Precision Agric (2011) 12:146–163 DOI 10.1007/s11119-010-9163-8
Use of soil nitrogen parameters and texture for spatially-variable nitrogen fertilization H. Shahandeh • A. L. Wright • F. M. Hons
Published online: 4 March 2010 Ó Springer Science+Business Media, LLC 2010
Abstract Recent studies have demonstrated the potential importance of using soil texture to modify fertilizer N recommendations. The objective of this study was to determine (i) if surface clay content can be used as an auxiliary variable for estimating spatial variability of soil NO3–N, and (ii) if this information is useful for variable rate N fertilization of nonirrigated corn [Zea mays (L.)] in south central Texas, USA across years. A 64 ha corn field with variable soil type and N fertility level was used for this study during 2004–2007. Plant and surface and sub-surface soil samples were collected at different grid points and analyzed for yield, soil N parameters and texture. A uniform rate (UR) of 120 kg N ha-1 in 2004 and variable rates (VAR) of 0, 60, 120, and 180 kg N ha-1 in 2005 through 2007 were applied to different sites in the field. Distinct yield variation was observed over this time period. Yield and soil surface clay content and soil N parameters were strongly spatially structured. Corn grain yield was positively related to residual NO3–N with depth and either negatively or positively related to clay content depending on precipitation. Residual NO3–N to 0.60 and 0.90 m depths was more related to corn yield than from shallower depths. The relationship of clay content with soil NO3–N was weak and not temporally stable. Yield response to N rate also varied temporally. Supply of available N with depth, soil texture and growing season precipitation determined proper N management for this field. Keywords Spatial soil N variability Residual NO3–N Soil texture Variable and uniform N rates Corn grain yield
H. Shahandeh (&) F. M. Hons Department of Soil and Crop Science, Texas A&M University, College Station, TX 77843-2474, USA e-mail:
[email protected] A. L. Wright Everglades Research and Education Center, University of Florida, 32 00E. Pal Beach Road, Belle Glade, FL 33430-4702, USA
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Introduction To improve N management in cropping systems, similar N fertilization rates should be applied to homogeneous sub-regions of a field that have similar yield limiting factors (Khosla et al. 2002; Koch et al. 2004). This practice is likely to achieve the greatest benefit when information about soil N reserves in surface and subsurface soil is also available (Eghball et al. 1997; Schmidt et al. 2002). Information about soil N reserve is usually obtained through a single criterion like soil NO3–N in surface and/or subsurface samples. The current recommended method for determining N fertilization for crop production in Texas involves soil testing for residual NO3–N in surface soil (0–0.15 m) and integrating with the anticipated yield goal for uniform application to the soil (McFarland et al. 1990). It has recently been suggested that to better determine the efficacy of variable-rate N fertilization and its contribution to yield, the spatial variation in NO3–N accumulated below 0.15 m depth should also be assessed (Katsvario et al. 2003; Shahandeh et al. 2005). However, some experiments show that residual soil NO3–N with depth is not enough and information on other soil N parameters, like mineralizable soil N, is also needed for variable rate N management (Schmidt et al. 2002; Eghball et al. 2003). For practical importance, knowledge about relationships between yield, soil properties, and soil N parameters is highly desirable. Evaluating N supply parameters for N fertilization in the field is time consuming and expensive, but if information on N supply is related to other soil physical and chemical properties within the field, these relationships can have significant importance for variable rate N fertilization with respect to cost and ease of measurements (Mamo et al. 2003; Baxter et al. 2003). Within-field yield variation has been attributed to changes in landscape position, nutrient availability, soil chemical and physical properties, cropping history and soil type (Inman et al. 2005; Baxter et al. 2003; Delin and Linde´n 2002; Pierce and Nowak 1999; Sawyer 1994; Wibawa et al. 1993). These attributes are known to be prime factors for variable rate nutrient technology (Machado et al. 2002). For, example, if soil surface clay content is related to soil N supply, then estimating clay content would be a more economical alternative for describing soil N supply and spatial distribution since it is less variable in time and could be determined accurately and for relatively low cost (Han et al. 2003; Chen et al. 2004). Clay content measured with only limited sampling for NO3–N has been suggested as a way to infer and estimate N availability for future N fertilization (Cox et al. 2003). The objectives of this study were to evaluate relationships between corn yield, soil N parameters and soil texture in a spatially variable field. Relationships between soil N supply with texture were used to assess the potential of variable rate N fertilization for nonirrigated corn production in Central Texas over time.
Materials and methods Experimental site Research was conducted in a 64 ha field adjacent to the Brazos River at the Texas AgriLife Research Farm in Burleson County near College Station, TX (30°320 5300 N, 96°250 2800 W) from 2004 to 2007 (Fig. 1a). The site had been managed under minimum tillage since 1996 and was planted to corn prior to this study. The alluvial soil used for the study is an intergrade of Weswood silt loam (fine-silty, mixed superactive, thermic Udifluventic
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WwA
A
WwB
A ShA
A WeA
Legend WwA WeA
Weswood silty clay loam, 0 to 1% slope Weswood silt loam, 0 to 1% slope
ShA WwB
Exp. Site in:
B
Ships clay, 0 to 1% slope Weswood silty clay loam, 1 to 3% slope
N Applied, kg ha-1
2004
UR, 120
2005-2007
VAR, 0-180
B
50 m
0
250 m
Fig. 1 Texas A&M research farm (a) and experimental site (b) in Burleson County near College Station, TX from 2004 to 2007
Haplustepts) and Ships clay (very fine, mixed, active, thermic Chromic Hapluderts) with pH of 7.9–8.1 (Fig. 1a). A point grid was laid out across this field in 2004 with 50 m between grid points in north and east directions using a Trimble GPS Pathfinder Pro XRS (Trimble, Sunnyvale, CA, USA) (Fig. 1b). The 64 ha field was planted in 0.91 m rows with corn variety Dekalb 687 (Monsanto, St. Louis, MO) on 12 March in 2004, 21 March
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in 2005, 2 March in 2006 and 7 March in 2007 with a Case/IH Early Riser planter (Racine, WI, USA) at a rate of *65 000 seed ha-1. BicepTM herbicide (metolachlor/atrazine) was used for weed control, along with in-season cultivation. The corn received a uniform N rate of 120 kg N ha-1 in 2004. The N source was urea ammonium nitrate solution (32-0-0) that was knifed into the furrow midway between plant rows at the six-leaf growth stage (V6) (Iowa State University 1993) using an eight-row cultivator. One hundred grids (plots) were superimposed on top of the 64 ha experimental field with each grid being 8 rows wide and 0.50 m long (Fig. 1b). Based on results from the 2004 study on the spatial structure of corn grain yield (Fig. 2a), field elevation (Fig. 2b), soil texture (Fig. 3), and NO3–N concentration with depth (Fig. 4), locations at the upper and lower elevation portions of a field section that comprised *20 ha were selected for variable rate N fertilization in 2005–2007 (Fig. 1b). The upper segment was located at higher elevation with lower clay content and higher residual soil NO3–N with depth and the bottom segment was located at lower elevation with higher clay and lower residual soil NO3–N content. Variable-rate N strips (0, 60, 120, and 180 kg ha-1) in three replications were applied to these sites in 2005–2007. Strips were 8 rows wide 9 150 m long. Soil and plant measurements Soil samples were collected to 0.90 m depth using a tractor-mounted hydraulic soil sampler near the center of each grid point before corn fertilization or after harvest in April or September of each year. Two cores were taken at 1-m radii from each grid point center and were sectioned into depths of 0–0.15, 0.15–0.30, 0.30–0.60, and 0.60– 0.90 m and composited with depth. Samples were dried in a forced-draft oven at 50°C, then ground with a flail-type soil grinder (Custom Lab, Orange City, FL, USA) to pass a 2-mm sieve. Soil N mineralization (Nmin), C mineralization, soil organic C (SOC), soil total N, and soil texture were determined on 0–0.15 m soil samples. Residual NO3–N and other plant essential nutrients were determined on soil samples from all depths. Soil C and N mineralization were determined according to Franzluebbers et al. (1994a, b). Approximately 20 g of oven-dried soil were placed in 50-ml beakers, wetted to -0.03 MPa, and incubated at 25°C in air-tight containers along with a vial containing 10 ml of 1.0 M KOH and another vial containing water to maintain humidity. Vials of KOH were replaced at 1, 10, and 24 d. Mineralized C as CO2 was determined at each sampling date by titrating the KOH with 1.0 M HCl to the phenolphthalein endpoint (Anderson 1982). Soil NH4– and NO3–N at 0 and 24 d were extracted with 2 M KCl and determined using autoanalyzer techniques (Technicon Industrial Systems 1977a, b). Initial inorganic N was subtracted from that measured at 24 d to determine net soil Nmin. Soil organic C was determined using the modified Mebius method (Nelson and Sommers 1982), while soil total N was determined by autoanalyzer techniques (Technicon Industrial Systems 1977a) following Kjeldahl digestion (Nelson and Sommers 1980). Soil particle size distribution was determined on all samples using the procedure of Day (1965), which utilizes hydrometer analysis following dispersion of soil by both chemical and physical means. A 3 m length of each of the middle two rows of each grid were hand-harvested for grain yield in August at maturity and shelled using a stationary plot sheller, before using a combine equipped with a calibrated Ag Leader PF3000 yield monitor with elevator mounted sensor (Ag Leader Technol., Ames, IA, USA) and a differential global
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Yield, t ha-1 Yield 11.8-13.1 10.5-11.8 9.2-10.5 7.8-9.2 6.5-7.8 5.2-6.5 3.9-5.2 2.6-3.9 1.3-2.6 0-1.3
11.8
A
Ela
Elevation, m 69.4-70.0 69-0-69.4 68.7-69.0 68.3-68.7 68.0-68.3 67.6-68.0 67.2-67.6 66.8-67.2 66.5-66.8 66.2-66.5
B
50 m
0
250 m
Fig. 2 Yield (a) and elevation contour maps (b) of the 64 ha in 2004
positioning system receiver to harvest the remainder of the field. Grain moisture was determined by electrical resistance and yields were calculated at a moisture content of 140 g kg-1.
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Corn Yield, kg ha-1 , 2004
kg ha-1 350 13500 13000 12500 12000 11500 11000 10500 10000 9500 9000 8500 8000 7500 7000 6500 6000 5500 5000 4500 4000 3500 3000 2500
300
Northing, m
250
200
150
100
50
0 0
50
100
150
200
250
300
350
400
450
Clay, %
%
350
68 66 64 62 60 58 56 54 52 50 48 46 44 42 40 38 36 34 32 30 28 26 24 22 20
300
Northing, m
250
200
150
100
50
0 0
50
100
150
200
250
300
350
400
450
Easting, m Fig. 3 Kriged contour maps of yield and clay content in the 20 ha field in 2004
Statistical and spatial variability analyses Correlation and spatial statistics were used to relate surface and profile residual soil NO3– N, soil Nmin, and other soil characteristics with corn grain yield. Geostatistical methods, variography and kriging (Isaaks and Srivastava 1989) were used to map variability of soil and plant parameters. Geostatistical software (GS? v5.0, Gamma Design Software,
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kg ha-1
350
36 35 34 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10
Northing, m
300 250 200 150 100 50 0 0
50
100
150
200
250
300
350
400
Total N, 2004
kg ha-1 125 120
300
115 110
250
105 100 95
200
90 85
150
80 75 70
100
65 60 55
50
50 45
0
450
40
0
mg kg-1
350
NO3-N-90 cm, 2004 350
50
100
150
200
250
300
350
400
Mineralized N-24d, 2004
450
mg kg-1
350
2200 2100
300
100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5
300
Northing, m
2000 1900
250
250
1800 1700
200
1600 1500
150
1400
200 150
1300
100
1200
100
1100
50
1000
50
900
0
800
0
50
100
150
200
250
300
Easting, m
350
400
450
0 0
50
100
150
200
250
300
350
400
450
Easting, m
Fig. 4 Kriged contour maps of soil N properties, residual soil NO3–N to 0.15 and 0.90 m depths, total N and Nmin in the 20 ha field in 2004
St. Plainwell, MI, USA) was used to analyze the spatial structure of the data and to define semi-variogram parameters. Contour maps of corn yield distribution and clay content were produced in Surfer (version 8, Golden Software, Golden, CO, USA) based on grid files created from the kriged values from GS?. A detailed description of analysis is presented in Shahandeh et al. (2005).
Results and discussion Spatial variability of corn grain yield in 64 ha field with uniform rate of N fertilization The yield map from data generated by the combine equipped with a yield monitor showed distinct spatial variability of corn yield within the 64 ha corn field in 2004 (Fig. 2a). The yield map is presented as a contour map with 1.2 t ha-1 contour intervals and with colored legends representing green for high and red for low values. Yield varied from about 13 to \2 t ha-1 and closely followed the elevation distribution map of the field generated by the mounted elevation sensor (Fig. 2b). High yields were obtained at higher elevation and low yields at lower elevation. However, to find the true spatial variation of yield in the field, information on soil-landscape features like elevation may not be enough and determination of plant and soil N properties at a finer scale may be required (Dobermann and Ping 2004; Scharf et al. 2006).
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Spatial variability of corn grain yield, soil nitrogen parameters and texture in 20 ha field A sub-field of about 20 ha in the upper section of the 64 ha field was selected to determine detailed spatial variability (Fig. 1b). Descriptive statistics and spatial variability of parameters measured for this sub-field are shown in Table 1. Yield and soil properties were highly variable in this 20 ha sub-field. For example, surface clay content varied from 18 to 67% and residual NO3–N concentration to 0.15 m depth varied from 7 to 59 kg ha-1. The coefficient of variation (CV) used for measuring spatial variability of plant and soil properties ranged between 26 to 52% (Table 1). The lowest variation was observed for soil total N, organic N and NO3–N concentration to 0.90 m depth, and the highest variation was noted for N mineralized at 24 d. The CV values reported for soil N parameters were similar to CV values reported in other spatially variable fields (Cambardella et al. 1994; Mahmoudjafari et al. 1997; Shahandeh et al. 2005). For example, Nmin had a CV of 52% with a mean concentration of 31 mg kg-1 and a range of 9–96 mg kg-1. The high CV for Nmin may have resulted from non-homogeneous incorporation of variable corn residues produced and/or incorporation of residue into a non-homogeneous soil (Fig. 1 and Table 1) (Rover et al. 1999). Variable yield and the associated variable residue produced support these possibilities. Mean corn grain yield was 8 350 kg ha-1, but varied from 1 783 on the west side to 12 548 kg ha-1 on the east side of field (Fig. 2 and Table 1). To characterize the structure of spatial variability in the field, variograms and spatial distribution maps of yield, soil clay content, and soil N parameters (NO3–N to 0.15 and 0.90 m depths, total N and Nmin) were constructed (Figs. 3 and 4). In each figure, light shading represents higher values while darker shading is associated with lower values. Grain yields and soil N characteristics were generally higher in the eastern portion of the field and lower in the western portion. In contrast, surface clay content (0 to 0.15 m)
Table 1 Descriptive statistics of parameters measured in the 20-ha plot within the 64 ha corn field in 2004 Parameter
Mean
Maximum
Minimum
Grain yield (kg ha-1)
8 350
12 548
1 783
34
Total N (mg kg-1)
1 151
2 160
550
26
Organic N (mg kg-1)
1 147
2 156
548
26
6
17
5
27
NH4–N at 24dincubationa (mg kg-1)
CV
NO3–N at 24dincubation (mg kg-1)
37
92
18
36
N mineralized at 24dincubationb (mg kg-1)
31
96
9
52
NO3–N to 0.15 m depth (kg ha-1)
24
59
7
39
NO3–N to 0.30 m depth (kg ha-1)
45
89
17
30
NO3–N to 0.60 m depth (kg ha-1)
68
117
29
27
NO3–N to 0.90 m depth (kg ha-1)
82
143
40
26
SOC (mg C g-1)c
11
27
8
28
Clay (%)
45
67
18
30
a
24dincubation = NH4–N, NO3–N, produced after 24 days of incubation
b
N Mineralized at 24d = incubated inorganic N at 24 d for 0.15 m depth minus initial inorganic N for 0.15 m depth
c
SOC, soil organic C
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tended to be higher in the western direction and followed a trend opposite to yield and N parameter distribution. In general, yield and soil N parameters were positively related with elevation, but were negatively related with clay content. Relationship between corn grain yield, soil nitrogen parameters and soil texture To better illustrate relationships in 2004 between corn grain yield, surface clay content and soil N parameters, yields [10 000 kg ha-1 and clay contents of \40% were separated and the contour lines for 40% and 10 000 kg ha-1 made bold in Fig. 3. This graphical separation tended to divide soil and plant properties in the 20 ha sub-field into higher (top) and lower (bottom) landscape positions. Table 2 shows the correlation coefficients for relationships between yield, soil N parameters and surface clay content in the 20 ha field and in the smaller top and bottom segments in 2004. Correlation coefficients for the 20 ha field indicated that grain yield was positively related to NO3–N concentration and Nmin, and negatively related to surface clay content. Nitrate N concentrations with depth were generally highly correlated and the highest correlation coefficients were at deeper depths. However, NO3–N concentration was not related to clay content at any depth. Clay content was positively related to SOC and Nmin. Clay protection of adsorbed organic compounds may partially explain this result. Similar results were reported by Shahandeh et al. (2005) and Johnson et al. (2003). Relationships between grain yield, N parameters and clay content were somewhat different, however, when analyzed separately for top or bottom field segments. For example, stronger relationships between clay content, yield, and mineralized N were observed in the bottom field segment, while clay content only was related to soil NO3–N to a depth of 0.90 m in the top segment (Table 2). In general, stronger relationships between N parameters and yield were observed in the top field segment compared to the whole field or bottom segment. The different relationships between clay content and N parameters in this field support Pierce and Nowak’s (1999) argument that N in soil will vary spatially with clay, soil N supply and organic matter content. Higher soil N supply (residual NO3–N with depth, Nmin, and total N) was generally associated with lower clay content in the top field segment, and lower N supply with higher clay content in the bottom segment (Tables 1 and 2). Kriged maps (Figs. 3 and 4) also supported a close spatial relationship between corn grain yield and soil texture and N supply parameters in the top and bottom field segments. When similar spatial structure exists, there is a possibility that results from one variable can be inferred from other properties (Baxter et al. 2003; Han et al. 2003). One approach to evaluate whether similar spatial structure exists is to apply variable N rates in more homogeneous sub-regions of the field. It has been suggested that it is preferable to evaluate variable N rates in areas that possess homogeneous attributes in landscape and soil conditions (Schepers et al. 2004; Franzen et al. 2002; Khosla et al. 2002; Diker et al. 2004). Variable rate N fertilization in homogeneous sub-regions and its relation to N supply over time Continued uniform application of N would probably have resulted in over application of N in the eastern areas of the field and under-fertilization in other parts of the field (Figs. 3 and 4). Variable rate N fertilization might help optimize grain yield in this field, but the success of variable N rate fertilization will also depend on the ability to predict and define the
123
Total N
NO3–N to 0.90 m
NO3–N to 0.60 m
NO3–N to 0.30 m
NO3–N to 0.15 m
Grain yield
Bottom field
N mineralized
SOC
Total N
NO3–N to 0.90 m
NO3–N to 0.60 m
NO3–N to 0.30 m
NO3–N to 0.15 m
Grain yield
Top field
N mineralized
SOC
Total N
NO3–N to 0.90 m
NO3–N to 0.60 m
NO3–N to 0.30 m
NO3–N to 0.15 m
Grain yield
Whole field
Parameters
–
0.36**
0.23*
NO3–N 0.15 m
0.63***
–
0.84***
–
0.84***
–
NO3–N 0.30 m
0.61***
0.40**
0.30*
0.89***
0.61***
0.32*
0.81***
0.59***
0.25*
NO3–N 0.60 m
0.63***
0.41**
0.40**
–
0.94***
0.71***
0.54***
0.50***
0.63***
0.51***
0.39**
0.41*
NO3–N 0.90 m
–
–
–
–
–
0.71***
0.30*
–
0.60***
0.39**
–
–
–
–
–
Total N
0.49***
–
–
–
–
–
0.89***
–
–
–
–
0.45**
0.54***
–
–
–
–
–
SOCa
0.43**
–
–
–
0.22*
–
0.66***
0.73***
0.54***
0.32*
–
0.34*
0.52***
0.38**
0.60***
–
–
–
0.28*
0.37**
Nmin at 24 d
–
–
–
–
–
-0.66***
–
–
–
0.29*
–
–
–
–
0.28*
0.37**
–
–
–
–
–
-0.47***
Clay
Table 2 Pearson correlation coefficients for corn grain yield and soil N parameters for the whole 20 ha corn field and smaller higher and lower elevation segments in 2004
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123
NO3–N 0.15 m
NO3–N 0.30 m
NO3–N 0.60 m
NO3–N 0.90 m
a
SOC, soil organic C
* Significant at the 0.05 level; ** Significant at the 0.01 level; *** Significant at the 0.001 level
‘‘–’’ indicates nonsignificant
N mineralized
SOC
Parameters
Table 2 continued Total N
SOCa –
Nmin at 24 d
-0.41**
–
Clay
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Table 3 Descriptive statistics of parameters measured for N rate transects in the upper elevation segment of the 20 ha corn field experiment in 2005, 2006, and 2007 Parameter
N transect
Mean (kg ha-1)
0 kg N ha-1
6978 a
7283
6250
6.7
60 kg N ha-1
6993 a
7362
6394
5.9
120 kg N ha-1 7190 a
7489
6433
5.5
180 kg N ha-1 7252 a
7817
6893
5.3
107
24
6.6
Maximum (kg ha-1)
Minimum (kg ha-1)
CV (%)
2005 Grain yield
NO3–N to 0.90 m 120 kg N ha-1 65 2006 Grain yield
0 kg N ha-1
9940 a
10407
9092
15.8
60 kg N ha-1
10307 ab
10714
9161
12.3
120 kg N ha-1 10669 b
11136
10677
5.3
180 kg N ha-1 10926 b
11696
11111
6.1
88
19
9.7
NO3–N to 0.90 m 120 kg N ha-1 45 2007 Grain yield
0 kg N ha-1
6579 a
6704
4213
14.3
60 kg N ha-1
9393 b
11600
7102
17.0
120 kg N ha-1 11058 c
12055
10710
5.8
180 kg N ha-1 11886 c
12987
11038
4.3
71
15
5.6
31.0
22.0
9.7
NO3–N to 0.90 m 120 kg N ha-1 33 Clay (%)
All
27.0
Means within a column and characteristic followed by the same letter are not significantly different (LSD0.05)
magnitude of the dynamics of soil N supply with depth over time (Mamo et al. 2003; Khosla et al. 2006). To determine corn response to variable N rate over time, studies were conducted in more homogeneous soil textural locations in the higher and lower elevation field segments in 2005–2007 (Fig. 2b). The top segment was located at higher elevation with greater yield, higher residual NO3–N with depth, and lower clay content (22–31% clay, CV = 9.7%) (Tables 1, 2, 3, 4; Figs. 2, 3, 4). The bottom segment was located at lower elevation with lower yield, lower residual NO3–N with depth, and higher clay content (54–68% clay, CV = 5.8%). Descriptive statistics for corn yield and residual NO3–N to 0.90 m depth for higher and lower elevation field segments are given in Tables 3 and 4, respectively. Similar to 2004 results, corn grain yield was greater in the higher than the lower elevation segment of the field during 2005 to 2007. Corn grown in higher and lower elevation segments also responded differently to variable N rate fertilization in each year. For example, there was no response to N fertilization in the higher elevation portion of the field in 2005. To achieve maximum yield in the higher elevation field segment, no N fertilization was needed in 2005, while 120 kg N ha-1 was required in 2006 and 2007. In the higher elevation field segment, the yield increase from the highest rate of applied N (180 kg N ha-1) was only 274 kg ha-1 (from 6 978 to 7 252 kg ha-1) in 2005, but was 5 307 kg ha-1 in 2007 with greater precipitation (Fig. 5) and lower residual NO3–N.
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Table 4 Descriptive statistics of parameters measured for N rate transects in the lower elevation segment of the 20 ha corn field experiment in 2005, 2006, and 2007 Parameter
N transect
Mean (kg ha-1)
0 kg N ha-1
1806 a
2300
892
44.0
60 kg N ha-1
2518 b
3158
1533
33.3
120 kg N ha-1 3715 c
4014
2741
24.1
180 kg N ha-1 4531 d
4689
2900
21.0
75
16
30.0
Maximum (kg ha-1)
Minimum (kg ha-1)
CV (%)
2005 Grain yield
NO3–N to 0.90 m 120 kg N ha-1 32 2006 Grain yield
0 kg N ha-1
6469 a
7548
5156
11.4
60 kg N ha-1
8091 b
9668
7124
9.6
120 kg N ha-1 8404 b
9286
7815
9.5
180 kg N ha-1 9107 bc
10028
8250
7.5
65
15
9.5
NO3–N to 0.90 m 120 kg N ha-1 28 2007 Grain yield
0 kg N ha-1
3308 a
4519
2136
22.6
60 kg N ha-1
6020 b
6909
4310
13.2
120 kg N ha-1 8384 c
8874
7805
6.5
180 kg N ha-1 9227 d
9983
9034
3.9
40
12
11.4
68.0
54.0
NO3–N to 0.90 m 120 kg N ha-1 21 Clay (%)
All
61.0
5.8
Means within a column and characteristic followed by the same letter are not significantly different (LSD0.05) 0.30
Corn Growing Season Rainfall Apr-May-Jun
Precipitation, m
0.25
Mar-Jul 50 yr. Ave.
0.20
0.15
0.10
0.05
Jun-07
May-07
Jul-06
Mar-07
Jun-06
Apr-06 May-06
Jul-05
Mar-06
Jun-05
May-05
Jul-04
Mar-05 Apr-05
May-04 Jun-04
Mar-04
0.00
Date Fig. 5 Growing season precipitation (March–July) during 2004 to 2007 near College Station, TX
Corn yield response to N in the lower elevation segment was very different than that observed for the higher elevation segment, especially in 2005 (Table 4). In fact, a significant grain yield response to the first increment of N fertilizer was observed in all years
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in the lower elevation segment. Yield responded positively to the greatest added N rate in both 2005 and 2007. Corn in the lower elevation segment may have been more responsive to N fertilization due to lower residual soil NO3–N in this section. The temporal response to N supply in higher and lower elevation field segments might have been caused by different weather conditions (Fig. 5). The 50-year average seasonal rainfall was 0.415 m and growing season rainfall was 0.627, 0.375, 0.413, and 0.606 m for 2004, 2005, 2006, and 2007, respectively. Based on the fifty-year rainfall average, corn growing season rainfall was divided into wetter (2004 and 2007), drier (2005) and average (2006) rainfall seasons. Researchers have demonstrated that a significant interaction can exist between crop N response and moisture availability (Mamo et al. 2003; Machado et al. 2002). Available soil moisture was likely influenced by both clay content and elevation. Average clay content was 27% in the higher elevation segment and 61% in the lower elevation segment. The higher clay content in the lower elevation segment could have influenced the amount of plant available water for grain production later in the season, especially in a drier year. In the drier season (2005), N application had no significant effect on yield in the higher elevation field segment. Nitrogen applied to the lower elevation segment, however, had a positive effect on yield. Schepers et al. (2004) and Kravchenko and Bullock (2000) reported a similar positive relationship between yield and moisture at lower elevation in bottom lands during dry years. However, in the wetter year (2007), the dominant factor influencing corn grain yield was probably soil N supply with depth. For example, corn grain yield with 180 kg N ha-1 was about doubled (from 6 579 to 11 886 kg ha-1) in the higher elevation field segment, and almost tripled (from 3 308 to 9 227 kg ha-1) in the lower elevation segment in 2007 (Tables 3 and 4). Yield increases in the average rainfall year (2006) were also influenced by N fertilization. Corn grain yield that year was relatively high in part due to the large amount of rainfall in June, which was about 0.05 m above the 50 year average (Fig. 5). In general, corn produced the highest grain yield in both higher and lower elevation field segments when 120 or 180 kg N ha-1 were applied in a wetter year. Reasons for the significant response to higher N rates in the wetter year were the decrease in residual soil NO3–N with time (Tables 3 and 4) and greater water available for growth. Mean residual NO3–N to 0.90 m depth was 33 and 21 kg ha-1 in 2007 versus 65 and 32 kg ha-1 in 2005 for higher and lower elevation field segments, respectively. Machado et al. (2002) similarly found that in wet years the most limiting factor for corn production was NO3–N supply with depth. Nitrogen may also be lost in the bottom segment in wetter years because of higher clay content. Schepers et al. (2004) and Kravchenko and Bullock (2000) reported crop N stress in lower areas during wet seasons partly because of N loss through leaching and/or denitrification associated with excess water. The CV of yield within the N transect is an indicator of the interaction of corn grain yield response to variable N rate and growing season precipitation. Kravchenko et al. (2005) found that variability of corn yield response to added N can increase in high rainfall years. However, in our experiment, the greatest yield variation was observed in transects with 0 kg N ha-1 in the drier year (CV 44.0%) in the lower elevation field segment and potentially may be related to its clay content (Cox et al. 2003). The least variation in yield was observed in transects receiving 180 kg N ha-1 in either higher (CV 4.3%) or lower (CV 3.9%) elevation field segment in the wetter year. In general, less yield variation was observed as N rates increased in the wetter year in this experiment. Correlation coefficients between corn grain yield and NO3–N with depth and clay content in transects receiving 120 kg N ha-1 in higher and lower elevation field segments
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Table 5 Pearson correlation coefficients for corn grain yield, surface soil clay content and residual NO3–N with depth in 120 kg N ha-1 transects in top and bottom segments of the 20 ha field in 2005, 2006, and 2007 Parameter
Landform segment
Correlation (r) NO3–N 0.15 m
NO3–N 0.30 m
NO3–N 0.60 m
NO3–N 0.90 m
Clay
2005 Grain yield
Higher
0.340**
0.365**
0.348**
0.312**
–
Clay
Higher
0.362*
0.269*
–
–
–
Grain yield
Lower
0.362**
0.269*
–
–
0.301*
Clay
Lower
–
–
–
–
–
Grain yield
Higher
0.289*
0.451**
0.460**
0.512***
–
Clay
Higher
0.355*
–
–
–
–
Grain yield
Lower
0.509***
0.557***
0.476**
0.468**
–
Clay
Lower
–
–
–
–
–
Grain yield
Higher
0.678***
0.746***
0.776***
0.772***
–
Clay
Higher
–
–
–
–
–
Grain yield
Lower
0.518***
0.580***
0.559***
0.584***
-0.521***
Clay
Lower
–
–
–
–
–
2006
2007
*, **, *** Significant at the 0.05, 0.01, or 0.001 level
were calculated for 2005, 2006 and 2007 (Table 5). Corn grain yield was significantly and positively related to residual NO3–N in all years regardless of field segment or landform conditions. The highest correlations for corn grain yield and NO3–N were obtained at deeper depths of either 0.60 or 0.90 m in the higher elevation field segment in 2007. The high correlation of corn grain yield and residual NO3–N in this experiment supported Kravchenko’s et al. (2005) findings that yield response to N could increase in higher rainfall years. The lowest correlations between corn grain yield and NO3–N with depth were observed in both higher and lower elevation segments in the drier year. The relationship between corn grain yield and soil clay content was not consistent in either higher or lower elevation field segments across years (Table 5). For example, corn grain yield in the higher elevation field segment containing less clay had no relation with clay content in 2005, 2006, or 2007. In the lower elevation segment, however, corn yield had a positive, no, or a negative relationship with clay content in 2005, 2006, and 2007, respectively. Corn yield in the lower elevation segment was negatively related to clay content in the wetter year and positively related in the drier year. Clay content was not related to residual soil NO3–N at any depth in the lower elevation segment in any year (Table 5). Clay content in the higher elevation field segment was positively related with residual NO3–N to 0.30 m depth in 2005, to 0.15 m in 2006, and showed no relationship in 2007. Clay content was related to residual soil NO3–N to 0.90 m depth, Nmin and SOC in the 20 ha field in 2004 (Table 2). It was anticipated that these relationships might be stable and would continue over time. Stable relationships may have allowed us to describe soil N supply using clay content as an auxiliary variable for estimating soil NO3–N for variable rate fertilization (Baxter et al. 2003; Chen et al. 2004; Han et al. 2003).
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At least two factors influence variable rate N fertilization for a spatially variable field. The first is the degree of N spatial variability within the field, and the second is yield response variability within management zones to achieve yield goals with recommended N rates. Both these factors in our study were affected by soil N reserve with depth and growing season precipitation. In addition, seasonal rainfall and N reserve with depth affected corn yield differently depending on position in the field. More homogeneous subregions were delineated based on clay content and elevation. Clay content interacted with precipitation to influence soil N supply. Schepers et al. (2004) found management zones within a field for variable rate N fertilization for corn would only have been beneficial 3 out of 5 seasons even under irrigation. In general, a major difficulty in implementing variable rate technology in our field study was the inability to relate and accurately depict the variation in residual N supply and its interaction with seasonal precipitation over time (Schepers et al. 2004; Miao et al. 2006; Derby et al. 2007).
Conclusions Strong relationships between the spatial distribution of corn grain yield, soil clay content, and several soil N parameters were observed in a 64 ha field experiment in 2004. Corn grain yield was negatively related to clay content and positively related to residual soil NO3–N to depths of 0.60 and 0.90 m. These relationships were tested along with variable rate N fertilization in more homogeneous sub-regions from 2005 to 2007. Corn yield responded differently to variable rate N fertilization within these sub-regions across years. Our results indicated the difficulty in consistently associating yields with soil conditions and to clearly establish the benefit of variable rate N addition over conventional uniform application in this field across years. Nitrogen is both spatially and temporally dynamic and its availability to plants at any one location and time depends on many factors. Predictions of growing season precipitation must become more accurate if residual soil NO3–N, clay content, and other factors are to be effectively used as bases for variable rate N application. However, site-specific management zones for corn production in this field may be warranted if information about residual NO3–N with depth, clay content, and elevation are known.
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