimental combine, removing an area of 2.0 x 4.6 m from the center of each plot. A Harvest Master yield-monitoring computer installed on the combine was used to record yield data. Grain yield from each plot was determined and a sub-sample was taken for total N analysis. Grain samples were dried in a forced air oven at 66^oC, ground to pass a 140 mesh sieve (100 um), and analyzed for total N content using a Carlo-Erba NA 1500 automated dry combustion analyzer (Schepers et al., 1989). Using the difference method, NUE was calculated by subtracting the grain N uptake of the 0 N plot from the N fertilized plot grain N uptake, then dividing by the N rate applied to the N fertilized plot. Analyses of variance and single degree of freedom contrasts were performed using SAS (SAS, 1990).

RESULTS AND DISSCUSION

Grain Yield

Delayed planting due to inclement weather at planting time (Table 2) resulted in poor establishment and limited tillering and wheat yield responses to applied N were minimal in 2001 at both locations. However, grain yields were much higher in all three of the following years, particularly in 2003 and 2004 when response to applied N was high. While there were no differences between tillage systems at Efaw in 2001 and 2004, conventional tillage did result in significantly higher grain yields at Efaw in 2002 and 2003 (Table 3). Similar advantages in grain yield were observed for conventional tillage in 2001, 2002, and 2004 at Lahoma, but in 2003 the no-till treatments had significantly higher grain yields with three out of eight treatments averaging 5 Mg ha^-1 (Table 4).

Grain yield was significantly affected by N application method for the Lahoma no-till in 2002 when the V-blade applicator increased yield over the knife applicator (P < 0.001), otherwise there was no significant difference between the applicators on grain yield for either tillage system at either location. At Efaw a positive linear response to N using both applicators was observed in the no-till treatments in all four years (P < 0.05). A positive linear trend was also observed at Efaw in the conventional till treatments with the knife applicator in 2001 (P < 0.1) and 2004 (P < 0.001) and with the V-blade applicator in 2001 (P < 0.01). A quadratic grain yield response to N occurred with the V-blade applicator for conventional till in 2003 (P < 0.01) and 2004 (P < 0.01).

At Lahoma a positive linear grain yield response to N was observed with the knife applicator for no-till in 2001 (P < 0.05) and 2004 (P < 0.01) and for conventional till in 2003 (P < 0.1). Highly significant quadratic grain yield responses to N rate occurred at Lahoma with the knife applicator for no-till in 2002 (P < 0.05) and 2003 (P < 0.001), as well as in 2004 for conventional till (P < 0.001). Quadratic N responses for the V-blade application method occurred at Lahoma for no-till in 2001 (P < 0.01), 2002 (P < 0.05), and 2004 (P < 0.05), but not for conventional till except in 2003 (P < 0.05). Although not specifically measured, increased N immobilization was likely present in no-till plots as in previous research (Smith and Sharpley, 1990; Knowles et al., 1993). Highly significant interactions were found between tillage and N rate in 2001 (P < 0.001), 2002 (P < 0.01), and 2004 (P < 0.05) at Lahoma. The lack of an interaction at the Efaw site in all four years would imply that immobilization did not greatly affect N availability for wheat production, but wheat response to N was high in 2003 and 2004 as were grain yields. This indicates that N availability was only limited at Efaw during high yielding years possibly due to limited sinks of plant available N in the soil.

Nitrogen Use Efficiency

Nitrogen use efficiency was generally low for the first three years at Efaw and the first two years at Lahoma. At Efaw, NUE was significantly higher for no-till in 2003 when compared to conventional till (Table 5). Nitrogen use efficiency was significantly higher at Lahoma for the no-till in 2001, but in 2003 the conventional till was higher (Table 6). Although at Lahoma NUE response to conventional tillage was high for both application methods in 2003 and for the knife application method in 2004, no NUE response to N was observed for the V-blade application method in 2004. This indicates that experimental error effected NUE response to N for the V-blade application method and created the false-positive result of no-till significantly improving NUE in 2004. At Efaw, the knife application method improved NUE over the V-blade in no-till the initial year (P = 0.002), but there were no significant differences in NUE between the AA application methods in either tillage practice the following years. The V-blade applicator increased NUE over knife applied AA at Lahoma in the no-till during the 2001 (P = 0.011), 2002 (P < 0.001), and 2004 (P < 0.1) crop years.

A NUE quadratic response to N with the knife applicator was observed in the no-till during the 2003 crop year at Efaw (P < 0.02). At Lahoma, negative linear N responses to NUE occurred with the knife applicator in conventional till during the 2002 (P < 0.05), 2003 (P < 0.1), and 2004 (P < 0.01) crop years. Negative linear NUE responses to N were observed with the V-blade applicator at Lahoma in conventional till the first three years (P < 0.05) and in no-till during the 2001 (P < 0.01), 2002 (P < 0.05), and 2004 (P < 0.05) crop years. Significant interaction between tillage and N method was detected the initial year at Efaw (P < 0.001) as well as in 2002 (P < 0.001) and 2004 (P < 0.1) at Lahoma, further revealing that tillage practices affected applicator efficiency.

Additional Parameters

In light of conflicting results being evaluated for the first two years of data collection, additional parameters were measured starting in 2003. Of the four site years of residue coverage data, a significant difference was only found between the AA applicators at Efaw in 2003 where the knife applicator caused less disturbance than the V-blade (data not shown). Furthermore, residue disturbance averaged less than fifteen percent each year. However, while the modified, line-intersect method prevented potential bias and inaccuracy from using a visual estimate of residue coverage and disturbance, the modified method limited accuracy to plus or minus five percent. As a result, small differences in residue disturbance that occurred between the AA applicators may have been overlooked.

Significant differences in plant population were noted for tillage and N method at Efaw in 2003 with the no-till treatments having higher plant populations than the conventional till and V-blade application method having higher plant populations in both tillage practices. However, a linear relationship failed to exist between plant population and residue coverage at Efaw in 2003 and no significant differences were found at Efaw in 2004 or either year (2003-2004) at Lahoma (data not shown). This data indicates that while plant population was influenced by AA application method at Efaw in 2003, an additional soil characteristic other than residue coverage was improved by the V-blade applicator over the knife. Furthermore, no significant differences were found between the AA applicators for grain yield or NUE at Efaw in 2003, signifying that some other effect has caused the V-blade advantage since plant population did not influence either grain yield or NUE.

Bulk density was considerably lower at the Lahoma site compared to Efaw, but no significant differences in bulk density were found in the treatments at either location (data not shown). At Lahoma, the knife applicator had greater soil compaction between the 0-15 and 25-30 cm depths in the no-till and at the 7.5 and 22.5-25 cm depths in the conventional till compared to the V-blade (Figure 1). No significant differences resulted between the tillage systems at Lahoma. However, at Efaw no-till significantly reduced soil compaction at the 0-5 and 15-20 cm depths (Figure 2). Furthermore, the knife applicator had greater soil compaction between the 2.5-12.5 and 27.5-30 cm depths in no-till at Efaw compared to the V-blade, but no differences were found between the AA applicators in conventional till. No-till crop production reduced soil compaction at Efaw and the V-blade applicator reduced soil compaction within the no-till at both locations, but NUE was only improved with the V-blade in no-till at Lahoma and no-till only improved NUE at Efaw one of four years. However, across both tillage systems, the Efaw soil penetrometer data confirms the presence of a plow-pan or root-limiting layer between 1the 2.5 and 17.5 cm depths, characteristic of intensively tilled soil (Figure 2). Potentially this explains the limited effectiveness of the V-blade at Efaw and a possible component of the generally lower grain yields observed at Efaw compared to Lahoma. Soil compaction was measured by soil resistance in this study, which, as reported by Vaz et al. (2001), does not always relate well to bulk density due to multiple variables that influence soil resistance that does not necessarily affect bulk density.

CONCLUSIONS

Although the no-till system showed comparable grain yield at two site years and significantly higher in a third compared to conventional tillage, the latter had a distinct advantage in grain yield over the four-year duration of this study. Over time N response in conventional till increased tremendously along with grain yield and eventually exceeded no-till. Furthermore, the 61 kg ha^-1 N rate appeared optimum in conventional till while the 123 kg ha^-1 N rate appeared optimum in no-till, suggesting that no-till required higher additional N to maximize grain yield. The V-blade application method improved grain yield and NUE in no-till at one site, potentially due to reduced soil compaction, but neither AA applicator showed an advantage in conventional tillage. Previous crop residue disturbance averaged less than 15% for both AA applicators all four site years and residue disturbance did not influence the AA application effects. Mid-season plant populations taken during the 2003 and 2004 crop years were insignificant in three of the four site years and plant population did not influence grain yield and NUE. Although four years of data were collected in this study, additional data may be needed to properly evaluate no-till cropping systems. Over the four years of this study no-till reduced soil compaction and conventional tillage produced higher grain yields, but no conclusive advantages were found in NUE for either tillage practice or AA application method.

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Received:

Revised:

Accepted :

Table 1. Initial surface (0-15 cm) and sub-soil (15-30 cm) test results prior to experiment initiation at Efaw and Lahoma OK.

NH₄-N and NO₃-N – 2 M KCL extract; P and K – Mehlich-3 extraction; pH – 1:1 soil:deionized water

Table 2. Planting, fertilizer, and harvest dates at Efaw and Lahoma, OK, 2000-04.

Treatment				2001	2002	2003	2004	Avg.
Tillage	App/ Source	N rate kg N ha^-1		Grain yield ------------------------ (Mg ha^-1) ----------------------------
CT	Knife	0		2.26	3.57	3.40	3.03	3.12
		61		2.27	3.59	3.82	4.18	3.46
		123		2.50	3.80	3.78	4.48	3.64
		185		2.53	3.64	3.68	4.98	3.71
	Avg.			2.39	3.65	3.67	4.17	3.49
	V-blade	0		2.13	3.83	2.67	2.34	2.58
		61		2.48	3.73	3.89	4.29	3.60
		123		2.63	3.99	3.84	4.96	3.85
		185		2.61	3.76	3.87	4.91	3.79
	Avg.			2.51	3.85	3.57	4.13	3.51
NT	Knife		0	1.90	2.98	2.67	2.48	2.57
			61	2.29	3.42	3.50	3.56	3.19
			123	2.55	3.66	3.04	4.54	3.50
			185	2.66	3.81	3.50	4.81	3.67
	Avg.			2.35	3.43	3.18	3.85	3.23
	V-blade		0	2.10	2.94	2.70	2.49	2.53
			61	2.11	3.11	3.23	3.13	2.87
			123	2.48	3.25	3.40	4.40	3.38
			185	2.44	3.67	3.56	4.78	3.61
	Avg.			2.28	3.24	3.22	3.70	3.12
SED				0.13	0.27	0.12	0.36	---

SED is the standard error of the difference between two equally replicated means.

Treatment				2001	2002	2003	2004	Avg.
Tillage	App/ Source	N rate kg N ha^-1		Grain Yield ----------------------- (Mg ha^-1) -----------------------------
CT	Knife	0		1.77	3.98	3.63	2.25	2.97
		61		2.13	4.29	4.07	4.51	3.75
		123		1.29	3.92	4.71	4.31	3.56
		185		2.07	3.86	4.48	3.98	3.60
	Avg.			1.82	4.01	4.22	3.66	3.48
	V-blade	0		1.90	3.74	2.28	4.24	3.15
		61		2.45	4.07	4.31	4.27	3.73
		123		1.70	3.80	4.50	4.04	3.51
		185		1.97	3.94	4.65	4.15	3.68
	Avg.			2.01	3.91	3.93	4.18	3.53
NT	Knife		0	1.12	2.50	3.87	2.46	2.56
			61	1.43	2.92	5.03	3.48	3.16
			123	1.54	2.97	5.09	3.71	3.33
			185	1.87	2.60	4.53	3.94	3.24
	Avg.			1.49	2.79	4.63	2.86	3.10
	V-blade		0	0.78	2.62	4.64	1.39	1.85
			61	1.68	3.33	4.80	3.10	3.10
			123	1.89	3.62	4.96	3.78	3.56
			185	1.66	3.59	5.13	3.63	3.50
	Avg.			1.50	3.32	4.97	2.89	3.11
SED				0.31	0.20	0.42	0.46	---

SED is the standard error of the difference between two equally replicated means.

Figure 1. Effect of tillage and N method on soil compaction over four years for depths 0-30 cm at Lahoma, OK. SED is the standard error of the difference between two equally replicated means. SED by depth from 0 to 30 cm with an interval of 2.5 cm is 156, 200, 241, 262, 292, 307, 278, 297, 327, 331, 332, 341, and 348.

Figure 2. Effect of tillage and N method on soil compaction over four years for depths 0-30 cm at Efaw, OK. SED is the standard error of the difference between two equally replicated means SED by depth from 0 to 30 cm with an interval of 2.5 cm is 165, 200, 212, 222, 280, 283, 291, 241, 190, 190, 205, 207, and 199.